Formation and characterization of the radical cation of pentamethylbenzyl trifluoroacetate from the oxidation of hexamethyl (Dewar benzene) by thallium(III) trifluoroacetate in trifluoroacetic acid—a slow and complex reaction

Abstract

The reaction between hexamethyl (Dewar benzene)(HMD) and Tl^III trifluoroacetate (Tl^III) in trifluoroacetic acid has been investigated in detail. The first step involves a slow acid-catalysed conversion of HMD into hexamethylbenzene (HMB) which is then oxidized by Tl^III to pentamethylbenzyl trifluoroacetate in a process which overall is about 15 times faster than the HMD → HMB reaction. In the formal one-electron transfer reaction between HMB and Tl^III, the corresponding radical cation, HMB˙⁺, appears in significant concentration and with a lifetime which makes it easy to monitor by EPR spectroscopy. It is shown that the true lifetime of HMB˙⁺ in trifluoroacetic acid is shorter by a factor of ≈0.03 (22 °C) or ≈0.005 (–11 °C) than the ‘decay’ lifetime recorded during an experiment in which it is generated by the reaction between HMB and Tl^III under otherwise identical conditions. Thus the ‘decay’ rate constant of HMB˙⁺ is actually a reflection of its slow rate of formation from HMB-Tl^III. The fact that the formal one-electron transfer reaction between HMB and Tl^III exhibits a significant kinetic isotope effect of both substrate and solvent type, indicates that this apparently simple step must be complex.

Pentamethylbenzyl trifluoroacetate, as well as the corresponding acetate, alcohol, methyl ether or chloride, exhibits a characteristic 13 × 8 line EPR spectrum when irradiated in trifluoroacetic acid with Tl^III trifluoroacetate at –11 °C. The similarity of this spectrum to the previously described, less well-resolved 13 line EPR spectrum from the oxidation of HMD in matrices at low temperatures or on a solid substrate, is profound. The fact that HMD on oxidation by Tl^III in trifluoroacetic acid gives the 13 × 8 line spectrum of pentamethylbenzyl trifluoroacetate, indicates that the transformation of HMD into a suitable pentamethylbenzyl derivative might be the origin of the 13-line EPR spectrum recorded by the matrix technique.