Sakurai addition and ring annulation of allylsilanes with α,β-unsaturated esters.: Experimental results and ab initio theoretical predictions examining allylsilane reactivity
Sakurai addition and ring annulation of allylsilanes with α,β-unsaturated esters.: Experimental results and ab initio theoretical predictions examining allylsilane reactivity
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DOI：
10.1021/jo991177i
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发表时间：
2000-06-16
影响因子：
3.6
通讯作者：
Goddard, JD
中科院分区：
文献类型：
--
作者：
Organ, MG;Dragan, V;Goddard, JD
三甲基烯丙基硅烷已被证明添加到丙烯酸甲酯在良好的产率时，在室温下由TiCl 4催化，尽管文献报道相反。此外，即使在金属上具有这些小的烷基配体，除了简单的烯丙基化（樱井加成）之外，环成环在很大程度上发生。动力学产物是（（三甲基甲硅烷基）甲基）环丁烷衍生物，如果在催化剂存在下，其可以异构化为热力学产物环戊烷。与该领域的其他文献一致，增加硅上配体的尺寸增加了产物形成的速率和环化相对于烯丙基化的比例。为了开发一个预测模型的烯丙基硅烷的反应性，从头计算气相计算已与不同的配体的金属上的父烯丙基硅烷和这些烯丙基硅烷和丙烯醛，丙烯酸，丙烯酸甲酯之间的反应。预测表明，随着硅上的n-烷基配体的长度增加，烯丙基硅烷的Si-C α键与C β上的空p轨道超共轭的明显能力也增加，因为烯丙基硅烷开始攻击亲电体。这是证实了一个逐渐增加的HOMO在基态烯丙基硅烷的配位体从甲基到正己基和增加的Si-C α键长和减少的Si-C α-C β键角的质子化的物种。气相中的这些结果反映了这些N-烷基取代的烯丙基硅烷在实验中的反应性;即，随着烷基链长度的增加，反应性显著增加。三异丙基烯丙基硅烷是一种活性很高的硅烷，与其它取代烯丙基硅烷体系相比，其电荷分布和几何特征都出现异常，这可能是由于空间位阻效应的影响。对质子化物质的计算表明，基于促进Si-C α键和C β上的空p轨道之间良好轨道重叠所需的键长和角度，几乎不可能发生超共轭稳定。然而，气相反应的三异丙基烯丙基硅烷与丙烯醛和丙烯酸甲酯导致相对较低的能量势垒分别为13.1和24.5，这是一致的，其高的实验反应性。总之，这种计算分析产生了一个用于预测烯丙基硅烷反应性的有用模型以及对这种反应性的一些可能的解释。
Trimethylallylsilane has been shown to add to methyl acrylate in good yield when catalyzed by TiCl4 at room-temperature despite literature reporting to the contrary. Further, even with these small alkyl ligands on the metal, ring annulation occurs to a large extent, in addition to simple allylation (Sakurai addition). The kinetic product is the ((trimethylsilyl)melhyl)cyclobutane derivative which can be isomerized to cyclopentanoid, the thermodynamic product, if left in the presence of the catalyst. Consistent with other literature in this area, increasing the size of the ligands on silicon increases both the rate of product formation and the proportion of ring annulation relative to allylation. To develop a predictive model for allylsilane reactivity, ab initio gas-phase calculations have been made on the parent allylsilane with different ligands on the metal and on the reaction between these allylsilanes and acrolein, acrylic acid, and methyl acrylate. Predictions indicate that as the length of n-alkyl ligands on silicon increase, so does the apparent ability of the Si-C alpha bond of the allylsilane to hyperconjugate with developing vacant p orbital on C beta as the allylsilane begins to attack an electrophile. This is corroborated by a gradually increasing HOMO in the ground-state allylsilane as the ligands are changed from methyl through to n-hexyl and an increasing Si-C alpha bond length and decreasing Si-C alpha-C beta bond angle in the protonated species. These results in the gas phase mirror the reactivity of these n-alkyl-substituted allylsilanes in experiment; i.e., as the length of the alkyl chain increases, reactivity increases significantly. Triisopropylallylsilane, a very reactive silane, appears to anomalous in charge distribution and geometrical features compared with other substituted allylsilane systems which is due, presumably to steric effects. The calculations on the protonated species would indicate that almost no hyperconjugative stabilization can occur on the basis of the bond lengths and angles necessary to promote good orbital overlap between the Si-C alpha bond and the empty p orbital on C beta. However, the gas-phase reaction of the triisopropylallylsilane with acrolein and methyl acrylate led to comparatively low energy barriers of 13.1 and 24.5, respectively, which is consistent with its high experimental reactivity. Together, this computational analysis has produced a useful model for predicting allylsilane reactivity and some possible explanations for this reactivity.