Shrenik K. Shah

Selenium stabilized carbanions. Preparation of .alpha.-lithio selenides and applications to the synthesis of olefins by reductive elimination of .beta.-hydroxy selenides and selenoxide syn elimination

The deprotonation of several alkyl aryl selenides with lithium amide bases has been studied. The kinetic acidity of methyl m-trifluoromethylphenyl selenide was found to be 1 / 3 3 that of the sulfur analogue. The introduction of a m-trifluoromethyl substituent into methyl phenyl sulfide increased the kinetic acidity by a factor of 22.4. A variety of @-hydroxy selenides have been prepared by the reduction of a-phenylseleno ketones and the addition of a-lithio selenides (prepared by deprotonation of benzyl phenyl selenide, bis(phenylseleno)methane, methoxymethyl m-trifluoromethylphenyl selenide, and phenylselenoacetic acid, and by n-butyllithium cleavage of bis(pheny1seleno)methane) to carbonyl compounds. These @-hydroxy selenides are converted to olefins on treatment with rnethanesulfonyl chloride and triethylamine. This reductive elimination proceeds exclusively or predominantly with anti stereochemistry. Simple olefins, styrenes, cinnamic acids, vinyl ethers, and vinyl selenides have been prepared using this technique. Attempts to carry out the syn reductive elimination of @-hydroxy selenoxides or related compounds have not been successful. a-Li thio selenides can also be alkylated, and the derived selenides converted to olefins by selenoxide syn elimination. The unique chemical properties of organoselenium compounds have been successfully exploited for a variety of olefin-forming proce~ses .~ The synthetic utility of these reactions is amplified by the capacity of the selenium function to serve as an activating group for carbon-carbon bond formation, in addition to its role in the introduction of the double bond. The preparation, properties, and reactions of a-lithio selenides will be the subject of this paper. The following paper4 describes our work on the chemistry of a-lithio selenoxides. All of the standard procedures for the preparation of alkyllithium reagents are, in principle, applicable to the preparation of a-lithio selenides. In practice, only two methods have been used widely: the n-butyllithium cleavage of selenoacetals and -ketals5s6 and the deprotonation of selenides using strong b a s e ~ . l ~ % ~ . ~ The former procedure developed by Seebach and c o w ~ r k e r s ~ . ~ ~ , ~ is generally applicable to systems 1 where Rl and Rl can be hydrogen or an alkyl or aryl group. The resulting lithium reagents 2 have high nucleophilicity; they react with