A diastereomerically mixed complex [Ru(S)-phgly}2(±)-biphep}] is readily prepared from achiral diphosphine BIPHEP in two steps. These diastereomers are then separated by silica gel column chromatography. A 61:39 equilibrium mixture of [Ru(S)-phgly}2(S)-biphep}] and [Ru(S)-phgly}2(R)-biphep}] with Li2CO3 is used to catalyze cyanosilylation of benzaldehyde to afford the R cyanated product in 92% ee. The enantioselectivity is just slightly lower than that by using the pure [Ru(S)-phgly}2(S)-biphep}]/Li2CO3 catalyst system of 96%. The high enantioselective ability of the diastereomerically mixed catalyst is revealed through a series of kinetic experiments in which the highly enantioselective [Ru(S)-phgly}2(S)-biphep}]/Li2CO3 system is shown to catalyze the reaction 16.8 times faster than the less selective [Ru(S)-phgly}2(R)-biphep}]/Li2CO3 system, affording the product in 2.6% ee. An equation is derived to approximate the relationship between the diastereomeric ratio of the catalyst and the ee value of the product.
用非手性
二膦 BIPHEP 分两步即可制备出非对映混合复合物 [Ru(S)-phgly}2(±)-biphep}]。然后通过
硅胶柱色谱法分离这些非对映异构体。[Ru(S)-phgly}2(S)-biphep}]和[Ru(S)-phgly}2(R)-biphep}]与 Li2CO3 的 61:39 平衡混合物被用来催化
苯甲醛的
氰化反应,以 92% 的ee 得到 R
氰化产物。其对映体选择性略低于使用纯[Ru(S)-phgly}2(S)-联
菲}]/Li2CO3 催化剂体系的 96%。在一系列动力学实验中,高对映体选择性的 [Ru(S)-phgly}2(S)-biphep}]/Li2CO3 体系催化反应的速度是选择性较低的 [Ru(S)-phgly}2(R)-biphep}]/Li2CO3 体系的 16.8 倍,产物的ee值为 2.6%。得出了催化剂非对映比率与产物ee值之间的近似关系式。