Mechanistic studies on C–C reductive coupling of five-coordinate Rh(iii) complexes

Abstract

A series of five-coordinate Rh(III) vinyl complexes [Rh(N^C)(PAr₃)₂CHCHR]PF₆ have been isolated as an intermediate in the coupling of a Rh(III) hydride with terminal alkynes. These Rh(III) vinyl complexes underwent aryl–vinyl reductive coupling to afford the Rh(I) chelating complex [Rh(N^C–CHCHR)–(PAr₃)₂]PF₆ in high yields. Kinetic studies on the C–C reductive elimination revealed that the reaction kinetics is first order for a Rh(III)(4-trifluoromethyl)styryl complex with activation parameters of ΔH^≠ = 20.9 kcal mol⁻¹ and ΔS^≠ = −6.1 eu. The electronic effects of the styryl group and the phosphine ligands on the rate of C–C reductive elimination were studied, and the rate constant decreases for a more electron-poor styryl group but increases for a less donating phosphine. The inhibitive effect of the added phosphine indicates that the dissociation of phosphine to afford a four-coordinate intermediate is involved, which was further supported by DFT calculations. Although intermediacy of a 4-coordinate species has been suggested, the active intermediate that directly undergoes C–C coupling was pinpointed to a five-coordinate cis phosphine complex on the basis of DFT studies. Significant accelerating effects were observed for oxygen donor solvents (THF-d₈ and acetone-d₆), possibly via efficient stabilization of the four-coordinate intermediate. However coordination of CO forms an inert six-coordinate Rh(III) complex. Thus an overall detailed mechanism of alkyne insertion and subsequent aryl–vinyl reductive elimination from the Rh(III) center has been proposed based on the experimental and theoretical data.