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+BF4– and Br2–SO2

Abstract

Sterically congested (Z)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene 1 is ring protonated in FSO₃H·SbF₅(1 : 1)-SO₂ClF or in FSO₃H·SbF₅(4:1 )–SO₂ClF superacid media and undergoes a rapid transannular cyclization eventually leading to mixtures of persistent phenanthrenium mono- and dications. The cofacial relationship of the phenyl groups is required for phenanthrenium ion formation. Low-temperature protonation of the corresponding (E)-stilbene 2gives 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 Br₂–SO₂ led to disubstitution at the para positions in both cases. The arenium ion precursors could not be observed. The (Z)-dinitro- and (Z)-dibromo-stilbenes are stable compounds.