Across Conventional Lines

The following sections are included:

• Preface

• Contents

Please refer to full text.

A series of aliphatic alcohols have been studied in HSO₃F–SbF₅–SO₂ solution. O-Protonation was observed by nmr spectroscopy with negligible exchange rates at temperatures ranging from -60 to +60°. The protonated alcohols cleave at higher temperatures to carbonium ions. The kinetics of cleavage of protonated 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol have been measured by nmr spectroscopy, and the mechanism of the reaction is discussed.

A series of aliphatic ethers have been studied in HSO₃F–SbF₅–SO₂ solution. O protonation was observed at -60° by nmr spectroscopy with negligible exchange rates. At higher temperatures, cleavage to carbonium ions takes place. The kinetics of cleavage of sec-butyl methyl ether to trimethylcarbonium ion and protonated methanol was measured.

The following sections are included:

• Acknowledgment

Trisilyloxonium ions, key intermediates in understanding the mechanism of cationic polymerization of cyclosiloxanes, were for the first time prepared by reacting trimethysilane with trityl tetrakis(pentafluorophenyl)-borate (TPFPB) in the presence of siloxanes and directly observed by NMR spectroscopy at -70 °C. Polymerization or oligomerization of siloxanes can be initiated by in-situ formed trisilyloxonium ions. The mechanism of the reactions is discussed. Structural parameters and ²⁹Si NMR chemical shifts of a number of silyloxonium ions were calculated by ab initio/IGLO methods. The results are in good agreement with the obtained experimental data.

The following sections are included:

• Results and Discussion

• Experimental Section

• Acknowledgment

Please refer to full text.

Protonation of hydrogen cyanide, alkyl- and alkenyinitriles, dinitriles, and bifunctional nitriles was studied by H¹, C¹³, and N¹⁵ nuclear magnetic resonance spectroscopy in the strong acid system FSO₃H–SbF₅–SO₂. Nuclear magnetic resonance and infrared spectroscopic studies of Meerwein's N-alkylnitrilium ions were also undertaken and the results compared with those of protonated nitriles.

Both nitrous and nitric acids were studied in sulfuric acid solutions of varying acid strengths by ¹⁵N NMR spectroscopy. The study gives new insights into the nature of intermediates present at different acid strengths. Furthermore, we have also discovered a novel redox reaction between NO₂+ and NO⁺ ions involving the intermediacy of their respective acids. A mechanism is proposed to explain the observed results.

Protonation of hydrazoic acid and alkyl azides with FSO₃H/SbF₅, HF/SbF₅, or HF/BF₃ results in the formation of stable aminodiazonium ions as shown by ¹H, ¹³C, and ¹⁵N NMR spectroscopic studies. Molecular orbital calculations of the thermodynamics of the protonation of hydrazoic acid also support preferential formation of the aminodiazonium ion. The aminodiazonium ion was also prepared in situ from NaN₃/AICI₃/HCI or (CH₃)₃SiN₃/AIC1₃/HCI [(CH₃)₃SiN₃/HF/BF₃]. Aminodiazonium salts were found to affect electrophilic amination of aromatics in high yields, with generally high regioselectivity and low substrate selectivity.

The following sections are included:

• Acknowledgment

Nitrous oxide is methylated with CH₃F → SbF₅ in SO₂F₂ or with CH₃O⁺SOCIF in SO₂ClF to give the stable methoxydiazonium ion CH₃ON₂⁺ (1), which was characterized by NMR (¹⁵N, ¹³C, ¹H) and FT IR spectroscopic studies. It is stable below -30 °C, above which it decomposes, regenerating N₂O. When reacted with aromatics, such as toluene, 1 gives only methylation products and no methoxy derivatives are formed. Spectroscopic and chemical data indicate that the mesomeric form CH₃O-N=N⁺ is a significant contributor to the overall structure of 1. Consideration of computed charge distribution (4-31 G with full geometry optimization and 4-31 G*) also supports this conclusion. Independent generation of 1 was also studied by solvolysis of methylazoxy triflate and diazotization of methoxylamine with NO⁺BF₄^-. Preparation of the elusive hydroxydiazonium ion HON₂⁺ (4) was attempted by protonation of nitrous oxide in superacids, but no long-lived ion could be observed. Diazotization of hydroxylamine with NO⁺BF₄^- gives nitrous oxide indicative of the intermediacy of 4.

The following sections are included:

• Acknowledgment

• References and Notes

The thermal decomposition of NF₄HF₂ was studied by using ¹⁸F-labeled HF₂^-. The observed distribution of ¹⁸F among the decomposition products indicates that within experimental error the attack of HF₂^- on NF₄⁺ occurs exclusively on fluorine and not on nitrogen, contrary to the predictions based on bond polarities. These results confirm the previous suggestion that the lack of pentacoordinated nitrogen species is due mainly to steric reasons.

Please refer to full text.

The following sections are included:

• Contents

The following sections are included:

• Acknowledgment

A comprehensive study of the structure of acyclic and cyclic halonium ions was carried out by cmr spectroscopy. The proton decoupled carbon-13 nmr spectra of 10 dialkylhalonium ions, 17 arylalkylhalonium ions, 14 cyclic ethylenehalonium ions, and 11 cyclic tetramethylenehalonium ions have been obtained using the Fourier transform method. The structure of the ions is discussed on the basis of the carbon-13 chemical shift data. For the cyclic halonium ions the structures considered were the symmetrically and unsymmetrically bridged ions, an equilibrium mixture of the open chain halocarbenium ions, and of bridged halonium ions with open chain β-haloalkylcarbenium ions. The α- and β-substituent effects of positively charged halogen atoms on carbon-13 chemical shifts are summarized and correlated with similar effects for halogens. Substituent effects of positively charged chlorine, bromine, and iodine are compared.

The following sections are included:

• Acknowledgment

The following sections are included:

• Acknowledgment

The following sections are included:

• Acknowledgment

• References and Notes

The following sections are included:

• Acknowledgment

The following sections are included:

• Acknowledgment

• References and Notes

Please refer to full text.

The parent hexa-, hepta-, and octacoordinate boronium ions, BH₆⁺ (1), BH₇²⁺ (4), and BH₈³⁺ (6), respectively, were found as stable minima by ab initio MP2/6-31G** and QCISD(T)/6-311G** level of calculations. The C_2ν symmetrical structure 1 with two 3c–2e bonds and C_3ν symmetrical structure 4 with three 3c–2e bonds are isostructural with their isoelectronic carbon analogues CH₆²⁺ and CH₇³⁺, respectively . Eight hydrogen atoms of T_d symmetrical structure 6 are bonded to the boron atom by four 3c–2e bonds. The protonation of BH₅ to form BH₆⁺ was found to be strongly exothermic. The reaction of BH₄⁺ (3) and H₂ might be a suitable way to generate BH₆⁺ (1) in the gas phase.

Hydrogen–deuterium exchange accompanying protolysis (deuterolysis) of the borohydride (BH₄^-) and aluminum hydride (AlH₄-) anions suggests the intermediacy of pentahydroboron (BH₅) and -aluminum (AlH₅). Pentahydroboron (BH₅) is both isoelectronic and isosteric with the methonium ion (CH₅⁺). Theoretical calculations (CNDO/2 method) favor the BH₅ structure with C_s. symmetry. These data suggest that the electrophilic attack on BH₄^- (AlH₄^-) is on the boron–hydrogen σ bond via the formation of a two-electron three-center bond and not through direct linear hydride ion transfer. In addition, trialkylboranes (alanes) and tetraalkylborates also undergo similar protolytic cleavage reaction with HF to give alkanes and BF₃ (AlF₃).

