Secondary enamines, structure

In streptocyanine dyes both ends of the methine chain are joined directly to nitrogen atoms, and a double enamine structure is thus present. The dyes are extremely susceptible to hydrolysis, particularly if they contain secondary nitrogen atoms. Stable dyes are obtained if the nitrogen is part of a heterocyclic ring system. Streptocyanine dyes are brilliant yellow dyes that dye polyacrylonitrile and acid-modified polyamide fibers with outstanding lightfastness [1],... 

The typical behavior of enamines has been mainly observed for compounds possessing a tertiary nitrogen atom.1 The analogous derivatives with a secondary amino group (the a,j8-unsaturated secondary amines) could, in principle, possess either the imino or the tautomeric enamine structure, but the first possibility is preferred practically without exception. In the text, some examples of their properties are quoted for the sake of comparison with those of tertiary enamines on these occasions, the group designation imines is used. Nucleophilic reactions of a limited number of aromatic heterocyclic systems are also included when they are similar to the reactions of enamines and illustrate the specific character of the enamine grouping. 

Although it is a generally accepted classification of enamines as (i) primary, (ii) secondary and (iii) tertiary, it was not necessary to form separate sets comprising these types of enamines for two reasons. The first was that (i) contained only 7 and (ii) 18 entries, which are insufficient for statistical analysis in comparison with the tertiary enamines (iii 449 entries). The second reason was a lack of special structural features which would justify such a separate treatment, as it was found that primary and secondary enamines contribute to all the four classes (a)-(d). The structures of the compounds in set S together with their reference codes (REFCOD s) ascribed to crystal structures in CSDS and the appropriate reference are given in the Appendix at the end of this chapter. It should be noted that the number of published crystal structures is less than the number of entries (enamine fragments), because more than one fragment may occur in a particular molecule. 

IR, UV and NMR spectra obtained so far are compatible with the structure of the secondary enamines. The tautomeric enamine structure of some enaminones in the solid state has also been established unambiguously by single-crystal X-ray diffraction analyses21. In agreement with the experimental evidence, theoretical calculations22,23 performed on enaminones 6-8 and nitroenamine 9 indicate that the enamine forms are... 

The subsequent oxidation at the secondary carbon in the enamine structure (center of Formula 18) is then clearly a very favorable reaction, the CHj group being doubly activated by allylic double bonds ... 

The pK values of the secondary enamines given in Table 3 are ca 2 units lower than those of the corresponding tertiary enamines. This is largely due to the fact that these compounds have imine structures ", so we shall not deal with them here. 

Scheme 5.34. (a) Suprafacial Michael addition-proton transfer of a tertiary enamine [180], (b) aza-ene-like transition structure for secondary enamine Michael additions [179]. 

Scheme 5.37. AT3 strain raises the energy of the transition structure for Michael addition of the less substituted secondary enamine [179]. The dashed lines in the transition structures indicate the primary MO interactions, according to Scheme 5.34c.

Enamine (Section 17.11) Product of the reaction of a secondary amine and an aldehyde or a ketone. Enamines are characterized by the general structure... 

The oxidation of amines by mercuric acetate is an old reaction (54) which up until recent years was employed primarily to modify alkaloid structures (55). A systemic study of the oxidizing action of mercuric acetate by Leonard and co-workers led to the development of a general method for the synthesis of enamines from cyclic tertiary amines. An observation made after a large number of compounds were oxidized, but which is worth noting at the onset, is that a tertiary hydrogen alpha to the nitrogen atom is removed preferentially to a secondary a-hydrogen. 

The addition of secondary amines to 1-cyanoallenes (161) results in the formation of enamines in 80-90% yield (124). Addition can occur at the 1,2 or 2,3 double bonds so that a mixture of isomeric enamines (162 and 163) is formed. The ratio of products is influenced by the alkyl substituents on the cyanoallenes and the structure of the secondary amine. 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

A recent adaptation of the procedure employing perchlorate and fluoro-borate salts has been reported by Leonard and Paukstelis (J5). This report includes proof of structure by direct comparison to iminium salts prepared by protonation of enamines. The general reaction reported was that of a ketone or aldehyde with a secondary amine perchlorate to give iminium salts. A large structural variety of carbonyl compounds and several amine... 

