meso-Tetraphenylporphinatoiron catalysed reductive cleavage of some S–O, S–N, and S–C bonds in sulphoxides and sulphilimines

Abstract

meso-Tetraphenylporphinatoiron chloride (TPPFe^IIICl) has been found to catalyse the reductive cleavage of semipolar S–O and S–N bonds in aromatic sulphoxides and sulphilimines respectively with both 1-benzyl-1,4-dihydronicotinamide (BNAH) and sodium borohydride. Sulphoxides bearing electron-withdrawing substituents are much more readily reduced than those with electron-donating groups. The rate of reduction by the TPPFe^III/BNAH system decreases in the following order: N-unsubstituted sulphilimines > N-tosylsulphilimines > aromatic sulphoxides. During the reaction with sulphoxides, the formation of the µ-oxo dimer of TPPFe^III has been observed. Under the same conditions, concurrent reductive cleavage of the S–C bond was observed with sulphoxides bearing an electron-withdrawing group at the α-position. Control experiments revealed that the reductive S-dealkylation of these sulphides is accompanied by the major S-deoxygenation reaction. Plausible mechanisms are discussed.