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.
Arrieta
Ana
A
Kimika Organikoa I. Saila, Kimika Fakultatea, Universidad del País Vasco-Euskal Herriko Unibertsitatea, P.K. 1071, 20080 San Sebastián-Donostia, Spain.
Otaegui
Dorleta
D
Zubia
Aizpea
A
Cossío
Fernando P
FP
Díaz-Ortiz
Angel
A
de la Hoz
Antonio
A
Herrero
M Antonia
MA
Prieto
Pilar
P
Foces-Foces
Concepción
C
Pizarro
José L
JL
Arriortua
María I
MI