The influence of structural changes on reaction outcomes can be quantitatively described through the utilization of physical organic parameters in linear free energy relationships. In a pioneering work by Hammett, the acid dissociation constants of benzoic acids were used to obtain a set of values, called Hammett values, which account for electronic effects of various substituents on aromatic rings. Following this effort, Hammett values have been a standard electronic parameter to describe aromatic ring substituents in a wide array of chemical reactions. The infrared carbonyl stretching frequency and NBO charge of carbonyl oxygen of para-substituted benzoic acids have been previously established as alternatives to the Hammett value. We have compiled an extended set of these computationally-derived IR carbonyl stretching frequencies and carbonyl oxygen NBO charges to eliminate the dependence on empirically obtained Hammett value and allow a straightforward evaluation of electronic influence from a particular aromatic ring substitution. A major limitation of Hammett parameter is its inability to account for electronic effects from ortho-substituents due to the presence of proximal steric effects which can disrupt the conjugation between the carbonyl and the arene. To account for the influence of ortho steric effects as well as remote steric effects on reaction outcome (site selectivity and enantioselectivity), the computationally-derived Hammett alternatives in combination with Sterimol steric parameter were then applied in a multivariate linear regression strategy.

Electronic and steric descriptors for aryl substituents: Revisiting the ortho effect

253rd National Meeting (2017)

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