Transition state stucture and frontier orbital influence on the cycloaddition reactions of 2,5-dimethyl-3,4-dimethylenethiophene with styrenes and benzaldehydes

Abstract

The biradical 2,5-dimethyl-3,4-dimethylenethiophene reacts with styrenes and benzaldehydes to give fused cycloadducts. The effect of substituents on the efficiency of the benzaldehyde trapping reaction resembles that of normal electron-demand Diels-Alder reactions and is linear with the Hammett σ-constant. With the styrenes, the Hammett plot is non-linear. Frontier orbital theory provides a rationalization of these effects in terms of the uniquely HOMO-LUMO gap characteristic of biradicals.