Formation of 2-[1-(Trimethylsilyl)alkylidene]-4-cyclopentene-1,3-dione from Lewis Acid-Catalyzed Reaction of Cyclobutenedione Monoacetal with Alkynylsilane:  Novel Cationic 1,2-Silyl Migrative Ring Opening and Subsequent 5-Exo-Trig Ring Closure

An ethoxycarbenium ion intermediate, which was produced by the catalytic action of a Lewis acid on a cyclobutenedione monoacetal, reacted with phenyl(trimethylsilyl)acetylene to give a normal electrophilic substitution product. In sharp contrast, the same catalytic reaction with bis(trimethylsilyl)acetylene afforded a 2-methylene-4-cyclopentene-1,3-dione derivative as a ring expansion product instead of an alkynylation product. Butyl(trimethylsilyl)acetylene showed reactivity between the aforementioned compounds as a result of the formation of both types of products. In the reactions of such alkyl-substituted silylacetylenes, both E- and Z-isomers of 2-(1-silylalkylidene)cyclopentenediones were obtained in ratios dependent on the reaction temperature and the amount of Lewis acid. This rearrangement resulted from unprecedented cationic 1,2-silyl migration on the alkynylsilane and subsequent ring expansion promoted by the formed vinyl cation intermediate. A detailed mechanism of the novel ring-expansion route is discussed with the aid of PM3 calculations, especially for the reclosure step, which is explained by a 5-exo-trig cyclization rather than a pentadienyl cation electrocyclization.