Experimental Verification of the Homoaromaticity of 1,3,5-Cycloheptatriene and Evaluation of the Aromaticity of Tropone and the Tropylium Cation by Use of the Dimethyldihydropyrene Probe

By use of a dimethyldihydropyrene experimental probe
for aromaticity,
1,3,5-cycloheptatriene (16) is demonstrated to be a neutral
homoaromatic hydrocarbon! On the basis of 1H NMR results, 16 is judged to be ∼30%, tropone 18 ∼20%,
and tropylium 22 ∼50% as aromatic as benzene.
The latter result may be an underestimation because of charge delocalization.
The B3LYP/6-31G* calculated geometries and GIAO-HF/6-31G*//B3LYP/6-31G*
calculated NMR chemical shifts and nucleus-independent chemical shifts
(NICS) support these conclusions. These estimates were obtained by
synthesis of the annelated dihydropyrenes 7 (tropone
fused), 9 (1,3,5-cycloheptatriene fused), and 10 (tropylium fused). [4 + 3] Cycloaddition of the isofuran 5 with an oxyallyl cation (prepared from 2,4-dibromopentan-3-one)
gave the C7 fused dihydropyrene 6 in 77% yield. Elimination
of water gave tropone 7 in 61% yield, which, via LiAlH4 reduction to alcohol 8 (48% yield) and treatment
with HBF4, gave quantitatively tropylium cation 10. When ketone 7 was reduced with AlH3 (generated
from AlCl3/LiAlH4) in ether/benzene at 25 °C,
the isomeric cycloheptatrienes 11 (70% yield) and 9 (15% yield) were obtained.