Effective Aromaticity of Tricarbonylchromiumbenzene, about 25 Enhanced over that of Benzene: Structural Evidence from a Complexed Benzannulene

The tricarbonylchromium complex of 2,7-di-tert-butyl-trans-12c,12d-dimethylbenzo[e]dihydropyrene is prepared in 65% yield from the parent annulene. Analysis of the 1H NMR spectrum leads to coupling
constant data and Günther's “Q” value data which unequivocally indicate that the tricarbonylchromiumbenzene moiety behaves like an aromatic system of greater bond fixing ability, aromaticity, than benzene
itself; this analysis is supported by X-ray structural data and by the rotational position of the tricarbonyl
podal. Analysis of chemical shift data indicates that the tricarbonylchromium benzene moiety has about
25% enhanced “aromaticity” over that of benzene.