Asymmetric transfer hydrogenation catalyzed, metal-ligand

Asymmetric transfer hydrogenation of imines catalyzed by chiral arene-Ru complexes achieves high enantioselectivity (Figure 1.34). Formic acid in aprotic dipolar solvent should be used as a hydride source. The reaction proceeds through the metal-ligand bifunctional mechanism as shown in the carbonyl reduction (Figure 1.24). 

In this HAM protocol, ammonia was generated by the hydrolysis of urea in an acidic medium. Final cleavage of the oxazolidinone units produces the desired polyamino alcohols, which can be utilized as ligands in metal-catalyzed asymmetric transfer hydrogenations. 

This is mainly due to the fact that by means of chiral ligands it is comparatively facile to transfer absolute stereochemical information to a cat-alytically active metal center. However, the success of some of these reactions (e.g. the Sharpless asymmetric epoxidation or the Noyori hydrogenation) must not hide the fact that the number of powerful transition metal-catalyzed C-C coupling reactions, which proceed reliably with high enantioselectivity, is still rather small. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

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