Going Anhydrous Modifies TEMPO-H

In a bid to avoid unwanted side reactions caused by a solvating water molecule in the nitrogen heterocycle TEMPO-H, a team of Canadian-based chemists has devised a synthetic route to an anhydrous version of the popular reagent. To their surprise, the researchers found that the anhydrous version is more stable and exhibits modified reactivity relative to the standard hydrated version (Org. Biomol. Chem., DOI: 10.1039/c0ob00999g). TEMPO-H, which contains the weakest O-H bond observed in any molecule, is a valued source of hydrogen atoms for probing organic, inorganic, and organometallic reactions. It’s the protonated cousin of the stable radical TEMPO, also a widely used reagent in organic synthesis. During the course of their studies on the reactivity of low-valent compounds such as N-heterocyclic carbenes (NHCs) and ketenes with TEMPO-H, Jason D. Masuda and Jason A. C. Clyburne of Saint Mary’s University, in Halifax, Nova Scotia, were finding that the water could result in unwanted reactivity. They hit on the idea of making TEMPO-H under dry conditions, which they did by treating the sodium salt of TEMPO with triethylamine hydrochloride under an inert argon atmosphere in anhydrous solvents. Their work with anhydrous TEMPO-H confirmed that the water can be a confounding factor and led to unusual reactivity such as formation of a rare C∂ ∂ ∂H–O hydrogen bond in an NHC complex.