(Z)-1,2-二苯基-1,3-丁二烯与单取代的乙烯衍生物区位选择性环加成反应

(Z)-1,2-二苯基-1,3-丁二烯分别与甲基乙烯基酮、丙烯酰胺、丙烯腈以及丙烯醛发生环加成反应,均生成顺式和反式的连位取代环己烯衍生物。以IR、¹HNMR和MS鉴定了产物的结构。利用CNDO/2法计算了分子轨道能量和系数,由前线分子轨道理论解释了这种环加成反应的区位选择性。     

Criteria for pericyclic and pseudopericyclic character of electrocyclization of (Z)-1,2,4,6-heptatetraene and (2Z)-2,4,5-hexatriene-1-imine

The electrocyclic reaction mechanisms of (Z)-1,2,4,6-heptatetraene and (2Z)-2,4,5-hexatriene-1-imine were studied by ab initio MO methods. The activation energy barrier height of the electrocyclic reaction of (Z)-1,2,4,6-heptatetraene is extremely a low energy barrier of 8.58 kcal/mol by a MRMP method. The activation energy barrier height of the electrocyclic ring closure of the trans-type of (2Z)-2,4,5-hexatriene-1-imine is lower by 3.18 kcal/mol than that of (Z)-1,2,4,6-heptatetraene. These low energy barriers come from some orbital interactions relating to allene group. For the reaction of (Z)-1,2,4,6-heptatetraene, the interactions of the vertical and side π orbitals of the allene group with another terminal π orbital are important at the transition state. The interaction of the vertical π orbital of allene group with a lone pair orbital of N atom is dominant at the transition state of the reaction of the trans-type of (2Z)-2,4,5- hexatriene-1-imine. The electrocyclic mechanism of the cis-type of (2Z)-2,4,5-hexatriene-1-imine was also discussed. Contribution of the Mark S. Gordon 65th Birthday Festschrift issue.     

卡宾与醚C-H键插入反应的理论研究——(Ⅰ)卡宾与二甲醚的插入反应

用量子化学从头计算方法在MP2/6-31G(d)水平上计算了单重态的CH₂与二甲醚中C-H键插入反应的过程,并在MP₄/6-31G(d)水平上计算了反应物、过渡态和产物的能量。反应仅具有一个8.1kJ/mol的早期势垒,反应过程是卡宾的一个亲电-亲核过程,在插入过程中,卡宾空的p轨道和占有一对孤电子的σ轨道分别指向C-H键的H原子和C原子。     

Examination of the cleavage and formation of the disulfide bond in poly[dithio-2,5-(1,3,4-thiadiazole)] by redox reaction

Reduction and oxidation peak potentials of poly[dithio-2,5-(1,3,4-thiadiazole)] were observed in hot γ-butyrolactam (90°C) at −0.1 and 0.1 V vs. Ag respectively. To clarify the redox reaction of the polymer (oligomer), bis(2-methyl-1,3,4-thiadiazoyl)-5,5′-disulfane was synthesized as a model compound and its redox reaction examined by experiment and molecular orbital calculation. Reduction and oxidation peak potentials of this model were observed at −0.65 and 0.2 V respectively, potentials corresponding to the cleavage and formation reactions of the disulfide bond. The bond cleavage reaction was also suggested by molecular orbital calculations. From a comparison of the shape and response of the cyclic voltammogram between the monomeric and polymeric disulfides, it became clear that reduction and oxidation of the polymer meant the cleavage and formation reactions of the disulfide bond respectively, and that the redox reaction is quasi-reversible.     

