首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
    
The study of the reaction systems “R3N – proton-donor reagent ( Nu H) – epichlorohydrin” using a complex of various kinetic methods of investigation allowed the reaction pathway and the catalysis mechanism of the oxirane ring opening by proton-donor reagents in the presence of R3N to be detailed. It was confirmed that the catalytic activity of R3N consists in their quaternization by oxirane under the obligatory condition of its electrophilic assistance with the Nu H reagent. The quaternization process of tertiary amines was studied using 1H NMR and UV spectroscopy, quantum chemical modeling, and correlation analysis. It was shown that the transition states of the quaternization stage are dissociative, and their degree of looseness depends on the electrophilic activation of the oxirane ring.  相似文献   

2.
    
The asymmetric oxirane ring-opening reaction leading to the formation of regioisomeric chlorohydrin esters was studied in the reaction series “acetic acid–epichlorohydrin–tetrahydrofuran (nitrobenzene)–trialkylamine” by kinetic methods and FT-IR spectroscopy. The effect of solvent polarity and the structure of tertiary amines on the regioselectivity and reaction rate were studied. Tertiary amines with comparable basicity but different nucleophilicity and spatial structure were chosen as catalysts. It was shown that in solvents of different polarity, the components of the initial reaction system are present both as hydrogen-bonded complexes and as individual substances. The reaction orders with respect to acid and amine in solvents of different polarity were established. Correlations between the reaction rate and the parameters of nucleophilicity and structure of amines as well as the polarity of the solvent were established. The regioselectivity of the reaction was studied by 1H NMR spectroscopy using the ratio of regioisomeric reaction products. It was shown that the regioselectivity and rate of catalytic acetolysis of epichlorohydrin are effectively controlled by the structure of tertiary amines and the polarity of the solvent. The scheme of reaction regio-flows was detailed.  相似文献   

3.
    
The catalytic hydrolysis of bis(4‐nitrophenyl)phosphate (BNPP) and bis(2,4‐dinitrophenyl)phosphate (BDNPP) catalyzed by α‐nucleophiles in gemini micellar media was investigated at 27 °C. The cationic gemini surfactants, i.e., alkanediyl‐α‐ω‐bis(hydroxyethylmethylhexadecylammonium bromide) (16‐s‐16 MEA 2Br?, where s = 4 and 6) were used. Nucleophilic reactivity of α‐nucleophiles such as hydroperoxide (HOO?), acetohydroxamate (AHA?), and butane 2,3‐dione monoximate ions (BDMO?) were compared. The kinetic rate data were treated by applying the pseudophase model. The cationic gemini surfactants show unusual rate acceleration toward the cleavage of phosphodiesters with nucleophiles. These studies reveal that the hydroperoxide ion shows the highest catalytic activity reported so far with an unprecedented acceleration rate, 107 times faster than that of the uncatalyzed reaction. The possible mechanism for the BNPP and BDNPP cleavage promoted by α‐nucleophiles is proposed on the basis of kinetic analysis. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

4.
    
This paper presents a comparative study of the solvent effects on the kinetics of the reactions between 1‐fluoro‐2,6‐dinitrobenzene and hexahydro‐1H‐azepine in ethyl acetate + chloroform or acetonitrile and acetonitrile + chloroform binary solvent mixtures. The purpose was to asses the contribution of each molecular‐microscopic solvent property to the overall solvation effect of the reaction critical state. The influence of the dipolarity/polarizability, hydrogen‐bond donor acidity and hydrogen‐bond acceptor basicity of the solvent mixtures on the reaction rate was analyzed by the application of the multiparametric approach of Kamlet, Abboud and Taft. The evaluation of the correlation coefficients shows that the solvation effects are dominated by the non‐specific interactions. Moreover, the incidence of the hydrogen‐bond acceptor solvent ability is more important than that corresponding to the hydrogen‐bond donor character, except for the reactions that manifest kinetic retarding effects. The influence of the solvation effects on the base catalysis was analyzed, describing the response patterns of the partial rate coefficients to the mixture composition and also to each solvent property. The application of a preferential solvation model to the kinetic results yields information on the preferential solvation of the reaction critical state by any of the pure solvents, or by the hydrogen‐bonded complexed media. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

5.
    