Trialkylsilanes undergo H / D exchange of their tertiary Si–H(D) bond with Dl (HI)/AlI₃. The H/D exchange was followed by ²H NMR and GC-MS analysis. The exchange reaction is an electrophilic substitution at silicon involving a five coordinate siliconium ion.

The following sections are included:

• Acknowledgment

Please refer to full text.

Electrophiles that are capable of further interaction (coordination, solvation) with strong Brensted or Lewis acids can be activated in this way. The resulting enhancement of their reactivity is great compared to that of their parent compounds under conventional conditions and indicates superelectrophile formation (i.e., electrophiles with doubly electrodeficient (dipositive) nature whose reactivity substantially exceeds that of their parent electrophiles). As representative examples the protolytic or electrophilic activation of the following electrophiles in superacidic and related strongly electrophilic media is discussed: oxonium, carboxonium, sulfonium, selenonium, and telluronium ions; acyl cations; protonated CO, CO₂, COS, and carbonic and thiocarbonic acids; heteroatom-substituted carbocations (and some of their boron analogs); halonium ions; azonium (including nitronium) and carbazonium ions. Emphasis is placed on both experimental chemical investigations and theoretical treatment of the involved systems. As prototypes for protolytically activated onium ions, the protohydronium dication (diprotonated water) and its sulfur analogue, the protosulfonium dication, are of particular significance. The protoacetyl and protoformyl dication (diprotonated carbon monoxide), diprotonated carbonic acid and carbon dioxide, diprotonated hydrogen cyanide and nitriles, as well as the protonitronium dication—among others—and their emerging chemistry are discussed as examples of superelectrophiles.

Isotopic hydronium ions D₂H¹⁷O⁺ and DH₂¹⁷O⁺ in 1:1 (molar) HF:SbF₅/SO₂ and DF:SbF₅SO₂ solutions, respectively, at -15 °C undergo slow hydrogen–deuterium exchange as monitered by ¹⁷O NMR spectroscopy. The rate of such exchange increases with the increase in the acidity of the fluoroantimonic acid medium (1:2 molar composition). The previously observed lack of exchange of isotopic hydronium ions in the somewhat weaker Magic Acid, FSO₃H:SbF₅, medium (H₀ ≈ -21.5) suggests that in the stronger HF:SbF₅ medium (H₀ ≈ -25 to -28), the exchange occurs through the intermediacy of protonated hydronium dication . Consequently, the structure and stability of protonated hydronium dication has been probed by ab initio theory. The T_d symmetry structure, 2, was found to be the minimum energy structure at the HF/6.31G* level. Although 2 at the HF/6.31G* level is thermodynamically unstable ( dissociation preferred by 59.2 kcal/ mol), it seems to have significant kinetic stability (deprotonation barrier 39.4 kcal/mol).

Isotopomeric hydridosulfonium ions undergo proton/deuterium exchange in (Magic Acid) FSO₃D·SbF₅ and FSO₃H–SbF₅–SO₂ solutions as monitored by ¹H NMR spectroscopy. The rate of exchange increases with the increase in acidity of the superacidic medium, indicating the involvement of isotopomeric tetrahydridosulfonium dications in the exchange process. The structure of tetrahydridosulfonium ion, H₄S²⁺, 2, was also probed by ab initio theory. The tetrahedral symmetry (T_d) structure of H₄S²⁺ 2, was found to be the minimum energy structure at the HF/6-31G* level. Although structure 2 is thermodynamically unstable (dissociation to H₃S⁺+H⁺ is preferred by 25.2 kcal/ mol) it appears to have significant kinetic stability (deprotonation barrier 59.2 kcal/mol).

The following sections are included:

• Acknowledgment

The following sections are included:

• Acknowledgment

• References and Notes

Hydrogen/deuterium exchange was observed by ²H-NMR spectroscopy at the CH₃ groups of the long-lived alkanoyl cations CH₃CH₂CO⁺ (4), (CH₃)₂CHCO⁺ (8), and (CH₃)₃CCO⁺ (13) when treated with excess DF·SbF₅ superacid. The intermediacy of the corresponding protio(deuterio)acylium dications is suggested to account for the exchanges. Under similar conditions, no exchange was observed in the acetyl ion CH₃CO⁺ (1) in DF·SbF₅, but at the same time its electrophilic reactivity is greatly enhanced in superacids. The acetyl cation in a superacidic medium also abstracts tertiary hydrogens of isoalkanes to give protonated acetaldehyde. Density functional theory calculations at the B3LYP/6-31G** level were carried out to support the suggested exchange mechanism as well as lack of exchange in the acetyl cation. On the basis of the calculated results, two alternative mechanisms are also suggested for the Nenitzescu rearrangement of pivaloyl chloride in isobutane with excess AlCl₃.

Protonitronium dication (1, NO₂H²⁺), the assumed superelectrophile responsible for the nitration of deactivated aromatics as well as alkanes with nitronium salts in superacidic media, was found to be a minimum at both HF/6-31G* and MP2/6-31G** levels of ab initio calculations. IGLO ¹⁵N NMR chemical shift as well as infrared frequencies were also calculated for 1. Attempts to experimentally observe 1 under superacidic conditions by ¹⁵N NMR and FT-IR (Raman) spectroscopy were so far unsuccessful, due to the expected low equilibrium concentration of 1 in the studied systems.

Under superacid conditions benzaldehyde reacts readily with benzene to give triphenylmethane in high yield. Experimental evidence supports the involvement of diprotonated benzaldehyde in the reaction. Ab initio calculations at the correlated MP2/6-31G* level on the structures and energetics of the possible dicationic intermediates indicate that the reactive intermediate involved is preferably the O,C(aromatic)-diprotonated dication and not the O,O-diprotonated system. IGLO-calculated ¹³C NMR chemical shifts of protonated benzaldehyde were compared with the experimentally observed data.

3,3-Diaryloxidoles are prepared in high yields (62–99%) by reaction of isatin or substituted isatins with aromatics in triflic acid. The reaction shows a significant dependence on acid strength which suggests the formation of diprotonated, superelectrophilic intermediates . Reaction of isatin, benzene, and acid systems of varying strength (composed of CF₃SO₃H and CF₃CO₂H) showed that the acid strength must be more acidic than H₀ = -11.5 for complete conversion to product. Reaction of isatins with mixtures of aromatics allows for the preparation of libraries of 3,3-diaryloxindole products by combinatorial synthesis.

Please refer to full text.

The following sections are included:

• INTRODUCTION

• NITRATION

• HALOGENATION

• ALKYLATION

• ACYLATION, SULFONYLATION

• OXIDATION AND OXYGENATION

• MISCELLANEOUS REAGENTS

• Acknowledgment

• References

The isolation of oxocarbenium ions (acyl cations) as stable, crystalline salts has been reported². The most stable, isolable salts are the hexafluoroantimonates^3,4. Their reported preparation involved first preparing and isolating an acyl fluoride, followed by reaction with antimony(V)-fluoride (SbF₅), a very reactive, difficult to handle, viscous Lewis acid. The present improved procedure involves a one-step reaction of readily available acyl chlorides with anhydrous hexafluoroantimonic acid (1:1 HF:SbF₅). Fluoroantimonic acid is a stable, nonviscous liquid which is conveniently handled and is also commercially available⁵…

The simplest perparation of triphenylcarbenium (trityl) salts in that of Dauben². The method utilizes the reaction of triphenylmethanol with aqueous complex acids, such as perchloric acid or tetrafluoroboric acid, in propanoic or acetic anhydride, the anhydride serving both as the solvent and the dehydrating agent…

The last decade has seen a rapid development in the observation of long-lived carbocation intermediates, generally in superacid solution². The isolation of simple alkyl and cycloalkyl carbenium ions, however, remained a challenging problem…

