Onium Ylide Chemistry. 1: Bifunctional Acid–Base-Catalyzed Conversion of Heterosubstituted Methanes into Ethylene and Derived Hydrocarbons. The Onium Ylide Mechanism of the C1 → C2 Conversion
A preliminary report of the mechanism was presented at the IUPAC Symposium on Physical Organic Chemistry at Santa Cruz, CA August 1980; see also ref 18.
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.
