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 Bu^t) 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 Fe_π–C_β interaction and a more obtuse Fe_σ–C_α–C_β 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-d¹_α and [Fe₂(CO)₆(μ-HCCDPh)(μ-PPh₂)] 2a-d¹_β. 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.