Characterization of HMPA-coordinated titanium(III) species in solutions of zinc-reduced titanocene dihalides

The effect of adding HMPA to THF solutions of metal-reduced titanocene dihalides (CP2TiX2) has been investigated by means of cyclic voltammetry and kinetic measurements. It is shown that the high coordination affinity of HMPA toward the titanium nucleus results in the formation of a mixture of mainly CP2Ti(hmpa)(+), Cp2Ti(hmpa)(2)(+), and Cp2TiX(hmpa) with an increase in the amount of ionic constituents as X is changed from Cl to Br and I. The oxidation potentials of all HMPA-coordinated Ti-III species are more negative than those of the uncoordinated analogues, but nevertheless an inverse correlation is observed between the HMPA concentration and the rate of reduction of electrophiles such as benzyl chloride and benzaldehyde. This indicates that the reactive reagents in solution remain the uncoordinated Ti-III species despite them being in low concentrations. In keeping with this interpretation, HMPA was found to exert no influence on the diastereoselectivity in the pinacol coupling of benzaldehyde.