GAS-PHASE REACTIVITY OF 5-MEMBERED HETEROAROMATICS TOWARD ELECTROPHILES, AN EXPERIMENTAL CHECK ON THEORETICAL PREDICTIONS

Pyrrole, N-methylpyrrole, furan, and thiophene have been first employed as model structures for a comprehensive evaluation of heteroaromatic reactivity in the gas phase, where complications due to the solvent, catalyst, counterion, etc., are minimized. Under such conditions, direct evaluation of the intrinsic reactivity properties of the selected heteroarenes is allowed and gas-phase kinetic data can be used with some confidence for testing the validity of the most advanced reactivity models, including Klopman's Charge and Frontier Orbital Control concept and Shaik and Pross' Curve Crossing model. On the basis of the quality of the correlations between the experimental kinetic data and the energy gap between the unperturbed orbitals of the donor and the acceptor, which in both models is a major factor determining reactivity, it emerges that Klopman's approach to the description of the gas-phase reactivity of heteroarenes toward ionic electrophiles can be thought as a special case of the more general Curve Crossing model. A quantitative application of Shaik and Pross' approach allowed to rationalize intrinsic heteroaromatic reactivity in terms of physically meaningful quantities, namely the vertical ionization potential of the donor and the vertical electron affinity of the acceptor.