N.m.r. studies of the keto–enol tautomerism of acylthioacetamides

Acetylthioacetamides exist as different keto and enol isomers in chloroform solutions. The keto form with intramolecular hydrogen bonding between the NH and the carbonyl group is the dominant keto isomer. On the other hand the enol forms with intramolecular hydrogen bonding between the OH and the thioketo group are the dominant enol isomers in the temperature range 60°C to ?60°C. The thermodynamic data of the keto-enol equilibria were obtained by measuring the intensities of appropriate high resolution proton signals as a function of temperature. At low temperatures all lines characteristic of the enol forms are doubled in the N-phenyl-substituted derivatives because the rotation of the NH? C₆H₅ group around the C? N bond becomes slow and the chemical shifts characteristic of the E and Z isomers are different. We estimated approximate thermodynamic data of the E/Z equilibrium in some of the compounds. The changes of the line shape as well as the chemical shifts as a function of temperature indicate the presence of various additional exchange processes. In order to obtain further information we performed curve fittings of the chemical shifts of one acetylthioacetanilide and of a series of monothio-β-diketones (studied in another paper) assuming a fast two site exchange process. On the basis of the results obtained a reaction scheme for N-substituted acylthioacetanilides in solution is proposed.     

3-Aryl(hetaryl)-3-hydroxy-2-phosphorus-substituted acrylonitriles. Synthesis and experimental and theoretical conformational analysis

Under conditions of the phase transfer catalysis, acylation of (thio)phosphorylacetonitriles by (het)aroyl chlorides affords the Z-enol forms of C-acylation products in high yields. Their configurations were studied by IR spectroscopy, dipole moment measurements, and ab initio quantum-chemical calculations [B3LYP/6-31G(d)]. The C=C double bond and the phosphoryl or thiophosphoryl group have an s-cis arrangement. The possibility of strong intramolecular hydrogen bonding in these conformers is the governing factor responsible for the three-dimensional structures of the compounds under investigation. Derivatives of nicotinic acid existing in the individual form as zwitterions are the only exceptions.     

Preparation and Characterization of N,N′-Dialkyl-1,3-propanedialdiminium Chlorides,N,N′-Dialkyl-1,3-propanedialdimines,and Lithium N,N′-Dialkyl-1,3-propanedialdiminates

We describe convenient preparations of N,N′-dialkyl-1,3-propanedialdiminium chlorides, N,N′-dialkyl-1,3-propanedialdimines, and lithium N,N′-dialkyl-1,3-propanedialdiminates in which the alkyl groups are methyl, ethyl, isopropyl, or tert-butyl. For the dialdiminium salts, the N₂C₃ backbone is always in the trans-s-trans configuration. Three isomers are present in solution except for the tert-butyl compound, for which only two isomers are present; increasing the steric bulk of the N-alkyl substituents shifts the equilibrium away from the (Z,Z) isomer in favor of the (E,Z), and (E,E) isomers. For the neutral dialdimines, crystal structures show that the methyl and isopropyl compounds adopt the (E,Z) form, whereas the tert-butyl compound is in the (E,E) form. In aprotic solvents all four dialdimines (as well as the lithium dialdiminate salts) adopt cis-s-cis conformations in which there presumably is either an intramolecular hydrogen bond (or a lithium cation) between the two nitrogen atoms.     

NMR Study of the Steric and Electronic Structure of 2-(2-Acylethenyl)pyrroles

According to the ¹H and ¹³C NMR data, 2-(2-acylethenyl)- and 2-(2-acyl-1-phenylethenyl)pyrroles in chloroform exist exclusively in the keto form. The Z isomers of these compounds are characterized by coplanar arrangement of the olefinic fragment, carbonyl group, and pyrrole ring. The strong intramolecular hydrogen bond NÄH···O is responsible for the presence of only one rotamer. The corresponding E isomers give rise to equilibrium between conformers with syn and anti arrangement of the olefinic fragment with respect to the pyrrole ring. A very strong conjugation between the ketovinyl group and the pyrrole ring is likely to arise from an appreciable contribution of the zwitterionic structure.     

