Iminium salts, from

The formation of NNN, NNK, and NNA from nicotine probably involved the intermediacy of cyclic iminium salts, as shown in Figure 5 (28). These salts can undergo hydrolysis to the free amines which are nitrosated, or at near neutral pH, can be directly nitrosated to give nitrosamines. The formation of nitrosamines from iminium salts under neutral conditions has been demonstrated in at least two studies and is of interest because iminium salts are known to be intermediates in the mammalian metabolism of nicotine (26,29,30,31). The possibility that tobacco bacteria could nitrosate nicotine via this pathway is currently under investigation. 

Tertiary amines are made from iminium salts by catalytic hydrogenation or by LiAlH4 reduction. The iminium salts are usually unstable, and so are reduced as they are formed by a reducing agent already in the reaction mixture. A mild reducing agent, e.g. sodium cyanoborohydride (NaBHsCN), can also be used. 

It is very easy to release enamines from iminium salts by reaction with alkali. 

The reaction of ketone derivatives (89) with chloromethyleneiminium salt (1) gives iminium salts (90) which yield, after hydrolysis, P-chloroacrolein derivatives (91 Scheme 7)." Numerous P-chloroacrolein derivatives have been prepared by this method and yields are generally moderate to good. p-Chloroacryl-onitrile derivatives (92) are also available in good yield from iminium salts (90 = H) by treatment... 

The E-isomer is generally more stable than the Z-isomer due to diminished steric hindrance, so it is assumed that the E-isomer is the major product (shown for 83). Water, a reaction by-product, may be removed to give a better yield of product and azeotropic distillation is used as well as molecular sieves (see Section 18.6.3). Enamines are structurally related to an end (HO—C=C) in that the heteroatom is directly attached to the alkene unit. Enamines are often isolable compounds, whereas enols tautomerize spontaneously to the carbonyl form. Note that when imine 76 is formed from iminium salt 80, there is no enamine product. In fact, the C-H in 80 is much less acidic that the N-H unit, so the product is the imine rather than the enamine. It is noted that there is an equilibrium between an imine and an enamine, known as imine-enamine tautomerism, but it will be ignored in this book. Many different secondary amines can be used in this reaction, including cyclic amines (see Chapter 26, Section 26.4.1) such as pyrrolidine (90), piperidine (91), and morpholine (92). It is important to note that it is generally easier to form an enamine from a ketone than from an aldehyde. 

Enamines are also prepared from azirines by thermolysis/ and from iminium salts by dehydrobromination/" Enaminones are prepared from ketones by reaction with hexamethydisilazane " and from vinamidinium salts by reaction with enolates/ and thioenaminones are now conveniently prepared directly from enaminones/" ... 

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