Quantitative preparation of allyl telechelic polyisobutylene under reflux conditions

Allyl telechelic polyisobutylene (allyl‐PIB‐allyl) is of great commercial and scientific interest produced by living polymerization of isobutylene followed by functionalization (allylation with allyltrimethylsilane) under external cooling, typically to ?78 °C. Cooling is cumbersome and costly, and temperature control is far from ideal. Herein we describe the quantitative preparation of allyl‐PIB‐allyl under ideal internal temperature control at ～?40 °C using refluxing propane/methyl chloride mixtures. The exact composition of the nonpolar/polar solvents and polymerization time crucially affect product quality. Well‐defined allyl‐PIB‐allyl is obtained using 60/40 (v/v) refluxing propane/methyl chloride and terminating not more than 5 min after monomer depletion. In pure refluxing propane or methyl chloride, or at longer reaction times, byproducts form that compromise product quality. A mechanism is presented to explain the observations. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1784–1789     

Insertion of CO2 into a palladium allyl bond and a Pd(II) catalysed carboxylation of allyl stannanes

The complex 2,6-bis[(di-t-butylphosphino)methyl]phenyl allyl palladium (PCP(tBu)Pd-allyl, 3) reacts with CO(2) in a very fast insertion reaction to give the corresponding butenoate complex. The reaction is thought to occur via a cyclic six-membered transition state (7), where the gamma-carbon of the allyl group is linked up with the CO(2)-carbon. A group of related PCP complexes were investigated as catalysts for the carboxylation of tributyl(allyl)stannane. A catalytic cycle is proposed for this reaction where the rate determining step is the transmetallation between tin and palladium. The carboxylation reaction is faster using less sterically crowded catalysts whereas the electron richness of the palladium complexes seems less important for reactivity. Thus, there was no apparent difference in reactivity between 2,6-bis[(di-phenylphosphino)methyl]phenyl palladium triflouroacetate (13) and resorcinolbis(diphenyl)phosphinite palladium triflouroacetate (10). Both of these complexes give high turnovers for the carboxylation of tributyl(allyl)stannane (80% in 16 h using a ca. 5% catalyst loading and 4 atm CO(2) pressure). On the other hand complex 3 was inactive in the catalytic carboxylation reaction.     

Gas-phase thermolysis of sulfur compounds. Part IV. n-propyl allyl sulfide

The pyrolysis of n-propyl allyl sulfide has been studied in static and stirred-flow systems at temperatures between 270 and 400°C. Propene and 2,4,6-triethyl-1,3,5-trithiane were the only reaction products. The order of the reaction was 0.99 ± 0.05 at 377°C. The first-order rate coefficients followed the Arrhenius equation The rate coefficients and the product distribution remained unchanged when cyclohexene was used as carrier gas. A molecular mechanism involving a six-centered cyclic transition state is proposed to explain the present results. This mechanism is further supported by the pyrolysis of 4-thia-5-dideutero-1-heptene at 377°C, where only 3-deuteropropene is formed. The kinetic deuterium isotope effect had a value of 2.6 ± 0.3 at this temperature. The results are compared with those obtained in the pyrolysis of n-butyl allyl sulfide previously reported.     

Computational study of the thermal decomposition and the thermochemistry of allyl ethers and allyl sulfides

In spite of the several experimental and computational studies on the thermal decomposition of allyl ethers and allyl sulfides, there are still disagreements on aspects of the reaction mechanism, such as the true nature of the transition states and the grade of synchronicity of the reactions. This work presents a computational study of the gas-phase thermolysis reaction of allyl ethers and allyl sulfides substituted at α-carbon, at the M05-2X/6-31+G(d,p) level of theory and a temperature range from 586.15 to 673.15 K. The substituent groups were methyl, ethyl, n-propyl, i-propyl, allyl, benzyl and acetonyl. It was found that the sulfides react faster than the homologous ethers and that the substituent groups with the capacity of delocalize charge increase the reaction rate. Through natural bond orbital calculations, the transition states were characterized. The synchronicities and atomic charges of the studied reactions were determined. A computational study at the G3 level of theory on the thermochemistry of allyl ethers and sulfides was also carried out.     

Gas-phase thermolysis of sulfur compounds. Part VIII. Chloromethyl allyl,cyanomethyl allyl, 1-cyanoethyl allyl and neopentyl allyl sulfides

The pyrolyses of four alkyl allyl sulfides with substituents on the α? C atom of the alkyl moiety have been studied in a stirred-flow system over the temperature range 340-400°C and pressures between 2 and 12 torr. The only products formed are propene and thioaldehydes. The reactions showed first-order kinetics with the rate coefficients following the Arrhenius equations: Chloromethyl allyl sulfide: Cyanomethyl allyl sulfide: 1-cyanoethyl allyl sulfide: Neopentyl allyl sulfide: The effects of these and other substituents on the reactivity is discussed in relation with the stabilization of a polar six-centered transition state. The results support a non-concerted mechanism where the 1–5 α? H atom shift is assisted by its acidic character.     

