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Abstract
Container-shaped host molecules provide structured environments that impart geometric boundaries on the motions, conformations, and chemical reactions of their smaller molecular occupants. They behave as fixed shells of solvation, constrained in time and space. When a functional group is fixed in the inner space of a host, it can be studied in isolation. One such host, an introverted acid cavitand, is a receptor for amines. The energetics and dynamics of its complexes are used to study salt bridges in structured, hydrophobic environments. The reactivity of the acid with diazoalkanes leads to the synthesis of introverted ester cavitands. Their ester groups function as probes for the ability of hydrocarbons to coil in response to restricted space. These results are complemented by a study of oligoethylene glycol and perfluoro-n-alkane binding in a cylindrical molecular capsule. Guest binding studies with the introverted methyl ester cavitand lead to the observation of its remarkably fast and selective Menschutkin reactions with tertiary amines; more than a 50,000-fold rate enhancement is reported. In a simpler system, quinuclidine is bound by an unfunctionalized deep cavitand, and exhibits more than 1000-fold greater reactivity towards electrophiles free in solution. Together, these results report upon the remarkable ability of spatially-organized host molecules to act as specialized environments that control and facilitate chemical transformations.
