Organic peroxide assisted transition metal hydrosilylation catalysis

Abstract

Summary:The catalytic activity of rhodium complexes for the hydrosilylation of substrates such as alkenes, 1,3-dienes, 1-alkynes, or ketones, is enhanced by the addition of organic oxidizing agents, such as t-butyl hydroperoxide, hydrogen peroxide, or m-chloroperbenzoic acid. Similar enhancement is found for the Group VIA hexacarbonyls in the hydrosilylation of 1,3-dienes. Article: IntroductionThe concept of using a co-catalyst in conjunction with an inactive or weakly active transition metal complex is exemplified by the activation of transition metal salts by alkyl aluminum compounds to form the extensively used Ziegler-Natta systems (1) . These systems have been used to catalyze the hydrosilylation of a variety of unsaturated organic species, often with novel results (2) . Another type of co-catalysts which has been used, involves the oxidation of a ligand to form a weakly co-ordinating ligand oxide, thus generating a coordinatively unsaturated catalytically active complex (3)(4) . Oxidizing agents that have been used include molecular oxygen (in systems such as W(CO) 6 -EtAlCl 2 -O 2 for the metathesis of olefins (3) ) and peroxides [such as the system (PPh 3 ) 3 RhCl-H 2 O 2 for the hydrogenation of olefinsn . We have been investigating the use of co-catalysts such as organic peroxides or molecular oxygen and report here on our use of organic peroxides as co-catalysts in hydrosilylation reactions. During the course of our studies reports by others have appeared where an organic peroxide has been used as a co-catalyst for the hydrosilylation reaction (5,6) Results and Discussion Rhodium complexesWe have made use of bis(triphenylphosphine)carbonylchlororhodium(I), (1), as this is only moderately active for hydrosilylation reactions at room temperature (7) . As can be seen from Table 1, the activity of this catalyst is greatly increased by the addition of an approximately 50% molar excess of t-butyl hydroperoxide as co-catalyst. The activity increases by a factor of four to five for most combinations of organic unsaturate and silane. No attempt to optimize the reaction conditions was undertaken. The yields increase for the hydrosilylation of 1-octene by triethoxysilane, from 4% to 75% as the t-BuOOH : Rh ratio increases from zero to fifteen. (Table 2).The yield is maximized with respect to t-BuOOH: Rh ratio at a value of 7.4 :1. An optimum ratio of t-BuOOH : M (M = Rh, Ir) of 4 : 1 was observed for [RhCl(PPh 3 ) 3 ] or [IrCl-(CO)(PPh 3 ) 2 ] in hydrosilylation reactions, but decomposition of the catalysts was observed at higher t-BuOOH : M ratios (10 : 1) (6) . Decomposition was not observed in our case, the catalyst system remaining active at ratios of t-BuOOH : Rh of 15 :1. As the catalysts contain different ligands it is not surprising that their responses differ.The increase in activity has been attributed (6) to the oxidation of either co-ordinated carbon monoxide to weakly coordinating carbon dioxide, as has been shown to occur stoichiometrically (8) , or by oxidation of co-ordinated triphenylphosphine ...

Hydrosilylation of CC and CC

“…The cis ligand alkene then inserts into the M-H bond (hydrometallation) of (6) to afford an alkyl metal complex (7), which interacts with another molecule of alkene to induce the coupling of the alkyl and silylligands and to regenerate the active catalyst (5). This last reaction is the rate-determining step of the catalytic cycle.…”