Substituen Effect of Chelated Cobalt. 5. Acidities of (Carboxymethyl)- and (1-Carboxyethyl)cobaloximes. A Quantitative Analysis of the β Effect

Article abstract of DOI:10.1021/ja00379a013

pK_a's of the weakly acidic (carboxymethyl)(ligand)cobaloximes and (1-carboxyethyl)(ligand)cobaloximes with 16 different axial ligands have been determined and correlated with those of 11 substituted acetic acids or 9 1-substituted propionic acids, respectively.Comparison of apparent ?₁ values thus calculated with those previously determined by correlation of (carboxyethyl)(ligand)cobaloxime pK_a's with the pK_a's of 2-substituted propionic acid indicates that the (1-carboxyalkyl)cobalt comlexes show a substantial β effect as an apparent extreme donation density to the carboxyl carbon.The β effect in these complexes has been quantitated by use of a dual substituent parameter equation, the results of which show that the effect is only some 8-10percent enhanced in (1-carboxyethyl)cobaloximes relative to (carboxymethyl)cobaloximes.This result is consistent with the β effect being mediated by ?-? hyperconjugation rather than neighboring group participation.Structural effects on the extent of ?-? conjugation and the effects of ?-? conjugation on reactivity of the cobalt atom in these complexes are discussed.