PHOTOCHEMISTRY OF CYCLOALKENES .6. INTRAMOLECULAR ADDITIONS

The effect of the availability of an intramolecular proton source on the photochemical behavior of three simple olefin systems (9,20, and 22) was studied. Photosensitized irradiation of the norbornenes 9a-c in methanol or benzene solution afforded principally three tricyclic ethers to which the structures 10a-c are assigned. This intramolecular addition of the hydroxyl group of an alcohol to the norbornene chromophore is in marked contrast to the exclusive reaction with the α-CH bond previously observed for intermolecular photoreactions of norbomenes with alcohols. An attempt to induce an analogous photoprotonation of an acyclic olefin through the introduction of an intramolecular proton source was made by irradiation of the allyl cyclohexanol 20. However, photosensitized irradiation under a variety of conditions led to a slow disappearance of 20 without the concomitant formation of a cyclic ether such as 21; thus, there are still no examples of the photoprotonation of an acyclic olefin. Photosensitized irradiation of the cyclohexenyl alcohol 22 afforded methylenecyclohexane (2) and acetaldehyde, which presumably arose via photoprotonation and subsequent fragmentation as shown in 23. In m-xylene solution the resulting acetaldehyde underwent photoaddition to the solvent to afford the alcohol 25. The relationship of these results to the intermolecular photochemical behavior of cycloalkenes observed previously is discussed. © 1969, American Chemical Society. All rights reserved.