Chemo- and periselectivity in the addition of [OsO2(CH2)2] to ethylene: a theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.     

Theoretical investigation of the mechanisms of reaction of NCN with NO and NS

Quantum-chemical calculations were performed on the mechanisms of reaction of NCN with NO and NS. Possible mechanisms were classified according to four pathways yielding products in the following four possible groups: N2O/N2S + CN, N2 + NCO/NCS, N2 + CNO/CNS, and CNN + NO/NS, labeled in order from p1/p1s to p4/p4s. The local structures, transition structures, and potential-energy surfaces with respect to the reaction coordinates are calculated, and the barriers are compared. In the NCN + NO reaction, out of several adduct structures, only the nitroso adduct NCNNO lies lower in energy than the reactants, by 21.89 kcal/mol; that adduct undergoes rapid transformation into the products, in agreement with experimental observation. For the NS counterpart, both thionitroso NCNNS and thiazyl NCNSN adducts have energies much lower than those of the reactants, by 43 and 29 kcal/mol, respectively, and a five-membered-ring NCNNS (having an energy lower than those of the reactants by 36 kcal/mol) acts as a bridge in connecting these two adducts. The net energy barriers leading to product channels other than p4s are negative for the NS reaction, whereas those for the NO analogue are all positive. The channel leading to p1 (N2O + CN) has the lowest energy (3.81 kcal/mol), whereas the channels leading to p2 (N2 + NCO) and p2s (N2 + NCS) are the most exothermic (100.94 and 107.38 kcal/mol, respectively).     

An ab initio study on the allene-isocyanic acid and ketene-vinylimine [2 + 2] cycloaddition reaction paths

The reaction paths of [2 + 2] cycloadditions of allene (H2C=C=CH2) to isocyanic acid (HN=C=O) and ketene (H2C=C=O) to vinylimine (H2C=C=NH), leading to all the possible 14 four-membered ring molecules, were investigated by the MP2/aug-cc-pVDZ method. In the two considered reactions, the 2-azetidinone (beta-lactam) ring compounds were predicted to be the most stable thermodynamically in the absence of an environment. Although 4-methylene-2-azetidinone is the most stable product of the ketene-vinylimine cycloaddition, its activation barrier is higher than that for 4-methylene-2-iminooxetane by ca. 6 kcal/mol. Therefore, the latter product can be obtained owing to kinetic control. The activation barriers in the allene-isocyanic acid reactions are quite high, 50-70 kcal/mol, whereas in the course of the ketene-vinylimine cycloaddition they are equal to ca. 30-55 kcal/mol. All the reactions studied were found to be concerted and mostly asynchronous. Simulation of the solvent environment (toluene, tetrahydrofuran, acetonitrile, and water) by using Tomasi's polarized continuum model with the integral equation formalism (IEF-PCM) method showed the allene-isocyanic reactions remained concerted, yet the activation barriers were somewhat higher than those in the gas phase, whereas the ketene-vinylimine reactions became stepwise. The larger the solvent dielectric constant, the lower the activation barriers found. The lowest-energy pathways in the gas phase and in solvent were confirmed by intrinsic reaction coordinate (IRC) calculations. The atoms in molecules (AIM) analysis of the electron density distribution in the transition-state (TS) structures allowed us to distinguish pericyclic from pseudopericyclic from nonplanar-pseudopericyclic types of reactions.     

Theoretical study on reactions of nitroethylene with the Si(100)-2 x 1 surface

Possible reaction pathways of nitroethylene with the Si(100)-2 x 1 surface have been investigated by unrestricted density functional theory. The facile occurrence of the studied reactions was demonstrated by the low activation energies of the rate-determining steps (1.07-5.23 kcal/mol). It was found that the [4 + 2] cycloaddition reaction of nitroethylene is most kinetically favorable. The isomerization reactions of the addition products were also investigated. The [3 + 2] cycloaddition product may further undergo a rearrangement by overcoming a 12.37 kcal/mol activation energy barrier into an isomer, with an oxygen atom of the nitryl group inserted between two silicon atoms of the Si(100) surface.     

2+3] Cycloaddition of ethylene to transition metal oxo compounds. analysis of density functional results by marcus theory.

