Hydrogen-bonded Capsules

The following sections are included:

• Dedication
• Contents

Research on molecular recognition became a fashionable topic for physical organic chemists in the 1980s. At the time there was growing disenchantment with the nonclassical ion controversy, and orbital symmetry fatigue was taking hold in the community. But the discovery of crown ethers by Pedersen in 1962 ignited the study of their complexes with ions, and macrocyclic polyethers dominated the scene for decades. The weak intermolecular interactions of these complexes gave rise to the concepts of molecular recognition: complementarity of sizes, shapes and chemical surfaces. The culmination of this activity was the award of the Nobel Prize in Chemistry to Pedersen, Cram and Lehn in 1987 for establishing host–guest chemistry and, on a grander scale, supramolecular chemistry…

The following sections are included:

• A Drop Shot with a Tennis Ball
• Sieves
• Other Resorcinarene Modules
• References

At this time (1995–1996) Ken Shimizu, surrounded by the encapsulation studies going on in the research group, decided to invent his own capsule, 1.1 (Figure 2.1). The concave surface he chose was based on the calixarenes developed by Gutsche, and the hydrogen bond motif expected to hold the two calixarene hemispheres together was inspired by Peggy Etter's studies of urea hydrogen-bonding patterns in the solid state. Unbeknownst to us, two other research groups were arriving at the calixarene capsules: those of Volker Böhmer in Mainz and David Reinhoudt in Enschede. In Europe, calixarenes were already much-admired as platforms for crown ethers and had been well developed for the selective sequestration of Cs+ in their “pinched cone” conformations. These compounds are so reliable that they are still in use today in the American nuclear waste industry…

The following sections are included:

• Introduction
• Capsules
• Single Large Guests
• Coencapsulation
• Guest Exchange
• Other Cylindrical Capsules
• Capsule Exchange
• References

In other chapters we have described how resorcinarenes have provided the structural curvature for several other capsules. But these molecules, and their pyrogallolarene cousins, have a capsular existence all of their own, and we explore them here. The recognition properties were the result of a convenient synthesis devised by Högberg, and explored in the 1980s by Aoyama. Reviews cover these and related efforts up to 1996, but the Atwood and MacGillivray X-ray structure of 1a (Figure 4.1) showed an unprecedented assembly: a capsule resembling an inflated cube with six resorcinarenes as the sides and eight water molecules as the corners. The space inside was nearly 1400ų. Mattay showed that pyrogallol[4]arenes (2) also form hydrogen-bonded hexameric capsules in the solid state — and without the need for water. A number of smaller (dimeric) capsules with these modules were subsequently reported. These systems are included in a recent review, so they will not be covered here. Instead, we will limit our descriptions to the hexamers, and mostly to the work in the last decade…

The translational consequence of molecular encapsulation is fairly obvious: the guest molecule's entropy is reduced as a result of its confinement in a small space. But what about its other motions — spinning, tumbling, internal rotations and vibrations? While there is never a fit between host and guest that is snug enough to effectively alter vibrations, we will see that other internal motions such as sigma bond rotations can be profoundly affected and result in otherwise unknown contorted conformations.

There are many approaches to the creation of chiral capsules and we have tried them all. Generally speaking, the chirality can be outside the capsule, in its lining or — most desirably — inside the capsule's space. We take up these locations here for different capsules but warn the reader that none proved deeply satisfying: hydrogen bonds are elastic and the shapes of the space they enclose are easily distorted.

As we have described in earlier chapters, some shapes — helical and folded alkanes — are unknown in solution; and even enzyme interiors show only bent alkyl structures. Tight-fitting alkane chains appear in other synthetic assemblies and covalent complexes. Their contortions and tight packing are now becoming advantageous in catalysis.

The following sections are included:

• Introduction
• The Diels–Alder Reaction
• The “Click” Reaction
• Chemical Amplification
• Chain Reaction Kinetics
• The Danishefsky–Gautier Reaction
• Hydration of Isonitriles
• Hydrolysis
• Rearrangements Inside Capsules
• Catalysts in Capsules
• Reaction Selectivity
• Epilogue
• References

The following sections is included:

• Index