Alkynyl Silyl Sulfides as Versatile Thioketene Equivalents

Abstract

This account covers the chemistry of alkynyl silyl sulfides and their use as substitutes for aldothioketenes. In contrast to the latter heterocumulenes, which can only be prepared and used in situ, many alkynyl silyl sulfides are stable at room temperature. Exceptions are bissilylated derivatives that are capable of rearranging to form the corresponding thioketenes. This reaction is promoted thermally or by the addition of silyl-accepting Lewis bases, but is suppressed by bulky S-silyl substituents. The reactivity of the highly electrophilic alkynyl silyl sulfides is comparable to that of the corresponding ketenes and thioketenes. Nucleophiles usually add to the alkyne α-carbon atoms to give, after desilylation, thiocarbonyl compounds. Particularly useful are the reactions with amines, imines, or ynamines, which give thioamides, β-thiolactams, and cyclobutenethiones, respectively. These transformations proceed under mild conditions and provide access to derivatives that could not previously be prepared by classical means. The scope of the [2+2]-cycloaddition reaction with imines to give β-thiolactams can be expanded by using Lewis acid catalysts, particularly zinc iodide. The mechanism of this reaction has been elucidated by careful characterization of byproducts. Further modifications of the β-thiolactams prepared by this methodology are also described. Reactions of alkynyl silyl sulfides with cyclic imines lead to bicyclic systems that can be considered as thio analogues of the core structures of β-lactam antibiotics. However, there is a competition from the ringexpansion reaction to form seven-membered thiolactams and dithiolactones.