Nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to amines.: Intrinsic and apparent reactivity

We have studied the nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to primary and secondary amines, observing that the rate of the reaction increases as a consequence of the presence of electron withdrawing groups on the aromatic ring of the nitrosating agents. The rate constants determined for the nitroso group transfer, k(tr), give good Bronsted-type relationships between log k(tr) (rate constant for nitroso group transfer) and pK(a)(R2NH2-) and pK(a)(leaving) (group). The study of the nitrosation processes of secondary amines catalyzed by ONSCN and denitrosation catalyzed by SCN-, in combination with the formation equilibrium of ONSCN, has enabled us to calculate the value of the equilibrium constant for the loss of the NO+ group from a protonated N-nitrosamine (pK(NO)(R2+HNO)) which can be defined by analogy with pK(a)(R2NH2+). The value of pK(NO)(X-No) for the loss of the NO+ group from an N-methyl-N-nitrosobenzenesulfonamide was obtained in a similar way. By using values of Delta pK(NO) = pK(NO)(R2N+HNO) - pK(NO)(X-NO) ,we were able to calculate the equilibrium constant for the nitroso group transfer and characterize the transition state. On the basis of Bronsted-type correlations, we have obtained values of beta (norm)(nucl) and alpha (norm)(lg) congruent to 0.55, showing a perfectly balanced transition state. In terms of the Marcus theory, the calculation of the intrinsic barriers for the nitroso group transfer reaction shows that the presence of electron withdrawing groups on the aromatic ring of the N-methyl-N-nitrosobenzenesulfonamides does not cause these barriers to vary.