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.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

“…DFT calculations have suggested that the Pd-catalyst is not only involved in the ketene formation process but also plays a role in the [2 + 2] cycloaddition process which may affect the diastereoselectivity of the products (Scheme e). Compared with the Co 2 (CO) 8 -catalyzed or -mediated carbonylation of EDA with CO involving the formation of ketene intermediate, − the Pd-catalyzed protocol provides a general route to ketene compounds and is expected to be widely used in organic synthesis owing to the broad substrate scope of carbene precursors and the mild reaction conditions (with atmospheric pressure of CO). Later, similar transformations were also achieved under the catalysis of other transition-metal complexes. − …”

Computational Study on the Intramolecular Carbene-CO Coupling in M(CH2)(CO)3 Radicals (M = Co, Rh, Ir)

“…The process is more exergonic for the cobalt complex in comparison to that for the ethoxycarbonyl carbene analogue (−18.3 kcal/mol as opposed to −7.0 kcal/mol) and takes place with a much smaller barrier (1.6 kcal/mol as opposed to 15.5 kcal/mol) [8]. For the rhodium pathway Δ = −13.3 kcal/mol has been obtained as reaction free energy with a small barrier of 5.4 kcal/mol.…”

“…The main goal of this study is to further explore computationally the reactivity of simple carbonyl carbene radicals, which have already proven their applicability in the carbonylation of ethyl diazoacetate [8]. The second purpose is to scrutinize the vertical trends in the cobalt group of transition metals and to investigate the reaction profile for the rhodium and iridium analogues and to check whether the equilibrium can be on the carbene side in some cases, like it was reported by Urtel and coworkers [7].…”

“…The most common method to access ketenes is from acyl halides via dehalogenation promoted by bases. Therefore, carbonylation of metal carbene provides an alternative, straightforward approach to the generation of ketene species [7][8][9][10]. There are also many ketenes that cannot be synthetized by classical synthetic methods due to their high reactivity.…”

DFT Study on the Mechanism of Iron-Catalyzed Diazocarbonylation

“…Triphenylphosphine-substituted cobalt carbonyl complexes were found to be even more active precatalysts in the carbonylation of EDA. With two equivalents of PPh 3 , the presence of the ion pair [Co(CO) 3 (PPh 3 ) 2 ][Co(CO) 4 ] could be observed, which was assumed to interconvert to the corresponding radical pair under catalytic conditions [ 21 ].…”