α–βAlkenyl isomerisation at diiron centres

Abstract

α-Substituted alkenyl complexes [Fe2(CO)6(μ-RCCH2)(μ-PPh2)] 1a–1e (R = Ph, Me, Prn, Bun or But) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(μ-PPh2)]. Thermolysis in toluene results in isomerisation to the β-substituted complexes [Fe2(CO)6(μ-HCCHR)(μ-PPh2)] 2a–2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe2(CO)5(PR′3)(μ-HCCHR)(μ-PPh2)] 3a–3e in which the phosphine is coordinated to the σ-bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe2(CO)6(μ-PhCCH2)(μ-PPh2)] 1a at 60–70 °C affords mono- and di-substituted α-alkenyl complexes [Fe2(CO)5{P(OMe)3}(μ-PhCCH2)(μ-PPh2)] 4 and [Fe2(CO)4{P(OMe)3}2(μ-PhCCH2)(μ-PPh2)] 5 respectively. Above 100 °C, conversion into the β-substituted isomers [Fe2(CO)5{P(OMe)3}(μ-HCCHPh)(μ-PPh2)] 6 and [Fe2(CO)4{P(OMe)3}2(μ-HCCHPh)(μ-PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on 1a, [Fe2(CO)6(μ-PrnCCH2)(μ-PPh2)] 1c, [Fe2(CO)6(μ-HCCHPh)(μ-PPh2)] 2a, [Fe2(CO)5(PPh3)(μ-HCCHPh)(μ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, β isomers being characterised by a longer Feπ–Cβ interaction and a more obtuse Feσ–Cα–Cβ angle. The mechanisms of alkyne addition to [Fe2H(CO)7(μ-PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both β sites in 1a, and since they do not in exchange on the NMR timescale suggests that a radical process is operating. Thermolysis of this mixture leads to a 3∶1 mixture of [Fe2(CO)6(μ-DCCHPh)(μ-PPh2)] 2a-d1α and [Fe2(CO)6(μ-HCCDPh)(μ-PPh2)] 2a-d1β. A mechanism for α–β alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) β-protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition.