Homogeneous Catalytic Synthesis of Alkyl Formates from the Reaction of Alkyl Halides, CO2, and H2 in the Presence of Anionic Group 6 Carbonyl Catalysts and Sodium Salts

Article abstract of DOI:10.1021/ja00245a024

Group 6 metal pentacarbonyl chlorides and bridging hydrides, (CO)5MCl^- and (μ-H)^- (M = Cr, Mo, or W), have been found to be effective homogeneous catalyst precursors for the production of long chain formate esters (HCO2R, R = n-butyl or n-octyl) from CO2, H2, and alkyl halides (RX; X = Cl, Br, or I) in the presence of alkali-metal salts (NaHCO3, Na2CO3, or K2CO3).For all the catalytic reactions conversions were found to be between 26 and 40percent with respect to RX.The selectivities toward the alkyl formates were found to be higher than 70percent.The only other organic products detected (by GC and GC-IR) were the corresponding alcohols, which were produced by the hydrolysis of the formate esters.The proposed catalytic cycle involves the reaction of (CO)5MCl^- with dihydrogen in the presence of sodium salts to produce the terminal hydride (CO)5WH^-.The latter complex dimerizes under the reaction conditions with subsequent formation of the bridging hydride (μ-H)^-, which was detected in the reaction solution.The terminal hydride rapidly inserts CO2 to produce the formate complex (CO)5MO2CH^-, which in turn reacts with the alkyl halide to yield the formate esters and regenerate the halide complex (CO)5MCl^-.This latter process is shown to proceed via loss of a carbon monoxide ligand.Subsequent oxidative addition of the alkyl halide to the metal center followed by reductive elimination of alkyl formate, or direct alkylation of the distal oxygen of the carboxylate ligand to afford alkyl formate and the metal halide complex, completes the reaction cycle.The overall process has been found very useful for the synthesis of long-chain alkyl formates starting from inexpensive starting materials such as CO2 and alkyl halides.