Oxidation of Primary Alcohols to carboxylic acids Reagent Guide
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Carboxylic acids are very widely used synthons in the synthesis of pharmaceuticals and fine chemicals. One route to prepare these compounds is by the oxidation of primary alcohols. Often, aldehydes are intermediates (or indeed the target of choice), and many reagents employed for aldehyde synthesis will over-oxidize aldehydes to carboxylic acids at the right stoichiometry.
Many classical reagents for the CH2OH to CO2H transformation, like SeO2 and Pd(OAc)4, have thankfully been mostly phased out, but some heavy metal reagents based on Cr (VI) and Mn (VII) are still used, or at least taught, as reagents for this oxidation. Much greener alternatives are now available using metal catalysts and green co-oxidants, including NaOCl, air(O2) and H2O2, as well as organocatalytic transformations using TEMPO and related nitroxyl radical catalysts. A number of biocatalytic routes to oxidize alcohols to acids are being developed. Generally, these biological systems use two enzymes – an alcohol oxidase and an aldehyde oxidase – or laccase enzymes as part of a catalytic cycle with TEMPO/O2.