Branched Polyphenylenes by Repetitive Diels−Alder Cycloaddition

Diels−Alder reactions between 1,4-bis(2,4,5-triphenylcyclopentadienone-3-yl)benzene and either phenylacetylene (model reaction) or 1,4-diethynylbenzene (polymer formation) were studied. NMR spectra suggest that the main product in the model reaction is the m,m-isomer (up to 83% yield). X-ray crystal structure analysis convincingly proved the structure of the above isomer. The polymer-forming reaction was carried out using different concentrations of the monomeric building block and different reaction times. As a result, branched polyphenylenes with Mw in the range of 1.2 × 104−12 × 104 g mol-1 were obtained. Both the model compound and the polymers were subjected to intramolecular oxidative cyclodehydrogenation with copper(II) trifluoromethanesulfonate and aluminum chloride. According to LD-TOF mass spectrometry, the cyclodehydrogenation of the model compound afforded the planarized polycyclic aromatic hydrocarbon C66H26. This polycyclic aromatic compound was isolated in 91% yield. The extended π-conjugation and ordering of cyclodehydrogenated products were demonstrated by Raman spectroscopy.