Ketones aromatic, optical yield

The condensation of the asymmetric oxazoline (125) and 2-propanone is reported to give the adduct (126 equation 40). The Darzens condensation of aromatic aldehydes with phenacyl halides in the presence of catalytic bovine serum albumin affords epoxy ketones in optical yields as high as 62% ee. ... 

Highly enantioselective hydrosilylation of aliphatic and aromatic carbonyl compounds such as acetophenone, methyl phenethyl ketone 1813, or deuterobenz-aldehyde 1815 can be readily achieved with stericaUy hindered silanes such as o-tolyl2SiH2 or phenyl mesityl silane 1810 in the presence of the rhodium-ferrocene catalyst 1811 to give alcohols such as 1812, 1814, and 1816 in high chemical and optical yield [47] (Scheme 12.14). More recently, hydrosilylations of aldehydes... 

A chiral catalyst consisting of Irans-RuC]2(xy]binap)(daipen) and (CH3)3COK in 2-propanol effects asymmetric hydrogenation of a-, / -, and y-amino aromatic ketones [128]. Hydrogenation of 2-(dimethylamino)acetophenone catalyzed by the (R)-XylBINAP/(R)-DAIPEN-Ru complex [(R,R)-31D] gives the R amino alcohol in 93% ee (Fig. 32.36). The optical yield is increased up to 99.8%, when... 

Asymmetric reduction of ketonesLithium aluminium hydride in conjunction with this chiral ligand reduces prochiral aromatic ketones to (S)-secondary alcohols in 90-95% optical yields. Optical yields are lower (10-40% ee) in the case of alkyl aryl ketones. It is superior to (S)-2-(anilinomethyl)pyrrolidine for this reduction. Evidently the two methyl groups enhance the enantioselectivity. 

The excellent enantioselectivity and wide scope of the CBS reduction have motivated researchers to make new chiral auxiliaries [3]. Figure 1 depicts examples of in situ prepared and preformed catalyst systems reported since 1997. Most of these amino-alcohol-derived catalysts were used for the reduction of a-halogenated ketones and/or for the double reduction of diketones [16-28]. Sulfonamides [29,30], phosphinamides [31], phosphoramides [32], and amine oxides [33] derived from chiral amino alcohols were also applied. The reduction of aromatic ketones with a chiral 1,2-diamine [34] and an a-hydroxythiol [35] gave good optical yields. Acetophenone was reduced with borane-THF in the presence of a chiral phosphoramidite with an optical yield of 96% [36]. 

Ti complexes prepared from Ti[OCH(CH3)2]4 and one equivalent of chiral diol catalyzed the reduction of ketones with catecholborane [44], Scheme 5 shows a recent example utilizing anisyl-BODOL (7) as a chiral auxiliary [45], Several aromatic ketones and 2-acetylcyclohexanone were reduced with an optical yield of up to 98%. Reduction of 2-octanone, a simple aliphatic ketone, gave the corresponding alcohol in 87% ee. 

DPEN/KOH [95] catalyst system is decreased by 10-15% in the hydrogenation of acetophenone or 2 -acetonaphthone. Pivalophenone, a sterically demanding aromatic ketone, is hydrogenated by RuCp Cl(> -cod)/(S)-(l-ethyl-2-pyrrolidinyl)-methylamine/KOH catalyst to afford the R alcohol in 81% e.e. [96], [NH2(C2H5)2j-[ RuCl[(S)-tolbinap] 2(,M-Cl)3] hydrogenates 2 -halo-substituted acetophenones under 85 atm H2 in up to >99% optical yield [97], A stable six-membered intermediate where the Ru metal is chelated by carbonyl oxygen and halogen at the 2 position is supposed [5cj. 

Modification of LAH with (-)-)V-methylephedrine (14) and )V-ethylaniline affords another chiral reducing agent (50). " A variety of aromatic ketones are reduced by (SO) to (S)-carbinols in high optical yields (Scheme 8). Acyclic a,3-unsaturated ketones are also reduced to (S)-carbinols with high selectivity (76-92% ee), but cyclic enones are reduced with only moderate selectivity. 