Hydrogen abstraction from copolymers of fluorinated olefins and allyl or vinyl ethers by hydroxyl radicals: a computational quantum semi-empirical study

The theoretical study of hydrogen abstraction by hydroxyl radicals on two substrates (copolymers of fluorinated olefins and allyl or vinyl ethers) was carried using the MNDO, AM1 and PM3 quantum semi-empirical methods. This study was performed as a function of the site of hydrogen abstraction and of the computational method. The results of the calculations clearly show that the transition state is early along the reaction coordinate and pinpoint that the reactions are not under enthalpic control. The results provide evidence of the importance of the polar effects due to the fluorine atoms.     

气态烯丙醇光异构化反应的理论研究

本文采用自洽场分子轨道UHF/6-31G从头计算法, 辅以能量梯度法研究气相烯丙醇光异构化反应的机理。全部优化了T1态势能面上反应物、过渡态、中间体和产物的几何构型。基于Fukui提出的内禀反应坐标理论(IRC)计算这一体系的反应途径, 并针对各驻点进行MP2/6-31G的相关能校正, 得到该反应在激发态进行并为一经历双自由基中间体的分步反应的结论。支持了实验工作者提出的机理。     

Transition metal-free tandem pyridinyl–allyl–allyl cross-coupling reaction

A transition metal-free, direct one-pot domino allylation reaction of 2-pyridinyl Grignard reagents with polysubstituted allyl chlorides for the regioselective synthesis of pyridinyl-substituted 1,5-diene derivatives has been disclosed. The reaction presumably proceeded through the coupling of polysubstituted allyl chloride to 2-PyMgX, which was in situ generated from 2-bromopyridine with i-PrMgCl·LiCl.     

1,3-Dipolar cycloaddition between substituted phenyl azide and 2,3-dihydrofuran

A theoretical study was performed on the 1,3-dipolar cycloaddition between 2,3-dihydrofuran and substituted phenyl azide using Density Functional Theory (DFT) in combination with a 6-311++G(d,p) basis set. The optimum geometries for reactant, transition state and product, as well as the kinetic data, rate constants and reaction constant (ρ) were investigated to rationalise the substitution effects and reaction rates of the 1,3-dipolar cycloaddition process in various solvents. The DFT calculation and Frontier Molecular Orbital (FMO) theory as well as the atomic Fukui indices show that the electron-withdrawing substituents enhance the reaction constant (ρ > 0), especially in polar aprotic solvents. Consequently, small changes in the rate constant of the reaction in various solvents and geometric similarity between reactants and transition state structures were suggested as the early transition state mechanism for electron-withdrawing substituents. In addition, the slope of the Hammett plot and susceptibility of the reaction to electron-withdrawing substituents in various solvents confirmed the mechanism.     

DFT calculations on the retro-ene reactions, part II: allyl n-propyl sulfide pyrolysis in the gas phase

The mechanism and kinetic aspects of the retro-ene reaction of the Allyl n-propyl sulfide and its deuterated derivative were studied using four different types of density functional theory methods with eight different levels of the basis sets. The activation energies were determined at 550.65 K. As a consequence of our calculations, a transition state is concluded that consists of a polar six-center cyclic structure. We found that the combination B3PW91/6-311++G** produces activation energy values closer to the experimental ones, but the simpler combination B3LYP/6-31G* produces excellent values too in less time. Our calculations show that the activation parameters obtained from the B3 methods are better than those obtained using the BLYP method. The mechanistic studies on the reaction show that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations indicate that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K.     

Solvent effect on the sensitized photooxygenation of cyclic and acyclic α-diimines

The reaction of singlet molecular oxygen with a series of cyclic and acyclic α-diimines was studied. Time-resolved methods were used to measure total reaction rate constants and steady-state methods were used to determine chemical reaction rate constants. GC-MS was used to tentatively assign the reaction products. 5,6-Disubstituted cyclic α-diimines are singlet oxygen quenchers, but become more effective in polar solvents. A reaction mechanism involving a perepoxide intermediate or transition state leading to a hydroperoxide seems to be a key reaction path for product formation. A replacement of the phenyl substituent for a methyl substituent opens up an additional reaction involving a perepoxide-like exciplex, which increases singlet oxygen quenching of the cyclic α-diimines. The reactivity of 5,6-disubstituted cyclic α-diimines towards singlet oxygen is highly dependent on steric interactions arising from vicinal phenyl rings and from electronic effects. 1,4-Disubstituted acyclic α-diimines are, by comparison, moderate or poor singlet oxygen quenchers. Total rate constants are scarcely dependent on solvent properties, but instead correlate with the Hildebrand parameter. These results are explained in terms of a mechanism involving a dioxetane-like exciplex that gives rise to a charged intermediate leading to products.     

