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 C[double bond, length as m-dash]C 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)[double bond, length as m-dash]CH2 into its silylated CH2[double bond, length as m-dash]C(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.