Matching and Mismatching Effects of Hybrid Chiral Biaxial Bisphosphine Ligands in Enantioselective Hydrogenation of Ketoesters

Abstract

Catalytic asymmetric synthesis is a significant component in modern organic chemistry. It has widely been used to produce a number of optically active compounds, from agrochemicals, to pharmaceuticals, to flavors, and fragrances as well as functional materials.[1] One of the most important methods to increase the stereoselectivity of reactions is multiple stereoselectivity (multiple stereodifferentiation, multiple asymmetric induction), when the stereochemical process proceeds under the control of more than one chiral auxiliary.[2] During the last two decades, these double stereoselective strategies have been successfully applied in a variety of reactions, such as Sharpless dihydroxylation, Michael additions, addition to allyl metals, the Reformatsky reaction, the Mukaiyama reaction, photochemical reactions, alkylation, cycloadditions and the synthesis of heteroatom compounds.[2] In contrast, catalytic systems with multiple stereogenic axial elements in chiral ligands are much less explored in transition metal catalyzed asymmetric hydrogenations. The multiple chiral elements in ligands may be situated favorably ("matched") or unfavorably ("mismatched") for stereocontrol when the catalyst interacts with a prochiral substrate. Ultimately, the matched cases may lead to dramatically higher asymmetric induction in the products, whereas the mismatched cases often result in significantly lower selectivity in hydrogenation reactions.[3] Syntheses of these molecules have facilitated a preliminary study of matching and mismatching effects, relating backbone chirality and phosphorus-based chirality with the performance of these ligands in asymmetric catalysis. The most notable achievements of catalytic systems with double asymmetric induction exist in hydrogenation chemistry, focusing primarily on rhodium or ruthenium with bidentate phosphine ligands, [4] in particular, modifications of DIOP, [5] BINAP, [6] dialkylphospholane bis(phospholane) ligands, [7] and many others.[8] As reported, configurational changes in ligands brought by introducing additional chirality are generally unpredictable. There are no reliable predictive models to explain the structural subtleties of catalysts with multiple chiral elements that lead to dramatic changes in stereoselectivities. [9] As part of our continued interest in the synthesis and use of new chiral bisphosphine ligands in asymmetric catalysis, we have developed a novel class of conformationally rigid C n -TunePhos (n = 1-6) by introducing a bridge with variable length to link the chiral atropisometic biaryl groups.[10] This family of TunePhos ligands has proven to be highly efficient in a variety of asymmetric reactions.[11] We envision that changes of relatively flexible alkyl linker in the C n -TunePhos family with an aromatic group or a bulky chiral bridge may enhance the rigidity and steric hindrance of the structure. Consequently, unique steric or electronic effects and good chiral discrimination could be expected. Herein we report a convenient strategy for the synthesis of ...

Catalyst Optimisation for Asymmetric Synthesis by Ligand Chirality Element Addition: A Perspective on Stereochemical Cooperativity

“…For the most part these are biaryl‐based compounds containing substituents that prevent rotation about the Ar−Ar single bond. An analysis of matching and mismatching effects was performed by Zhang and co‐workers using ligands L12 b and L12 c containing two atropisomeric biaryl units (Figure ) . These are related to the TunePhos series of ligands introduced by the same group, and for this study ligand L12 a was synthesised for comparison.…”

Organosoluble zirconium phosphonate nanocomposites and their supported chiral ruthenium catalysts: The first example of homogenization of inorganic-supported catalyst in asymmetric hydrogenation

“…The effect of solvents on the catalytic property of ruthenium catalyst 3c played an important role in the asymmetric hydrogenation (Table 3). 38, 39 Low catalytic activities (below 10% conversion and yield for 3a) in aprotic polar solvents such as CH 2 Cl 2 , CHCl 3 , and THF were observed, in which ruthenium catalyst 3c can be dissolved and homogeneous catalysis was realized. In entries 4-11 in Table 3, the highest conversion of ethyl pyruvate (100%), yield (92.6%) and enantioselectivity (76.4% ee) of (S)-ethyl-2hydroxylpropionate in the mixed ethanol-CH 2 Cl 2 (v/v = 1) were Fig.…”