Regiospecific oxidation of unsaturated fatty esters with palladium(II) chloride/p-benzoquinone: a sonochemical approach

Article abstract of DOI:10.1016/0009-3084(96)02533-9

Oxidation of unsaturated fatty esters to the corresponding oxo derivatives was accomplished in moderate to high yields when the substrates were treated with palladium(II) chloride and p-benzoquinone in aqueous tetrahydrofuran (THF/H2O, 4:1 v/v) under ultrasound (20 kHz).Methyl 10-undecanoate furnished methyl 10-oxoundecanoate in 92percent yield.Methyl oleate and trans-2-hexadecenoate furnished methyl 9(10)-oxostearate (65percent) and methyl 3-oxohexadecanoate (41percent) respectively.Unsaturated fatty esters with a terminal double bond were more readily oxidized than those containing an internal double bond.Methyl trans-3,12-tridecadienoate gave only methyl 12-oxo-trans-3-tridecenoate (86percent), while a cyclopentenoic fatty ester (methyl chaulmoograte) furnished a mixture of positional oxo-isomers: viz. methyl 1-oxocyclopentane-2-tridecanoate and 1-oxocyclopentane-3-tridecanoate (51.5percent total).Oxidation of methyl 12-hydroxy-, 12-azido-, 12-chloro- and 12-oxo-cis-9-octadecanoate furnished the corresponding 9-oxo-derivatives only.Methyl iso-ricinoleate (methyl 9-hydroxy-cis-12-octadecenoate) gave exclusively methyl 9-hydroxy-12-oxostearate.Without ultrasonication the same oxidation reactions at room temperature furnished low yields (<30percent) of the corresponding oxo derivatives after prolonged stirring at room temperature.Aqueous THF is a superior solvent system than the commonly used aqueous N,N-dimethylformmaide (DMF). - Keywords: Aqueous THF; p-Benzoquinone; Oxidation; Oxo fatty esters; Palladium(II) chloride; Ultrasound; Unsaturated fatty esters; Wacker reaction