Substituent, Temperature, and Solvent Effects in the Singlet Oxygenation of 7‐Alkyl‐ and 7‐Aryl‐substituted 1,3,5‐Cycloheptatrienes

Singlet oxygen is a sufficiently reactive dienophile to intervene in the tropilidene‐norcaradiene valence isomerization equilibria of 7‐aryl‐ and 7‐alkyl‐1,3,5‐cycloheptatrienes, leading to both the norcaradiene and tropilidene endoperoxides 2N and 2T , respectively, besides the thermally rather labile 1,2‐dioxetanes 3 and the substituted benzaldehydes 4 , derived from the latter on thermal decomposition. Quantitative product studies by 1 H‐NMR reveal that the proportion of 2N and 2T endoperoxides is a delicate function of (a) the electronic nature of the 7‐aryl substituent, i.e. the 2N/2T ratio increases in the order p‐Cl > H > p‐OMe, (b) the steric demand of the 7‐alkyl substituent, i.e. the 2N/2T ratio increases in the order tBu > iPr > Et > Me, (c) the temperature of singlet oxygenation, i.e. the 2N/2T ratio increases with increasing temperature, and (d) the polarity of the solvent, i.e. the 2N/2T ratio decreases in the order CCl 4 > C 6 H 6 > CH 2 Cl 2 > CH 3 CN. A mechanism is proposed in which substituent and temperature effects are interpreted in terms of appropriate shifts in the 2T ⇌ 2N equilibria. The solvent effect is rationalized in terms of reversible exciplex formation between the tropilidene isomer and singlet oxygen (electronically excited), leading to the 1,4‐dipolar intermediate 7 .