Triphenylphosphane-Modified Cobalt Catalysts for the Selective Carbonylation of Ethyl Diazoacetate

Abstract

The triphenylphosphane-substituted carbonyl cobalt complexes Co2(CO)7(PPh3), Co2(CO)6(CHCO2Et)(PPh3), and [Co(CO)3(PPh3)2][Co(CO)4] were found to be more effective precatalysts in the carbonylation of ethyl diazoacetate under atmospheric pressure of carbon monoxide at 10 °C in dichloromethane solution than the parent Co2(CO)8 and Co2(CO)7(CHCO2Et) complexes. The highly reactive (ethoxycarbonyl)ketene is the primary product of the catalytic carbonylation, which dimerizes in the absence of a proper scavenger. In the presence of ethanol as the trapping reagent diethyl malonate is the final product of the carbonylation reaction. The formation of (ethoxycarbonyl)ketene using the catalyst precursor Co2(CO)7(PPh3) occurs in a catalytic cycle, where Co2(CO)6(PPh3) and Co2(CO)6(CHCO2Et)(PPh3) are the repeating species. The 16e species Co2(CO)6(PPh3) is involved in the deazotization of ethyl diazoacetate, and Co2(CO)6(CHCO2Et)(PPh3) leads to the (ethoxycarbonyl)ketene formation. In the absence of carbon monoxide or at low CO concentration the reaction of Co2(CO)6(CHCO2Et)(PPh3) with ethyl diazoacetate is the source of Co2(CO)5(CHCO2Et)2(PPh3), which is not an active catalyst for the carbonylation of ethyl diazoacetate. Using [Co(CO)3(PPh3)2][Co(CO)4] as the catalyst precursor, the intermediary formation of [Co(CO)3(PPh3)2][Co(CO)3(O═C═CHCO2Et)] through radical pairs is assumed. Substituting PPh3 in Co2(CO)7(PPh3), Co2(CO)6(CHCO2Et)(PPh3), and [Co(CO)3(PPh3)2][Co(CO)4] by polymer-bound PPh3 results in active and reusable catalysts for the selective carbonylation of ethyl diazoacetate in dichloromethane solution at 40 °C and 11 bar of pressure with up to 5.1 mol of product/((mol of catalyst) h) turnover frequency.