1-Methyl-5-(trifluoromethyl)azafulvenium Methide, an Intermediate ThatUndergoes Reaction through “Unusual”cis-exo-1,3- andtrans-exo-1,7-Cycloadditions

Abstract

The generation and reactivity of a new 1-methyl-5-(trifluoromethyl)azafulvenium methide are described. Under microwave-induced pyrolysis conditions this intermediate could be trapped by dipolarophiles, acting either as a 4π or as an 8π dipole. Quantum chemical calculations carried out at the DFT level of theory allowed the rationalization of the results observed in the cycloaddition of 1-methyl-5-(trifluoromethyl)azafulvenium methide and N-substituted maleimides. The study revealed that for 1,7-cycloadducts the major products are obtained in the trans configuration through the unusual exo-cycloaddition mode, whereas for 1,3-cycloadducts the cis counterparts are obtained from this unexpected approach. In addition, under flash vacuum pyrolysis or conventional thermolysis conditions 1-methyl-5-(trifluoromethyl)azafulvenium methide undergoes an allowed suprafacial sigmatropic [1,8]H shift leading to the formation of 2-methyl-3-(trifluoromethyl)-1-vinyl-1H-pyrrole.