Organoselenium Chemistry. Conversion of Ketones to Enones by Selenoxide Syn Elimination

Abstract

The scope and limitation of the transformation of ketones to enones by selenenylation followed by selenoxide elimination have been examined. Several procedures for the preparation of α-phenylseleno ketones have been developed. The most useful are direct selenenylation of ketone enolates using PhSeBr and the reaction of enol acetates with electrophilic selenium species such as benzeneselenenyl trifluoroacetate. Several oxidants (ozone, hydrogen peroxide, sodium metaperiodate) and reaction conditions are described to allow optimization of the yield obtained in the transformation of α-phenylseleno ketones to enones. The reaction is quite general for acyclic carbonyl compounds and for tertiary selenides. Difficulties in achieving high yields may be anticipated when very strained double bonds are introduced, when the α-phenylseleno ketone is cyclic and has an α-hydrogen, or when the product is extremely reactive. Qualitative mechanistic studies have revealed two types of side reactions: (1) Pummerer-like transformations to give α-diketones and (2) reactions between the enolate or enol of α-phenylselenino ketones and selenenylating species formed during the disproportionation of benzeneselenenic acid. Reaction conditions which minimize these side reactions have been developed. The utility of benzeneseleninyl chloride as a seleninylating agent has been explored. One pot transformations of ketones to enones using this reagent can be achieved in satisfactory yield, but the procedure is prone to side reactions because of the sensitivity of the selenoxide function. © 1975, American Chemical Society. All rights reserved.