Catalytic air oxidation of benzoin in the presence of dioxomolybdenum(VI) complexes with sulphur chelate ligands

Abstract

Catalytic air oxidation of benzoin to benzil in N,N-dimethylformamide (dmf) occurred in the presence of [MoO₂(cysS-OR)₂][cysS-OR = S-deprotonated cysteine alkyl ester; R = Me (1) or Et (2)], [MoO₂(S₂CNEt₂)₂](3) or [MoO₂(cysS-NHC₁₀H₂₁)₂](4)(cysS-NCH₁₀H₂₁= S-deprotonated cysteine n-decylamide), [Mo₂O₃(cysS-OMe)₄](5), or [Mo₂O₃(S₂CNEt₂)₄](6) with conversions of 1 500, 1 200,1 000, 1 800, 1 300, and 900%([Mo]/[benzoin]= l/20, 20 h, 30 °C), respectively. The initial first-order rate constants ranged from 0.012 to 0.048 s^–1(per mol of catalyst). Slow deactivation of the catalysis by complexes (1)–(3) was observed due to the formation of di-µ-oxo binuclear molybdenum(V) complexes from µ-oxo binuclear molybdenum(V) complexes. The oxidation in the presence of (4) proceeds without deactivation. In the stoicheiometric reduction of complex (2) with benzoin the absence of any e.s.r. signal due to a mononuclear molybdenum(V) species indicates that a molybdenum(IV) species formed by a two-electron transfer process is aerobically converted into a µ-oxo binuclear molybdenum(V) complex. An ¹H n.m.r. study showed that the initial proton transfer from the benzoin OH group to MoO is followed by elimination of a proton from the benzoin methine group to form a carbonyl group.