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Synlett 2003(11): 1757-1758
DOI: 10.1055/s-2003-41440
SPOTLIGHT
© Georg Thieme Verlag Stuttgart · New York

2,2,6,6-Tetramethylpiperidine-Based Oxoammonium Salts

Nabyl Merbouh*
Department of Chemistry, 55 North Eagleville Road, Storrs CT 06269-3060 USA
e-Mail: Nabyl.merbouh@uconn.edu;
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Publication History

Publication Date:
22 September 2003 (online)

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Introduction

Oxoammonium salts are derived from nitroxide free radicals by a one-electron oxidation (Figure 1). Their discovery in 1965 by Golubev et al. [1] has led to the synthesis of a series of oxidizing agents with diverse properties. The parent nitroxides have been the subject of numerous oxidation studies, [2] which involve the in situ synthesis of oxoammonium salt by a secondary oxidant, thus making the nitroxide a catalyst. These catalytic reactions and many stoichiometric reactions of oxoammonium salts have been reviewed. [2] [3] Oxoammonium salts used for stoichiometric oxidations can be obtained by the oxidation of nitroxides with halides or by the acid catalyzed disproportionation of nitroxides. Oxoammonium salts are stable and highly specific oxidants. When the counter anion is bromide or chloride, the salts are usually quite hygroscopic, however the tetrafluoroborates are not. The perchlorates are well known, but due to their latent ability to detonate their use cannot be recommended.

Oxoammonium salts, generated in situ, by the oxidation of a catalytic amount of nitroxide with a stoichiometric amount of a secondary oxidant have found extensive use in the oxidation of alcohols to aldehydes, ketones and, under special circumstances, to carboxylic acids. The secondary oxidant can be bleach, [4] oxone, [5] bromine or chlorine, [6] iodine, [7] MCPBA, [8] sodium bromite and chlorite, [9] and many other oxidants. Oxoammonium salts can also be generated in situ electrochemically. [10] [11]

Figure 1