Amino-imino Tautomerism Research Articles

Amino–imino tautomerism of mono- and diaminothiazoles: Quantum chemical study

Abstract The amino–imino tautomerism of mono- and diaminothiazoles in the gas, water and CCl 4 phases was investigated using the B3LYP and MP2(full) levels with the 6-311++G(d,p) basis set. The MP2 method predicted accurately the geometrical parameters of the compounds studied and showed that the amino forms predominate in solution as well as the gas phases. The tautomeric process is endothermic with a small entropy contribution to Gibbs free energy. Single point calculations at the MP4/6-311++G**, CISD/6-31+G**or QCISD/6-31+G**levels were also performed to improve the thermochemical values. 2-Aminothiazole-4(5 H )-imino was found to be the most stable structure in gas and solution phases in good agreement with the experimental data. At the QCISD/6-31G*and CISD/6-31G*levels the diimino form was more stable than the corresponding diamino form 24ATH whereas the MP2 level resulted in stabilization of the diamino form 24ATH over the imino form 2A5ITH. For the 2,5-diamino isomer 25ATH the calculations at all levels showed that the diamino form is the stable configuration in the gas or solution phases.

Amino–imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system

The amino–imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353 nm), imino-tautomer (at 414 nm), and 3M4PMI monomer (at 437 nm) exhibit the single-exponential decays of 98, 73, and 19 ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex → 3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be ∼2 kJ mol −1 using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer → 4APM/FoOH complex was estimated to be almost zero kJ mol −1 at B3LYP/6-31G(d) level.

The hydrogen bonding and amino–imino tautomerization of the alkoxy-aminopyridines and amino-methoxypyrimidines with acetic acid: The effects of the methoxy group

The hydrogen bonding and amino–imino tautomerization of the systems of 2-amino-3-methoxypyridine (2A3MOP), 2-amino-6-methoxypyridine (2A6MOP), 2-amino-6- n-propoxypyridine (2A6NPOP), 2-amino-6- iso-propoxypyridine(2A6IPOP), 2-amino-4-methoxypyrimidine (2A4MOPM), 4-amino-2-methoxypyrimidine (4A2OPM), 4-amino-6-methoxypyrimidine (4A6MOPM), 2-amino-4-methoxy-6-methylpyrimidine (MMPM), and 2-amino-4,6-dimethoxypyrimidine (DMOPM), with acetic acid (AcOH) in n-hexane at room temperature were investigated by means of the UV absorption and fluorescence spectroscopy. From the UV absorption spectra the presence of the dual hydrogen-bonded complexes that linked by a 1:1 molar ratio with AcOH were found, since the enthalpy changes accompanying the hydrogen bond formation between 2A3MOP, 2A4MOPM, 4A2MOPM, 4A6MOPM, or MMPM, and AcOH were ca. 42.8–61.1 kJ mol −1 in n-hexane. The fluorescence spectra of the 2A3MOP/AcOH, 2A4MOPM/AcOH, 4A6MOPM/AcOH, and MMPM/ AcOH systems revealed that the imino-tautomers were produced through double proton transfer in the amino hydrogen-bonded 1:1 complexes in the S1 state, but the imino-tautomer formation for the 4A2MOPM/AcOH system was not found on account of the steric hindrance due to the inversion of the methoxy group in the S1 state. The imino-tautomer for the MMPM/AcOH system fluoresces most intensely among these systems investigated. On the other hand, not only the formation of the corresponding amino dual hydrogen-bonded complex and but also that of imino-tautomer were prevented for the 2A6MOP/AcOH, 2A6NPOPM/AcOH, 2A6IPOP/AcOH, and DMOPM/AcOH systems, because of the steric hindrance of the methoxy group in both the S0 and S1 states. The theoretical approaches by an ab initio molecular orbital calculation were in accord with the experimental results.

