Mono and double polar [4 + 2(+)] Diels-Alder cycloaddition of acylium ions with O-heterodienes

Gas-phase reactions of acylium ions with alpha,beta-unsaturated carbonyl compounds were investigated using pentaquadrupole multiple-stage mass spectrometry. With acrolein and metacrolein, CH(3)-C(+)(double bond)O, CH(2)(double bond)CH-C(+)(double bond)O, C(6)H(5)-C(+)(double bond)O, and (CH(3))(2)N-C(+)(double bond)O react to variable extents by mono and double polar [4 + 2(+)] Diels-Alder cycloaddition. With ethyl vinyl ketone, CH(3)-C(+)(double bond)O reacts exclusively by proton transfer and C(6)H(5)-C(+)(double bond)O forms only the mono cycloadduct whereas CH(2)(double bond)CH-C(+)(double bond)O and (CH(3))(2)N-C(+)(double bond)O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O-heterodienophiles, and mono cycloaddition occurs readily across their C(+)(double bond)O bonds to form resonance-stabilized 1,3-dioxinylium ions which, upon collisional activation, dissociate predominantly by retro-addition. The mono cycloadducts are also dienophiles activated by resonance-stabilized and chemically inert 1,3-dioxonium ion groups, hence they undergo a second cycloaddition across their polarized C(double bond)C ring double bonds. (18)O labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6-311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO(2)COR(1) molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH(3))(2)N-C(+)(double bond)S with acrolein dissociates to (CH(3))(2)N-C(+)(double bond)O in a sulfur-by-oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge-remote type of polar [4 + 2(+)] Diels-Alder cycloaddition reaction.     

Theoretical investigation on the GaCl3-catalyzed ring-closing metathesis reaction of N-2,3-butadienyl-2-propynyl-1-amine: three-membered ring versus four-membered ring mechanism

The gallium chloride (GaCl(3))-catalyzed ring-closing metathesis reaction mechanism of N-2,3-butadienyl-2-propynyl-1-amine has been studied at the Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (B3LYP)/6-31G(d), B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p)//B3LYP/ 6-31G(d) and the second-order M?ller-Plesset perturbation (MP2)/6-311++G(d,p)//B3LYP/6-31+G(d,p) levels. It was found that the final metathesis product can be yielded via a three-membered or four-membered ring mechanism. The three-membered ring pathway is favorable due to its low energy barrier at the rate determining step. The whole reaction is stepwise and strongly exothermic.     

A computational investigation on the sequential rearrangement mechanism of 2-allyl-2,4,5-hexatrienaldehyde involving [1,5]-hydrogen migration and 8pi-electrocyclization

The sequential rearrangement reaction mechanism of the 2-allyl-2,4,5-hexatrienaldehyde has been studied at the unrestricted Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (UB3LYP)/6-31G**, Complete Active Space Self-Consistent Field (CAS (8,8))/6-31G**, Configuration Interaction with Single and Double Excitations (CISD)//UB3LYP/6-31G** and the second-order perturbation theory based on the CASSCF reference wave function (CASPT2)//CAS(8,8)/6-31G** levels. Two pathways have been found to be involved for this reaction. The first pathway includes four processes of the rotation of the C3--C4 single bond, the stepwise [2 + 2] cycloaddition, the [1,5]-hydrogen migration, and the ring opening isomerization, while the second pathway undergoes only two processes of the [1,5]-hydrogen migration and the 8pi-electrocyclization. The calculation results indicate that the second pathway is favorable, in good agreement with the recent experimental observation. The whole reaction is stepwise and strong exothermic.     

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.     

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found. This was shown to be due to the formation of an intermediate charge-dipole complex which led to the transition state. Reaction field calculations found the complex to disappear in a high dielectric constant medium, and the range of activation energies for the reaction in solution became quite small. MP2 calculations were carried out in order to have a comparison with the DFT results. MP2-MP4 gave unusual results for calculations on permanganate ion as well as chromate ion and iron tetraoxide. They also gave markedly unreasonable results for the activation energy of the reaction of permanganate with ethane. CCSD/6-311++G** calculations gave satisfactory results for permanganate ion and chromate ion. At this level of theory, the reaction of permanganate with ethene was found to have a very early transition state, when the bond lengths of the reactants just began to change. The reaction was calculated to be very exothermic (-69 kcal/mol), and this was confirmed via calorimetry. The rates of permanganate oxidation of allyl alcohol and acrylonitrile were determined, and they had similar reactivities. The kinetics and the products of the reaction of permanganate with crotonate ion were examined in some detail.     

