In recent years, we have seen more reports about using phase-transfer catalysis for oxidations of primary alcohols to aldehydes using hypochlorite to reoxidize spent catalytic TEMPO in-situ. PTC-hypochlorite-TEMPO oxidations are often reported using high shear mixers and short reaction times from less than 1 second to 8 minutes.
One reason for short contact times is to minimize over-oxidation of the aldehyde to the carboxylic acid. Two reasons to use a phase-transfer catalyst are [1] to control the amount of inorganic hypochlorite in the organic reaction phase that presumably minimizes overoxidation of the aldehyde to the acid and [2] when the phase-transfer catalyst transfers hypochlorite into the bulk organic phase from its hydrated form in the aqueous phase, the hypochlorite anion is more active and we can then work at a lower temperature which can further enhance selectivity the greater control over the reaction.
This patent reports a TBAB-TEMPO-hypochlorite oxidation of an alcohol to an aldehyde at 0 deg C with a 30-min dropwise addition of hypochlorite and a 10-min post-addition reaction time. No mention was made of the type of agitation used.
The original PTC-hypochlorite oxidation of primary alcohol to aldehyde by Lee and Friedman in the 1970’s showed that ethyl acetate and methylene chloride were the best solvents for this reaction. The inventors of this patent used methylene chloride.
It is not known why the inventors added extra catalytic KBr to the reaction mixture. If any of our readers have an idea why, please share. One thought might be to form the more reactive hypobromite in-situ, but if so, then the bromide from the TBAB might be sufficient.
No isolation or even a phase separation was reported between piperidinemethanol and phenyl chloroformate, so a full equivalent of NaCl was present when the reactants and catalysts for the oxidation were added.
The reaction was performed on a sub-gram scale, so the yield after two steps and after flash chromatography does not suggest low yield. It is not kniwn how much over-oxidation to carboxylic acid may have occurred.
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