Density functional results on the [2+3] cycloaddition of ethylene to various transition metal complexes MO(3)(q) and LMO(3)(q) (q = -1, 0, 1) with M = Mo, W, Mn, Tc, Re, and Os and various ligands L = Cp, CH(3), Cl, and O show that the corresponding activation barriers DeltaE(double dagger) depend in quadratic fashion on the reaction energies DeltaE(0) as predicted by Marcus theory. A thermoneutral reaction is characterized by the intrinsic reaction barrier DeltaE(0) of 25.1 kcal/mol. Both ethylene [2+3] cycloaddition to an oxo complex and the corresponding homolytic M-O bond dissociation are controlled by the reducibility of the transition metal center. Indeed, from the easily calculated M-O bond dissociation energy of the oxo complex one can predict the reaction energy DeltaE(0) and hence, by Marcus theory, the corresponding activation barrier DeltaE. This allows a systematic representation of more than 25 barriers of [2+3] cycloaddition reactions that range from 5 to 70 kcal/mol.     

Cycloaddition reactions of acrylonitrile on the Si(100)-2 x 1 surface

Multi-reference as well as single-reference quantum mechanical methods were adopted to study the potential energy surface along three possible surface reaction mechanisms of acrylonitrile on the Si(100)-2 x 1 surface. All three reactions occur via stepwise radical mechanisms. According to the computed potential energy surfaces, both [4+2] and [2+2](CN) cycloaddition products resulting from the reactions of surface dimers with the C[triple bond]N of acrylonitrile are expected, due to the negligible activation barriers at the surface. Another possible surface product, [2+2](CC), requires a 16.7 kcal/mol activation energy barrier. The large barrier makes this route much less favorable kinetically, even though this route produces the thermodynamically most stable products. Isomerization reactions among the surface products are very unlikely due to the predicted large activation barriers preventing thermal redistributions of the surface products. As a result, the distribution of the final surface products is kinetically controlled leading to a reinterpretation of recent experiments. An intermediate Lewis acid-base type complex appears in both the [4+2] and [2+2](CN) cycloadditions entrance channels, indicating that the surface may act as an electrophile/Lewis acid toward a strong Lewis base substrate.     

Ab initio study of the pathways and barriers of tricyclo[4.1.0.0(2,7)]heptene isomerization

The thermal isomerization of tricyclo[4.1.0.0(2,7)]heptene has been studied using computational chemistry with structures determined at the MCSCF level and energies at the MRMP2 level. Both the allowed conrotatory and forbidden disrotatory pathways have been elucidated resulting in cycloheptatriene isomers. Four reaction channels are available for the conrotatory pathway depending on which bond breaks first in the bicyclobutane moiety leading to enantiomeric pairs of (E,Z,Z)-1,3,5-cycloheptatriene and (Z,E,Z)-1,3,5-cycloheptatriene intermediates. The activation barrier is calculated to be 31.3 kcal·mol?1 for two channels and 37.5 kcal·mol?1 for the other two. The lower activation barrier leading to the (E,Z,Z)-1,3,5-cycloheptatriene enantiomeric pair is proposed to be due to resonance within the transition state. The same behavior was observed for the disrotatory pathway with activation barriers of 42.0 kcal·mol?1 and 55.1 kcal·mol?1 for the two channels, again with one transition state resonance stabilized. The barriers for trans double bond rotation of the intermediate cycloheptatrienes are determined to be 17.1 and 17.4 kcal·mol?1, about 5 kcal·mol?1 more than that for the seven carbon diene (E,Z)-1,3-cycloheptadiene. The electrocyclic ring closure of the trans cycloheptatrienes have been modeled and barriers determined to be 11.1 and 11.9 kcal·mol?1 for the formation of bicyclo[3.2.0]hepta-2,6-diene. This structure was previously reported as the end product for thermolysis of the parent tricyclo[4.1.0.0(2,7)]heptene. The thermodynamically more stable cycloheptatriene can be formed from bicyclo[3.2.0]hepta-2,6-diene through a two step process with a calculated pseudo first-order barrier of 36.4 kcal·mol?1. The trans-cycloheptatrienes reported herein are the first characterization of a small seven-membered ring triene with a trans double bond.     

