Hemiacetal anions: A model for tetrahedral reaction intermediates

Article abstract of DOI:10.1021/ja00003a012

A deprotonated hemiacetal ion is used as a model intermediate for nucleophilic addition reactions at a carbonyl group. The acidity of the cyclic hemiacetal 2-hydroxytetrahydropyran has been estimated to be ΔG^o_acid ≥ 351 ± 2 kcal/mol. The basicity of the deprotonated ion was found to be 347 ± 2 kcal/mol, different from the acidity by 4 kcal/mol. This difference is ascribed to an isomerization reaction in the ion. The electron affinity of the neutral radical corresponding to removal of an electron from the ion was measured with use of electron photodetachment spectroscopy and was found to be 49.6 ± 2.5 kcal/mol. The structure of the isomerized ion is assigned as a hydrogen bond stabilized enolate ion. The implications of the strong acidity of the hemiacetal for the stability of tetrahedral reaction intermediates are discussed. Thermochemical arguments suggest that tetrahedral adducts of this type are often global minima on the reaction potential surface. The addition reactions of different alkoxide-alcohol complexes with benzaldehyde are discussed in terms of the stability of the corresponding tetrahedral addition product.