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

Abstract

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 LiAlH 4 reduction to alcohol 8 (48% yield) and treatment with HBF 4, gave quantitatively tropylium cation 10. When ketone 7 was reduced with AlH 3 (generated from AlCl 3/LiAlH 4) in ether/benzene at 25 °C, the isomeric cycloheptatrienes 11 (70% yield) and 9 (15% yield) were obtained. © 2012 American Chemical Society.