Imine-enamine tautomer

Enamines are normally stable only when there is no hydrogen on the nitrogen (R2C=CR—NR2). Otherwise, the imine form predominates. The energy of various imine-enamine tautomers has been calculated. ... 

Reaction of aryl hydrazines and hydrazine with dimethyl acetylene dicarboxylate to afford imine-enamine tautomers [192]. 

Deprotonation of the imine 86 (R1 = H, R2 = Bn, entry 1) with LDA (1 equiv.) at -35 °C afforded an azaenolate which was diastereoselectively attacked at -78 °C by the electrophilic reagent DBAD (1.5 equiv.). After 3 min the reaction was quenched with a 10 % aqueous NH4C1 solution. After classical work-up and flash chromatography, the aminated product 87 was obtained in 66% yield as a 7 3 mixture of diastereomers 87a 87b (method A). The major diastereomer was characterized by NMR spectroscopy (DMSO-d6,100 °C). Due to an imine-enamine tautomer-ism, 86 could be aminated without previous deprotonation 86 reacted with DBAD (1.4 equiv.) in THF at 0 °C for 1 h. After evaporation of the solvent and flash chromatography, 87a and 87b were isolated in 72 % yield with a diastereomeric ratio of 75 25 (method B). 

Imine-enamine tautomers 1-Aminoenedisulfides from iminodisulfides... 

Exists as an imine-enamine tautomer. This substance and its derivs. are used as corrosion inhibitors and vulcanisation accelerators. Used as a 5% soln. in cone. H2SO4 for detn. of Bi, Fe(/7), NO2 . Yellow prisms (H2O or DMSO aq.). Sol. CHCI3, mod. sol. Me2CO, Eton, insol. Et20, H2O. Mp 200-202°. 

Spectroscopic investigation of enamines conjugated with ketone, ester and nitrile groups established the prevalence of enamine rather than imine-enol tautomers in examples of secondary amines (206-212). Similar studies have been made with enamines of acylpyridines and acetophenones (213,214). 

Another approachl2a e makes use of bilenimincs which can be photochemically cyclized to isobacteriochlorins. A plausible mechanism for this ring-closure process involves the formation of an enamine tautomer from the imine at the 1-position, which is then cyclized in a photochemical 1 STc-n-isomcrization process followed by elimination of the leaving group at the 19-... 

The spiro compound 206 was prepared in five steps from (S)-l-naphthyl-ethylamine and was composed of a mixture of imine and enamine tautomers. Reduction of the imine function by sodium borohydride occurred on the less hindered si face, leading to the diamine with the R configuration of the newly formed stereo center, then the N-benzyl substituent was removed by hydrogenolysis to give 207 with good overall yield [98] (Scheme 30). 

Since 1960, almost all papers have provided at least some spectroscopic evidence that these compounds (1 and 2) exist as the enamine tautomer in solid phase or in solution. Imine tautomers have been detected for only those aminomethylenemalonates in which R2 and R3 formed a ring (e.g., 83IZV1687, 83MI2 88ZOR1793). This chapter will examine only certain papers in which details of spectroscopic investigations or quantum-chemical calculations are given. 

The prototropic tautomerism of 8-azaadenine has been studied theoretically in both the gas phase and aqueous solution by means of ab initio methods. It has been shown that dehydrovaline (399 R =Me, R = H, R = Me), dehydrophenylalanine (399 R = Ph, R, R = H), and dehydropipecolinic acid [399 R R = (CH2)3, R = H] hydrolyse rapidly via the imine tautomer (400) even when the corresponding esters and sodium salts exist as the enamine tautomers. The 3-methoxy-substituted deriva-... 

Imines derived from ketones with an a-methylene group can react via their enamine tautomers, and mixtures of triazoles are also isolated from these systems. The triazoline adducts of the enamine tautomers are aromatized by treating with acid, and in these conditions the triazoline appears to undergo a Dimroth rearrangement before elimination of the amine, because two triazoles are obtained, one of which has... 

The reaction is exactly analogous to the chemical aldol reaction (also shown), but it utilizes an enamine as the nucleophile, and it can thus be achieved under typical enzymic conditions, i.e. around neutrality and at room temperature. There is one subtle difference though, in that the enzyme produces an enamine from a primary amine. We have indicated that enamine formation is a property of secondary amines, whereas primary amines react with aldehydes and ketones to form imines (see Section 7.7.1). Thus, a further property of the enzyme is to help stabilize the enamine tautomer relative to the imine. 

A way forward might be to form the imine 7.3 [and hence its enamine tautomer 7.4] by reacting the phenylamine 7.2 with cyclohexanone (Scheme 7.18). Then to generate the benzyne anion 7.5 by treating the tautomers with sodamide and sodium fcr/-buloxide in THF. Cydization to the required indole 7.1 occurs through nucleophilic addition to the benzyne, followed by protonation during work-up. 

The UV spectra of 9-(phenylamino)tetrahydro-4f/-pyrido[ 1,2-a] pyrimidin-4-ones 23 indicated the presence of an imine-enamine type of tautomerism in solution. A carboxyl or ester group in position 3 (R1 = COOR3 R3 = H, Et) and a methyl group in position 6 (R = Me) shifted the equilibrium toward the enamine form in a polar solvent [85JCS(P1) 1015]. For the imine tautomer, UV maxima at 235-250 nm and 280-300 nm and for the enamine absorbances at 260-265 nm and 355— 365 nm are characteristic. 

Now consider condensation of ammonia with ketoester 1.18. The isolated product is not imine 1.19 but the thermodynamically more stable enamine tautomer 1.20 which has a conjugated double bond system and a strong intramolecular hydrogen-bond. Although not a heterocyclic example, 1.20 illustrates that an enamine-like linkage, as in generalised heterocycle 1.21, is also accessible by a condensation reaction. 

Spectroscopic studies of imine-enamine tautomerism have shown that the equilibrium is almost completely in favour of the imine form for simple aldehydes and ketones372-374. Nevertheless, some secondary enamines are sufficiently stable to exist in detectable amounts in equilibrium with the corresponding imines for example, the f-butylamine imine of cyclohexanone shows signals due to the secondary enamine tautomer in the NMR spectrum (<5=CH 4.6)375. Studies of the imine-enamine equilibria have shown, as expected, that the enamine form is stabilized by methyl or aryl substituents at the -position (Scheme 189). 

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