Carbonyl enantioselective catalysis

Corey, E.J. Helal, C.J. (1998) Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method. Angewandte Chemie International Edition, 37, 1986-2012. 

Enantioselective Catalysis of the Aldol Addition Reaction. There are also several catalysts that can effect enantioselective aldol addition. The reactions generally involve enolate equivalents, such as silyl enol ethers, that are unreactive toward the carbonyl component alone, but can react when activated by a Lewis acid. The tryptophan-based oxazaborolidinone 15 has proven to be a useful catalyst.148... 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

The role of multicomponent ligand assembly into a highly enantioselective catalyst is shown in the enantioselective catalysis for the carbonyl-ene reaction (Table 8.9). The catalyst is prepared from an achiral precatalyst, Ti(0 Pr)4 and a combination of BINOL with various chiral diols such as TADDOL and 5-Cl-BIPOL in a molar ratio of 1 1 1 (10mol% with respect to the olefin and glyoxylate) in... 

Corey EJ, Helal CJ (1998) Reduction of carbonyl compounds with chiral oxazaborolidine catalysts a new paradigm for enantioselective catalysis and a powerful new synthetic method. Angew Chem Int Ed 37 1986-2012... 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

An indirect route to a-hydroxy carbonyl compounds uses enol ethers as substrates for dihydroxylations (Scheme 8.24). The primary product is a vicinal hydroxy-hemiacetal which fragments to afford an a-hydroxyketone, rendering the overall route a two-step conversion of ketone to a-hydroxy ketone. The stereochemically important step can use a chiral auxiliary or enantioselective catalysis [64]. The sense of asymmetric induction found in Oppolzer s sulfonamide, shown in Scheme 8.24a... 

E. J. Corey, C. J. Helal, Angew. Chem. 1998, 110, 2092 Angew. Chem. Int. Ed. 1998, 37, 1986 (Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method.), S. Itsuno, in Organic Reactions , Ed. L. A. Paquette, John Wiley Sons, New York, 1998, Vol. 52, pp. 395-576 (Enantioselective Reduction of Ketones). 

Although there has been little research into the hydrosilylation of functionality other than carbonyls (and alkenes), what work has been published provides some exceptionally good examples of enantioselective catalysis. ... 

Enantioselective Catalysis of Carbonyl-Ene Reaction with Fluoral. 

The a-arylation of carbonyl compounds (sometimes in enantioselective version) such as ketones,107-115 amides,114 115 lactones,116 azlactones,117 malonates,118 piperidinones,119,120 cyanoesters,121,122 nitriles,125,124 sul-fones, trimethylsilyl enolates, nitroalkanes, esters, amino acids, or acids has been reported using palladium catalysis. The asymmetric vinylation of ketone enolates has been developed with palladium complexes bearing electron-rich chiral monodentate ligands.155... 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

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