An investigation of intramolecular hydrogen bonds and the spatial conformations of the stereoisomeric α-oxoalcohols of the cyclohexane,piperidine, and decahydroquinoline series

The influence of the conformation of the geometric isomers of 1-acetyl-4-tert-butylcyclohexan-1-ol and of 4-acetyl-1, 2-dimethyldecahydroquinoline on the nature of the intramolecular hydrogen bonds has been studied by IR spectroscopy, and on the basis of the results obtained the configurations and conformations of the geometric isomers of the 4-acyl-1,2,5-trimethylpiperidin-4-ols have been established.Conformational stability of the stereoisomers of ketols of the piperidine series has been found.It has been shown that the geometric isomers of each of the compounds studied with axially orientated acyl groups exist in solutions in the form of an equilibrium mixture of two rotameric forms. The enthalpy of an intramolecular hydrogen bond in these compounds is 0.5–0.7kcal/ mole.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Ab Initio Study of the Conformational and Geometrical Isomerism in Heteroallyl and Heteropropenyl Systems

Conformational isomerism has been studied by ab initio methods (RHF/6-31+G*, MP2/6-31+G*) for CH₂=CHCH₂X heteroallyl and CH₃CH=CHX heteropropenyl systems (X = H, Me, NMe₂, OMe, PMe₂, SMe, ONCH₂). In 3-heteroprop-1-enes, substituents preferably occupy the AC position relative to the C=C double bond. The E isomers of 1-methylthio- and 1-methoxyprop-1-enes, which are thermodynamically more stable, have two stable forms, SP and AC; for 1-dimethylamino- and 1-imethylphosphinoprop-1-enes, the stable forms are AP and SC. The molecule of the E isomer of formoxime propenyl ether exists in two stable rotamer forms, SC and AP, the latter being predominant. The Z isomers preferably exist in the form of AC (X = CH₃O, CH₃S) and AP (X = (CH₃)₂N, (CH₃)₂P, CH₂=NO) conformations. Migration of the double bond toward an heteroatom in formoxime allyl ether, forming the E and Z isomers, is energetically favorable, the Z isomer being thermodynamically preferable.     

Molecular and crystal structure of 5Z-carboxymethylene-2-chloro-4,4-dimethoxy-3-N,N-dimethylaminocyclopent-2-en-1-one

In the crystal, the 5Z-carboxymethylene-2-chloro-4,4-dimethoxy-3-N,N-dimethyl-aminocyclopent-2-en-1-one molecule has an intramolecular hydrogen bond (stabilized in the chelate form) between the proton of the carboxy group and the oxygen atom of the keto group. The methoxy groups are in an antiperiplanar conformation and are synclinal with respect to the dimethylamino group and the double bond of the carboxylmethylene fragment. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1979–1981, November, 1997.     

Tautomeric preferences and electron delocalization in biurets,thiobiurets, and dithiobiurets: An ab initio study

In several literature reports biuret and its sulfur analogs are reported to exist in their diketo form with general formula H₂N? CX? NH? CY? NH₂ (X = O, Y = O, biuret; X = Y = S, dithiobiuret; and X = O, Y = S, thiobiuret). On the other hand, recently reported results on the electronic structure of biguanide analogs (X = Y = NH)demonstrated that a form equivalent to diketo is not the preferred structure. Thus, a systematic ab initio study on the tautomeric preferences of biuret and its sulfur analogs (dithiobiuret and thiobiuret) has been carried out. The results indicate that an interplay of conjugative stabilization and intramolecular hydrogen bonding to play a role in tautomeric preferences. Energy and geometric parameters, natural bond orbital analyses have been employed to understand the chemistry of the title compounds. The results indicate that unlike biguanides, these compounds prefer diketo forms containing hydrogen on the bridging nitrogen (N4) and in a trans‐arrangement (1a–4a). However, tautomerization of these keto forms to the corresponding enol isomers was also found to be highly probable. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

NMR-spektroskopische Untersuchungen zurE/Z-Isomerie von substituierten Dithiocarbazinsäurederivaten

Summary NMR-spectroscopic investigations of substituted dithiocarbacinic acid derivatives show the existence of configurational isomers relative to the C=N double bond, but no tautomerism. We applied chemical shift differences in the¹H-,¹³C- and¹⁵N-NMR-spectra, N - H and C - H coupling constants and NOE difference measurements in the solvents CDCl₃,DMSO-d ₆ and partly CD₃OD in order to discriminate between the isomers and to determine the configurational composition. Compounds4 and5 exist asE-isomers in all solvents, compounds1, 2 and3 as a mixture ofE/Z-isomers. InDMSO-d ₆ theE-form is the preferred isomer.

     

Steric structure of alkyl 2-aryl(hetaryl)hydrazono-3-fluoroalkyl-3-oxopropionates

Steric structure of fluorinated 2-arylhydrazono-3-oxo esters was studied by ¹H, ¹⁹F, and ¹³C NMR spectroscopy and X-ray analysis. It was found that these compounds in the crystalline state and in solutions in acetone-d ₆, DMSO-d ₆, and CDCl₃ exist as Z isomers with the ester fragment involved in intramolecular hydrogen bond with the hydrazone NH proton. Exceptions are alkyl 2-arylhydrazono-4,4-difluoro-3-oxobutanoates which exist in acetone-d ₆ as mixtures of Z and E isomers, the former prevailing. Unlike fluorinated analogs, ethyl 2-(4-methylphenyl)hydrazono-3-oxobutanoate in crystal has the structure of E isomer in which intramolecular hydrogen bond is formed between the NH proton and acetyl carbonyl group. The same compound in acetone-d ₆, DMSO-d ₆, and CDCl₃ gives rise to a mixture of Z and E isomers, the latter prevailing.     

