Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VIII. The 1-position of biphenylene

Abstract

1-Biphenylen-1-ylethyl acetate has been prepared and its rate of pyrolysis (togther with that of 1-phenylethyl acetate) measured between 399·3 and 349·2 °C. The former ester eliminates 1·205 times faster than the latter at 625 K which leads to a σ⁺-value of –0·13 for the 1-position of biphenylene in this reaction. The weak activation of the 1-position is less than in hydrogen exchange but contrasts with protodesilylation where weak deactivation occurs. The reactivity in the pyrolysis is predicted by our theory attributing the differential reactivity of the α- and β-positions in biphenylene (and strained aromatic molecules in general) to bond strain in the transition state, and is counter to the prediction of the alternative theory based upon increased electronegativity of bridgehead carbons in strained rings.