8-羟基鸟嘌呤自由基的开环反应机理

使用经实验校准的B3LYP/DZP++方法研究了8-羟基鸟嘌呤自由基的开环反应机理. 计算结果表明, 该反应先后历经C8—N9键的断裂、羟基H原子向N7原子转移两个步骤完成, 转移中的H原子具有阳离子的特征. 当没有水存在时, 羟基H原子的转移反应需经历一个四元环的过渡态, 具有较高的活化能, 反应较困难. 但如果有水分子存在, 羟基H原子的转移步骤将经历一个低活化能的六元环过渡态, 使整个8-羟基鸟嘌呤自由基的开环反应可以在较温和的条件下顺利完成. 在无水催化时, 羟基H转移是反应的速率控制步骤; 而有水催化时, 反应的速率由C8—N9键的断裂步骤控制.     

Hydrogen-bonding catalysis and inhibition by simple solvents in the stereoselective kinetic epoxide-opening spirocyclization of glycal epoxides to form spiroketals

Mechanistic investigations of a MeOH-induced kinetic epoxide-opening spirocyclization of glycal epoxides have revealed dramatic, specific roles for simple solvents in hydrogen-bonding catalysis of this reaction to form spiroketal products stereoselectively with inversion of configuration at the anomeric carbon. A series of electronically tuned C1-aryl glycal epoxides was used to study the mechanism of this reaction based on differential reaction rates and inherent preferences for S(N)2 versus S(N)1 reaction manifolds. Hammett analysis of reaction kinetics with these substrates is consistent with an S(N)2 or S(N)2-like mechanism (ρ = -1.3 vs ρ = -5.1 for corresponding S(N)1 reactions of these substrates). Notably, the spirocyclization reaction is second-order dependent on MeOH, and the glycal ring oxygen is required for second-order MeOH catalysis. However, acetone cosolvent is a first-order inhibitor of the reaction. A transition state consistent with the experimental data is proposed in which one equivalent of MeOH activates the epoxide electrophile via a hydrogen bond while a second equivalent of MeOH chelates the side-chain nucleophile and glycal ring oxygen. A paradoxical previous observation that decreased MeOH concentration leads to increased competing intermolecular methyl glycoside formation is resolved by the finding that this side reaction is only first-order dependent on MeOH. This study highlights the unusual abilities of simple solvents to act as hydrogen-bonding catalysts and inhibitors in epoxide-opening reactions, providing both stereoselectivity and discrimination between competing reaction manifolds. This spirocyclization reaction provides efficient, stereocontrolled access to spiroketals that are key structural motifs in natural products.     

Synthesis and Characterisation of a New Schiff‐Base‐like Ligand and its Iron(II) Complexes

The reaction of iron(II) acetate with the tetradentate Schiff base like ligand H₂L [(E,E)‐[{diethyl 2,2’‐[4,5‐dihydroxy‐1,2‐phenylenebis(iminomethylidyne)]bis3‐oxobutanato}]) leads to the formation of the octahedral N₂O₄ coordinated complex [FeL(MeOH)₂] · MeOH ( 1 ). Conversion of 1 with N‐methylimidazole (N‐meim) leads to the N₄O₂ coordinated complex [FeL(N‐meim)₂] · MeOH ( 2 ). Both complexes are pure HS compounds that were characterised using magnetic measurements and X‐ray crystallography. A special attention was given to the role of the two hydroxyl groups at the phenyl ring on the formation of a hydrogen bond network and the influence of this network on the magnetic properties.     

Next-generation quantum theory of atoms in molecules for the ground and excited state of the ring-opening of cyclohexadiene

The factors underlying the experimentally observed branching ratio (70:30) of the (1,3-cyclohexadiene) CHD → HT (1,3,5-hexatriene) photochemical ring-opening reaction are investigated. The ring-opening reaction path is optimized by a high-level multi-reference DFT method and the density along the path is analyzed by the quantum theory of atoms in molecules (QTAIM) and stress tensor methods. The performed density analysis suggests that, in both S₁ and S₀ electronic states, there exists an attractive interaction between the ends of the fissile σ-bond of CHD that steers the ring-opening reaction predominantly in the direction of restoration of the ring. It is suggested that opening of the ring and formation of the reaction product (HT) can only be achieved when there is a sufficient persistent nuclear momentum in the direction of stretching of the fissile bond. As this orientation of the nuclear momentum vector can be expected relatively rare during the dynamics, this explains the observed low quantum yield of the ring-opening reaction.     

