Smith-Loeppky mechanism

The Smith-Loeppky mechanism [2) convincingly rationalized much of what was known about tertiary amine nitrosations. However, there was some evidence in the literature that there might be more to the mechanistic story. For one thing, it was not clear why the nitrosation of tertiary amines should have a higher pH optimum than that of secondary amines (2). Even more troubling was the somewhat controversial later report by Mai ins (15) that dimethylnitrosamine formed at pH 6 more readily from trimethyl amine than from dimethyl amine. If that report is correct, then free dimethylamine and the corresponding ammonium ion could not be the only kinetically significant intermediates in the trimethyl amine nitrosation. 

The first clues that compounds of structure I might be involved in nitrosamine-forming reactions came during the study of tertiary amine nitrosations. Smith and Loeppky had proposed ( ) in their detailed, classical investigation of the mechanism of this reaction that the first steps involve nitrosammonium ion formation followed by elimination of nitroxyl (HNO). The resulting immonium ion was postulated to hydrolyze to the secondary amine, which reacted with nitrosating agent to form the observed product. These mechanistic proposals are summarized in Fig. 2a. 

Figure 2. Nitrosative dealkylation of tertiary amines (a) mechanism postulated by Smith and Loeppky (2) (b) composite of proposals by Lijinsky et al. (S) and...

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