ENANTIOMER RECOGNITION OF ASYMMETRIC CATALYSTS - THERMODYNAMIC PROPERTIES OF HOMOCHIRAL AND HETEROCHIRAL DIMERS OF THE METHYLZINC ALKOXIDE FORMED FROM DIMETHYLZINC AND ENANTIOMERIC 3-EXO-(DIMETHYLAMINO)ISOBORNEOL

Reaction of dimethylzinc and (2S)- or (2R)-3-exo-(dimethylamino)isobor neol forms methylzinc alkoxide 1, which forms a dimeric structure 2 in a reversible manner. Depending on the chirality of the monomer 1, thr ee stereoisomeric dinuclear complexes are formed; the homochiral dimer ization of 1 leads to (S,S)- or (R,R)-2, whereas the heterochiral inte raction forms (S,R)-2. The heterochiral dimer is thermodynamically fav ored over the homochiral dimer in both toluene solution and crystallin e states. The vapor pressure osmometry method has revealed that the st ereoisomeric dimers 2 in toluene are in equilibrium with the monomer 1 with dissociation constants K-homo = (3.0 +/- 1.0) x 10(-2) and K-het ero congruent to 1 x 10(-5) at 40 degrees C, respectively. Calorimetri c determination of the enthalpies of solution of the crystalline compo unds indicates that the S,R heterochiral dimer is more stable than the S,S or R,R homochiral dimer by 3.4 +/- 0.8 kJ mol(-1) in the solid st ate.