Solvent-Free Thermal and Microwave-Assisted [3 + 2] Cycloadditions between Stabilized Azomethine Ylides and Nitrostyrenes. An Experimental and Theoretical Study

The stereochemical outcomes observed in the thermal and microwave-assisted [3 + 2] cycloaddition
between stabilized azomethine ylides and nitrostyrenes have been analyzed using experimental and
computational approaches. It has been observed that, in the absence of solvent, three stereoisomers are
formed, both under classical heating conditions and under microwave irradiation. This result contrasts
with that observed in solution under classical thermal conditions. The 4-nitropyrrolidines obtained in
this way can be aromatized under further microwave irradiation to yield mixtures of pyrroles and
4-nitropyrroles. It is found that ground state cycloadditions between imines and nitrostyrenes take place
by three-step mechanisms. The first step involves enolization of the starting imine, and this is followed
by a pseudopericyclic 10-electron [1.4]-hydrogen shift. Finally, the cycloaddition takes place by a relatively
asynchronous aromatic six-electron supra−supra thermal mechanism.