Analogous compounds with a secondary amino group (a,j8-unsaturated secondary amines) can, in principle, exist in either the form of imines (6) or the tautomeric form of enamines (7). As they practically occur and react in the former structure, it is more convenient to use the group designation imines. ... 

Enamines formed in this way may be distilled or used in situ. The ease of formation of the enamine depends on the structure of the secondary amine as well as the structure of the ketone. Thus pyrrolidine reacts faster than morpholine or piperidine, as expected from a rate-controlling transition state with imonium character. Six-membered ring ketones without a substituents form pyrrolidine enamines even at room temperature in methanol (20), and morpholine enamines are generated in cold acetic acid (21), but a-alkylcyclohexanones, cycloheptanone, and linear ketones react less readily. In such examples acid catalysis with p-toluenesulfonic acid or... 

In the reactions of benzyne with enamines, arylated enamines or amino-benzocyclobutenes can be obtained, depending on reaction conditions and the structure of the enamine. Thus the presence of a proton source such as a secondary amine will favor the enamine product through capture of the zwitterionic intermediate, whereas in the absence of protons one sees... 

The formation of enamines from carbonyl compounds and secondary amines usually entails as only questionable structural feature, the possible isomeric position of double bonds in the product. Molecular rearrangements have not presented synthetic limitations. A notable exception is the generation of o-aminophenols on distillation of enamines derived from 2-acylfurans 620,621). 

Abstract The reversible reaction of primary or secondary amines with enolizable aldehydes or ketones affords nncleophilic intermediates, enamines. With chiral amines, catalytic enantioselective reactions via enamine intermediates become possible. In this review, structure-activity relationships and the scope as well as cnrrent limitations of enamine catalysis are discnssed. 

Stork enamine synthesis takes advantage of the fact that an aldehyde or ketone reacts with a secondciry cimine to produce an enamine. Enamines cire resonance stabilized (see Figure 15-25) and have multiple applications. In the first resonance structure, the nitrogen is the nucleophile, while in the second resonance structure, the Ccirbanion is the nucleophile. Some commonly used secondary amines, pyrrolidine, piperidine, and morpholine, are shown in Figure 15-26. 

Furin et al. [90] have conducted the electrochemical fluorination of a series of amines, tripropylamine, tributylamine and triamylamine, and a series of enamines formed by the reaction of hexafluoropropene, its dimers and trimers with the secondary amines dipropylamine, dibutylamine and dial-lylamine. As well as the anticipated saturated perfluoroanalogues of the starting materials by-products resulting from structural breakdown were also produced. 

Secondary amino compounds of the type R2N—H add to aldehyde and ketone carbonyl groups in an acid-catalyzed reaction in much the same way as do RNH2 compounds—with one important difference. The product contains the structural unit C=C—N rather than C—C—N and because there is a carbon-carbon double bond, such a substance is called an enamine (alkene + amine). An example is ... 

The tertiary enamines, in contrast to the secondary derivatives, cannot exhibit enamine-imine tautomerism. As the free bases, they exist only in the vinylamino form. Their physico-chemical properties are in agreement with this structure, especially the spectral properties. The bands due to the stretching frequency of the carbon-carbon double bond in their infrared spectra1-25-27 (situated at 1630-1660 cm-1 according to the nature of the substituents) occur at somewhat lower frequencies, but their intensities are greatly increased in comparison to those of simple olefins because of conjugation with the free electron pair on the nitrogen atom. Indications of cis-trans isomerism... 

The choice of the secondary amine for formation of the enamine is not completely arbitrary even though it does not end up in the final alkylated product. Simple dialkyl amines can be used but cyclic amines such as pyrrolidine, piperidine, and morpholine are popular choices as the ring structure makes both the starting amine and the enamine more nucleophilic (the alkyl groups are tied back and can t get in the way). The higher boiling points of these amines allow the enamine to be formed by heating. 

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