Theoretical study of C-H and N-H sigma-bond activation reactions by titinium(IV)-imido complex. Good understanding based on orbital interaction and theoretical proposal for N-H sigma-bond activation of ammonia

The C-H sigma-bond activation of methane and the N-H sigma-bond activation of ammonia by (Me3SiO)2Ti(=NSiMe3) 1 were theoretically investigated with DFT, MP2 to MP4(SDQ), and CCSD(T) methods. The C-H sigma-bond activation of methane takes place with an activation barrier (Ea) of 14.6 (21.5) kcal/mol and a reaction energy (DeltaE) of -22.7 (-16.5) kcal/mol to afford (Me3SiO)2Ti(Me)[NH(SiMe3)], where DFT- and MP4(SDQ)-calculated values are given without and in parentheses, respectively, hereafter. The electron population of the CH3 group increases, but the H atomic population decreases upon going to the transition state from the precursor complex, which indicates that the C-H sigma-bond activation occurs in heterolytic manner unlike the oxidative addition. The Ti atomic population considerably increases upon going to the transition state from the precursor complex, which indicates that the charge transfer (CT) occurs from methane to Ti. These population changes are induced by the orbital interactions among the d(pi)-p(pi) bonding orbital of the Ti=NSiMe3 moiety, the Ti d(z2) orbital and the C-H sigma-bonding and sigma*-antibonding orbitals of methane. The reverse regioselective C-H sigma-bond activation which leads to formation of (Me3SiO)2Ti(H)[NMe(SiMe3)] takes place with a larger Ea value and smaller exothermicity. The reasons are discussed in terms of Ti-H, Ti-CH3, Ti-NH3, N-H, and N-CH3 bond energies and orbital interactions in the transition state. The N-H sigma-bond activation of ammonia takes place in a heterolytic manner with a larger Ea value of 19.0 (27.9) kcal/mol and considerably larger exothermicity of -45.0 (-39.4) kcal/mol than those of the C-H sigma-bond activation. The N-H sigma-bond activation of ammonia by a Ti-alkylidyne complex, [(PNP)Ti(CSiMe3)] 3 (PNP = N-[2-(PH2)2-phenyl]2-]) ,was also investigated. This reaction takes place with a smaller E(a) value of 7.5 (15.3) kcal/mol and larger exothermicity of -60.2 (-56.1) kcal/mol. These results lead us to predict that the N-H sigma-bond activation of ammonia can be achieved by these complexes.     

A Three-Center Orbital Interaction in the Diels-Alder Reactions Catalyzed by Lewis Acids

Ab initio calculations were performed on title reactions between butadiene and acrolein with BCl(3), AlCl(3), GaCl(3), InCl(3), ZnCl(2), SnCl(2), and SnCl(4). A dimethyl ether molecule is explicitly considered in various reaction systems to examine solvent effects. First, the reaction path of an AlCl(3)-promoting reaction was examined thoroughly. This reaction has two channels. The first one involves a weak reactant-like complex (precursor) and a normal [4 + 2] addition. The second does three elementary processes, one-center addition, ring closing, and Claisen shift. The first channel is more favorable by 12.1 kcal/mol (B3LYP/6-311+G(2d,p) SCRF//B3LYP/6-31G SCRF) than the second one. Then the first channels with other Lewis acids were traced with and without an ether molecule. The ether molecule has an appreciable effect not on geometries but on activation energies. BCl(3) is desolvated and has extraordinarily strong catalytic ability. Even with the strongest catalyst, not a [2 + 4] but a normal [4 + 2] cycloaddition takes place. Except for BCl(3), SnCl(4) is the strongest Lewis acid with the ether molecule. The frontier orbital, LUMO, of acrolein is distorted in the course of the reaction so that the formation of two C-C covalent bonds is possible. The precursor formation and the one-center addition were discussed also by the frontier orbital theory.     

Theoretical and experimental studies on the thermal ring-opening reaction of cyclobutene having a stannyl substituent at the 3-position

The ring-opening reaction of 3-(trimethylstannyl)cyclobutene gave a mixture of the (Z)- and (E)-1,3-dienes, whereas that of 3-tert-butylcyclobutene exclusively afforded the (E)-1,3-diene due to the steric influences. The contrasting rotational behaviors suggest that there is some effect operating with the 3-stannylcyclobutene to stabilize the inward transition state, counteracting the steric influences. This contrasteric effect is ascribed to negative hyperconjugation. The stannyl group in the inward transition state accommodates electron density from the breaking sigma orbital of the cyclobutene into its low-lying tin-carbon sigma orbital. Theoretical studies supported this interpretation.     