Aromatic dienophiles. 1. A theoretical study of an inverse-electron demand Diels-Alder reaction between 2-aminopyrrole and 1,3,5-triazine

This study is devoted to a detailed theoretical study of an inverse-electron demand Diels-Alder reaction (IDA) with 1,3,5-triazine as the diene and 2-aminopyrrole 1A(alpha) as the dienophile, which is a key step in a cascade reaction for the one-pot synthesis of purine analogues. Geometries were optimized with the B3LYP/6-31G* method and energies were evaluated with the MP2/6-311++G** method. This IDA reaction occurs through a stepwise mechanism, where the first step corresponds to the nucleophilic attack of 2-aminopyrrole to triazine to form a zwitterionic intermediate, which is in equilibrium with a neutral intermediate through a hydrogen transfer process, followed by a rate-determining ring-closure step. It is shown that the B3LYP method significantly overestimates the activation energy, whereas the MP2 method offers a reasonable activation barrier of 27.9 kcal/mol in the gas phase. The solvation effect has been studied by the PCM model. In DMSO, the calculated activation energy of the IDA reaction is decreased to 24.0 kcal/mol with a strong endothermicity of 17.4 kcal/mol due to the energy penalty of transforming two aromatic reactants into a nonaromatic IDA adduct. The possible stepwise [2+2] pathway is ruled out based on its higher activation and reaction energies than those of the [4+2] pathway. By comparing the IDA reactions of triazine to 2-aminopyrrole and pyrrole, we address two crucial roles of the alpha-amino substituent in lowering activation and reaction energies and controlling the reaction regiochemistry.     

Proton as the simplest of all catalysts for [2 + 2] cycloadditions: DFT study of acid-catalyzed imine metathesis

The mechanism of imine metathesis was studied as a prototype reaction for the impact that heteroatom substitution has on thermally forbidden [2 + 2] addition reactions using high-level density functional theory in combination with a continuum solvation model. The intuitively expected high activation barriers were confirmed for N-alkyl- and N-aryl-substituted imine reactants with transition state free energies of 78.8 and 68.5 kcal/mol, respectively, in benzene. The computed reaction energy profiles were analyzed to discover possible strategies for lowering the transition state energy. Protonation of the imine nitrogen was proposed as a possible catalytic route and was explicitly modeled. The computed reaction energy profile shows that protonation of one of the imine reactants has an enormous effect on the overall rate of metathesis and lowers the activation barrier by as much as 37.3 and 30.6 kcal/mol for the N-alkyl and N-aryl reactants, respectively. These results suggest that acid-catalyzed imine metathesis should be amenable at elevated temperatures. Furthermore, the protonation of both reactants of the metathesis reaction is predicted to be not productive owing to electrostatic repulsion of the reactants, thus suggesting that there should be an optimum pH for the catalytic turnover. A detailed analysis of the catalytic mechanism is presented, and the primary driving force for the catalysis is identified. Upon protonation of the imine nitrogen, the key [2 + 2]-addition step becomes asynchronous and one of the two intermolecular N-C bonds is formed before traversing the transition state, resulting in a substantial net decrease of the overall energy requirement. The general applicability of this intuitively understandable mechanism for designing structural features for lowering the energy of transition state structures is explored.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

Thinking out of the black box: accurate barrier heights of 1,3-dipolar cycloadditions of ozone with acetylene and ethylene

Accurate barriers for the 1,3-dipolar cycloadditions of ozone with acetylene and ethylene have been determined via the systematic extrapolation of ab initio energies within the focal point approach of Allen and co-workers. Electron correlation has been accounted for primarily via coupled cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [CCSD, CCSD(T), CCSDT, and CCSDT(Q)]. For the concerted [4 + 2] cycloadditions, the final recommended barriers are DeltaH(0K) = 9.4 +/- 0.2 and 5.3 +/- 0.2 kcal mol(-1) for ozone adding to acetylene and ethylene, respectively. These agree with recent results of Cremer et al. and Anglada et al., respectively. The reaction energy for O3 + C2H2 exhibits a protracted convergence with respect to inclusion of electron correlation, with the CCSDT/cc-pVDZ and CCSDT(Q)/cc-pVDZ values differing by 2.3 kcal mol-1. Recommended enthalpies of formation (298 K) for cycloadducts 1,2,3-trioxole and 1,2,3-trioxolane are +32.8 and -1.6 kcal mol(-1), respectively. Popular composite ab initio approaches [CBS-QB3, CBS-APNO, G3, G3B3, G3(MP2)B3, G4, G4(MP3), and G4(MP2)] predict a range of barrier heights for these systems. The CBS-QB3 computed barrier for ozone and acetylene, DeltaH(0K) = 4.4 kcal mol(-1), deviates by 5 kcal mol(-1) from the focal point value. CBS-QB3 similarly underestimates the barrier for the reaction of ozone and ethylene, yielding a prediction of only 0.7 kcal mol(-1). The errors in the CBS-QB3 results are significantly larger than mean errors observed in application to the G2 test set. The problem is traced to the nontransferability of MP2 basis set effects in the case of these reaction barriers. The recently published G4 and G4(MP2) approaches perform substantially better for O3 + C2H2, predicting enthalpy barriers of 9.0 and 8.4 kcal mol(-1), respectively. For the prediction of these reaction barriers, the additive corrections applied in the majority of the composite approaches considered lead to worse agreement with the reference focal point values than would be obtained relying only on single point energies evaluated at the highest level of theory utilized within each composite method.     

