Theoretical investigation of solvent effects on tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol

The density functional theory has been used to study the tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol(DDNP) in the gas phase and in 14 solvents at the B3LYP/6‐31G* level. The solvent effects on the tautomeric equilibria were investigated by the self‐consistent reaction field theory (SCRF) based on conductor polarized continuum model (CPCM) in apolar and polar solvents and by the hybrid continuum‐discrete model in protic solvent, respectively. Solvent effects on the computed molecular properties, such as molecular geometries, dipole moments, E_LUMO, E_HOMO, total energies for DDNP tautomers and transition state, tautomerization energies and solvation energies have been found to be evident. The tautomeric equilibrium of DDNP is solvent‐dependent to a certain extent. The tautomer I (cyclic azoxy form) is preferred in the gas phase, while in nonpolar solvents tautomer I and II (quinold form) exist in comparable amounts, and in highly polar solvents, the tautomeric equilibrium is shifted in favor of the more polar tautomer II . © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Tautomeric equilibria of 2‐ and 4‐thiouracil in gas phase and in solvent: A density functional study

The relative stabilities of the five favored tautomers of 2‐ and 4‐thiouracil in gas phase and in water solution were determined by density functional theory employing the Becke, Lee, Yang, and Parr (B3LYP) exchange–correlation potential and the three 6‐31G(d,p), 6‐311++G(d,p), and triple‐zeta valence (TZVP) basis sets. Zero‐point vibrational corrections were also computed. Bulk solvent effects were studied in the framework of the self‐consistent reaction field approach by the polarizable continuum model. All calculations indicate that the most stable tautomer for both species, in the gas phase as well as in solution, has the oxo‐thione form, in full agreement with the previous ab initio and experimental studies. The tautomeric stability orders obtained in the aqueous solution are sensibly different from that in the gas phase. At B3LYP/6‐311++G(d,p) level in the gas phase, the following orders of stability for 2‐ and 4‐thiouracil tautomers were observed, respectively: S2U1>S2U2>S2U4>S2U5>S2U3 and S4U1>S4U2>S4U3>S4U4>S4U5. The corresponding trends in the aqueous phase are S2U1>S2U3>S2U2>S2U5>S2U4 and S4U1>S4U2>S4U3>S4U5>S4U4. On the basis of the computed energy differences we can hypothesize that only the oxo‐thione forms of 2‐ and 4‐thiouracil should exist in the gas phase and in water solution. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 44–52, 2001     

Ring‐opening polymerization of 1,4‐dioxan‐2‐one initiated by lanthanum isopropoxide in bulk

Lanthanum isopropoxide (La(OiPr)₃) has been synthesized and employed for ring‐opening polymerization of 1,4‐dioxan‐2‐one in bulk as a single‐component initiator. The influences of reaction conditions such as initiator concentration, reaction time, and reaction temperature on the polymerization were investigated. The kinetics indicated that the polymerization is first‐order with respect to the monomer concentration. The Mechanistic investigations according to ¹H NMR spectrum analysis demonstrated that the polymerization of PDO proceeded through a coordination‐insertion mechanism with a rupture of the acyl‐oxygen bond of the monomer rather than the alkyl‐oxygen bond cleavage. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5214–5222, 2008     

Rapid ring‐opening polymerization of 1,4‐dioxan‐2‐one initiated by titanium alkoxides