(R)-N-(β-三氯锗丙酰基)四氢噻唑-2-硫酮-4-羧酸甲酯的合成及其晶体结构

三氯锗丙酰氯与(R)-四氢噻唑-2-硫酮-4-羧酸甲酯反应, 得到标题化合物1, [α]~D^2^0-89.40°。经水解得到取代丙酰四氢噻唑-2-硫酮-4-羧酸甲酯基锗倍半氧化物2, X射线衍射法测出标题化合物的晶体结构, 属于正交晶系, 晶胞参数: a=0.6192(1)nm,b=1.1147(4)nm,c=2.1796(8)nm, V=1.5045nm^3, Z=4, 空间群P2~12~12~1。分子中酰胺羰基C=O与C=S基团处于C(4)NC(3)键两侧呈反式。用MNDO分子轨道方法研究了该化合物的电子结构, 电荷和键序分布,前沿轨道性质,讨论了电子光谱性质。     

The effect of the axial ligand on distinct reaction tunneling for methane hydroxylation by nonheme iron(iv)-oxo complexes

Comprehensive density functional theory computations on substrate hydroxylation by a range of nonheme iron(iv)-oxo model systems [Fe(IV)(O)(NH(3))(4)L](+) (where L = CF(3)CO(2)(-), F(-), Cl(-), N(3)(-), NCS(-), NC(-), OH(-)) have been investigated to establish the effects of axial ligands with different degrees of electron donor ability on the reactivity of the distinct reaction channels. The results show that the electron-pushing capability of the axial ligand can exert a considerable influence on the different reaction channels. The σ-pathway reactivity decreases as the electron-donating ability of the axial ligand strengthens, while the π-pathway reactivity follows an opposite trend. Moreover, the apparently antielectrophilic trend observed for the energy gap between the triplet π- and quintet σ-channel (ΔG(T-Q)) stems from the fact that the reaction reactivity can be fine-controlled by the interplay between the exchange-stabilization benefiting from the (5)TS(H) relative to the (3)TS(H) by most nonheme enzymes and the destabilization effect of the orbital by the anionic axial ligand. When the former counteracts the latter, the quintet σ-pathway will be more effective than the other alternatives. Nevertheless, when the dramatic destabilization effect of the orbital by a strong binding axial σ-donor ligand like OH(-) counteracts but does not override the exchange-stabilization, the barrier in the quintet σ-pathway will remain identical to the triplet π-pathway barrier. Indeed, the axial ligand does not change the intrinsic reaction mechanism in its respective pathway; however, it can affect the energy barriers of different reaction channels for C-H activation. As such, the tuning of the reactivity of the different reaction channels can be realised by increasing/decreasing the electron pushing ability.     

A spin-coupled study of the Claisen rearrangement of allyl vinyl ether

The spin-coupled (SC) form of modern valence bond (VB) theory is utilised to examine the electronic structure of the transition state (TS) and the electronic reaction mechanism of the Claisen rearrangement of allyl vinyl ether. The differences between the spin-coupling patterns and orbital overlap integrals at the optimised TS geometries obtained using B3LYP/6-31G^*, MP2/6-31G^* and MP4(SDQ)/6-31G^* wavefunctions are minimal, and the SC picture suggests that the TS is non-aromatic. SC calculations along the intrinsic reaction coordinates computed at these three levels of theory also produce near identical results. The SC wavefunctions at different stages of the reaction provide easily interpretable orbital diagrams which, in combination with the changes in the orbital overlap integrals, indicate an electronic reaction mechanism involving concerted, though not entirely synchronous, bond breaking and bond formation processes. The evolution of the active space spin-coupling pattern, which is closely related to the classical VB concept of resonance, combined with the changes in the orbital overlap integrals, show that the reaction path involves a region in which the electronic structure of the reacting system becomes similar to that of benzene. This suggests that during the Claisen rearrangement the reacting system can attain moderately aromatic character but that this does not necessarily happen at the TS. The results of the SC analysis indicate that the most appropriate schematic representation of the Claisen rearrangement is furnished by a homolytic mechanism in which six harpoons describe the changes in the bonding pattern from reactant to product     

Mechanism of cysteine oxidation by a hydroxyl radical: a theoretical study.

