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Triphenylphosphane-modified cobalt catalysts for the selective carbonylation of ethyl diazoacetate

The triphenylphosphane-substituted carbonyl cobalt complexes Co 2(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(PPh 3) 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(CHCO 2Et)(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=CHCO 2Et)] 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.

Kinetics of the Reactions of (Ethoxycarbonyl)methylcobalt Tetracarbonyl with 13CO, Ph3P, HCo(CO)4, and H2. A Comparison of the Reactivities of RCo(CO)4 (R=CH2COOEt, COOEt, and H) Complexes

The kinetics of the reactions of (ethoxycarbonyl)methylcobalt tetracarbonyl (1) with 13CO, Ph3P, HCo(CO)4, and H2 are consistent with initial reversible dissociation of EtOOCH2Co(CO)4 (1) to EtOOCCH2Co(CO)3 and CO.The alkylcobalt tricarbonyl then reacts competitively with the other reaction partner.The relative reactivities of 2 toward Ph3P, CO, HCo(CO)4, and H2 are 1.82, 1.0, 0.078, and 0.0006 at 25 deg C in n-heptane.The rate of 13CO substitution has also been measured for HCo(CO)4, Co2(CO)8, and (ethoxycarbonyl)cobalt tetracarbonyl (2).The half-life for HCo(CO)4 at -30 deg C is 45 s, t1/2 for EtOOCCH2Co(CO)4 at 15 deg C is 18.8 min, and t1/2 for EtOOCCo(CO)4 (2) at 15 deg C is 24.2 min.The complex 2 shows a slow 13CO incorporation into the acyl carbonyl group, t1/2 ca. 50 h at 28 deg C, presumably through an ethoxycobalt tetracarbonyl intermediate.The rate of 13CO exchange with Co2(CO)8 is not influenced by the presence of HCo(CO)4 at 0 deg C in n-octane, indicating that exchange of cobalt centers between HCo(CO)4 and Co2(CO)8 is slow compared to carbonyl exchange.