Krenske Elizabeth H, Lam Sarah, Ng Jerome P L, Lo Brian, Lam Sze Kui, Chiu Pauline, Houk Kendall N
School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD 4072 (Australia).
Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong (P. R. China).
Angew Chem Int Ed Engl. 2015 Jun 15;54(25):7422-5. doi: 10.1002/anie.201503003. Epub 2015 May 7.
Silyl-triflate-catalyzed (4+3) cycloadditions of epoxy enolsilanes with dienes provide a mild and chemoselective synthetic route to seven-membered carbocycles. Epoxy enolsilanes containing a terminal enolsilane and a single stereocenter undergo cycloaddition with almost complete conservation of enantiomeric purity, a finding that argues against the involvement of oxyallyl cation intermediates which have been previously proposed for these types of reactions. Reported are theoretical and experimental investigations of the cycloaddition mechanism. The major enantiomers of the cycloadducts are derived from S(N)2-like reactions of the silylated epoxide with the diene, in which stereospecific ring opening and formation of the two new C-C bonds occur in a single step. Calculations predict, and experiments confirm, that the observed small losses of enantiomeric purity are traced to a triflate-mediated double S(N)2 cycloaddition pathway.
三氟甲磺酸硅酯催化的环氧烯基硅烷与二烯的(4+3)环加成反应为合成七元碳环提供了一条温和且具有化学选择性的路线。含有末端烯基硅烷和单一立体中心的环氧烯基硅烷进行环加成反应时，对映体纯度几乎完全保留，这一发现与之前针对这类反应所提出的involvement of oxyallyl cation intermediates观点相悖。本文报道了该环加成反应机理的理论和实验研究。环加成产物的主要对映体源自硅烷化环氧化物与二烯的类似SN2的反应，其中立体专一性的开环以及两个新C-C键的形成在一步反应中发生。计算预测并经实验证实，观察到的对映体纯度的少量损失可追溯到三氟甲磺酸酯介导的双SN2环加成途径。
