Mechanistic studies of nitrosation reactions of N-.O- and S-nitroso compounds
A kinetic study of the N-nitrosation of 2,3-diamlnonaphthalene in acid solution at 25 C was undertaken. Reaction occurred via both the protonated and unprotonated forms of 2,3-dlaminonaphthalene to yield 2,3-naphthotrlazole. The reaction is acid-catalysed and is also catalysed by added nucleophiles, The mechanism is consistent with the rate-limiting formation of the diazonium ion (N-nitrosation followed by rapid proton transfer and loss of a water molecule), which undergoes a rapid cyclisation process, to yield the product, Reactions of nitrosamines involving the N-N heterolytic cleavage with direct transfer of the nitroso group was also investigated. In particular, the denitrosation of N-nitrosoproline was studied. The reaction was carried out In the presence of an excess of nitrite trap to ensure irreversibility, in the presence of added nucleophlles, in high acidic conditions. All kinetic results are consistent with the mechanism involving a rapid reversible N-protonation followed by a rate-limiting attack by the nucleophile (added Br(^-), SCN(^-) or thiourea) giving the nitrosyl derivative, which is rapidly destroyed with the nitrite traps used in these experiments. However, at high nucleophile concentration and for reaction in ethanol, the reaction becomes Independent of added nucleophile. This is because the rate-limiting step is now the proton transfer to the nltrosamine rather than the attack of the nucleophile. N- nitrosopyrrolidine, a heterocycle which does not contain a β-electron withdrawing group reacted less readily in the denitrosation reaction. N- nitrososarcoslne, an acyclic compound containing a g-electron withdrawing group showed similar behaviour to NNP, suggesting that it is the β-electron withdrawing group, rather than the cyclic structure in NMP which is the dominant factor in the loss of the nitroso group. The reaction of N-nitroso compounds containing an electron-withdrawing group was studied. The high reactivity of N-methyl-N-nltrosotoluene-p- sulphonamide (MNTS) with nucleophlles, with the loss of the nitroso group has been explained in terms of either a rapid blmolecular reaction between the nucleophile and the more susceptible nitrosamine, due to the β-electron withdrawing substituent, or the weakening of the N-N bond by –S0(_2), resulting in extensive bond breaking. MNTS was reacted with powerful nucleophlles, L- cysteine, L-cystelne methyl and ethyl esters and N-acetyl-L-cystelne, In water at 25ºC in the pH range 6-13. The pH dependence of the rate constant Is consistent with a mechanism Involving a direct nitrosatlon by MNTS with the thiolate anion of the thiol. A quantitative kinetic analysis yielded microscopic pKa values for RSH ionlsatlon in good agreement with literature. Nitrosatlon by 0- and S-nitroso compounds were also studied. The substrates used in this case Include phenols, ascorbic acid and haems. Reaction of iso-amyl nitrite (IAN) (an alkyl nitrite) with phenols and ascorbic acid occurs via a rapid reversible acid-catalysed hydrolysis of iso-amyl nitrite giving nitrous acid, which then In Its protonated form effects nitrosatlon. Reaction of S-nitroso-N-acetylpenicillamine (SNAP) with phenols and ascorbic acid, however, showed different characteristics: a possible radical mechanism is suggested on the basis of e.s.r. experiments. Preliminary Investigations of the reactions of both IAN and SNAP with haems showed a complex reaction scheme: the reaction is not a simple single step reaction. The properties of the N-, 0- and S-nitroso compounds used in this study are also discussed.