Nitrosation primary aliphatic amines

The serious problem of disentangling diazonium ion reactions from carbocation reactions makes diazonium ion rearrangements difficult to study. It seems likely that products of primary aliphatic amine nitrosation are derived from the diazonium ion rather than a primary aliphatic carbocation formed by spontaneous loss of nitrogen, since primary carbocations are unknown in solution. Products from secondary aliphatic amine nitrosation are formed from either the diazonium ion or the carbocation. 

Because primary aromatic amines are weaker bases/nucleophiles than aliphatic (due to interaction of the electron pair on N with the n orbital system of the aromatic nucleus), a fairly powerful nitrosating agent is required, and the reaction is thus carried out at relatively high acidity. Sufficient equilibrium concentration of unprotonated... 

In the aliphatic series, primary amines are deaminated by nitrous acid, secondary aliphatic amines are A-nitrosated, while tertiary aliphatic amines form salts in the acidic medium followed by secondary reactions such as dealkylation [1, 21a, b]. 

Secondary aliphatic amines form stable N-nitrosoamines, however, and therefore, their investigation will be discussed briefly in this section, although N-nitroso derivatives of secondary aliphatic amines do not fall within the scope of this book. We will see that kinetics and mechanisms of nitrosation of secondary amines display many similarities with the diazotization of primary aromatic amines. 

The nitrosation mechanism of aliphatic amines by metal nitrosyl complexes was studied with various secondary amines and one primary amine (ethylamine) using pentacyanonitrosylferrate (sodium nitroprusside). Very little mechanistic information is available for reactions with other metal nitrosyl complexes (see also Sects. 2.3, 4.3, and 7.2). 

A mechanistically different type of nitrosation was discovered by Keefer and Roller (1973), namely a nitrosation of secondary aliphatic amines with nitrite anions in alkaline solution, catalyzed by aldehydes. Although it is unlikely to be applicable to diazotization, i. e., to primary amines, it will be mentioned here because it is a good example of the fact that, in chemistry, particularly in organic chemistry, for a certain type of reaction, e. g., nitroso-de-protonation (which includes substitution of protons bonded to C, N, O, S, etc., atoms), practically all methods follow the same basic pattern (in the case of nitrosation substitution by an electrophilic nitrosating reagent). The Keefer-Roller nitrosation is apparently different if one looks at the stoichiometric equation (4-8). A careful kinetic investigation (Casado et al., 1981b, 1984 a) on the concentration and pH dependence of this reaction revealed that the nitrite anion and free amine base enter the substitution process and that formaldehyde is a true catalyst, as it is not required in equimolar amounts. 

One of the best leaving groups is molecular nitrogen from diazonium ions. Diazonium salts are usually prepared by nitrosation of primary aliphatic or aromatic amines ... 

Amines and amides can be N-nirra/erf549 with nitric acid,55" N205,551 or N02, 552 and aromatic amines can be converted to triazenes with diazonium salts. Aliphatic primary amines can also be converted to triazenes if the diazonium salts contain electron-withdrawing groups.555 C-Nitrosation is discussed at 1-3 and 2-8. 

Besides this iron-nitrosyl complex, nitrosyl complexes of other transition metals can be used for nitrosation. As discussed by Bottomley et al. (1973, see also review by Bottomley, 1978), these complexes are not only sources of nitrosyl ions (NO" ) as two-electron acceptors, but also of nitroxide (NO ) as one-electron donor. Bottomley found that they are nitrosating reagents only if their NO stretching frequency is greater than 1886 cm The ruthenium nitrosyls are particularly interesting with respect to their reaction with aliphatic and aromatic primary amines. We discuss them in the context of metal dinitrogen complexes (Sect. 3.3). 

Attention towards an aliphatic diazonium ion as a transient intermediate in nitrosation of alkylamines was subsequently further distracted by investigations of Linnemann and Siersch (1867), who found that in the reaction of propylamine even more propan-2-ol was detected than the propan-l-ol expected. Kinetics of nitrosa-tions of alkylamines were determined as early as 1928 by Taylor, but it was not until Hammett, in his pioneering book on physical organic chemistry (1940, p. 295), that the analogy to the aromatic series was established. He suggested that diazonium ions are also intermediates in the reactions of aliphatic primary amines. 

With nitrous acid, primary amines are diazotised (equation 58), secondary amines undergo iV nitrosation (section II.C.2.e), and tertiary aliphatic or alicyclic amines fail to react while tertiary aromatic amines may undergo C-nitrosation. 

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