An interesting case is the reduction of prochiral benzophenones with one parasubstituted phenyl ring. It is surprising that optical yields as great as 26% could be attained, allowing the preparation of chiral benz-hydrols (23,24). Any of the proposed schemes of asymmetric induction with the rhodium-DlOP catalyst (27, 28) can hardly explain such results. Obviously very subtle interactions between the ketone and the complex are involved. Maybe charge transfer interactions between aromatic... 

Palladium complexes carrying chiral BINAP ligands catalyze the addition of disilanes to a,)8-unsaturated ketones these are subsequently converted into either j8-silylketones or -hydroxyketones (Scheme 29). Silylketones are obtained in up to 92% ee. Complexes made in situ from [(COD)RhCl]2 and the new chiral ligands (42) (R = H, Me) catalyze the reaction of Ph2SiH2 with aromatic ketones to give, after hydrolysis, chiral secondary alcohols in good optical yields (>80% ee). 

Recently, Wang [64] prepared by radieal copolymerization a cinchona alkaloid copolymer the methyl acrylate-co-quinine (PMA-QN (71)) (Scheme 34). Complexed with palladium(II), its catalytic activity in the heterogeneous catalytic reduction of aromatic ketones by sodium borohydride was studied. High yields in their corresponding alcohols are obtained but it is found that the efficiency of the catalyst depended on the nature of the solvent and the ketone which related to the accessibility of the catalytic active site. The optical yields in methanol and ethanol 95% were lower than in ethanol. This ability was attributed to a bad coordination between PMA-QN-PdCl2 and sodium borohydride and a reaction rate which was very rapid. The stability of the chiral copolymer catalyst was studied... 

Dong and co-workers reported the synthesis of chiral phthalides (76) by intramolecular enantioselective ketone hydroacylation. Excellent yields and exceedingly high optical purities of 76 were found. Good tolerance to substitutions at positions 4, 5 or 6 (but not at 3) of the aromatic ring of 75 was observed. To achieve high enantioselectivities and avoid undesired reactions, an appropriate choice of the silver salt was found to be crucial. 

Reduction of ketones with sodium borohydride in the presence of a carboxylic acid and 1,2 5,6-di-0-cyclohexylidene-a -D-glucofuranose gave 35-50% enantiomeric enhancement values.Another group has reported a similar reaction with the corresponding di-O-isopropylidene-glucose derivative and prochiral aromatic ketones. Optical yields of up to 64% were claimed. The chiral reagents appear to be sodium acyloxyborohydrides, which complex with the carbohydrate before reduction takes place. 

Cyanohydrins eliminate HCN under basic conditions, giving the corresponding planar aldehyde or ketone. When combined with an asymmetric reaction, the equilibrium can be used for an efficient in situ racemization of cyanohydrins, leading to a DKR process. For example, chiral secondary cyanohydrins can be acylated by isopropenyl acetate in the presence of lipase and solid base such as anion-exchange resin (OH" form) [8a,b] or silica-supported ammonium hydroxide [8c] (Scheme 5.31). A range of aromatic cyanohydrin acetates can be obtained in high chemical and optical yields, although the efficiency is lower for aliphatic precursors [8a]. The success of DKR is ascribable not only to the stereochemical... 

Oxidation of terminal olefins to methyl ketones by aqueous palladium chloride and oxygen is very slow, but addition of micellar sodium lauryl sulphate increases the rate of formation of 2-octanone from 1-octene twentyfold at 50 °C. There is weaker catalysis by the non-ionic surfactant Brij-35 and inhibition by cationic surfactants. " Oxidation of diosphenol (35) in basic aqueous tetradecyltrimethylammonium chloride is faster and more effective than in water, giving a higher yield of (36). Two attempts at effecting the enantioselective reduction of aromatic ketones, one in micelles of R-dodecyl-dimethyl-a-phenylethylammonium bromide and the other in sodium cho-late micelles, both give optical yields of less than 2%. Rather more success was obtained in the catalysed oxidation of L-Dopa, 3,4-dihydroxyphenyI-alanine. In the presence of the Cu complex of N-lauroyl-L-histidine in cetyl-trimethylammonium bromide micelles reaction was 1.42 (pH 6.90, 30 °C) to... 

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