Site-selective imination of an anthracenone sensor: selective fluorescence detection of barium(II)

Site-selective imination of anthraquinone-based macrocyclic crown ethers using titanium tetrachloride as the catalyst yields imines where only the external carbonyl group of the anthraquinone forms Schiff-bases. The following aromatic amines yield monomeric compounds (aniline, 4-nitroaniline, 4-pyrrolaniline, and 1,3-phenylenediamine). Reaction of 2 equiv of the macrocyclic anthraquinone host with 1,2- and 1,4-phenylenediamine yields dimeric imine compounds. The 1,2-diimino host acts as a luminescence sensor, exhibiting enhanced selectivity for Ba(II) ion. Spectroscopic data indicate that two barium ions coordinate to the sensor. Due to E/Z isomerization of the imine, the monomeric complexes are nonluminescent. Restricted rotation about the 1,2 oriented C═N groups or other noncovalent/coordinate-covalent interactions acting between neighboring crown ether rings may inhibit E/Z isomerization in this example, which is different from current examples that employ coordination of a metal cation with a chelating imine nitrogen atom to suppress E/Z isomerization and activate luminescence. The 1,4-diimino adduct, where the crown rings remain widely separated, remains nonluminescent.     

Light-Fueled Transformations of a Dynamic Cage-Based Molecular System

In a chemical equilibrium, the formation of high-energy species—in a closed system—is inefficient due to microscopic reversibility. Here, we demonstrate how this restriction can be circumvented by coupling a dynamic equilibrium to a light-induced E/Z isomerization of an azobenzene imine cage. The stable E-cage resists intermolecular imine exchange reactions that would “open” it. Upon switching, the strained Z-cage isomers undergo imine exchange spontaneously, thus opening the cage. Subsequent isomerization of the Z-open compounds yields a high-energy, kinetically trapped E-open species, which cannot be efficiently obtained from the initial E-cage, thus shifting an imine equilibrium energetically uphill in a closed system. Upon heating, the nucleophile is displaced back into solution and an opening/closing cycle is completed by regenerating the stable all-E-cage. Using this principle, a light-induced cage-to-cage transformation is performed by the addition of a ditopic aldehyde.     

Isomerization of an imine intermediate in a reductive amination reaction over metal catalysts

The reductive amination reaction of acetone by cyclohexylamine over hydrogenation metal catalysts was investigated. The study is focused on the formation of side products in the reaction. It was verified that the formation of amines having unusual combinations of alkyls is caused by the metal-catalyzed rearrangement of the double bond around the nitrogen atom in an imine intermediate and consequent reactions of the isomeric imine. It was found that the isomerization reactions occur over virtually all of the hydrogenation catalysts studied, while their respective activities for the imine isomerization decreases in the order Ni = Co > Ru > Pt = Rh > Pd.     

Pmr spectra and cis-trans isomerism of 2-methylenepyrrolidine and 2-methylenetetrahydrofuran derivatives

A number of secondary enamines and cyclic enol ethers (2-methylenetetrahydrofuran derivatives) were synthesized by the reaction of O-methylbutyrolactim and 2,2-diethoxytetrahydrofuran, respectively, with compounds that have an active methylene link. The absence of reversible cis-trans isomerization was established in a study of the PMR spectra of these compounds. The configuration of the secondary enamines is determined by the possibility of the formation of a strong intramolecular hydrogen bond between the carbonyl-containing substituent and the NH group. In the case of the cyclic enol ethers it was shown that the energy barrier to cis-trans isomerization is greater than 25 kcal/mole and that, as a consequence of this, the geometrical isomers can be separated preparatively.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 914–918, July, 1982.     

