Mechanism for the Karstedt Catalyzed Hydrosilylation Reaction

Francesco (Cecco) Faglioni, Mario Blanco, William A. Goddard III

Materials and Process Simulation Center (MSC), Beckman Institute, Mail Code 139-74

California Institute of Technology, Pasadena, CA 91125

Caltech, March 23, 2000



The term hydrosilylation refers to the addition of Si-H bonds to double bonds like C=C. It is one of the most fundamental methods for the laboratory and industrial synthesis of organosilicon compounds and organic silyl derivatives.

Despite the chemical and economical importance of the process, a good understanding of its mechanism is missing and several possible reaction paths have been proposed in the literature.

Current catalytic hydrosilylation techniques often involve platinum(0) complexes. Of these, Karstedt catalyst Pt2{[(CH2=CH)Me2Si]2O}3 is attractive for theoretical investigations because it is both widely used and well characterized.

A theoretical investigation (DFT) was performed on all intermediates and transition states for a typical Karstedt catalyzed reaction in order to elucidate its mechanism.

The reaction profile we obtain indicates that the catalytic cycle is fast and that the two main mechanisms proposed in the literature (by Lewis and Chalk-Harrod) are essentially equivalent in the rate determining steps.

The main source of problems in experimental settings is hence due to the initialization steps for the catalytic cycle and to side reactions that can poison the active catalyst.

We investigated the role of a few catalyst inhibitors used in industrial processes. We predict the inhibiting mechanism to involve phase separation between the inhibitor and the substrate. The inhibitor phase surrounds the catalyst thus preventing the substrate from reacting. Hence, in order for the reaction to occur the inhibitor must be evaporated.

We suggest a candidate for one of the deleterious side reactions observed experimentally.

Reaction profile for Karstedt catalyzed hydrosilylation