DFT Study on Ru-II-Catalyzed Cyclization of Terminal Alkynals to Cycloalkenes

摘要

The reaction mechanism of [CpRu(MeCN)(3)]PF6-catalyzed cyclization of terminal alkynals 1 to cycloalkenes 2 was investigated by means of density functional methods combined with polarizable continuum models. Calculations indicate that the coordination of the cationic catalyst [CpRu(CH3CN)(2)](+) to the carbon-carbon triple bond of the Substrate 1 enhances the electrophilicity of alkynyl group, and the Subsequent nucleophilic attack of the carbonyl oxygen to the electron-deficient alkyne forms ate complex IM2, which would further isomerize into 2H-oxete complex IM3. Then a replacement of MeCN by AcOH Occurs, followed by two proton-migrations, which leads to a Fischer-type carbene complex IM6. Finally, a decarbonylation takes place leading to cycloalkene 2. The terminal alkynal is activated by its combination with ruthenium, which leads to a decrease in the natural bond orbital energy of pi(star)((C1 C2)). The four-membered ring formation is the rate-controlling step. However, AcOH helps proton shift through coordination with metal center and decreases the reaction energy barriers. Throughout the reactions, all the Ru-II complexes obey the 18-electron-rule. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 109: 679-687, 2009