The first isolable 2-silenolate
The first isolable 2-silenolate
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DOI：
10.1002/anie.200250400
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发表时间：
2003-01-01
影响因子：
16.6
通讯作者：
Ottosson, H
中科院分区：
文献类型：
--
作者：
Guliashvili, T;El-Sayed, I;Ottosson, H
烯醇酸盐在有机化学中占有中心地位。[1]然而，对硅烯醇酸盐，即烯醇酸盐的硅类似物的关注要少得多(图式1)。到目前为止，还没有得到1-硅烯醇类化合物，甚至没有对这些物种进行计算研究。关于2-硅烯醇类化合物，只有Bravo-Zhivotovskii和Apeloig[2，3]和Ishikawa和Ohshita等几个基团的报道。[4]研究发现，在羰基碳原子上带有大量烷基或芳基取代基的2-硅烯醇类化合物在低温下是中等稳定的，因此可以用核磁共振光谱对其进行表征。在室温下，这些化合物在几个小时内发生降解，推测在此之前是二聚反应。[4]我们感兴趣的是表现出反转的SiC键极性(Sid？Cd+)的Silenes，[5]这是由于2-位的π-给电子取代基到Silene Si原子的负电荷离域而实现的。[6]这些反极化的Silenes对水分和醇的反应比自然极化的Silenes(Sid+？Cd？)小，并且醇的加成以意想不到的区域化学进行，因为形成的是C？O而不是Si？O键。8]反极化的二烯烃也选择性地与二烯反应，只生成[4，5，7]加合物。[4，5，7]这与自然极化的二烯烃形成对比，自然极化的二烯烃还生成[2，2]和烯加合物。[9]2-硅烯醇酸盐中的负电荷存在于O和/或Si原子上(方案1)。如果共振结构II占主导地位，那么电子结构类似于反转极化的硅烯。这样的2-硅烯醇酸盐应该含有Si？C单键(%1.87)，[10]，并且硅原子将类似于硅基阴离子的单键，即强金字塔结构。[11]另一方面，如果2-硅烯醇酸盐以结构I为主，则碳化硅的键长将接近于硅？C双键的键长(在H_2SI？CH2中为1.704)，[12]，硅原子将是平面的。关于2-硅烯醇类化合物的结构信息很少。根据HF/6-31G(D)计算，[(H3Si)2SiC(O)Me][(H3Si)2SiC2SiCMe]的碳化硅键为1.926，相当于Si±C单键。[4B]然而，Hartree-Fock能正确地描述2-硅烯醇酸盐吗？气相如何转变为凝聚相？事实上，变温~1H核磁共振谱表明，部分Si？C双键的成键转动活化能为14.3kcalmol？1。[4B]作为我们研究反转极化如何影响Si？C键合化合物的一部分，我们现在已经生成了可结晶的2-硅烯酸酯。这也是第一次实验确定了重的14族烯醇的晶体结构。我们从三(三甲基硅基)酰基硅烷(1，方案2)开始，但我们没有使用硅基锂试剂来提取三甲基硅基，而是使用了tBuOK。
Enolates hold a central position in organic chemistry.[1] However, there has been much less focus on silenolates, that is, Si analogues of enolates (Scheme1). So far 1-silenolates have not been made, and even computational studies of these species are absent. With regard to 2-silenolates, there are only a handful of reports from the groups of Bravo-Zhivotovskii and Apeloig [2, 3] and Ishikawa and Ohshita.[4] It was found that 2-silenolates with bulky alkyl or aryl substituents on the carbonyl carbon atom are moderately stable at low temperatures, thus allowing their characterization by NMR spectroscopy. At room temperature these compounds undergo degradation within a few hours, presumably preceded by dimerization.[4] Our interest lies in silenes that display reversed Si¼C bond polarity (SidÀ¼Cd+),[5] which is enabled by delocalization of the negative charge from π-electron-donating substituents in the 2-position to the silene Si atom.[6] These reverse-polarized silenes are less reactive towards moisture and alcohols than naturally polarized silenes (Sid+¼CdÀ), and addition of alcohol proceeds by an unexpected regiochemistry because CÀO instead of SiÀO bonds are formed.[5, 7, 8] The reverse-polarized silenes also react selectively with dienes to yield only [4þ2] adducts.[4, 5, 7] This is in contrast to the naturally polarized silenes, which give [2þ2] and ene adducts as well.[9]The negative charge in 2-silenolates resides on the O and/or Si atom (Scheme 1). If resonance structure II dominates, then the electronic structure resembles that of a reversepolarized silene. Such 2-silenolates should contain SiÀC single bonds (% 1.87),[10] and the Si atoms will be similar to those of silyl anions, that is, strongly pyramidal.[11] On the other hand, if 2-silenolates are dominated by structure I the SiC bond lengths will be close to that of a Si¼C double bond (1.704 in H2Si¼CH2),[12] and the Si atoms will be planar. There is very little structural information available on 2-silenolates. According to HF/6-31G (d) calculations,[(H3Si) 2SiC (O) Me] À has a SiC bond of 1.926, corresponding to a SiÀC single bond.[4b] However, can Hartree–Fock correctly describe 2-silenolates, and how does the result obtained for the gas phase transfer to condensed phases? Indeed, variable temperature 1H NMR spectroscopy suggests a partial Si¼C double bond with an activation energy for bond rotation of 14.3 kcalmolÀ1.[4b] As part of our study on how reversed polarization affects Si¼C-bonded compounds, we have now generated a crystallizable 2-silenolate. This is also the first experimentally determined crystal structure of a heavy Group 14 enolate. We started from tris (trimethylsilyl) acylsilane (1, Scheme 2), but instead of applying a silyllithium reagent for abstraction of a trimethylsilyl group we used tBuOK.[13] This