Contribution of Pyrone-Type Structures to Carbon Basicity:  An ab Initio Study

The contribution to carbon basicity of pyrone-type structures is examined in this work by carrying out ab initio calculations on various cluster models. The different basic sites in a bicyclic pyrone structure are studied at the MP2/6-311+G(2d,2p)//MP2/6-31G(d) level of theory, rendering a reaction energy of −84.2 kcal/mol when a proton transfer takes place from H 3 O + to the carbonylic oxygen in pyrones. The interpretation of the theoretical results confirms that resonance stabilization is a crucial factor controlling the basic character of pyrone-type structures. Various effects, like the modification of the etheric position, the relative position of non-neighboring oxygen atoms, and the presence of an adjacent basal plane, are also taken into account. Due to the increase of resonance stabilization observed in large cluster models, a broad spectrum of base strength covered by pyrone-type structures is predicted. The reaction energies of these pyrone-type structures with H 3 O + are a few kilocalories per mole more exoergic than that of quinoline, an illustrative organic base. These results support the outstanding role played by pyrone-type structures in carbon basicity.