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(sp(3))-C(sp(2)) 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 pi-acceptor while the second one is a sigma-acceptor through the nitrogen lone pair. The conclusion is that N(sp(2))-N(sp(2)) migrations of hydrogen in aromatic azoles are outside the Woodward-Hoffmann domain of application.