Metal-catalyzed silylation of sp3C–H bonds

Abstract

Metal-catalyzed activations of inert sp²C–H and sp³C–H bonds have recently brought about a revolution in the synthesis of useful molecules and molecular materials. Among them, the catalytic silylation of sp³C–H bonds has been developed due to the interest in the formed sp³C–SiR₃ silanes, a stable organometallic species, for carrying out further functionalizations that cannot be directly achieved using sp³C–H bonds. Besides many examples of sp²C–H bond catalytic silylations, metal-catalyzed silylations of sp³C–H bonds have been mostly discovered during the last decade in spite of the high reactivity of the sp³C–SiR₃ group. This review will present all the methods of metal-catalyzed silylations of sp³C–H bonds into sp³C–SiR₃ functions, discuss the catalytic mechanisms according to various metal-catalysts, and illustrate their applications in synthesis. The review describes successively the intermolecular sp³C–H bond silylations directed first by N-containing heterocycles with silanes using various Ru, Rh, and Ir catalysts and then directed by an amide type function using a Pd(II) catalyst and R₃Si–SiR₃ reagent. The catalytic intramolecular silylations of sp³C–H bonds can be performed after the catalytic formation of CH–OSiR₂H or CH–N(R)SiR₂H groups from alcohols, ketones, esters, or amine NH bonds by catalytic hydrosilylation with R₂SiH₂. Both catalytic processes can be performed using Ir(I) and Rh(I) catalysts with an alkene to capture the formed H₂. This reaction with Rh(I) and Ir(I) catalysts can be applied to the formation of 5-membered cyclic silanes from aryl silanes and from alkyl silanes arising from hydrosilylated terminal CC bonds of alkenes. Oxidation of the cyclic silane derivatives easily leads to 1,3- and 1,4-diols, from alcohol or ketone precursors and to 1,2-amino alcohols from amines. Several methods show how to transform various heteroatom–methyl groups X–CH₃: B–CH₃, O–CH₃, Si–CH₃, N–CH₃, Ge–CH₃ and S–CH₃ into their reactive functionalized X–CH₂SiR₃ group, using various Ru(0), Ir(I), pincer-Ru(II), or Y catalysts. Examples are shown of catalytic transformations of the allylic moiety CH₃–C(R)CH₂ into its silylated CH₂C(R)–CH₂SiR′₃ form via (i) Pd(II) allyl activation, (ii) silyl radical generation with photocatalyst and (iii) dual Ir(I) and Fe(II) catalysts for hydrosilylation of alkanes, via alkene formation, isomerization and hydrosilylation. Finally, a Ru(II)-catalyzed sp³C–H silylation of a methyl group of arylphosphine, directed by a P(III) atom, will be presented.