Factors influencing the oxidative addition of iodomethane to [Rh(CO) 2I 2] −, the key step in methanol and methyl acetate carbonylation

Abstract

The oxidative addition of MeI to A + [Rh(CO) 2I 2] − [A = n-Bu 4N, Ph 4P, and Ph 4As], the rate determining step in the Rh and I − catalysed conversion of methanol into acetic acid and of methyl acetate into acetic anhydride, is second order overall, first order in both complex and methyl iodide. The reaction is slower in less polar solvents ( k 2 1.9 × 10 −5 M −1 s −1 in MeOAc, 3.1 × 10 −5 in THF, and 10.0 × 10 −5 M −1 s −1 at 298 K, in MeOH). Protic solvents accelerate the reaction; e.g. adition of ca. 3% water quadruples the rate in THF. Values of Δ G ≠ have been measured in MeOH and MeOAc between 288 and 318 K, and values of Δ H ≠ [60 (MeOH), 46 (MeOAc) kJ mol −1], and Δ S ≠ [−120 (MeOH), −180 (MeOAc) J mol −1 K −1] calculated from them; these numbers are very close to those for the catalytic carboxylation. Iodide also accelerates the reaction in methyl acetate up to two-fold, for addition of 20 equivalents of [I −]; this may be due to a “general” salt effect, stabilising the transition rather than the ground state. LiOAc behaves analogously; this is due to the reaction, MeI + LiOAc ⇌ MeOAc + LiI, which increases the LiI concentration, not to the formation of acetato-complexes of unusual reactivity. n-Bu 4NBF 4 has no effect on the reaction in methyl acetate. The oxidative addition is insensitive to the nature of the counter-ion except for C 12H 25NH + 3[Rh(CO) 2I 2] − in less polar solvents, where the IR spectra and also the slower rates of oxidative addition are consistent with some form of interaction, probably a N ⋯ H ⋯ Rh hydrogen-bond.