THERMAL AND PHOTOCHEMICAL-REACTIONS OF AROMATIC-COMPOUNDS IN TRIFLUOROACETIC-ACID, INVOLVING CARBOCATIONS AND RADICAL CATIONS

Upon dissolution of 2,2',3,3',4,4',5,5',6,6'-decamethyldiphenylmethane (1) in trifluoroacetic acid (TFA), a deep-seated transformation takes place with eventual formation of 1,2,3,4,5,6,7,8-octamethylanthracene (OMA) and its radical cation. The formation of OMA is initiated by the establishment of an equilibrium involving proton-induced cleavage of 1 to give pentamethylbenzyl cation and pentamethylbenzene. Species 1-H+ undergoes intramolecular methyl transfer to pentamethylbenzene, to give a nonamethyldiphenylmethane with one free ortho position (NMDM) and hexamethylbenzene. Pentamethylbenzyl cation then acts as a hydride-transfer oxidant toward NMDM (at the 2-methyl position), yielding 9,10-dihydro-OMA, from which OMA is obtained by a second hydride-transfer step and proton loss. The oxidation of OMA to OMA.+ in TFA is light-dependent, most likely by excitation of OMA to 1OMA* followed by electron transfer to the TFA dimer. If electron transfer is synchronous with or very rapidly followed by proton transfer within the dimer, the back electron transfer step is blocked. As an experimentally convenient model, the electron transfer reaction between 9,10-dimethyl-2,3,6,7-tetramethoxyanthracene and TFA was studied in detail, showing that this process is strongly dependent on light. Under dark conditions, one can also detect a thermal component of the electron transfer reaction, but it is very slow (k = 10(-6)-10(-7) M-1 s-1) and not definitively established.