Anhydrous TEMPO-H: reactions of a good hydrogen atom donor with low-valent carbon centres

Abstract

In this paper, we report a novel synthesis of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H). An X-ray crystal structure and full characterization of the compound are included. Compared to hydrated TEMPO-H, its anhydrous form exhibits improved stability and a differing chemical reactivity. The reactions of anhydrous TEMPO-H with a variety of low-valent carbon centres are described. For example, anhydrous TEMPO-H was reacted with 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes), an unsaturated NHC. Crystals of CHNC[subscript 6H[subscript 2](CH[subscript 3])[subscript 3]][subscript 2]C...HO-(NC[subscript 5]H[subscript 6](CH[subscript 3])[subscript 4]), IMes...TEMPO-H, were isolated and a crystal structure determined. The experimental structure is compared to the results of theoretical calculations on the hydrogen-bonded dimer. Anhydrous TEMPO-H was also reacted with the saturated NHC, 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), giving the product [CH[subscript 2]Ni-Pr[subscript 2]C[subscript 6]H[subscript 3]][subscript 2]CH...O(NC[subscript 5]H[subscript 6](CH[subscript 3])[subscript 4]). In contrast, the reaction of hydrated TEMPO-H with 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene gave small amounts of the hydrolysis product, N-(2,6-diisopropylphenyl)-N-[2-(2,6-diisopropylphenylamino)ethyl]formamide. Finally, anhydrous TEMPO-H was reacted with (triphenylphosphoranylidene)ketene to generate Ph[subscript 3]PC(H)C(=O)O(NC[subscript 5]H[subscript 6](CH[subscript 3])[subscript 4]). A full characterization of the product, including an X-ray crystal structure, is described.