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

Abstract

Abstract The scope of the B(C 6 F 5 ) 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 ‐NO 2 , m ‐NO 2 , p ‐Et; R = Me or CF 3 ). Reactions proceed at room temperature with high chemoselectivity in a host of solvents (toluene, benzene, CCl 4 , 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(C 6 F 5 ) 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(C 6 F 5 ) 3 ‐catalyzed reactions, when MeCN was used as solvent. Mechanistic implications of these Lewis acid catalyzed reactions are discussed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)