Emergence of Hemicryptophanes: From Synthesis to Applications for Recognition, Molecular Machines, and Supramolecular Catalysis

In the wide area of host-guest chemistry, hemicryptophanes, combining a cyclotribenzylene (or cyclotriveratrylene CTV) unit with another different C₃-symmetrical moiety, appears as a recent family of molecular cages. The synthesis and recognition properties of the first hemicryptophane were reported in 1982 by Collet and Lehn, but the very little attention received by this class of host compounds in the 20 years following this first promising result can account for their apparent novelty. Indeed, in the last 10 years hemicryptophanes have aroused growing interest, and new aspects have been developed. Thanks to the rigid shaping unit of the north part (CTV) and also the variable and easily functionalized south moiety, hemicryptophanes are revealed to be inherently chiral ditopic host compounds, able to encapsulate various guests, including charged and neutral species. They also enter the field of stimuli-responsive supramolecular systems exhibiting controlled functions. Moreover, endohedral functionalization of their inner cavity leads to supramolecular catalysts. The confinement of the catalytic center affords nanoreactors with improved catalytic activities or selectivities when compared to model systems without a cavity. The current trend shows that reactions in the confined space of synthetic hosts, mimicking enzyme behavior, will expand rapidly in the near future.