Enamines properties

Interaction of substituted dienediamines with ethylenediamine yields im-idazo[3,2-n]pyridine derivatives (95JHC477). 3-Arylaminoindoles, with distinct enamine properties, are the starting materials for the synthesis of indolo[3,2-b]quinoline derivatives [96KFZ(7)42]. a-Carboline derivatives can be obtained from enamine-based 3-dimethylamino-2-indolinones [96KFZ(9)35, 96KFZ(10)32], The synthesis and transformation of enamines based on the pyrido[l,2-n]pyrazine ring system into imidazo[l,2-n]pyridine and imidazo[l,2-a]pyrimidine derivatives have been reported... 

Rifamycin S also undergoes conjugate addition reactions to the quinone ring by a variety of nucleophiles including ammonia, primary and secondary amines, mercaptans, carbanions, and enamines giving the C-3 substituted derivatives (38) of rifamycin SV (117,120,121). Many of the derivatives show excellent antibacterial properties (109,118,122,123). The 3-cycHc amino derivatives of rifamycin SV also inhibit the polymerase of RNA tumor vimses (123,124). 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

This chapter is devoted mostly to the discussion of the structure and physical properties of the enamines with a tertiary nitrogen atom, the emphasis being on enamines of cyclic ketones. 

The following reference numbers, some of which have been cited previously, are good sources for lists of simple enamines, their physical properties, methods of preparation or references thereto, and yields I, 9, 18,19, 24,35,36,36a, and 37. They provide a reasonable starting place for a research scientist who requires this kind of information. 

Electrophilic attack can occur on the /3-carbon atom as well as on the nitrogen atom. The fact that enamines are basic compounds is a further characteristic property. 

The difunctionality of the enamines of cyclic ketones toward phenyl isocyanate provides the ideal situation for a potential polymer. The properties of the polyamides produced by addition of various diisocyanates to the enamines of cyclic ketones have been reported Ilia). 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

Stretching vibration in the infrared region. The imine salts possess an active hydrogen, whereas their quaternization products exhibit the same spectral properties as the enamine salts (187). 

Enamines in which the double-bond shift is sterically prevented afford only the ammonium salts. Their spectra in the C=C stretching vibration region does not differ greatly from that of the free amine spectrum (171). For example, neostrychnine (159) has vc c 1666 cm and its perchlorate at 1665 cm . Salts of quinuclideine (92) and the polycyclic alkaloid trimethylconkurchine have similar properties. 

The bulk of enamine studies since Stork s original publication have focused on establishing the breadth and limitations of individual substitution reactions and on extending the list of useful electrophiles. In addition, auxiliary studies have enriched our knowledge about the ambident nature of the vinyl nitrogen system, stereoelectronic factors governing its reactivity, its stability and spectroscopic properties. An increasing number of synthetic applications of these fundamental studies can be expected in future years. 

The objective of this book is to review and correlate in depth the synthetic, mechanistic, and physical properties of enamines. This has been done by surveying the field from eight different perspectives. Since these perspectives have certain common areas between them, and since it seemed desirable to allow each contributor to tell his story in its fullest scope and in his own style, there is some overlapping of material between the chapters. However, it is hoped that the difference of viewpoints and context in which this material appears in these chapters will provide the reader with fresh insights and stimulate further research. 

In 1887, Conrad and Limpach described the condensation of ethyl acetoacetate 5 with aniline 1 to provide enamine 6. Subsequent warming of the mixture provided quinoline 7. Limpach reported several years later that the yield of the cyclization step was improved when an inert solvent (e.g., mineral oil) was employed. While the cyclization step was normally quite facile at 240-280 °C, the physical properties and the methods described for the preparation of enamino-esters were inconsistent. 

Beneficial Micro Reactor Properties for Formation of Enamines... 

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