Structural variety of zinc and copper complexes based on a 2,3-disubstituted 1,2,3,4-tetrahydroquinazoline ligand

The ring-chain tautomerism of 2-(3-tosyl-1,2,3,4-tetrahydroquinazolin-2-yl)quinolin-8-ol (H(2)L(ring)) has been exploited to produce mononuclear complexes or, alternatively, dinuclear complexes, as desired, by varying the stoichiometry of the ligand. Cu(2+) and Zn(2+) stabilise the ring tautomeric form of the ligand in their mononuclear complexes M(HL(ring))(2). The structural characterisation of Zn(HL(ring))(2)·2MeOH·0.5H(2)O shows O,N-donor behaviour of the ring tautomer. The 1,2,3,4-tetrahydroquinazoline undergoes a ring-opening reaction upon formation of phenoxo-bridged dinuclear complexes M(2)(L(chain))(2) in which the chain tautomer is acting as O,N,N,N-donor. The crystal structure of Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH evidenced the sensitivity of H(2)L(ring) to the copper-mediated aerobic oxidation, which results in two derivatives of the ligand, a quinazoline and an amide. The quinazoline ligand is acting as monoanionic and mononucleating through its O,N,N binding site, while the amide ligand behaves as a trianionic and binucleating through its O,N,N,N and O,O binding sites in Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH.     

Specific features of the interaction of methyl ethers of methyl (methyl-α-D-galactopyranoside)uronate with trimethylsilyl ions in the gas phase

Specific features of the interaction between trimethylsilyl ions and methyl (methyl-α-d-galactopyranoside)uronate and its methyl ethers were revealed. It was shown that a hydrogen atom is generated when the trimethylsilyl ion is located at hydroxyl group. This atom migrates over the methoxy and hydroxyl groups toward the glycoside methoxy group, resulting in the formation of [Me+SiMe₃−MeOH]⁺ ions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1317–1319, July, 1997.     

Synchrotron photoionization measurements of OH-initiated cyclohexene oxidation: ring-preserving products in OH + cyclohexene and hydroxycyclohexyl + O2 reactions

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck-Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H(2)O(2) photolysis in the presence of C(6)D(10) establish that the addition-elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-d(4) confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-d(10) reaction with O(2) are observed upon adding a large excess of O(2) to the OH + C(6)D(10) system.     

Theoretical Analysis of Structural Characteristics of Morin Rearrangement for Cephalosporins Prepared from Penicillin Sulfoxide

The structure of penicillin sulfoxide is rearranged to cephalosporins by the Morin rearrangement. It is a unit reaction for the preparation of various types of cephalosporins. In order to make better use of the reaction and in view of the shortage of the reaction theory, this study used m062x/6-311++G(d, p) to explore the possible ring-opening reaction of the penicillin sulfoxide. It is found that the isomer of(S)-sulfoxide is a necessary structure. At the same time, the intramolecular hydrogen bonding effect between the side-chain amide proton(-CONH-) and the sulfinyl oxygen(-SO) is the decisive structure factor for the formation of alkenyl in ring-opening reaction, and the best reaction path is S0- TS2- IN1 channel. The main effect of acid catalysis is to catalyze the dehydration reaction of sulfenic acid to form sulfur cations for subsequently ring closing reaction.     

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states (1A1/3A' ') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ (1Sigma/3Sigma); formation of propynaldehyde and the moiety V-(OH2)+; and two elimination processes of water molecule to yield cationic products, Prod-fc+ and Prod-dc+ where the vanadium atom adopts a four- and di-coordinate structure, respectively.     

