π-Extended o-quinoidal tropone derivatives: experimental and theoretical studies of naphtho[2,3-c]tropone and anthro[2,3-c]tropone

Abstract

π-Extended o-quinoidal tropone derivatives, naphtho[2,3-c]tropone (3) and anthro[2,3-c]tropone (4), have been investigated theoretically as well as experimentally. The geometrical optimization of 3, 4, and related compounds at the B3LYP level employing 6-31G* basis set as well as the GIAO calculations at the RHF level employing the 6-31+G* basis set have been performed to evaluate the contributions of the polarized resonance forms to these molecules. The GIAO calculated NICS(1) values indicate that the aromaticities of the tropone rings of o-quinoidal 3 and 4 are significantly increased as compared with that of parent tropone (1) at the expense of the fused benzenoid rings, consistent with the significant electronic polarization of these molecules in the ground state. On the other hand, the fusion of a benzene or naphthalene ring to the 2,3- or 4,5-position of tropone leads to diminution of aromaticity in the resulting tropone moiety. Experimentally, irradiation of 6,7-(2′,3′-naphtho)bicyclo[3.2.0]hepta-3,6-dien-2-one (10) in a rigid glass at −196 °C leads to the formation of 3, which exhibits a characteristic UV-Vis absorption extending to 700 nm and undergoes rapid [π12 + π14] dimerization upon thawing the glass. In contrast, 6,7-(2′,3′-anthro)bicyclo[3.2.0]hepta-3,6-dien-2-one (11) showed no sign of isomerization to 4 under the same reaction conditions.