Entrapment of cyclopentadiene in zeolite NaY and its application for solvent-free Diels-Alder reactions in the nanosized confined environment

Nonpolar cyclopentadiene was stably entrapped as a monomer in the hydrophilic supercages of zeolite NaY. A monomer form of cyclopentadiene in the supercages was confirmed by ¹³C MAS NMR spectroscopy. The adsorbed cyclopentadiene in the confined cavities performed the Diels-Alder reaction with typical dienophiles under solvent-free conditions more effectively than in organic and water media.     

Unusual regioselectivity in a Diels-Alder reaction of isoprene with levoglucosenone

The ZnCl₂-catalyzed [4+2]-cycloaddition of isoprene with levoglucosenone is highly regio- and stereoselective.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2047–2049, August, 1996.     

Quantum chemical study on the thermal Diels-Alder reaction of cyclohexadiene and methyl crotonate

AM1 molecular orbital method using the unrestricted Hartree-Fock(UHF) calculations has been applied to investigate the thermal reaction of cyclohexadiene and methyl crotonate. The calculated results indicate that the thermal Diels-Alder reaction proceeds to product through the concerted path and two radical pathways.     

Diels-Alder reaction volumes in the solid state and solution

The densities of anthracene, tetracyanoethylene, maleic anhydride, N-phenylmaleineimide, trans, trans-1,4-diphenylbuta-1,3-diene, and their Diels-Alder adducts were measured in the solid state and in solution at 25 °C. The reaction volumes in the solid state were calculated from the difference in molar volumes. They turned out to be low, close to each other (–4 to –11 cm³ mol^–1), and slightly different from the reaction volumes (–8±1 cm³ mol^–1) calculated from the van der Waals radii. The reaction volumes in solutions (–15 to –32 cm³ mol^–1) were found from the difference in partial molar volumes of the reactants in dioxane, acetonitrile, and 1,2-dichloroethane, The experimental Diels-Alder reaction volumes in solution are determined not only by the formation of new bonds in an adduct: a considerably higher contribution (to 75%) is made by a change in the volume of intermolecular empty spaces in solution on going from reactants to adducts.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2386–2390, November, 2004.     

Reactivity and stereoselectivity of the Diels-Alder reaction using cyclic dienophiles and siloxyaminobutadienes

Several bicyclic compounds were synthesized by the Diels-Alder reaction using aminodiene and a cyclic dienophile. The stereochemistries of the obtained adducts were determined by X-ray crystallography or NMR analysis. The stereoselectivity of this Diels-Alder reaction was based on the interaction of molecular orbitals between the diene and dienophile. The reactivities of these Diels-Alder reactions were estimated, and the generality of this reaction is discussed.     

Experimental and theoretical FMO interaction studies of the Diels-Alder reaction of 5-acetyl-3-methythio-1,2,4-triazine with cyclic enamines

Diels-Alder reaction of 5-acetyl-3-methylthio-1,2,4-triazine with five cyclic enamines has been reinvestigated in its preparative and theoretical aspects. Its regioselectivity has been developed practically, which is in agreement with theoretical consideration of the FMO interactions, including secondary orbital interactions in the transition state. Since the energetic demands are similar for all five pairs, it has been indicated that their reactivity differences can be explained by an influence of steric hindrance in the considered transition state. In result, the synthesis of the 3-acetyl-1-methylthiocycloalka[c]pyridines, as synthons for preparation of sempervirine and its analogues has been optimized. The subsequent side reaction has been detected as a serious problem, especially in the case of the six-membered enamine, which reacts with the acetyl group of the final product formed in the reaction mixture.     

Study on the oligomerization of cyclopentadiene and dicyclopentadiene to tricyclopentadiene through diels-alder reaction

Summary The oligomerization of cyclopentadiene (CPD) and its dimer, dicyclopentadiene (DCPD), to tricyclopentadiene (TCPD) through Diels-Alder reaction at temperatures between 120 and 150°C was investigated. The results show that reaction temperature, pressure and solvent influence the product yield; at 150°C up to 50% yield of TCPD was obtained in the absence of solvents. The ratios of isomers A to B in the product can be adjusted by using different solvents. The kinetics indicate that the rate is more sensitive to the concentration of CPD than to that of DCPD.     