Ring‐opening polymerization of 1,4‐dioxan‐2‐one in bulk was initiated by three titanium alkoxides, titanium dichlorodiisopropoxide (TiCl₂(OiPr)₂), titanium chlorotriisopropoxide (TiCl(OiPr)₃), and titanium tetraisopropoxide (Ti(OiPr)₄). The results indicate that the polymerization rate increased with number of OiPr groups in the initiator. High conversion of monomer (90%) and high molecular weight (11.9 × 10⁴ g/mol) of resulting polymer can be achieved in only 5 min at 60 °C with Ti(OiPr)₄ as an initiator. Analysis on nuclear magnetic resonance (NMR) spectra suggests the initiating sites for TiCl₂(OiPr)₂, TiCl(OiPr)₃, and Ti(OiPr)₄ to be 1.9, 2.6, and 3.8, respectively. Coordination‐insertion mechanism for the polymerization via cleavage of the acyl–oxygen bonds of the monomer was proved by NMR investigation. Kinetic studies indicate that polymerization initiated by Ti(OiPr)₄ followed a first‐order kinetics, with an apparent activation energy of 33.7 kJ/mol. It is noteworthy that this value is significantly lower than earlier reported values with other catalysts, namely La(OiPr)₃ (50.5 kJ/mol) and Sn(Oct)₂ (71.8 kJ/mol), which makes it an attractive catalyst for reactive extrusion polymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis and nuclear magnetic resonance analysis of starch‐g‐poly(1,4‐dioxan‐2‐one) copolymers

A new biodegradable starch graft copolymer, starch‐g‐poly(1,4‐dioxan‐2‐one), was synthesized through the ring‐opening graft polymerization of 1,4‐dioxan‐2‐one onto a starch backbone. The grafting reactions were conducted with various 1,4‐dioxan‐2‐one/starch feed ratios to obtain starch‐g‐poly(1,4‐dioxan‐2‐one) copolymers with various poly(1,4‐dioxan‐2‐one) graft structures. The microstructure of starch‐g‐poly(1,4‐dioxan‐2‐one) was characterized in detail with one‐ and two‐dimensional NMR spectroscopy. The effect of the feed composition on the resulting microstructure of starch‐g‐poly(1,4‐dioxan‐2‐one) was investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3417–3422, 2004     

Synthesis and hydrolytic degradation of a random copolymer derived from 1,4‐dioxan‐2‐one and glycolide

Novel biodegradable copolymers, poly(1,4‐dioxan‐2‐one‐co‐glycolide) [P(DON‐co‐GA)] containing a high proportion of 1,4‐dioxan‐2‐one (DON), were synthesized by copolymerizations of DON and glycolide (GA) at 120 °C for 16 h using stannous octoate as catalyst. Chemical composition and microstructural variation of the resulting copolymer were investigated by ¹H‐ and ¹³C NMR and thermal properties by differential scanning calorimetry (DSC). From the ¹³C NMR spectra, it was observed that, apart from the expected preponderance of DON sequences, the minor component, GA, was indeed distributed at various points along the copolymer chain rather than incorporated as distinct blocks, which is consistent with a random sequence distribution. This view also was supported by the DSC results, which showed that most copolymers were amorphous except for one with a relatively high fraction of DON. The conclusion that it was a random structure rather than a statistical copolymer is discussed, using the theories about the mechanism of this type of polymerization in current as a reference. P(DON‐co‐GA) films were prepared by casting the copolymer solution in hexafluoroisopropanol (HFIP) with two concentrations of the polymeric solution (10 and 25 wt %). The in vitro hydrolytic degradation behaviors of these films were studied in phosphate buffer solution (pH = 7.4) at 37 °C and characterized by DSC, scanning electron microscopy, weight loss, and change in inherent viscosity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2558–2566, 2004     

Rate and Equilibrium Constants for the Addition of N‐Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2‐Substituent Effect

Rate and equilibrium constants for the reaction between N‐aryl triazolium N‐heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3‐(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2‐substituent in these reactions and provide insight into the chemoselectivity of cross‐benzoin reactions.     

Synthesis of metal‐free poly(1,4‐dioxan‐2‐one) by enzyme‐catalyzed ring‐opening polymerization

To avoid the harmful effects of metallic residues in poly(1,4‐dioxan‐2‐one) (PPDO) for medical applications, the enzymatic polymerization of 1,4‐dioxan‐2‐one (PDO) was carried out at 60 °C for 15 h with 5 wt % immobilized lipase CA. The lipase CA, derived from Candida antarctica, exhibited especially high catalytic activity. The highest weight‐average molecular weight (M_w = 41,000) was obtained. The PDO polymerization by the lipase CA occurred because of effective enzyme catalysis. The water component appeared to act not only as a substrate of the initiation process but also as a chain cleavage agent. A slight amount of water enhanced the polymerization, but excess water depressed the polymerization. PPDO prepared by enzyme‐catalyzed polymerization is a metal‐free polyester useful for medical applications. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1560–1567, 2000     

