Lewis acid catalysts stable in water. Correlation between catalytic activity in water and hydrolysis constants and exchange rate constants for substitution of inner-sphere water ligands
Lewis acid catalysts stable in water. Correlation between catalytic activity in water and hydrolysis constants and exchange rate constants for substitution of inner-sphere water ligands
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DOI：
10.1021/ja980715q
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发表时间：
1998-08-19
影响因子：
15
通讯作者：
Busujima, T
中科院分区：
文献类型：
--
作者：
Kobayashi, S;Nagayama, S;Busujima, T
当今的环境问题需要不使用有害有机溶剂的清洁反应过程。1水无疑是最环保的溶剂;然而，它在有机反应过程中的使用相当有限，因为许多有机材料不溶于水，因此在大多数情况下反应进行缓慢。2此外，许多活性中间体和催化剂被水分解。这是刘易斯酸催化的反应的情况，由于它们可以实现的独特反应性和选择性以及所使用的温和条件，这些反应受到了极大的关注。3刘易斯酸被认为在水中不稳定，因此在水溶液中不可用。另一方面，我们最近发现了水稳定的刘易斯酸，镧系元素三氟甲烷磺酸盐（镧系元素三氟甲磺酸盐），其可用于在水介质中的几种碳-碳键形成反应。4镧系三氟甲磺酸盐在水中的稳定性和催化活性归因于它们大的离子半径和刘易斯酸与水之间的平衡。我们现在已经阐明，镧系元素以外的某些金属盐在水中也是稳定的刘易斯酸，并可用作催化剂。5此外，共同的特点，一定范围内的水解常数，内球水配体取代的交换速率常数较高本文以苯甲醛与（Z）-1-苯基-1-甲基苯为模型反应，筛选了1-15族金属的氯化物，（三甲基甲硅烷氧基）丙烯（Mukaiyama羟醛缩合反应）（表1）。6该反应适合于测试金属氯化物作为刘易斯酸催化剂在水介质中的催化能力，因为甲硅烷基烯醇醚是水敏感的（特别是在酸性条件下），并且如果刘易斯酸在水中水解，烯醇醚迅速分解，并且所需的反应不再进行。在第一次放映中，
Today’s environmental concerns demand clean reaction processes that do not use harmful organic solvents. 1 Water is no doubt the most environmentally friendly solvent; however, its use in organic reaction processes is rather limited because many organic materials do not dissolve in water, and therefore in most cases reactions proceed sluggishly. 2 In addition, many reactive intermediates and catalysts are decomposed by water. This is the case for Lewis acid catalyzed reactions, which are of great current interest because of the unique reactivities and selectivities they can achieve and for the mild conditions used. 3 Lewis acids have been believed to be unstable in water and therefore unusable in aqueous solution. On the other hand, we have recently found water-stable Lewis acids, lanthanide trifluoromethanesulfonates (lanthanide triflates), which can be used in several carbon-carbon bond-forming reactions in aqueous media. 4 The stability and catalytic activity of lanthanide triflates in water were ascribed to their large ionic radii and an equilibrium between the Lewis acids and water. We have now clarified that some metal salts other than lanthanides are also stable Lewis acids in water and work as catalysts. 5 In addition, common characteristics, a certain range of hydrolysis constants, and a high order of exchange rate constants for substitution of inner-sphere water ligands (water exchange rate constant (WERC)) have been found among these water-stable Lewis acids.We screened group 1-15 metal chlorides in a model reaction of benzaldehyde with (Z)-1-phenyl-1-(trimethylsiloxy) propene (the Mukaiyama aldol reaction)(Table 1). 6 The reaction is suitable for testing catalytic ability of the metal chlorides as Lewis acid catalysts in aqueous media, because the silyl enol ether is watersensitive (especially under acidic conditions) and if the Lewis acids hydrolyze in water, the enol ether decomposes rapidly and the desired reaction proceeds no further. In the first screening,