STEREOCHEMISTRIES AND MECHANISMS OF REACTIONS OF ELECTROPHILES WITH ORGANOTIN COMPOUNDS
STEREOCHEMISTRIES AND MECHANISMS OF REACTIONS OF ELECTROPHILES WITH ORGANOTIN COMPOUNDS
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DOI：
10.1021/om00145a029
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发表时间：
1987-02-01
期刊：
影响因子：
2.8
通讯作者：
JENSEN, FR
中科院分区：
文献类型：
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作者：
FUKUTO, JM;NEWMAN, DA;JENSEN, FR
研究了溶剂和亲电试剂对碳锡键卤化脱金属反应的影响。立体化学结果和n9Sn核磁共振都被用作机理探针。溶剂的作用是极端的；在极性溶剂中进行的反应，如乙腈和二甲基甲酰胺，产生的裂解产物主要是构型反转，而非极性溶剂产生的产物主要是保留。极性非质子溶剂在促进转化立体化学方面表现出很高的效率。极性质子溶剂的效率不高。这是用一些非常特殊的溶剂化现象来解释的。此外，几种三烷基卤化锡在不同溶剂中的核磁共振研究表明，这些特定的溶剂化现象可以定性地评估。亲电试剂的性质在最终的立体化学中也起着重要的作用；对Br2、12、IC1和IBr进行了比较和讨论。早期对聚合物亲电裂解的研究表明，SE2反应的一般立体化学结果是构型在碳上的保留。为了证明反转机制的可行性，Jensen和Davis2试图设计一个系统，该系统具有有利于反转途径的特性。对SE2反应可能的过渡态的考察（图1）表明，导致碳构型反转的过渡态（图lc）需要(a)留下的基团Y和M都有足够的电荷稳定，(b)对亲电试剂YZ的正面攻击具有空间位阻。于是，Jensen和Davis合成了三新戊基叔丁基锡，在甲醇中与溴反应，并加入溴化钠，得到了碳上的转化。自这一发现以来，其他研究小组也证明了碳锡键上SE2过程的倒置立体化学。Rahm和Pereyre证明了锡上新戊基取代引起的严重空间充血导致了异常的立体化学结果，保留仍然是首选途径。在后来由McGahey和Jensen进行的一项研究中，发现反转途径与保留途径竞争，并且没有普遍首选的立体化学。他们研究了一系列四烷基醇的溴化脱金属反应，其形式为（仲丁基）Sn（异丙基）3_jv（新戊基）2v， N=
The effect of solvent and electrophile on halodemetalation reactions of carbon-tin bondshas been investigated. Both stereochemical results and n9Sn NMR have beenused as mechanistic probes. The effect of solvent is extreme; reactions performedin polarsolvents such as acetonitrile and dimethylformamide yield cleavage products with predominantly inversion of configuration, whereas nonpolar solvents yield products with predominantly retention. Polar-aprotic solvents appear to be highly efficient in promoting inversion stereochemistry. Polar-protic solvents are not as efficient. This is explained in terms of some very specific solvation phenomena. Also, 119Sn NMR studies of several trialkyltin halides in various solvents show that these specificsolvation phenomena can be qualitatively assessed. The nature of the electrophile also plays an important role in eventual stereochemistry; Br2, 12, IC1, and IBr are compared and discussed.Early work investigating electrophilic cleavages of or-ganomercurials indicated that retention of configuration at carbon was the general stereochemical outcome for SE2 reactions. 1 In an attempt to demonstrate the viability of an inversion mechanism, Jensen and Davis2 sought to devise a system which would have properties favorable to an inversion pathway. An examination of the possible transition states for SE2 reactions, Figure 1, reveals that the transition state leading to inversion of configuration at carbon, Figure lc, would require (a) sufficient charge stabilization of both leavinggroups Y and M and (b) steric hindrance to frontside attack by the electrophile YZ. Thus, Jensen and Davis synthesized trineopentyl-sec-bu-tyltin, reacted it with bromine in methanol with added sodium bromide, and obtained inversion at carbon. 2 Since this finding, othergroups have also demonstrated inversion stereochemistry for SE2 processes at carbon-tin bonds. Rahm and Pereyre demonstrated that the severe steric congestion caused by neopentyl substitution on tin resulted in an abnormal stereochemical outcome and that retention was still the preferred pathway. 3 In a later study per-formed by McGahey and Jensen it was found that inver-sion pathways compete with retention pathways and there is no general preferred stereochemistry. 4 They studied the bromodemetalation of a series of tetraalkyltins of the form (sec-butyl) Sn (isopropyl) 3_jv (neopentyl) 2v, N=