A spontaneous asymmetric synthesis of optically active polymers by crystallization, lattice-controlled reaction, and autocatalytic induced recrystallization

Abstract

A model for the generation and amplification of optical activity, involving crystallization of achiral monomers in chiral crystals combined with topochemical polymerization, is described. Taking into consideration some empirical rules of crystal-packing, seven achiral or chiral racemic monomers were selected as reactants for these studies, which were indeed found to pack in chiral crystals with the motif suitable for a topochemical solid-state polymerization with quantitative enantiomeric yield. The chiral products (dimers, trimers, and oligomers), obtained in a first successful generation experiment, proved to be powerful asymmetry inducing agents in a fresh crystallization of the starting materials. From the totally consistent results of a large number of experiments, the following “inversion rule” was derived: dimers, trimers, and oligomers generated in a “d” crystal always induce preferential crystallization of the monomers in the enantiomorphic “l” phase and vice versa. A mechanism of preferential adsorption of the chiral impurity on that crystalline phase which is stereochemically similar to it is proposed, and evidence for the validity of this hypothesis is presented. Viewed in more general terms, the inversion rule can be extended to include conglomerates as well, and examples from the literature as well as new systems have been demonstrated to undergo resolution via this mechanism. Finally the implications of this mechanism for the possible resolution of racemic polymers will be discussed.