Origin of the relative stereoselectivity of the beta-lactam formation in the Staudinger reaction

The relative (cis, trans) stereoselectivity of the beta-lactam formation is one of the critical issues in the Staudinger reaction. Although many attempts have been made to explain and to predict the stereochemical outcomes, the origin of the stereoselectivity remains obscure. We are proposing a model that explains the relative stereoselectivity based on a kinetic analysis of the cis/trans ratios of reaction products. The results were derived from detailed Hammett analyses. Cyclic imines were employed to investigate the electronic effect of the ketene substituents, and it was found that the stereoselectivity could not be simply attributed to the torquoelectronic model. Based on our results, the origin of the relative stereoselectivity can be described as follows: (1) the stereoselectivity is generated as a result of the competition between the direct ring closure and the isomerization of the imine moiety in the zwitterionic intermediate; (2) the ring closure step is most likely an intramolecular nucleophilic addition of the enolate to the imine moiety, which is obviously affected by the electronic effect of the ketene and imine substituents; (3) electron-donating ketene substituents and electron-withdrawing imine substituents accelerate the direct ring closure, leading to a preference for cis-beta-lactam formation, while electron-withdrawing ketene substituents and electron-donating imine substituents slow the direct ring closure, leading to a preference for trans-beta-lactam formation; and (4) the electronic effect of the substituents on the isomerization is a minor factor in influencing the stereoselectivity.     

Infrared spectra of a species of astrochemical interest: aminoacrylonitrile (3-amino-2-propenenitrile)

Ammonia easily reacts on cyanoacetylene in the gas phase or in a solvent to form the Z- and E-isomers of aminoacrylonitrile (3-amino-2-propenenitrile, 2). This kinetically stable enamine presents interest for its possible presence in the interstellar medium, the comets, the atmospheres of Planets including the Primitive Earth, and from a theoretical point of view. B3LYP/6-311+G(3df,2p) and G2 calculations indicate that the imine isomer is significantly less stable than the enamine 2. DFT and G2 calculations indicate that the Z-isomer of compound 2 lies ca. 8.0 kJ mol(-1) lower in energy than the E-isomer. The infrared spectra of the aminoacrylonitrile, in both the gas and condensed phases were recorded in the range 500-4000 cm(-1). Consistent with the theoretical calculations, the imine and the E-isomer of the enamine have never been detected in the infrared spectrum of a gaseous sample and only the Z-isomer has been observed. With a neat sample in the condensed phase, IR spectra of a 1:1 and 20:1/Z:E mixtures were recorded. The comparison of these data with the spectrum of the Z-isomer in the gas phase allowed us to deduce the IR spectrum of the E-isomer. The E-Z isomerization takes place through a torsion around the C=C bond. A possible mechanism involving a previous enamine-imine tautomerism must be discarded because it implies a much larger barrier than the direct isomerization process. Consistently, the presence of a deuterium atom has not been observed on the sp2 carbon of the products of distillation of a 1:1/E:Z mixture of the NCCH=CHND2.     

Conjecture: imines as unidirectional photodriven molecular motors-motional and constitutional dynamic devices

Compounds containing the C==N group, such as imines and their derivatives, may undergo syn-anti isomerization by two different routes: 1) photochemically, by out-of-plane rotation around the carbon-nitrogen double bond through a "perpendicular" form, and 2) thermally, by in-plane nitrogen inversion through a "linear" transition state. When the two interconversions occur in sequence, a full, closed process is accomplished, restoring the initial state of the system along two different steps. In a chiral imine-type compound, for example, with an asymmetric center next to the C==N function, photoinduced rotation may be expected to occur in one sense in preference to the opposite one. Thus, photoisomerization followed by thermal isomerization in a chiral imine compound generates unidirectional molecular motion. Generally, imine-type compounds represent unidirectional molecular photomotors converting light energy into mechanical motion. As they are also able to undergo exchange of the carbonyl and amine partners, they present constitutional dynamics. Thus, imine-type compounds are double dynamic, motional, and constitutional devices.     

Staudinger – Reaktionen am Bornanylen(dimethylphosphino)methylimin. Phosphorylierung des dabei gebildeten Trimethylsilylphosphinimids mit Diorganochlorphosphinen: unerwartete Bildung von Verbindungen mit P=N–P–P-Einheiten

Staudinger Reactions with Bornanylene(dimethylphosphino)methylimine. Phosphorylation of the Trimethylsilylphosphine Imide formed with Diorganochlorophosphines: unexpected Formation of Compounds with P=N–P–P Units Phosphinimido derivatives of the chiral bornanylene(dimethylphosphino)methyl imine have been obtained from its reactions with trimethylsilyl-, phenyl- and 4-nitrobenzoyl azide. A single crystal X-ray structure determination has been conducted on the 4-nitro-benzoylphosphinimido derivative of bornanylene(dimethylphosphino)methyl imine. The compound was found to crystallize with two independent molecules in the chiral monoclinic space group P2₁. The trimethylsilylphosphinimido derivative of bornanylene(dimethylphosphino)methyl imine was allowed to react with diorgano-chlorophosphines, with unexpected formation of compounds with PP-bonded RR′₂P=N–P⁽⁺⁾R″₂–PR″₂ groups. The ³¹P-NMR spectra of two representative compounds are simulated and discussed.     

