Keto-enamine Form Research Articles

Syntheis of Novel Anil-Like Compounds from Heteroaryl Amines, Naphthols, and Triethylorthoformate Under Solvent-Free Conditions

One-pot, three-component reactions of naphthols, triethylorthoformate, and heteroaryl amines in the presence of formic acid catalyst under solvent-free conditions lead to the formation of the corresponding new hydroxynaphthalidene heteroaryl amines (anil-like) in moderate to good yields in an environmentally friendly reaction in organic synthesis. Investigation of 1H NMR and infrared spectra indicated that the compounds are in tautomeric equilibrium between the enol-imine and keto-enamine forms. Furthermore, intramolecular hydrogen bonds have been observed in the synthesized Schiff bases.

4-[(3-Hydroxyanilino)(phenyl)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

In the title compound, C23H19N3O2, the dihedral angles formed by the pyrazolone ring with the three benzene rings are 30.91 (6), 60.96 (4) and 57.01 (4)°. The ligand is in the enamine–keto form and its structure is stabilized by an intra­molecular N—H⋯O hydrogen bond. In the crystal, O—H⋯N hydrogen bonds link mol­ecules into chains parallel to [01-1].

(4Z)-4-[(2-Chloroanilino)(phenyl)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

The title compound, C23H18ClN3O, exists in an enamine–keto form with the amino group involved in an intra­molecular N—H⋯O hydrogen bond. The five-membered ring is nearly planar, the largest deviation being 0.0004 (7) Å, and makes dihedral angles of 16.62 (6), 41.89 (5) and 71.27 (4)° with the phenyl rings. In the crystal, weak C—H⋯O hydrogen bonds link the mol­ecules into supra­molecular chains along the b axis.

(Z)-4-[(Ethylamino)(furan-2-yl)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

In the crystal of the title compound, C17H17N3O2, the mol­ecules exist in the keto–enamine form. The pyrazole ring is oriented at 10.59 (4) and 57.98 (5)° to the phenyl and furyl rings, respectively, and the dihedral angle between phenyl and furyl rings is 73.30 (11)°. An intra­molecular N—H⋯O hydrogen bond occurs between imino and carbonyl groups. In the crystal, weak C—H⋯O hydrogen bonds link the mol­ecules into supra­molecular chains along the b axis.

(E)-N′-(5-Chloro-2-hydroxybenzylidene)-2,4-dihydroxybenzohydrazide methanol monosolvate

In the title compound, C14H11ClN2O4·CH3OH, the mol­ecule adopts an E conformation about the C=N bond. The compound is in the enamine–keto form. The two terminal benzene rings make a dihedral angle of 10.53 (9)°. Intra-mol­ecular O—H⋯O and O—H⋯N hydrogen bonding stabilizes the mol­ecular structure. In the crystal, O—H⋯O hydrogen bonds link the mol­ecules, forming chains running along the b axis.

4-[(Z)-(n-Butylamino)(phenyl)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

The title compound, C21H23N3O, exists in an enamine–keto form with the amino group involved in an intra­molecular N—H⋯O hydrogen bond. The dihedral angle between the phenyl rings is 73.59 (6)°. The five-membered ring is nearly planar, the largest deviation being 0.0004 (7) Å, and makes dihedral angles of 4.81 (6) and 69.81 (5)° wth the phenyl rings. In the crystal, pairs of weak C—H⋯O inter­actions link the mol­ecules into centrosymmetric dimers.

Studies on molecular structure and tautomerism of a vitamin B6 analog with density functional theory

This work presents a computational study on the molecular structure and tautomeric equilibria of a novel Schiff base L derived from pyridoxal (PL) and o-phenylenediamine by using the density functional method B3LYP with basis sets 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p) and 6-311++G(d,p). The optimized geometrical parameters obtained by B3LYP/6-31G(d,p) method showed the best agreement with the experimental values. Tautomeric stability study of L inferred that the enolimine form is more stable than its ketoenamine form in both gas phase and solution. However, protonation of the pyridoxal nitrogen atom (LH) have accelerated the formation of ketoenamine form, and therefore, both ketoenamine and enolimine forms could be present in acidic media.

