Influence of Lewis Acid and Solvent in the Hydrosilylation of Aldehydes and Ketones with Et3SiH; Tris(pentafluorophenyl)borane B(C6F5)3 versus Metal Triflates [M(OTf)3; M = Sc, Bi, Ga, and Al] - Mechanistic Implications

The scope of the B(C6F5)(3)-catalyzed hydrosilylation of (X)Ph-CH=O and (X)Ph-C(R)=O was expanded to include a large set of substitutents (X = H, p-Me, o-Me, p-F, o-F, p-Cl, p-Br, p-NO2, m-NO2, p-Et; R = Me or CF3). Reactions proceed at room temperature with high chemoselectivity in a host of solvents (toluene, benzene, CCl4, 1,2-dichloroethane, and methylcyclohexane), or under solventless conditions, with hydrosilylation yields ranging from 85 to 95% (for aldehydes) and 71 to 100% (for ketones) and no noticeable solvent dependency of hydrosilylation yields. Replacing B(C6F5)(3) for M(OTf)(3) (M = Bi, Al, Ga, Sc) causes a dramatic change in chemoselectivity, forming dibenzyl ether and benzylated solvent (with toluene and benzene), with hydrosilylation products becoming negligible in most cases. The M(OTf)(3)-catalyzed reactions thus represent a practical method for the synthesis of dibenzyl ethers. Remarkably, substantial amounts of dibenzyl ether was formed in the B(C6F5)(3)-catalyzed reactions, when MeCN was used as solvent. Mechanistic implications of these Lewis acid catalyzed reactions are discussed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)