One-pot reduction of TEMPO derivatives by triethylsilane and trifluoroacetic acid: isolation and structural analysis of the air-stable TEMPOH ammonium trifluoroacetate

Abstract 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) derivatives were found to be efficiently converted to the corresponding air-stable hydroxylammonium salts (TEMPOH salts) by treatment with triethylsilane and trifluoroacetic acid. This reaction is thought to proceed via disproportionation of the TEMPO derivatives under strong acidic conditions, a key pathway in the preparation of oxidants such as the well-known Bobbitt's salt. A variety of TEMPOH salts have been synthesized to date, and their structures have been elucidated by X-ray crystallography. However, most reported salts are derived from non-substituted TEMPO and are water-soluble due to limitations of previous synthetic methods. Our method is simple and applicable to water-insoluble TEMPO derivatives. In addition, a crystalline, hydrophobic TEMPOH salt was designed and synthesized, and its structure was determined by X-ray crystallographic analysis.