Desymmetrization of meso 1,3- and 1,4-diols with a dinuclear zinc asymmetric catalyst.

A dinuclear asymmetric zinc catalyst generated by mixing a 2:1 ratio of diethylzinc and 2,6-bis[5-2-diarylhydroxy methyl-1-pyrrolidinyl]-4-methylphenol has been contrasted with enzymes for the desymmetrization of some meso diols. The best ligand has a p-biphenylyl group as the aromatic substituent defining the chiral space. A series of 2-substituted propanediols were examined. The best acyl transfer agent proved to be vinyl benzoate. Diacylation normally did not occur. The phenyl substituted substrate gave 91-95% ee which compares favorably with the best ee of 92% reported for an enzymatic desymmetrization. The methyl substituted substrate gave significantly better results with the dinuclear zinc catalyst (89% yield, 82% ee) as compared to the best enzymatic esterification (70% yield, 60% ee). One case of a 1,4-diol, cis-1,2-bis(hydroxymethyl) cyclohexane, also gave much better results with the dinuclear zinc catalysts (93% yield, 91% ee) as compared to the reported enzymatic process (44% yield, 7% ee). A model to rationalize the results is presented.