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-nitr osobenzenesulfonamides 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 rat e constants determined for the nitroso group transfer, k(tr), give good Bro nsted-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 n itrosation processes of secondary amines catalyzed by ONSCN and denitrosati on catalyzed by SCN-, in combination with the formation equilibrium of ONSC N, has enabled us to calculate the value of the equilibrium constant for th e loss of the NO+ group from a protonated N-nitrosamine (pK(NO)(R2+HNO)) wh ich 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)(R2 N+HNO) - pK(NO)(X-NO) ,we were able to calculate the equilibrium constant f or 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 balance d transition state. In terms of the Marcus theory, the calculation of the i ntrinsic barriers for the nitroso group transfer reaction shows that the pr esence of electron withdrawing groups on the aromatic ring of the N-methyl- N-nitrosobenzenesulfonamides does not cause these barriers to vary.