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)isoborneol forms methylzinc alkoxide 1, which forms a dimeric structure 2 in a reversible manner. Depending on the chirality of the monomer 1, three stereoisomeric dinuclear complexes are formed; the homochiral dimerization of 1 leads to (S,S)- or (R,R)-2, whereas the heterochiral interaction forms (S,R)-2. The heterochiral dimer is thermodynamically favored over the homochiral dimer in both toluene solution and crystalline states. The vapor pressure osmometry method has revealed that the stereoisomeric 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-hetero congruent to 1 x 10(-5) at 40 degrees C, respectively. Calorimetric determination of the enthalpies of solution of the crystalline compounds 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 state.