Substituent Effects on Site Selectivity (C=C vs C=N) in Heterocumulene-Heterodiene [4 + 2] Cycloadditions: Density Functional and Semiempirical AM1 Molecular Orbital Calculations

The effect of substituents on the site selectivity (C=C vs C=N) in the [4 + 2] cycloaddition between heterocumulenes (ketene imines) 2a-g with heterodienes (acroleines 9a-n and 4-acylfuran-2,3-diones 1a-d) is treated by semiempirical AM1 molecular orbital and density functional calculations using Becke's three-parameter hybrid method (B3LYP/6-31G). For some reactions calculations were also done at the B3LYP/6-31+G level of theory. For reaction of the oxa 1,3-dienes with ketene imines unsubstituted at the terminal carbon invariably addition across the C=C heterocumulene double bond has a lower activation energy than addition across the C=N double bond. Substitution of methyl or especially phenyl groups at the ketene imine C-terminus leads to a reversal of the respective activation energies. Incorporation of the oxa 1,3-diene system into the heterocyclic dione 1 substantially enhances the reactivity ( approximately 10 kcal mol(-1) lower activation energies) as compared to similarly substituted acroleins. At the DFT level of theory all reactions are found to proceed via a concerted asynchronous mechanism.     

A DFT study for the regioselective 1,3-dipolar cycloadditions of nitrile N-oxides toward alkynylboronates

The mechanism for the 1,3-dipolar cycloaddition of benzonitrile oxide toward ethynyl and propynylboronate has been studied by using density functional theory (DFT) at B3LYP/6-31G* level. These cycloadditions are concerted [3+2] processes. The presence of the two oxygens on the boronic ester precludes the participation of the boron atom on [3+3] processes. The two regioisomeric channels associated to the formation of the isoxazoles bearing the boronic ester unit on the 4- or 5-positions have been characterized. The B3LYP/6-31G* activation parameters are in acceptable agreement with the experiments, allowing to explain the factors controlling these regioselective cycloadditions.     

Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene

The electronic mechanism for the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane (CH2N2) to ethene (C2H4) is described through spin-coupled (SC) calculations at a sequence of geometries along the intrinsic reaction coordinate obtained at the MP2/6-31G(d) level of theory. It is shown that the bonding rearrangements occurring during the course of this reaction follow a heterolytic pattern, characterized by the movement of three well-identifiable orbital pairs, which are initially responsible for the pi bond in ethene and the C-N pi bond and one of the N-N pi bonds in diazomethane and are retained throughout the entire reaction path from reactants to product. Taken together with our previous SC study of the electronic mechanism of the 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne (C2H2) (Theor. Chim. Acc. 1998, 100, 222), the results of the present work suggest strongly that most gas-phase concerted 1,3-dipolar cycloaddition reactions can be expected to follow a heterolytic mechanism of this type, which does not involve an aromatic transition state. The more conventional aspects of the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane to ethene, including optimized transition structure geometry, electronic activation energy, activation barrier corrected for zero-point energies, standard enthalpy, entropy and Gibbs free energy of activation, have been calculated at the HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p), QCISD/6-31G(d) and CCD/6-31G(d) levels of theory. We also report the CCD/6-311++G(2d, 2p)//CCD/6-31G(d), MP4(SDTQ)/6-311++G(2d,2p)//CCD/6-31G(d) and CCSD(T)/6-311++G(2d, 2p)//CCD/6-31G(d) electronic activation energies.     

Cyclic vinyl p-tolyl sulfilimines as chiral dienophiles: theoretical study on the stereoselectivity, Lewis acid catalysis, and solvent effects in their Diels-Alder reactions

The theoretical study reported in the present work deals with chiral cyclic vinyl sulfilimines and their reactivity as dienophiles in [4 + 2] cycloaddition reactions, using B3LYP/6-31G(d)//AM1 and B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistries. Consideration of Lewis acid catalysis, illustrated by BF(3), decreases the activation energies of the cycloaddition process while the charge transfer from the diene to the sulfilimine is augmented. The [4 + 2] cycloaddition reactions of sulfilimines with both furan and cyclopentadiene occur in the gas phase with endo stereoselectivity, which is more pronounced with the latter diene. Endo-exo energy differences in the gas phase with the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-31G(d)//B3LYP/6-31G(d), and B3LYP/6-31G(d)//AM1 model chemistries are almost the same. Solvent effects are responsible for the inversion of the stereoselectivity in the reactions of sulfilimines with furan because of the great difference in the dipole moments in endo and exo approaches.     