A density functional theory study on diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole

The reaction mechanism, thermodynamic and kinetic properties for diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole were studied by a density functional theory. The geometries of the reactants, transition states, and intermediates were optimized at the B3LYP/6‐31G (d, p) level. Vibrational analysis was carried out to confirm the transition state structures, and the intrinsic reaction coordinate (IRC) method was used to explore the minimum energy path. The single‐point energies of all stagnation points were further calculated at the B3LYP (MP2)/6‐311+G (2d, p) level. The statistical thermodynamic method and Eyring transition state theory with Wigner correction were used to study the thermodynamic and kinetic characters of all reactions within 0–25°C. Two reaction channels are computed, including the diazotization and nitration of 3‐NH₂ or 5‐NH₂, and there are six steps in each channel. The reaction rate in each step is increased with temperature. The last step in each channel is the slowest step. The first, second, and fifth steps are exothermic reactions, and are favored at lower temperature in the thermodynamics. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Density functional theory study of tautomerization of 2‐aminothiazole in the gas phase and in solution

The amino/imino tautomeric equilibrium in the isolated, mono‐, di‐, and trihydrate forms and dimer of 2‐aminothiazole, and the effects of hydration or self‐assistance on the transition state structures corresponding to proton transfer from the amino to imino form, have been investigated by the B3LYP method in conjunction with 6‐31+G(d,p) and 6‐311+G(3df,2p) basis sets in the gas phase and in solution. The amino form has been found to be the predominant tautomer. The tautomeric barrier heights for water‐ and self‐assisted tautomerization reactions are significantly lower than that from the amino to imino form by the intramolecular proton transfer, showing the catalytic effect of water molecule(s) and the important role of 2‐aminothiazole itself for intermolecular proton transfer. Comparison between the tautomeric barriers demonstrates that the self‐association tautomerization through the dimerization is the most favorable pathway. Bulk solvent effects have been taken into account using the polarizable continuum model (PCM) of water and CCl₄. The polar medium is favorable for the population of the imino form. The amino/imino equilibrium is also analyzed using the aromaticity index nucleus‐independent chemical shift (NICS); the NICS values for the amino form (about ?10 ppm) are more negative than the imino species (about ?8 ppm), showing that the amino form is more stable. The time‐dependent density functional theory (TDDFT) calculations of electronic absorption spectra suggest that the λ_max of dimer is 255 nm. The oscillator strength of the imino forms is less than the amino form, and increases with the polarity of the solvents. All calculations for the tautomerization of 2‐aminothiazole are in reasonable line with the available experiments. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Copolymerization of poly(vinyl alcohol)‐graft‐poly(1,4‐dioxan‐2‐one) with designed molecular structure by a solid‐state polymerization method

Poly(vinyl alcohol)‐graft‐poly(1,4‐dioxan‐2‐one) (PVA‐g‐PPDO) with designed molecular structure was synthesized by a solid‐state polymerization. The solid‐state copolymerization was preceded by a graft copolymerization of PDO initiated with PVA as a multifunctional initiator, and Sn (Oct)₂ as a coininitiator/catalyst in a homogeneous molten state. The polymerization temperature was then decreased and the copolymerization was carried out in a solid state. The products prepared by solid‐state polymerization were characterized by ¹H NMR and DSC, and were compared with those synthesized in the homogeneous molten state. The degree of polymerization (D_p), degree of substitution (D_s), yield and the average molecular weight of the graft copolymer with different molecular structure were calculated from the ¹H NMR spectra. The results show that the crystallization process during the solid‐state polymerization may suppress the undesirable inter‐ or intramolecular side reactions, then resulting in a controlled molecular structure of PVA‐g‐PPDO. The results of DSC measurement show that the molecular structures determine the thermal behavior of the PVA‐g‐PPDO. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3083–3091, 2006     

Substituent effects on the Bergman cyclization of (Z)‐1,5‐hexadiyne‐3‐enes: a systematic computational study

