THEORETICAL-ANALYSIS OF SELECTIVITY IN THE CATION-RADICAL DIELS-ALDER
THEORETICAL-ANALYSIS OF SELECTIVITY IN THE CATION-RADICAL DIELS-ALDER
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DOI：
10.1021/ja00349a038
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发表时间：
1983-01-01
影响因子：
15
通讯作者：
GARDNER, SA
中科院分区：
文献类型：
--
作者：
BELLVILLE, DJ;BAULD, NL;GARDNER, SA
用电荷密度分析和PMO理论从理论上解释了阳离子-自由基Diels-Alder反应的化学选择性和区域选择性。双烯阳离子自由基作为Diels-Alder亲双烯分子，往往在正电荷密度最高的位置反应，但空间效应和产物性质的发展也是重要的因素。中性二烯中负电荷密度最高的位置倾向于与亲双烯阳离子自由基中的最大活性中心结合。还发展了一个适用于阳离子自由基周环反应的PMO理论，并成功地应用于预测这些相同的选择性元素。用PMO理论解释了在某些阳离子-自由基Diels-Alder反应中观察到的极高的内切立体选择性。通过电子转移将中性或富电子的双烯基团转变为高度缺电子的阳离子自由基来增强其亲双烯性的策略已被证明是非常成功的。1·2阳离子自由基催化的Diels-Alder反应具有极高的反应速率、表面立体选择性和令人印象深刻的立体选择性，在某些情况下接近排他性。共轭双烯、苯乙烯和富含电子的烯烃等，当被三芳胺盐或光敏电子转移转化为阳离子自由基时，可以作为强有力的双烯亲和剂发挥作用。根据轨道关联图对该反应的立体专一性进行了合理化，表明该反应是可行的。此外，环加成是角色选择性的，允许将电离化的双烯基加成到中性二烯组分，但禁止将中性双烯加成到电离的二烯组分。从理论上解释了双烯亲电离对Diels-Alder反应速率的深刻影响，并将其推广到其他类型的阳离子自由基周环反应。3本论文的重点是阳离子自由基Diels-Alder的特殊选择性轮廓，这一点以前只被简单地评论过。2所给出的理论分析为阳离子自由基Diels-Alder的定量化和预测观察到的内向/外向、区域和化学选择模式提供了必要的基础。
Chemo-and regioselectivities in the cation-radical Diels-Alder reaction are interpreted theoretically using a charge-denisty analysis and also PMO theory. Diene cation radicals acting as Diels-Alder dienophiles tend to react at the position of highest positive charge density, but steric effects and product character development are also significant factors. The position of highest negative charge density in the neutral diene tends to combine with themost reactive site in the dienophile cation radical. A PMO theory appropriate to cation-radical pericyclic reactions is also developed andsuccessfully applied to the predictionof these same selectivity elements. The extremely high endo stereoselection observed in some cation-radical Diels-Alder reactions is interpreted, via PMO theory, in terms of secondary interactions.The tactic of enhancing the dienophilicityof neutral or elec-tron-rich dienophilesby converting them, via electron transfer, to highly electron-deficient cation radicals has proved enormously successful. 1· 2 The resulting cation-radical-catalyzed Diels-Alder reaction is characterized by exceptionally high reaction rates, suprafacial stereospecificity, and an impressive arrayof stereo-selectivities which, in some instances, approaches exclusivity. Conjugated dienes, styrenes, and electron-rich alkenes, among others, can function as potent dienophiles when converted to cation radicals by triarylaminium salts or by photosensitized electron transfer. The stereospecificity of the reaction has been rationalized on the basis of orbital correlation diagrams, which reveal it to be allowed. Moreover, the cycloaddition is role selective, being allowed for the addition of an ionized dienophile to a neutral diene component, but forbidden for the addition of a neutral dienophile to an ionized diene component. The profound effect of dienophile ionization on the Diels-Alder reaction rate has been interpreted theoretically and extended to other types of cation-radical pericyclic reactions. 3 The focus of the present paper is the exceptional selectivity profileof the cation-radicalDiels-Alder, which has previously been commented on only rather briefly. 2 The theoretical analysis to be presented provides the necessary basis for ration-alizing and predicting the endo/exo-, regio-, and chemoselection patterns observed for the cation-radicalDiels-Alder.