Enantioselective Noncovalent Synthesis of Hydrogen-Bonded Double-Rosette Assemblies

摘要

The noncovalent synthesis of enantiomerically pure hydrogen-bonded assemblies (M)- and (P)-1_3(centre dot)(CA)_6 is described. These dynamic assemblies are of the one single handedness (M or P), but do not contain any chiral components. They are prepared by using the "chiral memory" concept: the induction of supramolecular chirality is achieved through initial assembly with chiral barbiturates, which are subsequently replaced by achiral cyanurates. This exchange process occurs quantitatively and without loss of the M or P handed-ness of the assemblies. Racemization studies have been used to determine an activation energy for racemization of 105.9 +- 6.4 kJ mol~(-1) and a half-life time to racemization of 4.5 days in benzene at 18 deg C. Kinetic studies have provided strong evidence that the rate-determining step in the racemization process is the dissociation of the first dimelamine component 1 from the assembly 1_3(centre dot)(CA)_6. In addition to this, it was found that hte expelled chiral barbiturate (RBAR or SBAR) acts as a catalyst in the racemization process Blocking the dissociation process of dimelamines 1 from assembly 1_3(centre dot)(CA)_6 by covalent capture through a ring-closing metathesis (RCM) reaction produces an increase of more than two orders of magnitude in the half-life time to racemization.