Iminium salts, from enamines

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. ... 

With strong acids, enamines give iminium salts, from which the parent enamine is regenerated by treatment with piperidine or triethylamine.115... 

Preparation of imines and enamines from carbonyl compounds and amines can be achieved with a dehydrating agent under acid/base catalysis [563]. Basically, primary amines afford imines unless isomerization to an enamine is favored as a result of conjugation, etc (see Eq. 252), and secondary amines afford iminium salts or enamines. These transformations can be conducted efficiently with a catalytic or stoichiometric amount of a titanium salt such as TiCU or Ti(0-/-Pr)4. Equation (247) illustrates an advantageous feature of this method in the imination of a hindered ketone. f-Butyl propyl ketone resisted the formation of the imine even by some methods reported useful for sterically hindered ketones [564,565]. The TiCU-based method works well, however, for this compound, giving the desired imine in high yield within a relatively short reaction period [566]. Imine derivatives such as iV-sulfonylimines could be... 

In this section we shall look at (mostly) heterocyclic syntheses where a vital intermediate is an unstable imine, iminium salt, or enamine that cannot be made stoichiometrically. The only way to use such an intermediate is to prepare it in low concentration as part of a tandem process in the presence of the third reagent. If the second step is fast enough, the unstable intermediate will be removed from the equilibrium and the process continues. 

If one keeps in mind that this process can provide N-oxides of polysubstituted heterocyclic compounds from simple olefins, further improvement of face selectivity would certainly be welcome, one of the reasons being that the N-oxides obtained this way, as well as those prepared by peracid oxidation, have proven to be broadly applicable precursors of iminium salts and enamines, which are both of high value in alkaloid synthesis. 

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). 

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). 

Iminium salts are readily available from C protonation of the corresponding enamines (7). Experimentally the procedure is very simple The enamine dissolved in ether or some other solvent is treated with an appropriate acid such as anhydrous hydrogen chloride or 70% perchloric acid. The iminium salt usually separates and is then collected. Protonation at low temperatures... 

The close agreement of the three methods supports the contention that protonation at low temperatures first occurs at nitrogen and is followed by a proton shift to give the iminium salt (M). The rate of this rearrangement is dependent on temperature, the nature of the amine, and the nature of the carbonyl compound from which the enamine was made. Even with this complication the availability of iminium salts is not impaired since the protonation reaction is usually carried out at higher temperatures than —70°. Structurally complicated enamines such as trichlorovinyl amine can be readily protonated (17,18). 

The previous sections have dealt with stable C=N-I- functionality in aromatic rings as simple salts. Another class of iminium salt reactions can be found where the iminium salt is only an intermediate. The purpose of this section is to point out these reactions even though they do not show any striking differences in their reactivity from stable iminium salts. Such intermediates arise from a-chloroamines (133-135), isomerization of oxazolidines (136), reduction of a-aminoketones by the Clemmensen method (137-139), reductive alkylation by the Leuckart-Wallach (140-141) or Clarke-Eschweiler reaction (142), mercuric acetate oxidation of amines (46,93), and in reactions such as ketene with enamines (143). 

This is the most common method for the preparation of enamines and usually takes place when an aldehyde or ketone containing an a hydrogen is treated with a secondary amine. The water is usually removed azeotropically or with a drying agent, but molecular sieves can also be used. Stable primary enamines have also been prepared.Enamino-ketones have been prepared from diketones and secondary amines using microwave irradiation on silica gel. ° Secondary amine perchlorates react with aldehydes and ketones to give iminium salts (2, p. 1178). Tertiary amines can only give salts (12). 

The reaction from an enamine is initiated by the addition of a trace of a strong acid, e.g. /7-toluenesulfonic acid (TsOH, 4-methylbenzene-sulfonic acid), to the ketone and pyrrolidine in a solvent such as toluene. When the mixture is at reflux in a Dean-Stark apparatus, water is liberated and is removed through azeotropic distillation, leaving the enamine in the reaction vessel. After a follow-up reaction between the enamine and a suitable electrophile, an iminium salt is produced that liberates both the a-substituted ketone and pyrrolidine when it is treated with aqueous acid (Scheme 6.20). 

