The return of the succinimidyl radical

The aqueous kinetics of the succinimidyl radical. S-. has been re-examined in the presence of oxidizable substrates and oxygen. The results indicate a rapid equilibrium between S-. and its ring-opened analogue, the beta B-(isocyanatocarbonyl)ethyl radical, PI.. The equilibrium constant K-1, is ca, 10, with k(1) approximate to 10(7) s(-1) and k(-1) approximate to 10(6) s(-1). The glutarimidyl radical, G(.), was produced by one-electron reduction of N-chloroglutarimide, GCI. The rate constants of several oxidation and hydrogen abstraction reactions with S-. and G(.) have been determined. Furthermore. halogen abstraction reactions from haloimides by some selected alkyl radicals were also scrutinised. Most striking is the finding that the 2-cyanoethyl radical abstracts Br from SBr ca. 25 times slower than does the ethyl radical. This demonstrates a strong beta-effect and rationalises a relatively slow Br abstraction rate by PI. from SBr. While the closure rate of the PI. radical appears to be solvent-insensitive, the ring opening rate of S-., k(1), is estimated to be ca. 100 times faster in, e.g., CH2Cl2, than in water. This suggests hydrogen-bonded stabilisation of S-..