Baeyer-Villiger reaction catalysts

The original library of 10 000 clones used in the Baeyer-Villiger reaction [89] was screened for performance as potential catalysts in the sulfoxidation [32]. This led to the discovery of at least 20 mutants having enantiomeric excess values in the range of 85-99%, some being (R) selective and others being (S) selective. Five mutants resulting in enantiomeric excess values of >95% were sequenced (Table 2.2) [32]. 

Another method for the asymmetric version of the Baeyer-Villiger reaction was presented by Lopp and coworkers in 1996 . By employing overstoichiometric quantities of Ti(OPr-t)4/DET/TBHP (1.5 eq./1.8 eq./1.5 eq.), racemic andprochiral cyclobutanones were converted to enantiomerically enriched lactones with ee values up to 75% and moderate conversions up to 40% (Scheme 171). Bolm and Beckmann used a combination of axially chiral C2-symmetric diols of the BINOL type as ligands in the zirconium-mediated Baeyer-Villiger reaction of cyclobutanone derivatives in the presence of TBHP (or CHP) as oxidant (Scheme 172) . With the in situ formed catalysts 233a-d the regioisomeric lactones were produced with moderate asymmetric inductions (6-84%). The main drawback of this method is the need of stoichiometric amounts of zirconium catalyst. 

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

Co2 +-Substitution at the addenda atoms gives catalysts for the epoxidation of olefins in the presence of aldehyde [293). PWM-Co is the most active among the mono-transition-metal-substituted polyanions the order of activity is PWn-Co > -Mn 2= -Fe 2= -Cu > -Ni. Here, PWll(M + )0(379", ) (M = Co2 +, Cu2+, Fe3 +, Ni2 +, Mn2 + ) is denoted by PWn M. The same order was observed for the oxidation of isobutyraldehyde, suggesting that the oxidation of aldehyde to give peracid is an important step in the reaction. It has been reported that substitution of V5+ for Mo6+ in PMo O3 gives a good catalyst for epoxidation and the Baeyer-Villiger reaction [294). 

Free-radical autoxidation of aldehydes with 02 is facile and affords the corresponding peradds, which are used as oxidants for carbonyl compounds. The peracid can transfer an oxygen atom to a substrate such as an olefin or ketone, resulting in the formation of one equivalent of epoxide or ester and add as a co-produd in the absence of metal catalysts [59]. Kaneda and coworkers have developed several HT materials that are active for heterogeneous Baeyer-Villiger reactions with 02/aldehyde [60]. Combination with Lewis addic metals improved the reaction by allowing coordination of the peracid and the intermediate. 

Compounds in this class are usually made from specific precursors with the ultimate functionalities appearing in the starting materials. Syntheses in which one new bond is formed (cyclizations) usually involve substitution or condensation reactions. Two new closures with potentially some generality have been noted in this chapter. One of the intramolecular Diels-Alder reactions (see Section 13.12.8.1) and the other RCM (see Section 13.12.8.1). The Baeyer-Villiger reaction is still very much used for the preparation of lactones. A supported catalyst and a recyclable catalyst carried in an ionic liquid have been introduced for use in Baeyer-Villiger oxidations from other areas. 

A combined theoretical and experimental study of the Baeyer-Villiger reaction of acetone and pentan-3-one, including their fluorinated and chlorinated derivatives, with performic acid revealed that the first step is rate determining for all substrates examined, even in the presence of acid. Acid catalysts generally decrease the A of the first transition state (TS1), but increase the Aof the second (TS2). The optimal... 

Using the same approach, Mikami etal. [213] were able to increase the reaction rate of a Sc[N(S02CgF17)2]3 catalyzed Baeyer-Villiger reaction. Only one regioisomeric lactone was obtained in high yields even at very low-catalyst concentrations (0.05 mol%) (Figure 4.21). 

ArCHO — ArOH. Aromatic aldehydes undergo Baeyer-Villiger reaction when treated with H202 (30%) in the presence of several selenium compounds as catalysts, of which areneseleninic acids are most effective. The resulting aryl formates are readily hydrolyzed to phenols. 

Bis(trimethylsilyl) peroxide, (CH3)3SiOOSi(CH3)3, is prepared from trimethylsilyl chloride, l,4-diaza[2,2,2]bicyclooctane, and Dabco s complex with 2 mol of hydrogen peroxide [127]. It is used alone [228] or in the presence of catalysts such as pyridinium dichromate [236] trimethylsilyl trifluoromethanesulfonate, CF3S03Si(CH3)3 [228, 237] or tris-(triphenylphosphine)ruthenium dichloride, [(C6H5)3P]3RuCl2 [236]. This reagent oxidizes primary alcohols to aldehydes (in preference to the oxidation of secondary alcohols to ketones [236]), ketones to esters or lactones Baeyer-Villiger reaction) [238], and nucleoside phosphites to phosphates [228]. All these oxidations require anhydrous conditions. 

Oxidations with m-chloroperoxybenzoic acid are carried out in solutions in hexane, dichloromethane, chloroform, methanol, or tetrahydro-furan at temperatures ranging from -78 to 40 C. The applications of m-chloroperoxybenzoic acid are epoxidation [287, 314, 315, 316] the Baeyer-Villiger reaction [286, 315, 317, 378] and the oxidation of primary amines to nitro compounds [379], of tertiary amines to amine oxides [320], of sulfides to sulfoxides [327, 322, 323, 324], and of selenides to selenones [325]. Secondary alcohols are oxidized to ketones in the presence of hydrogen chloride [326], and acetals are oxidized to esters with boron trifluoride etherate as a catalyst [327]. The addition of potassium fluoride to reaction mixtures facilitates product isolation, because both m-chloroben-zoic acid and the unreacted m-chloroperoxybenzoic acid are precipitated... 

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