1,2-Proton shifts in pyrazole and related systems: a computational study of [1,5]-sigmatropic migrations of hydrogen and related phenomena
Three different approaches are used to discuss the possible analogy between the [1,5] hydrogen shift in cyclopentadiene and the prototropy in 1H-pyrazole. In the first, a series of NH→HN hydrogen shifts in cyclic conjugated molecules are considered demonstrating that the case of pyrazole is not intrinsically different from the other systems which are unrelated to [1,5] H shifts. The second approach compares pyrazole and cyclopentadiene with their open-ring structures, pentadiene and aminoazadiene, proving that the N–N bond is essential to describe pyrazole while the C(sp3)–C(sp2) bond in cyclopentadiene can be considered as a perturbation. Finally, the third approach is a study of cyclopentadienide and pyrazolide anions as hydrogen-bond acceptors, the first one being a π-acceptor while the second one is a σ-acceptor through the nitrogen lone pair. The conclusion is that N(sp2)–N(sp2) migrations of hydrogen in aromatic azoles are outside the Woodward–Hoffmann domain of application.