Photoinduced Amino−Imino Tautomerism: An Infrared Study of 2-Amino-5-methylpyridine in a Low-Temperature Argon Matrix

The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopy and DFT calculation. Photoinduced reversible amino (N=C-NH(2))-imino (NH-C=NH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changes to the imino tautomer by UV irradiation (340>lambda>or= 300 nm) and the reverse change occurs by longer-wavelength light irradiation (420>lambda>or= 340 nm). The results of the CASSCF calculation revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. The IR spectra of 2-amino-5-methylpyridine in the T(1) state and 5-methyl-2-pyridinamino radical were also obtained by UV irradiation (lambda>or= 300 nm).

Photoinduced amino–imino tautomerization reaction in 2-aminopyrimidine and its methyl derivatives with acetic acid

The electronic absorption and fluorescence spectra of 2-aminopyrimidine (2APM), 2-amino-4-methylpyrimidine (2A4MPM), and 2-amino-4,6-dimethylpyrimidine (2ADMPM) with acetic acid (AcOH) were measured in isooctane (2,2,4-trimethylpentane) at room temperature. From the absorption spectra, a hydrogen-bonded complex formation of the 2APM/AcOH, 2A4MPM/AcOH, and 2ADMPM/AcOH systems was recognized in isooctane. The enthalpy changes (−Δ H) for the complex formation were estimated to be ca. 41.2–45.1 kJ mol −1 and increased in proportion to the numbers of the methyl group introduced into the 2APM. The −Δ H values refer to the formation of the hydrogen-bonded 1:1 complex between the ring nitrogen atom and NH 2 group of the aminopyrimidine and the OH and C O groups of AcOH, respectively. In the 2A4MPM/AcOH double hydrogen-bonded complex the OH group of AcOH is thought to be linked to the ring nitrogen at the 1-postion of 2A4MPM. The fluorescence spectral results indicate that the double proton transfer reaction takes place during the excited state, and gives rise to an imino-tautomer vibration emission, from analogy with the fluorescences of 1-methyl-2(1H)-pyrimidinimine (MPMI), 1,4-dimethyl-2(1H)-pyrimidinimine (DMPMI), and 1,4,6-trimethyl-2(1H)-pyrimidinimine (TMPMI). The fluorescence quantum yields of the imino-tautomers also increased in proportion to the numbers of the methyl group introduced into the 2APM.

NMR and DFT investigations of the substituent and solvent effect on amino—imino tautomerism in acridin‐9‐amines substituted at the exocyclic nitrogen atom

AbstractThe 1H and 13C NMR spectra of 9‐(methoxyamino)acridine (1) and 9‐hydrazinoacridine (2) show that these compounds exist principally in the imino tautomeric form in CDCl3, acetone‐d6, CD3CN, DMSO‐d6 and Py‐d5, all solvents with different polarities and abilities to participate in specific interactions. The spectra of the other two compounds investigated—N‐(2‐chloroethyl)acridin‐9‐amine (3) and N‐(5‐methylpyridin‐2‐yl)acridin‐9‐amine (4)—indicate that they coexist in the amino and imino forms. The amino tautomer of compound 3 predominates in CDCl3, CD3CN and Py‐d5 and that of compound 4 in CDCl3 and Py‐d5. On the other hand, the amino and imino forms of compound 3 coexist in acetone‐d6 and probably DMSO‐d6, whereas those of compound 4 coexist in acetone‐d6 and DMSO‐d6. The positions of the signals in the NMR spectra compare qualitatively with those predicted computationally at the GIAO/DFT level of theory. The equilibrium constants predicted by the DFT(PCM) method are in agreement with the results of NMR spectral analysis. In general, both the data predicted at the DFT level of theory and x‐ray structural data show that the imino tautomers display a ‘butterfly’‐type geometry, whereas the amino forms are characterized by an almost flat acridine moiety. Electron‐attracting substituents at the exocyclic N atom improve the stability of the imino form, and electron‐withdrawing substituents do likewise for the amino form. The importance of tautomeric phenomena in the context of the ability of acridin‐9‐amines to participate in specific interactions is outlined in brief, as are the possible applications of these compounds as probes of environmental properties. Copyright © 2005 John Wiley & Sons, Ltd.

