Substituent effects on a reversible cycloaddition reaction

Abstract

The effect of substituents in the aryl group on the reversible dimerisation of 3-aryl-2-cyanothioacrylamides (a [2_s+ 4_s] cycloaddition) has been determined from measurement of the equilibrium constants by ¹H n.m.r. spectroscopy. For a series of seven such compounds the free-energy change and enthalpy change show opposite linear correlation with the substituent Hammett σ values and these trends are rationalised in terms of the substituent perturbation to the frontier orbitals. The parameters ΔH° and ΔS° show a reverse correlation with ΔG° and a direct correlation with each other. The isokinetic temperature has been evaluated.