Magic Blue-Promoted Synthesis of a Highly Planar Tetraphenylbenzidine: Evaluation of Its Fundamental Properties and Unraveling the Intriguing Mechanism for Its Formation

Magic blue [MB, (p-BrC6H4)3N•+SbCl6–]-promoted oxidation of the tetracyclodecane derivative mm-1 containing two triphenylamine (TPA) moieties and subsequent reduction using triethylamine results in formation of the uncharged pentacyclic product 4 that possesses a novel, cage-framework-constrained tetraphenylbenzidine (TPB) moiety. This process follows an electron-transfer mechanism involving formation of the intermediate radical cation 4•+ through intramolecular aryl coupling of a di(radical cation) mm-12(•+). The TPB biphenyl type conjugated moiety in 4 is highly planar. Moreover, 4 has a higher energy HOMO and greater electrochemical stability than does the parent TPB. Notably, because it undergoes a fully reversible two-step–two-electron oxidation-reduction reaction, 4 is an ideal electron donor for use in electronic devices. Finally, conversion of 4•+ to the corresponding dication 42+ by SbCl6–-promoted "auto"oxidation is described.