DFT (density functional theory) studies on cycloisomerization of 15–membered triazatriacetylenic macrocycle

The mechanism as well the stereochemistry of cascade cycloisomerization of 15–membered triazatriacetylenic macrocycle was investigated theoretically by using M062X/6–31+G(d,p) and M062X/LANL2DZ calculations. The results showed that the mechanism and outcome of the reaction depended on the absence and presence of a transition metal catalyst. So that, in thermal-induced condition, the reaction had to experience several suprafacial concerted reactions including Ene-reaction (DG^#=35.38 kcal/mol), Diels–Alder cycloaddition (DG^# = 17.16 kcal/mol), and sigmatropic H-shift rearrangement (DG^# = 56.21 kcal/mol) to produce diastereoselective fused cis–tetracyclic aromatic bearing a pyrrole moiety by following kinetic considerations. Also, the [2+2+2] cycloaddition mechanism was neglected in thermal–induced conditions because of high activation free Gibbs energy (DG^# = 63.90 kcal/mol). In the presence of palladium catalyst, Pd(0) formed an adduct by coordinating to C = C bonds and decreased the DG^# of the process to 29.58 kcal/mol, and consequently provided a facilitated media for the reaction to follow the [2+2+2] to produce more stable fused tetracyclic benzenoid aromatic by passing through the lower energy barrier.     

A DFT study of SiH(4) activation by Cp(2)LnH

A theoretical study of SiH(4) activation by Cp(2)LnH complexes for the entire series of lanthanides has been carried out at the DFT-B3PW91 level of theory. The reaction paths corresponding to H/H exchange and silylation, formation of Cp(2)Ln(SiH(3)), have been computed. They both occur via a single-step sigma-bond metathesis mechanism. For the athermal H/H exchange reaction, the calculated activation barrier averages 1.8 kcal.mol(-)(1) relative to the precursor adduct Cp(2)LnH(eta(2)-SiH(4)) for all lanthanide elements. The silylation path is slightly exogenic (DeltaE approximately -6.5 kcal.mol(-1)) with an activation barrier averaging 5.2 kcal.mol(-1) relative to the precursor adduct where SiH(4) is bonded by two Si-H bonds. Both pathways are therefore thermally accessible. The H/H exchange path is calculated to be kinetically more favorable whereas the silylation reaction is thermodynamically preferred. The reactivity of this familly of lanthanide complexes with SiH(4) contrasts strongly with that obtained previously with CH(4). The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent.     

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.     

Reaction of the i-C4H5 (CH2CCHCH2) radical with O2

The resonantly stabilized radical i-C(4)H(5) (CH(2)CCHCH(2)) is an important intermediate in the combustion of unsaturated hydrocarbons and is thought to be involved in the formation of polycyclic aromatic hydrocarbons through its reaction with acetylene (C(2)H(2)) to form benzene + H. This study uses quantum chemistry and statistical reaction rate theory to investigate the mechanism and kinetics of the i-C(4)H(5) + O(2) reaction as a function of temperature and pressure, and unlike most resonantly stabilized radicals we show that i-C(4)H(5) is consumed relatively rapidly by its reaction with molecular oxygen. O(2) addition occurs at the vinylic and allenic radical sites in i-C(4)H(5), with respective barriers of 0.9 and 4.9 kcal mol(-1). Addition to the allenic radical form produces an allenemethylperoxy radical adduct with only around 20 kcal mol(-1) excess vibrational energy. This adduct can isomerize to the ca. 14 kcal mol(-1) more stable 1,3-divinyl-2-peroxy radical via concerted and stepwise processes, both steps with barriers around 10 kcal mol(-1) below the entrance channel energy. Addition of O(2) to the vinylic radical site in i-C(4)H(5) directly forms the 1,3-divinyl-2-peroxy radical with a small barrier and around 36.8 kcal mol(-1) of excess energy. The 1,3-divinyl-2-peroxy radical isomerizes via ipso addition of the O(2) moiety followed by O atom insertion into the adjacent C-C bond. This process forms an unstable intermediate that ultimately dissociates to give the vinyl radical, formaldehyde, and CO. At higher temperatures formation of vinylacetylene + HO(2), the vinoxyl radical + ketene, and the 1,3-divinyl-2-oxyl radical + O paths have some importance. Because of the adiabatic transition states for O(2) addition, and significant reverse dissociation channels in the peroxy radical adducts, the i-C(4)H(5) + O(2) reaction proceeds to new products with rate constant of around 10(11) cm(3) mol(-1) s(-1) at typical combustion temperatures (1000-2000 K). For fuel-rich flames we show that the reaction of i-C(4)H(5) with O(2) is likely to be faster than that with C(2)H(2), bringing into question the importance of the i-C(4)H(5) + C(2)H(2) reaction in initiating ring formation in sooting flames.     

