Photoelimination concerted

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. 

The number of photoelimination reactions that occur in a concerted manner is very small. One of the best-characterized examples does not lead to the formation of a C—C bond between the two a-carbons. Rather, it involves the stereospecific photoelimination of CO from 3,5-cydoheptadienone 15 to 1,3,5-octatriene 16 (Scheme 2.5) [18]. The reaction occurs in a stereospecific manner with the relative stereochemistry of the two a-carbons determining the configuration of the resulting 1,5-double bonds. Another interesting and more relevant example involves the photodecarboxylation of enantiomerically pure aromatic esters derived from (+ )-or (—)-2-methylbutyric acid and 2,4,6-trimethylphenol 17, which occurs with 100% retention of configuration in the product 18. [19] While this is a promising lead that... 

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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