The kinetics of nucleophilic bimolecular substitution reactions of γ‐functionalized allyl bromides with non‐substituted and p‐substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second‐order rate constants k, activation energy EA, and changes in the entropy ΔS≠, enthalpy ΔH≠, and free energy ΔG≠ of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the SN2‐reactions as follows: p‐toluenesulfinate ion > benzenesulfinate ion > p‐chlorobenzenesulfinate ion and 4‐bromo‐2‐butenenitrile > 1,3‐ dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ‐functionalized allyl bromides. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
    
The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R1O‐CR2CH‐COCF3 ( 1a – e ) [( 1a ), R1C2H5, R2H; ( 1b ), R1R2CH3; ( 1c ), R1C2H5, R2C6H5; ( 1d ), R1C2H5, R2V−pNO2C6H4; ( 1e ), R1C2H5, R2C(CH3)3] with four aliphatic amines ( 2a – d ) [( 2a ), (C2H5)2NH; ( 2b ), (i‐C3H7)2NH; ( 2c ), (CH2)5NH; ( 2d ), O(CH2CH2)2NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k1) and ‘uncatalyzed’ decomposition of transition state (k2) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k3/k2 ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pKa and small bulk. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

7.
    
Computational studies are reported for reactions of 4‐substituted‐1‐chloro‐2,6‐dinitrobenzenes 1 , 6‐substituted‐1‐chloro‐2,4‐dinitrobenzenes 2 and some of the corresponding 1‐phenoxy derivatives 3 and 4 with aniline in the gas phase. The effects of substituent groups in the calculated energy values for reactants 1–4 , transition states structures, intermediates and products formed in the reactions between the compounds and anilines have been compared. Calculated bonds length and angles from optimized structures of the reactants were comparable with values reported for some of compounds 1–4 obtained by X‐ray crystal structures analysis. Generally, the decomposition of the Meisenheimer intermediate to the products requires more energy compared with the reactants except for when R = H. The order of stabilization of the intermediate was found to reflect the relative order of activation by substituents in the substrates. The 4‐substituted‐1‐chloro‐2,6‐dinitrobenzenes 1 and the phenoxy derivatives 3 were found to be more stable than their corresponding 6‐substituted analogues. This is an indication that the rate of nucleophilic attack at 1‐position will increase with increasing ring activation but may be reduced by steric repulsion at the reaction centre that increases in the order Cl < OPh. However, the steric hindrance to the steps involved in nucleophilic substitution by aniline is significantly increased when the substrates contain two ortho‐substituents. In most cases, the rate determining step is the decomposition of the σ‐adduct intermediate except with 1‐chloro‐2,6‐dinitrobenzenes 1 and 6‐substituted‐1‐chloro‐2,4‐dinitrobenzenes 2 , either because of reduction in ring activation or the presence of bulky ortho‐substituents in the chloro compounds 1 and 2 . Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

8.
    
The reaction path of the Interacting‐State Model is used with the Transition‐State Theory and the semiclassical correction for tunneling (ISM/scTST) to calculate proton transfer rates in rate‐determining C H bond breaking by enzymes such as mandelate racemase, triose‐phosphate isomerase, methylamine dehydrogenase (MADH), and aromatic amine dehydrogenase (AADH), as well as of the analogous uncatalyzed proton transfers in aqueous solution. This method employs the reaction energy, the bond distances in reactants and products, and properties of the reactive bonds to calculate the proton transfer rates. The comparison between the molecular factors that control reactivity in solution and in the enzymes reveals the following salient features of enzyme catalysis: (1) the change from a bimolecular reaction in solution to an intramolecular reaction with negligible activation entropy in the enzyme; (2) energetic stabilization of the intermediates; and (3) extreme tunneling corrections when hydrogen bonding to a carbon atom is present in the reaction path. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
    
Transesterification of a phosphodiester bond of RNA models has been studied in various buffer solutions, under neutral and slightly alkaline conditions in H2O and D2O. The results show that imidazole is the only buffer system where a clear buffer catalysis on the cleavage of a phosphodiester bond is observed. The rate enhancement in sulphonic acid buffers is smaller, and a sulphonate base, particularly, is inactive as a catalyst. The rate‐enhancing effect of imidazole is, however, catalytic, and the catalytic inactivity of sulphonate buffers can be attributed to their structure and/or charge. The catalysis by imidazole is a complex system which, in addition to first‐order reactions, involves a process that shows a second‐order dependence in imidazole concentration. The latter reaction becomes significant in acidic imidazole buffers (pH < pKa), as the buffer concentration increases. The kinetic solvent deuterium isotope effect kH/kD, referring to first‐order catalysis by imidazole base, is 2.3 ± 0.3. That referring to second‐order catalysis is most probably much larger, but an accurate value could not be obtained. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
    
The conversion of the Z‐phenylhydrazone of 5‐amino‐3‐benzoyl‐1,2,4‐oxadiazole ( 1a ) into the relevant 1,2,3‐triazole ( 2a) has been quantitatively studied in toluene in the presence of several halogenoacetic acids ( HAA s, 3a – h ). Again, the occurrence of two reaction pathways has been pointed out: they require one or two moles of acid, respectively, thus repeating the situation previously observed in the presence of trichloroacetic acid. The observed rate constant ratios (kIII/kII) are only slightly affected by the nature of the acid used. To gain a deeper insight into the action of the acids used we have measured the association constants of the HAA s ( 3a – h) with 4‐nitroaniline ( 4 ) in toluene. Also in this case, the formation of two complexes requiring one (K2) or two (K3) moles of acid has been evidenced, but now the K3/K2 ratios are significantly affected by the strength of the acid examined. The variation of the K3/K2 ratios larger than those concerning the kIII/kII ratios appears useful to enlighten the very nature of the acid‐catalyzed pathways in toluene, which has been elucidated also carrying out the rearrangement in the presence of mixtures of tribromo‐ and trichloro‐acetic acids at different concentrations. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
    
