Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Three
N-substituted tetracyclo­(2,7-carbazole)­s were synthesized to investigate
the inner regions of nanohoops. One compound has a 5,5-dimethylnonane
bridge between two neighboring anti-carbazoles, which
can be used as covalently bonded “methane probes”. These
probes near the ring center are strongly shielded by local ring currents
and exhibit a singlet at δ = −2.70 ppm in 1H NMR. To visualize local and macrocyclic ring currents separately,
we drew nucleus-independent chemical shift contour maps of tetracyclo­(9-methyl-2,7-carbazole)
and [n]­cycloparaphenylenes (CPPs). Local ring currents
make the interior diatropic, and paratropic regions exist only outside
the ring. Macrocyclic ring currents in [5]­CPP to [7]­CPP generate deshielding
cones, which are typical of antiaromatic [4n]­annulenes.