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 mechanism s of platinum(N)-catalyzed alkyne-to-conjugated diene conversion reaction, which involves two subsequent triple bond activation steps followed by viny l-vinyl coupling. Calculations have shown that acetylene triple bond activa tion by PtI62- in water or methanol solution may proceed through either ext ernal nucleophile addition or intramolecular insertion, with the former mec hanism occurring with a lower barrier and leading to thermodynamically favo red product. The rate-determining step of the entire catalytic cycle is fou nd to be the formation of a platinum(IV) cis-divinyl derivative. Although v inyl-vinyl coupling reaction may take place from both six-coordinated octah edral and five-coordinated square-pyramidal platinum(IV) divinyl complexes, the five-coordinated derivative was found to react with a significantly lo wer barrier. The results obtained here are in good agreement with available experimental data and reveal important details of the catalytic reaction m echanism. The present investigation also has shown that no reliable conclus ions may be drawn for the system studied without taking solvent effects int o account.