Quantum mechanical study of the potential energy surface of the ClO + NO2 reaction

In the study of the reaction pathways of the ClO + NO2 reaction including reliable structures of the reactants, products, intermediates, and transition states as well as energies the MP2/6-311G(d), B3LYP/6-311G(d), and G2(MP2) methods have been employed. Chlorine nitrate, ClONO2, is formed by N-O association without an entrance barrier and is stabilized by 29.8 kcal mol(-1). It can undergo either a direct 1,3 migration of Cl or OCl rotation to yield an indistinguishable isomer. The corresponding barriers are 45.8 and 7.1 kcal mol(-1), respectively. ClONO2 can further decompose into NO3 + Cl with an endothermicity of 46.4 kcal mol(-1). The overall endothermicity of the NO2 + ClO --> NO3 + Cl reaction is calculated to be 16.6 kcal mol(-1). The formation of cis-perp and trans-perp conformer of chlorine preoxynitrite, ClOONO(cp) and ClOONO(tp), are exothermic by 5.4 and 3.8 kcal mol(-1), respectively. Calculations on the possible reaction pathways for the isomerization of ClOONO to ClONO2 showed that the activation barriers are too high to account for appreciable nitrate formation from peroxynitrite isomerization. All quoted relative energies are related to G2(MP2) calculations.     

MnZSM-5催化剂上NH3-SCR反应机理的研究

通过密度泛函理论（DFT）对Mn/ZSM-5催化剂上NH3-SCR反应机理进行了理论研究。一种为气态NO直接参与反应的E-R机理,NO分子与[NH2]反应生成中间体[NH2NO],该反应路径的能垒为43.35 kcal/mol;另一种为吸附态NO参与反应的L-H机理,[NO]与[NH3]反应生成[NH2NO],该反应路径的能垒为44.73 kcal/mol。因两种机理的反应能垒相差不大,因此在一定温度下遵循两种机理的反应皆能进行。     

Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

In this paper, the mechanisms of the intermolecular [3+2] and [1+2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C═O or C═C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3+2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1+2] cycloaddition and then a rearrangement from the [1+2] cycloadducts to the final [3+2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C═O [3+2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C═C dipolarophile favors undergoing [1+2] cycloaddition rather than concerted [3+2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C═O concerted [3+2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C═O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C═O to the C═C dipolarophile and the origins of cis-stereoselectivity for C═C [1+2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH(3)CN, DMF, THF, CH(2)Cl(2), toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.     

Iridium-catalyzed borylation of benzene with diboron. Theoretical elucidation of catalytic cycle including unusual iridium(v) intermediate

