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

The reaction between hexamethyl (Dewar benzene) (HMD) and T1(III) trif luoroacetate (T1(III)) in trifluoroacetic acid has been investigated i n detail. The first step involves a slow acid-catalysed conversion of HMD into hexamethylbenzene (HMB) which is then oxidized by T1(III) to pentamethylbenzyl trifluoroacetate in a process which overall is about 15 times faster than the HMD --> HMB reaction. In the formal one-elec tron transfer reaction between HMB and T1(III), the corresponding radi cal cation, HMB(.+), appears in significant concentration and with a l ifetime which makes it easy to monitor by EPR spectroscopy. It is show n that the true lifetime of HMB(.+) in trifluoroacetic acid is shorter by a factor of approximate to 0.03 (22 degrees C) or approximate to 0 .005 (-11 degrees C) than the 'decay' lifetime recorded during an expe riment in which it is generated by the reaction between HMB and T1(III ) under otherwise identical conditions. Thus the 'decay' rate constant of HMB(.+) is actually a reflection of its slow rate of formation fro m HMB-T1(III). The fact that the formal one-electron transfer reaction between HMB and T1(III) exhibits a significant kinetic isotope effect of both substrate and solvent type, indicates that this apparently si mple step must be complex. Pentamethylbenzyl triffluoroacetate, as wel l as the corresponding acetate, alcohol, methyl ether or chloride, exh ibits a characteristic 13 x 8 line EPR spectrum when irradiated in tri fluoroacetic acid with T1(III) trifluoroacetate at -11 degrees C. The similarity of this spectrum to the previously described, less well-res olved 13-line EPR spectrum from the oxidation of HMD in matrices at lo w temperatures or on a solid substrate, is profound. The fact that HMD on oxidation by T1(III) in trifluoroacetic acid gives the 13 x 8 line spectrum of pentamethylbenzyl trifluoroacetate, indicates that the tr ansformation of HMD into a suitable pentamethylbenzyl derivative might be the origin of the 13-line EPR spectrum recorded by the matrix tech nique.