Promoting factors for CO insertion

In many palladium-catalyzed reactions, generation of cationic intermediates has been often found to enhance the catalytic activities [60]. A number of reports have demonstrated that the CO insertion into metal-carbon a-bonds can be accelerated by formation of cationic complexes from neutral complexes [50,61-63]. 

The stepwise alternating insertions of CO and an alkene into Pd-C bonds comprise important steps in living catalysts for the alternating copolymerization [65]. The olefin insertion into acyl complexes provides cationic alkyl species in which a carbonyl oxygen is coordinated to the palladium center as shown in Eq. 7.4 The chelating alkyl complexes, whose presence has been confirmed by several research groups, would give extra stabilization to prevent occurrence of /I-elimination. 

Modification of the behavior of a palladium methyl complex by coordination of a second transition metal complex was reported to enhance the CO insertion rate. The CO insertion into palladium-methyl bond in a Pd-Co heterodinuclear complex, (dppe)(CH3)Pd-Co(CO)4, proceeded with a higher reaction rate than into the mononuclear palladium complex, Pd(CH3)Cl(dppe) (Eq. 7.5) [66]. The outcome of the reaction using CO was consistent with the mechanism of the preferential insertion of the carbonyl on Co center into the Pd-C bond and the coordination of incoming CO to Co. Theoretical studies suggested that the 

The CO insertion can be promoted by le oxidation of metals. A partial positive charge on the CO carbon induced by increase of electrophilicity of the metal would facilitate the nucleophilic alkyl migration. The effect of electron deficiency is to promote incorporation of the solvent molecules that may assist the CO insertion [67]. For example, rate enhancement of CO insertion into CpFeCH3(CO)2 (Cp = 7 -cyclopentadienyl) by treatment with Ce(IV) and trityl salts has been demonstrated (Eq. 7.6) [68]. 

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