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

Article abstract of DOI:10.1016/S0040-4039(00)92371-2

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-Adler 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 small HOMO-LUMO gap characteristic of biradicals.