The cyclopropa-arenes cyclopropabenzene'' -31 and cyclopropa [b]naphthale~~e[~! are relatively stable compounds without pronounced bond fixation, although the presence of the threemembered ring gives rise to extensive strain and distortion of the benzene rings[5. 6!. It therefore appeared conceivable that cyclopropa-arenes having more than one three-membered ring might also be capable of existence, e. g. 1,3-dihydrodicyclopropa [a,d]benzene ( I ) and 1,4-dihydrodicyclopropa[b,g]naphthalene (2). As we found, at least compound (2) is isolable.The cis-trans isomeric 4,4,10,lO-tetrabromotetracycl0[5.5.0.0~~~.0~~' ']dodec-l(7)-enes [(3a) and (36) respectively] which we encountered in the synthesis of heptalene~ [~] provided us with an opportunity of directly obtaining compound (2) by the action of strong bases (Billups' methodL2]). On treatment of (3a) or (3b) with a tenfold excess of potassium tert-butoxide in tetrahydrofuran (room temperature, 4 h), clean dehydrohalogenation does in fact take place, forming the desired compound (2) in both cases. The new cyclopropa-arene was obtained after chromatography on alumina (elution with pentane) and subsequent recrystallization from ether as slightly yellow tinted rhombic crystals (yield 30 and 33 %, respectively) which undergo explosive decomposition at 132--133°C. Since the crystals are also sensitive to shock their handling requires caution. is sensitive to acidic reagents"'! As an example we cite the extremely fast reaction of (2) with acetic acid in the presence of Age ion, which leads expectedly to a mixture of 2,6-and 2,7-naphthalenedimethyl diacetate. of strain energy and resonance loss) compared with the strainfree aromatic model[61. Hence it can be reasonably assumed that the corresponding destabilization energy will considerably exceed lOOkcal/mol in the case of (2). Since (2) itself already tends to undergo explosive decomposition, it may well be that the limit of existence at room temperature has been reached in ( I ) , where the possibility of a certain internal equalization of strain is even less favorable than in (2). 12, 762 (1973).[9] J . R. Plafr. J. Chem. Phys. 17, 484 (1949). [lo] See the review by B. Halron, Chem. Rev. 73, 113 (1973). The still unknown Ion-heterocycle 1P-dithiocin (2), which is to be regarded as a potential aromatic compound according to the Hiickel rule, has been the goal of several synthetic attempts[" 21 and the subject of quantum mechanical calculat i o n~[~. 41 during recent years. Whereas the experimental work on (2) is not very informative with regard to its a-electron structure, Hiickel calculations-employing Coulomb and resonance integrals derived from thermodynamic data for divalent sulfur-predict, contrary to expectations, that (2) will not possess pronounced electronic stability[4!. As already discussed previouslyr5~ 6!, symmetry-allowed valence isomerization (n2, + u2s + .2, process) of syn-benzene bisepisulfide ( 1 )constitutes an apparently compelling synthetic route to (2), provided that the eight-membered heterocy...