Solvent-controlled intramolecular [2 + 2] photocycloadditions of alpha-substituted enones

The regio- and stereoselectivity of intramolecular [2 + 2] photocycloadditions of 2'-hydroxyenones are shown to be solvent-dependent. In the presence of aprotic solvents, 2'-hydroxyenones undergo photocycloadditions in a manner consistent with the presence of an intramolecular hydrogen bond between the carbonyl group and the tether's hydroxy functionality. In protic solvents, intermolecular interactions appear to disrupt the intramolecular hydrogen bond, providing products with complementary diastereoselectivity. If the facial accessibility of the alpha-tethered olefin is limited, the cycloadditions proceed to give head-to-tail or head-to-head regioisomers, depending on the nature of the solvent employed.     

Excited-state charge coupled proton transfer reaction via the dipolar functionality of salicylideneaniline

Based on design and synthesis of salicylideneaniline derivatives(1a-1d),we demonstrate a prototypical system to investigate the excited-state intramolecular charge transfer(ESICT) coupled excited-state intramolecular proton transfer(ESIPT) reaction via the dipolar functionality of the molecular framework. In solid and aprotic solvents 1a-1d exist mainly as E conformers that possess an intramolecular sixmembered -ring hydrogen bond.Compounds 1a-1c exhibit a unique proton-transfer tautomer emission, while compound 1d exhibits remarkable dual emission due to the different solvent-polarity environment between ESICT and ESIPT states.Time-dependent density functional theory(TDDFT) calculations are reported on these Schiff bases in order to rationalize their electronic structure and absorption spectra.     

Z and E isomers of butenedioic acid with 2‐amino‐1,3‐thiazole: 2‐amino‐1,3‐thiazolium hydrogen maleate and 2‐amino‐1,3‐thiazolium hydrogen fumarate

Maleic acid and fumaric acid, the Z and E isomers of butenedioic acid, form 1:1 adducts with 2‐amino‐1,3‐thiazole, namely 2‐amino‐1,3‐thiazolium hydrogen maleate (2ATHM), C₃H₅N₂S⁺·C₄H₃O₄⁻, and 2‐amino‐1,3‐thiazolium hydrogen fumarate (2ATHF), C₃H₅N₂S⁺·C₄H₃O₄⁻, respectively. In both compounds, protonation of the ring N atom of the 2‐amino‐1,3‐thiazole and deprotonation of one of the carboxyl groups are observed. The asymmetric unit of 2ATHF contains three independent ion pairs. The hydrogen maleate ion of 2ATHM shows a short intramolecular O—H...O hydrogen bond with an O...O distance of 2.4663 (19) Å. An extensive hydrogen‐bonded network is observed in both compounds, involving N—H...O and O—H...O hydrogen bonds. 2ATHM forms two‐dimensional sheets parallel to the ab plane, extending as independent parallel sheets along the c axis, whereas 2ATHF forms two‐dimensional zigzag layers parallel to the bc plane, extending as independent parallel layers along the a axis.     

A Facile,One-Pot Synthesis of Azoic Compounds and Anthraquinone Derivatives Containing Dialkyl Phosphoryl Moieties in Multicomponent Reactions

Protonation of the reactive 1:1 intermediates produced in the reactions between triphenylphosphine and dialkyl acetylenedicarboxylates by 1-amino-anthraquinone or 1,5-diphenylcarbazone as a core dye leads to vinyl phosphonium salts, which undergo Michael addition with conjugate base of NH compounds to produce stable phosphorus ylides as novel dyes in fairly good yields. These ylides can exist in two geometrical isomers (Z) and (E) for 3, because the negative charge of the ylide moiety of these compounds are strongly conjugated with the adjacent carbonyl group. Rotation around the carbon–carbon double bond is slow in the (Z) and (E) geometrical isomers on the NMR time scale at ambient temperature. These compounds are assigned by their IR, ¹H, ¹³C NMR spectral data as well as their mass spectroscopic data.     

