Salts iminium

The products of interest here are the salts of the iminium radical cations (4.9) and diiminium dications (4.10), especially the latter. These are highly coloured compounds, which absorb well into the near-lR, (4.9) at 920 nm and (4.10) at 1064 nm,e 81 300.  

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The action of ammonia on N-(5-aryl-l,3-oxathioI-2-ylidene) tertiary iminium salts (7) affords linear intermediates that cyclize to 2-amino-4-phenylthiazoles (Scheme 7) (45). 

TL2733,82TL2737). The reaction proceeds through an aza-Cope rearrangement of the initially formed iminium salt, followed by intramolecular cyclization. 

Isoxazolidine reacted with acetone and perchloric acid to produce the iminium salt (189) <80JA1649). 

With strong acids, enamines give iminium salts, from which the parent enamine is regenerated by treatment with piperidine or triethylamine. Enamines are usually cleaved by warming in acetic acid-sodium acetate solution. The more labile enamines of saturated 3-ketones are hydrolyzed simply by heating in 95% ethanol. ... 

The formation of 88 is postulated to be occurring by the nucleophilic attack of a hydride ion (47), abstracted from the secondary amine, on the a-carbon atom of the iminium salt (89). The resulting carbonium ion (90) then loses a proton to give the imine (91), which could not be separated because of its instability (4H). In the case of 2-methyIhexamethylenimine, however, the corresponding dehydro compound /l -2-methylazacyclo-heptene (92) was isolated. The hydride addition to the iminium ion occurs from the less hindered exo side. 

Protonation with strong acids such as hydrochloric acid and hydro-bromic acids occurs, however, at the 6 position (53) to give the thermo- ynamically more stable zl -iminium salt (116), as shown by UV spectro- <=opy, = 274-278 m x. 

A hypsochromic shift of 20-50 cm is observed in the double-bond stretching region, when the enamines are converted to the corresponding iminium salts by the electrophilic addition of a proton at the /3-carbon atom. The shift is accompanied by an enhancement in the intensity of the band. Leonard and co-workers (68,71-74) have used this absorption shift as a diagnostic tool for the determination of the position of the double bond... 

The reduction was studied in more detail by Cook and Schulz (52). They demonstrated conclusively that reduction of iminium salts by secondary amines is possible as illustrated in Eqs. (7) and (8). The oxidation... 

Conformational effects appear to be important in determining which tertiary a-hydrogen is removed. For example, 4-phenyl-, 4-p-nitrophenyl-, and 4-/)-methoxyphenylquinolizidine (83) all are oxidized to the corresponding /j < -iminium salts (84) and not to the conjugated zJ -iminium salts (85) (86). The authors Judged that steric hindrance was responsible or that the conformation of the 4-sabstituted quinolizidines did not contain ideal... 

The preceding section described the preparation of enamines by mercuric acetate oxidation of tertiary amines. The initial product in these oxidations is the ternary iminium salt, which is converted to the enamine or mixture of enamines by reaetion with base. Thus iminium salts synthesized by methods other than the oxidation of tertiary amines or the protonation of enamines are potential enamine sources. 

The alkylation of imines by an alkyl halide to give an iminium salt will be illustrated by selected reactions over a period of years. A more complete survey is available (88). Decker and Becker (89) prepared a number of iminium salts (91, for example) by mixing methyl iodide and aromatic imines in benzene. 2,5-Dimethyl-2-pyrroline (92) has been alkylated and the... 

In the case of enamines protonation on nitrogen or carbon is possible and gives the conjugate acids 4 and 5, respectively. Whereas the final isolated product has the iminium salt structure (5), recent work 10-12) has shown... 

For purposes of characterization of enamines the perchlorate salts are preferred, as they crystallize well, and the perchlorate anion has no tendency to add to the iminium cation. Other salts, including hexachlorostannates (13), hexachloroantimonates (13), chlorides, bromides, tetraphenylborates, and nitrates, have also been used. Recently a method for the preparation of iminium salts directly from aldehydes or ketones and the amine perchlorate has been reported (16). 

When trichloroacetic acid is used to protonate an enamine (17,17a), the salt has only limited stability. The trichloroacetate anion undergoes decarboxylation and the trichloromethyl anion which is generated adds to the iminium salt, giving an a-amino trichloromethyl derivative (8). 

The iminium salts are of course especially Subject to attack by nucleophiles, and reactions of this type are discussed in Chapter 5. See also Section V.H. 

The course of the reaction appears to involve dehydrohalogenation of the intermediate iminium salt (30) to the new enamine (31), which then undergoes further alkylation. Evidence that alkylation in this case is directly on... 

The alkylation of dienamines has not been studied extensively. Thus the dienamine (44) is reported (43,44) to alkylate at Cj with methyl iodide to give the iminium salt (45), which on hydrolysis gives the alkylated a,)3-unsatur-ated ketone (46). Similar dienamines prepared from A -2-oko steroids. On the other hand, are reported to undergo only N alkylation (45). 

More recently the acylation of aldehyde enamines has been reinvestigated (75) and shown to proceed normally when the enamine is added to the acid chloride. The morpholine enamine of isobutyraldehyde (98), on being added to an ether solution of acetyl chloride, afforded the iminium salt (99), from which the ketoaldehyde (100) was obtained in 66% yield by hydrolysis (75). 

This result has been rationalized by consideration of the stability of the intermediate iminium salts (93). O acylation would give 134, whereas C acylation would give 135. The latter can undergo loss of a proton to give the product, whereas 134 cannot, but can revert to reactants, so that in this... 

The reduction of the double bond of an enamine is normally carried out either by catalytic hydrogenation (MS) or by reduction with formic acid (see Section V.H) or sodium borohydride 146,147), both of which involve initial protonation to form the iminium ion followed by hydride addition. Lithium aluminum hydride reduces iminium salts (see Chapter 5), but it does not react with free enamines except when unusual enamines are involved 148). 

In most reviews of enamine chemistry the reactions of iminium salts are scattered throughout the review and are consequently not covered in a comprehensive manner. This chapter will be an attempt to look at reactions that, at one stage or another, proceed by nucleophilic addition to the iminium intermediate. The subject of enamines has been reviewed 1-4) and certain aspects of iminium salt chemistry such as reduction of aromatic quaternary salts have been treated in detail (5). Consequently, the reduction of aromatic quaternary salts with complex hydrides will be presented here only briefly. Although the literature (especially 1950-1967) has been checked with care, the author can make no claim to completeness. The... 

II. Structure, Preparation, and Detection of Iminium Salts A. Structure oe Iminium Salts The positively charged carbon-nitrogen double bond... 

Iminium salts can be made in a number of ways such as protonation of enamines (7), alkylation of aldimines and ketimines (5,9), cleavage of a covalent bond in a... 

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