Diastereoselective inter- and intramolecular pinacol coupling of aldehydes promoted by monomeric titanocene(III) complex Cp2TiPh

A monomeric titanocene(III) derivative, Cp2TiPh, effectively promoted the pinacol coupling of both an aromatic aldehyde, benzaldehyde, and an aliphatic aldehyde, 3-phenylpropionaldehyde. The same reactive complex was successfully generated by a catalytic amount of a precursor, Cp2Ti-(PH)Cl and its stoichiometric amount of Zn. The Cp2TiPh-catalyzed pinacol coupling of benzaldehyde derivatives and aliphatic aldehydes afforded the corresponding 1,2-diols in high yields with moderate to good three-selectivity. On the other hand, Cp2TiPh-catalyzed pinacol cyclization of dials gave cyclic 1,2-diols with excellent diastereoselectivity. The extension of this protocol to chiral dials demonstrated that the phenyltitanium complex catalytically transmitted an axial chirality or a central chirality of the starting dials to the central chirality of the resultant l,a-diols.