Methyl 2-{[(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-ylidene)(thiophen-2-yl)methyl]amino}-3-phenylpropionate

In the title compound, C25H23N3O3S, an intra­molecular N—H⋯O inter­action generates an S(6) ring, which stabilizes the enamine–keto form of the compound. This S(6) ring and the pyrazole ring are essentially coplanar, making a dihedral angle of 1.49 (6)°. The bond lengths within the S(6) ring of the mol­ecule lie between classical single- and double-bond lengths, indicating extensive conjugation. The structure exhibits a thienyl-ring flip disorder, with occupancy factors in the ratio 64.7 (3):35.3 (3).

Methyl 2-{1-[(Z)-3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-ylidene]ethylamino}-3-phenylpropanoate

The mol­ecule of the title compound, C22H23N3O3, exists in the enamine–keto form. A strong intra­molecular N—H⋯O hydrogen bond occurs, generating an S(6) ring. The dihedral angle between the heterocycle and the adjacent phenyl ring is 3.75 (15)°.

Ethyl 2-{[(1Z)-(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-ylidene)(p-tolyl)methyl]amino}-3-phenylpropanoate

The asymmetric unit of the title compound, C29H29N3O3, contains two mol­ecules, which exist in their enamine–keto form, being stabilized by strong intra­molecular N—H⋯O hydrogen bonds, which generate S(6) loops. In the crystal, inter­molecular C–H⋯O hydrogen bonds link the mol­ecules into chains, which are further linked by weak C—H⋯π inter­actions, forming a two-dimensional network.

Synthesis and characterization of a trinuclear CuII3 complex bridged by an extended phloroglucinol-ligand: implications for a rational enhancement of ferromagnetic interactions

The synthesis and characterization of the extended phloroglucinol-ligand H(3)felden based on the trialdehyde 2,4,6-triformylphloroglucinol and its trinuclear Cu(II)(3) complex [(felden){Cu(bpy)}(3)](ClO(4))(3) is presented. This study is motivated to optimize analogous extended phloroglucinol-ligands based on the triketone 2,4,6-triacetylphloroglucinol, which transmit ferromagnetic interactions by the spin-polarization mechanism between three Cu(II) ions and have therefore been applied as the central ligand backbone for a class of heptanuclear single-molecule magnets in a supramolecular approach. A detailed NMR spectroscopic study reveals that the ligand H(3)felden is not in the usually anticipated enol-imine form but in the tautomeric keto-enamine form. The presence of a C(3h) and a C(s) symmetric isomer results in a set of four different signals for each proton. In conjunction with FTIR, electronic absorption spectroscopy, and bond length considerations, it also appears that the complex [(felden){Cu(bpy)}(3)](ClO(4))(3) must be considered as a resonance hybrid of an enolate-imine and a keto-enamine form. A strong contribution of the keto-enamine resonance structure with loss of the central π system explains the weak but ferromagnetic interactions between the Cu(II)S = 1/2 spins. This detailed analysis identifies the strong resonance with unsaturated groups in 2,4,6-position of phloroglucinol as the main source for the low ferromagnetic couplings by the spin-polarization mechanism in these ligands. This provides a synthetic handle to improve the spin-polarization mechanism in these ligands by replacing the imine with amine functions.

Z/E (C=C)-isomerization and fluorescence modulation of imines of 7-N,N-dialkylamino-4-hydroxy-3-formylcoumarins in organic solvents

Imines of 7-dialkylamino-3-formyl-4-hydroxycoumarins have been found to exist in Z-ketoenamine form in crystal state and undergo Z/E-isomerization around C=C bond in organic solvents at room temperature. Activation energies of isomerization have been measured experimentally and calculated by the DFT B3LYP method. Transition of ketoenamine form of p-nitrophenylimines into (hydroxy)imine form at low concentration (10-5 m) in polar solvents is accompanied by strong increase in fluorescence.

MethylN-[(4-chlorophenyl)(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-ylidene)methyl]glycinate

The title compound, C20H18ClN3O3, is in an enamine–keto form, stabilized by two strong intra­molecular N—H⋯O hydrogen bonds. The pyrazole ring is oriented at dihedral angles of 4.13 (3) and 85.60 (3)° with respect to the aromatic rings. The dihedral angle between the aromatic rings is 81.79 (3)°. In the crystal structure, inter­molecular C—H⋯O hydrogen bonds link the mol­ecules into double chains, which are further linked by weak C—H⋯π inter­actions, forming a two-dimensional network.