Biomimetic Reductive Amination of Fluoro Aldehydes and Ketones via [1,3]-Proton Shift Reaction.(1) Scope and Limitations

A systematic study of azomethine-azomethine isomerizations of the N-benzylimines 2, derived from fluorinated aldehydes or ketones and benzylamine, has been made. The results reveal that, in sharp contrast to hydrocarbon analogs, fluorinated imines of 2 in triethylamine solution undergo isomerizations to give the corresponding N-benzylidene derivatives 5 (for 5/2 K > 32) in good isolated yields. The rates of the isomerizations depend on the starting imine structures and increase in the following order: aryl perfluoroalkyl ketimine 2m, per(poly)fluoroalkyl aldimine 2a,d-g, perfluoroaryl aldimine 2h, alkyl perfluoroalkyl ketimine 2i,j. The presence of chlorine or bromine atoms in the alpha-position to the C=N double bond of the starting imine favors a dehydrohalogenation reaction, giving rise to unsaturated products 6-9. The azomethine-azomethine isomerization was studied and proven to proceed essentially (>98%) intramolecularly with isotope exchange experiments. High chemical yields, the simplicity of the experimental procedure, and the low cost of all reagents employed make this biomimetic transamination of fluorocarbonyl compounds a practical method for preparing fluorine-containing amines of biological interest.     

DFT study on the isomerization in vitamin B6

Isomerization and tautomerism reactions of the active form of vitamin B6, pyridoxal phosphate, are studied at B3LYP level of theory using 6-311++G(2df,p) basis set in gas and aqueous phases. Twenty-three transition state (TS) structures for vitamin B6 isomerization are optimized, including 13 TS structures for O–H and C–C rotations, 8 TS structures for imine–enamine tautomerism, and 2 TS structures for keto–enol tautomerism. Activation energy (E _a), imaginary frequency (υ), and Gibbs free energy of activation (ΔG ^#) for the isomerization reactions are calculated. The activation energies of the imine–enamine tautomerism are in the range of 190–280 kJ/mol and of O–H and C–C rotations are mainly less than 60 kJ/mol. Also, our calculation shows that the imine forms of B6 are mainly more stable than the enamine forms. Effect of microhydration on the TS structures and activation energies is also investigated. It is found that the presence of water molecules catalyzes only the imine–enamine tautomerism.     

Confirmation of the presence of imine bonds in thermally cured polyimides

Model compounds for imines formed during the thermal curing of short chain polyimides have been synthesized and characterized. These compounds have imine bonds (C?N) formed by the nucleophilic attack of primary amines on imide carbonyls. The C?N stretching mode appears at 1649–1664 cm^?1 in the Raman and infrared spectra of these compounds and the band assigned to the carbonyl mode in an imide ring with an imine bond appears near 1740 cm^?1. These compounds have been prepared and characterized to verify the conclusions of a previously reported study in which bands observed in thermally cured short chain polyimides at 1656 and 1742 cm^?1 were assigned as the C?N and associated C?O modes, respectively. It has also been confirmed that the C?N stretching mode in the imide model compounds is inherently IR weak and can only be seen if the concentration of imine species is high. © 1993 John Wiley & Sons, Inc.     

MO Study of the photochemical behavior of the imine bond

The photoisomerization of imine compounds is studied in terms of an ab initio MO CI calculation. The potential curves of the syn-anti isomerization via the rotation and the inversion are examined for benzaldimine. It is suggested that the photoisomerization is initiated through the rotation around the C? N bond in both singlet and triplet states. The ease of the photoisomerization is found to be determined by both the conformation of phenyl ring in the ground state and the energy difference of vertically excited states between two isomers.     

