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首页> 外文期刊>Chemistry: A European journal >Regio- and Stereoselectivity in the Decatungstate Photocatalyzed Alkylation of Alkenes by Alkylcyclohexanes
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Regio- and Stereoselectivity in the Decatungstate Photocatalyzed Alkylation of Alkenes by Alkylcyclohexanes

机译：十环态烷基环己烷光催化烯烃的烷基化中的区域和立体选择性

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摘要

Tetrabutylammonium decatungstate (WO) photocatalyzes radical alkylation reactions starting directlyfrom alkanes. When using tert-butylcyclohexane and methylcyclohexane asthe radical precursors, the addition to electrophilic alkenes (β,β-dialkylmethylenemalononitriles, acrylonitrile, methyl acrylate, methyl vinyl ketone) is regioselective and exclusively gives 3- and 4-substituted cyclohexyl adducts, with no significant functionalization of the other positions. Furthermore, when a β,β-disubstituted alkene is used, the reaction is stereoselective (cis stereochemistry for the 1,3-cyclohexane derivatives and trans for the 1,4 isomers). Some of the reactions have also been carried out by using benzophenone as the photocatalyst, giving the same product distribution. However, the decatungstate anion is a superior catalyst from a preparative point of view, because it is efficient at low concentrations (0.002m, 2 mol%) and allows for a simple work up. From a mechanistic point of view, the role of both the alkyl radicals and the radical adducts has been assessed by trapping experiments in the presence of suitable additives (α-phenyl- N-tert-butylnitrone, PBN, and 2- methyl-2-nitrosopropane, MNP) and by EPR spectroscopic detection of the resulting nitroxides in solution. Furthermore,trapping by the nitroxide TEMPO (TEMPO=2,2,6,6-tetramethylpiperidine N-oxide) gives O-(tertbutylcyclohexyl) hydroxylamines, again as only the 3- and the 4-substituted isomers. We conclude that the observed regioselective activation originates from the initial hydrogen abstraction step (the statistically corrected ratio for positions 3 and 4 ranges from 1.1 to 1.35 for all of the trapping products). The selectivity is mainly due to steric hinderence.
机译：四丁基癸酸铵（WO）光催化直接从烷烃开始的自由基烷基化反应。当使用叔丁基环己烷和甲基环己烷作为自由基前体时，向亲电烯烃（β，β-二烷基亚甲基丙二腈，丙烯腈，丙烯酸甲酯，甲基乙烯基酮）的加成是区域选择性的，并且仅产生3-和4-取代的环己基加合物，而没有明显的官能化其他职位。此外，当使用β，β-二取代的烯烃时，反应是立体选择性的（对于1，3-环己烷衍生物为顺式立体化学，对于1，4异构体为反式）。通过使用二苯甲酮作为光催化剂还进行了一些反应，得到相同的产物分布。然而，从制备的观点来看，十碳酸盐阳离子是优异的催化剂，因为它在低浓度（0.002m，2 mol％）时是有效的，并且易于后处理。从机理的观点来看，烷基自由基和自由基加合物的作用已通过在合适的添加剂（α-苯基-N-叔丁基硝基，PBN和2-甲基-2-亚硝基丙烷（MNP）并通过EPR光谱法检测溶液中生成的氮氧化物。此外，通过硝基氧化物TEMPO（TEMPO ＝ 2,2,6,6-四甲基哌啶N-氧化物）的捕获，再次形成O-（叔丁基环己基）羟胺，仅是3-和4-取代的异构体。我们得出的结论是，观察到的区域选择性活化源自最初的氢提取步骤（位置3和4的统计校正比率，对于所有捕集产物，其范围从1.1到1.35）。选择性主要是由于位阻。

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