Synthesis and thermal reactivity of 3-benzyl-7-trifluoromethyl-1H,3H-pyrrolo[1,2-c]thiazole-2,2-dioxide

The generation and reactivity of 1-benzyl-5-trifluoromethyl-azafulvenium methide are described. Under microwave induced pyrolysis this intermediate could be trapped by dipolarophiles acting as a 4π as well as 8π dipole. It was observed that with dimethyl acetylenedicarboxylate the 1,3-dipolar cycloadduct was the major product whereas with N-substituted maleimides the major product results from the addition across the 1,7-position. FMO analysis of the cycloadditions corroborated the rationalization of the observed reactivity. Quantum chemical calculations carried out at the DFT level of theory allowed the rationalization of the stereoselectivity observed in the cycloaddition of 1-benzyl-5-trifluoromethyl-azafulvenium methide with N-substituted maleimides. The study revealed that exo-cycloaddition is the main reaction path for the 1,7-cycloaddition, while the endo-approach is the main mode of reaction leading to 1,3-cycloadducts. In addition, under flash vacuum pyrolysis or conventional thermolysis, 1-benzyl-5-trifluoromethyl-azafulvenium methide undergoes an allowed suprafacial sigmatropic [1,8]H shift leading to the efficient formation of 2-methyl-1-styryl-3-trifluoromethyl-1H-pyrrole.