Oxoammonium salts (1) and their nitroxide (2) counterparts have enjoyed particular attention in recent years as effectors of oxidation; the former as mainly a stoichiometric oxidant and the latter as a catalyst. These materials hold many advantages over traditional metal oxidants: stability, diminished toxicity, and recyclability. In the accepted mechanism for nitroxide catalyzed oxidations, 2 is first oxidized to 1, which subsequently undergoes reduction to its hydroxyammonium salt (3) counterpart. To regenerate 2 from 3, terminal oxidants are used, often in the presence of additives. However, not all additives currently used are environmentally benign, and still may be necessary in stoichiometric quantities for the reaction to work. While oxoammonium salts are used primarily as stoichiometric oxidants, their presence in the nitroxide oxidation catalytic cycle shows that they hold promise to be used in a catalytic sense as well. The challenge in foundationally describing 1 as a catalyst lies in the re-oxidation of 3 to 1. By first understanding the redox chemistry of 3, we have developed conditions that utilize 3 in catalytic quantities to affect oxidation, in conjunction with cheap, readily available NaOCl ● 5 H2O as the terminal oxidant. Under these conditions, 3 is oxidized to 1 which then performs the desired oxidation. In acidic media, 3 is regenerated and rapidly re-enters the catalytic cycle. We show that a wide range of alcohol substrates are rapidly oxidized to their carbonyl derivatives. Of particular note is that α,α,α-trifluoromethyl alcohols, a class of compounds which are notoriously hard to oxidize, can be converted to the corresponding trifluoromethyl ketones in a short time with excellent isolated yields.

Hydroxyammonium salts as simple, versatile, highly effective oxidation catalysts

256th National Meeting (2018)

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