Basicity of five-membered cyclic allenes: Proton and cation affinity evaluation using density functional theory calculations

Abstract

The proton affinity (PA) of stable five-membered cyclic allenes 1–13 in the gas phase was studied by DFT/B3LYP computational method in conjunction with the 6-311+G(d,p) basis set. The PAs of the title compounds are large. Their protonation at the central carbon introduces super- and hyperbases as evidenced by their enthalpies of protonation ΔH = 272–315 kcal/mol, which are above the threshold of superbasicity and in some cases close to the hyperbasicity. The trends of PA correlate quite well with the energy level of the HOMO orbitals and the molecular electrostatic potential maps of the studied allenes. The Wiberg bond order analysis gives a reasonable explanation of the exocyclic delocalization involving the π-donor substituents with the allenic unit of the studied compounds. Protonation of central carbon in the cyclic allenes leads to a change in the structural arrangement and an increment in the aromaticity of the pyrazole ring, which was evaluated by the HOMA index. Furthermore, the B3LYP/6-311+G(d,p) calculated cation affinities for the cyclic allene 6 increases as K+<Na+<Li+<Cu+<Mg2+<Fe2+<Cu2+. Cation affinity is strongly dependent on the charge-to-size ratio of the Mn+.