Rhodium catalytic compounds asymmetric reactions

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

These catalytic reactions of dihydrosilanes make possible the use of asymmetric catalysts to produce chiral silicon compounds. Introduction of a chiral ligand L on the rhodium complex will not change the validity of the kinetic Scheme 12. However, in this case complexes 56 and 57 will be diastereomeric and their equilibrium concentrations will be different. The ratio of the substituted silanes will be close to k, [56] k2 [57]. 

Asymmetric Transfer Hydrogenation of Ketones. The first reports on asymmetric transfer hydrogenation (ATH) reactions catalyzed by chiral metallic compounds were published at the end of the seventies. Prochiral ketones were reduced using alcohols as the hydrogen source, and Ru (274,275) or Ir (276) complexes were used as catalysts. Since then, many chiral catalytic systems for homogeneous ATH of ketones, imines, and olefins have been developed (37,38,256,257,277-289). The catalytic systems are usually based on ruthenium, rhodium, or iridium, and the ATH of aryl ketones is by far the most studied. Because of the reversibility of this reaction, at high conversions, a gradual erosion of the ee of the product has been frequently reported. An azeotropic 5 2 mixture of formic acid/triethylamine can be used to overcome this limitation. 

First, for reasons of clarity, the currently-accepted mechanism of transition-metal complex catalyzed-hydrosilylation reactions will be described briefly. Furthermore, consideration of selective, if not asymmetric, reduction of certain carbonyl compounds by way of rhodium(I)-catalyzed hydrosilylation (Section 4) is included in this review because the catalytic process and stereochemical course of this reaction correlate closely with those of their asymmetric reduction under similar conditions that will be described in the succeeding section. 

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