Generation, Spectroscopic Characterization, and Reactions of 3,4-Benzotropone. Distinctive Photochemical Behavior of 6,7-Benzobicyclo[3.2.0]hepta-3,6-dien-2-one in Rigid Glass at Low Temperature and in Fluid Solution

Abstract

Irradiation of 6,7-benzobicyclo[3.2.0]hepta-3,6-dien-2-one (2) in a rigid glass at -196°C leads to the formation of 3,4-benzotropone (1), which exhibits a characteristic UV-vis absorption extending to 535 nm and dimerizes to give syn-and anti-[π8 + π10] dimers 7 and 8 upon thawing the glass. The rate constant of the thermal dimerization of 1 is 12 ( 3 M -1 s -1 at -78°C. Photochemically, 1 is stable under matrix isolation, but undergoes [π10 + π10] dimerization in solution. Compound 1 in a CFCl 3 -CF 2 BrCF 2 Br-CHCl 3 glass at -155°C shows an IR band at 1506 cm -1 which is assigned to a vibration primarily involving CdO stretching mode by comparing the spectrum with that of 18 O-labeled 1. Under the same conditions tropone exhibits a CdO vibrational band at a significantly higher frequency (1553 cm -1 ), suggesting that the 3,4-benzo annulation makes the tropone ring more strongly polarized. The large contribution of a polar resonance form 1b is reflected in the high affinity of 1 for protonation: 1 is nearly half-protonated in a 0.1 M solution of CHCl 2 CO 2 H in EPA (a 5:5:2 mixture of ether, isopentane, and ethanol). Furthermore, the electronic absorption spectrum undergoes a characteristic blue shift when the medium is changed from hydrocarbons to alcohols, suggesting that 1 is more polarized in the ground state than in the excited state. The generation of 1 from 2 is thermally effected also and, upon heating a mixture of 2 and olefin such as maleic anhydride or ethyl vinyl ether in benzene at 220°C, adducts of the latter to 1 are obtained. The photochemical behavior of 2 is dependent on the state of medium in which 2 is irradiated; the photolysis of 2 in rigid glass results almost exclusively in the valence isomerization providing 1, while in fluid solution predominantly in the di-π-methane rearrangement affording 3,4-benzotricyclo[3.2.0.0 2,7 ]hept-3-en-6-one (19). Plausible causes of the medium-dependent photochemical behavior of 2 are discussed.

One hundred years of benzotropone chemistry

“…The aromaticity of 3,4-benzotropone ( 13A ) is considered to depend on the contribution of electronically polarized form 13B such as tropone 1A ( Fig. 8 ) [ 153 – 154 ]. Kurihara’s group reported the calculated resonance energies and bond currents for benzotropones and troponoid compounds [ 155 ].…”

“…9 ) [ 153 ]. In a subsequent study, Tsuji’s group described details of the spectral and chemical properties of 13 [ 154 ]. The IR spectroscopic results showed a substantial contribution of 13B to 13A in the ground state.…”

“…Moreover, it was found that the photochemical behavior of 230 depended on the state of the irradiation medium. For example, the smooth [π10 + π10] dimerization of 13 to give dimeric product 233 was realized with the irradiation (>420 nm) of 13 in a fluid EPA solution below −100 °C [ 154 ]. Furthermore, the IR spectra of 3,4-benzotropone ( 13 ) generated in matrices at 13 K by the photoisomerization of 230 were directly observed [ 156 ].…”

“…Furthermore, the IR spectra of 3,4-benzotropone ( 13 ) generated in matrices at 13 K by the photoisomerization of 230 were directly observed [ 156 ]. In addition, the thermal generation of 13 from 230 was investigated [ 153 – 154 ]. When 230 with 10 equiv of maleic anhydride in benzene at 220 °C was reacted, [π2 + π8] cyclo-adduct 234 as a single product was isolated in 52% yield ( Fig.…”

“…When 230 with 10 equiv of maleic anhydride in benzene at 220 °C was reacted, [π2 + π8] cyclo-adduct 234 as a single product was isolated in 52% yield ( Fig. 9 ) [ 153 – 154 ]. The thermolysis of 230 in the presence of ethyl vinyl ether gave three volatile products 235 – 237 in GLC yields of 10%, 7%, and 15%, respectively ( Fig.…”