Meerwein's pioneering studies² established trialkyloxonium, trialkylcarboxonium and dialkoxycarbenium tetrafluoroborates (as well the related hexachloroantimonates) as versatile, widely used reagents in alkylation reactions (Meerwein-salts)…

Reagents for nonsaponificative ester hydrolysis are generally powerful nucleophiles which promote O-alkyl cleavage. They are exemplified by lithium iodide/alkylpyridines^[1], potassium thiocyanate/dimethylformamidet^[2], and alkali thiolates in aprotic solvents^[3]…

We have recently developed a new non-saponicative hydrolysis of esters using iodotrimethylsilane². This method is superior to many known procedures based on O-alkyl bond cleavage because its applicability is not limited to only methyl and activated (e.g. benzyl, anthrylmethyl) esters; moreover, strictly neutral conditions can be maintained if needed. The only drawback associated with this method is the commercial unavailability of the iodosilane reagent. This prompted us to investigate a number of other variations resulting in a simplified procedure reported in this communication…

New, efficient cleavage of carboxylic esters with iodotrimethylsilane or a mixture of phenyltrimethylsilane and iodine is described. Essentially neutral conditions can be maintained throughout the reactions. Ethers can be dealkylated by the latter method in high yields. The mechanism of the cleavage reactions is considered to include six-membered ring homopolar transition states.

A new, convenient, inexpensive alternative to iodotrimethylsilane reagent is explored. A mixture of chlorotrimethylsilane/sodium iodide in acetonitrile is found to be a better reagent than iodotrimethylsilane for the cleavage of esters, lactones, carbamates, and ethers. Cleavage of esters and lactones (10 examples) occurred somewhat slower with the present system than with iodotrimethylsilane. On the other hand, ethers (7 examples) cleaved much more readily with the present system. A feasible mechanism is proposed for this disparity. Carbamates (6 examples) also underwent facile cleavage to give the corresponding amines. The general applicability of the method has been shown using various types of substrates. The facile conversion of alcohols to iodides using the present method is also reported. Conversion of alcohols to iodides is much faster with chlorotrimethylsilane/sodium iodide than with iodotrimethylsilane, and iodides are formed in excellent yield.

Facile synthesis of symmetrical ethers is achieved by either trimethylsilyl triflate or trimethylsilyl iodide catalyzed reductive coupling of carbonyl compounds (aldehydes and ketones) with trialkylsilanes. The method was also extended to the trimethylsilyl iodide catalyzed preparation of unsymmetrical ethers by reductive condensation (of carbonyl compounds) with alkoxysilanes. The scope and limitations of the reactions are discussed with emphasis on diastereoselectivity.

Formic anhydride (1), the parent carboxylic acid anhydride, has long eluded preparation and characterization. Olah et al.^[1] first reported in 1955 the possible formation of (1) in the reaction of formyl fluoride with metal formates at low temperature. No spectroscopic observations or physical data were, however, obtained. In 1964. Muramatsu et al.^[2] assumed the intermediacy of (1) in the formylation of p-nitrophenol and L-leucine with an ethereal solution of formic acid and dicyclohexylcarbodiimide (DCC); no isolation was attempted nor were physical data obtained. Stevens et al.^[3], in the same year, reported the formation of (1) by the thermal disproportionation of formic-acetic anhydride and of mixtures of formic acid and carboxylic anhydrides; proof of formation of (1) was based on the appearance of a ¹H-NMR singlet at δ = 9.0, which, however, might have been due to a mixed anhydride or other species…

Using formyl fluoride as acylating agent a new Friedel–Crafts-type aldehyde synthesis and formylation method was developed for the preparation of C-, O-, S- and N-formyl derivatives.

The following sections are included:

• Introduction

• Results and Discussion

• Experimental Section

• Acknowledgment

Carbon monoxide, carbon dioxide, methyl alcohol, formic acid, carbonyl sulfide and carbon disulfide are reduced to methane by ionic hydrogenation using sodium borohydride and superacidic trifluoromethanesulfonic acid. Experimental details are given and the mechanism is discussed.

It has long been known that magnesium in methanol (or in ethyl or isopropyl alcohol) can been used for the reduction of carbon-heteroatom double bonds^[1] and N-oxides^[2]; more recently, the selective reduction of α,β-unsaturated nitriles^[3a] and aryl-substituted ethylenes has also been achieved in this way^[3b]. In all these reactions, however, isolated, nonactivated double or triple bonds were found unaffected by the reducing system. We now wish to report that addition of catalytic palladium metal on carbon to the Mg/CH₃OH reagent dramatically enhances its reactivity, thus allowing rapid and facile reduction of nonactivated multiple bonds (Table 1)…

Aryl and heteroaryl boronic acids react with N-iodosuccinimide and N-bromosuccinimide to give the corresponding iodoand bromo- arenes in good to excellent yields. The reaction is usually highly regioselective and yields only the ipso-substituted product. Esters of arylboronic acids react similarly, but less readily.

Please refer to full text.

The reaction of ozone with a series of alkanes in superacid–SO₂CIF solution has been investigated. Product analysis reveals that the reaction pathway involves oxygenation followed by a carbon to oxygen alkyl group migration analogous to the acid-catalyzed cleavage–rearrangement reaction of hydroperoxides. Mechanistic studies are consistent with electrophilic insertion into a σ bond by protonated ozone, O₃H⁺.

The oxygenation of alkanes with hydrogen peroxide in the presence of FSO₃H–SbF₅, FSO₃H, H₂SO₄, and HF was studied under typical electrophilic conditions. From the results obtained, it is concluded that the reactions proceed in strong acids via initial electrophilic hydroxylation of the appropriate σ bonds of the alkanes by the incipient hydroxyl cation, formed through protolytic cleavage of H₂O₂ through the hydroperoxonium ion H₃O₂⁺.

The reaction of ozone with aliphatic alcohols, ketones, and aldehydes in magic acid–SO₂CIF or FSO₃H solution has been investigated. Experimental results indicate the reactions proceed via electrophilic insertion of protonated ozone into the C–H σ bonds of the substrates at positions γ- or further removed from the oxonium centers present in the superacidic media. No oxidation products of the protonated (and thus protected) carbinol or carbonyl groups were observed. The reactions allow the preparation of bifunctional oxygenated derivatives.

Bis(trimethylsilyl) peroxide/trifluoromethanesulfonic (triflic) acid was found to be an efficient electrophilic oxygenating agent for adamantane and diamantane. With adamantane the major reaction product is 4-oxahomoadamantane (isolated in 79% yield) obtained through C-C σ-bond insertion with very little 1-adamantanol, the C-H insertion product. In the case of diamantane two isomeric oxahomodiamantanes were obtained along with two isomeric bridgehead diamantanols corresponding to C-C and C-H σ-bond insertions.

The following sections are included:

• Acknowledgment

Please refer to full text.

A new method of nitration of aromatic compounds has been developed by use of stable nitronium salts, first, with nitronium tetrafluoroborate.

The infrared spectra of complexes of NO₂F with BF₃, PF₅, and SbF₅ have been studied. In each case bands at about 2360 cm^-1 were recorded and have been attributed to the nitronium ion NO_2⁺. Bands consistent with a complex anion of the form MF_n+1^- have been observed, though with less accuracy. It is considered correct to designate these complexes as NO_2⁺BF_4^-, NO_2⁺PF_6^-, and NO_2⁺SbF_6^-.

The competitive rates and isomer distributions of the BF₃ catalyzed nitration of benzene, alkyl-, halo-, and substituted polymethylbenzenes with methyl nitrate in nitromethane solution were determined. Relative nitration rates of alkylbenzenes were shown to correlate more closely with the stability of the corresponding π complexes, than those of the σ complexes. This trend is greatly enhanced with increasing alkyl substitution. Relative rates of nitration of substituted benzenes, 3-substituted toluenes, 2-substituted p-xylenes, and 3-substituted durenes were correlated with help of the Yukawa–Tsuno equation. The correlation of r values with the nature of the involved transition states is discussed.

