carbo-Naphthalene: A Polycyclic carbo-Benzenoid Fragment of α-Graphyne.

A ring carbo-mer of naphthalene, C32 Ar8 (Ar=p-n-pentylphenyl), has been obtained as a stable blue chromophore, after a 19-step synthetic route involving methods inspired from those used in the synthesis of carbo-benzenes, or specifically devised for the present target, like a double Sonogashira-type coupling reaction. The last step is a SnCl2 /HCl-mediated reduction of a decaoxy-carbo-decalin, which is prepared through successive [8+10] macrocyclization steps. Two carbo-benzene references are also described, C18 Ar6 and o-C18 Ar4 (C≡C-SiiPr3 )2 . The carbo-naphthalene bicycle is locally aromatic according to structural and magnetic criteria, as revealed by strong diatropic ring current effects on the deshielding of 1 H nuclei of the Ar groups and on the negative value of the DFT-calculated NICS at the center of the C18 rings (-12.8 ppm). The stability and aromaticity of this smallest fused molecular fragment of α-graphyne allows prediction of the same properties for the carbon allotrope itself.