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

The triphenylphosphane-substituted carbonyl cobalt complexes Co-2(Co)(7)(PPh3), Co-2(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 degrees C in dichloromethane solution than the parent Co-2(CO)(8) and Co-2(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 Co-2(CO)(7)(PPh3) occurs in a catalytic cycle, where Co-2(CO)(6)(PPh3) and Co-2(CO)(6)(CHCO2Et)(PPh3) are the repeating species. The 16e species Co-2(CO)(6)(PPh3) is involved in the deazotization of ethyl diazoacetate, and Co-2(CO)(6)(CHCO2Et)(PPh3) leads to the (ethoxycarbonyl)ketene formation. In the absence of carbon monoxide or at low CO concentration the reaction of Co-2(CO)(6)(CHCO2Et)(PPh3) with ethyl diazoacetate is the source of Co-2(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 Co-2(CO)(7)(PPh3), Co-2(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 degrees C and 11 bar of pressure with up to 5.1 mol of product/((mol of catalyst) h) turnover frequency.