We are investigating the oxidation in flow with O2 of alpha hydroxy acids into alpha-keto acids. Employing the classical oxidation catalysts with an excess of base and using oxygen at 150 C (at lower T the conversion is reduced) the reaction is not selective bringing to decarboxylation products. How is possible to reduce this side reaction?
It seems to me that oxidative decarboxylation is an intrinsic feature of these compounds. For example it is known that some metal complexes like Fe, Co catalyze the oxidative decarboxylation of alfa-hydroxy acids under oxygen or air as primary reaction. The formed keto-acids are even more susceptible for oxidative decarboxylation, as they can undergo spontaneous decarboxylation. So, it looks like catch in 22 to me. Perhaps by forming an ester or silyl ester would increase the stability of your compound? Why do you insist on this oxidation route with O2 at 150 C anyway? This wouldn't be welcome in industry. Why don't you try to dehydrogenate the hydroxy group instead under a lot safer conditions or to use air for oxidation with catalyst (Pt-Bi?) at much lower temperatures?
Perhaps run the reaction in the presence of excess CO2/ buffered carbonate solution while using the minimal O2 concentration?
Thank you for the replies. I would like to avoid the use of CO2 and also the use of the esters because it means that I have to add another step to the reaction. From the literature I know that supported metals like, Pd, Pt, Au doped with e.g. Bi, Mo, Pb can direct the reaction towards the alpha-keto acid product but still I didn't find the right combination. Even the support on which the metals are deposited should play an important role (basic supports permit to avoid the use of the added base). Are there any other parameters or conditions that anyone would suggest me to try?
yes but i was thinking about the the problem of the accumulation of CO2..but in principle i can try to check if it is the real problem...which kind of buffered solution would you suggest me, how would you prepare it?
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