Other classical synthetic approaches to 2-furanamine have failed, including the Curtius method and Beckmann rearrangement of 2-benzoylfuran oxime. However, hydrazinolysis of AT-(2-furyl)phthalimide, obtained from phthalimide and 2,5-dimethoxy-2,5-dihy-drofuran, gives 2-furanamine which was not isolated but detected by GLC-MS and H NMR spectroscopy. The latter reveals the absence of imino tautomers (75AP713). The chemistry of 2-dialkylamino-5-phenylfurans is typical of enamines protonation occurs on carbon to produce iminium salts. They are stable to base but afford 5-phenylfuran-2(3//)-one on hydrolysis with dilute acid. 2-Morpholino-5-phenylfuran couples with diazonium salts and affords Diels-Alder adducts with maleic anhydride and IV-phenylmaleimide (73JCS(P1)2523). 

With the procedure for constructing the quaternary carbon stereocenter in hand, the conversion of the ris-form to the trans form was explored in accordance with the synthetic plan shown in Scheme 9. The ketone moiety of the 1,4-conjugated adduct 61 was protected by an acetal group, followed by decarboxylation of compound 65 using sodium ethylthiolate to yield lactam trans-62 and cis-62 as an 8 1 diastereomixture [31]. The reason why the lactam trans-62 was obtained as a major product is that the subsequent protonation after decarboxylation proceeded kinetically. This assertion is supported by experimental results in which the trans- and cis-lactam diastereomixture (8 1) in ethanol was refluxed in the presence of potassium hydroxide to afford a 1 5 mixture [15,32,33]. The mixture of the lactam trans-62 and cis-62 was reduced with DIBALH, followed by treatment with sodium hydroxide to give bicyclic enamine 63. The kinetic iminium salt prepared from bicyclic enamine 63 with hydrochloric acid was reduced with sodium cyanoborohydride, leading to the frans-decahydroisoquinoline structure [22], The acetal moiety of the resultant 67 was removed to provide the objective ketones 68a and 2c. This method enabled the construction of the tra s-decahydroisoquinoline structure without an intermediate resembling the neurotoxic MPTP, and in fewer steps. 

Among the best enol equivalents for aldehydes are enamines.19 They are stable compounds, easily made from aldehydes 95 and secondary amines, reacting with electrophiles in the same way as enols 96 to give iminium salts 97, hydrolysed to substituted aldehydes 98. 

If a proton is added to enamine, diastereomers are possible products when an optically active secondary amine is used, assuming the iminium salt has been produced. To investigate this matter, the series of reactions shown in Figure 4 is performed. The salt is made from the acid using HC1, and D20 hydrolysis of the resultant salt is carried out. Since deuterium is not incorporated into the recovered a-phenylpropionaldehyde, chirality is believed to be induced before the hydrolysis (4). 

The chirality is decided when a porton is added to the enamine nitrogen on the amine group. Furthermore, when rearrangement to the iminium salt occurs, the chirality of the iminium proton is decided by this first proton. Upon hydrolysis of the enammonium salt, water is added from the opposite side of the enammonium proton and asymmetry of the recovered carbonyl compounds results. When an iminium salt with established chirality is hydrolyzed, the resultant carbonyl compound will have the reversed chirality because the hydrolysis occurs at the carbon-nitrogen double bond. 

Enamines are intermediate in reactivity more reactive than an enol, but less reactive than an enolate ion. Enamine reactions occur under milder conditions than enolate reactions, so they avoid many side reactions. Enamines displace halides from reactive alkyl halides, giving alkylated iminium salts. The iminium ions are unreactive toward further alkylation or acylation. The following example shows benzyl bromide reacting with the pyrrolidine enamine of cyclohexanone. 

Enamines are made from secondary amines and aldehydes or ketones via the iminium salt you met them in Chapter 14 and have seen them in action in Chapters 21,26, and 27. 

FIGURE 8. UV absorption spectra of the enamine (a) l-piperidino-2-ethyl-l-butene (219f) in hexane and the corresponding C-protonated iminium salt (b) V-[2-ethylbutylidene]piperidinium chloride (220) in acetonitrile. (Reproduced with permission from Reference 343)... 

The imine, prepared in the usual way from a carbonyl compound and a primary amine, can be converted into the tertiary enamine by direct N-alkylation to tertiary iminium salt, which can be converted into the corresponding enamine by base181-183 (equation 5). 

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

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