title: 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 (1)H 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.
authors.affiliation: Department of Chemistry, University of Idaho, Post Office Box 442343, Moscow, Idaho 83844-2343, USA. williams@uidaho.edu
authors.forename: Richard Vaughan
authors.initials: RV
authors.lastname: Williams
authors.forename: W Daniel
authors.initials: WD
authors.lastname: Edwards
authors.forename: Pengrong
authors.initials: P
authors.lastname: Zhang
authors.forename: David J
authors.initials: DJ
authors.lastname: Berg
authors.forename: Reginald H
authors.initials: RH
authors.lastname: Mitchell