Asymmetric catalysis based on tropos ligands

Abstract

All enantiopure atropisomeric (atropos) ligands essentially require enantiomeric resolution or synthetic transformation from a chiral pool. In sharp contrast, the use of tropos (chirally flexible) ligands, which are highly modular, versatile, and easy to synthesize without enantiomeric resolution, has recently been the topic of much interest in asymmetric catalysis. Racemic catalysts bearing tropos ligands can be applied to asymmetric catalysis through enantiomeric discrimination by the addition of a chiral source, which preferentially transforms one catalyst enantiomer into a highly activated catalyst enantiomer. Additionally, racemic catalysts bearing tropos ligands can also be utilized as atropos enantiopure catalysts obtained via the control of chirality by a chiral source followed by the memory of chirality. In this feature article, our results on the asymmetric catalysis via the combination of various central metals and tropos ligands are summarized.

Design and synthesis of a stereodynamic catalyst with reversal of selectivity by enantioselective self‐inhibition

“…A self‐amplifying reaction also necessitates occurrence of nonlinear effects, which can lead to a significant growth of the enantiomeric excess as demonstrated first by Kagan and Noyori . Along similar lines, Mikami and coworkers applied concepts of stereoselective activation and deactivation by combining racemic ligands with chiral coligands to induce enantioselectivity in various transformations …”

“…[3][4][5] Along similar lines, Mikami and coworkers applied concepts of stereoselective activation and deactivation by combining racemic ligands with chiral coligands to induce enantioselectivity in various transformations. [6][7][8] Even before the first experimental realization of stereoamplification, Frank described a "mutual antagonism," where the desired reaction is assisted by the inducing product and the undesired reaction is simultaneously suppressed in order to prevent a decrease in selectivity, as a fundamental requirement. 9 The first experimental examples that combine autoinduction and amplification were delivered by Wynberg and Alberts and later by Wynberg and Feringa, who observed autoinductive properties during C-C bond formation.…”

Supramolecular chirality transfer in a stereodynamic catalysts

“…[10][11][12][13] So far, the stereochemistry of these ligands has been controlled via hydrogen bonding, 14 hydrogen bonding and formation of supramolecular structures, [15][16][17] coordination of additional chiral coligands, 18 chiral counter-ions, 19,20 and chiral ionic liquids. [10][11][12][13] So far, the stereochemistry of these ligands has been controlled via hydrogen bonding, 14 hydrogen bonding and formation of supramolecular structures, [15][16][17] coordination of additional chiral coligands, 18 chiral counter-ions, 19,20 and chiral ionic liquids.…”

Rotational Barriers of Substituted BIPHEP Ligands: A Comparative Experimental and Theoretical Study

“…[1] The BIPHEP ligand 2,2′-bis(diphenylphosphino)-1,1′-biphenyl (1a) is of particular interest because of its established use in dynamic asymmetric activation in combination with chiral additives. [2,3] Schlosser et al first determined the rotational barrier, ΔG ‡ , of partially deuterated 1 to be 22 ± 1 kcal mol -1 (92 ± 4 kJ mol -1 ) by dynamic 1 H NMR spectroscopic analysis. [4,5] During that time, enantioselective dynamic high performance liquid chromatography (DHPLC) methods [6][7][8] were introduced to systematically determine the thermodynamic properties, ΔG ‡ , ΔH ‡ , and ΔS ‡ of the interconversion barriers of tropos ligands.…”