Reactions of hydride reagents with alkylmolybdenum carbonyl complexes. Reaction of LiBHEt3with [MoMe(CO)3(η-C5H5)], formation of an anionic acetaldehyde complex, and a stoicheiometric cycle for the synthesis of acetaldehyde

Abstract

The molybdenum alkyl complex [MoMe(CO)3(η-C5H5)](1) reacts with LiBHEt3 in tetrahydrofuran (thf) solution at ambient temperature to give the anionic acetaldehyde complex [Mo(η2-MeCHO)(CO)2(η-C5H5)]–(2) as the predominant organomolybdenum product together with very small quantities of the anion [Mo(CO)3(η-C5H5)]–. Monitoring experiments using low-temperature i.r. and n.m.r. spectroscopy allows the spectroscopic identification of the formyl [MoMe(CHO)(CO)2(η-C5H5)]–(3) as the primary product at –66 °C. Slow warming of the reaction mixture results in the spectroscopic observation of the successive rearrangements of (3) into the hydrido-acyl [MoH(COMe)(CO)2(η-C5H5)]–(6) and finally of (6) into the ultimate product (2). Treatment of a solution of (2) with PhCH2Br at ambient temperature results in formation of predominantly the η3-benzyl [Mo(CO)2(η3-CH2Ph)(η-C5H5)] and the evolution of acetaldehyde. In a related reaction (2) reacts with MeI and PPh3 to form acetaldehyde and trans-[MoMe(CO)2(PPh3)(η-C5H5)]. Complex (2) reacts with MeI under a CO atmosphere to regenerate (1) with the evolution of acetaldehyde, thus completing a stoicheiometric cycle for the synthesis of acetaldehyde. Possible mechanisms for the above reactions are discussed.