Si–H Bond Activation with Bullock’s Cationic Tungsten(II) Catalyst: CO as Cooperating Ligand

Abstract

An in-depth investigation of the reaction of tertiary hydrosilanes with [CpW(CO)2(IMes)] + [B(C6F5)4] − reveals a fundamentally new Si-H bond activation mode. Unlike the originally proposed oxidative addition of the Si-H bond to the tungsten(II) center, there is strong experimental and NMR spectroscopic evidence for the involvement of one of the CO ligands of the cationic complex in the Si-H bond breaking event. The Si-H bond is heterolytically cleaved to form a tungsten(II) hydride and a silylium ion, which is stabilized by one of the CO ligands. This reactive hydrosilane adduct was eventually isolated and characterized by X-ray diffraction analysis. Quantum-chemical calculations support a cooperative mechanism but a stepwise process consisting of oxidative addition and subsequent tungsten-to-oxygen silyl migration in the tungsten(IV) silyl hydride is also energetically feasible. However, our combined spectroscopic and computational analysis favors the cooperative pathway. The newly identified hydrosilane adduct is the key intermediate of Bullock's ionic carbonyl hydrosilylation.

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…catalyst of type [CpW(CO)2(IMes)] + and suggested a new Si-H bond activation mode where the CO ligand is cooperating and interacting with the Si-H bond. 118 The Si-H bond is heterolytically cleaved to generate a W-H species and a CO-stabilized silylium ion.…”

Ligand-Metal Cooperation Enables C–C Activation Cross-Coupling Reactivity of Cyclopropyl Ketones

“…The reason for this change in reactivity with TMSCl is that a transient silyl carboxonium ion, formed by interaction of TMSCl, ZnX2, and the ketone, is a better electron acceptor that reacts with I to generate an intermediate (II), which is primed for C-C activation (Figure 4D). 58 Binding of the silyl carboxonium ion to I confers significant spin density (i.e., partial radical character) from the tpy •ligand to the pz orbital of the carbonyl carbon (Figure 4A-C). While free cyclopropylmethyl radicals undergo rapid radical ringopening processes (krearrangement ~ 10 7 s -1 ), 59 the bound substrate of II performs sequential oxidative ring opening and Ni-C bond formation to yield the metallacycle species III (Figure 4D).…”

Ligand-Metal Cooperation Enables C–C Activation Cross-Coupling Reactivity of Cyclopropyl Ketones

“…3 A-C) that rearranges and recombines with the catalyst to form III (Fig. 3D) (37). The silyl carboxonium ion, upon binding to I, accepts electron density from the tpy •ligand through an intramolecular SET event that confers significant spin density (i.e., radical character) to the pz orbital of the carbonyl carbon to form an O-nickel-coordinated a-silyloxy radical (Fig.…”