6,12-Diphenyldibenzo[b,f][1,5]diazocine as an Electron-Capture Agent: Efficient Mechanistic Probe for SET Processes and Reagent for the Oxidative Dimerization of Benzylic Organometallics

In the present study, 6,12‐diphenyldibenzo[b,f][1,5]diazocine, which X‐ray diffraction measurements have now shown to possess a tub‐shaped, eight‐membered central ring, has been treated with sodium or lithium metal at 25 °C in THF, in an attempt to form the planar, Hückel‐aromatic dianion by the addition of two electrons to the central diazocine. Hydrolysis of such an aromatic dianion should have led to the isomeric 5,12‐ or 5,6‐dihydro derivative of the original diazocine. In actuality, the only product obtained quantitatively upon hydrolytic workup was the interesting quadricyclic transannular reduction product, 4b,9b‐diphenyl‐4b,5,9b,10‐tetrahydroindolo[3,2‐b]indole, whose 3D structure has now been confirmed by X‐ray crystallography and 13C NMR spectroscopy. Preferential SET transannular reduction of the diazocine to yield the quadricyclic indolo[3,2‐b]indole dianion, rather than the planar, Hückel‐aromatic anion, is ascribed to the transannular electronic stabilization operative in the tub‐shaped diazocine radical‐anion. The quantitative generation of the indolo[3,2‐b]indole dianion can be employed for the oxidative dimerization of the organic groups in benzylic lithium reagents. Thus treating one equivalent of the diazocine with two equivalents of benzyllithium, benzhydryllithium, or trityllithium yields quantitatively bibenzyl, 1,1,2,2‐tetraphenylethane, or (4‐benzhydrylphenyl)triphenylmethane, respectively. This oxidative dimerization is potentially of practical preparative scope, since the hydrolysis byproduct, the indolo[3,2‐b]indole, is conveniently reconverted into the starting diazocine reagent by oxidation with chromium trioxide in acetic acid. The formation of the indolo[3,2‐b]indole as a byproduct in the carbometalation of the diazocine by various RLi and Grignard reagents offers a clue as to the SET mechanism of carbometalation. 6,12‐Diphenyldibenzo[b,f][1,5]diazocine readily undergoes transannular SET reductions with sodium, lithium, or even benzylic lithium or magnesium reagents to yield, upon hydrolysis, 4b,9b‐diphenyl‐4b,5,9b,10‐tetrahydroindolo[3,2‐b]indole. This diazocine can thereby serve as a mechanistic probe for SET processes and as a reagent for the oxidative dimerization of benzylic organometallics.