Elimination–addition. Part XXI. Addition–dealkylation reactions of acetylenic sulphonium salts with oxygen, sulphur, and nitrogen nucleophiles

Abstract

Reactions of nucleophiles with dimethylprop-2-ynylsulphonium and S-prop-2-ynyltetrahydrothiophenium bromides have been investigated. These salts readily isomerise to the allenic isomers ([graphic omitted]·C:C:C), which are susceptible to nucleophilic addition. The initially formed adducts, according to their structure, undergo several types of subsequent reaction. Most commonly, isomerisation of the initial non-conjugated adduct to the conjugated isomer is either followed by addition of a second nucleophile or the nucleophile dealkylates the sulphonium group by attack at carbon adjacent to sulphur. In the case of the cyclic sulphonium salt, dealkylation results in ring cleavage and formation of open-chain sulphides. Nucleophilic reactions with alcohols, phenols, and thiols have been studied. The formation of the products obtained is interpreted in terms of the basicity of the nucleophile and its nucleophility for sp³ and sp² carbon centres. Double addition to the allene system is restricted to small nucleophiles irrespective of the nucleophilic atom. Comparisons are drawn with earlier work on allenic sulphones and, where appropriate, the stereochemistry of the products is discussed.