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

Introduction

Oxoammonium salts have recently attracted much attention as environmentally friendly, shelf-stable alternatives to traditional oxidants.[ ¹ ] In addition to being more sustainable, these salts have several advantages: 1) the spent ­oxidant is easily recovered and recyclable; 2) oxidations are performed under mild conditions and are simple to conduct; 3) oxidations are typically colorimetric processes and reaction progress can be monitored visually.[ ² ]

Scheme 1 Preparation of oxoammonium salts

Much of the work involving oxoammonium salts focusses on the conversion of alcohols into the corresponding carbonyl species.[3] [4] [5] [6] A range of functional groups can be ­accessed using this strategy including aldehydes,[ ² ] carboxylic acids,[ ³ ] ketones (including the electron-deficient CF₃ ketone)[ ⁶ ] and even esters.[ ⁵ ]

While highly effective for this type of transformation, several new applications for oxoammonium salts have been recently explored.[8] [9] [10] [11] These include several mechanistically interesting transformations such as exhaustive oxidation to give ene-triketones,[ ⁷ ] tandem dehydrogenative Povarov/oxidation reactions,[ ⁸ ] tandem α-alkylation/cyclization, α- and γ-alkylation of aldehydes,[ ⁹ ] and de­hydrogenative coupling of benzylic nitrogen/oxygen compounds with enolizable carbonyl groups.[ ¹⁰ ]

• References

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