Silylium ion-catalyzed challenging Diels-Alder reactions: the danger of hidden proton catalysis with strong Lewis acids.
Silylium ion-catalyzed challenging Diels-Alder reactions: the danger of hidden proton catalysis with strong Lewis acids.
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DOI：
10.1021/ja211856m
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发表时间：
2012-02
影响因子：
15
通讯作者：
Ruth K. Schmidt;Kristine Muether;Christian Mück‐Lichtenfeld;S. Grimme;M. Oestreich
中科院分区：
文献类型：
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作者：
Ruth K. Schmidt;Kristine Muether;Christian Mück‐Lichtenfeld;S. Grimme;M. Oestreich
三配位硅阳离子具有明显的路易斯酸性，在路易斯酸催化中具有不同寻常的反应活性。然而，用强路易斯酸催化的缺点是，它们确实有可能通过各种机制介导质子的形成，因此释放的Brønsted酸甚至可能比刘易斯酸更有可能成为真正的催化剂。这是一个经常被忽视的问题。消除隐藏的质子催化途径的一种方法是添加质子清除剂。由二茂铁稳定硅阳离子催化的低温Diels-Alder反应就是这样一种情况，必须仔细研究质子催化的可能性。然而，共同受阻碱2,6-二叔丁基吡啶的加入导致分解缓慢，并形成相应的吡啶离子。用更碱的（Mes）(3)P定量地观察到硅阳离子脱质子生成磷离子。氘标记实验证实质子是从二茂铁主链中提取出来的。在量子化学计算的基础上，提出了质子形成的合理机理。诚然，这是一种特殊情况，但这表明必须仔细审查质子清除剂的使用，因为质子的形成可能会引起而不是阻止。质子催化Diels-Alder反应在文献中没有很好的记录，这里包括一个使用TfOH的代表性调查。这些催化的结果与我们的硅离子催化Diels-Alder反应进行了比较，从而清楚地证实了隐藏的Brønsted酸催化不与我们的Lewis酸起作用。报道了几种看似简单但具有挑战性的Diels-Alder反应，其中异常罕见的二酚/酚组合。另一个指示是由催化剂的化学选择性得到的。硅离子催化的Diels-Alder反应对于α，β-不饱和亲二酚（羰基和羧基）的氧化水平是一般的，而质子催化仅限于羰基化合物。
The pronounced Lewis acidity of tricoordinate silicon cations brings about unusual reactivity in Lewis acid catalysis. The downside of catalysis with strong Lewis acids is, though, that these do have the potential to mediate the formation of protons by various mechanisms, and the thus released Brønsted acid might even outcompete the Lewis acid as the true catalyst. That is an often ignored point. One way of eliminating a hidden proton-catalyzed pathway is to add a proton scavenger. The low-temperature Diels-Alder reactions catalyzed by our ferrocene-stabilized silicon cation are such a case where the possibility of proton catalysis must be meticulously examined. Addition of the common hindered base 2,6-di-tert-butylpyridine resulted, however, in slow decomposition along with formation of the corresponding pyridinium ion. Quantitative deprotonation of the silicon cation was observed with more basic (Mes)(3)P to yield the phosphonium ion. A deuterium-labeling experiment verified that the proton is abstracted from the ferrocene backbone. A reasonable mechanism of the proton formation is proposed on the basis of quantum-chemical calculations. This is, admittedly, a particular case but suggests that the use of proton scavengers must be carefully scrutinized, as proton formation might be provoked rather than prevented. Proton-catalyzed Diels-Alder reactions are not well-documented in the literature, and a representative survey employing TfOH is included here. The outcome of these catalyses is compared with our silylium ion-catalyzed Diels-Alder reactions, thereby clearly corroborating that hidden Brønsted acid catalysis is not operating with our Lewis acid. Several simple-looking but challenging Diels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported. Another indication is obtained from the chemoselectivity of the catalyses. The silylium ion-catalyzed Diels-Alder reaction is general with regard to the oxidation level of the α,β-unsaturated dienophile (carbonyl and carboxyl), whereas proton catalysis is limited to carbonyl compounds.