Experimental and quantum-mechanical investigation of the vinylsilane-iminium ion cyclization
Experimental and quantum-mechanical investigation of the vinylsilane-iminium ion cyclization
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DOI：
10.1039/b212333a
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发表时间：
2003-01-01
影响因子：
3.2
通讯作者：
Tanner, D
中科院分区：
文献类型：
--
作者：
Kværno, L;Norrby, PO;Tanner, D
乙烯基硅烷酮基离子环化反应涉及亚胺物种来自胺6和7的实验研究作为一种可能的方法，一些生物有趣的1-氮杂螺环。然而，即使在高温下微波辐射的条件下，也没有观察到这样的环化，而（与先前的结果一致）相应的乙烯基硅烷-铝离子环化更成功。取代氮的Aldiminium物种显示没有非对映选择性在环化的前体衍生自6，而高的反式非对映选择性可以获得亚胺物种衍生自7。量子力学的一般反应机理的调查强调缺乏反应性的ketiophosphate物种，也令人信服地解释所观察到的aldiminium物种的非对映选择性。计算进一步揭示了（Z）乙烯基硅烷通过硅稳定的β-碳阳离子环化，并且起始材料到烯丙基硅烷-亚胺鎓物质的任何形式的氮杂-Cope重排不会以协同的方式发生。然而，计算表明，氮杂-Cope重排先于相应的（E）-乙烯基硅烷的环化，整个反应在能量上略低于（Z）-异构体的环化。
A vinylsilane-ketiminium ion cyclization involving iminium species derived from amines 6 and 7 was investigated experimentally as a possible approach to some biologically interesting 1-azaspirocycles. However, even under conditions of microwave irradiation at high temperatures no such cyclization was observed whereas (in line with previous results) the corresponding vinylsilane-aldiminium ion cyclizations were more successful. Aldiminium species substituted to nitrogen displayed no diastereoselectivity in the cyclization of precursors derived from 6 while high trans diastereoselectivity could be obtained for iminium species derived from 7. Quantum-mechanical investigations of the general reaction mechanism underlined the lack of reactivity of ketiminium species and also convincingly explained the observed diastereoselectivities of aldiminium species. The calculations further revealed that (Z) vinylsilanes cyclize via a silicon-stabilized beta-carbocation, and that any formal aza-Cope rearrangement of the starting material to an allylsilane-iminium species does not take place in a concerted fashion. However, the calculations show that the aza-Cope rearrangement precedes cyclization for the corresponding (E)-vinylsilanes, the overall reaction being energetically slightly less favoured than cyclization of the (Z)-isomers.