Reactions of aryl isothiocyanates with dicyclohexylcarbodi-imide. Search for the mechanism of cycloaddition and cycloreversion

Abstract

The [2 + 2] thermal cycloadditions of dicyclohexylcarbodi-imide (DCC) with para-substiuted phenyl isothiocyanates (NO₂, CI, H, OMe, or NMe₂) give 1 : 1 adducts, all showing spectral characteristics of 1,3-thiazetidine derivatives. The reactions are reversible. The kinetics of the forward and reverse reactions of a selected example (p-O₂NC₆H₄NCS with DCC) were measured at different temperatures in four solvents (cyclohexane, carbon tetrachloride, butan-2-one, and acetonitrile); the equilibrium constants were determined under the same conditions. The absence of a substantial solvent effect on rate, as well as the failure of trapping experiments point to concerted cycloaddition and cycloreversion reactions with differing degrees of bond formation (or breaking) at the reaction termini in the transition state. Altenative mechanisms via open-chain intermediates are not definitively ruled out.