Time‐dependent density functional theory study on the electronic excited‐state geometric structure,infrared spectra,and hydrogen bonding of a doubly hydrogen‐bonded complex

The geometric structures and infrared (IR) spectra in the electronically excited state of a novel doubly hydrogen‐bonded complex formed by fluorenone and alcohols, which has been observed by IR spectra in experimental study, are investigated by the time‐dependent density functional theory (TDDFT) method. The geometric structures and IR spectra in both ground state and the S₁ state of this doubly hydrogen‐bonded FN‐2MeOH complex are calculated using the DFT and TDDFT methods, respectively. Two intermolecular hydrogen bonds are formed between FN and methanol molecules in the doubly hydrogen‐bonded FN‐2MeOH complex. Moreover, the formation of the second intermolecular hydrogen bond can make the first intermolecular hydrogen bond become slightly weak. Furthermore, it is confirmed that the spectral shoulder at around 1700 cm^?1 observed in the IR spectra should be assigned as the doubly hydrogen‐bonded FN‐2MeOH complex from our calculated results. The electronic excited‐state hydrogen bonding dynamics is also studied by monitoring some vibraitonal modes related to the formation of hydrogen bonds in different electronic states. As a result, both the two intermolecular hydrogen bonds are significantly strengthened in the S₁ state of the doubly hydrogen‐bonded FN‐2MeOH complex. The hydrogen bond strengthening in the electronically excited state is similar to the previous study on the singly hydrogen‐bonded FN‐MeOH complex and play important role on the photophysics of fluorenone in solutions. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

Theoretical study of pyridoxine (vitamin B6) photolysis

Two different reaction types for the photolysis of pyridoxine-aromatic ring-opening and photodissociation-have been studied in the Density Functional Theory (DFT) framework. Our results show that neither photolytic ring-opening, dehydroxymethylation, demethylation nor dehydroxylation from the aromatic ring can be induced spontaneously in UV-irradiated pyridoxine, due to the high barriers along the reaction coordinates in the excited states. However, the simultaneous dehydroxylation of the C4-bound hydroxymethyl group and dehydrogenation of the ring bound hydroxyl substituent, selectively generating ortho-quinone methide and water, does occur after UV exposure. The findings correlate very well with available experimental data. The geometries of pyridoxine, its various transition states and products are optimized in the ground and first excited states in vacuum within the TD-DFT formalism.     

Elucidation of the Mechanistic Pathways of the Hydroxyl Radical Scavenging Reaction by Daidzein Using Hybrid QM/MM Dynamics

Employing a hybrid QM/MM simulation we explored the reaction dynamics of the hydroxyl radical scavenging activity of daidzein, a soy isoflavone. Our simulations illustrate that the highly reactive hydroxyl radical can participate in hydrogen abstraction reaction with both OH functional groups of daidzein and can form stable daidzein radicals. We found that the reaction involving the 4'-OH site of daidzein is energetically favorable over the other reaction pathway involving the 7-OH site of daidzein by ～29 kcal/mol. The high enthalpic stabilization involved in daidzein radical formation at the 4'-OH site can be partly attributed to better solvation through hydrogen-bonding interactions with water and higher electron density delocalization of radical over the adjacent aromatic ring. As evident from the QM/MM dynamics, both HAT pathways led to formation of ketones at the 7-OH and 4'-OH sites of daidzein, respectively, and the adjacent aromatic rings appear in a p-quinonoid form, a highly stable resonating structure. The suitability of the QM/MM methodology to study the reaction mechanism, identification of intermediate states, and pathways of flavonoid radical stabilization reported here opens up a new possibility to study a similar reaction mechanism in other systems.     

