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Abstract: A theoretical study of the mechanism, regio- and stereoselectivity of Diels–Alder reactions of 5-aryl-2-pyrones with substituted alkenes is carried out at the B3LYP/6-31G(d) level of theory. The PES (potential energy surface) analysis and IRC (intrinsic reaction coordinate) calculations show that the studied reactions are undergoing through concerted asynchronous mechanisms yielding to the formation of the 5-endo compound as the major cycloadduct. The calculation of activation energies and reaction energies indicate that the 5-endo cycloadduct is favored both kinetically and thermodynamically. The experimental ratios of regioisomers (5/6) and stereoisomers (endo/exo) are correctly estimated at the MP2/6-31G(d)//B3LYP/6-31G(d) level of theory. [Copyright &y& Elsevier]

Abstract: Theoretical calculations are carried out to predict gas- and aqueous-phase acidities of a series of 21 carbon acids with pK a values varying from −6.20 to 50. Acceptable linear correlations (R 2 >0.93, SD<4) are obtained between calculated deprotonation Gibbs free energy changes and experimental pK a values (measured in water). Solvent effects are taken into account by means of the polarizable continuum model (PCM). Our calculations also show that the high acidity of α-proton carbonyl compounds and electron-withdrawing substituted methanes can be related to the strong increase in the vicinal charge interactions n c → π ∗ in anion species. Calculations are performed at the B3LYP/6-311++G(d,p) level of theory. [Copyright &y& Elsevier]