新型含2-取代-1,3-噻唑烷和噻唑环的亚胺类化合物的合成及生物活性

通过4-[(2-氰基亚胺基-1,3-噻唑烷-3-基)甲基]-2-氨基噻唑与取代苯甲醛的缩合反应, 合成了14个新型含2-取代- 1,3-噻唑烷和噻唑环的亚胺类化合物5. 所有化合物的结构均经1H NMR和元素分析确证, 并通过X射线单晶衍射分析测定了化合物5a的晶体结构. 初步生物活性试验结果表明, 部分目标化合物具有一定的杀菌活性和植物生长调节活性.     

3-羟基-2-吡啶亚胺异构反应的机理

在RHF-6-31G,MP2/6-31G和MP2/6-31G水平上,对3-羟基-2-吡啶亚胺的气相、水分子作为催化剂的异构化反应进行了研究,结果表明,气象异构难于进行,水分子作为催化剂参与反应过程是目标反应所循的反应路径。     

Some new Ag(I), VO(II) and Pd(II) chelates incorporating tridentate imine ligand: Design,synthesis, structure elucidation,density functional theory calculations for DNA interaction,antimicrobial and anticancer activities and molecular docking studies

Some new complexes derived from VO(II), Ag(I) and Pd(II) metal ions and HNA imine ligand (L), i.e. (2‐((6‐allylidene‐2‐hydroxycyclohexa‐1,3‐dienylmethylene)amino)benzoic acid), have been prepared and their structures elucidated via molar conductance measurements, elemental analyses, infrared, NMR and electronic spectra and magnetic susceptibility estimations. Moreover, stability constants of the synthesized complexes were evaluated utilizing a spectrophotometric technique. On the basis of molar conductance and elemental analyses, the metal imine chelates have structure [M(L)], where M = Pd(II), VO(II) and Ag(I). The results indicate that the prepared HNA imine ligand acts as a tridentate moiety via nitrogen atom of azomethine group and two oxygen atoms of phenolic and carboxylic groups. All the complexes are found to be monomeric with 1:1 stoichiometry with square planar geometry for Pd(II), tetrahedral geometry for Ag(I) and distorted square pyramidal for VO(II). Theoretical density functional theory calculations were applied to verify the molecular geometry of the chelators and their metal chelates. The geometry optimization results are in agreement with experimental observations. The antimicrobial properties of the prepared HNA imine ligand and its metal chelates were evaluated against numerous plant pathogenic fungi and bacteria. The results of these studies indicate that the metal complexes exhibit a stronger antibacterial and antifungal effect compared to the imine ligand. In addition, the interaction of the metal imine chelates with calf thymus DNA was observed by way of viscosity, gel electrophoreses and spectral studies. Absorption titration studies reveal that each of the complexes is an avid binder to calf thymus DNA. Also, there are appreciable changes in the relative viscosity of DNA, which are consistent with enhanced hydrophobic interaction of the aromatic rings and intercalation mode of binding. Additionally, the cytotoxic activity of the investigated compounds against various cancer cell lines shows promising results which makes them prospective compounds for antibiotic and anticancer medicament studies. Furthermore, docking studies of the prepared compounds were conducted for confirming the biological results.     

Mechanisms of acid-catalyzed Z/E isomerization of imines.

The kinetics and mechanism of acid-catalyzed Z/E isomerization of O-methylbenzohydroximoyl chloride (1Za and 1Ea), methyl O-methylbenzohydroximate (1Zb and 1Eb), ethyl O-methylbenzohydroximate (1Zc and 1Ec and five para and meta substituted derivatives), O-methylcinnamohydroximoyl chloride (2Za and 2Ea), and methyl O-methylcinnamohydroximate (2Zb and 2Zb) have been investigated. The kinetics of Z/E isomerization of these imines have been studied in glacial acetic acid (1Ea and 1Zc) and in dioxane solutions containing HCl, trifluoromethanesulfonic acid, or tetrafluoroboric acid (1Ea, 1Zb, 2Ea, and 2Zb). The isomerization takes place by either (a) rotation about the carbon-nitrogen double bond of the protonated imine (iminium ion rotation) or (b) nucleophilic attack on the protonated imine to form a tetrahedral intermediate that undergoes stereomutation and loss of the nucleophile (nucleophilic catalysis). The hydroximoyl chlorides 1Eaand 2Ea only isomerize by the nucleophilic catalysis mechanism. The hydroximate 1Zb appears to be capable of isomerizing by either mechanism. The hydroximate 2Zb may be isomerizing only by iminium ion rotation. Theoretical calculations support the notion that increased conjugation in the protonated imine increases the rate of iminium ion rotation.     