The effects of several substituents (? BH₂, ? BF₂, ? AlH₂, ? CH₃, ? C₆H₅, ? CN, ? COCH₃, ? CF₃, ? SiH₃, ? NH₂, ? NH₃⁺, ? NO₂, ? PH₂, ? OH, ? OH₂⁺, ? SH, ? F, ? Cl, ? Br) on the Bergman cyclization of (Z)‐1,5‐hexadiyne‐3‐ene (enediyne, 3 ) were investigated at the Becke–Lee–Yang–Parr (BLYP) density functional (DFT) level employing a 6‐31G* basis set. Some of the substituents (? NH₃⁺, ? NO₂, ? OH, ? OH₂⁺, ? F, ? Cl, ? Br) are able to lower the barrier (up to a minimum of 16.9 kcal mol^?1 for difluoro‐enediyne 7rr ) and the reaction enthalpy (the cyclization is predicted to be exergonic for ? OH₂⁺ and ? F) compared to the parent system giving rise to substituted 1,4‐dehydrobenzenes at physiological temperatures. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1605–1614, 2001     

Theoretical investigations of 1,4‐butanediol and 2‐butene‐1,4‐diol cyclodehydration using postprocessing visualization of quantum chemical calculation data

Cyclodehydration of 1,4‐butanediol and 2‐butene‐1,4‐diol to the corresponding cyclic ethers was studied using the AM1 semiempirical method. It was established that the cyclodehydration reaction of 1,4‐butenediol and 2‐butene‐1,4‐diol is effected by converting of semicyclic conformers in the presence of acidic and basic active centers. The calculation results indicate that a concerted mechanism is probably realized in the cyclodehydration of both diols, while the sequences of the predicted steps in the cyclodehydration reaction for 1,4‐butanediol and 2‐butene‐1,4‐diol are different. The calculated reaction heats for 1,4‐butanediol and 2‐butene‐1,4‐diol transformations are ?184.029 and ?308.746 kcal/mol, respectively. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Functionalization of Hexa‐peri‐hexabenzocoronenes: Investigation of the Substituent Effects on a Superbenzene

We have demonstrated that the iridium‐catalyzed direct borylation of hexa‐peri‐hexabenzocoronene (HBC) enables regioselective introduction of boryl groups to the para‐, ortho‐, and meta‐substituted HBCs in high yields. The boryl groups have been transformed into various functionalities such as hydroxy, cyano, ethynyl, and amino groups. We have elucidated that the substituents significantly influence the photophysical properties of HBCs to enhance fluorescence quantum yields. DFT calculations revealed that the origin of the substituent effect is the lift in degeneracy in the frontier orbitals by an interaction with electron‐donating and electron‐withdrawing substituents at the para‐ and ortho‐positions. The change in molecular orbitals results in an increase of the transition probability from the S₀→S₁ states. In addition, the two‐photon absorption cross‐section values of para‐substituted HBCs are significantly larger than those of ortho‐ and meta‐substituted HBCs.     

Density functional theory study of the interaction between 3‐nitro‐1,2,4‐triazole‐5‐one and ammonia

Two fully optimized geometries of 3‐nitro‐1,2,4‐triazol‐5‐one (NTO)–NH₃ complexes have been obtained with the density function theory (DFT) method at the B3LYP/6‐311++G** level. The intermolecular interaction energy is calculated with zero point energy (ZPE) correction and basis set superposition error (BSSE) correction. The greatest corrected intermolecular interaction of the NTO–NH₃ complexes is ?37.58 kJ/mol. Electrons in complex systems transfer from NH₃ to NTO. The strong hydrogen bonds contribute to the interaction energies dominantly. Natural bond orbital (NBO) analysis is performed to reveal the origin of the interaction. Based on vibrational analysis, the changes of thermodynamic properties from the monomer to complexes with the temperature ranging from 200 K to 800 K have been obtained using the statistical thermodynamic method. It is found that two NTO–NH₃ complexes can be produced spontaneously from NTO and NH₃ at normal temperature. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Conductivity of I2‐Doped High trans‐1,4‐Polybutadiene

High trans‐l,4‐polybutadiene ( ?96% (trans)) was prepared by lanthanum naphthenate catalytic system. The conductivity of obtained polybutadiene doped with iodine reaches about ?100 s/cm, which is 2 orders of magnitude higher than the value reported.^4,5 During the I₂‐doping, the conjugated sequence was formed through double bond shifting reaction. According to the relationship between conductivity and temperature, conducting mechanism of doped high trans‐l,4‐polybutadiene is fit on variable range hoping (VRH) model.     

