Kinetics and Mechanism of Aromatic Thallation. Identification and Proof of Competiting Electrophilic and Electron-Transfer Pathways

Article abstract of DOI:10.1021/ja00335a039

The unusual occurrence of simultaneous electrophilic (two-electron) and electron-transfer (one-electron) pathways during the thallation of the homologous methylbenzenes ArCH3 is demonstrated by (1) the careful analysis and identification of three major types of products, (2) the complete dissection of the complex kinetics, and (3) the identification of the reactive intermediates by time-resolved UV-vis and ESR spectroscopy.Side-chain substitution S, dimerization D, and oxidative nuclear substitution O derive from the radical cation ArCH3^+. produced as a common intermediate by electron transfer from the methylbenzene to thallium(III) trifluoroacetate in trifluoroacetic acid.The importance of ArCH3^+., which is detected by both its electronic and ESR spectra, decreases in the following order, hexamethylbenzene > pentamethylbenzene > durene >> mesitylene, with a concomitant rise in electrophilic nuclear thallation R to account for the complete material balance.The striking color changes that accompany thallation are identified as charge-transfer transition in the series of transient 1:1 ?-complexes of the methylbenzene donors and the thallium(III) acceptor.Quantitative spectrophotometry employing the Benesi-Hildebrand analysis establishes the cationic Tl(O2CCF3)2⁺ formed by the dissociation of a single trifluoroacetate ligand from the parent thallium tris(trifluoroacetate) as the active electron acceptor.The complete analysis of the complex kinetics including kinetic isotope effects with accompany the nuclear thallation R of mesitylene as well the side-chain substitution S of hexamethylbenzene shows that the cationic Tl(O2CCF3)2⁺ also serves the dual function as the active electrophile and the active oxidant, respectively.The close competition between these apparently disparate pathways is quantitatively evaluated by the second-order rate constants which differ by less than an order of magnitude.Therefore, the thallation of arometic hydrocarbons represents one of the few systems in which such dual pathways, electrophilic and free radical, apparently occur side under the same experimental conditions of solvent, temperature, etc.Accordingly, it represents an unusual opportunity to delineate two-electron (concerted, electrophilic) from one-electron (stepwise, free radical) mechanism-especially as two whether they represent parallel or sequential events.