Distinctive Activation and Functionalization of Hydrocarbon C-H Bonds Initiated by Cp*W(NO)(η3-allyl)(CH2CMe3) Complexes

Converting hydrocarbon feedstocks into value-added chemicals continues to offer challenges to contemporary preparative chemists. A particularly important remaining challenge is the selective activation and functionalization of the C(sp(3))-H linkages of alkanes, which are relatively abundant but chemically inert. This Account outlines the discovery and development of C-H bond functionalization mediated by a family of tungsten organometallic nitrosyl complexes. Specifically, it describes how gentle thermolyses of any of four 18-electron Cp*W(NO)(eta(3)-allyl)(CH2Me3) complexes (Cp* = eta(5)-C5Me5; eta(3)-allyl = eta(3)-H2CCHCHMe, eta(3)-H2CCHCHSiMe3, eta(3)-H2CCHCHPh, or eta(3)-H2CCHCMe2) results in the loss of neopentane and the transient formation of a 16-electron intermediate species, Cp*W(NO)(eta(2)-allene) and/or Cp*W(NO)(eta(2)-diene). We have never detected any of these species spectroscopically, but we infer their existence based on trapping experiments with trimethylphosphine (PMe3) and labeling experiments using deuterated hydrocarbon substrates. This Account first summarizes the syntheses and properties of the four chiral Cp*W(NO)(eta(3)-allyl)(CH2CMe3) complexes. It then outlines the various types of C-H activations we have effected with each of the 16-electron (eta(2)-allene) or (eta(2)-diene) intermediate nitrosyl complexes, and presents the results of mechanistic investigations of some of these processes. It next describes the characteristic chemical properties of the Cp*W(NO)(eta(3)-allyl)(eta(1)-hydrocarbyl) compounds formed by the single activations of C(sp(3))-H bonds, with particular emphasis on those reactions that result in the selective functionalization of the original hydrocarbon substrate. We are continuing development of methods to release the acyl ligands from the metal centers while keeping the Cp*W(NO)(eta(3)-allyl) fragments intact, with the ultimate aim of achieving these distinctive conversions of alkanes into functionalized organics in a catalytic manner.