Photochemistry of acyclic βγ-unsaturated ketones: the effect of α-methyl substitution

Abstract

The effect of irradiation of three acyclic βγ-unsaturated ketones (1f–h) at 313 nm has been investigated, with 2,2-dimethylbutane, hexadecane and t-butyl alcohol as solvents. The photoproducts are those formed by recombination of the allyl radicals resulting from initial α-cleavage of the excited ketone, by a 1,3-acyl shift [from (1f)], and by decarbonylation [ from (1h)]. With (1f and g) small amounts of biacetyl were found and no carbon monooxide, whereas with (1h) substantial amounts of carbon mono-oxide and no α-diketone were observed. The quantum yields for ketone disappearance and product formation are reported. The effects of α- and α′-methyl substitution on the quantum yield of ketone disappearance is very small. Solvents of higher viscosity (hexadecane and t-butyl alcohol) were found to promote the solvent-cage radical combination reactions. By use of tri-n-butylstannane as a radical scavenger, it was shown (i) that at least 40% of the initially formed free radicals from (1f and g) recombine to yield the starting compound, (ii) that with (1f) at least 45% of the 1,3-acyl shift product is formed via a free radical pathway, (iii) that with (1h) at least 85% of the decarbonylation product is formed from free radicals, and (iv) that the decarbonylation of the isobutyryl radical scarcely occurs in the solvent cage. The acetonephotosensitized conversions of (1f–h) are slow in comparison with the photodecomposition of acetone.