Migration of aryl and methyl onto carbonyl in iodorhodium(III) complexes

Abstract

The complexes [(C₅Me₅)RhR(CO)I][R = Me, (1a); Ph, (1b); p-MeC₆H₄, (1c);p-ClC₆H₄, (1d);p-OHCC₆H₄(1e);p-NCC₆H₄, (1f) or p-O₂NC₆H₄, (1g)] were synthesised by the route [(C₅Me₅)RhMe₂(Me₂SO)]+ RCHO →[(C₅Me₅)RhR(CO)Me]; [(C₅Me₅)RhR(CO)Me]+ I₂→[(C₅Me₅)RhR (CO)I]. The migration reaction, [(C₅Me₅)RhR(CO)I](1)+ PPh₃→[(C₅Me₅)Rh(COR)(PPh₃)I](2), proceeded easily and essentially quantitatively for (1a)–(1d) in a variety of solvents; in a side-reaction, (1e)–(1g) gave the substitution products [(C₅Me₅)RhR(PPh₃)I](3e)–(3g). The reactions of complexes (1a)–(1g) with PPh₃ in toluene obeyed second-order kinetics, rate =k₃[(1)][PPh₃], implying a direct bimolecular attack by the triphenylphosphine on the metal complex. A Hammett plot of k_obs. for the reaction of (1b)–(1g) to (2b)–(2g) against σ gave a good linear relationship (ρ=–2.9) indicating that migration is favoured by electron-releasing substituents, R =p-MeC₆H₄ > Ph > p-ClC₆H₄ > p-OHCC₆H₄ > p-NCC₆H₄≈p-O₂NC₆H₄. Values of ΔG^‡, ΔH^‡, and ΔS^‡ for the reactions of complexes (1a)–(1d) with PPh₃ were determined over the temperature range 286–343 K; ΔS^‡ was smaller for (1b) than for the others, leading to the phenyl [in (1b)] migrating faster than methyl [in (1a)]. There were only small rate differences for the reactions of (1a) and (1b) between toluene, tetrahydrofuran, and dichloromethane, but they proceeded significantly faster in nitromethane or acetonitrile; for (1b) the migration in these solvents showed a first-order (unimolecular) as well as a second-order (bimolecular) path. Both the second-order rate constant, k₃, and the first-order rate constants, k₁, were several orders of magnitude larger than for any previously measured unpromoted migration reaction.