Cycloadditions of two thiocarbonyl ylides with α,β-unsaturated esters and nitriles: steric course and mechanism

In the probably concerted cycloadditions of the sterically hindered thiocarbonyl ylide with fumaronitrile, maleonitrile, and dimethyl fumarate, the dipolarophile configuration is retained whereas retention/inversion 99:1 for dimethyl maleate (51 times less reactive than fumarate) signals a small involvement of a two-step pathway. The latter becomes dominant when two acceptor groups stabilize the anionic terminus of a zwitterionic intermediate. Nonstereospecific cycloadditions of with dimethyl 2,3-dicyanofumarate (, retention/inversion 60:40) and dimethyl 2,3-dicyanomaleate (, 76:24) were observed. Special conditions were required to avoid a preceding cis, trans isomerization, ⇄, catalyzed by thiadiazoline , the precursor of . In the case of the related thiocarbonyl ylide , this catalysis could not be suppressed and the same ratio of adducts (55:45) was obtained with and .