Stable Axial Chirality in Metal Complexes Bearing 4,4′-Substituted BIPHEPs: Application to Catalytic Asymmetric Carbon–Carbon Bond-Forming Reactions
作者:Kohsuke Aikawa、Yoshitaka Miyazaki、Koichi Mikami
DOI:10.1246/bcsj.20110309
日期:2012.2.15
Not only electronic but also steric effects of 4,4′-substituents in BIPHEP derivatives and metal (Pd, Pt, and Au) complexes are shown to influence the stability of the biphenyl single bond rotation. While electron-donating or sterically demanding substituents on the 4,4′-positions destabilize the axial chirality of BIPHEP derivatives, electron-withdrawing or sterically less demanding ones on the 4,4′-positions stabilize the axis chirality. Particularly, the axial chirality of palladium dichloride complexes bearing BIPHEP with t-Bu and CF3 substituents on the 4,4′-positions is most labile and stable, respectively (ΔG≠ = 29.22 and 30.49 kcal mol−1 at 300 K; t1/2 = 7 and 56 years at 300 K). These enantiopure dicationic BIPHEP–Pd complexes can be employed for catalytic enantioselective arylation, alkenylation, and ene reactions to give the corresponding products in good-to-excellent yields and enantioselectivities. Significantly, in the carbonyl-ene reaction of trifluoropyruvate with isobutene, the turnover frequency (TOF) reached 58200 h−1. The remarkable effects of 4,4′-substituents in BIPHEP derivatives can be employed as a guiding principle in the design of versatile and efficient ligands.
不仅是电子效应,4,4′-取代基在BIPHEP衍生物及金属(Pd、Pt和Au)配合物中的空间效应也被证明会影响联苯单键旋转的稳定性。在4,4′-位置的电子供体或空间占位需求较大的取代基会使BIPHEP衍生物的轴向手性不稳定,而电子吸引或空间占位需求较小的取代基则会稳定轴向手性。特别是,带有t-Bu和CF3取代基的钯二氯化物配合物的轴向手性表现出最大的易变性和稳定性(在300 K时,ΔG≠ = 29.22和30.49 kcal mol−1;t1/2 = 7年和56年)。这些对映体纯的二阳离子BIPHEP–Pd配合物可用于催化对映选择性的芳烃化、烯烃化和烯反应,以良好至优异的产率和对映选择性生成相应的产品。重要的是,在三氟丙酮酸与异丁烯的羰基-烯反应中,周转频率(TOF)达到了58200 h−1。4,4′-取代基在BIPHEP衍生物中的显著影响可作为设计多用途高效配体的指导原则。