Homogeneous Catalytic Carbonylation of Nitroaromatics. 7. Transesterification and an Intramolecular Ligand-Hopping Mechanism for Isomerization in the Bis(methoxycarbonyl) Complex Ru(Ph₂PCH₂CH₂PPh₂)(CO)₂[C(O)OCH₃]₂

The bis (methoxycarbonyl) complex (OC-6-32)-dicarbonylbis(methoxycarbonyl)[1,2-bis(diphenylphosphino)ethane]ruthenium(II) (3) isomerizes to (OC-6- 2′2)-dicarbonylbis(methoxycarbonyl) [ 1,2-bis (diphenylphosphino)ethane] ruthenium(II) (3′) via an intramolecular methoxide migration to an adjacent metal carbonyl. Isolation of pure 3, followed by dissolution in benzene-d₆ at ambient temperature, results in the formation of an equilibrium mixture of the two species within 3 h. The assignment of methoxide migration as the mechanism of isomerization is based on kinetic analysis of the isomerization via NMR spectroscopic methods, ¹³CO ligand exchange with 3, and attempted isotopic crossover experiments. Reacting 3 with Ru(dppe)(CO)₂[C(O)OCD₃]₂ in a 1.15:1 ratio under ambient conditions over 2 days did not result in observable scrambling of the isotopic labels. Transesterification of the two methoxycarbonyl groups occurs rapidly in the presence of excess alcohol. The rates of exchange with CD₃OD for the two methoxycarbonyl ligands on 3 were found to be independent of each other and were quantified by NMR spectroscopic method.