Cysteine oxidation by HO(.) was studied at a high level of ab initio theory in both gas phase and aqueous solution. Potential energy surface scans in the gas phase performed for the model system methanethiol+HO(.) indicate that the reactants can form two intermediate states: a sulfur-oxygen adduct and a hydrogen bound reactant complex. However these states appear to play a minor role in the reaction mechanism as long as they are fast dissociating states. Thus the main reaction channel predicted at the QCISD(T)/6-311+G(2df,2pd) level of theory is the direct hydrogen atom abstraction. The reaction mechanism is not perturbed by solvation which was found to induce only small variations in the Gibbs free energy of different reactant configurations. The larger size reactant system cysteine+HO* was treated by the integrated molecular orbital+molecular orbital (IMOMO) hybrid method mixing the QCISD(T)/6-311+G(2df,2pd) and the UMP2/6-311+G(d,p) levels of theory. The calculated potential energy, enthalpy, and Gibbs free energy barriers are slightly different from those of methanethiol. The method gave a rate constant for cysteine oxidation in aqueous solution, k=2.4 x 10(9) mol(-1) dm(3) s(-1), which is in good agreement with the experimental rate constant. Further analysis showed that the reaction is not very sensitive to hydrogen bonding and electrical polarity of the molecular environment.     

DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2-H heptafluoropropane and CF3Br

Reaction enthalpies and barrier heights of the reactions CF3Br+H-->CF3+HBr {reaction (1)} and CF3CHFCF3+H-->CF3CFCF3+H2 {reaction (2)} have been calculated at the near state-of-the-art ab initio level, and also by employing the B3LYP, BH&HLYP, BB1K, MPW1K, MPWB1K and TPSS1KCIS functionals. In addition, the integrated molecular orbital+molecular orbital (IMOMO) method has been used to study reaction (2). The ab initio benchmark values of the reaction enthalpy (298 K) and barrier height (0 K) of reaction (2) are reported for the first time {-(0.7+/-0.7) and 13.3+/-0.5 kcal/mole respectively}. When density functional theory (DFT) results are compared with ab initio benchmarks for both reactions (1) and (2), the MPWB1K functional is found to have the best performance of the six functionals used. The IMOMO method with the RCCSD/aug-cc-pVTZ and/or RCCSD(T)/aug-cc-pVTZ levels, as the high levels of calculation on the model system, gives reaction enthalpies and barrier heights of reaction (2), which agree with ab initio benchmark values to within 1 kcal/mole. Computed key geometrical parameters and imaginary vibrational frequencies of the transition state structures of reactions (1) and (2) obtained at different levels of calculation are compared. The magnitudes of the computed imaginary vibrational frequencies of the transition states of both reactions considered are found to be very sensitive to the levels of calculation used to obtain them. The heat of formation (298 K) of CF3CFCF3 calculated at the near state-of-the-art level has a value of -(318+/-3) kcal/mole.     

Ab initio molecular orbital study of the reactivity of active alkyl groups. VI. Modified reaction model for the elimination process of nitrosation reaction

The mechanisms of nitrosation of acetone through sodium enolate [CH3COCH2]- Na+ (1) or naked enolate [CH3COCH2]- (2) with tert-butyl nitrite (CH3)3CONO (3) were studied using ab initio molecular orbital (MO) methods. When the modified complex model was used in the elimination process, our results demonstrated the predominant formation of E-1-hydroxy-imino-2-oxo-propane CH3COCH=NOH (4E), in which a counter-cation of the base catalyst did not participate during the reaction. On the other hand, participation of the counter-cation during the reaction contributed to the formation of the Z-isomer of 4 (4Z).     

