A paradigm shift in rate determining step from single electron transfer between phenylsulfinylacetic acids and iron(III) polypyridyl complexes to nucleophilic attack of water to the produced sulfoxide radical cation: a non‐linear Hammett

Abstract

Mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAAs) by iron(III) polypyridyl complexes in aqueous acetonitrile medium has been investigated spectrophotometrically. 3+ is rationalized on the basis of coordination of a water molecule at the carbon atom adjacent to the ring nitrogen of the metal polypyridyl complexes by nucleophilic attack at higher concentrations. Electron-withdrawing and electron-releasing substituents in PSAA facilitate the reaction and Hammett correlation gives an upward 'V' shaped curve. The apparent upward curvature is rationalized based on the change in the rate determining step from electron transfer to nucleophilic attack, by changing the substituents from electron-releasing to electron-withdrawing groups. Electron-releasing substituents in PSAA accelerate the electron transfer from PSAA to the complex and also stabilize the intermediate through resonance interaction leading to negative reaction constants (ρ). Conversely, electron-withdrawing groups, while retarding the electron transfer exert an accelerating effect on the nucleophilic attack of H 2 O which leading to low magnitude of ρ + compared to high ρ À values of electron-releasing groups. Marcus theory is applied, and a fair agreement is seen with the experimental values.

Reductive N-methylation of amines using dimethyl carbonate and molecular hydrogen: Mechanistic insights through kinetic modelling

“…Hammett plots shown in Supporting Information of [11] and redrawn here illustrate an upward concave behaviour (Figure 3) apart from the case of fluorine as a substituent. In general, non-linear Hammett plots with upward and downward concave behaviour have been reported in the literature [29][30][31][32][33][34][35]. While concave downwards plots can be explained by a complex reaction mechanism composed of a reversible and a subsequent irreversible step [36] upwards concave Hammett plots cannot be explained by changing the rate determining step in a multistep mechanism [37].…”