Enantioselective Noncovalent Synthesis of Hydrogen-Bonded Assemblies

The noncovalent synthesis of enantiomerically pure hydrogen-bonded assemblies (M)- and (P)-1(3)(.)(CA), is described. These dynamic assemblies are of 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 handedness of the assemblies. Racemization studies have been used to determine an activation energy for racemization of 105.9 +/- 6.4 kJ mol I and a half-life time to racemization of 4.5 days in benzene at IS 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 1v(.)(CA), In addition to this, it was found that the expelled chiral barbiturate (RBAR or SBAR) acts as a catalyst in the racemization process. Blocking the dissociation process of dimelamines 1 from assembly 1v(.)(CA), 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.