Iridium-catalyzed borylation of benzene with diboron was theoretically investigated with the DFT method, where an iridium(I) boryl complex, Ir(Beg)(NN) 1, and an iridium(III) tris(boryl) complex, Ir(Beg)(3)(NN) 14, (eg (ethyleneglycolato) = -OCH(2)CH(2)O-, NN = HN=CHCH=NH (diim) or 2,2'-bipyridine (bpy)) were adopted as models of active species and B(2)(eg)(2) was adopted as a model of bis(pinacolato)diboron (pinacolato = -OCMe(2)CMe(2)O-). Oxidative addition of a benzene C-H sigma-bond to 1 takes place with an activation barrier (E(a)) of 11.2 kcal/mol, followed by reductive elimination of phenylborane, Ph-Beg, from Ir(Beg)(H)(Ph)(diim) with an activation barrier of 15.6 kcal/mol. Though the oxidative addition and the reductive elimination occur with moderate activation barriers, B(2)(eg)(2) much more easily reacts with 1 to afford 14 than does benzene, of which the activation barrier is very small (2.9 kcal/mol). Oxidative addition of the benzene C-H sigma-bond to 14 occurs with a moderate activation barrier of 24.2 kcal/mol to afford an unusual seven-coordinate iridium(V) complex, Ir(H)(Ph)(Beg)(3)(bpy) 16. From this complex, phenylborane Ph-Beg is produced through the reductive elimination with concomitant formation of IrH(Beg)(2)(bpy) 17, where the activation barrier is 4.9 kcal/mol. Complex 17 further reacts with diboron to form Ir(H)(Beg)(4)(bpy) (E(a) = 8.0 kcal/mol), followed by the reductive elimination of borane H-Beg (E(a) = 2.6 kcal/mol) to regenerate Ir(Beg)(3)(bpy), when diboron exists in excess in the reaction solution. After consumption of diboron, IrH(Beg)(2)(bpy) reacts with borane, H-Beg, to form Ir(H)(2)(Beg)(3) (E(a) = 21.3 kcal/mol) followed by the reductive elimination of H(2), to regenerate Ir(Beg)(3)(bpy) with concomitant formation of H(2). Formation of the iridium(III) tris(boryl) complex 14 from IrCl(diim) and diboron was also theoretically investigated; IrCl(diim) undergoes two steps of oxidative addition of diboron to afford a seven-coordinate iridium(V) complex, IrCl(Beg)(4)(NN), from which the reductive elimination of Cl-Beg takes place easily to afford 14. From these results, it should be clearly concluded that the iridium(III) tris(boryl) complex is an active species and an unusual iridium(V) species is involved as a key intermediate in the reaction. Detailed discussion is presented on the full catalytic cycle and the importance of a seven-coordinate iridium(V) intermediate.     

Relative reactivity of peracids versus dioxiranes (DMDO and TFDO) in the epoxidation of alkenes. A combined experimental and theoretical analysis

Comparative analysis of the calculated gas-phase activation barriers (DeltaE++) for the epoxidation of ethylene with dimethyldioxirane (DMDO) and peroxyformic acid (PFA) [15.2 and 16.4 kcal/mol at QCISD(T)// QCISD/6-31+G(d,p)] and E-2-butene [14.3 and 13.2 kcal/mol at QCISD(T)/6-31G(d)//B3LYP/6-311+G(3df,2p)] suggests similar oxygen atom donor capacities for both oxidants. Competition experiments in CH(2)Cl(2) solvent reveal that DMDO reacts with cyclohexene much faster than peracetic acid/acetic acid under scrupulously dried conditions. The rate of DMDO epoxidation is catalyzed by acetic acid with a reduction in the classical activation barrier of 8 kcal/mol. In many cases, the observed increase in the rate for DMDO epoxidation in solution may be attributed to well-established solvent and hydrogen-bonding effects. This predicted epoxidative reactivity for DMDO is not consistent with what has generally been presumed for a highly strained cyclic peroxide. The strain energy (SE) of DMDO has been reassessed and its moderated value (about 11 kcal/mol) is now more consistent with its inherent gas-phase reactivity toward alkenes in the epoxidation reaction. The unusual thermodynamic stability of DMDO is largely a consequence of the combined geminal dimethyl- and dioxa-substitution effects and unusually strong C-H and C-CH(3) bonds. Methyl(trifluoromethyl)dioxirane (TFDO) exhibits much lower calculated activation barriers than DMDO in the epoxidation reaction (the average DeltaDeltaE++ values are about 7.5 kcal/mol). The rate increase relative to DMDO of approximately 10(5), while consistent with the higher strain energy for TFDO (SE approximately 19 kcal/mol) is attributed largely to the inductive effect of the CF(3) group. We have also examined the effect of alkene strain on the rate of epoxidation with PFA. The epoxidation barriers are only slightly higher for the strained alkenes cyclopropene (DeltaE++ = 14.5 kcal/mol) and cyclobutene (DeltaE++ = 13.7 kcal/mol) than for cyclopentene (DeltaE++ = 12.1 kcal/mol), reflecting the fact there is little relief of strain in the transition state. Alkenes strained by twist or pi-bond torsion do exhibit much lower activation barriers.