Singlet photosensitization of simple alkenes. Part 4. cis-trans Photoisomerization of cycloheptene sensitized by aromatic esters. Some aspects of the chemistry of trans-cycloheptene

Abstract

Upon photosensitization at –78°C of cis-cycloheptene (1c) with methyl benzoate as sensitizer, trans-cycloheptene (1t) was produced in a high trans : cis photostationary ratio of 0.24. trans-Cycloheptene thus formed was entirely stable at –78°C and, on warming to room temperature, regenerated the cis-isomer quantitatively, but was found to be trapped efficiently by methanol in acid and by diazomethane at low temperature. An anomalous enhancement of the rate of protonation was observed for (1t) compared with (1c) or even with trans-cyclo-octene, for which the constrained structure of (1t) must be responsible. The thermodynamic parameters for the thermal trans–cis isomerization of (1t) were also determined from a study of the lifetime at temperatures between –15.0 and +1.0 °C: Ea 17.4 kcal mol–1, log A 11.1, ΔG‡266 19.4 kcal mol–1, ΔH‡266 17.0 kcal mol–1, ΔS‡266–9 cal K–1 mol–1. A potential energy curve for the ground-state cycloheptene was presented.