Enamines. III. A theoretical and photoelectron spectroscopic study of the molecular and electronic structures of aziridine enamines

The photoelectron (PE.) spectra of N-vinylaziridine ( 1 ) and some methyl and ethyl substituted derivatives are discussed in the light of quantum-chemical model calculations using the PRDDO SCF method. All aziridine enamines are found to exist as equilibrium mixtures of variable compositions, with an enamine-type conformation ( I ) generally as the major and a trans-bisected form (II) as the minor component.     

Recent advances in transition metal-catalyzed enantioselective hydrogenation of unprotected enamines

Transition metal-catalyzed enantioselective hydrogenation of enamines is undoubtedly a useful and environment-friendly method for the preparation of optically pure chiral amines and amine derivatives. Over the last few decades, the use of transition metal catalysts containing chiral phosphorus or phosphine-oxazoline ligands attracted much attention for the hydrogenation of unprotected enamines. A number of efficient chiral catalysts have been developed, and some of them have shown high potential for the application in the synthesis of optical chiral amines in both laboratory and industry. This tutorial review focuses on the contributions concerning the transition metal-catalyzed enantioselective hydrogenation of unprotected enamines for the synthesis of chiral amines and amine derivatives.     

Vicarious nucleophilic substitution in enamine derivatives of 1-hydroxynaphthalene-2,4-dicarbaldehyde

The reaction of aromatic amines with highly stable Schiff base enamines formed from an alkyl amine and 1-hydroxynaphthalene-2,4-dicarbaldehyde resulted in nucleophilic substitution of the alkyl amine with the aromatic amine in ethyl alcohol at room temperature within 1-2 min. This reactivity, regioselectivity and formation of stable derivatives are due to extra stabilization through extended conjugation in these systems.     

Enamines I. Vinyl Amine,a Theoretical Study of its Structure,electrostatic potential,and proton affinity

The structure of vinyl amine and its reactivity towards a proton is studied by the PRDDO SCF MO method. The equilibrium structure is found to be non-planar and barriers to inversion- and rotation-dominated processes are calculated. Proton addition to vinyl amine, as a model of enamine protonation, is examined by means of electrostatic molecular potentials and C- versus N-proton affinities.     

A new synthesis of 4-thiouracils

Ethoxycarbonyl isothiocyanate has been found to form adducts with enamines which, on treatment with primary amines or ammonia, undergo apparent “amine exchange” and cyclization to 4-thiouracil derivatives. Evidence for the 4-thiouracil structure includes both spectral data and chemical transformations.     

Highly chemoselective formation of aldehyde enamines under very mild reaction conditions

Although ketone enamines are widely used in organic synthesis, aldehyde enamines are rarely employed due to the limitations of their preparation using known methods (need for acid or base, excess of amine, and/or elevated temperature). We have successfully developed rapid and particularly mild condensation conditions (1 h, 0 degrees C, 1.2 equiv of amine) leading to di- and trisubstituted enamines with excellent conversion (84-100%). Remarkably high chemoselectivity was observed with complete discrimination between aldehyde and ketone, among other functional groups positively tested.     

Acetals of lactams and acid amides. 50. 1-Cyanomethylpyrrolid-2-one diethylacetal in the synthesis of derivatives of pyrrolo[1,2-a]pyrroles

We have synthesized a series of enamines — derivatives of 1-cyanomethyl-2-methy-enepyrrolidine-by reaction of 1-cyanomethylpyrrolid-2-one diethylacetal with compounds having a reactive methylene link. The enamines undergo a Thorpe-Zeigler cyclization in the presence of bases with the formation of pyrrolo [1,2-a]-pyrrole derivatives. The configuration of the enamines obtained were studied using PMR spectroscopy.For Communication 49, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1616–1620, December, 1987.     

Reductive cleavage of an SiCl3 group by amine hydrochlorides and base

A new procedure has been developed for removing the SiCl₃ group from α-trichlorosilyl-substituted amines. In this procedure the trichlorosilyl substrate is treated first with amine hydrochloride in CH₃CN, followed by hydrolysis with aqueous hydroxide. Using this sequence, amines corresponding to replacement of the trichlorosilyl group with hydrogen can be conveniently isolated in good yields for all tested cases. Evidence suggests the intermediacy of an unstable chlorosilane species formed by probable nucleophilic attack of chloride. In conjunction with the addition reaction fo HSiCl₃ to enamines to form the α-trichlorisilyl amines, the cleavage completes a new method for the transformation of enamines to amines which could be a useful alternative to either catalytic hydrogenation or metal hydride reduction.     