Alkylbenzenes are nitrated in high yields with butyl nitrate or acetone cyanohydrin nitrate catalyzed by a perfluorinated resinsulfonic acid (Nafion-H) catalyst. The reactions are very clean and do not require any aqueous basic workup. Nitrations were also carried out with nitric acid, under azeotropic distillation conditions to remove water and thus prevent dilution of the acid, and with dinitrogen tetroxide.

The reversibility of electrophilic aromatic nitration was proven in certain, specific cases of superacid-catalyzed transfer nitration reactions. In the reaction of benzene, toluene, and mesitylene with 9-nitroanthracene and pentamethylnitrobenzene, nitrobenzene, nitrotoluenes, and nitromesitylene were obtained in the presence of superacids. The primary kinetic hydrogen isotope effect, k_H/k_D, of 2.25 ± 0.05 in the nitration of anthracene-d₁₀ with nitronium hexafluorophosphate in nitromethane solution is also in accord with the reversibility of the nitration of anthracene.

The transfer nitration of aromatics with various N-nitropyridinium and quinolinium salts (PF₆^- or BF₄^-) was studied. The nitrations were found to take place via a nucleophilic displacement pathway, involving the N-nitropyridium ions themselves and not free nitronium ion. Steric factors were, however, shown to play an insignificant role in determining the positional selectivity of nitration. Positional and substrate selectivities were found to be independent of one another and are suggested to be determined in two separate steps.

Naphthalene was nitrated with a variety of nitrating agents. Comparison of data with Perrin's electrochemical nitration [Perrin, C. L. (1977) J. Am. Chem. Soc. 99, 5516–5518] shows that nitration of naphthalene gives an α-nitronaphthalene to β-nitronaphthalene ratio that varies between 9 and 29 and is thus not constant. Perrin's data, therefore, are considered to be inconclusive evidence for the proposed one-electron transfer mechanism for the nitration of naphthalene and other reactive aromatics. Moodie and Schoefield [Hoggett, J. G., Moodie, R. B., Penton, J. R. & Schoefield, K. (1971) Nitration and Aromatic Reactivity (Cambridge Univ. Press, London)], as well as Perrin, independently concluded that, in the general scheme of nitration of reactive aromatics, there is the necessity to introduce into the classical Ingold mechanism an additional step involving a distinct intermediate preceding the formation of the Wheland intermediate (σ complexes). This view coincides with our two-step mechanistic picture [Kuhn, S. J. & Olah, G. A. (1961) J. Am. Chem. Soc. 83, 4564–4571] of the nitronium salt nitration of aromatic hydrocarbons (including benzene and toluene), in which low substrate selectivity but high positional selectivity was found, indicating the independence of substrate from positional selectivity.

Electrophilic nitration of toluene and benzene was studied under various conditions with several nitrating systems. It was found that high orthopara regioselectivity is prevalent in all reactions and is independent of the reactivity of the nitrating agent. The methyl group of toluene is predominantly ortho-para directing under all reaction conditions. Steric factors are considered to be important but not the sole reason for the variation in the ortho/para ratio. The results reinforce our earlier views that, in electrophilic aromatic nitrations with reactive nitrating agents, substrate and positional selectivities are determined in two separate steps. The first step involves a π-aromatic- ion complex or encounter pair, whereas the subent step is of arenium ion nature (separate for the ortho, meta and para positions). The former determines substrate selectivity, whereas the latter determines regioselectivity. Thermal free radical nitration of benzene and toluene with tetranitromethane in sharp contrast gave nearly statistical product distributions.

The nitration of toluene and anisole was studied with nitrating systems of varying reactivity. High regioselectivity of ortho-para over meta substitution was maintained in all nitrations, regardless of the reactivity of the nitrating system. At the same time, the amount of meta substitution stayed low (3% or less), even when the fast reactions may have reached the encounter-controlled limit. Because the nitration of o-xylene, in which both ring positions are activated by the effect of a methyl group, also does not show any diminishing of regioselectivity, the possibility of a dual mechanistic pathway, in which the activated position would react by a fast, encounter-controlled path, whereas the nonactivated meta position by a slower σ-type path, can be ruled out. The data unambiguously prove that the high regioselectivity of electrophilic aromatic nitration is independent of the reactivity of the reagent, because no significant increase of meta substitution of toluene or anisole was observed, regardless of the activity of the nitrating system. No selectivity-reactivity relationship is thus evident and the ortho-para directing effect of primary substituents over meta substitution is always maintained. The variation in the amount of the meta isomer, up to the observed limit of about 3% in the case of toluene and <2% for anisole, is probably significant but, at the present time, cannot be quantitatively evaluated with the ±0.5% overall reproducible accuracy of the nitrations. Steric factors, such as increasing bulkiness of the nitrating agent, also can affect the ortho-para isomer ratios but are not considered to be the only reason for the observed variations, which reflect the specific nitrating systems, affecting the nature and position of the transition state of highest energy on the reaction pathway.

The following sections are included:

• CONTENTS

The reactivity of nitronium tetrafluoroborate in the nitration of deactivated di- and trifluoronitrobenzenes is enhanced in superacidic trifluoromethanesulfonic (triflic) acid compared with aprotic methylene chloride and sulfolane solutions. The enhanced reactivity is discussed in terms of better solubility and higher dissociation of the nitronium salts, as well as protosolvation of by superacids.

The nitration of various deactivated arenes (including methanesulfonyl-, nitro-, and polyhalobenzenes) was carried out in good yield with mixed nitric–triflatoboric superacid. For example pentafluorobenzene gave pentafluoronitrobenzene in 99% yield, nitrobenzene to m-dinitrobenzene in 92% selectivity with 85% overall yield, and methyl phenyl sulfone gave only the m-nitro isomer in 78% isolated yield. Thus the new nitrating system gives high regioselectivity and yields under generally mild reaction conditions. The reagent system is compatible with many functional groups of arenes.

Toluene is cyanated and nitrated with cyano- and nitrodiazonium ion, generated via in situ diazotization of cyanamide and nitramide, respectively, with NO⁺BF₄^-. Attempted fluorination with fluorodiazonium ion, prepared from cis-difluorodiazene and arsenic pentafluoride, gave only trace amounts of fluoroaromatics.

The following sections are included:

• Results and Discussion

• Experimental Section

• Acknowledgment

• References and Notes

Desilylative nitration of methyl- and ethylsilanes with nitronium tetrafluoroborate in sulfolane solution gives nitromethane and nitroethane, respectively. Higher alkylsilanes are also nitrated, but the reactions are followed by HNO₂ elimination. The method represents the first successful nitrodesilylations at saturated carbon. Allylsilanes react cleanly with NO₂BF₄ in methylene chloride solution to give nitropropenes in good to excellent yield. These reactions, however, involve initial attack by NO₂⁺ on the allylic π-system followed by fluoride-induced trimethylsilyl elimination and not direct desilylative nitration at saturated carbon. 1-(Trimethylsilyl)but-2-ene gives exclusive secondary 3-nitrobut-1-ene and not the primary direct nitrodesilylation product 1-nitrobut-2-ene.

