Secondary to normal alkyl group rearrangements in octahedral iridium(III) complexes. 1. Monoalkyl derivatives

Article abstract of DOI:10.1021/om00140a006

sec-Alkyliridium(III) complexes IrYI-sec-R(CO)L₂ (Y = Cl, I; L = PMe₃, PMe₂Ph) formed by oxidative addition of sec-alkyl iodides to IrY(CO)L₂ rearrange cleanly by a first-order process to the n-alkyl isomers on dissolution in dichloromethane containing protic solvents. The order of efficacy of these solvents in promoting alkyl group rearrangement is CF₃CO₂H ? CH₃OH ? C₂H₅OH > CH₃CO₂H ≈ CH₃CH₂CH₂OH > (CH₃)₂CHOH, while in the more strongly coordinating medium THF the order is H₂O ? CH₃OH. These orders correlate with the anion-solvating ability of the solvents and, together with the observed retardation by added iodide ion, suggest that the rate-determining step in the rearrangement is dissociation of iodide ion trans to the sec-alkyl group. Rapid, reversible β-hydride elimination in the resulting cation and stereospecific return of iodide ion trans to the resulting n-alkyl group complete the process. The rearrangement is promoted by increasing bulk, both of the alkyl group, up to a certain limit, and of the tertiary phosphine (PMe₂Ph > PMe₃). Treatment of IrClI{CH(CH₃)₂}(CO)(PMe₂Ph)₂ (2c) with AgBF₄ in acetonitrile induces immediate alkyl group rearrangement to give the n-propyliridium(III) salt [IrCl(CH₂CH₂CH₃)(CO)(NCMe)(PMe ₂Ph)₂]BF₄ (6). Studies of analogous CD₂CH₃ compounds suggest that they, and presumably other n-alkyliridium(III) complexes, undergo reversible β-hydride elimination more slowly than the sec-alkyl complexes. The deuterium labels in the isobutyl-d₂ complex IrClI{CD₂CH(CH₃)₂}(CO)(PMe₃) ₂ (1e′) scramble over all the alkyl carbon atoms when the compound is heated in CD₂Cl₂/CD₃OD, indicating that a tert-butyliridium(III) species is accessible. Surprisingly, the complexes IrClI{CH₂CH(CH₃)CH₂CH₃}(CO)(PMe ₃)₂ (1h) and IrClI{CH₂CH₂CH(CH₃)₂}(CO)(PMe ₃)₂ (1i) do not interconvert under the same conditions, implying that a tert-pentyliridium(III) species cannot be formed. The results are compared with alkyl group rearrangements that occur in other transition-metal systems, especially those promoted by dissocation of triphenylphosphine in (η-C₅H₅)FeR(CO)(PPh₃).