Observation of 13C rearrangement in [13C2]biphenylene formed from benzyne on pyrolysis of [1,6-13C2]phthalic anhydride and [2a,3-13C2]benzocyclobutenedione

Abstract

Examination of [13C2]biphenylene formed by gas phase pyrolysis of doubly labelled benzyne precursors shows that the principal pyrolytic process leads to overall 1,2→1,3 rearrangement of the C6H4 carbon skeleton either in an intermediate C7H4O before decarbonylation or in benzyne itself. A minor process involves an apparent 1,3-hydrogen shift. [1,2-13C2]Ethyne-1,2-diylbistrimethylsilane was acylated with 3-(2,5-dihydro-1,1-dioxothien- 2-yl)propanoyl chloride and the resulting ketone was desilylated to yield 5-(2,5-dihydro-1,l-dioxo-thien-2-yl)[1,2-13C2]pent-1-yn-3-one. Thermal elimination of sulfur dioxide and cyclization followed by dehydrogenation yielded [7,7a-13C2]-2,3-dihydro-1H-inden-1-one which was oxidized and dehydrated to give [3a,4-13C2]isobenzofuran-1,3-dione. This doubly labelled phthalic anhydride was diluted to approximately 5% 13C2 and the resulting material was converted via benzenediazonium- 2-carboxylate into biphenylene at 84�, and pyrolysed at 830� to yield biphenylene, and a sample diluted to 7.5% was converted into [2a,3-13C2]benzocyclobutenedione which was pyrolysed at 650�, 750� and 830� to yield further samples of biphenylene. The biphenylene samples were examined by mass spectrometry at 20 eV to determine their isotopic composition and by 13C n.m.r. spectroscopy to determine the distribution of labelling.