Elimination of water from the molecular ion of cycloheptanol

Under electron impact cycloheptanol decomposes by four fragmentation paths: (1) α-cleavage with subsequent losses of C¹-C⁵ fragments, (2) elimination of water, (3) loss of the hydrogen atom from C-1 and (4) loss of the hydroxyl group. The mechanism of water elimination was investigated by means of deuterium labelling. 1,4-Elimination of water predominates in cycloheptanol, with the stereospecific cis-1,3-elimination also being operative. The loss of water is preceded by extensive exchange of the hydroxyl hydrogen with those of the ring. This is attributed to a very facile transannular interaction of the hydroxyl group with the C-3 to C-6 positions that are made accessible due to conformational properties of the 7-membered ring. A kinetic model is proposed, describing migrations of the ring hydrogen atoms.     

n-sigma charge-transfer interaction and molecular and electronic structural properties in the hydrogen-bonding systems consisting of p-quinone dianions and methyl alcohol

Molecular and electronic structural properties of the hydrogen-bonded complexes of p-quinone dianions (PQ(2)(-)) were investigated by electrochemistry and spectroelectrochemistry of PQ in MeCN combined with ab initio MO calculations. Hydrogen bonding between PQ(2)(-) and MeOH was measured as the continuous positive shift of the apparent second half-wave reduction potentials with increasing concentrations of MeOH. Detailed analyses of the behavior reveal that PQ(2)(-) forms the 1:2 hydrogen-bonded complexes at low concentrations of MeOH and the 1:4 complexes at high concentrations, yielding the formation constants. Temperature dependence of the formation constants allows us to yield the formation energy as 76.6 and 118.9 kJ mol(-)(1) for the 1:2 and 1:4 complex formation of the 1,4-benzoquinone dianion (BQ(2)(-)) with MeOH, respectively. These results show that the pi-dianions involving the quinone carbonyl groups exhibit very strong hydrogen-accepting ability. The longest wavelength band of the spectra of BQ(2)(-) and the chloranil dianion (CL(2)(-)) is assigned to the (1)B(3u) <-- (1)A(g) band mainly contributed from an intramolecular charge-transfer (CT) configuration. Hydrogen bonding allows the band of BQ(2)(-) and CL(2)(-) to be blue-shifted, depending on the strength of the hydrogen bonds. CNDO/S-CI calculations reveal that the blue shift is ascribed to stabilization of the ground state by the hydrogen bonding involving strong n-sigma-type CT interaction. The HF/6-31G(d) calculation results show that the structure of PQ(2)(-) is characterized by a lengthening of the C=O bonds and a benzenoid ring. The geometrical properties of the hydrogen-bonded complexes of PQ(2)(-) are a slight lengthening of the C=O bonds and a short distance of the hydrogen bonds. It is demonstrated that this situation is due to the strong n-sigma CT interaction in the hydrogen bonds. The results suggest that the differing functions and properties of biological quinones are conferred by the n-sigma CT interaction through hydrogen bonding of the dianions with their protein environment.     

A systematic investigation of the ring size effects on the free radical ring-opening polymerization (rROP) of cyclic ketene acetal (CKA) using both experimental and theoretical approach

Radical ring-opening polymerization (rROP) reaction of cyclic ketene acetals (CKA) is an interesting route to biodegradable polymers. Contrary to their tremendous potential, fundamental understanding of their reaction kinetics and thermodynamics is still limited. We present experimental and theoretical investigations for rROP reactions of CKA to systematically elucidate the effects of monomer ring sizes on the homopolymerization. We aim to provide insights on the structural-reactivity relationship of CKA by studying the thermodynamics and kinetics of the forward ring-opening propagation reactions and key side reactions, namely ring-retained propagation and radical back-biting reaction leading to branching. Experimental results show that for the CKA with smaller ring sizes, significant amount of ring-retained side products are formed when up to 90% of the monomers are converted. However, for the larger ring sizes (7 and 8 membered), almost complete ring-opening polymerization with <1% of ring-retained products are formed. Density functional theory (DFT) calculations show that kinetic effects from the collision frequency dominate in differentiating between ring-opening propagation, ring-retained propagation, and backbiting. The results corroborate well with experiments and reports in the literature. Our systematic study from the first principle and experimental validation provide insights into CKA rROP to apply radical polymerization to generate biodegradable polymers.     

