Breaking Conventional Site Selectivity in C-H Bond Activation: Selective sp3 versus sp2 Silylation by a Pincer-Based Pocket.

Abstract

A deeply ingrained assumption in the conventional understanding and practice of organometallic chemistry is that an unactivated aliphatic C(sp3)-H bond is less reactive than an aromatic C(sp2)-H bond within the same molecule given that they are at positions unbiasedly accessible for activation. Herein, we demonstrate that a pincer-ligated iridium complex catalyzes intramolecular dehydrogenative silylation of the unactivated δ-C(sp3)-H (δ to the Si atom) with exclusive site selectivity over typically more reactive ortho δ-C(sp2)-H bonds. A variety of tertiary hydrosilanes undergo δ-C(sp3)-H silylation to form 5-membered silolanes, including chiral silolanes, which can undergo further oxidation to produce enantiopure β-aryl-substituted 1,4-diols. Combined computational and experimental studies reveal that the silylation occurs via the Si-H addition to a 14-electron Ir(I) fragment to give an Ir(III) silyl hydride complex, which then activates the C(sp3)-H bond to form a 7-coordinate, 18-electron Ir(V) dihydride silyl intermediate, followed by sequential reductive elimination of H2 and silolane. The unprecedented site selectivity is governed by the distortion energy difference between the rate-determining δ-C(sp3)-H and δ-C(sp2)-H activation, although the activation at sp2 sites is much more favorable than sp3 sites by the interaction energy.