Electrophilic aromatic substitution. Part V. Partial rate factors for protodesilylation of biphenylene: a novel polycyclic reactivity
Both trimethylsilyl derivatives of biphenylene have been prepared and their rates of protodesilylation by aqueous perchloric acid and methanol (2:5, v/v) at 50 °C measured spectrophotometrically giving the partial rate factors 27·8 (2-position) and 0·52 (1-position). The 2-position is close in reactivity to that of the 2-position of fluorene, as found previously in protodetritiation and pyrolysis of 1-arylethyl acetates. The deactivation at the 1-position contrasts with its considerable activation in detritiation, and this therefore constitutes the first example of a polycyclic aromatic position being deactivated or activated according to the demands for resonance stabilisation of the transition states.
The ratio of 2 : 1 substitution (53) is considerably greater than for the related benzocyclobutene (9·6) in accord with the prediction of theories which attribute this reactivity difference to strain in the four-membered ring. The similar reactivities of the 2-positions of fluorene and biphenylene is predicted by our theory but not by that of Streitwieser et al.