Nitrosation and denitrosation of substituted N-methylbenzenesulfonamides. Evidence of an imbalanced concerted mechanism

Abstract

The kinetics of the nitrosation reaction of several substituted sulfonamides and of the denitrosation of the resulting products have been studied. The denitrosation rate is first-order with respect to both the nitroso compound and acid concentration and no effect of added nucleophiles was observed. The denitrosation reaction is general-acid catalysed, with a Brønsted parameter, α_d, of 0.7, which is independent of the substituents on the aromatic ring. Kinetic solvent isotope effects range from k_d^H₃O+/k_d^D₃O+ = 1.20 ± 0.05 to 2.04 ± 0.06 for denitrosation by L₃O⁺ and from k_d^AH/k_d^AD = 1.5 ± 0.2 to 2.3 ± 0.3 for denitrosation by dichloroacetic acid, which suggest that a rate-determining proton transfer is involved in this reaction. For nitrosation reaction, the absence of catalysis by nucleophilic anions, the observed general-base catalysis (β_NO = 0.3) and the substituent effects suggest a concerted nitrosation–denitrosation process. The Leffler parameters obtained for N· · ·H bond formation (α_nuc = 0.7) as well as for N· · ·NO bond breaking (α_lg = 0.17) are in favour of an imbalance in the transition state (α_imbalance = 0.53) with the development of a positive charge on the nitrogen adjacent to the nitroso group.