Miranda-Pizarro J, Luo Z, Moreno JJ, Dickie DA, Campos J, Gunnoe TB. Reductive C-C Coupling from Molecular Au(I) Hydrocarbyl Complexes: A Mechanistic Study.
J Am Chem Soc 2021;
143:2509-2522. [PMID:
33544575 PMCID:
PMC8479859 DOI:
10.1021/jacs.0c11296]
[Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2020] [Indexed: 02/07/2023]
Abstract
Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C-C bond formation. In this study, we investigate C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au2(μ-CH3)(PMe2Ar')2][NTf2], [Au2(μ-CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar' = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au2(μ-CH3)(PMe2Ar')2][NTf2] increases as the steric bulk of the phosphine substituent Ar' decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C-C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar')2][NTf2] to form Au(PR2Ar')(NTf2) and Au(CH3)(PMe2Ar'), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar')2][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar')]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar')][NTf2] and [Au2(PMe2Ar')]+, and (3) recombination of [Au2(CH3)(PMe2Ar')][NTf2] and Au(CH3)(PMe2Ar') to eliminate ethane.
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