Regioselective Baeyer-Villiger rearrangement

Fig. 14.36. Regioselective Baeyer-Villiger rearrangement of an unsymmetrical ketone with HCPBA (meta-chloro-peroxybenzoic acid). The aryl group is [l,2]-shifted in all cases and irrespective of whether the acetophenone is electron-rich or electron-poor.

Fig. 14.37. Regioselective Baeyer-Villiger rearrangement of an electron-poor aromatic aldehyde. This reaction is part of the autoxidation of benz-aldehyde to benzoic acid. Both alternative reaction mechanisms are shown the [1,21-rearrangement (top) and the /3-elimination (bottom).

Fig. 14.38. Regioselective Baeyer-Villiger rearrangement of an electron-rich aromatic aldehyde.

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. 

Fig. 14.34. Regioselective and stereoselective Baeyer-Villiger rearrangement of an unsymmetrical ketone with magnesium monoperox-ophthalate hexahydrate (in the drawing, Mg is omitted for clarity).

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. 

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