Reactions of hydride reagents with alkylmolybdenum carbonyl complexes. Reaction of LiBHEt3 with [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.