Reaction of ω-cyanoacetophenone and some of its derivatives with secondary amines. New synthesis of aminopyridine derivatives

ω-Cyanoacetophenone (1a) and its derivatives 1b-c react with morpholine (or piperidine) to give mainly 2,4-diaryl-3-eyano-6-morpholino- (or piperidino-) pyridine derivatives (5) ; relative β-aminoeinnamonitriles 6 and very small amounts of amidines 4 are also obtained. When pyrrolidine is used compounds 5 cannot be detected and enamines 6 are the main product. A mechanism involving the intermediate formation of enamines 6 (as electrophiles) and of carbanions 11 (as nucleophiles) is proposed to explain this new synthesis of aminopyridine derivatives.     

Reaction of nitric oxide at the beta-carbon of enamines. A new method of preparing compounds containing the diazeniumdiolate functional group

The reaction of nitric oxide (NO) with enamines has been investigated. Unlike previously reported reactions of NO as a free radical with alkenes, the electrophilic addition of NO to the beta-carbon of enamines results in the formation of compounds containing the diazeniumdiolate functional group (-[N(O)NO](-)). This reaction between NO and enamines has been shown to be quite general and a variety of enamine-derived diazeniumdiolates have been isolated and characterized. While enamines derived from aldehydes and ketones whose structures allow for sequential multiple electrophilic additions tended to undergo overreaction leading to unstable products, it has been shown that this complication may be overcome by suitable choice of reaction solvent. The products obtained may exist as zwitterionic iminium salts or as neutral species depending upon the structure of the parent enamine. The diazeniumdiolate derived from 1-(N-morpholino)cyclohexene is unique among the new compounds in that it spontaneously releases NO upon dissolution in buffered aqueous solution at pH 7.4 and 37 degrees C. While the total quantity of NO released by this material (ca. 7% of the theoretical 2 moles) is apparently limited by a competing reaction in which it hydrolyzes to an alpha-diazeniumdiolated carbonyl compound and the parent amine, this feature may prove to be of great value in the development of multiaction pharmaceuticals based upon this new type of NO-releasing compound. Reports of enzymatic (oxidative) release of NO from previously known carbon-bound diazeniumdiolates also suggest that analogues of these compounds may be useful as pharmaceutical agents. This new method of introducing the relatively rarely studied diazeniumdiolate functional group into organic compounds should lead to further research into its chemical and biological properties.     

n-π-Interaction in Enamines and Enaminoketones. A 15N-NMR. Study

¹⁵N-Chemical shifts of 32 enamines, 11 enaminoketones and 28 closely related amines have been determined with the isotope in natural abundance. In order to eliminate substituent effects, differential chemical shifts Δδ(N) are defined as δ_N(amine)-δ_N(enamine). This parameter is shown to correlate well with the free enthalpy of activation ΔG# for restricted rotation about the N? C(α) bond in enamines with extended conjugation. Δδ(N) values of substituted anilinostyrenes correlate also with ¹³C-chemical shifts of the β-carbon in the enamine system and with Hammett σ-constants of the aniline substituents. The experimental results suggest that differential ¹⁵N shifts are a useful probe to study n, π-interaction in enamines.     

Benzotriazole-assisted synthesis of enamines

A new, facile preparation of enamines was achieved a two-step sequence: (i) the ready formation of an N-(-aminoalkyl)benzotriazole derivative from equimolar amounts of benzotriazole, an aldehyde, and a secondary amine, (ii) the elimination of benzotriazole from the derivative by sodium hydride in tetrahydrofuran (THF). The method provides enamines in good overall yields based on the quantity of amine used.     

Amines as Catalysts: Dynamic Features and Kinetic Control of Catalytic Asymmetric Chemical Transformations to Form C−C Bonds and Complex Molecules

Carbonyl transformations involving enolates and/or enamines have been used for various types of bond-forming reactions. In this account, catalysts and catalyst systems that have amino acids or primary, secondary, and/or tertiary amines as key catalytic functional groups that we have developed to accelerate chemical transformations, including regio-, diastereo- and enantioselective reactions, are discussed. Our chemical transformation strategies and methods that use amine derivatives as catalysts are also discussed. As amines can have different functions depending on protonation and on the species formed during the catalysis (such as enamines and iminium ions), dynamics and kinetic controls are the keys for understanding the catalysis. Further, strategies that harness dynamic steps and kinetic control in amine-catalyzed reactions have enabled the synthesis of complex molecules in stereocontrolled manners. Understanding the dynamic features and the kinetic controls of the catalysis will further the design of the catalysts and the development of chemical transformation strategies and methods.     

