Reaction of a stable N ∴ N bonded radical cation with free radicals generated by pulse radiolysis: exceedingly rapid hydrogen abstraction from C–H bonds

Abstract

Absolute rate constants have been measured for the reaction of the N ∴ N three-electron bonded 1,6-diazabicyclo[4.4.4]dodecane radical cation ([4.4.4]⁺˙) with various free radicals produced by means of pulse radiolysis. Reduction and oxidation reactions occur with rate constants generally somewhat below the diffusion limit. This is considered to reflect the inwardly oriented structure of the [4.4.4]⁺˙. High rate constants (ca. 10⁹ mol^–1 dm³ s^–1) have been measured for hydrogen-atom abstraction from [4.4.4]⁺˙ by almost all radicals except e_eq^–. The most remarkable of these reactions appears to be H-atom abstraction by a thiyl radical [(CH₃)₃CS˙], which occurs with k 3.2 × 10⁹ mol^–1 dm³ s^–1. This indicates highly labile C–H bonds in [4.4.4]⁺˙, which are considered to be those located on CH₂ groups α to the nitrogen atoms. The fast radical–radical H-atom transfer is considered to be energetically assisted by favourable stereoelectronics and least heavy atom motion.