Facile Synthesis of Chiral Cyclic Ureas through Hydrogenation of 2‐Hydroxypyrimidine/Pyrimidin‐2(1H)‐one Tautomers

A facile access to optically active cyclic ureas was developed through palladium‐catalyzed asymmetric hydrogenation of pyrimidines containing tautomeric hydroxy group with up to 99 % ee. Mechanistic studies indicated that reaction pathway proceed through hydrogenation of C=N of the oxo tautomer pyrimidin‐2(1H)‐one, acid‐catalyzed isomerization of enamine–imine, and hydrogenation of imine pathway. In addition, the chiral cyclic ureas are readily converted into useful chiral 1,3‐diamine and thiourea derivatives without loss of optical purity.     

Differentiation of isomeric sulfur heterocycles by electron ionization mass spectrometry: 1,4-dithiins, 1,4-dithiafulvenes and their analogues tetrathianaphthalenes,tetrathiafulvalenes and tetrathiapentalenes

The electron ionization mass spectra of the title compounds have been studied along with the product ion spectra of their metastable or collisionally activated molecular ions. The relative abundances of ions in the spectra allow unambiguous isomer differentiation. Isomerization of the molecular ions was observed in the metastable ion spectra of tetrathianaphthalenes and tetrathiafulvalenes. This isomerization reaction parallels the electrochemical or base-induced isomerization observed in solution. The studied tetrathiapentalene derivative does not rearrange to the corresponding tetrathiafulvalene or tetrathianaphthalene isomers.     

Synthesis and characterization of 7, 16‐bis(phenylazo)tetraaza[14]annulenes

The diazo coupling reaction between a tetraaza[14]annulene ( 1 ) and a series of 4‐substituted phenyl‐diazonium tetrafluoroborates yielded the corresponding 7, 16‐disubstituted products. Mass spectra indicate the presence of molecular ion peaks that substantiate the 7,16‐disubstituted products ( 4 ); the lack of olefinic proton signals corresponding to the 7,16‐ position in the ¹H nmr spectra of 4 also show that diazo coupling has taken place. Analysis of signals corresponding to the methyl groups of 4 in their ¹H and ¹³C nmr spectra indicate that the imine‐bis‐hydrazone form is present for compounds that do not contain p‐methoxyphenyl groups. However, analysis of methyl signals in the nmr specta of compounds 4 containing p‐methoxyphenyl groups show that the bis‐azo form and the imine‐bis‐hydrazone form are present through tautomerism. Complexation with nickel(II) ion induces the formation of the bis‐ azo structure.     

High-resolution mass spectrometric analysis of secondary organic aerosol produced by ozonation of limonene

Chemical composition of secondary organic aerosol (SOA) formed from the ozone-initiated oxidation of limonene is characterized by high-resolution electrospray ionization mass spectrometry in both positive and negative ion modes. The mass spectra reveal a large number of both monomeric (m/z < 300) and oligomeric (m/z > 300) condensed products of oxidation. A combination of high resolving power (m/Deltam approximately 60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the molecular composition of hundreds of individual compounds in SOA samples. Van Krevelen analysis shows that the SOA compounds are heavily oxidized, with average O : C ratios of 0.43 and 0.50 determined from the positive and negative ion mode spectra, respectively. A possible reaction mechanism for the formation of the first generation SOA molecular components is considered. The discussed mechanism includes known isomerization and addition reactions of the carbonyl oxide intermediates generated during the ozonation of limonene. In addition, it includes isomerization and decomposition pathways for alkoxy radicals resulting from unimolecular decomposition of carbonyl oxides that have been disregarded by previous studies. The isomerization reactions yield numerous products with a progressively increasing number of alcohol and carbonyl groups, whereas C-C bond scission reactions in alkoxy radicals shorten the carbon chain. Together these reactions yield a large number of isomeric products with broadly distributed masses. A qualitative agreement is found between the number and degree of oxidation of the predicted and measured reaction products in the monomer product range.