Fluorous hydrosilylation

Abstract

In this review, we describe the papers and patents dealing with the fluorous biphasic system (FBS) hydrosilylation reactions reported to date. Despite the limited number of reports, the FBS hydrosilylation reaction has been extremely successful. In all cases fluorous monophosphines (either alkylic or perfluoroalkylsilyl-substituted derivatives of triphenylphosphine) have been employed as ligands to synthesize and inmobilize the metal catalysts (either rhodium(I) or gold(I) derivatives) in the fluorous solvent (including a fluorous ionic liquid). The hydrosilylation of alkenes, ketones and enones with fluorous rhodium analogs to the Wilkinson's catalyst [RhCl(PPh(3))(3)], have afforded high TON/TOF and a very efficient separation and recycling of the fluorous catalyst. Modification of the fluorous content and position of the fluorous tails in the aryl groups of the phosphines have allowed for further optimization of the process and a better recovery of the catalyst with minimal leaching of rhodium and fluorous ligand to the organic phase. Moreover, the use of the so-called second generation methods which eliminate the need of fluorous solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents (thermomorphic properties) have permitted the use and separation of fluorous alkyl-phosphine rhodium catalysts in hydrosilylation reactions in conventional organic solvents. The addition of an insoluble fluorous support such as Teflon tape allowed for an exceptionally easy and efficient recovery of fluorous rhodium catalysts ("catalyst-on-a-tape") in the hydrosilylation of ketones. In the case of the FBS gold-catalyzed hydrosilylation of aldehydes, new fluorous gold catalysts with alkylic phosphines have led to an efficient separation and recycling of the gold catalysts although the TON/TOF are lower than in the rhodium-catalyzed hydrosilylation of alkenes and ketones. A detailed study of the non-fluorous gold-catalyzed version has helped to explain how this catalytic system could be improved.