Tetrahydrochromeno is a structural variant of spiropyran that undergoes a reversible ring‐opening to generate a colored nitrophenolate intermediate. Earlier work confirmed this intermediate through trimethylsilyl cyanide trapping under continuous irradiation. We have performed transient absorption spectroscopy to further characterize the mechanism of the ring‐opening reaction. Excitation at 355 nm produced a transient species with an absorption maximum at 445 nm, which we assign to the nitrophenolate unit of the ring‐opened product. The transient absorption decays after ~970 ns with small optical density changes corresponding to a 0.15 quantum yield. Exposure to oxygen did not exhibit a significant deleterious effect on the photoisomerization of the chromeno dye. Time‐dependent density functional theory corroborated spectroscopic assignments of the starting chromeno and the putative ring‐opened intermediate. The excited state behavior of this system parallels the structurally similar oxazine system reported by Raymo and coworkers. The one significant difference is the longer lifetime of the photochemically generated intermediate from chromeno. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

12.
    
Nitronaphthalimides are a particularly interesting group of nitroaromatics used for staining cells in hypoxia and in photodynamic therapy. The kinetics of the aromatic nucleophilic substitution of the nitro‐group of 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide (4‐NBN) by n‐heptanethiol (RSH), thiophenol (ArSH) and 2‐hydroxy‐1‐ethanethiol (HESH) in aqueous solutions containing micelles and in water–methanol mixtures were analyzed. The substitution of the nitro group by aromatic or aliphatic thiols led to fluorescent substitution products. Hexadecyltrimethylammonium chloride micelles (CTAC) increased the rate of thiolysis of 4‐NBN by 1 × 105 (RSH) and 4 × 105 (ArSH) fold. For the reaction of ArSH with 4‐NBN the calculated second‐order rate constant in the micellar phase, k2m, is 1.4 times higher than that in the aqueous phase, k2w. For the reaction of ArSH with 4‐NBN, k2m/k2w = 0.05. Both k2w and k2m decreased with increasing pKa of the thiol, suggesting that thiolate attack is not rate limiting in either the aqueous phase or micellar pseudophase. The high micellar rate acceleration is therefore largely due to concentration of the reagents in the micellar phase and effects on the thiol acid–base equilibrium. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

13.
    
The kinetics of the reactions of 1‐fluoro‐2,4‐dinitrobenzene with morpholine and piperidine were studied at 25 °C in several binary solvent mixtures of the polar aprotic hydrogen bond acceptor solvent + chloroform or dichloromethane type, which, in some cases, exhibit synergism for the ET(30) solvent polarity parameter. In each case, the kinetic response model was analyzed as a function both of the amine concentration and of the solvent mixture composition. The kinetic data show that for some reaction systems under certain experimental conditions, the reactions are faster in binary solvent mixtures than in the corresponding pure solvents, manifesting a special kinetic synergetic behavior. This kinetic effect was attributed to a complex combination of factors related to the variations of the influence of base catalysis with the solvent composition, and to the consequence of specific solvent effects. The kinetic results were related to the ET(30) solvent polarity parameter, analyzing the general validity of the chemical probe for interpreting the solvent effects in the aromatic nucleophilic substitution reactions explored. The applicability of the preferential solvation model allowed the special behavior observed to be attributed to the influence of preferential solvation phenomena. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

14.
    
The usual idea on the two‐steps mechanism of aromatic electrophilic substitution reactions is that the first step (the attack of the electrophilic reagent on the activated substrate) is rate limiting, while the driving force of the reaction is the fast proton departure to recover the resonance energy of the aromatic substrate. The now examined systems allow the formation of stable σ cationic complexes (Wheland intermediates) which may be investigated by simple procedures. Data here reported represent a clear and simple instance of a measurement of the rate of the proton abstraction from a Wheland intermediate and they indicate that this proton abstraction occurs by base catalysis in a rate determining step. Probably, this feature is more frequent than that usually conceived in the mechanism of electrophilic aromatic substitution reactions, because these reactions are often carried out in reaction mixtures containing large amounts of proton acceptor species which might mask the possible base catalysis. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

15.
    
