Unsaturated FAMEs can be modified to higher value compounds by hydroboration of the double bond. The added boron atom can be replaced in a subsequent oxidation by a hydroxy group. This way methyl 10-undecenoate (1b), methyl oleate (3a), methyl ricinoleate ((9Z,12R)-6a), and methyl linoleate (9) were converted into methyl 11-hydroxyundecanoate and methyl hydroxyoctadecanoates in 82–92% yield. Organoboranes isomerize at higher temperatures. Hydroboration of oleic acid and subsequent heating to 220°C led after oxidative work-up to 70% of 1,4-octadecanediol. Presumably a 1,4-oxaborinane intermediate is involved. Alkyl groups of organoboranes can be attached to the carbonyl-equivalent: dichloromethyl methyl ether (DCME). With this reaction the boranes of the esters 1b and 3a were converted in 55–80% yield to dimers with an inserted ketocarbonyl group. Ester 9 afforded in the same reaction methyl nonadecanoates with an integrated cyclopentene ring.
Practical applications: Hydroboration–oxidation allows the partially regioselective conversion of unsaturated FAMEs into hydroxy- and dihydroxyesters. Thermal isomerization of the boranes from methyl oleate, oleic acid, and oleyl alcohol leads after oxidative work-up to the 1,4-diol: 1,4-octadecanediol as main product. These products are useful as components for polyesters, polyurethanes, and precursors for heterocycles. Reaction of organoboranes from unsaturated FAMEs with the carbonyl equivalent: DCME provides a one-step access to 1,ω-diesters with an internal keto group, which can be applied for coupling, cross-linking, and further conversion. The borane · THF complex is commercially available in amounts of 0.8 mol and more to do exploratory research toward such applications.