Abstract
In the course of studies on the iodine-sensitized conjugation of 1, 4-diene compounds, the effects of iodine compounds on this reaction were investigated. A solution containing 10% methyl linoleate and iodine compound (0.1% : calcd. for I2) in petroleum ether was irradiated with a 100 W high-pressure mercury lamp for 8 h.
Iodine compounds, especially CHI3, CH2I2, IBr, ICl, (CH3) 2CHI, ICl3 and C6H5I, were found effective for this sensitized reaction. Methyl linoleate was converted to conjugated dienoates in approximately 80% yields along with small quantities of methyl trans-octadecenoates and dimers. The ratio of cis, trans-, cis, cis- and trans, trans-forms was 8 : 1 : 24 in the conjugated dienes. Conjugation with CH3I was quite slow, the yield being 10% after 8 h.
The activities of iodine compounds in this sensitized reaction decrease in the following order :
CHI3 ≥ I2 ≥CH2I2 ≥ IBr ≥ ICl > (CH3) 2CHI ≥ ICl3 ≥ C6H5 I>
p-CH3C6H4I >> CH3 (CH2) 3I > CH3 (CH2) 2I >> CH3CH2I > CH3I
The rates of conjugation of methyl linoleate thus appear closely correlated to the dissociation energies of the I-X bonding (X=C, Cl, Br, I) and absorption spectra of iodine compounds.
This isomerization proceeds fundamentally in the same way as noted in the previous paper. That is, an iodine compound first dissociates with light, and conjugation subsequently proceeds through a radical mechanism involving a bridged iodoalkyl radical.
