Chiral compounds catalyst controlled stereoselectivity

Dynamic Resolution of Chirally Labile Racemic Compounds. In ordinary kinetic resolution processes, however, the maximum yield of one enantiomer is 50%, and the ee value is affected by the extent of conversion. On the other hand, racemic compounds with a chirally labile stereogenic center may, under certain conditions, be converted to one major stereoisomer, for which the chemical yield may be 100% and the ee independent of conversion. As shown in Scheme 62, asymmetric hydrogenation of 2-substituted 3-oxo carboxylic esters provides the opportunity to produce one stereoisomer among four possible isomers in a diastereoselective and enantioselective manner. To accomplish this ideal second-order stereoselective synthesis, three conditions must be satisfied (1) racemization of the ketonic substrates must be sufficiently fast with respect to hydrogenation, (2) stereochemical control by chiral metal catalysts must be efficient, and (3) the C(2) stereogenic center must clearly differentiate between the syn and anti transition states. Systematic study has revealed that the efficiency of the dynamic kinetic resolution in the BINAP-Ru(H)-catalyzed hydrogenation is markedly influenced by the structures of the substrates and the reaction conditions, including choice of solvents. 

Up to this point, we have considered cases in which there is a single major influence on the stereo- or enantioselectivity of a reaction. We saw examples of reactant control of facial selectivity, such as 1,2- and 1,3-asymmetric induction in carbonyl addition reactions. In the preceeding section, we considered several examples in which the chirality of the catalyst controls the stereochemistry of achiral reagents. Now let us consider cases where there may be two or more independent influences on stereoselectivity, known as double stereodijferentiation For example, if a reaction were to occur between two carbonyl compounds, each having an a-stereocenter, one carbonyl compound would have an inherent preference for R or S product. The other would have also have an inherent preference. These preferences would be expressed even toward achiral reagents. 

The reaction of terminal allyl alcohols proceeds in a 5-endo fashion to give five-membered ring compounds regioselectively and stereoselectively. Subsequent oxidation affords 2,3-5 y -l,3-diols preferentially, regardless of the nature of the catalyst (eq 1). The stereoselectivity increases with increased bulkiness of the al-lylic substituent and the nature of the alkene substituent (see below). 5-Exo type cyclization occurs with homoallyl alcohols to form five-membered heterocycles and 1,3-diols after oxidation. Two chiral centers are produced in this reaction. The 2,3-relationship (anti) is controlled by the allylic substituent, while the 3,4-relationship is determined by the stereochemistry of the alkene the hydrosilation occurs by cis addition of Si-H to the alkene (eq 2). ... 

Control of Enantioselectivity. In the previous sections, the most important factors in determining the syn or anti stereoselectivity of aldol and Mukaiyana reactions were identified as the nature of the transition state (cyclic versus acyclic) and the configuration (E or Z) of the enolate. Additional factors affect the enantioselectivity of aldol additions and related reactions. Nearby chiral centers in either the carbonyl compound or the enolate can impose facial selectivity. Chiral auxiliaries can achieve the same effect. Finally, use of chiral Lewis acids as catalysts can also achieve enantioselectivity. Although the general principles of control of the stereochemistry of aldol addition reactions have been developed for simple molecules, the application of the principles to more complex molecules and the selection of the optimum enolate system requires analysis of the individual cases.Not infrequently, one of the enolate systems proves to... 

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