Nucleophilic substitution by a hydroxide ion at a vinylic carbon: Ab initio and density functional theory studies on methoxyethene, 3-methoxypropenal, 2,3-dihydro-4H-pyran-4-one, and 4H-pyran-4-one

Nucleophilic substitutions by a hydroxide ion at vinylic carbons of methoxy ethene (system A), 3-methoxypropenal (system B), 2,3-dihydro-4H-pyran-4-one (system C), and 4H-pyran-4-one (system D) were calculated by Becke's three -parameter hybrid density functional-HF method with the Lee Yang-Parr corre lation functional (B3LYP//B3LYP) and the second-order Moller-Plesset theory (MP2//B3LYP) using the 6-31+G(d) and AUG-cc-pVTZ basis sets. In addition, bulk solvent effects (aqueous solution) were estimated by the polarized con tinuum (overlapping spheres) model (PCM-B3LYP//B3LYP) and the polarizable c onductor PCM model (CosmoPCM-B3LYP//B3LYP). The mechanisms as well as the i nfluence of resonance, cyclic strain, aromatic, and polar effects on the re activity of the calculated systems were determined. In the gas phase the ra te-determining step of nucleophilic vinylic substitutions by a hydroxide io n may be either addition of hydroxide ion at the vinylic carbon (systems A and B) or elimination of the leaving group (systems C and D). In aqueous so lution, for all four systems investigated, addition of hydroxide ion at the vinylic carbon is rate determining.