Oxidative desymmetrizations

A series of meso-dihydrobenzoins was also subjected to oxidative desymmetrization. Three equivalents of the chiral ketone 88 again provided the chiral dioxirane as the active species [138, 139]. As shown in Table 10.14, enantiomeric excesses up to 60% were achieved. In addition to the meso diols themselves, acetonides also proved suitable substrates in two instances (Table 10.14). 

Adam et al. subjected a series of meso-dihydrobenzoins to oxidative desymmetrization, using three equivalents of the chiral Shi ketone 54 as catalyst. In the presence of peroxomonosulfate (Oxone, Curox etc.), the latter generates a chiral dkmrane as... 

Chiral (nitrosyl)ruthenium(salen) complexes have been found to be efficient catalysts for aerobic oxidative desymmetrization of mc.vo-diols under photoirradiation to give optically active lactols. With the suitable catalysts, high enantioselectivities up to 93% has been achieved. The kinetics of the oxidation depend on the nature of the ligand. On the basis of kinetic parameters and the kinetic isotope effect, it has been suggested that when a ligand with an apical hydroxy group is used, the hydrogen atom... 

Keywords C-H oxidation, Benzylic hydroxylation, AUylic oxidation, Desymmetrization of meso compound, Kharash-Sosnovsky reaction. Iron-porphyrin complex, (Salen)manganese(lll) complex... 

Table 4. Products and Methods of Enantioselective Reductive or Oxidative Desymmetrization...

Katsuki recently reported the use of Zr-salen complexes to promote the oxidative desymmetrization of 2-Ph-cyclobutanone 64. Katsuki utilized UHP and the C2-symmetric salen ligand, Zr-salen, which proved to be the best ligand screened. Reaction of ketone 64 with 5 mol% Zr-salen and UHP provided lactone 65 in good yield and 87% ee. These conditions also worked well with racemic bicyclooctanone 66 which provided a mixture of normal 67 and abnormal 68 Baeyer-Villiger products.34 The remaining ketone was recovered in 92% ee. 

In the case of the pinene-derived acyl nitroxide 31 [50], as well as the phthalimide-iV-oxyl radical 32 [56], the mechanism of the oxidation involves hydrogen abstraction by the reactive acyl nitroxide [58] (or phthalimidyl nitroxide), followed by trapping of the resulting carbon radical with either a second equivalent of nitroxide or by molecular oxygen (Scheme 19). Table 2 summarizes the results of oxidative desymmetrization using optically active nitroxides. 

Table 2. Oxidative desymmetrization of meso substrates with optically active nitroxides ... 

Shimizu, H., Onitsuka, S., Egami, H., et al. (2005). Ruthenium(Salen)-Catalyzed Aerobic Oxidative Desymmetrization of Meso-Diols and its Kinetics, J. Am. Chem. Soc., 127, pp. 5396-5413. 

Zhou, L., Liu, X., Ji, J., Zhang, Y, Hu, X., Lin, L., Feng, X. (2012). Enantioselective Baeyer-Villiger oxidation desymmetrization of meso cyclic ketones and kinetic resolution of racemic 2-arylcyclohexanones. Journal of the American Chemical Society, 134, 17023-17026. 

Prospective Substrates and Oxidation Agents Acceptorless Dehydrogenative Oxidations Oxidative Desymmetrizations... 

Other examples include OKR of racemic secondary alcohols (Scheme 25A), oxidative desymmetrizations of meso-diols, etc. The kinetic resolution is generally defined as a process where two enantiomers of a racemic mixture are transformed to products at different rates. Thus, one of the enantiomers of the racemate is selectively transformed to product, whereas the other is left behind. This method allows to reach a maximum of 50% yield of the enantiopure remaining sec-alcohol. To overcome this fim-itation, a modification of the method, namely dynamic kinetic resolution (DKR), was introduced. In this case, the kinetic resolution method is combined with a racemization process, where enantiomers are interconverted while one of them is consumed (e.g., by esterification. Scheme 25B). Therefore, a 100% theoretical yield of one enantiomer can be reached due to the constant equifibrium shift. In most of the proposed DKR processes, several catalytic systems, e.g., enzymes and transition-metal catalysts, work together. Both reactions (transfer hydrogenation of ketones and the reverse oxidation of secondary alcohols using ketone as a hydrogen acceptor) can be promoted by a catalyst. The process can involve a temporary oxidation of a substrate with hydrogen transfer to a transition-metal complex. 

Suzuki T, Morita K, Matsuo Y, Hiroi K. Catalytic asymmetric oxidative lactonizations of meso-diols using a chiral iridium complex. Tetrahedron Lett. 2003 44 2003—2006. Moritani J, Hasegawa Y, Kayaki Y, Ikariya T. Aerobic oxidative desymmetrization of meso-diols with bifunctional amidoiridium catalysts bearing chiral N-sulfonyldiamine ligands. Tetrahedron Lett. 2014 55 1188-1191. 

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