Iron Carbonyl-Promoted Isomerization of Olefin Esters to Their alpha,beta-Unsaturated Esters: Methyl Oleate and Other Examples

Ultraviolet photolysis of stoichiometric amounts of methyl oleate and Fe(CO)(5) in hexanes solvent at 0 degrees C gives Fe(CO)(3)(eta(4)-alpha,beta-ester) in which the alpha,beta-unsaturated ester isomer of methyl oleate is stabilized by eta(4)-oxadiene pi coordination of the olefin and ester carbonyl groups to the Fe(CO)(3) unit. Treatment of the Fe(CO)(3)(eta(4)-alpha,beta-ester) with pyridine or CO liberates the free alpha,beta-ester, methyl octadec-trans-2-enoate, in 70% yield. The Fe(CO)(3) unit both catalyzes the olefin isomerization and stabilizes the alpha,beta-unsaturated ester, which results in the formation of the alpha,beta-ester in a yield far above that (3.5%) observed for simple catalyzed methyl oleate isomerization. The much smaller olefin esters, methyl 3-butenoate and ethyl 4-methyl-4-pentenoate, are isomerized under the same conditions to their alpha,beta-unsaturated esters in 94 and 90% yields, respectively. The effects of reaction conditions on the yield, the use of Fe(CO)(3)(cis-cyclooctene)(2) as a nonphotolytic catalyst, and the mechanism of this useful synthetic process are discussed.