The insertion of CO into a metal alkyl bond (or, more precisely, alkyl migration to coordinated CO) is one of the most important and characteristic reactions of transition metal complexes. It is a key step in important commodity chemical transformations such as hydroformylation as well as numerous reactions widely applied to the synthesis of fine chemicals. Decades ago it was found that rates of such CO insertions can be markedly increased by polar solvents such as THF or DMF. Bergman subsequently demonstrated that the ability to promote CO insertion correlated strongly with the ability of the promoter to act as an electron-pair donor, rather than with its polarity or dielectric constant.1 Halpern and co-workers later showed that certain nucleophiles such as Ph3PO were remarkably effective. Kinetic analysis revealed that the nucleophile dissociates from the complex prior to attack by the trapping molecule, thus indicating that the nucleophile catalyzes insertion of CO (and must therefore catalyze deinsertion as well).2 The putative ability of a nucleophile to catalyze a deinsertion reaction is particularly puzzling, and over 30 years after Halpern's elucidation of the kinetic role only a highly speculative mechanism has been tentatively advanced. We have conducted a computational (DFT) study of this catalysis, which has revealed a completely unanticipated reaction mechanism. We have found that the nucleophile does not actually promote CO insertion. Instead, its role is to catalyze the rearrangement of the initial CO-insertion product, an agostic-C-H acyl complex, to give an κ2-(C,O)-acyl complex, which reacts with the trap to give the final product. The calculations explain the unusual kinetics obtained with Ph3PO as catalyst. They also capture, remarkably well, the ordering of catalytic effectiveness of an extensive range of nucleophiles that was determined in the Halpern lab.

Promotion of CO insertion into metal-alkyl bonds by nucleophiles: Elucidation of an unanticipated mechanism of the "Halpern insertion reaction"

256th National Meeting (2018)

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