Our research continues to focus on problems in the general area of physical organic chemistry. As this is written, there are two ongoing projects in our group.
In the first of these projects, the syntheses, stabilities and reactivities of various diphenylamines and their corresponding nitranions are being evaluated.
Syntheses of Substituted Diphenylamines
Using a scheme that involves nucleophilic aromatic substitution reactions, three families of diphenylamines (1, 2, and 3, where G is any remote substituent) have been prepared.
The reactions used to generate 1, 2, and 3 involved nucleophilic aromatic substitutions utilizing nitranions derived from G-substituted anilines as nucleophiles and variously substituted benzenes as electrophiles. During these investigations, an interesting dependence on leaving group was discovered...examples of which are shown in the graphic below eq 1. Further research into these reactions is warranted and underway in our laboratory.
Stabilities of Nitranions Derived from Diphenylamines
Acidity data for 1 - 3 indicate that the resonance saturation effect attenuates the effects of substituents on the pKa's of 1 - 3, and therefore also attenuates the effects of substituents on the stabilities of the nitranions derived from these compounds. Specifically, the Hammett r values for 1 - 3 are 4.3, 2.6, and 1.8, respectively. Additional investigations of the phenomena of resonance saturation are ongoing.Reactivities of Nitranions Derived from Diphenylamines
Kinetic data for the nitranions derived from 1 - 3, when allowed to react with SN2-type electrophiles, indicate substantial solvent assisted resonance effects for G-substituents such as -NO2 and -CN. These effects retard reaction rates, and complicate interpretation of linear free energy relationships in the context of the generality of the Hammond Postulate and the Reactivity-Selectivity Principle. We are currently examining other reaction families that may shed light on the question of the Hammond Postulate and RSP.In the second ongoing project, the concept of dipole stabilization is being examined, particularly as it pertains to carbanion and nitranion stabilities.
Synthetic chemists have long utilized dipole stabilized anions as useful synthons. A simple explanation of dipole stabilized anions is shown below, where resonance between X and Y creates positive charge immediately adjacent to the negative charge present on Z.
In other words, protons on Z are more acidic than expected due to the influence of the incipient dipole.
In preliminary pKa experiments using DMSO as solvent and K+ as the counterion, we have determined that approximately 75% of what is generally referred to as dipole stabilization can instead be attributed to the inductive effect of X (where X=O). This estimate relies upon data for analogous molecules in which Y = N (an atom in which dipole stabilization is possible) and Y = C (where dipole stabilization is forbidden).
In an effort to make these investigations more real-world like, future acid-base investigations are planned in which ethereal solvents and Li counterions are employed...reaction systems more like those utilized by most practicing synthetic organic chemists.