CpRu(PPh3)2SSiiPr3与SCNR(R=Ph,1-Naphthyl)反应的结构、成键与机理的理论研究

应用密度泛函理论(DFT), 通过CpRu(PH₃)₂SSiⁱPr₃ (Cp＝环戊二烯负离子; ⁱPr＝异丙基)与SCNH模型化反应, 探讨了CpRu(PPh₃)₂SSiⁱPr₃ 与SCNR (R＝苯基, 萘基)的反应机理, 分析了反应所涉及的各相关化合物的结构与成键特征. 反应中首先失去一个膦配体, 生成一个中间体. 该中间体中, 硫原子采取sp²杂化, 硫原子剩余的一个p轨道与金属中心上的d轨道具有相同的对称性, 因而该p轨道上的孤电子对可与金属中心上的d轨道形成π键, 导致Cp环中心, Ru, S1, P和Si原子在同一平面内, 而不是S1, P和Si原子偏离该平面. 计算结果预测, S＝C双键中的p键打开, 生成含金属中心的四元环螯合物一步为反应的决速步骤. 空间位阻的减小、p共轭体系的生成以及螯合环的存在, 是导致该反应热力学有利的重要原因.     

H3PO→H2POH异构化反应的LMO研究

The localized molecular orbital (LMO) theory is used to study the reaction mechanism of the isomerization reaction: H_3PO→H_2POH. The energy transition state (TS) of the reaction is also obtained by Powell's mehtod using 6-31G basis set. The resluts show that the lone pair electrons of oxygen atom play an forortant role in this reaction.     

Ab initio study on the decomposition of first excited state HOOO radicals

The HOOO radical plays a crucial role in atmospheric processes involving the OH radical and O(2) molecule. We present an ab initio molecular orbital theory study on the decomposition reaction of the first excited state HOOO((2)A') with respect to OH and O(2). The geometries and harmonic vibrational frequencies of all stationary points are calculated at the CASSCF and MRCI levels of theory in conjunction with the 6-31+G(d,p) basis set. The potential energy profile of the decomposition reaction is studied at the CASSCF/6-31+G(d,p) level of theory, in which the complete valence orbitals and electrons are included in the active space. The energies of the potential energy profile are further refined at the CASPT2 and MRCI levels of the theory. Additionally, we have determined the interesting reaction process: the HOOO((2)A') radical with C(s) symmetry does not dissociate to OH((2)Pi) and O(2)((3)Sigma(-)(g)) directly as this is forbidden by orbital symmetry, but dissociates to OH((2)Pi) and O(2)((3)Sigma(-)(g)) via the change in symmetry from C(s) to C(infinity v) symmetry with a low barrier.     

Thermal rearrangements of 2-vinylcyclopropylidene to cyclopentadiene and vinylallene: a theoretical investigation

In an attempt to clarify the favored rearrangement reaction of vinylcyclopropylidenes, the prototype thermal rearrangements of singlet 2-vinylcyclopropylidene (1) leading to 1,3cyclopentadiene (2) and 1,2,4-pentatriene (vinylallene) (3) were investigated by means of ab initio quantum-mechanical electronic-structure calculations. The B3LYP functional with the 6-31G(d) basis set was employed for geometry optimization of the equilibrium and transition-state structures relevant to the two reaction pathways and for computing their harmonic vibrational frequencies. Final energies were evaluated by single-point calculations at the CCSD(T) level of theory with the 6-311 + G(3df,2p) basis set. The rearrangement of s-cis 1 to 2 is found to occur by a three-step pathway. The first step involves the formation of a nonclassical carbene (5), which is an internal pi complex between the pi molecular orbital of the double bond and the empty p atomic orbital of the carbene carbon. In the second step, the nonplanar five-membered ring geometry of 5 flattens to reach the planar structure of 3-cyclopentenylidene (4). The last step is the 1,2-migration of a alpha-hydrogen atom to the carbene center in 4. The rate-determining step for the rearrangement of s-cis 1 to 2 is the formation of 5, with a predicted global deltaG++(220 K) of only 0.6 kcalmol(-1). The rearrangement of s-trans 1 to 2 requires an initial conversion of s-trans 1 to the s-cis conformer, with a predicted deltaG++(220 K) of 1.8 kcalmol(-1). The transition structure for the ring-opening of s-trans 1 into s-trans 3 (deltaG++(220 K)=4.7 kcalmol(-1)) is more energetic than that for the ring-opening of s-cis 1 into s-cis 3 (deltaG++(220 K)=2.5 kcalmol(-2)) due to larger repulsive nonbonded H...H interactions in the former transition structure. On the basis of these results, it is suggested that if the reaction of 1,1-dibromo-2-vinylcyclopropane with methyllithium at -78 degrees C leads to the initial formation of carbene 1, then the reaction should yield 2 as the main product together with small amounts of 3. This theoretical prediction nicely agrees with experimental findings.     

