CO2 insertion into M-OR bonds

The reaction of CO2 with M-OR (R = alkyl, aryl) bonds is of industrial interest as it is related to the synthesis of molecular organic carbonates and polycarbonates based on the direct carboxylation of substrates such as alcohol, polyols, and epoxides. In this paragraph the kinetics and thermodynamics of the elementary step (4.18) are discussed, and the synthesis of organic carbonates (acyclic and cyclic) is discussed in Chap. 6. 

Because of the well defined polarity of the M-O bond, there is no ambiguity about the mode of insertion of CO2. The electrophilic carbon of the heterocumulene is attacked by the electron-rich terminal of the polar M -O bond. Therefore, the insertion product is a metal-alkylcarbonate complex (4.18). 

A well studied case of insertion of CO2 into the M-OR bond is represented by the insertion of CO2 into the M-OCH3 bond of the complex/ac-(CO)3(dppe)Mn (OCH3) to afford/ac-(CO)3(dppe)Mn-OC(0)OCH3, a complex with scarce application as a copolymerization catalyst. Kinetic and mechanistic studies have been aimed at the definition of the TS and kinetic parameters [64]. Such reaction was reported to be very fast already at 195 K [65, 66]. Darensbourg and coworkers have used low temperature in situ IR for their studies. Despite the difficulty linked to the experimental conditions, they were able to determine at 195 K the value of 2.0 X 10 M s for the rate constant ki for the second-order reaction (4.19). The authors say that such a value is most likely underestimated. 

The reaction of the carbonato complex with C02 has allowed the demonstration of a facile insertion-deinsertion equilibrium. The study of the deinsertion reaction has allowed estimation of the activation parameters as being = 130 4.0 kJ mor and AS = 121.6 11.9 J moP K . From the above values the authors have calculated an approximate value of the equiUbrium cmistant for the carboxylation reaction equal to 3 x 10 M at 195 K (or a AG value for the same reaction of 50 kJ moP ), showing that the insertion of CO2 into the M-O-alkyl bond is both kineticaUy and thermodynamically very favored. This trend has also been confirmed for the insertion of CO2 into the Nb-OR bond in [Nb(OR)5]2 (R = methyl, ethyl, aUyl) (see Sect. 6.2.2.1), a catalyst for the synthesis of dialkyl carbonates [67]. Very recently, the facile insertion of CO2 into metal-phenoxide bonds has been reported [68] for cobalt and zinc complexes (Fig. 4.2). It should be noted that such metal systems are used as catalysts in the copolymerization of CO2 with epoxides. 

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