DL-2-Amino-2-thiazoline-4-carboxylic acid trihydrate

In the title compound, C4H6N2O2S.·3H2O, the 2-amino-2-thia­zoline-4-carboxyl­ic acid mol­ecule is in a zwitterionic form and shows amino–imino tautomerism. There are intermolecular O—H⋯O, O—H⋯N, N—H⋯O and C—H⋯O interactions, forming a three-dimensional network.

1H and 13C chemical shifts for acridines: Part XVIII. 9‐Chloroacridine and 9‐(N‐allyl)‐ and 9‐(N‐propargyl)acridinamine derivatives

AbstractThe1H and 13C NMR resonances for acridine derivatives 9‐substituted with chloro, allylamino and propargylamino groups were completely assigned using a concerted application of gs‐COSY, gs‐HMQC and gs‐HMBC experiments. 9‐(N‐Allyl)‐ and 9‐(N‐propargyl)acridinamine derivatives present amino–imino tautomerism including a large broadening of 1H and 13C NMR signals at room temperature. To obtain suitable resolution, therefore, these latter compounds were studied at 370 K in DMSO‐d6 solutions and showed a complete shift towards the imino tautomers. Copyright © 2003 John Wiley & Sons, Ltd.

Bis(2-aminopyridinium) maleate.

Two cyclic eight-membered hydrogen-bonded rings exist in the title compound, 2C(5)H(7)N(2)(+) x C(4)H(2)O(4)(2-), involving the 2-aminopyridinium and maleate ions. The dihedral angle between the two pyridinium rings hydrogen bonded to the maleate ion is 74.80 (4) degrees. The maleate anion lies on a twofold axis and is linked to the pyridinium cations by intermolecular N-H.O hydrogen bonds. The heterocycle is fully protonated, which enables amino-imino tautomerization.

1H and 13C chemical shifts for acridines: Part XVII. (1,3‐Benzothiazol‐2‐yl)amino‐9(10H)‐acridinone derivatives

AbstractThe 1H and 13C NMR resonances for 16 acridin‐9(10H)‐ones substituted with amino or (1,3‐benzothiazol‐2‐yl)amino groups were completely and unequivocally assigned by the concerted application of gs‐COSY, gs‐HMQC and gs‐HMBC experiments. Evidence for hydrogen bond and amino–imino tautomerism is presented for 1‐ and 4‐substituted acridin‐9(10H)‐ones. Copyright © 2002 John Wiley & Sons, Ltd.

Thermal decomposition of aminotetrazoles

The thermal decomposition of 5-aminotetrazole (5-AT), 1-methyl-5-aminotetrazole (MAT), 1,5-diaminotetrazole (DAT), poly-1-vinyl-5-aminotetrazole (PVAT) and sodium salt of 5-aminotetrazole (SAT) have been studied by thermogravimetry, thermal volumetric analysis (TVA), DSC, DTA and evolved gas analysis (EGA). The kinetic parameters of the thermal decomposition of aminotetrazoles were calculated either by the Ozawa method or by the method of invariant kinetic parameters (IKP). The gaseous products, volatile condensed products and solid residues were identified by FTIR and gas chromatography–mass-spectrometry (GS/MS). The total energies and the energies of chemical bonds of various isomeric forms of 5-AT and MAT have been calculated ab initio using MP2/6-31G ∗∗ theory level and MNDO approximation. Based on the content of products of thermal decomposition and the kinetic consideration, the mechanism of thermal decomposition of aminotetrazoles has been derived. Two routes of the splitting of tetrazole ring leading either to elimination nitrogen or hydrogen azide are suggested. The contribution of each route is changing upon the advancement of the process. It was assumed, that hydrogen azide splits out from the prothotropic forms of the tetrazole ring, which have hydrogen atoms by nitrogens in the ring. Experimental study as well as literature data on the amino-imino tautomerism are in agreement with the suggested mechanism of the decomposition of the tetrazole ring. It is shown that secondary reactions significantly extend variety of the products of thermal decomposition of aminotetrazoles.

