Silica-Supported Rhodium Complex Research Articles

Ortho-C–H Borylation of Arenes Using Silica-Supported Rhodium Complexes

ortho-C–H Borylation of Arenes Using Silica-Supported Rhodium Complexes

Catalytic asymmetric hydrogenation of activated keto compounds by some homogeneous and silica-supported di(μ-carboxylato)bis(aminophosphinephosphinite)dirhodium complexes

New soluble and silica-supported rhodium complexes have been synthesized and tested as catalyst precursors for the enantioselective hydrogenation of α-ketoesters and an α-ketoamide. The homogeneous catalysts have been found to be efficient in terms of activity and asymmetric induction (up to 94% enantiomeric excess). On the other hand, the heterogenized systems exhibited lower performances and were gradually deactivated.

Silica-supported rhodium complexes. relation between catalyst structure and activity

A series of heterogenized catalysts has been prepared by the reaction of chlorobis(ethylene)rhodium(I) dimer and phosphinated silica. The structures of these catalysts have been varied by changing the length of the phosphine-alkylene chain, -(CH2)nPPh2, used to couple the rhodium center to the silica surface. The dependence of the activity of the silica-attached catalysts on the structure has been studied in the hydrosilylation of hexene-1. It was found that the activity was ten times higher for the complex linked to silica through a short chain, containing one methylene group, than for the other rhodium complexes linked via alkylene chains containing two to six carbon atoms. Variatons in the activity have been attributed to the differences in the nature of the catalytic species formed on the surface. A binuclear structure for the catalysts of lower activity and a mononuclear one for the catalyst of higher activity have been postulated and confirmed by e.s.c.a. and i.r. measurements.