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 H-1 NMR results, 16 is judged to be similar to 30%, tropone 18 similar to 20%, and tropylium 22 similar to 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 LiAIH(4) 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 degrees C, the isomeric cycloheptatrienes 11 (70% yield) and 9 (15% yield) were obtained.