Nitrosation and denitrosation of substituted N-methylbenzenesulfonamides. Evidence of an imbalanced concerted mechanism
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, αd, of 0.7, which is independent of the substituents on the aromatic ring. Kinetic solvent isotope effects range from kdH3O+/kdD3O+ = 1.20 ± 0.05 to 2.04 ± 0.06 for denitrosation by L3O+ and from kdAH/kdAD = 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 (βNO = 0.3) and the substituent effects suggest a concerted nitrosation–denitrosation process. The Leffler parameters obtained for N· · ·H bond formation (αnuc = 0.7) as well as for N· · ·NO bond breaking (αlg = 0.17) are in favour of an imbalance in the transition state (αimbalance = 0.53) with the development of a positive charge on the nitrogen adjacent to the nitroso group.