Baeyer-Villiger oxidation promoted

Table 5. Direct catalytic asymmetric aldol reactions promoted by heteropolyme-tallic asymmetric catalyst and following Baeyer-Villiger oxidations.

The addition of benzyltriethylammonium chloride promotes the perborate-mediated Baeyer-Villiger oxidation of water-immiscible ketones [20]. 

Baeyer-Villiger oxidation of the 5a-6-keto steroid 1 with trifluoroperacedc acid is 1000 times faster than oxidation with m-chloroperbenzoic acid and also is more regioselective. This oxidation was used in the last step in a synthesis of brassinolide (2), a natural steroid that promotes plant growth.2... 

The TFE- or HFIP-activated H2O2 oxidant promotes not only epoxidation [34, 47, 48], but also Baeyer-Villiger oxidation [48], oxidation of sulfides to sulfoxide [34, 49], and oxidation of thiols to disulfides [5, 50]. The nature of the weaker nucleophilicity and higher acidity of TFE as compared with ethanol is useful as a solvent for Pd-catalyzed asymmetric hydrogenation of trifluoromethylimines [51]. 

A variety of metals have been used to promote the Baeyer-Villiger oxidation. Using catalytic metal complexes has allowed the use of inexpensive H202 or 02. Yamada and co-workers reported the use of a Ni(II) complex with 02 to promote the Baeyer-Villiger oxidation on simple ketones. In this work was reported the use of (dipivaloylmethanato)-nickel(II) to be a good complex with isovaleraldehyde or benzaldehyde under an atmosphere of 02 to provide good conversions of ketones to lactones or esters. These conditions proved to be amenable to promote the Baeyer-Villiger oxidation of a variety of ketones in 33-96% yield and is exemplified by the conversion of ketone 51 into lactone 52,29... 

Subsequent to this disclosure, Bolm reported the use of Ni(oxa)2 or Cu(OAc)2 to provide good yields of the lactone 52. Again, a similar set of keto-substrates were oxidized in 23-93% yield using these 02 promoted Baeyer-Villiger oxidation conditions. 

Strukul has recently shown that platinum catalyst [(triphosPO)Pt]2+ used to promote the Baeyer-Villiger oxidation with H2O2 operated by Lewis acid catalysis and promoted the leaving of the OH group.33 This dual catalysis property was considered unique to Pt catalysts. Chiral platinum systems have been applied to cyclic ketones to give moderate enantioselectivity of lactones.32... 

Molybdenum and rhenium complexes have also been reported to promote the Baeyer-Villiger oxidation using O2. Although limited to cyclobutanones, Sharpless asymmetric epoxidation conditions have provided optically active lactones from hydroxymethylbutanone.35... 

Over the past few years, there have been reports of directed evolution of Baeyer-Villiger MOs and their application in synthetic chemistry. In the evolution and synthetic application of these mutated enzymes to previous and new substrates, Mihovilovic and co-workers have improved the selectivity of a variety of CHMOs and CPMOs toward the oxidation of bicyclic ketone rac-87. It was shown that rac-87 provides 99% ee of each lactone 88 and 91 as a 1 1 mixture.50 In addition, it was shown that high % ee s and conversions could be obtained for a variety of substrates for the enzyme-promoted Baeyer-Villiger oxidation.51... 

Application of dynamic kinetic resolution has also been reported in a whole-cell enzymatic Baeyer-Villiger process. While this was initially done using slightly basic conditions, Furstoss and Alphand have more recently reported the use of a weakly basic anion exchange resin to promote racemization of the slow oxidizing enantiomer 99 to the fast enantiomer 100.53 Baeyer-Villiger oxidation using recombinant E. coli to overexpress the CHMO from A. calcoaceticus provided excellent yield and % ee of lactone 101. 

The Ti-silicalite catalysed Baeyer-Villiger oxidation of cyclohexanone by H2O2 in aqueous solution promotes a high concentration of OH radicals, which accelerate the reaction and the consecutive formation of adipic acid (and of lighter diacids as well) as the kinetically preferred product. In the absence of the catalyst, the primary reaction product is e-caprolactone, which quickly undergoes consecutive transformations to form adipic acid even when a stoichiometric amount of the oxidant is used, since it is more reactive than cyclohexanone. The choice of solvent, which could act as a radical scavenger, also allows control over the reaction rates. ° ... 

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

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. 

The oxidation of thiols to disulfides with molecular oxygen has been achieved by Go(ii)-phthalocyanine complexes in [C4CiIm]BF4. Hydrogen peroxide/methyltrioxorhenium in [C4GiIm]PF6 or [G4GiIm]BF4 promotes the oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols (Scheme 26) or the Baeyer-Villiger reaction. ... 

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