Issue 7, 2015

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)2CH[double bond, length as m-dash]CHR]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–CH[double bond, length as m-dash]CHR)–(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.

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

Supplementary files

Article information

Article type
Research Article
Submitted
09 Nah 2015
Accepted
28 Agd 2015
First published
29 Agd 2015

Org. Chem. Front., 2015,2, 783-791

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

S. Chen, Y. Su, K. Han and X. Li, Org. Chem. Front., 2015, 2, 783 DOI: 10.1039/C5QO00049A

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