Tuning the Lewis Acidity of Boranes in Frustrated Lewis Pair Chemistry: Implications for the Hydrogenation of Electron-Poor Alkenes
Tuning the Lewis Acidity of Boranes in Frustrated Lewis Pair Chemistry: Implications for the Hydrogenation of Electron-Poor Alkenes
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DOI：
10.1002/chem.201301158
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发表时间：
2013-08-12
影响因子：
4.3
通讯作者：
Alcarazo, Manuel
中科院分区：
文献类型：
--
作者：
Nicasio, Juan A.;Steinberg, Sebastian;Alcarazo, Manuel
对缺电子烯烃无金属还原的路易斯对分析表明，速率决定步骤可能是H-2的异裂裂解形成硼氢化盐，或随后的氢化部分在迈克尔型加成反应后转移到底物。虽然使用强路易斯酸如B(C6F5)(3)有利于第一个过程，但使用弱路易斯酸反而有利于氢化物向烯烃的转移，因为同样的原因，弱路易斯酸可能无法促进先前的H-2分裂。在系统测试了几种不同刘易斯酸度的硼烷（用Childs方法评估）和立体需求后，得出了使用三（2,4,6-三氟苯基）硼烷的最佳情况。该硼烷与1,4-重氮杂环[2.2.2]辛烷（DABCO）的混合物对丙二酸烷基二烯的加氢反应表现出优异的催化活性。事实上，这种转变可以在比我们之前报道的更温和的条件下实现。此外，反应范围可以扩展到其他含酯、砜或硝基官能团的缺电子烯烃作为吸电子取代基。
An analysis of the metal-free reduction of electron deficient olefins by frustrated Lewis pairs indicates that the rate-determining step might be either the heterolytic cleavage of H-2 to form an -onium borohydride salt, or the subsequent transfer of the hydride moiety to the substrate following a Michael-type addition reaction. While the use of strong Lewis acids such as B(C6F5)(3) facilitates the first of these processes, hydride transfer to the olefin should be contrarily favoured by the use of weak Lewis acids which, for this very same reason, might be unable to promote the prior H-2 split. After systematic testing of several boranes of different Lewis acidity (assessed by using the Childs' method) and steric demand, an optimal situation that employs tris(2,4,6-trifluorophenyl)borane was reached. Mixtures of this borane with 1,4-diazabicyclo[2.2.2]octane (DABCO) exhibited excellent catalytic activity for the hydrogenation of alkylidene malonates. In fact, this transformation could be achieved under milder conditions than those we reported previously. Moreover, the reaction scope could be expanded to other electron deficient olefins containing esters, sulfones or nitro functionalities as electron-withdrawing substituents.