The Baeyer-Villiger rearrangement

Ab initio and semiempirical studies on the transition structure of the Baeyer-Villiger rearrangement of acetone with performic acid have been undertaken,510 and a theoretical investigation has been made511 of the mechanism of the Baeyer-Villiger reaction in non-polar solvents. The effect of fluoromethyl groups on the Baeyer-... 

RB3LYP calculations indicate that the s-cis conformer of peroxy acids is more stable than the s-trans conformer. Calculations on the reaction of prop-2-enol with some peroxy acids showed that trans-transition states collapse to the epoxide via a 1,2-shift, whereas a 1,4-shift is operable for cis-transition states.195 Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger rearrangements of some substituted acetophenones with m-chloroperbenzoic acid (m-CPBA). The energy barriers, charge distributions and frontier molecular orbitals, determined for the aryl migration step, have been used to explain the effects of substituents on the reactivity of the ketones.196... 

Enol ethers of 1,2- and 1,3-diketones afford on ozonolysis products that are not in full agreement with the Criegee mechanism, because in some cases products of the Baeyer-Villiger rearrangement are formed. The main product in the ozonolysis of the enol ether 180 is a mixture of spiranic stereoisomers 181 involving a lactone and a 1,2,4-trioxolane ring (Scheme 62) <2004HCA2025>. 

The hydrogenation of 63 occurred quantitatively and with high stereoselectivity (25 1 in favour of 62). The regioselectivity was similarly good in the Baeyer-Villiger rearrangement (12 1) in favour of 61 over the isomeric lactone. Attempts to convert the lactone directly to 60 X = z-Bu gave only low yields. 

Similar migrations to electron-deficient oxygen also occur. In the Baeyer-Villiger rearrangement a ketone is reacted with a peracid. The product is an ester in which one of the alkyl groups of the ketone has migrated to oxygen ... 

The Baeyer-Villiger rearrangement often is called Baeyer-Villiger oxidation (see the last subsection of Section 14.3.2, Oxidative Cleavage of Ketones). In the Baeyer-Villiger rearrangement a carbonyl compound (ketones are almost always used) and an... 

One of the most important methods for lactone preparation, and hence of the corresponding hydroxy acids (or halogeno acids, Section 5.14.1 above) is the Baeyer- Villiger rearrangment of cyclic ketones by the action of peracids. A wide variety of peracids have been used in this reaction but currently the reagents of choice are pertrifluoroacetic acid, m-chloroperbenzoic acid, and permaleic acid. The mechanism is formulated below for the conversion of an acyclic ketone into an ester. 

Scheme 11/24 gives the proposed mechanism [133]. The central intermediate, 11/195, is tetrahedral. From studies of a series of unsymmetrical substituted ketones, the relative ease of migration of various groups has been found to be tert-alkyl (best) > cyclohexyl, sec-alkyl, benzyl, phenyl > prim-alkyl > methyl [134]. To illustrate this method, some examples of the Baeyer-Villiger rearrangement are given in Scheme II/2512). 

The oxidation of the bicyclic amino-ketone shown in the margin demonstrates how useful this effect can be. This is the Baeyer-Villiger rearrangement, which you will meet in Chapter 37. The mechanism is not important here all you need to know is that it inserts an oxygen atom on one side or the other of the ketone C=0 group. The question is—which side ... 

The same alcohol could be made by the Baeyer-Villiger rearrangement but the stereochemistry would have to be set up before the Baeyer-Villiger step. Hydroboration has the advantage that stereochemistry is created in the hydroboration step. We have discussed the details of this step. In drawing the mechanism it is usually best to draw it as a simple concerted four-centre mechanism providing you remember that the regioselectivity is controlled by the initial interaction between the nucleophilic end of the alkene and the empty p orbital on boron. 

Why the difference between the Baeyer-Villiger rearrangement and boron chemistry ... 

The transition state for the Baeyer-Villiger rearrangement has a positive charge in the important area. Anything that can help to stabilize the positive charge, such as a tertiary migrating group (R1), stabilizes the transition state and makes the reaction go better. 

The Baeyer-Villiger rearrangement of cyclohexanone and acetophenone with TS-I/H2O2 proved to be poorly selective [117]. Notably, Ti-P and Sn-P have different chemoselectivities in the oxidation of unsaturated ketones, leading selectively to corresponding epoxides and lactones, respectively [118]. The different oxidation pathways were attributed to the preferential adsorption of hydrogen peroxide on Ti-sites and of the carbonyl group on Sn-sites. 

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