Nuclear quadrupole resonance of amidinium tetrachlorophosphorates

The³⁵C1 NQR spectra and relaxation parameters of the cyclic amidinium tetrachlorophos-phorates PCl₄(NCH₃)₂CX, where X=C₆H₅ (1), CCl₃ (2), and CF₃ (3), have been studied at 77 °K. The crystal of (3) has been found to contain two coexisting phases of the compound, which consist, respectively, of zwitterionic molecules of a hexacoordinated phosphorus atom with square-pyramidal symmetry of the PCl₄ group and molecules of chlorotropic isomers with a trigonal-bipyramidal environment for the phosphorus atom. The structural features of the crystalline state and the nature of the phosphorus—ligand bonds have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 727–730, November–December, 1985.     

Sigmatropic mechanism of the chlorotropic isomerization of phosphazopentachloroethane

The sigmatropic and dissociative-recombination mechanisms of the chlorotropic isomerization of benzodioxachlorophosphazopentachloroethane and its 1:2 solvate with chloroform have been studied by the MNDO-PM3 method in the supermolecular approximation. It is observed that the thermodynamically more stable chlorotropic isomer corresponds to the phosphorane structure with axial-equatorial disposition of two chlorine atoms and the two oxygen atoms in the dioxaphosphole ring, in complete agreement with literature data on teh³⁵Cl NQR spectrum. It is shown that the most likely pathway for phosphorus-carbon chlorotropic rearrangement for both phosphazopentachloroethane and its solvate with two molecules of chloroform is the sigmatropic route. Institute of Bioorganic and Oil Chemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev 02094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 3, pp. 177–181, May–June, 2000.     

A quantum-chemical study of alternative chlorotropic isomerization mechanisms for bis(diethylamino)-chlorophosphonium methylide

The MNDO-PM3 method has been used in the supermolecular approximation to examine alternative mechanisms for the chlorotropic isomerization of unsolvated bis(diethylamino)chlorophosphonium methylide and solvated forms of it containing chloroform of 1∶1 and 1∶2 compositions. It is shown that the most favorable channel for the chlorotropic rearrangement of unsolvated P-chloroylide corresponds to the sigmatropic mechanism. There are effects from specific interactions with the chloroform that make themselves felt in the preferential chlorotropic transformation of the isomers by the dissociation-recombination mechanism, whose activation energy E_1∶2 ^≠=kcal/mol is comparable with the observed activation barrier for the 1,2-shift of the chlorine atom in the related P-chloroylide. Institute for Bioorganic Chemistry and Petroleum Chemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev 253094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 4, pp. 215–221, July–August, 1999.     

丁烯异构化L-H机理复杂反应动力学

用玻璃外循环无梯度反应器研究了丁烯异构化L-H机理复杂反应动力学. 用正交设计法估计了动力学方程中的参数. 色谱法测定了丁烯吸附热, 并研究了异构化生成反-2-丁烯及顺-2-丁烯的动力学方程.     

The mechanism of cis-trans isomerization of prolyl peptides by cyclophilin

The mechanism of cis-trans isomerization of prolyl peptides catalyzed by cyclophilin (CyP) was studied computationally via molecular dynamics (MD) simulations of the transition state (TS) and the cis and trans forms of the ground state (GS), when bound to CyP and when free in aqueous solution. The MD simulations include four enzyme-bound species of tetrapeptide (Suc-Ala-XC([double bond]O)-NPro-Phe-pNA; X = Gly, Trp, Ala, and Leu). In water, the prolyl amide bond is favorably planar with the presence of conformers exhibiting +/-20 degrees twist of the C-N dihedral. In the active site a hydrogen bond between the cis-prolyl amide carbonyl O and the backbone amide N-H of Asn102 retains the 20 degrees twist of the C-N dihedral. The TS structure is characterized by a 90 degrees twist of the amide C-N bond and a more favorable interaction with Asn102 due to the shorter distance between Asn102(HN) and the amide carbonyl O. The conformational change of cis --> TS also involves pyramidalization of the amide N, which results in the formation of a hydrogen bond between the amide N and the guanidino group of Arg55. Both Asn102 and Arg55 are held in the same position in CyP.cis-isomer as in CyP.TS. In the ligand-free CyP the Arg55 guanidino group is highly disorganized and Asn102 is displaced 1 A from the position in the ligand-bound CyP. Thus, the organization of Arg55 and Asn102 occurs upon substrate binding. The geometrical complimentarity of the organized enzyme structure to the TS structure is a result of preferential binding of the proline N and the amide carbonyl of the TS compared to that of GS. However, the N-terminal part (Suc-Ala) becomes repositioned in the TS such that two hydrogen bonds disappear, one hydrogen bond appears and two other hydrogen bonds becomes weaker on the conversion of CyP.cis to CyP.TS. During this conversion, total hydrophobic contact between enzyme and the peptide is preserved. Thus, the interaction energies of GS and TS with enzyme are, as a whole, much alike. This does not support the contention that TS is bound more tightly than GS by K(m)/K(TS) = 10(6) in the cis --> trans reaction. Repositioning of the N-terminal part of the peptide on CyP.TS formation becomes more pronounced when the substrate X residue is changed from Gly < Trp < Ala < Leu. We propose that the larger turning of the N-terminus is responsible for the larger value of the experimentally observed Delta S(++) and Delta H(++), which sum up to little change in Delta G(++). The positioning of the Arg55 and the degree of 20 degrees twist of the amide C-N bond are considered as criteria for Near Attack Conformers (NACs) in cis-trans isomerization. NACs account for approximately 30% of the total GS populations of the cis-isomer. Similar NAC populations were observed with four different substrates. This is consistent with the insensitivity of enzymatic activity to the nature of the X residue. Also, the NAC population in CyP.trans-AAPF was comparable to that in CyP.cis-AAPF, in accord with similar experimentally measured rates of the cis --> trans and trans --> cis reaction in CyP. These NACs, found in CyP.cis and CyP.trans, resemble only one of the four possible TS configurations in the water reaction. The identity of this TS structure (syn/exo) is in accord with experimentally determined KIE values in the enzymatic reaction. However, the geometry of the active site was also complementary to another TS structure (anti/exo) that was not detected in the active site by the same KIE measurements, implying that the geometrical fitness of the TS cannot be a single determining factor for enzymatic reactions.     

