Stereochemically Dynamic 2,2‘-Biphosphole Ligands for Asymmetric Catalysis

Abstract

The stereochemically dynamic 2,2′-biphospholes 8, 9, and 10 have been synthesized by asymmetric alkylation of 2,2′-biphospholyl anion with enantiomerically pure diol ditosylates. A good carbon-to-phosphorus induction has been obtained in particular in the case of compound 8. The introduction of a chiral linker between the two phosphorus atoms resulted in partial chirality control of the central and axial chiralities of the 2,2′-biphosphole framework, as three diastereoisomers a, b, and c are obtained among the six expected. In solution at room temperature a, b, and c diastereoisomers exist as an equilibrium mixture; the interconversion occurred by phosphorus-inversion inducing atropoinversion. Reaction of the equilibrium mixture of 2,2′-biphosphole with [MCl 2 (CH 3 CN) 2 ] resulted in dynamic resolution leading to diastereo-and enantiopure Pd-and Pt-complexes. Palladium-catalyzed asymmetric allylic substitution can be achieved with the ligands 8 and 10 with high activities but still moderate enantioselectivities.

Chiral diphosphines and diphosphinites derived from 2,2-biphosphole: Stereodynamic ligands for asymmetric catalysis

“…4 In phospholes, the inversion barrier for the pyramidal phosphorus is reduced, relative to that in phosphines, as a result of the increase in aromatic character of the phosphole in the transition state. The activation barrier to phosphorus inversion in 2,2'-biphosphole is measured to be only 16.5 kcal/mole leading to phosphorus inversion at À60 8C [8,10]. 5 The refinement of the Flack's parameter clearly indicates that these compounds are enantiomerically pure in solid state.…”

“…(5). 10 The broadness of the line can be explain by a short T2 relaxation associated to an aggregation phenomena in solution as no dynamic effect have been observed by variable temperature experiment.…”

“…Some years ago, we reported the first application of the chiral stereochemically dynamic 2,2'-biphophole ligand (BIPHOS) to asymmetric allylic substitution involving crystallization-induced spontaneous resolution and kinetic stabilization by coordination to Pd centre 2 [9]. In a more convenient procedure, we have discovered that a dual chirality control can be achieved by introducing a chiral linker between the two phosphorus atoms that favours a single enantiomeric form upon coordination to a metal [10]. In this article, we report our approach to stereodynamic diphosphines and diphosphinites derived from 2'2-biphospholes for asymmetric catalysis.…”

Recent Advances of [1,5]-Sigmatropic Shift of Phospholes

“…[24] . 参照该动态拆分的方法, 一系列 Pd [25] , Pt 等手性配 合物被合成出来并应用于烯丙基化(Pd) [22,26] 、烯烃脂基 化(Pd) [27] 和氢甲酰化(Pt) [28] 等不对称催化反应中. 整体 来说, 以手性诱导和动态拆分获得的联磷杂环戊二烯配 合物在位置选择性和化学选择性方面表现出较为优异 的性能, 但在立体选择性方面的表现还不是非常理想.…”

New enantiopure palladium(II) complexes from a stereodynamic 2,2′-biphosphole ligand

“…By asymmetric alkylation of a 2,2 H -biphospholyl dianion, (2), under highly dilute conditions, using various enantiomerically pure diol ditosylates or mesylates, an equilibrium mixture of diastereoisomeric diphosphines was obtained (Robe  et al, 2005). The reaction of this equilibrium mixture with transition metals such as Pd, Pt and Rh resulted in dynamic resolution leading to diastereo-and enantiopure complexes.…”