Diphenylprolinol silyl ethers as efficient organocatalysts for the asymmetric Michael reaction of aldehydes and nitroalkenes
Diphenylprolinol silyl ethers as efficient organocatalysts for the asymmetric Michael reaction of aldehydes and nitroalkenes
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DOI：
10.1002/anie.200500599
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Shoji, M
中科院分区：
文献类型：
--
作者：
Hayashi, Y;Gotoh, H;Shoji, M
催化剂结构的细微变化有时可以显著提高催化活性，正如我们最近对α-氨氧基化、串联O-亚硝基-羟醛/迈克尔反应和由高活性的脯氨酸替代物硅氧基-4催化的曼尼希反应(方案1)所发现的那样。[1]在脯氨酸结构中简单地引入硅氧基可以提高催化活性，从而在不影响对映体选择性的情况下减少催化剂的负载量和缩短反应时间，并伴随着底物范围的扩大。这种较高的活性可以归因于4在有机溶剂中的溶解度增加。羟基取代硅氧基可以极大地影响催化活性这一事实促使我们研究了其他催化体系，这些研究将我们引向了二苯基-2-吡咯烷基硅醚2。由Corey和他的同事开发的母体二苯基-2-吡咯烷甲基乙醇(1，二苯基-2-吡咯烷甲基乙醇)已被证明是用于不对称合成的非常有用的配体：B-烷基化的恶唑硼烷是CBS还原的有用催化剂，[2]而由B-芳基-恶唑硼酸和B-芳基恶唑-硼酸制备的有效的不对称Lewis酸催化剂促进了广泛的底物的Diels-Alder反应
A subtle change in catalyst structure can sometimes improve catalytic activity dramatically, as we found in our recent study on α-aminoxylation, tandem O-nitroso aldol/Michael reactions, and Mannich reactions catalyzed by the siloxyproline 4, a highly active proline surrogate (Scheme 1).[1] The simple introduction of a siloxy group into the proline structure leads to an increase in the catalytic activity, thus allowing a decrease in catalyst loading and shorter reaction times, without compromising the enantioselectivity, and is accompanied by broadening of the substrate scope. This higher activity can be attributed to the increased solubility of 4 in organic solvents. The fact that the substitution of a hydroxy group for a siloxy group can dramatically affect catalytic activity prompted us to investigate other catalytic systems, and these investigations led us to the diphenylprolinol silyl ether 2. The parent diphenyl-2-pyrrolidinemethanol (1, diphenylprolinol), a commercially available amino alcohol developed by Corey and co-workers, has proved to be a very useful ligand for asymmetric synthesis: B-alkylated oxazaborodine is a useful catalyst for the CBS reduction,[2] while an effective, asymmetric Lewis acid catalyst prepared from B-aryl oxazaborodine and a Brønsted acid promotes the Diels–Alder reaction of a broad range of substrates with excellent enantioselectiv-