Denmark Scott E, Lee Wheeseong
Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA.
Chem Asian J. 2008 Feb 1;3(2):327-41. doi: 10.1002/asia.200700300.
A model compound was designed to study the relative orientation of enol silane and carbonyl moieties in the Mukaiyama aldol reaction. The cyclization must proceed with either a synclinal or an antiperiplanar orientation of the aldehyde with respect to the enol silane. These two orientations lead to diastereomeric products, thus allowing for unambiguous correlation between product configuration and transition-state geometry. As steric bias is minimal, the product distribution should reflect the intrinsic preferences for reactant geometry in the transition state. Cyclizations of the model compound showed a modest preference for reaction via an antiperiplanar (open transition state) orientation of reactants in the presence of a wide range of Lewis acids (TiCl4, SnCl4, SnCl2, BF3OEt2, TMSBr, trityl perchlorate, EtAlCl2) and triflic acid. The cyclizations promoted by tin(II) salts were syn-selective and dependent on the nature of the counterion. Fluoride ion promoted reactions were anti-selective and were independent of the nature of the cation.
设计了一种模型化合物，用于研究在木山羟醛反应中烯醇硅烷和羰基部分的相对取向。环化反应必须以醛相对于烯醇硅烷的顺叠或反叠取向进行。这两种取向会导致非对映体产物，从而能够明确关联产物构型和过渡态几何结构。由于空间位阻偏向最小，产物分布应反映过渡态中反应物几何结构的内在偏好。在多种路易斯酸（四氯化钛、四氯化锡、二氯化锡、三氟化硼乙醚、三甲基溴硅烷、三苯基高氯酸盐、二氯乙基铝）和三氟甲磺酸存在下，模型化合物的环化反应显示出适度倾向于通过反应物的反叠（开放过渡态）取向进行反应。由锡（II）盐促进的环化反应具有顺式选择性，且取决于抗衡离子的性质。氟离子促进的反应具有反式选择性，且与阳离子的性质无关。