Single‐ended transition state finding with the growing string method

Reaction path finding and transition state (TS) searching are important tasks in computational chemistry. Methods that seek to optimize an evenly distributed set of structures to represent a chemical reaction path are known as double‐ended string methods. Such methods can be highly reliable because the endpoints of the string are fixed, which effectively lowers the dimensionality of the reaction path search. String methods, however, require that the reactant and product structures are known beforehand, which limits their ability for systematic exploration of reactive steps. In this article, a single‐ended growing string method (GSM) is introduced which allows for reaction path searches starting from a single structure. The method works by sequentially adding nodes along coordinates that drive bonds, angles, and/or torsions to a desired reactive outcome. After the string is grown and an approximate reaction path through the TS is found, string optimization commences and the exact TS is located along with the reaction path. Fast convergence of the string is achieved through use of internal coordinates and eigenvector optimization schemes combined with Hessian estimates. Comparison to the double‐ended GSM shows that single‐ended method can be even more computationally efficient than the already rapid double‐ended method. Examples, including transition metal reactivity and a systematic, automated search for unknown reactivity, demonstrate the efficacy of the new method. This automated reaction search is able to find 165 reaction paths from 333 searches for the reaction of NH₃BH₃ and (LiH)₄, all without guidance from user intuition. © 2015 Wiley Periodicals, Inc.     

微量热法研究单底物酶促反应的热力学和动力学性质及过渡态的分析

用LKB-2107型微量热系统, 测定了漆酶催化氧化3, 4-二羟基苯甲醛、邻甲氧基酚、邻苯三酚、3, 4, 5-三羟基苯甲酸反应的热谱图, 利用热谱图计算了米氏常数(Km)、反应速率常数(k2)和热力学参数(ΔrHm, ΔG0, ΔT^≠, Ea, ΔST^≠)。并应用过渡态理论对其催化过程进行了分析。结果表明: 稳定过渡态结构有利于酶促反应, 酶-底物在反应物时相互作用仅仅是降低酶的催化效率。提出两种可能提高酶催化效率的方法。由活化熵(ΔST)<0得出酶-底物在过渡态的结构较酶-底物复合物的结构更为有序。     

Multiconformer transition state theory rate constants for the reaction between OH and -dimethoxyfluoropolyethers

In this work, we have calculated rate constants for the tropospheric reaction between the OH radical and -dimethoxyfluoropolyethers. The latter are a specific class of the hydrofluoropolyethers family with the general formula , from which we have selected three case studies: , , and . The calculations were performed by applying a cost-effective protocol developed for bimolecular hydrogen-abstraction reactions and based on multiconformer transition state theory relying on computationally accessible M08-HX/apcseg-2//M08-HX/pcseg-1 calculations. Within the protocol's uncertainties and approximations, the results show that (1) the calculated rate constants have the same order of magnitude and (2) if observed together with previous experimental and theoretical investigations, the chain length (that varies with q and p) is seen to have a small effect on the rate constant, which is consistent with the “no discernible effect” reported in the experimental work.     

A chiral probe for the asynchronous transition state of diels-alder reactions with acetylenedicarboxylate

Diels-Alder reactions took place with modest diastereoselectivity between dimethylcyclo-hexadiene derivative 3a and di-(−)-menthyl acetylenedicarboxylate, whereas the dimethylcyclohexadiene 2 showed no selectivity whatsoever. These results can be rationalized in terms of a complete lack of any endo-exo preference for the carboxylate groups and a more synchronous addition with 3a than with 2.     

基于水中的Diels-Alder反应制备水凝胶的研究

首先通过N,N-二甲基丙烯酰胺与甲基丙烯酸糠酯的自由基共聚合成了水溶性的大分子双烯体,然后通过聚乙二醇与N-甘氨酸马来酰亚胺的缩合反应合成了水溶性的大分子亲双烯体,接着,将合成的大分子双烯体与亲双烯体溶解于水中,通过水中的Diels-Alder反应合成了水凝胶,考察了不同温度下的凝胶化时间。研究发现凝胶化时间与温度有很大关系,温度越高,凝胶化时间越短。且合成的水凝胶在水中比较稳定,水中回流8h,凝胶并没有消失。但在DMF中,容易发生逆Diels-Alder反应。本文对合成的水凝胶进行了表征,并探讨了该凝胶体系在药物控释方面的应用。     

AM1法研究取代乙烯与环已-1，3-二烯热加成

用ＡＭ１方法（采用非限制的Ｈａｒｔｒｅｅ－Ｆｏｃｋ ＵＨＦ计算）研究取代乙烯ＣＨ２＝ＣＨＲ（Ｒ＝ＣＨ３，ＣＨＯ和ＣＮ）与环己－１，３－二烯的热Ｄｉｅｌｓ－Ａｌｄｅｒ加成反应，结果表明，反应存在协同途径和两条自由基途径。     

