Ketones, aliphatic, photoelimination

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

Since an appreciable amount of the photoelimination of aliphatic ketones occurs from an excited singlet, the reported partial retention of optical activity in photocyclobutanol formation355,356 is easily rationalized. The reaction from a singlet could be either concerted or proceed via a spin-paired biradical which would cyclize extremely rapidly. 

Wagner, P. J., Differences between Singlet and Triplet State Type II Photoelimination of Aliphatic Ketones, Tetrahedron Lett. 1968, 9, 5385 5388. 

Upon irradiation in solution aliphatic ketones containing 7-hydrogens from cyclobutanols as well as undergo photoelimination Cc g-) Eq. (46) Two mechanisms have been considered to account for the photochemical formation of cyclobutanols. The first is a stepwise mechanism, Eq. (59), and the second is a concerted process, Eq. (CO). 

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