The crystal structure of an anomalous and tautomeric salt: 2:1 complex of (E)-2(1H)-6-methyl-pyridiniminium hydrogen maleate with maleic acid

An anomalous salt in the 2:3 ratio of the original partners of 2-amino-3-methylpyridine and maleic acid, named tautomerically and structurally as the 2:1 complex of (E)-2(1H)-6-methyl-pyridiniminium hydrogen maleate with maleic acid, has been prepared and characterized by X-ray crystallography. The salt crystallizes in the monoclinic space group C2/c with a=15.891(2)Å,b=5.4040(10)Å,c=30.761(4)Å,β=94.810(10)°, V=2632.3(7)Å3,Z=4. The maleic acid partner adopts two forms of configuration, the hydrogen maleate anion and the original maleic acid. The hydrogen maleate anion is tied by O–H⋯O short intramolecular hydrogen bonds with a non-centered hydrogen atom, and bridged by maleic acid with hydrogen bond of OH⋯O to form whole crystal. The hydrogen maleate anion is linked to the individual pyridinium cation by intermolecular hydrogen bond of N(sp2)–H⋯O and N(sp3)–H⋯O. The heterocycle is fully protonated, which arouses amino–imino tautomerization. The geometrical arrangement in the crystal structure is characterized by the formation of laminar dimers of centrosymmetric pyridiniminium hydrogen maleate that is arranged in a manner of layer upon layer.

Proton-Transfer Tautomerism of β-Carbolines Mediated by Hydrogen-Bonded Complexes

The carboxylic acid catalyzed excited-state amino-imino tautomerism for β-carboline (β-CB) and its analogues has been investigated. Thermodynamics and microsolvation (i.e., stoichiometry of the complex formation) of various β-CB/acetic acid complexes in nonpolar solvents have been studied by means of absorption and emission titration experiments. Supplementary support of the stoichiometric ratio and structure for the hydrogen-bonding formation was provided by molecular design and syntheses of various β-CB analogues incorporating either only one hydrogen bonding site or dual hydrogen bonding sites where interplay between two sites are sterically prohibited. The results in combination with time-resolved measurements and theoretical approaches suggest the 1:2 β-CB/acetic acid complex with a structure of triple hydrogen bonding formation to be responsible for the excited-state proton-transfer tautomerism in cyclohexane. The proton transfer time is beyond the response limit of the detecting system of 15 ps, indicating that only a negligibly small geometry adjustment is required for the guest molecule (i.e., acetic acid) to a correct geometry, i.e., a hydrogen-bond relay configuration, for the triple proton transfer to proceed. In comparison, for the 1:1 β-CB/acetic acid non-hydrogen-bond relayed complexes, amino−imino tautomerism is prohibited during the excited-state lifetime, giving rise to a normal Stokes shifted emission. The results provide detailed ground-state thermodynamics of β-CB HB complexes as well as the dynamics of proton-transfer tautomerism mediated by the hydrogen-bonding structures.

1H and 13C chemical shifts for tetracycle‐fused acridine analogues of amsacrine

AbstractThe 1H and 13C NMR resonances for 18 tetracyclic acridines 9‐substituted on the acridine ring with Cl, NH2 and AMS [N‐(4‐amino‐3‐methoxyphenyl)1methanesulfonamide] groups were completely and unequivocally assigned by the concerted application of gs‐HMQC and gs‐HMBC experiments. For NH2 and AMS substituents, the potential amino–imino tautomerism is completely shifted towards the imino tautomer. Copyright © 2001 John Wiley & Sons, Ltd.

