carbo-Naphthalene: A polycyclic carbo-benzenoid fragment of alpha-graphyne
Résumé
A ring carbo-mer of naphthalene, C32Ar8 (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, C18Ar6 and o-C18Ar4(C C-SiiPr(3))(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 H-1 nuclei of the Ar groups and on the negative value of the DFT-calculated NICS at the center of the C-18 rings (-12.8 ppm). The stability and aromaticity of this smallest fused molecular fragment of alpha-graphyne allows prediction of the same properties for the carbon allotrope itself.