Corey E J, Shibata Takanori, Lee Thomas W
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
J Am Chem Soc. 2002 Apr 17;124(15):3808-9. doi: 10.1021/ja025848x.
This paper reports a new method for the generation of chiral Lewis superacids by protonation of a non-Lewis acidic oxazaborolidine (1) with triflic acid. The resulting cationic species (3) are powerful and highly enantioselective catalysts for the Diels-Alder reaction of various 1,3-dienes with alpha,beta-enals. An optimization study (see Table 1) led to the selection of reaction conditions and catalysts (6A and 6B) which are very effective. The reactions are simple to conduct, reproducible, and economical, since only ca. 6 mol % of catalyst is required. In addition, the chiral catalyst precursor is readily recovered for reuse (>95% efficiency) and is commercially available. The broad scope of the process is documented by the 14 examples listed in Table 2. The absolute stereochemical course of the Diels-Alder reactions catalyzed by 6A and 6B was successfully predicted on the basis of the mechanistic principles which have recently been formulated for this type of catalytic enantioselective reaction involving re-face attack by the diene on complex 7. The mode of generation of Lewis superacids 6A and 6B allows an approximate comparison (or scale) connecting the catalytic power Lewis and protic acids.
本文报道了一种通过用三氟甲磺酸质子化非路易斯酸性恶唑硼烷（1）来生成手性路易斯超强酸的新方法。所得的阳离子物种（3）是用于各种1,3 - 二烯与α,β - 烯醛的狄尔斯 - 阿尔德反应的强大且高度对映选择性的催化剂。一项优化研究（见表1）导致选择了非常有效的反应条件和催化剂（6A和6B）。这些反应易于进行、可重复且经济，因为仅需要约6摩尔％的催化剂。此外，手性催化剂前体易于回收再利用（效率> 95％）且可商购获得。表2中列出的14个实例证明了该方法的广泛适用范围。基于最近针对这种涉及二烯对配合物7进行再面进攻的催化对映选择性反应所制定的机理原理，成功预测了由6A和6B催化的狄尔斯 - 阿尔德反应的绝对立体化学过程。路易斯超强酸6A和6B的生成方式允许对路易斯酸和质子酸的催化能力进行近似比较（或标度）。
