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 (together 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 a t 625 K which leads to a a+-value of -0.1 3 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 6-and p-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.THE chemistry of biphenylene is interesting not only meant however that very few quantitative studies of because it is a relatively simple aromatic molecule about its electrophilic reactivity, for example, have been which comparatively little is known, but also because made. Previously, one of us showed that the 2-position it is a strained molecule and the manner in which this is much more reactive than the l-position in both strain affects the reactivity is important, especially protodetritiation and protodesilylation ,* and that since similar strains occur in a wide range of aromatic the latter position was weakly activated in the molecules.2 The inaccessibility of the derivatives has former reaction (fl = 104, p = ca. --8.5), and weakly