Chemical studies of the antioxidant mechanism of theaflavins: radical reaction products of theaflavin 3,3′-digallate with hydrogen peroxide

The objective of the current study is to characterize the reaction products of theaflavin 3,3′-digallate, one of the major characteristic polyphenols of black tea, with hydroxyl radicals generated by hydrogen peroxide, with the aim of gaining insights into specific mechanisms of antioxidant reactions in physiological systems. Two major reaction products were isolated and identified using high-field 1D and 2D NMR spectral analysis. Both of them are A-ring fission products. The observation of these compounds indicates that the A ring rather than the benzotropolone moiety is the initial site for formation of reaction products in the hydrogen peroxide oxidant system.     

Photo‐Induced Hydrogen Exchange Reaction between Methanol and Glyoxal: Formation of Hydroxyketene

We study the structure and photochemistry of the glyoxal–methanol system (G–MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non‐hydrogen‐bonded complex, G–MeOH‐1, is present in an inert environment of solid argon. MP2/aug‐cc‐pVDZ calculations indicate that G–MeOH‐1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry‐adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non‐hydrogen‐bonded G–MeOH‐1 complex than compared to the hydrogen‐bonded ones. The irradiation of G–MeOH‐1 with the filtered output of a mercury lamp (λ>370 nm) leads to its photo‐conversion into the hydroxyketene–methanol complex HK–MeOH‐1. The identity of HK–MeOH‐1 is confirmed by both FTIR spectroscopy and MP2/aug‐cc‐pVDZ calculations. An experiment with deuterated methanol (CH₃OD) evidences that hydroxyketene is formed in a photo‐induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo‐conversion of G–MeOH‐1 to HK–MeOH‐1 is studied by a coupled‐cluster method [CR–CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.     

Theoretical study on the thermodynamic properties and self-decomposition of methylbenzenediol isomers

Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H?C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2-diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T ≤ 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways.     

2,2,6,6-四甲基哌啶-1-氧自由基促进的钒基催化剂催化苯直接氧化制苯酚

以钒基化合物为催化剂,在TEMPO(2,2,6,6-四甲基哌啶-1-氧自由基)存在下,能形成快速催化分子氧氧化苯制苯酚的催化体系.在反应过程中,由类似芬顿试剂反应过程生成的羟基自由基亲核进攻苯环,形成羟基环己二烯自由基;该羟基氢可在TEMPO存在的催化体系中消除,同时苯环氢可立即转移至氧原子而生成苯酚.在以[(CH3)...     

Interaction of triols with formaldehyde and acetone: Experimental and theoretical study

Experimental and theoretical aspects of the condensation of glycerol and its homologs (1,2,3- and 1,2,4-butanetriols) with formaldehyde and acetone are studied under conditions of acid catalysis. Calculation of the thermodynamic parameters of the resulting products by the composite method CBS-QB3 shows that the six-membered heterocycles, the products of the interaction of triols with formaldehyde, are thermodynamically more stable than the five-membered acetals, while the reaction of the same triols with acetone is preferable for the formation of the five-membered acetals. This is due to the fact that the regioselectivity of the studied reactions is determined by the structural features and reactivity of the carbocations formed in a condensed medium during the course of the reaction. According to the theoretical data obtained experimentally, during the condensation of glycerol and 1,2,4-butanetriol with formaldehyde in the most stable form of the six-membered cyclic carbocation, intramolecular hydrogen bonding and anomeric stabilization due to the axially oriented hydroxyl group take place. As a result, cation 1b–1 is 1.2–1.6 kJ/mol more stable than its five-membered isomers ( 1a–1 and 1b–2 ). It leads to the predominant formation of 1,3-dioxane ( 3b ). However, upon condensation of butanetriol-1,2,3 with formaldehyde, the intermediate cation 4a–1 turns out to be significantly more stable than the other isomers due to the strong intramolecular hydrogen bond in the six-membered ring with the participation of the hydroxyl group of the substituent and the hydroxyl group of the cationic center, leading to the predominant formation of the dioxolane 6a .