"Formal" and Standard Ruthenium-Catalyzed [2 + 2 + 2] Cycloaddition Reaction of 1,6-Diynes to Alkenes: A Mechanistic Density Functional Study

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• 作者：Jesú ; s A. Varela ; Silvia G. Rubí ; n ; Luis Castedo ; Carlos Sa&aacute
• 刊名：Journal of Organic Chemistry
• 出版年：2008
• 出版时间：February 15, 2008
• 年：2008
• 卷：73
• 期：4
• 页码：1320 - 1332
• 全文大小：1073K
• 年卷期：v.73,no.4(February 15, 2008)
• ISSN：1520-6904

文摘

"Formal" and standard Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes 1 to alkenes gave bicyclic1,3-cyclohexadienes in relatively good yields. The neutral Ru(II) catalyst was formed in situ by mixingequimolecular amounts of [Cp*Ru(CH₃CN)₃]PF₆ and Et₄NCl. Two isomeric bicyclic 1,3-cyclohexadienes3 and 8 were obtained depending on the cyclic or acyclic nature of the alkene partner. Mechanisticstudies on the Ru catalytic cycle revealed a clue for this difference: (a) when acyclic alkenes were used,linear coupling of 1,6-diynes with alkenes was observed giving 1,3,5-trienes 6 as the only initial reactionproducts, which after a thermal disrotatory 6e^- electrocyclization led to the final 1,3-cyclohexadienes3 as probed by NMR studies. This cascade process behaved as a formal Ru-catalyzed [2 + 2 + 2]cycloaddition. (b) With cyclic alkenes, the standard Ru-catalyzed [2 + 2 + 2] cycloaddition occurred,giving the bicyclic 1,3-cyclohexadienes 8 as reaction products. A complete catalytic cycle for the formaland standard Ru-catalyzed [2 + 2 + 2] cycloaddition of acetylene and cyclic and acyclic alkenes withthe Cp*RuCl fragment has been proposed and discussed based on DFT/B3LYP calculations. The mostlikely mechanism for these processes would involve the formation of ruthenacycloheptadiene intermediatesXXIII or XXVII depending on the alkene nature. From these complexes, two alternatives could beenvisioned: (a) a reductive elimination in the case of cyclic alkenes 7 and (b) a -elimination followedby reductive elimination to give 1,3,5-hexatrienes 6 in the case of acyclic alkenes. Final 6e^-electrocyclization of 6 gave 1,3-cyclohexadienes 3.