Quantum-chemical and NMR study of nitrofuroxanoquinoline cycloaddition

Using DFT/B3LYP and ab initio RHF quantum-chemical calculations in the triple-zeta basis set 6-31++G** the endo and exo cycloaddition mechanism for the interaction of ethyl vinyl ether, trimethylsilyloxybutadiene, or cyclopentadiene with 5-nitro-7,8-furoxanoquinolines was studied in details. Considering that both in solutions and crystals nitrofuroxanoquinoline exists as an inseparable mixture of two N -oxide tautomers, all cycloaddition processes were studied for both of them. The studied mechanisms practically do not depend on the location of the exocyclic oxygen atom in nitrofuroxanoquinoline molecule. At the first step of all reactions the conjugated nitroarene fragments C=C–N=O react with nucleophilic reagents following the mechanism of endo -[4+2] cycloaddition with inverse electronic demand. Further (in the cases of trimethylsilyloxybutadiene and cyclopentadiene) endo -[4+2] cycloadducts recyclize spontaneously according to the mechanism of [3,3] sigmatropic rearrangement into more thermodynamically stable, experimentally detected endo -[2+4] cycloadducts. Both endo -[4+2] and endo -[2+4] cycloadducts obtained from cyclopentadiene and nitrofuroxanoquinoline have been experimentally isolated and characterized. For this case, the kinetic and activation parameters of [4+2] → [2+4] transformation have been studied by 1H NMR method, which have shown an excellent agreement with quantum-chemical results. In all cases the exo processes are one-step reactions, less favorable kinetically than their endo competitors.