MP2 and DFT calculations on circulenes and an attempt to prepare the second lowest benzolog, [4]circulene

Abstract

MP2 and DFT calculations have been carried out for [n]circulenes for n=3 to 20 in order to predict the strain energy and topology of these cyclically condensed aromatic systems. To synthesise [4]circulene (2), 1,5,7,8- tetrakis(bromomethyl)biphenylene (14) was prepared from the corresponding tetramethyl derivative (8) and subjected to various dehalogenation reactions; all attempts to obtain [2.2]biphenylenophane (7) as a precursor for 2 by this route failed. Treatment of 14 with sodium sulfide furnished the thiaphanes 16 and 17, thermal and photochemical desulfurization of which also failed to provide 7. In a second approach [2.2]paracyclophane was converted to the pseudo-geminal dithiol23, which was subsequently bridged to the thiaphanes 22 and 24. On flash vacuum pyrolysis at 800°C these were converted exclusively into phenanthrene (30). An approach to dehydrochlorinate the commercial product PARYLENE C® to the tetrahydro[4]-circulene 7 led only to polymerisation. The X-ray structures of the intermediates 8, 14, 17, 23, 24, 26, and 35 are reported. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.