title: Enantioselective noncovalent synthesis of hydrogen-bonded double-rosette assemblies.
abstract: The noncovalent synthesis of enantiomerically pure hydrogen-bonded assemblies (M)- and (P)-1(3).(CA)(6) 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(-1) and a half-life time to racemization of 4.5 days in benzene at 18 degrees 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).(CA)(6). 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 1(3).(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.
authors.affiliation: Laboratory of Supramolecular Chemistry and Technology, MESA+ Research Institute, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands.
authors.forename: Leonard J
authors.initials: LJ
authors.lastname: Prins
authors.forename: Jeroen J
authors.initials: JJ
authors.lastname: Verhage
authors.forename: Feike
authors.initials: F
authors.lastname: De Jong
authors.forename: Peter
authors.initials: P
authors.lastname: Timmerman
authors.forename: David N
authors.initials: DN
authors.lastname: Reinhoudt