Alcohols and polyols can be converted into the corresponding nitrate esters by O-nitronium with either mixed nitric-sulfuric acid^2,3, nitronium tetrafluoroborate⁴, or acetyl nitrate⁵. Nitrate esters can also be obtained by metathesis of the corresponding alkyl halides with silver nitrate². The first three methods involve acidic conditions with frequently elaborate safety precautions³, whereas the last method uses an equimolar amount of the expensive silver salt. Moreover, the presence of even very small amounts of nitrite ion give highly unstable nitrite esters. A recently published procedure³ claims to involve less hazards. It uses, however, highly acidic conditions with a strong oxidizing agent in contact with an organic solvent. Such mixtures of reagents are known to be potentially very dangerous. We have previously shown the efficiency of N-nitropyridinium salts for transfer nitration reactions^6,7. We now wish to report an improved, mild procedure for the preparation of alkyl nitrates via transfer nitration of the corresponding alcohols (as well as polyols) with N-nitrocollidinium tetrafluoborate…

Please refer to full text.

The following sections are included:

• Experimental
  • Preparation of benzyl fluoride
  • Reaction of benzene with paraformaldehyde and hydrogen fluoride
  • Preparation of p-fluorobenzyl fluoride
  • Reaction of fluorobenzene with paraformaldehyde and hydrogen fluoride

• SUMMARY

• LITERATURE

The following sections are included:

• EXPERIMENTAL

The following sections are included:

• Experimental

The following sections are included:

• Acknowledgment

The following sections are included:

• Acknowledgment

Pyridinium poly(hydrogen fluoride) (30% pyridine–70% hydrogen fluoride) reagent, a stabilized, less-volatile form of hydrogen fluoride, was found to be a convenient and effective fluorinating agent. Fluorination, halofluorination, nitrofluorination, and hydrofluorination of olefins were achieved using the reagent. The in situ diazotization and subsequent fluorinative dediazonization of α-amino acids, aminoarenes, and carbamates yielded α-fluorocarboxylic acids, aryl fluorides, and fluoroformates, respectively. Geminal dihalides and α-halo ketones were reacted with mercuric oxide in pyridinium poly(hydrogen fluoride) to form geminal difluorides and α-fluoro ketones. Solutions of alkali halides in pyridinium poly(hydrogen fluoride) were also found to be effective halogenating agents for aminoarenes, via in situ diazotization and subsequent nucleophilic dediazonization by the corresponding halides, as well as for alcohols, via S_N2 type displacement reactions. Diazo ketones and diazoalkanes also reacted smoothly with halide ions in pyridinium poly(hydrogen fluoride) solution to give the corresponding geminally halofluorinated compounds.

Poly-4-vinylpyridinium poly(hydrogen fluoride)(PVPHF), containing 35–60 % hydrogen fluoride by weight, was prepared as a solid hydrogen fluoride equivalent reagent. PVPHF with 60 % hydrogen fluoride by weight was found to be a versatile fluorinating agent for the hydrofluorination and bromofluorination of alkenes and alkynes, fluorination of alcohols as well as other fluorination reactions. Low hydrogen fluoride content PVPHF (3 equivalents of hydrogen fluoride to 1 equivalent of 4-vinylpyridine unit) was also found to be an efficient reagent for bromofluorination of alkenes in the presence of 1,3-dibromo-5,5-dimethylhydantoin. Fluorosulfonic acid-modified PVPHF showed enhanced reactivities for the fluorination of secondary alcohols.

The following sections are included:

• Acknowledgment

1-(chloromethyl)-4-fluoro-1,4-diazabicyclo[2.2.2]octane bis (tetrafluoroborate) [Selectfluor™ F-TEDA-BF₄ (TEDA = triethylenediamine)] in the presence of trifluoromethanesulfonic acid has been found to be a very effective reagent system for the direct electrophilic fluorination of a wide variety of aromatic compounds under mild reaction conditions to the corresponding fluoroaromatics in good to excellent yields.

The following sections are included:

• PREFACE

• CONTENTS

The following sections are included:

• Introduction

• Fluorinating agents

• Applications of some fluorinated reagents in general organic synthesis

• Acknowledgments

• References

• BIOGRAPHIC NOTES

Please refer to full text.

The following sections are included:

• Acknowledgment

Arenemercurinium ion complexes were prepared and studied by proton and carbon-13 NMR spectroscopy. It is concluded that the complexes are involved in fast exchange.

The long-lived ethylene, cyclohexene, and norbornenemercurinium ions prepared in superacidic, low-nucleophilic media have been studied by ¹³C and ¹⁹⁹Hg NMR spectroscopy. The norbornenemercurinium ion shows temperature-dependent ¹³C and ¹⁹⁹Hg NMR spectra, consistent with equilibration via rapid hydride and Wagner-Meerwin shifts. The ¹⁹⁹Hg NMR shifts of a series of alkylmercury bromides were also obtained in order to elucidate the effect of methyl substituents on ¹⁹⁹Hg NMR chemical shifts.

¹H and ¹³C NMR spectroscopic study of cyclobutadieneiron tricarbonyl in fluorosulfuric acid solution at low temperature shows iron protonation with formation of a static σ–π complex resulting from an unusual π to σ change in the metal–ligand bonding; the observation of the geminal ¹H₄–Fe–¹H_x (29 Hz) and ¹³C–Fe–H_x (81.2 Hz) coupling substantiates the proposed structure; evidence of σ–π complex formation was further provided by studies in deuterated fluorosulfuric acid solution.

Acyclic and cyclic cisoid π-pentadienyliron tricarbonyl cations were studied by ¹³C NMR spectroscopy in strong acids. The origin of the unusual stability of these ions and their fluxional behavior are discussed. The nature of bonding and the structure of the protonated norbornadieneiron tricarbonyl were also studied and are discussed.

The cumylchromium tricarbonyl cation was prepared under long-lived ion conditions, and studied by NMR (¹H and ¹³C) spectroscopy. For comparison, the carbon-13 NMR parameters of three cycloheptatrienyl–M(CO)₃ cations (M = Mo, Cr, and W) were also determined and are reported. Based on the ¹³C NMR studies the origin of the unusual stabilization of the cumylchromium tricarbonyl ion is discussed. The σ⁺ substituent constant for the Cr(CO)₃ group and the fraction of the unit positive charge transmitted into the M(CO)₃ groups were qualitatively estimated.

Diaryl phosphorofluoridates and diaryl phosphorofluoridothioates were prepared and their properties investigated.

Protonation, cleavage, and alkylation reactions of phosphorothioic acids, H₃PS_nO_4-n (n = 0-3), alkyl mono- and dithiophosphates, and systematically substituted phosphorothioites P(SC₂H₅)_n(OCH₃)_3-n, phosphorothiolates, OP(SC₂H₅)_n(OCH₃)_3-n, and phosphorothionates, SP(SC₂H₅)_n(OCH₃)_3-n (n = 0-3), were studied in fluorosulfuric acid solution by ¹H and ³¹P NMR spectroscopy (primarily at -60 to -80°). Trivalent phosphorus compounds are protonated at phosphorus, phosphoryl compounds at the phosphoryl oxygen, and thiophosphoryl compounds at the thiono sulfur atom. In the last case, the two-bond coupling, ²J_pSH, is observed below -50°. In general, sulfur is less able than oxygen to donate nonbonded electron pairs to phosphorus in the stabilization of phosphonium ions. The change in the ³¹P chemical shift of thiophosphates and thiophosphites upon protonation is a function of the site of protonation and the relative numbers of oxygen and sulfur atoms bonded to phosphorus. Isopropoxymercaptophosphonium ions undergo rearrangements by isopropyl group migration from oxygen to sulfur. All intermediates in these processes can be individually observed.

The following sections are included:

• Experimental Section

• Acknowledgment

• References and Notes

The reaction of arsenic fluoride with aliphatic amines was studied. Secondary amines gave N,N-dialkyl amidodifluoroarsenites. Primary amines while reacting similarly lose HF subsequently and yield N-alkyl imidofluoroarsenites.