Model transition states for methane diazonium ion methylation of guanine runs in oligomeric DNA

The DNA reaction pattern of the methane diazonium ion, which is the reactive intermediate formed from several carcinogenic methylating agents, was examined at N7 and O(6) sites in guanine runs occurring in oligonucleotides and model oligonucleotides. Density functional B3LYP/6-31G*, and SCF 3-21G and STO-3G energies of model transition states were calculated in the gas phase and in the CPCM reaction field. For nucleotides containing two, three, and four stacked guanines with counterions in the gas phase, O(6) reactivity is greater than N7 reactivity. In the reaction field, N7 reactivity is 9.0 to 9.8 times greater than O(6) reactivity. For a double-stranded oligonucleotide containing two stacked guanines with counterions in the reaction field, the N7 and O(6) reactivities of the 3'-guanine are 3.9 times greater than the corresponding sites in the 5'-guanine. For double-stranded oligonucleotides with three or four stacked guanines and counterions, the reactivities of the interior guanines are higher than corresponding reactivities of guanines at the ends. These reaction patterns agree with most of the available experimental data. Activation energy decomposition analysis for gas-phase reactions in a double-stranded dinucleotide containing two stacked guanines with counterions indicates that selectivity at O(6) is almost entirely due to electrostatic forces. Selectivity at N7 also has a large electrostatic interaction. However, the orbital interaction also contributes significantly to the gas-phase selectivity, accounting for 32% of the total interaction energy difference between the 3'- and 5'-guanine reactions. In aqueous solution, the relative orbital contribution to N7 selectivity is likely to be larger.     

DFT study on activation of carbon dioxide by (tBuArN)3M≡N (M=Nb,V,Ta): the electronic structure and activity

The reaction mechanism for the reduction of CO(2) gas activated by (tBuArN)(3)M≡N was studied by the means of density functional theory (DFT) calculations. The calculations indicated that this reaction has a two step reaction mechanism. From our calculations, we found that (tBuArN)(3)Ta≡N held the best activity among the three (tBuArN)(3)M≡N complexes studied. Our results also indicated that the reaction of (tBuArN)(3)M≡N with CO(2) occurred under orbital control involving the HOMO-3 orbital of (tBuArN)(3)M≡N, which could give higher overlapping with the LUMO of the CO(2) molecule. The substitutions on the amino donor ligands studied here took larger effect on the HOMO structure of the (tBuArN)(3)M≡N molecules. The electronic structure of the (tBuArN)(3)M≡N complexes also showed their ability for activating CO(2) molecules, in the order of M = V < Nb < Ta.     

Elimination of H<Subscript>2</Subscript> from CH<Subscript>3</Subscript>CH=N<Superscript>+</Superscript>HCH<Subscript>3</Subscript>: A synchronous,concerted 1,4-H<Subscript>2</Subscript> elimination

Most H2 eliminations from cations in the gas phase are formally 1,1- or 1,2- processes. Larger ring size H2 eliminations are rare and little studied. Thus, whether the 6-center, 1,4- elimination CH3CH=N+HCH3-->CH2=CHN+H=CH2+H2 is concerted and synchronous, as indicated by isotope effects and predicted by conservation of orbital symmetry, is a significant question. This reaction is characterized here by application of QCI and B3LYP theories. CH bond-breaking and H-H bond-making in this reaction are found by theory to be highly synchronized, consistent with previously established isotope effects and in contrast to "forbidden" 1,2-eliminations from organic cations in the gas phase. This reaction is made feasible by its conservation of orbital symmetry, the energy supplied by formation of the H-H bond, and a favorable geometry of the ion for eliminating H2.