Scrambling of the Label in [13C2] Biphenylene Obtained on Pyrolysis of [1,6,13C2] and [2,3,13C2]-2-(3',3'-Dimethyltriazenyl)\benzoic Acid

Abstract

Flash vacuum pyrolysis of 2-(3′,3′-dimethyltriazenyl)benzoic acid (6) above 350? gives biphenylene and N,N- dimethylaniline. The acid (6) was synthesized with 13C2 labels placed as in (13) to generate [1,6- 13C2] benzyne. F.v.p . of labelled acid (13) from 400-850° gave biphenylene samples which showed 13C spectra consistent with an increasing extent of carbon rearrangement from [1,6-13C2] benzyne to [1,3-13C2] benzyne with increasing temperature. This is in accord with direct rearrangement of benzyne (Scheme 1) but it is probable that more than one mechanism is operating. The labelling evidence suggests that N,N- dimethylaniline is formed by re-addition of dimethylamine to benzyne.