Changes in electronically excited states and photochemistry of troponoids on complexation with acids

Abstract

UV irradiation of 2-ethyltropone ( 2) in CH 3CHBF 3 or CH 3CNH + gave a mixture of 3-ethylbicyclo[3.2.0]hepta-3,6-dien-2-one ( 4) and its 1-ethyl isomer ( 5) (approximately 95 : 5) with good efficiency, whereas irradiation in CH 3CN resulted in the slow disappearance of 2 and neither 4 nor 5 was formed. In a similar irradiation in CH 3CN, tropone ( 1) also reacted sluggishly to form dimers, whereas with added acids bicyclo[3.2.0]hepta-3,6-dien-2-one ( 3) was formed. Irradiation of 2-methoxytropone ( 6) proved to be efficient in both CH 3CN and CH 3CN—acid to give 1-methoxybicyclo [3.2.0]hepta-3,6-dien-2-one ( 7) in the former case and the 3-methoxy isomer ( 9) in the latter. With 2-acetylaminotropone ( 10), irradiation in CH 3CN gave 1-acetylaminobicyclo[3.2.0]hepta-3,6-dien-2-one ( 11), whereas with added acids the 3-acetylamino isomer 13 was formed in competition with 11; however, 13 was observed to revert thermally to 10 and 12 was found to be unstable in the acidic medium. Irradiation of 4-methoxytropone ( 14) in CH 3CN gave 5-methoxybicyclo[3.2.0]hepta-3,6-dien-2-one ( 15) as well as tar, whereas with added acids 4-hydroxytropone ( 17) was formed, accompanied by 15 in only trace amounts. With a change from CH 3CN to CH 3CNBF 3 or CH 3CNH + as solvents, the UV absorption bands at 220 and 320 nm of compounds 1, 2, 6, 10 and 14 sharpened, whereas the emission intensity was increased and the Stokes' shift was reduced. Ab initio calculations on tropone ( 1) and protonated tropone - limited to the ground and excited singlet states since no quenching of these photochemical reactions by either naphthalene or 2,4-hexadiene was observed - showed that the lowest-lying excited state is n → π* for 1 and π → π* for protonated 1. A decreased CC/CC bond alternation is found in the former and, in contrast, CC bond localization in the latter. This feature, together with the free valence indices computed for the π → π* protonated 1, is consistent with the observed intramolecular cyclization.