Z/E(C=C)-isomerization of coumarin enamines induced by organic solvents

Imines of 3-formyl-4-hydroxycoumarin have been found to exist in E - and Z -ketoenamine forms in a crystal state and undergo Z/E -isomerization around the C=C bond in organic solvents ( e.g. , CHCl 3 ).

Theoretical study on the ground state intramolecular proton transfer (IPT) and solvation effect in two Schiff bases formed by 2-aminopyridine with 2-hydroxy-1- naphthaldehyde and 2-hydroxy salicylaldehyde

The tautomerization mechanism the isolated and monohydrated forms of two Schiff bases 1 and 2, and the effect of solvation on the proton transfer from enol-imine form to the keto-enamine form have been investigated using the B3LYP hybrid density functional method at the 6-31G** basis set level. The barrier heights for H(2)O-assisted reactions are significantly lower than that of unassisted tautomerization reaction in the gas phase. Nonspecific solvent effects have also been taken into account by using the continuum model (IPCM) of four different solvent. The tautomerization energies and the potential energy barriers are decreased by increasing solvent polarity.

Antidyslipidemic and antioxidative activities of 8-hydroxyquinoline derived novel keto-enamine Schiffs bases

8-Hydroxyquinoline when subjected to Duff reaction resulted in the formation of unexpected 7-methylaminomethylene-8-oxo-7, 8-dihydroquinoline-5-carbaldehyde 2, which existed in the keto-enamine form, in which the aromaticity of the relevant ring was disrupted, which upon subsequent treatment with various primary amines resulted in its nucleophilic substitution of aliphatic methyl amine. These interesting novel derivatives were evaluated in vitro for their antioxidant and in vivo for their antidyslipidemic and post-heparin lipolytic activities. Compound 6 was found to be most active antidyslipidemic and antioxidative agent in this series, respectively, and thus represent a new class of promising lead.

Metal complexes of Schiff bases derived from dicinnamoylmethane and aliphatic diamines

Two new Schiff bases containing olefinic linkages have been synthesized by condensing aliphatic diamines with dicinnamoylmethane under specified conditions. The existence of these compounds predominantly in the intramolecularly hydrogen bonded keto-enamine form was well demonstrated by their IR, 1H-NMR and mass spectral data. Dibasic tetradentate N2O2 coordination of the compounds in their [ML] complexes (M = Ni(II), Cu(II) and Zn(II)) was established on the basis of analytical and spectral data.

N–H…O Assisted Structural Changes Induced on Ketoenamine Systems

The tautomeric equilibrium between the ketoenamine and enolimine forms for a series of arylimines (1a–1c) derived from salicylaldehyde and enaminones (2a–2f) derived from benzoylacetone and substituted anilines has been analyzed in solution by 1H NMR and UV-vis spectroscopy, while IR and X-ray diffraction analyses were used in the solid state. The X-ray diffraction analysis of 1a, 2a–2c and 2e–2f showed that the most stable tautomer is the ketoenamine and all enaminones present intramolecular NH…O hydrogen bonds forming a six member ring in which the keto form is favored. In addition, the VT-NMR analysis for 2b in solvents of different polarities evidenced the behavior of the tautomeric equilibrium, showing that in DMF and acetone at low temperature, the enolimine tautomer is more stable.

Novel keto-enamine Schiffs bases from 7-hydroxy-4-methyl-2-oxo-2 H-benzo[h] chromene-8,10-dicarbaldehyde as potential antidyslipidemic and antioxidant agents

A series of Schiff bases have been synthesized from dicarbaldehyde of benzocoumarin, in which the reactions were regioselective and the products existed in the keto-enamine form, in which the aromaticity of the relevant ring was disrupted. The compounds were evaluated in vitro for their antioxidant and in vivo for their antidyslipidemic activity for the first time. Compounds 3 and 7 possess significant lipid lowering and antioxidant activity.

4-[(Z)-(n-Butylamino)(2-furyl)methylene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

In the title compound, C19H21N3O2, the pyrazolone ring and the attached C—NH group are essentially coplanar. The compound is in an enamine–keto form and its structure is stabilized by one strong intra­molecular N—H⋯O hydrogen bond.