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

Abstract

In the probably concerted cycloadditions of the sterically hindered thiocarbonyl ylide 1 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 1 with dimethyl 2,3-dicyanofumarate ( 16 , retention/inversion 60:40) and dimethyl 2,3-dicyanomaleate ( 17 , 76:24) were observed. Special conditions were required to avoid a preceding cis, trans isomerization, 16 ⇄ 17 , catalyzed by thiadiazoline 4 , the precursor of 1 . In the case of the related thiocarbonyl ylide 37 , this catalysis could not be suppressed and the same ratio of adducts (55:45) was obtained with 16 and 17 .