The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

The dynamics of the excited states of 3‐ and 4‐aminofluoren‐9‐ones (3AF and 4AF, respectively) are investigated in different kinds of solvents by using a subpicosecond time‐resolved absorption spectroscopic technique. They undergo hydrogen‐bonding interaction with protic solvents in both the ground and excited states. However, this interaction is more significant in the lowest excited singlet (S₁) state because of its substantial intramolecular charge‐transfer character. Significant differences in the spectroscopic characteristics and temporal dynamics of the S₁ states of 3AF and 4AF in aprotic and protic solvents reveal that the intermolecular hydrogen‐bonding interaction between the S₁ state and protic solvents plays an important role in its relaxation process. Perfect linear correlation between the relaxation times of the S₁ state and the longitudinal relaxation times (τ_L) of alcoholic solvents confirms the prediction regarding the solvation process via hydrogen‐bond reorganization. In the case of weakly interacting systems, the relaxation process can be well described by a dipolar solvation‐like process involving rotation of the OH groups of the alcoholic solvents, whereas in solvents having a strong hydrogen‐bond‐donating ability, for example, methanol and trifluoroethanol, it involves the conversion of the non‐hydrogen‐bonded form to the hydrogen‐bonded complex of the S₁ state. Efficient radiationless deactivation of the S₁ state of the aminofluorenones by protic solvents is successfully explained by the energy‐gap law, by using the energy of the fully solvated S₁ state determined from the time‐resolved spectroscopic data.     

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S₀ and S₁ states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S₁ state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S₁ state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.     

Structural studies on 9-hydrazono-6,7,8,9-tetrahydro-4-oxo-4H-pyrido[1,2-a]pyrimidines by 1H, 13C and 15N NMR spectroscopy

¹H, ¹³C and ¹⁵N NMR studies demonstrated that 9-hydrazono-6,7,8,9-tetrahydro-4-oxo-4H-pyrido-[1,2-a] pyrimidlnes exist as an equilibrium mixture of Z-E isomers in the hydrazono–imino tautomeric form having an exocyclic double bond. Proton-catalysed Z-E interconversion is fast. Substituent and solvent effects revealed that the decisive factors controlling the Z:E ratio are internal hydrogen bonding in the Z-isomer, stabilization by solvation and steric interaction.     

Structure of 1(5)-aryl-3-phenyl-5(1)-(2-benzothiazolyl)formazanes

1,3-Diphenyl-5-(2-benzothiazolyl)formazane exists in its crystalline state (based on x-ray crystal structure data) in the E ^1,2 Z ^2,3 Z ^3,4-conformation, which is stabilized by intramolecular hydrogen bonding (IMHB); the proton is localized on the nitrogen atom which is bound to the heterocycle. In chloroform solutions 1(5)-aryl-3-phenyl-5(1)-(2-benzothiazolyl)formazanes exist in the form of equilibrium mixtures of their chelate and open E ^1,2 Z ^2,3 Z ^3,4-forms, in which the first of these forms predominates. Based on their ¹³ C-NMR spectral data the tautomeric equilibrium involving these forms is shifted in favor of the benzothiazolylhydrazone form. The amount of tautomer containing the benzothiazolidene fragment structure increases in DMSO solution.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1268–1274, September, 1991.     

Structure of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone as chiral component of liquid-crystalline systems

The spatial structure of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone prepared by photochemical isomerization of the E-isomer was studied by analyzing the magnitudes and temperature dependence of the proton spin-spin coupling constants obtained by ¹H NMR spectroscopy and the results of molecular modeling using semiempirical quantum chemical AM1 and PM3 methods and the density functional theory (DFT). Comparison of the results obtained for the Z-and E-isomers shows that in both cases the conformational equilibrium for both isomers is characterized by significant preference of the chair conformer having an equatorial methyl group, namely, − ΔH (chair a ⇌ chair e) = 1.98–2.12 and 1.36–1.54 kcal mole⁻¹ for the Z-and E-isomers, respectively. Distinctions in the non-planarity of the enone fragment and cyclohexanone ring in the Z-and E-isomers under study following from the results of mathematical modeling were confirmed by the experimental values of the geminal spin-spin coupling constants of protons of the methylene groups in α,α ′-positions with respect to the enone group. Quantum chemical calculations of the Z-isomer revealed the existence of intramolecular hydrogen bond between the carbonyl oxygen and the nearest aromatic proton in ortho-position of the benzene ring. Possible reasons for different helical twisting power of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone and the E-and Z-arylidene derivatives of 1R, 4R-isomenthone in the mesophase are discussed based on the results of molecular structure studies for these compounds. In the text below the unsaturated ketones under study will be called “arylidene cyclohexanone derivatives” for convenience of comparing the characteristics of methylcyclohexanone and isomenthone derivatives. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 962–972, June, 2006.