Breaking Conventional Site Selectivity in C–H Bond Activation: Selective sp<sup>3</sup> versus sp<sup>2</sup> Silylation by a Pincer-Based Pocket
Chuan Qin, Zhidao Huang, Zhidao Huang, Song-Bai Wu, Zhuangxing Li, Yuhong Yang, Songgen Xu, Xin Zhang, Guixia Liu, Yun‐Dong Wu, Lung Wa Chung, Zheng Huang, Zheng Huang
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.