Photoconjugation of Methyl Linoleate in the Presence of Iodine as a Sensitizer

The authors previously reported that polyunsaturated fatty acids having 1, 4-diene structure can be easily conjugated at ordinary temperature using iodine as a photosensitizes. This paper describes the optimum conditions for the iodine-sensitized conjugation of methyl linoleate under irradiation with a 100W high-pressure mercury lamp and the mechanism involved. A nonpolar solvent such as petroleum ether was used in this reaction. The optimum conditions of the reaction were as follows : approximately 0.1% of the amount of iodine added; 10% of the concentration of linoleate; room temperature; and irradiation for 1.5h with a mercury lamp.Under these conditions, methyl linoleate was converted to conjugated dienoates in yields of approximately 80% along with small quantities of methyl trans-octadecenoates and polymers. No positional transfer selectivity of double bonds could be detected at all. The conjugated pro-ducts were present in equal amounts as a mixture of methyl 9, 11- and 10, 12-octadecadienoates (the ratio of the cis, trans and trans, trans forms was 3 : 7). It is considered that iodine atoms react only with the 1, 4-diene system in this iodine-sensitized conjugation to first form a bridged iodoalkyl radical intermediate which is subsequently converted mainly into cis, trans conjugated forms accompanied by a shift of hydrogen and finally the geometrical isomerization equilibrium mentioned above is attained.