Structurally characterized hetero-oligopolyphenylenes: Synthetic advances toward next-generation heterosuperbenzenes

The successful Diels-Alder [2+4] cycloaddition([1]) of dipyrimidyl acetylene and suitably substituted 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones (3-7) generates a series of selectively functionalized hexaarylbenzenes. Each has two pairs of peripheral functional groups (R' and R=tert-butyl 8 and R=methyl 9, methoxy 10, bromo 11, triisopropylsilylethynyl 12) and four ortho-imine nitrogen atoms. The dibromo derivative 11 is a useful precursor for the formation of a mono ethynyl 13 and diethynyl 14 substituted polyphenylene. Changing the dienophile to di(2-thienyl)acetylene gives an S-heteroatom polyphenylene 15. The compounds were fully characterized by using H-1, C-13 and a range of 2D NMR spectroscopic techniques, elemental analysis, and mass spectrometry. Oxidative cyclodehydrogenation of dimethoxy hexaphenylbenzene 10 by using iron(m) chloride results in the formation of a spirocyclic dienone 16, which in a separate reaction undergoes dienone/ phenol rearrangement to give the first 4-fused-ring, N-heterosuperbenzene (HSB) 17. Six single crystal molecular structures reveal the commonality of unidirectional twisting of the external aromatic rings in these heteroatom polyphenylenes. The twist angles and any H-bonding or interdigitation in these structures are discussed.