An experimental and theoretical study on the substituent effect of the permanganate oxidation of styrenes

The kinetic data obtained for the cycloadditions of the permanganate ion to a series of styrene derivatives in dichloromethane solution in the presence of a quaternary ammonium ion were examined with two theoretical approaches, on the assumption that the reactions proceed via a concerted [3 + 2] mechanism. The semi-quantitative frontier molecular orbital analysis of the kinetic data shows a linear free energy relationship with better correlation than the Hammett plot with a values when the point for p-NO2 group is omitted. Further examination of the results of the FMO analysis reveals that the deviation of the point for p-nitrostyrene is attributed to the transition structure being more reactant-like than that of the other derivatives. The plot of log k2 vs. -deltaG++ calculated by the density functional theory (Becke3LYP) follows a straight line with the desired correlation for all the substituents. A marked tendency was observed for the MO calculations to underestimate the deltaG++ value for electron-withdrawing substituents when the calculation was carried out excluding the quaternary ammonium ion. This inconsistency was much improved by the calculations incorporating the quaternary ammonium ion. The actual values of deltaG++ obtained from the Eyring analysis are in good agreement with those calculated at the B3LYP/6-311 +G(d,p)//B3LYP/ LanL2DZ level.     

Mechanism of metal chloride-promoted Mukaiyama aldol reactions

Ab initio calculations (MP2/6-311+G**//B3LYP/6-31G*) were employed to investigate the mechanism of metal chloride-promoted Mukaiyama aldol reaction between trihydrosilyl enol ether and formaldehyde. The metal chlorides considered include TiCl4, BCl3, AlCl3, and GaCl3. In contrast to the concerted pathway of the uncatalyzed aldol reaction, the Lewis acid-promoted reactions favor a stepwise mechanism. Three possible stepwise pathways were located. The lowest energy pathway corresponds to a simultaneous C-C bond formation and a chlorine atom shift in the first (rate-determining) step. This process is calculated to have a low activation barrier of 12 kJ mol-1 for the TiCl4-promoted reaction. The alternative [2+2] cycloaddition and direct carbon-carbon bond formation pathways are energetically competitive. BCl3, AlCl3, and GaCl3 are predicted to be efficient catalysts for the silicon-directed aldol reaction as they strongly activate the formaldehyde electrophile. Formation of a stable pretransition state intermolecular pi-pi complex between enol silane and the activated formaldehyde (CH2=O...MCln) is a key driving force for the facile metal chloride-promoted reactions.     

Computational study of the deamination reaction of cytosine with H2O and OH-

The mechanism for the deamination reaction of cytosine with H(2)O and OH(-) to produce uracil was investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d) levels and for anions at the B3LYP/6-31+G(d) level. Single-point energies were also determined at B3LYP/6-31+G(d), MP2/GTMP2Large, and G3MP2 levels of theory. Thermodynamic properties (DeltaE, DeltaH, and DeltaG), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway that was investigated. Intrinsic reaction coordinate analysis was performed to characterize the transition states on the potential energy surface. Two pathways for deamination with H(2)O were found, a five-step mechanism (pathway A) and a two-step mechanism (pathway B). The activation energy for the rate-determining steps, the formation of the tetrahedral intermediate for pathway A and the formation of the uracil tautomer for pathway B, are 221.3 and 260.3 kJ/mol, respectively, at the G3MP2 level of theory. The deamination reaction by either pathway is therefore unlikely because of the high barriers that are involved. Two pathways for deamination with OH(-) were also found, and both of them are five-step mechanisms. Pathways C and D produce an initial tetrahedral intermediate by adding H(2)O to deprotonated cytosine which then undergoes three conformational changes. The final intermediate dissociates to product via a 1-3 proton shift. Deamination with OH(-), through pathway C, resulted in the lowest activation energy, 148.0 kJ/mol, at the G3MP2 level of theory.     

Molecule-induced alkane homolysis with dioxiranes.

The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2.     

Ketene-acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio(G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH(2) and NMe(2)) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol(-1) for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for [small pi]-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol(-1) for X = NMe(2). In contrast, the 1,3-shifts of CH(3) and H have very high barriers.     

The Curtius rearrangement of cyclopropyl and cyclopropenoyl azides. A combined theoretical and experimental mechanistic study

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.     

