Hydrogen abstraction, ketones solvent effects

Other Hydrogen Transfers - Weigel and Wagner S have studied the 1,4-hydrogen abstraction reactions within the ketones (37) leading to the cyclopropanols (38). The study used wavelengths > 300 nm with benzene or methanol as the solvents. The yields of products are shown below the structures. Polar solvents appeared to have no effect on the outcome of the reaction. 

Irradiation in benzene of 4-butyroylpyrimidine (37) affords a 60 40 mixture of the cyclopropanol (38) and cleavage product (39). Only the former product is formed when t-butyl alcohol is used as solvent and only the latter in hexane. These results are best accounted for by formation of an n-n triplet which abstracts a y-hydrogen (a process rarely seen with nitrogen aromatic ketones), giving a 1,3-biradical which can ring-close. Fragmentation could come from the 1,3-biradical or from alternative 1,4-biradical formation. The solvent effects have not yet been clearly explained. ... 

---