Diastereoselective Cycloreductions and Cycloadditions Catalyzed by Co(dpm)2-Silane (dpm = 2,2,6,6-tetramethylheptane-3,5-dionate): Mechanism and Partitioning of Hydrometallative versus Anio

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• 作者：Long-Cheng Wang ; Hye-Young Jang ; Yeonsuk Roh ; Vincent Lynch ; Arthur J. Schultz ; Xiaoping Wang ; and Michael J. Krische
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：August 14, 2002
• 年：2002
• 卷：124
• 期：32
• 页码：9448 - 9453
• 全文大小：115K
• 年卷期：v.124,no.32(August 14, 2002)
• ISSN：1520-5126

文摘

In the presence of phenylsilane and 5 mol % cobalt(II) bis(2,2,6,6-tetramethylheptane-3,5-dionate),aryl-substituted monoenone monoaldehydes and bis(enones) undergo reductive cyclization to afford syn-aldol and anti-Michael products, respectively. For both aldol and Michael cycloreductions, five- and six-membered ring formation occurs in good yield with high levels of diastereoselectivity. Cycloreduction ofmonoenone monoaldehyde 1a in the presence of d³-phenylsilane reveals incorporation of a single deuteriumat the enone ges/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-position as an equimolar mixture of epimers, inferring rapid isomerization of the kineticallyformed cobalt enolate prior to cyclization. The deuterated product was characterized by single-crystal neutrondiffraction analysis. For bis(enone) substrates, modulation of the silane source enables partitioning of thecompetitive Michael cycloreduction and [2 + 2] cycloaddition manifolds. A study of para-substitutedacetophenone-derived bis(enones) reveals that substrate electronic features also direct partitioning ofcycloreduction and cycloaddition manifolds. Further mechanistic insight is obtained through examinationof the effects of enone geometry on product stereochemistry and electrochemical studies involving cathodicreduction of bis(enone) substrates. The collective experiments reveal competitive enone reduction pathways.Enone hydrometalation produces metallo-enolates en route to aldol and Michael cycloreduction products,that is, products derived from coupling at the ges/gifchars/alpha.gif" BORDER=0>-position of the enone. Electron-transfer-mediated enonereduction produces metallo-oxy-ges/gifchars/pi.gif" BORDER=0 >-allyls en route to [2 + 2] cycloadducts and, under Ni catalysis, homoaldolcycloreduction products, that is, products derived from coupling at the ges/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-position of the enone. Theconvergent outcome of the metal-catalyzed and electrochemically induced transformations suggests theproposed oxy-ges/gifchars/pi.gif" BORDER=0 >-allyl intermediates embody character consistent with the mesomeric metal-complexed anionradicals.