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

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 a-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 a-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 a-phenylseleno ketone is cyclic and has an a-hydrogen, or when the product is extremely reactive. Qualitative mechanistic studies have revealed two types of side reactions: (1) Pummerer-like transformations to give a-diketones and (2) reactions between the enolate or enol of a-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. The many synthetic transformations originating from a,P-unsaturated carbonyl compounds have made their preparation a long standing important synthetic problem. The most straightforward method is the dehydrogenation of carbonyl compounds. There are a number of methods for performing this the most important of which is the a-bromination-dehydrobromination method.' Orientational control is difficult to achieve in direct bromination of ketones, but Stotter and HillId have recently shown that bromination of cyclohexanone enolates can be carried out in high yield, and also that dehydrobromination can be performed without loss of regiospecificity. Isomerization of abromo ketones under conditions of the debrominations has been frequently reported,Ie however, particularly for bromides of 0-dicarbonyl compounds.If,g The vigorous reaction conditions (frequently temperatures in excess of 120') also severely limit this method because of the sensitivity of many enones. Direct dehydrogenations can be performed by a number of reagents, including selenium dioxide,2a%b dichlorodicyanoquinone,2c periodic acid,2d oxygen in the presence of transition metal catalysts,2e and pyridine N-oxide-acetic anhydride.2f The first two methods have been studied in great detail, and some excellent procedures have been developed, but yields vary greatly, and effective control of regioselectivity is frequently a problem. The discovery by Jones, Mundy, and Whitehouse3 that selenoxides undergo clean syn elimination to form olefins a t or below room temperature suggested that this reaction may offer a solution to the problem discussed above. W e report here full details of our ~ o r k ~ ~ ~ on the conversion of ketones to enones using the selenoxide elimination (eq 1).