ELECTROPHILIC CHEMISTRY (PROTONATION, NITRATION, BROMINATION) OF CROWDED (Z)-2,2,5,5-TETRAMETHYL-3,4-DIPHENYLHEX-3-ENE - FORMATION OF PHENANTHRENIUM IONS BY FACIAL RING PROTONATION TRANSANNULAR CYCLIZATION IN SUPERACID MEDIA - P,P-DINITRATION AND P,P-DIBROMINATION WITH NO2(-) AND BR-2-SO2()BF4()

Sterically congested (Z)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene 1 i s ring protonated in FSO3H.SbF5(1:1)-SO2ClF or in FSO3H.SbF5(4:1)-SO2C IF superacid media and undergoes,a rapid transannular cyclization even tually leading to mixtures of persistent phenanthrenium mono- and dica tions. The cofacial relationship of the phenyl groups is required for phenanthrenium ion formation. Low-temperature protonation of the corre sponding (E)-stilbene 2 gives Bu(t+) as the main product and a complex mixture of aromatic products. Low-temperature nitration of 1 with NO2 +BF4--SO2 and bromination with Br-2-SO2 led to disubstitution at the p are positions in both cases. The arenium ion precursors could not be o bserved. The (Z)-dinitro- and (Z)-dibromo-stilbenes are stable compoun ds.