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) × 10^-2 and K_hetero ≅ 1 × 10^-5 at 40°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.