Organocatalytic, enantioselective intramolecular [6+2] cycloaddition reaction for the formation of tricyclopentanoids and insight on its mechanism from a computational study

Diphenylprolinol silyl ether was found to be an effective organocatalyst for promoting the asymmetric, catalytic, intramolecular [6 + 2] cycloaddition reactions of fulvenes substituted at the exocyclic 6-position with a δ-formylalkyl group to afford synthetically useful linear triquinane derivatives in good yields and excellent enantioselectivities. The cis-fused triquinane derivatives were obtained exclusively; the trans-fused isomers were not detected among the reaction products. The intramolecular [6 + 2] cycloaddition occurs between the fulvene functionality (6π) and the enamine double bond (2π) generated from the formyl group in the substrates and the diphenylprolinol silyl ether. The absolute configuration of the reaction products was determined by vibrational circular dichroism. The reaction mechanism was investigated using molecular orbital calculations, B3LYP and MP2 geometry optimizations, and subsequent single-point energy evaluations on model reaction sequences. These calculations revealed the following: (i) The intermolecular [6 + 2] cycloaddition of a fulvene and an enamine double bond proceeds in a stepwise mechanism via a zwitterionic intermediate. (ii) On the other hand, the intramolecular [6 + 2] cycloaddition leading to the cis-fused triquinane skeleton proceeds in a concerted mechanism via a highly asynchronous transition state. (iii) The fulvene functionality and the enamine double bond adopt the gauche-syn conformation during the C-C bond formation processes in the [6 + 2] cycloaddition. (iv) The energy profiles calculated for the intramolecular reaction explain the observed exclusive formation of the cis-fused triquinane derivatives in the [6 + 2] cycloaddition reactions. The reasons for the enantioselectivity seen in these [6 + 2] cycloaddition reactions are also discussed.     

Theoretical study of the reaction of Ni with OCS

The reactivity of Ni⁺ with OCS on both doublet and quartet potential energy surfaces (PES) has been investigated at the B3LYP/6-311+G(d) level. The object of this investigation was the elucidation of the reaction mechanism. The calculated results indicated that both the CS and CO bond activations proceed via an insertion–elimination mechanism. Intersystem crossing between the doublet and quartet surfaces may occur along both the CS and CO bond activation branches. The ground states of NiS⁺ and NiO⁺ were found to be quartets, whereas NiCO⁺ and NiCS⁺ have doublet ground states. The CS bond activation is energetically much more favorable than the CO bond activation. All theoretical results are in line with early experiments.     

Toward an understanding of the mechanisms of the intramolecular

The molecular mechanism for the intramolecular [5 + 2] cycloaddition reaction of beta-silyloxy-gamma-pyrones bearing tethered alkenes has been characterized using ab initio methods. A comparative study for this sort of cycloaddition carried out at different computational levels points out that the B3LYP/6-31G calculations give similar barriers to those obtained with the MP3/6-31G level. Analysis of the energetic results shows that the reaction takes place along a stepwise process: first, the migration of the neighboring silyl group to the carbonyl group of the gamma-pyrone takes place to give a weak oxidopyrylium ylide intermediate, which by a subsequent concerted intramolecular [5 + 2] cycloaddition affords the final cycloadduct. The cycloaddition process is very stereoselective due to the constraints imposed by the tether. The [5 + 2] cycloaddition reaction has a large barrier, and the presence of the silyloxy group and the intramolecular character of the process are necessary to ensure the thermodynamic and kinetic feasibility of these cycloadditions.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

New insights into the bromination reaction for a series of alkenes--a computational study

Ab initio calculations were carried out for the reaction of Br2 with ethene, propene, isobutene, fluoroethene, chloroethene, (E)-1,2-difluoroethene, and (E)-1,2-dichloroethene. For ethene the calculations were also carried out for the reaction with 2Br2. Geometries were optimized at the HF, MP2, and B3LYP levels using the 6-31G(d) and 6-31+G(d) basis sets where for Br both the standard 6-31G and the Binning-Curtiss bromine basis sets were used. Energies were also calculated at the G3MP2 and G3MP2B3 levels. For a single Br2 one mechanism involves a perpendicular attack by Br2 to the C=C bond, and a second mechanism consists of sidewise attack by Br2. Alkenes can react with 2Br2 via several mechanisms, all leading to the dibromo product. The most likely pathway for the reaction of ethene and 2Br2 involves a trans addition of a Br atom from Br3- to one of the bromonium ion carbons. Activation energies, free energies, and enthalpies of activation along with thermodynamic properties (DeltaE, DeltaH, and DeltaG) for each reaction were calculated. We have found that the reaction of ethene with 2Br2 is favored over reaction with only Br2. There is excellent agreement between the calculated free energies of activation for the reaction of ethene and 2Br2 and experimental values in nonpolar aprotic solvents. However, the free energies of activation for the reaction with a single Br2 agrees well with the experimental results for polar protic solvents only when the reaction is mediated by a solvent molecule. A kinetic expression is proposed that accounts for the difference between bromination of alkenes in protic and nonprotic solvents. Some previously unknown heats of formation are reported.