A ¹³C nuclear magnetic resonance spectroscopic study of dimeric trimethyl-, triethyl-, tricycloproppyl-, and triarylaluminums is reported. The five-coordinated bridging carbons are found consistently more shielded than the terminal carbons, in accordance with the increased p-character of the former. The nature of bridging two-electron three-centered Al—C—Al bonds is discussed. ¹³C nuclear magnetic resonance shifts of several nido and closo carboranes containing five and six coordinated carbons and their ¹³C–¹H spin-spin coupling constants were also obtained. The relationship between the carbon chemical shifts and coordination number of the carbon atom is discussed. There is approximately a 20- to 40-ppm shielding of the ¹³C chemical shifts of five- and six-coordinated carbons, compared with those of four valent carbons, with a simultaneous general increase of J_C-H coupling constants.

Anhydrous aluminum trichloride or bromide when heated in a 2:1 molar ratio with aluminum powder as a suspension in dry n-heptane or methylcyclohexane was found to be partially reduced to aluminum dichloride or dibromide. Ultrasound treatment (sonication) significantly promotes the reaction. Aluminum dichloride in higher purity was obtained by the reaction of gaseous aluminum trichloride with aluminum metal in a high-vacuum reactor, allowing subsequent investigation by IR spectroscopy. An aluminum sub-halide of the form Al₂(i-Bu)_4-xCl_x was also prepared through the reaction of tetraisobutyldialane and HCl at low temperature. Both materials were investigated by IR spectroscopy and compared to AlCl₂ prepared and isolated through the codeposition of aluminum atoms and molecular chlorine in a solid argon matrix. The matrix study characterized AlCl₂ together with AlCl and AlCl₃, which were also formed in the system. The paramagnetic aluminum dihalides, i.e. AlCl₂ and AlBr₂, are associated in the condensed state (except under matrix isolation conditions where they are monomeric). An ESR study of the pyridinium complex of AlCl₂ was carried out and showed its paramagnetic nature. In the present study, for simplicity, the reactions of aluminum dihalides are considered as those of the dimers but could involve higher associated oligomers. MNDO calculations on the heats of formation of several possible isomeric structures of Al₂Cl₄ indicate the preference for both halogen bridging and significant Al–Al bonding in the dimer. Reaction of AlCl₃ + Al with ethylene, the Hall and Nash reaction, was reinvestigated by ¹³C and ²⁷Al NMR spectroscopy. The reaction was found to give, besides ethylaluminum sesquichloride, 1,2- and 1,1-bis(dichloroaluminio) ethanes. Cyclohexene in a similar reaction gives, although less readily, 1,2-bis(dichloroaluminio)cyclohexane. The reactions are indicative of addition of (AlCl₂)₂ to the olefins. Alkyl- and arylaluminum monohalides are intermediately formed in the reaction of alkyl halides or halobenzenes with active aluminum powder. These divalent aluminum halides are also considered to be dimeric in nature and immediately react with excess of the alkyl (aryl) halides to form the corresponding sesquihalides. In contrast, aluminum dihalides formed in the aluminum trihalide–aluminum metal systems react with alkyl or aryl halides to give alkyl(aryl)aluminum dihalides. Sonication was found to significantly promote these reactions.

The following sections are included:

• Acknowledgment

²⁹Si and ³³Cl NMR spectroscopic and X-ray crystallographic study of triphenylsilyl perchlorate shows it to be a covalent perchloryl ester in both solution and the solid state. The results are in accord with earlier studies notably those of Wannagat but contrast the recent claim by Lambert et al. (J. Am. Chem. Soc. 1986, 108, 2482) for NMR observation of long-lived ionic triphenylsilyl perchlorate.

Ab initio/IGLO (individual gauge for localized orbital) ²⁹Si NMR studies of trisubstituted silicenium and their related arenium ions have been carried out. Comparison of the obtained results with the reported experimental data indicate that to date no free long-lived trisubstituted silicenium cations have been observed in solution. The reported results in π-donor arene solvents clearly indicate the formation of silylated Wheland intermediates (arenium ions), whereas in previously discussed heteroatom n-donor solvents (such as ethers or nitriles) the related silylated onium (such as oxonium or nitrilium) ions are formed. At the same time due to the high electrophilicity of trivalent silicon and its affinity for oxygen, halogen, etc, counter ions, such as perchlorate or fluoroantimonates, are also unsuitable for the observation of long-lived trisubstituted silyl cations.

Although the parallel between ²⁹Si NMR chemical shifts in silicon compounds and ¹³C NMR chemical shifts in the analogous carbon compounds has been recognized, no systematic study was made to explore the relationship. We have carried out such a study extending the relationship from tetraalkyl-substituted silanes to polarized trimethylsilyl halides in search for still elusive stable silicenium ions. Due to the high affinity of silicon for oxygen and fluoride (or chloride), methods used for the preparation of stable carbocations were not applicable. Using bromosilanes and aluminum tribromide, in methylene bromide solution, strongly polarized donor–acceptor complexes of Me₃Si^δ+Br → ^δ-AlBr₃ were observed but no free silicenium ions.

A series of phenylsilyl anions, including Ph₃SiLi, Ph₂MeSiLi, PhMe₂SiLi, and Me₃SiK, has been studied by ¹³C and ²⁹Si NMR spectroscopy with particular emphasis to determine the effective delocalization of charge away from the silicon atom. The ¹³C and ²⁹Si data are discussed in this regard and evaluated as indicative of the trend of the delocalization of charge. As compared to the corresponding carbanions, the delocalization is reduced by a factor of about 10 for Ph₃SiLi and 4 for Ph₂MeSiLi and PhMe₂SiLi, indicating the substantially decreased degree of Si–C pπ–pπ interaction and thus Si=C character, as compared to carbanions showing strong C–C pπ–pπ interaction and thus double-bonded quinoidal character.

The reaction of gaseous diborane (hexadeuteriodiborane) with silane (tetradeuteriosilane), alkyl and arylalkylsilanes was studied on a high vacuum line at temperatures betwen 25 and 125°C. Hydrogen -deuterium exchange and cleavage reactions were observed indicative of electrophilic borane (or diborane) insertion into Si–H, Si–C, and Si–Si single bonds. For comparison, reaction of related alkanes (arylalkanes) was also studied under comparable mild reactions.

Please refer to full text.

The rôle of carbocations in cationic polymerizations is discussed, differentiating π- (i.e. alkene and arene), σ- (i.e. alkane) and n-donor (i.e. ether, sulfide, nitrogen compound) monomers as the reactive nucleophiles. The mechanism of the initiation step in alkene polymerization is evaluated considering both protic (i.e. coinitiator containing) and aprotic systems. The aluminum bromide catalyzed polymerization of isobutylene, as an example, can involve besides the conjugate protic acid, bromoalanation intermediates in aprotic media, which, however, themselves rapidly can undergo elimination, forming HBr in the originally acid free system. The involvement of dialkylhalonium ions in alkyl halide solvent systems is also of substantial interest and may explain increasing deactivation in the order RI > RBr > RCl. Oxidative polycondensation of π-donor arenes (such as benzene) and σ-donor alkanes (such as methane) is discussed in the context of the general carbocation concept, involving trivalent carbenium ion intermediates in the former and pentacoordinated carbonium ions in the latter case, as the reactive alkylating agents responsible for the ionic propagation reactions. n-Donor monomers, such as ethers, sulfides or miscellaneous nitrogen compounds, undergo polymerization via nucleophilic displacement reactions on the polarized carbocationic sites of intermediate alkylated or protonated onium ions.

