Up to six units of charge and twist-boat benzene moieties: Alkali metal reduction of phenyl-perisubstituted benzenes

Phenyl-perisubstituted benzenes, tetraphenylbenzene (1) and hexaphenylbenze ne (2), were reduced by lithium and sodium metal in THF-d(8) under high vac uum. The reduction process and the nature of the reduction products were st udied by NMR. Tetraphenylbenzene was reduced by both metals to yield the co rresponding dianionic salt. It was found that the addition of extra charge into the system, restricted the free rotation of the four phenyl substituen ts about the sigma bond connecting them to the central ring (G(181)(double dagger) = 7.8 +/- 0.2 kcal mol(-1)). The reaction of the alkali metals with 2 yielded four diamagnetic species: the first three were assigned to the d ianion, tetraanion, and the surprising hexaanion of 2. These species were c alculated using density functional theory (DFT) and were found to have cent ral benzene rings with an unusual twist-boat geometry. Computational and ex perimental evidences show that each phenyl ring and its attached carbon of the central ring behave like a benzyl anion. We therefore view the hexaanio n of 2 as a cyclohexa(benzylanion). The fourth diamagnetic species was a pr oduct of a double-sided cyclization, which yielded the dianion of dihydro-9 , 18-diphenylphenanthro[9,10-b]triphenylene (9,18-diphenyltetrabenz[a,c,h,j ] anthracene dianion, 3(2-)). Reaction of the dianion with oxygen gave 3 in improved yields compared to literature preparations.