Catalytic triple bond activation and vinyl-vinyl reductive coupling by Pt(IV) complexes. A density functional study

A density functional theoretical study has been performed for the mechanisms of platinum(N)-catalyzed alkyne-to-conjugated diene conversion reaction, which involves two subsequent triple bond activation steps followed by vinyl-vinyl coupling. Calculations have shown that acetylene triple bond activation by PtI62- in water or methanol solution may proceed through either external nucleophile addition or intramolecular insertion, with the former mechanism occurring with a lower barrier and leading to thermodynamically favored product. The rate-determining step of the entire catalytic cycle is found to be the formation of a platinum(IV) cis-divinyl derivative. Although vinyl-vinyl coupling reaction may take place from both six-coordinated octahedral and five-coordinated square-pyramidal platinum(IV) divinyl complexes, the five-coordinated derivative was found to react with a significantly lower barrier. The results obtained here are in good agreement with available experimental data and reveal important details of the catalytic reaction mechanism. The present investigation also has shown that no reliable conclusions may be drawn for the system studied without taking solvent effects into account.