Asymmetric Thermal Transformation, a New Way to Enantiopure Biphenyl-Bridged Titanocene and Zirconocene Complexes:  Efficient Catalysts for Asymmetric Imine Hydrogenation¹

Abstract

Enantiopure biphenyl-bridged titanocene and zirconocene complexes were obtained, by an asymmetric thermal transformation of the binaphthol complexes formed from the metallocene racemates and subsequent transformation to the corresponding dichlorides, in practically quantitative yields. Increased rates of this transformation in the presence of O2 gas or TEMPO indicate a radical reaction mechanism. The biphenyl-bridged titanocene enantiomers give rise to an efficient asymmetric catalysis for the hydrogenation of cyclic and noncyclic imines.

Asymmetric Hydrogenation of Imines and Olefins Using Phosphine‐Oxazoline Iridium Complexes as Catalysts

“…In recent years a variety of chiral metal catalysts have been successfully applied to this reaction. The chiral titanocene catalysts developed by Buchwald et al,5a,b and their Zr analogues,5c were found to be particularly effective in the hydrogenation of cyclic imines. While Ru6, 7 and Rh8 complexes have been successfully employed in imine hydrogenations6, 8a–e, g and in asymmetric transfer hydrogenations,7, 8f they have also been found to be effective in the reduction of cyclic and acyclic imines, sulfonimines,6a, 7c and the direct reductive amination of ketones 8c,g.…”

“…However, most of these procedures often require high catalyst loadings, elevated pressures, and long reaction times to obtain the desired amines in high yields and with high ee 's. Although several useful methods have been described for the hydrogenation of cyclic imines,5c, 7b, 9h, 14 the enantioselective hydrogenation of acyclic imines is more difficult to achieve 1. The process is also sometimes further complicated by the interconversion between E and Z isomers of an acyclic imine in solution 5a,b…”

Ligand distortion modes leading to increased chirality content of Katsuki‐Jacobsen catalysts

“…We are not the first to assert this claim; several research groups were cognizant of or invoked ligand distortions as being responsible for stereoinduction in Jacobsen-Katsuki-type catalysts, 11,[13][14][15][16][17] bisoxazolinyl-type catalysts, [18][19][20] porphyrins, 21,22 biaryls, [23][24][25] and there are examples in the literature where flexible ligands can be locked into an asymmetric shape so that good ee's are noted. [26][27][28][29][30] The difference between these studies and ours is that we can quantify how much chirality is associated with each of these distortions. In this article we report on several distortion modes available to this ligand and demonstrate that one or more such modes can significantly increase the chirality content of this catalyst.…”

Understanding Stereoinductionin Catalysis via Computer: New Tools for Asymmetric Synthesis

“…We are not the first to assert this claim; several research groups were cognizant of or invoked ligand distortions as being responsible for stereoinduction in Jacobsen-Katsuki-type catalysts, 48 [62][63][64][65][66] The difference between those studies and the computational assessment described here is that we can quantify how much chirality is associated with each of these distortions.…”