Catalytic hydrogenation enantioselective

The similarities of above experimental results inspired us to investigate the role of SE in heterogeneous catalytic enantioselective hydrogenation reactions. In heterogeneous catalytic reaction the SE means that a given template molecule interacts with the prochiral substrate in the liquid phase in such a way that one of the prochiral sites is preferentially shielded. If the substrate is shielded then its adsorption onto the metal can take place with its unshielded site resulting in ED. 

Norton and coworkers found that catalytic enantioselective hydrogenation of the C=N bond of iminium cations can be accomplished using a series of Ru complexes with chiral diphosphine ligands such as Chiraphos and Norphos [68], Even tetra-alkyl-substituted iminium cations can be hydrogenated by this method. These reactions were carried out with 2 mol.% Ru catalyst and 3.4—3.8 bar H2 at room temperature in CH2C12 solvent (Eq. (39)). 

Enantiomerically pure amines are extremely important building blocks for biologically active molecules, and whilst numerous methods are available for their preparation, the catalytic enantioselective hydrogenation of a C=N bond potentially offers a cheap and industrially viable process. The multi-ton synthesis of (S)-metolachlor fully demonstrates this [108]. Although phospholane-based ligands have not proven to be the ligands of choice for this substrate class, several examples of their effective use have been reported. 

Related catalytic enantioselective processes It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [27] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [28]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect the resolution of five- and six-membered allylic carbinols [29] in this instance, as with the Ti-catalyzed procedure, the presence of an unprotected hydroxyl function is required. Perhaps the most efficient general procedure for the enantioselective synthesis of this class of cyclic allylic ethers is that recently developed by Trost and co-workers, involving Pd-catalyzed asymmetric additions of alkoxides to allylic esters [30]. 

Related catalytic enantioselective processes Although great progress has been achieved in the area of metal-catalyzed hydrogenation reactions [124], examples of catalytic asymmetric hydrogenations of tetrasubstituted alkenes are rare. One other example, reported by Pfaltz and co-workers, is depicted in Eq. 6.26 (81 % ee, absolute stereochemistry of the product not determined) [125],... 

Recently, we established that several proton acids catalyze the metal-free reduction of ketimines under hydrogen-transfer conditions with Hantzsch dihydropyridine as the hydrogen source.Additionally, we were able to demonstrate a catalytic enantioselective procedure of this new transformation by employing a chiral Br0nsted acid as catalyst.(see Chapter 4.1). 

Table 9.7 Catalytic enantioselective oxidation of aryl alkyl sulfides 2 with hydrogen peroxide in water-SDS solution mediated by 1...

As the follow up to our studies in connection to the development of Ti-cat-alyzed cyanide additions to meso epoxides [4], we developed the corresponding catalytic enantioselective additions to imines [5]. A representative example is shown in Scheme 1 chiral non-racemic products maybe readily converted to the derived cx-amino acids (not available through catalytic asymmetric hydrogenation methods). In these studies, we further developed and utilized the positional optimization approach effected by examination of parallel libraries of amino acid-based chiral ligands (e.g., 1 and 2). Thus, the facile modularity of these ligands and their ease of synthesis were further exploited towards the development of a new catalytic enantioselective method that delivers various ar-... 

Another fact is the quite impressive functional group selectivity of this method. Because of their greater reactivity imines can be reduced in the presence of ketones, although chiral Ru-complex 9 catalyzes the transfer hydrogenation of ketones. Besides this catalytic enantioselective reduction of imines others are known.8... 

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