Lewis acid assisted reactions of N-acylimidazoles with transition-metal nucleophiles. A route to formyl transition-metal complexes

Article abstract of DOI:10.1021/om00145a022

Lewis acid assisted acylations of Na₂M(CO)₄ complexes (M = Fe, Ru, Os) by N-acylimidazoles have been investigated, and N-formyl- (1), N-acetyl- (6), N-pivaloyl- (7), and N-benzoylimidazole (8) have been examined as acyl transfer agents. The yields of NaFe(CO)₄CHO on treatment of Na₂Fe(CO)₄ with 1 were found to be dependent upon the types of solvent and Lewis acid employed. Optimum yields were obtained in HMPA by using 2 equiv of (MeO)₃B/equiv of 1. THF and N-methylpyrrolidinone could also be employed as solvents. These reaction conditions enabled the in situ synthesis of the new anionic formylmetal complexes Na-Ru(CO)₄CHO and NaOs(CO)₄CHO. The NaM(CO)₄CHO complexes and NaFe(CO)₄COPh were characterized in solution by NMR and infrared spectroscopy. [PPN]Fe(CO)₄COCH₃ and [PPN]Fe(CO)₄CO-t-Bu have been isolated by treatment of the NaFe(CO)₄COR product mixtures with PPNCl. Acylations of Na₂M(CO)₄ complexes by 1 and 6 occurred only in the presence of a Lewis acid. In the absence of a Lewis acid, 1 was decarbonylated to afford NaM(CO)₄H and sodium imidazolate and 6 underwent a Claisen-type condensation. Sodium imidazolate also caused the decarbonylation of 1 and the self-condensation of 6. 7 and 8 acylated Na₂Fe(CO)₄ in both the presence and the absence of R₃B Lewis acids. The results of the study show that the Lewis acid scavenges the sodium imidazolate byproduct, prohibiting its promotion of the decarbonylation of 1 and the enolization of 6. The Lewis acid also appears to increase the susceptibility of the N-acylimidazole to nucleophilic attack at the acyl carbon.