Isotopic splitting patterns in the 13C NMR spectra of some partially deuterated 1‐aryl‐2‐(phenyldiazenyl)butane‐1,3‐dione and 4‐hydroxy‐3‐(phenyldiazenyl)‐2H‐chromen‐2‐one: evidence for elucidation of tautomeric forms

Nuclear magnetic resonance spectra of synthesized azo dyes derived from aniline derivatives in reaction with benzoylacetone and 4‐hydroxycoumarin were studied in both CDCl₃ and (CD₃)₂SO (two drops of D₂O were added into solutions of dyes). All dyes showed intramolecular hydrogen bonding. Dyes derived from o‐nitro aniline in the reaction with benzoylacetone, and 4‐hydroxycoumarin showed bifurcated intramolecular hydrogen bonds. The solvent‐substrate proton exchange of dyes derived from benzoylacetone and 4‐hydroxycoumarin was examined in the presence of two drops of D₂O. Among ten dye samples, two dyes derived from benzoylacetone did not show deuteration, three dyes showed partial deuteration and five dyes showed full deuteration under similar conditions. For the partially deuterated dyes the β‐isotope effect in ¹³C splitting was investigated and was used for the determination of the predominant tautomeric form. Copyright © 2016 John Wiley & Sons, Ltd.     

One‐Pot Synthesis of 4‐(Alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐ dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones via a Multicomponent Reaction

An effective route to novel 4‐(alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones 10 is described (Scheme 2). This involves the reaction of an enamine, derived from the addition of a primary amine 5 to 1,4‐diphenylbut‐2‐yne‐1,4‐dione, with an arenesulfonyl isocyanate 7 . Some of these pyrrolones 10 exhibit a dynamic NMR behavior in solution because of restricted rotation around the C? N bond resulting from conjugation of the side‐chain N‐atom with the adjacent α,β‐unsaturated ketone group, and two rotamers are in equilibrium with each other in solution ( 10 ? 11 ; Scheme 3). The structures of the highly functionalized compounds 10 were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS), by elemental analyses, and, in the case of 10a , by X‐ray crystallography. A plausible mechanism for the reaction is proposed (Scheme 4).     

Reaction of 2,3‐Dichloro‐1,4‐Naphthoquinone with 1‐Phenylbiguanide and 2‐Guanidinebenzimidazole

When 2,3‐dichloro‐1,4‐naphthoquinone (DCHNQ) ( 1 ) is allowed to react with 1‐phenylbiguanide (PBG) ( 2 ), 4‐chloro‐2,5‐dihydro‐2,5‐dioxonaphtho[1,2‐d]imidazole‐3‐carboxylic acid phenyl amide ( 4 ), 6‐chloro‐8‐phenylamino‐9H‐7,9,11‐triaza‐cyclohepta[a]naphthalene‐5,10‐dione ( 5 ) and 4‐dimethyl‐amino‐5,10‐dioxo‐2‐phenylimino‐5,10‐dihydro‐2H‐benzo[g]quinazoline‐1‐carboxylic acid amide ( 6 ) were obtained. While on reacting 1 with 2‐guanidinebenzimidazole (GBI) ( 3 ) the products are 3‐(1H‐benzoimidazol‐2‐yl)‐4‐chloro‐3H‐naphtho[1,2‐d]imidazole‐2,5‐dione ( 7 ) and 3‐[3‐(1H‐benzoimidazol‐2‐yl)‐ureido]‐1,4‐dioxo‐1,4‐dihydronaphthalene‐2‐carboxylic acid dimethylamide ( 8 ).