PHOTOCHEMISTRY OF O-ALLYLPHENOL - IDENTIFICATION OF THE MINOR PRODUCTS AND NEW MECHANISTIC PROPOSALS

The photochemistry of o-allylphenol (1) in cyclohexane has been reinvestigated. Besides the previously reported cyclic ethers 2 and 3, seven additional minor photoproducts have been detected. Spectroscopic methods, coupled with independent synthesis, have allowed their identification as 2-methylbenzofuran (5), o-propylphenol (8), the epoxide 4, the dihydroxy compound 9, the cyclohexyl ether 6, o-(cyclohexylmethyl)phenol (10), and the dimer 7. Their formation is rationalized through new mechanistic pathways, which involve initial intermolecular electron and/or proton transfer between two molecules of o-allylphenol, as well as di-pi-methane rearrangement. Key intermediates appear to be radical V, carbenium ion IX, and carbene XI. This is supported by photolysis of o-allylphenyl acetate (I 1), which leads to the formation of a radical pair, followed by in cage recombination to the photo-Fries products 12 and 13 or, alternatively, diffusion of the radicals out of the solvent cage to afford the minor products 2, 5, and 6, identical to those obtained by photolysis of 1.