Catalysis of silicon alkoxide transesterification by early transition metal complexes

The transition metal alkoxides Ti(OPri)(4) and OV(OPri)(3) are shown to catalyze the transesterification of tetramethyl- and tetraethyl orthosilicates: Si(OR)(4) + R'OH <----> Si(OR)(3)-(OR') + ROH. A detailed study of metal-catalyzed transesterification reactions and the factors that affect their efficiency and selectivity are presented. Si-29 NMR techniques were used to quantify the extent of transesterification of the two different silicon alkoxides under a variety of conditions while mechanistic aspects of the reaction were probed using V-51 NMR. It was found that tetramethyl and tetraethyl orthosilicates, when reacted with smaller, less sterically demanding alcohols, are substituted to a similar extent by both metals. However, transesterification of bulkier alcohols (2-propanol) revealed dramatic differences in their reactivity, with titanium being significantly more efficient than vanadium. The catalytic process and the differences in reactivity between the two metals are explained in terms of a mechanism involving ligand migration between the metal and the silicon. A comparison of the catalytic activity of these Lewis acids with a strong Bronsted acid (triflic acid) showed the latter to have far greater catalytic activity.