The aza-xylylene Diels-Alder approach for the synthesis of naturally occurring 2-alkyl tetrahydroquinolines

The recently discovered intramolecular aza-xylylene Diels-Alder reaction, based on a 1,4-dehydrohalogenation reaction, was extended in terms of substrates and leaving groups allowing the assembly of tetrahydroquinolines in two synthetic steps. Intramolecular cleavage of a thiocarbamate using triphenylphosphine and tetrachloromethane (Appel conditions) to give chloromethyl phenylisocyanate has been presented for the first time. The synthetic feasibility of this process was demonstrated in the first total syntheses of the alkaloids rac-Angustureine and 1-methyl-2-propyltetrahydroquinoline.     

Curie transition,ferroelectric crystal structure,and ferroelectricity of a VDF/TrFE(75/25) copolymer 1. The effect of the consecutive annealing in the ferroelectric state on curie transition and ferroelectric crystal structure

We have completely deconvoluted multiple ferroelectric-to-paraelectric phase transitions of a VDF/TrFE copolymer often observed on DSC by consecutive annealing below the Curie transition temperature and obtained three clear endothermic peaks resolved well from each other. We could also increase the Curie transition temperature by using the consecutive annealing performed in the ferroelectric state. Annealing time, as well as annealing temperature, was found to affect the Curie transition behavior significantly. Each ferroelectric crystalline phase was characterized by IR, Raman scattering, x-ray diffraction, and DSC measurements. The ferroelectric phase, having higher Curie transition temperature, has been found to have more trans sequences and less gauche defects and to require larger thermal energy for the Curie transition. © 1994 John Wiley & Sons, Inc.     

Variational transition state theory without the minimum-energy path

In this paper we propose a method for carrying out variational transition state theory calculations without first obtaining a converged minimum-energy path (MEP). We illustrate the method in two ways, first of all by employing an unconverged MEP and secondly by using a dynamically optimized distinguished reaction path. Preliminary tests of the algorithm for the reactions OH+H₂→H₂O+H and C₂H₅→C₂H₄+H are very encouraging. Received: 22 January 1997 / Accepted: 11 March 1997     

The amorphous state equivalent of crystallization: new glass types by first order transition from liquids, crystals, and biopolymers

We review the normal state of glasses and explain some exceptional cases by referring to a mode of glass formation, which is distinct from the normal and involves a first order transition route. Important materials like amorphous water and silicon belong to the distinct class, which we expect will prove to have many members, and which we expect will occupy a position part way between quasi-crystals and ordinary glasses. There may also be many mesoscopic examples of this class of material, because the low energy tertiary structures obtained by the (first order) folding of specialized heteropolypeptides (proteins) satisfy many of the criteria that we utilize in defining the class. The mesoscopic examples have the advantage of undergoing the transition to the low energy state under conditions of relatively long-lived metastability so that the phenomenon can be studied at leisure. There is no obvious reason why the phenomenon should be confined to biomolecules. We discuss the relation of the new glass types to ordinary glasses, plastic crystals, folding proteins and quasi-crystals, within the energy landscape paradigm. The first order transition occurs in the lower levels of the landscape in all cases, implying that ‘funnels’ are the general rule.     

Studies on the Diels-Alder reaction of annulated furans: application to the synthesis of substituted phenanthrenes

In an attempt to utilize the synthetic power of the furan nucleus in the synthesis of complex natural products, we have developed an entry into annulated furan intermediates that involves an electrochemical annulation protocol. An approach to the eunicellin diterpenes based on this methodology required the use of an intermolecular Diels-Alder reaction on an annulated furan. Although well known for simple furans, there is a paucity of related examples on annulated furans. To examine the feasibility of such an approach to these diterpenes, we have studied this key cycloaddition reaction. Our studies indicate that the process can be hampered by a facile retro-Diels-Alder that is highly dependent on the dienophile employed.     

The trifluoromethoxy group as a fluorine twin in the Diels-Alder reactions of halogenated quinones

We describe here a study devoted to the comparison of the relative influence of chlorine, fluorine, and trifluoromethoxy substituents on the regiochemical outcome of the Diels-Alder reaction. For this purpose, we examined the behavior of mixed ‘halogenated’ quinones bearing these groups in their cycloadditions with simple dienes. Contrary to the expectation based on its known electronic properties, the trifluoromethoxy group behaves very much more like a fluorine than a chlorine atom in such reactions. On the basis on an endo transition state demonstrated here for these additions, we tentatively suggest that non-bonded interactions are the main factor controlling the regiochemistry.