Selenium Stabilized Carbanions. a-Lithio Selenoxides as Reagents for the Synthesis of Olefins, Allyl Alcohols, and Dienes'

Techniques for the preparation of a-lithio selenoxides have been developed. These reagents react cleanly with most aldehydes and ketones to give 0-hydroxy selenoxides, which can be thermolyzed to allyl alcohols or reduced to &hydroxy selenides, The P-hydroxy selenides are further transformed to olefins by reductive elimination. a-Lithio selenoxides can also be alkylated and acylated, although these reactions are of lesser scope and usefulness than the reaction with aldehydes and ketones. A synthesis of I , 1 -bis(phenylseleno)cyclopropane was developed based on an intramolecular alkylation of an a-lithio selenoxide. The compound is a suitable precursor for the preparation of 1 -phenylselenocyclopropyllithium, which was used to prepare cyclopropyl phenyl selenide and I -phenylselenocyclopropanecarboxylic acid. Heteroatom stabilized organometallic reagents are powcrful tools for the formation of functionalized carbon-carbon bonds. The most widely used reagents of this type are the phosphonium ylides (Wittig reagents), although sulfonium qlides and phosphorus, sulfur, and to a lesser extent silicon stabilized anions have become increasingly important. Developments in the preparation and reactions of selenium stabi!ized lithium and other organometallic reagents have been fostered by the interesting and useful chemistry available to selenides and ~e lenoxides .~ I n the preceding paper4 the chemistry of a-lithio selenides was presented. W e report here the results of our study of the preparation and reactions of tr-lithio selenoxides. The incentive for the study of a-lithio selenoxides is provided by the lack of satisfactory procedures for the general preparation of a-lithio selenides by deprotonation of selenides, ;I result of the limited acidifying power of the phenylseleno group, and the propensity of selenides to be fragmented upon treatment with powerful metalating agents. The substantially greater acidity expected for selenoxides, however, should allow their deprotonation in cases where the corresponding selenides are insufficiently acidic. The greater acidity of selenoxide vs. selenide can be inferred from comparisons with kinetic and thermodynamic acidity data available for sulfides, sulfoxides, and sulfones. Bordwell and co-workers5 have reported the following pK, data (Me2SO solvent and references).