Oxidation enantioselective metal catalysis

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

On the contrary, to achieve a successful cinchona-catalyzed asymmetric oxidation chemistry, cinchona-catalyzed asymmetric reduction has been explored very little despite the importance of this reaction. Previous reports on this subject are restricted to the reduction of aromatic ketones and, moreover, the enantioselectivities achieved to date remain far from satisfactory when compared with metal catalysis. 

In contrast to asymmetric oxidation chemistry, cinchona-catalyzed asymmetric reduction reactions have been explored very little, despite the importance of this reaction. Previous reports on this topic are restricted to the reduction of aromatic ketones, and the enantioselectivities achieved to date remain far from satisfactory when compared with metal catalysis. Moreover, Hantsch esters, another type of useful organic hydrides, have not yet been studied in combination with cinchona catalysts. However, as is well known, the structures of cinchona alkaloids are easily modifiable, thus permitting the easy tuning of the reaction course. The successful use of cinchona catalysts for this reaction will therefore likely be reported in the very near future. 

Apart from the asymmetric metal catalysis, enantioselective Baeyer-Villiger oxidations mediated by enzymes have been known for some time [32,33,34]. Both whole-cell cultures and isolated enzymes, usually flavin-dependent monooxygenases, can be used to oxidize ketones enantioselectively. For future improvements in the asymmetric Baeyer-VilHger oxidation the use of chiral Lewis acids in combination with an appropriate oxidant seems worthy of intensive investigation. 

Wang and co-workers reported an enantioselective metal-organic catalyzed aerobic oxidative aza-Morita-Baylis-Hilman reaction between N-aiyl THIQs and electron-deficient olefins such as acrolein and acrylonitrile (Scheme 7.7). Under co-operative catalysis with Cu(OTf)2 and quinine, sp C-H olefination at the benzylic position of THIQs occurred to afford the... 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

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