Molecular orbital treatments of hydrogen bonded systems. 2. Dimers of water and HCN

Calculations were performed to investigate the reliabilities of the CNDO/2, PRDDO, and MNDO approximate molecular orbital methods. Systems selected for study included the linear, cyclic, and bifurcated dimers of water as well as the linear and cyclic dimers of HCN. The PRDDO method was found to provide the most consistently accurate reproduction of ab initio and experimental data. CNDO/2 performed fairly well in a number of cases but yielded extremely poor results for the cyclic dimers of both H₂O and HCN. Hydrogen bond strengths were consistently underestimated by MNDO which also furnished erroneously large intermolecular separations. In addition, MNDO calculations indicate the bifurcated water dimer to be most stable in contrast to other quantum mechanical and experimental information.See Ref. [1] for Paper 1 of this series.     

Stille cross-coupling reaction of an alpha-stannyl enamide.

The Stille cross-coupling reaction of an N-tosyl alpha-stannyl enamine is clearly exemplified for the first time with a wide range of halogeno derivatives. This gives access to functionalized alpha-substituted enamines in fair yields.     

Detection of enantiomers of chiral primary amines by 1H NMR analysis via enamine formation with an enantiopure γ-position aldol product of a β-keto ester

A ¹H NMR analysis method that detects enantiomers of molecules bearing a primary amino group has been developed. The method uses a β-keto ester-derived probe that forms enamines with the amines and is able to discriminate enantiomers of functionalized amines and amines that have chiral centers at positions remote from the amine group.     

Solid-phase synthesis of 1,2,3,4-tetrahydro-2-pyridones via aza-annulation of enamines

An efficient solid-phase approach has been developed to prepare nitrogen heterocycles with a 1,2,3,4-tetrahydro-2-pyridone core via aza-annulation of enamines. Immobilized enamines were prepared from the reaction of primary amines with propynoic acid derivatives or ketones. Aza-annulation reactions were carried out by reacting resin-bound enamines with symmetrical alpha,beta-unsaturated acid anhydrides or alpha,beta-unsaturated acids in the presence of DPPA and TEA. The annulation products were isolated in good to high crude yields. Influence of sterically hindered amines as well as alpha- and beta-substituted acrylic acid derivatives on the annulation reaction was also investigated.     

Lactam and acid amide acetals. 58. New synthesis of 3-nitro-5-hydroxybenzofurans

Previously unknown 3-nitro-5-hydroxybenzofuran derivatives were obtained by condensation of p-benzoquinones with nitro enamines.See [1] for Communication 57.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 597–600, May, 1990.     

Studies in organic mass spectrometry—VI. The fragmentation of enol derivatives of acyclic β-diketones

The mass spectra of enol derivatives of β-diketones, such as enol ethers and enaminies are discussed. Their behaviour under electron-impact is in accordance with the fragmentation we suggested for an acyclic β-diketone in the enol form. Rearrangement of the enol functional group is observed in the spectra. This process is a general one, as it show not only migration of oxygen and nitrogen in the enol ethers and enamines, but also migeration of sulphur and chlorine in the thio-ether and in 4-chloro-3-pentene-2-one. Enol derivatives are suitable compounds for determining the branching of alkyl chains in β-diketones.     

Mechanism of cyclocondensation of isoprenoid acyclic α,Β-enals with monoethyl malonate under the conditions of the Knoevenagel reaction

It was shown that the formation of the di- and trisubstituted derivatives of 1.3-cyclohexadiene-1-carboxylic acid during the condensation of 3-methyl-2-butenal, citral, and farnesal with monoethyl malonate in the presence of secondary amines takes place through the enamines corresponding to these aldehydes, which add to the monoesters of the respective alkenylidenemalonic acids (the normal products of the Knoevenagel reaction) by a mechanism of [4 + 2]-cycloaddition. The free carboxyl group in the dienophile is required for the spontaneous transformation of the intermediate [4 + 2]-cycloadducts into the derivatives of cyclohexadiene-1-carboxylic acid in which the catalyst of the process (the secondary amine) is regenerated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1079–1088, May, 1991.