Chemoselectivity optically active compounds

Several functionalized trialkyltin hydrides have been prepared and used in organic synthesis. For example, an optically active organotin hydride with binaphthyl as chiral center underwent hydrostannylation with methyl methacrylate leading to a -stannyl ester diastereoselectivity, however, was not sufficient [234]. Although a bowl-shaped organotin hydride with bulky aromatic substituents was prepared, the structurally novel tin hydride resulted in quite high chemoselectivity. When tris(2,6-diphenylbenzyl)tin hydride (TDTH) was used for competitive reduction of carbonyls under the influence of a Lewis acid it was observed that unsaturated carbonyl compounds such as benzaldehyde and a,/ -enones are highly resistant to TDTH reduction (Scheme 12.131) [235]. 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

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