Efficient Biomimetic Decarboxylation of Diphenylacetic Acid by [Mn(TPP)X]n-Bu4NIO4 Catalytic Systems: Effect of Anionic Axial Ligands (X-)

Abstract

Highly efficient decarboxylation of diphenylacetic acid has been performed by tetrabutylammonium periodate in the presence of different [Mn(TPP)X] complexes (X- = anionic axial ligand) in CH2Cl2. In these catalytic systems, diphenylacetic acid was selectively (>97%) converted to benzophenone. The reactivity of manganese porphyrins are markedly influenced by the nature of anionic axial ligands and the efficiency of the catalysts decreases in the order: CN- > OCN- > N3- > SCN- > F- > OAc- ∼ Cl- ∼ Br- ∼ I-∼ ClO4-. Marked improvement in reactivities and selectivities has resulted from addition of imidazole and its derivatives to the reaction mixture. π-Bonding interaction in Mn–X moiety and the possibility of distal hydrogen bonding between imidazole and X- is presented to describe the enhanced catalytic activities observed for Mn(TPP)X (X- = SCN-, N3-, OCN-, CN-). A simple catalytic cycle is also proposed to show the role of X- in the possible intermediates.