Peculiarities of the mechanism of chlorotropic and conformational isomerism in a series of chloromethyl isocyanates

The thermodynamic and kinetic stability of chlorotropic isomers in a series of chloromethyl isocyanates Cl_3−nH_nC−N=C=O, where n=0, 1, 2, has been shown by quantum chemical calculations, using the MNDO method, and by³⁵Cl NQR spectroscopy. The phase transformations of two polymorphic modifications of N-chlorocarbonyl dichloride, which is a stable iminocarbonyl form of trichloromethyl isocyanate with small conformational activation barriers, have been studied. Institute of Bioorganic and Petrochemistry, National Academy of Sciences of Ukraine, 1 Murmansk ul., Kiev-94 252660, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 3, pp. 164–169, May–June, 1998.     

甲氧基硫代磷酰二氯异构化机理研究

甲氧基硫代磷酰二氯(1)在三乙胺存在下, 进行异构化反应, 首先形成离子对中间体3, 然后1将3烷在化成甲硫基磷酰二氯(2). 中间体3经NMR, IR和MS等鉴定, 确定了其结构及反应性能. 在此异构化反应中, 三乙胺不是催化剂, 具有催化作用的是中间体3. 3本身不能自行异构化, 异构化在3和1分子之间进行. 动力学实验证明异构化过程中, 存在着过渡态, 该反应属表观一级。     

The photochemical isomerization of cross-linked 1,3,5-tristyrylbenzene dendrimer with hula-twist mechanism

A cross-linked 1,3,5-tristyrylbenzene dendrimer 5 was synthesized to study the photochemical isomerization mechanism. On irradiation, 5 isomerized with the quantum yields (Φ_E→Z) of 0.063, which was not very much different from that for a model compound 1 (Φ_E→Z 0.080), supporting a volume-conserving hula-twist (HT) mechanism during the photochemical E-Z isomerization.     

The mechanism of allyl isomerization of unsaturated compounds catalyzed by organomagnesium clusters

The paper presents a density functional theory investigation of the mechanism of allyl isomerization of olefins on magnesium clusters for the example of the Mg₄-allylbenzene system. The rearrangement of the cluster compound corresponding to the insertion of the tetranuclear cluster at a C-H allylbenzene bond into a structure derivative from 2-methylstyrene is an activationless reaction. The limiting step is the liberation of the cluster nucleus capable of interacting with another allylbenzene molecule. Cluster decomposition competes with this process; this conclusion is in agreement with experimental short lifetimes of cluster magnesium hydride catalysts.     

Facile microwave-assisted synthesis of cyclic amidinium salts

The cyclization of N,N'-dialkyl or diaryl ethane-1,2-diamines or propane-1,3-diamines with inorganic ammonium salts and orthoesters proceeds briskly under microwave irradiation to afford the corresponding imidazolinium or tetrahydropyrimidinium salts. The transformation is highly versatile and tolerates a wide range of substituents and counterions. It could be scaled from 1 to 50 mmol without any difficulty. Because the workup is equally rapid and straightforward, this experimental procedure provides fast and convenient access to an important class of heterocyclic compounds that have found numerous applications as N-heterocyclic carbene precursors, organocatalysts, and ionic liquids.     