The initiation of the cationic ring-opening polymerization of lactones and tetrahydrofuran by electron-deficient organosilicon species has been investigated. The formation of the long-lived silylcarboxonium ion 1a from γ-butyrolactone and the silyloxonium ion 2a from THF has been evidenced by ¹H, ¹³C, and ²⁹Si NMR. The structure and NMR chemical shifts of 2a were also calculated using DFT/IGLO methods, and the results are in good agreement with experimental data. The nature of the ions is discussed in terms of the reactivity with respect to their methylated analogues. Polymerization of lactones initiated by in situ formed silylcarboxonium ions showed a much higher rate than that initiated by trimethylsilyl triflate, while polymerization of tetrahydrofuran in a similar manner failed. The mechanism of polymer initiation is discussed.

Fischer in 1943 described the AlCl₃ + Al + TiCl₄ catalyzed polymerization of ethylene to solid polyethylene at temperatures of 130–180 °C and pressures of 30–80 atm. No characterization of the polyethylene formed was given nor any mechanism suggested for the reaction. Recently, Martin et al reinvestigated the Fischer polymerization and reported that the polyethylene obtained in admixture with Friedel–Crafts-type hydrocarbon oils is highly branched. In contrast, our reinvestigation of the Fischer polymerization of ethylene showed that if the temperature of the reaction mixture is kept in the range suggested by Fischer (130–180 °C) or below, besides Friedel–Crafts-type oils, linear high-density polyethylene similar to Ziegler polyethylene is obtained. Without proper temperature control, however, the exothermic reaction can lead to the results reported by Martin et al. Study of the mechanism of the Fischer polymerization indicates that the system is well suited for the in situ formation of ethylalumiinum chlorides via the Hall and Nash reaction, as well as isomeric bis(dichloroaluminio)ethanes which, with TiCl₄, form the active Ziegler–Natta-type coordination polymerization catalysts.

The following sections are included:

• Acknowledgment

Please refer to full text.

Superacid catalyzed isomerization of butane was studied. Highly efficient isomerization of butane to 2-methylpropane (isobutane) was achieved using fluorosulfuric acid containing up to 5% of hydrogen fluoride acting as a protic co-acid. The isomerization when carried out in excess FSO₃H at 21° C in a flow system gave ~70% conversion to 2-methylpropane with generally less than 3% cracking. When 2-methylpropane was isomerized under similar conditions about 12% butane was formed. Attempted isomerization of pentane and hexane with prolonged reaction times in a static system results in predominant protolytic cleavage (cracking).

A process for alkylating an aliphatic hydrocarbon having isobutane with an alkenyl hydrocarbon with isobutylene in the presence of a liquid onium polyhydrogen fluoride complex as the reaction medium and catalyst at a temperature between -20° and 70° and pressure between atmospheric and 200 psi and at a time sufficient to form a high octane alkylate thereof.

The HF·BF₃ superacid-catalyzed hydrocracking of tar sands bitumens and asphaltenes leads to deep-rooted chemical changes which affect both the aliphatic and aromatic constituents of the feedstocks even under mild experimental conditions, resulting in high yields of volatiles and liquid products. In contrast to conventional catalytic hydrocracking following free-radical mechanisms, the superacid-catalyzed reaction follows ionic mechanisms, yielding products very different from those following the free-radical reactions. When methylcyclohexane (MCH) is employed as a hydrogen-donor solvent, new products resulting from the oligomerization of MCH appear. MCH neat, without bitumen or asphaltene, does not oligomerize under identical conditions. These preliminary results point to the commercial potential of using the volatile HF·BF₃ superacid in the upgrading of bitumen and the oligomerization of cycloalkanes.

HF:BF₃: H₂ catalysed depolymerization and hydroliquefaction of coal was studied. This superacidic system was found to be extremely effective for low temperature liquefaction of coal. Illinois No. 6 coal could be solubilized in pyridine to the extent of >90 % by treating it at ≈105°C for 4 h. Under somewhat more elevated temperature (150–170°C) cyclohexane extractabilities of up to 22 % and distillability of up to 28% is achieved. A hydrogen donor solvent such as isopentane is shown to improve the efficiency of the superacidic catalyst for the conversion of coal to cyclohexane soluble products.

The conversion of heterosubstituted methanes, such as methyl alcohol, dimethyl ether, methyl mercaptan, dimethyl sulfide, methylamines, and methyl halides, to ethylene and hydrocarbons derived thereof takes place over bifunctional acidic–basic-supported transition-metal oxide or oxyhalide catalysts, such as tungsten oxide supported on alumina, between 300 and 350 °C. The conversion of methyl alcohol starts with bimolecular dehydration to dimethyl ether followed by acid-catalyzed transmethylation giving trimethyloxonium ion (or related catalyst-bound methyloxonium ion). The trimethyloxonium ion then undergoes base-induced deprotonation forming a catalyst surface-bound methylenedimethyloxonium ylide. Intermolecular methylation of the ylide, indicated by experiments using singly ¹³C-labeled dimethyl ether, gives methylethyloxonium ion thus providing the crucial first C–C bond. No intramolecular Steven's-type rearrangement takes place, and methyl ethyl ether is not a significant intermediate as also shown in experiments comparing the products formed from reacting CD₃OCH₂CH₃ under similar conditions. The ethyloxonium ion readily undergoes β-elimination forming ethylene. Initialy formed ethylene subsequently can undergo further reaction with the ylide giving via cyclopropane propylene or it can undergo more complex alkylation/oligomerization/cracking reactions giving a mixture of alkanes, alkanes and via cyclization–dehydrogenation aromatics. The complexity of these processes was shown by reacting ethylene itself, as well as ¹³CH₃OH and ethylene, under conditions of the condensation reaction. It is also necessary to differentiate initially formed ethylene via direct C₁ → C₂ conversion from that formed in secondary processes together with higher condensation products. The conversion of methyl mercaptan (dimethyl sulfide), methyl halides, and methylamines to ethylene follows similar onium ylide pathways.

The catalytic monohalogenation (chlorination and bromination) of methane was achieved over either supported solid acid (such as FeO_xCl_y/Al₂O₃, TaOF₃/A1₂O₃, NbOF₃/A1₂O₃, ZrOF₂/Al₂O₃, SbOF₃/Al₂O₃, SbF₅/graphite, and Nafion-H/TaF₅) or platinum metal (Pt/A1₂O₃ and Pd/BaSO₄) catalysts. The reactions were carried out at temperatures between 180 and 250 °C, with GHSV of 50–1400 giving 8–58% conversions with selectivity in methyl chloride (bromide) generally exceeding 90%. Limited methylene halide formation accompanies the reactions, but no formation of haloforms or carbon tetrahalides was observed. The mechanism of the halogenations is considered to involve insertion of a surface -coordinated electrophilic halogen species or electron-deficient metal site into a methane C–H bond involving five-coordinate intermediate carbonium ion formation, with subsequent cleavage-halogenolysis giving the monohalogenated methane. Catalytic hydrolysis of methyl halides was also studied over γ-alumina-supported metal oxide/hydroxide catalysts, giving mixtures of methyl alcohol and dimethyl ether. Combining the selective monohalogenation of methane with subsequent hydrolysis and oxyhalogenative recycling of byproduct HX allows conversion of methane to methyl alcohol/dimethyl ether, a route which offers an alternative to the presently exclusively used preparation of methyl alcohol via syngas. The preparation of methyl halides and/or methyl alcohol/dimethyl ether directly from methane also offers a way to convert methane via previously described bifunctional acid–base-catalyzed condensation into ethylene and subsequently into homogeneous lower olefins and/or higher hydrocarbons.

Fuel cells that can operate directly on fuels such as methanol are attractive for low to medium power applications in view of their low weight and volume relative to other power sources. A liquid feed direct methanol fuel cell has been developed based on a proton-exchange membrane electrolyte and Pt/Ru and Pt-catalyzed fuel and air/O₂ electrodes, respectively. The cell has been shown to deliver significant power outputs at temperatures of 60 to 90 °C. The cell voltage is near 0.5 V at 300 mA/cm² current density and an operating temperature of 90 °C. A deterrent to performance appears to be methanol crossover through the membrane to the oxygen electrode. Further improvements in performance appear possible by minimizing the methanol crossover rate.