Halogenoarylation of allyl isothiocyanate: Synthesis of 2,5-disubstituted 2-thiazolines

A new approach to the synthesis of 2-R-5-benzyl-2-thiazolines with the use of chloro- and bromoarylation products of allyl isothiocyanate with arenediazonium halides was elaborated. The isothiocyanates obtained were reacted with ammonia, aliphatic or aromatic amines, and sodium methoxide. The use of ammonia or weakly basic amines in this reaction allowed. Intermediate thioureas to be isolated. On the basis of 1H NMR spectra, amino–imino tautomerism of the synthesized 2,5-disubstituted 2-thiazolines were analyzed. 2-Arylamino-5-benzyl-2-thiazolines exist mainly in the Z-configuration of the imino form. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 517–525, 1999

Synthesis of 3-heteroalkyl-2-N-organylaminothiophenes. The first proof for amino-imino tautomerism of N-monosubstituted aminothiophenes

The compounds 2- N -methylamino-3-methoxythiophene and 2- N -phenylamino-3-methoxythiophene obtained in fair to good yields in a novel one-pot synthesis from methyl or phenyl isothiocyanate and 1-lithiomethoxyallene have been shown to be in equilibrium with the imino tautomers. The analogous 3-methylthio derivatives obtained by a similar procedure from 1-lithiomethylthiollene exist exclusively in the amino form.

On the tautomerism of maleimide and phthalimide derivatives

The tautomerism of maleimide and phthalimide and their derivatives was examined by means of ah initio MO theory. The molecular geometry of the various tautomers was completely optimized employing the 6-31G ∗ basis set and the semiempirical AM 1 method, respectively. Correlation energy effects were estimated using the MP2 formalism. The solvent influence on the tautomeric equilibria was considered by means of the quantum chemical PCM formalism within the ab initio MO theory and the Cramer-Truhlar method (C.J. Cramer and D.G. Truhlar, J. Am. Chem. Soc., 113 (1991) 8305) incorporated into the semiempirical AM1 calculations. The results indicate the diketo tautomers of maleimide and phthalimide as the most stable ones both in the gas phase and in solution. The stepwise replacement of the oxygen atoms by nitrogen causes significant changes. Whereas the lactam-lactim tautomerism can still be neglected in all compounds, the amino-imino tautomerism gains considerable importance, in particular in the maleic and phthalic imidine systems, where the amino-imino forms are of comparable stability with the diimino forms.

Study of Amino-Imino Tautomerism in Derivatives of 2-, 4- and 6-Aminonicotinic Acid

13C NMR spectra of p-nitrobenzoyl 2-, 4-, and 6-aminopyridine-3-carboxylates, their hydrochlorides, trifluoroacetates and 1-benzyl derivatives were studied. As found from the chemical shifts of pyridine carbon atoms C-2, C-4 and C-6, the free bases exist in the amino form whereas hydrochlorides and 1-substituted pyridinium derivatives in the imino form. Trifluoroacetates of the 2- and 6-amino derivatives have structure similar to that of amidiniumcarboxylates (parallel hydrogen bonds and partially ionic character) whereas trifluoroacetate of the 4-amino derivative is structurally close to the corresponding hydrochloride. The found structures were confirmed by 1H NMR and IR spectroscopy.

1,4,5,8-Tetrakis(arylamino)anthraquinones/near IR dyes (II): Reaction of 4,8-diaminoanthrarufin with arylamines

Condensation of 4,5-dinitrochrysazin (DNCZ) and 4,8-diaminoanthrarufin (DAAR) with arylamines in the presence of boric acid and phenol gives 8-hydroxy-1,4,5-tris- and 1,4,5,8-tetrakis-(arylamino)anthraquinones, respectively. The dyes absorb in the near infrared at about 750–770 nm and colour polycarbonate (PC) film in green hues of very good light fastness. It is proposed that the initial enol-ketone and amino-imino tautomerism of DAAR facilitates the 1,4,5,8-tetrakis-arylamination.

Amino-imino tautomerism of N(6)-methylformycin, a potent inhibitor of E. coli, but not mammalian, purine nucleoside phosphorylase

Amino-imino tautomerism of N(6)-methylformycin, a potent inhibitor of E. coli, but not mammalian, purine nucleoside phosphorylase