基于C=N双键异构化传感机制的荧光探针

多种传感机制已经被用于荧光探针分子的设计中,其中,基于C=N双键异构化传感机制的荧光探针的研究近年来引起了较大关注。本文归纳总结了过去10年基于C=N双键异构化传感机制的阳离子、阴离子和中性分子探针的发展情况。文章最后对其应用前景及发展趋势进行了展望。     

The vinylidene-acetylene isomerization barrier

The barrier to rearrangement of vinylidene to acetylene has been determined using Møller-Plesset perturbation theory and an extended polarized basis set. With the inclusion of zero-point vibrational corrections, the barrier is found to be only 0.9 $\text{kcal}\text{mole}$, suggesting that vinylidene will have an extremely short lifetime.     

Cycloaddition and chlorotropic transformations in reactions of dichloromethylenetrifluoroacetamide with phosphites

Reactions of dichlormethylenetrifluoroacetamide with triphenyl phosphite, diethylchlorophosphite (o-phenylene)diethylamidophosphite, (o-phenylene)chlorphosphite, and 2-fluoro-3-isopropyl-5-tert-butyl-1,3,2-oxazaphospholine proceeded as cheletropic [4+1]-cycloaddition. The formed λ⁵-1,4,2-oxazaphospholines underwent 1,3-migration of the chlorine atom in C=N–C cyclic triad.     

Exploring the mechanism for the amine-catalyzed isomerization of dimethyl maleate. A computational study

The main objective of this study was to investigate the amine-catalyzed isomerization of dimethyl maleate into dimethyl fumarate in order to utilize the former as a prodrug for the latter. Mechanistic study of this reaction using DFT at B3LYP/6-31G(d,p) level revealed that the reaction is first order in dimethyl maleate, second order in the amine, and overall third order. Moreover, the calculations revealed the existence of a linear correlation between the basicity of the amine catalyst and the isomerization rate.     

Kinetics and mechanism of isomerization of alkyl radicals in solution

It has been shown that in alkyl radicals the 1, 5-rearrangement of hydrogen atoms takes place. The isomerization rate constants for radicals of various structure have been estimated. There is a compensation effect for their Arrhenius parameters.

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, 1,5- . . .

     

HNNO_2自身氢迁移及异构化机理的理论研究

采用CBS-QB3方法对二硝酰胺酸(HDN)裂解过程中的HNNO2自由基自身氢迁移及N—N键断裂异构化反应机理进行了研究.结果表明,HNNO2自由基自身氢迁移反应经历了N(4)—O(2)间的氢迁移、O(2)—O(3)间的氢迁移及内转化3个不同类型的基元反应,最终生成N2O分子与OH自由基.其中N(4)—O(2)间的氢迁移为HNNO2自由基自身氢迁移反应中的速率决定步.HNNO2自由基通过N(1)—N(4)键断裂以及O(2)—N(4)键形成异构化成产物NO+HNO,该过程的能垒为176.17kJ·mol-1,比氢迁移通道决速步能垒高出了47.59kJ·mol-1,表明氢迁移通道为HNNO2裂解过程中的优势通道.     

Axially chiral amidinium ions as inducers of enantioselectivity in Diels-Alder reactions

[reaction: see text] Enantioselective catalysis of Diels-Alder reactions is mostly achieved by coordinating the dienophile to relatively strong chiral Lewis acids. Here we report on a novel approach employing the hydrogen-bond-mediated association of dienophiles to chiral host molecules. In a reaction forming the steroid skeleton of norgestrel, chiral amidinium ions induce 5:ent-5 ratios of up to 2.5:1. Improved and simplified amidinium catalysts may become interesting candidates to perform stereoselective transformations.     

Kinetics and mechanism of selenium-catalyzed isomerization of some cis-diarylacrylonitriles

The kinetics of the cis-trans isomerization of some ,β-diarylacrylonitriles has been investigated in decahydronaphthalene at 150–190° with selenium as a catalyst. The reaction is first order with respect to olefin and 1/2 apparent order with respect to catalyst. Isomerization rate is increased by electron donating groups in the -phenyl and by electron withdrawing ones in the β-phenyl. Satisfactory correlations between log k and substituent σ⁺ values are observed. Evidence is obtained for a free radical mechanism. The results are consistent with a reaction path involving the formation of a σ-complex between the -C atom and selenium diradical, followed by a free rotation around the single bond and removal of the catalyst. The effects of heteroaromatic nuclei linked to -C atom on the isomerization rate (phenyl>2-furyl>2-thienyl) indicate the lack of conjugation from heteroatom to the side chain in the rate determining step.     

聚乙炔顺、反异构化机理研究

探讨了聚乙炔三种异构化-光、热、掺杂异构化的共性和特性,对光、热异构化过程提出了一种新的可能的"双自由基"机理,该机理在定性方面吻合了ESR实验结果,有限分子CNDO/2及INDO量子化学计算证明该机理在能量上最有利;估价了有限分子半经验MO方法在聚合物方面应用的可靠性,结果表明,这种简单易行的方法对一些规律性的预言是可靠的,从而为通常的半经验MO方法在高分子方面的推广应用提供了算例。