Cyclizative radical carbonylations of azaenynes by TTMSS and hexanethiol leading to α-silyl- and thiomethylene lactams. Insights into the E/Z stereoselectivities
作者:Mami Tojino、Noboru Otsuka、Takahide Fukuyama、Hiroshi Matsubara、Carl H. Schiesser、Hiroki Kuriyama、Hironari Miyazato、Satoshi Minakata、Mitsuo Komatsu、Ilhyong Ryu
DOI:10.1039/b309944j
日期:——
Free-radical mediated cyclizative carbonylations of azaenynes were carried out using TTMSS as a radical mediator to compare the efficiency and the stereochemistry with those using tributyltin hydride. Using a substrate concentration of 0.1 M, the reactions gave good yields of α-silylmethylene lactams having four to seven-membered rings. The observed E-diastereoselectivity of the resulting vinylsilane moiety is in sharp contrast to the Z-selectivity observed during the analogous carbonylation using tributyltin hydride. When hexanethiol was used as the radical mediator, α-thiomethylene lactams were formed with E-favoring stereoselectivity again. Ab initio and DFT molecular orbital calculations on the stability of E and Z products were carried out for a set of five-membered methylene lactams bearing SnH3, SiH3, and SMe groups. The distinct thermodynamic preference for the Z-isomer was only predicted for the Sn-bearing lactam. A steric effect due to the bulky (TMS)3Si group is proposed for the E-selectivity observed in the TTMSS-mediated reaction.
以TTMSS作为自由基介导剂,对azaenynes进行的环化碳基化反应,以比较其效率和立体化学与使用三丁基锡氢化物的反应。使用0.1 M的底物浓度,反应产生了良好的产率,得到具有四至七元环的α-硅烯基乳酸酰胺。所观察到的结果中乙烯基硅烷部分的E-异构体选择性与使用三丁基锡氢化物进行相应碳基化时观察到的Z选择性形成鲜明对比。当使用己硫醇作为自由基介导剂时,形成了同样偏向E选择性的α-硫烯基乳酸酰胺。对一组具有SnH3、SiH3和SMe基团的五元环亚甲基乳酸酰胺的E和Z产物稳定性进行了从头算和DFT分子轨道计算。仅预测到含Sn乳酸酰胺的Z-异构体具有明确的热力学偏好。提出由于笨重的(TMS)3Si基团所导致的空间效应,解释了在TTMSS介导反应中观察到的E选择性。