The aromatic nucleophilic substitution reaction of 3,6‐dichloro‐1,2,4,5‐tetrazine (DCT) with a series of biothiols RSH: (cysteine, homocysteine, cysteinyl–glycine, N‐acetylcysteine, and glutathione) is subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the disappearance of DCT at 370 nm. In the case of an excess of N‐acetylcysteine and glutathione, clean pseudo first‐order rate constants (kobs1) are found. However, for cysteine, homocysteine and cysteinyl–glycine, two consecutive reactions are observed. The first one is the nucleophilic aromatic substitution of the chlorine by the sulfhydryl group of these biothiols (RSH) and the second one is the intramolecular and intermolecular nucleophilic aromatic substitutions of their alkylthio with the amine group of RSH to give the di‐substituted compound. Therefore, in these cases, two pseudo first‐order rate constants (kobs1 and kobs2, respectively) are found under biothiol excess. Plots of kobs1 versus free thiol concentration at constant pH are linear, with the slope (kN) independent of pH (from 6.8 to 7.4). The kinetic data analysis (Brønsted‐type plot and activation parameters) is consistent with an addition–elimination mechanism with the nucleophilic attack as the rate‐determining step. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

16.
We investigate the kinetic behavior of a two-species catalysis-driven aggregation model, in which coagulation of species A occurs only with the help of species B. We suppose the monomers of species B are stable and cannot selfcoagulate in reaction processes. Meanwhile, the monomers are continuously injected into the system. The model with a constant rate kernel is investigated by means of the mean-field rate equation. We show that the Mneties of the system depends crucially on the details of the input term. The injection rate of species B is assumed to take the given time- dependent form K（t） -t^λ, and the sealing solution of the duster size distribution is then investigated analytically. It is found that the cluster size distribution can satisfy the conventional or modified scaling form in most cases.  相似文献   

17.
    
Eight 3‐alkyl‐1,3‐diaryltriazenes with methyl, ethyl, propyl, butyl, pentyl, isopropyl, sec‐butyl and cyclohexyl substituents were synthesized and their rate constants of decomposition by trichloroacetic acid (0.01–0.25 mol dm−3) in hexane at 25°C were measured. The kinetic model and mechanism thereof were studied by modelling of the dependences of kobs on the concentration of trichloracetic acid. On the basis of this kinetic model and the interpretation of solvent effects, a reaction mechanism was suggested according to which the triazene reacts with monomer and obviously also opens the dimer of trichloroacetic acid in a single reaction step. At the same time, a non‐reactive associate between the N1 nitrogen of triazene and two molecules of trichloroacetic acid is formed in the reaction mixture. The equilibrium and rate constants depend on the addition of trichloroacetic acid as the co‐solvent. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

18.
A competition model of three species in exchange-driven aggregation growth is proposed. In the model, three distinct aggregates grow by exchange of monomers and in parallel, birth of species A is catalyzed by species B and death of species A is catalyzed by species C. The rates for both catalysis processes are proportional to kj^v and ky respectively, where ν（ω） is a parameter reflecting the dependence of the catalysis reaction rate of birth （death） on the catalyst aggregate＇s size. The kinetic evolution behaviors of the three species are investigated by the rate equation approach based on the mean-field theory： The form of the aggregate size distribution of A-species αk（t） is found to be dependent crucially on the two catalysis rate kernel parameters. The results show that （i） in case of ν ≤O, the form of ak （t） mainly depends on the competition between self-exchange of species A and species-C-catalyzed death of species A; （ii） in case of ν 〉 0, the form of αk（t） mainly depends on the competition between species-B-catalyzed birth of species A and species-C-catalyzed death of species A.  相似文献   

19.
Ruthenium catalyzed ring opening cross-metathesis of resin-bound bicyclic alkenes with terminal aryl olefins was utilized for the construction of a combinatorial library containing highly functionalized cyclopentane derivatives. The technology described herein represents a convergent method for the diastereospecific synthesis of unique cyclopentane molecular scaffolds useful for exploratory medicinal chemistry. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

20.
         下载免费PDF全文
This report examines the effect of substrate design upon the Truce‐Smiles rearrangement, an intramolecular nucleophilic aromatic substitution reaction. The length of the molecular spacer that tethers the carbanion nucleophile to the substituted benzene ring was found to have a strong influence on the ability of the substrate to undergo the reaction successfully. Our experimental results show highest yield of desired aryl migration product for substrates designed with a 3‐atom tether, which proceed through a 5‐membered spirocyclic intermediate. The results are interpreted in comparison with a survey of Truce‐Smiles rearrangements described in the literature and found to be consistent. Computational studies support the observed reactivity trend and suggest an explanation of a favorable combination of ring strain and electrostatic repulsion leading to optimal reactivity of the substrate designed with a 3‐atom tether. Comparison of our results with trends for related ring‐closing reactions illustrate the unique electrostatic features of the system studied herein.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号