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.

Cross-Electrophile Silylation of Aryl Carboxylic Esters with Hydrochlorosilanes by SiH-Directed and Cr-Catalyzed Couplings

“…Silicon is a material and pharmaceutically interesting carbon isostere. − Because the incorporation of silicon bioisosters into material and pharmaceutical candidates often induces changes in the electrochemical stability, solubility, and pharmacokinetic properties of a compound, the development of strategies to incorporate silicon motifs into organic molecules by forming Si–C bonds is of great synthetic and mechanistic interest. − In general, cross-couplings provide valuable strategies to construct Si–C bonds, mainly by coupling silicon electrophiles with carbon nucleophiles or reactions of silicon nucleophiles with carbon electrophiles (Figure a). , The use of silicon electrophiles to couple orthogonally with carbon electrophiles in the formation of Si–C bonds has rarely been studied, particularly through reductive coupling between two strong electrophilic Si–X and C–Y bonds . Strategies that enable access to one and two Si–C bonds, by reductive coupling of strong Si–X/C–Y bonds using inexpensive starting materials would represent a value-addition to the synthetic toolbox of organosilanes …”

Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand

“…1−4 As a potentially mild and efficient route to olefins, the field of homogeneous alkane dehydrogenation reaction has been remarkably advanced in the last four decades. While the further enhancement of the catalytic efficiency is still in demand for industrial applications, the mild reaction conditions allowed with homogeneous catalysts led to the spectacular synthetic application in tandem with other reactions; examples include alkane metathesis, 5,6 alkane silylation, 7,8 and depolymerization of polyethylene. 9,10 The most privileged molecular catalysts for alkane dehydrogenation are arguably bis(phosphine)-based (κ 3 -P,C,P) pincer iridium complexes R A (R represents the substituents on the phosphorus atoms) discovered in the 1990s by Kaska, Jensen, and Goldman (Figure 1).…”

“…Catalytic alkane dehydrogenation is an important process to convert low-value saturated hydrocarbons into value-added olefins and arenes. − As a potentially mild and efficient route to olefins, the field of homogeneous alkane dehydrogenation reaction has been remarkably advanced in the last four decades. While the further enhancement of the catalytic efficiency is still in demand for industrial applications, the mild reaction conditions allowed with homogeneous catalysts led to the spectacular synthetic application in tandem with other reactions; examples include alkane metathesis, , alkane silylation, , and depolymerization of polyethylene. , …”

“…The most privileged molecular catalysts for alkane dehydrogenation are arguably bis­(phosphine)-based (κ 3 - P , C , P ) pincer iridium complexes R A (R represents the substituents on the phosphorus atoms) discovered in the 1990s by Kaska, Jensen, and Goldman (Figure ). − Since then, various iridium complexes with other combinations of ligating motifs ,− and non-iridium transition-metal complexes (Rh, ,− Ru, − Os, and Ti) have been examined, while most of them are less active than the original (PCP)Ir complexes. Recent noteworthy exceptions are the (κ 3 - P , P , P ) pincer iridium complex , and (κ 2 - P , P )­(chloro)­iridium complex, which are effective catalysts for the transfer dehydrogenation of terminal alkanes and 1,1-disubstituted ethanes, respectively.…”

Recyclable Iridium Catalyst Supported on Porous Organic Polymer for Acceptorless Dehydrogenative Silylation: Synthesis of Ring‐Fused Oxasilacycles

“…[5] Moreover, transition metal-catalyzed CÀ H silylation has emerged as a powerful tool for accessing organosilicon compounds, [6] which have high atom economy and good functional group tolerance. This strategy includes the intramolecular couplingcyclization of preprepared silicon reagents to construct silacycles [7] (Scheme 1c) and the intermolecular dehydrogenative silylation of CÀ H bonds with hydrosilane or chlorosilane assisted by hydroxyl group as directing groups (Scheme 1d). [8] However, most of these methods require equivalent amounts of hydrogen sacrificial agents and prefunctionalization of silicon reagents.…”