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首页> 外文期刊>Chemistry: A European journal >Predicting Reactivity and Stereoselectivity in the Nazarov Reaction: A Combined Computational and Experimental Study
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Predicting Reactivity and Stereoselectivity in the Nazarov Reaction: A Combined Computational and Experimental Study

机译：预测Nazarov反应中的反应性和立体选择性：计算和实验相结合的研究

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摘要

The Nazarov reaction of pentadienyl cations generated by protonation of either dienones or alkoxytrienes has been examined in detail both experimentally and by DFT calculations. In particular, calculations at the B3LYP/6-311G** level of theory accurately predicted, and accounted for, the outcome of the Bronsted acid catalyzed electrocyclization of 4 pi-electron systems in which one of the double bonds involved in the process was embedded in N- and S-heterocyclic rings. Calculations showed that both heteroatoms are capable of accelerating the ring closure by stabilizing the partial positive charge which develops at C-6 (C-2) in the transition state, with S-heterocyclic derivatives being more reactive than the corresponding N-containing compounds. In general, pentadienyl cations generated by protonation of alkoxytrienes were expected to react faster than those obtained by protonation of the corresponding dienones, as the latter were stabilized by a hydrogen bond. The presence of a substituent on the heterocyclic ring significantly affects the stereoselectivity (torquoselectivity) only in the case of the N-heterocyclic derivatives, in which a 2-alkyl group is axially oriented, providing the cis-2,5-disubstituted isomer only. Instead, with substituted S-heterocyclic compounds, the anticipated torquoselectivity was very low and, in fact, a 3:1 diastereomeric mixture between the trans and cis products was experimentally found after ring closure. For this study, the synthesis of the appropriate N- and S-containing dienones and alkoxytrienes was realized to evaluate the predictivity power of the DFT computations, which was very good in all of the cases examined, both in terms of reactivity and stereoselectivity. The consistency observed between computational and experimental results, therefore, shows the usefulness of DFT calculations at the B3LYP/6-311G** level of theory as a robust instrument for the prediction of reactivity and stereoselectivity in the Nazarov electrocyclic reaction.
机译：由二烯酮或烷氧基三烯的质子化产生的戊二烯基阳离子的纳扎罗夫反应已通过实验和DFT计算进行了详细研究。尤其是，在B3LYP / 6-311G **理论水平上的计算可准确预测并解释布朗斯台德酸催化4个π电子系统电环化的结果，该过程中涉及到一个双键之一在N和S杂环上计算表明，两个杂原子都能够通过稳定过渡状态下在C-6（C-2​​）处形成的部分正电荷来加速闭环，其中S杂环衍生物比相应的含N化合物更具反应性。通常，期望通过烷氧基三烯的质子化产生的戊二烯基阳离子比通过相应的二烯酮的质子化获得的戊二烯基阳离子反应更快，因为后者通过氢键稳定。仅在N-杂环衍生物的情况下，杂环上取代基的存在会显着影响立体选择性（torquoselectivity），其中2-烷基是轴向取向的，仅提供顺式-2,5-二取代的异构体。取而代之的是，用取代的S-杂环化合物，预期的对甲苯二酚的选择性非常低，实际上，在闭环后，通过实验发现反式和顺式产物之间的3：1非对映异构体混合物。对于本研究，实现了适当的含N和S的二烯酮和烷氧基三烯的合成，以评估DFT计算的预测能力，在反应性和立体选择性方面，在所有检查的情况下都非常好。因此，在计算结果和实验结果之间观察到的一致性表明，DFT计算在B3LYP / 6-311G **理论水平上是有用的，可作为预测Nazarov电环反应中反应性和立体选择性的可靠工具。

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