Elimination reactions phenyl selenoxides

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

Conjugate addition of phenyl trimethylsilyl selenide to a,/3-unsaturated aldehydes and ketones has been attained by using triphenylphosphine, zinc chloride, or trimethylsilyl trifluoro-methanesulfonate as the catalyst. Combination of this reaction with selenoxide elimination provides a one-pot procedure for a-alkoxyalkylation of a,/3-unsaturated ketones (eq 3). 

The selenoxide elimination of primary alkyl 2-pyridyl selenides to give terminal alkcnes also proceeds in higher yield than the corresponding reaction with alkyl phenyl selenides.2 Example ... 

Conversion of ketones into a,(3-unsaturated ketones has been effected by bromination-dehydrobromination, although a better method involves a-phenyl-seleno ketones as intermediates. These are normally obtained by reaction of the enolate of the ketone with a phenylselenyl halide or diphenyl diselenide at low temperature. Oxidation with hydrogen peroxide, sodium periodate or other oxidant gives the selenoxide which immediately undergoes syn p-elimination to form the a, -unsaturated ketone. The process is tolerant of many functional groups, such as... 

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