Oxidation of Alkylbenzenes with Cerium Complexes Containing a Tripodal Oxygen Ligand

Abstract

AbstractThe treatment of [Ce(LOEt)2(NO3)2] (LOEt– = [Co(η5‐C5H5){P(O)(OEt)2}3]–) with KMnO4 in water afforded a diamagnetic purple solid 1, which is tentatively formulated as a CeIV permanganate complex, “[Ce(LOEt)2(MnO4)2]”. The ZrIV analogue, [Zr(LOEt)2(MnO4)2] (2), has been prepared similarly from [Zr(LOEt)2(NO3)2] and KMnO4. The recrystallization of 1 from CH2Cl2/hexanes at –18 °C led to the isolation of dinuclear [(CeLOEt)2(μ‐LOEt′)2][MnO4]2 (3), which contains the dianionic tripodal ligand [LOEt′]2– ([(η5‐C5H5)Co{P(O)(OEt)2}2{P(O)2(OEt)}]2–). The reactions of 1 with K[ReO4] and [NH4][OsO3N] afforded the heterodimetallic complexes [Ce(LOEt)2(ReO4)2] (4) and [Ce(LOEt)2(NOsO3)2] (5), respectively. The crystal structures of 3 and 5 have been determined. Freshly prepared 1 can oxidize alkylbenzenes such as toluene, ethylbenzene, and cumene at room temperature to give the corresponding ketone and/or alcohol products. Ce–LOEt complexes are efficient promoters of the aerobic oxidation of alkylbenzenes by a radical mechanism. For example, cumene in the presence of [Ce(LOEt)2(H2O)2]Cl (1 mM) in air at 100 °C for 10 h afforded a ca. 6:1 mixture of 2‐phenyl‐2‐propanol and acetophenone with a total turnover number of 6810.