Efficient method for the preparation of pinacols derived from aromatic andaliphatic ketones by using low-valent titanium reagents in dichloromethane-pivalonitrile

The reductive coupling reaction of aldehydes and ketones, including unsymme trical aliphatic ketones, proceeded smoothly to give the corresponding pina cols in good to high yields under mild conditions by using combination of t itanium(II) chloride and zinc or titanium(IV) chloride and zinc in dichloro methane-pivalonitrile. Meso-selective formation of the coupling products wa s observed in the cases of some aliphatic ketones. The diastereoselectiviti es of coupling products depend on both difference of bulkiness of 2-, and 2 '-substituents of carbonyl group of the reactant, and overall steric effect around the carbonyl groups.