A regenerative electrochemical cell system based on a fuel cell to oxidize methyl alcohol or other oxygenated hydrocarbons coupled, with a regenerative cell to reduce carbon dioxide to form oxygenated hydrocarbons is disclosed. Methods to reversibly interconvert oxygenated hydrocarbons and carbon dioxide, to recycle carbon dioxide produced as a by-product of industrial processes, and to store and release electrical and chemical energy are also disclosed.

Please refer to full text.

A series of 6-X,2,4-dinitroanisoles and their carbanionic methoxide addition products (Meisenheimer complexes) have been examined by ¹³C NMR spectroscopy. Variation of X (CF₃, H, Cl, F, CH₃) does not affect the charge distribution pattern in the complexes as reflected by their ¹³C NMR shifts. Only in the case where X is NO₂ can a change be observed. The ¹³C NMR studies indicate that the cyclohexadienylic carbons carry about 0.3–0.4 e more negative charge than the corresponding carbons in their aromatic precursors. The additional charge is located on C₂, C₄, and C₆.

By analogy with the homocyclopropenyl (1) and homotropylium cations (2), cyclohexadienyl anions (3) might also be expected to be nonplanar, homoaromatic species. Several such anions, prepared by proton abstraction from the corresponding 1,3- and 1,4-cyclohexadienes, were studied by ¹H and ¹³C NMR spectroscopy in ND₃ solution. Interpretation of the chemical shifts, coupling constants, and the results of low-temperature studies at 300 (¹H NMR) and 67.88 MHz (¹³C NMR) indicated these cyclohexadienyl anions rather to be planar nonhomoaromatic species. MINDO/3 calculations on the parent cyclohexadienyl anion agreed; the planar form was found to be a shallow energy minimum. The large spatial separation between the π-system termini and the stabilizing interaction of π* (CH₂) with the HOMO of the pentadienyl fragment are responsible for the absence of the expected homoaromaticity.

A series of methylated biphenylene dianions were generated by reduction of the respective hydrocarbons with lithium metal in THF and 2-MeTHF solutions with the aid of ultrasound and characterized by ¹H, ¹³C, and ⁷Li NMR spectroscopy. The majority of these dianions were found to be more stable than the parent biphenylene dianion toward ring opening to the corresponding dilithiobiphenyls. The enhanced stability of the dianions substituted with methyl groups at the ortho positions appears to be of steric origin, whereas the diminished stability of the meta-substituted derivatives is of electronic origin. The direction of ring opening of these dianions to substituted dilithiobiphenyls was rationalized by MO calculations. Temperature-dependent ¹³C NMR studies as well as MNDO calculations give valuable insights concerning solvation (i.e., ion pairing) as well as the location of the lithium counterions in these delocalized dianionic networks. In the case of octamethylbiphenylene dianion an interesting aggregation phenomenon was observed, as evidenced by concentration as well as temperature-dependent NMR spectra.

The following sections are included:

• Acknowledgment

The reaction mechanism of singlet and triplet methylene with benzene and related aromatic compounds was investigated by kinetic isotope effects ( KIEs), solvent effects, and product studies. The results are further rationalized by a series of ab initio calculations at MP2/6-31G*//RHF/6-31G* and UMP2/6-31G*//UHF/6-31G* levels of theory. The proposed 1c intermediate for the triplet reaction was found by means of the calculations, whereas no singlet analog 1 could be found.

The highly hindered tetraneopentylethylene has been synthesized and was shown to exhibit a temperature dependent ¹H NMR spectrum demonstrating the nonequivalence of methylene protons, with the barrier of rotation across the sp²–sp³ bond being ΔG^‡ = 21.7 ± 5 kcal/mol at 145 °C.

An approach which characterizes the effect of nonbonded interactions on ¹³C NMR shifts has been developed and applied to a complete series of phenylethanes. Force field calculations using Warshel's QCFF/PI + MCA program have been performed on the compounds to evaluate the nonbonded force on each atom used in the multiple regression analysis. X-ray crystal structure derived geometries reported in the literature have been used to compare with those obtained through calculation. The results show (1) experimental evidence for the polarization of electrons induced by nonbonded interactions, (2) the importance of steric interactions for all positions in a molecule, and (3) previously unobserved deshielding of the NMR signal of carbon due to steric compression of its bonds.

Phosphorus pentachloride and trichloride are highly effective catalysts for the radical type chlorination of alkanes, cycloalkanes, and arylalkanes. Reactions can be carried out conveniently at or above room temperature and in the dark. The reaction can be performed in chlorinated solvents of low polarity or in hydrocarbon solvents. Polar solvents, such as nitromethane, cause exclusive ionic chlorination to occur. The scope and mechanism of the novel, Lewis acid catalyzed radical chlorination reaction are discussed.

The following sections are included:

• Introduction

• Results and Discussion

• Experimental Section

• Acknowledgment

The following sections are included:

• Acknowledgment

¹H and ¹³C NMR spectroscopic investigation of aliphatic and aromatic iminium ions was carried out for their structural study and to determine the extent of the contribution of their carbenium ion character based on a comparison of the iminium ions with isoelectronic model compounds. CNDO/2 calculations of simple aliphatic iminium ions were also performed and related to the ¹H and ¹³C NMR chemical shifts. N- and C-methyl substituents were found to polarize the charge density of the π bond, resulting in shielding and deshielding effects, respectively.

Methyl-tert-butylether (MTBE) is an oxygenate widely used in the United States as a motor vehicle fuel additive to reduce emissions and as an octane booster [National Research Council, Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fules, National Academy Press, Washington, DC, 1996]. But it is the potential for MTBE to enter drinking water supplies that has become an area of public concern. MTBE has been shown to induce liver and kidney tumors in rodents but the biochemical process leading to carcinogenesis is unknown. MTBE was previously shown to be non-mutagenic in the standard Ames plate incorporation test with tester strains that detect frame shift (TA98) and point mutations (TA100) and in a suspension assay using TA104, a strain that detects oxidative damage, suggesting a non-genotoxic mechanism accounts for its carcinogenic potential. These strains are deficient in excision repair due to deletion of the uvrB gene. We hypothesized that the carcinogenic activity of MTBE may be dependent upon a functional excision repair system that attempts to remove alkyl adducts and/or oxidative base damage caused by direct interaction of MTBE with DNA or by its metabolites, formaldehyde and tert-butyl alcohol (TBA), established carcinogens that are mutagenic in some Ames strains. To test our hypothesis, the genotoxicity of MTBE-induced DNA alterations was assayed using the standard Ames test with TA102, a strain similar to TA104 in the damage it detects but uvrB + and, therefore, excision repair proficient. The assay was performed (1) with and without Aroclor-induced rat S-9, (2) with and without the addition of formaldehyde dehydrogenase (FDH), and (3) with human S-9 homogenate. MTBE was weakly mutagenic when tested directly and moderately mutagenic with S-9 activation producing between 80 and 200 TA102 revertants/mg of compound. Mutagenicity was inhibited 25%–30% by FDH. TA102 revertants were also induced by TBA and by MTBE when human S-9 was substituted for rat S-9. We conclude that MTBE and its metabolites induce a mutagenic pathway involving oxidation of DNA bases and an intact repair system. These data are significant in view of the controversy surrounding public safety and the environmental release of MTBE and similar fuel additives.

The following sections are included:

• Monographs and Books for Reference and Additional Reading

• CURRICULUM VITAE OF GEORGE ANDREW OLAH

• Bibliography of George A. Olah
  • Publications
  • Patents

• Past and Present Graduate Students

• Past and Present Postdoctoral and Visiting Scholars