Abstract
In the course of studies on the iodine-sensitized photoconjugation of 1, 4-diene compounds, the effects of terminal functional groups on this reaction were investigated. Linoleic acid and its related compounds, such as methyl linoleate, linoleyl alcohol, linoleyl acetate and cis-6, cis-9-octadecadiene derived from linoleic acid, were used as samples.
Based on the results of sensitized isomerization, the carboxyl group was without any effect whatsoever. Linoleic acid reacted at the same rate as 6, 9-octadecadiene, or faster than that of methyl linoleate, so that maximum equilibrium was attained within 1h. Moreover, the photo-conjugation of linoleyl acetate occurred more slowly than that of methyl linoleate. The amounts of the conjugated compounds were about as much as 80% in either case. With linoleyl alcohol, isomerization was quite slow, the yield being about 10% after 8h. This is in agreement with the finding that this sensitized conjugation does not proceed in a polar solvent such as ethanol.
Therefore, the reactivities of the end-groups on this iodine-sensitized reaction decrease in the following order : CH3≥COOH>COOCH3>CH2OCOCH3>>CH2OH.
This reaction apparently occurs through a radical mechanism involving a bridged iodoalkyl radical as an intermediate. The products mainly consisting of cis, trans- and trans, trans-conjugated dienes were thus obtained at a ratio of about 3 : 7.