Origin of the regio- and stereoselectivity in palladium-catalyzed electrophilic substitution via bis(allyl)palladium complexes

Palladium-catalyzed allylic substitution of aryl allyl chlorides with aromatic and heteroaromatic aldehydes was performed in the presence of hexamethylditin. This procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis(allyl)palladium intermediate, which subsequently reacts with the aldehyde electrophile. The catalytic substitution reaction proceeds with high regio- and stereoselectivity. The stereoselectivity is affected by the steric and electronic properties of the allylic substituents. Various functionalities including NO(2), COCH(3), Br, and F groups are tolerated under the applied catalytic conditions. Density functional calculations at the B3PW91/DZ+P level of theory were applied to study the steric and electronic effects controlling the regio- and stereoselectivity of the electrophilic addition. The development of the selectivity was studied by modeling the various bis(allyl)palladium species occurring in the palladium-catalyzed substitution of cinnamyl chloride with benzaldehyde. It was found that the electrophilic attack proceeds via a six-membered cyclic transition state, which has a pronounced chair conformation. The regioselectivity of the reaction is controlled by the location of the phenyl group on the eta(1)-allyl moiety of the complex. The stereoselectivity of the addition process is determined by the relative configuration of the phenyl substituents across the developing carbon-carbon bond. The lowest energy path corresponds to the formation of the branched allylic isomer with the phenyl groups in anti configuration, which is in excellent agreement with the experimental findings.     

Pursuit of reinitiation efficiency of resonance‐stabilized monomeric allyl radical generated via “degradative monomer chain transfer” in allyl polymerization

For a deeper understanding of allyl polymerization mechanism, the reinitiation efficiency of resonance‐stabilized monomeric allyl radical was pursued because in allyl polymerization it is commonly conceived that the monomeric allyl radical generated via the allylic hydrogen abstraction of growing polymer radical from monomer, i.e., “degradative monomer chain transfer,” has much less tendency to initiate a new polymer chain and, therefore, this monomer chain transfer is essentially a termination reaction. Based on the renewed allyl polymerization mechanism in our preceding article, the monomer chain transfer constant in the polymerization of allyl benzoate was estimated to be 2.7 × 10^?2 at 80 °C under the polymerization condition, where the coupling termination reaction of growing polymer radical with allyl radical was negligible and, concurrently, the reinitiation reaction of allyl radical was enhanced significantly. The reinitiation efficiencies of monomeric allyl radical were pursued by the dead‐end polymerizations of allyl benzoate at 80, 105, and 130 °C using a small amount of initiators; they increased remarkably with raised temperature. Thus, the enhanced reinitiation reactivity of allyl radical at an elevated temperature could bias the well‐known degradative monomer chain transfer characteristic of allyl polymerization toward the chain transfer in common vinyl polymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Polymerization of allyl esters of unsaturated acids. I. Cyclopolymerization of methyl allyl maleate

Radical polymerization of methyl allyl maleate is kinetically discussed in terms of cyclopolymerization using 2,2′-azobisisobutyronitrile (AIBN) as an initiator and benzene as a solvent at 60°C. The ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allyl and vinyl radicals, K_A and K_V, are estimated to be 9.7 and 1.35 mole/liter by fitting the kinetic equations obtained here to the dependence of the degree of cyclization on monomer concentration, respectively; the large difference between K_A and K_V is also discussed in detail. On the basis of these results the formation mode and the sequence distribution of the structural units of the polymer produced are discussed in detail; thus, for the polymer obtained in the bulk polymerization, about 90% of the cyclic structures can be formed via the intramolecular attack of uncyclized allyl radical on maleic double bond and the probability of succession of the cyclic structural units in diad sequence is exemplified as 0.27.     

Substituent effects on the rate constants for the photo-Claisen rearrangement of allyl aryl ethers

The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH2-CH=CH2) has been examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisen rearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from the well-established radical pair mechanism. The excited singlet state properties (absorption spectra, fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallel set of unreactive 4-X- and 3-X-anisoles 4. The excited-state properties of three substituted 4-X-aryl 4-(1-butenyl) ethers 14 (ArOCH2CH2-CH=CH2) were also examined. The model compounds 4 and the reactive allyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of, the rate constant for homolytic cleavage from S(1) of the allyl ethers to give the radical pair. The difference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtain values of for all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2 orders of magnitude with electron-donating groups (CH3O, CH3) accelerating the reaction and electron-withdrawing ones (CN, CF3) slowing it down. The parallel range of rate constants observed in both methanol and cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantum yields of reaction (Phi(r)) for several of the more reactive ethers demonstrate that neither these values nor rate constants of reaction derived from them are reliable measures of the actual excited-state process. In fact, the values are significantly lower than the ones, indicating that the radical pairs undergo recombination to generate starting material. Finally, the rate constants were found to parallel a trend for the change in bond dissociation energy (deltaBDE) for the O-C (allyl) bond of the allyl ethers, indicating that other possible substituent effects are of minor importance.     

Solvophobic Acceleration of a Diels–Alder Reaction in True Solutions in Organic Solvents

The rate of Diels–Alder reaction of diene 9,10‐bis(hydroxymethyl)anthracene with dienophile N‐ethylmaleimide was studied in a series of solvents with different polarity and hydrogen‐bonding ability. Enthalpies and entropies of activation were determined from the temperature dependences of the rate constants. Rate acceleration in nonaqueous protic solvents such as glycerol, propylene, and ethylene glycols was observed. In addition, enthalpy versus entropy of activation plots show a compensation pattern different from the other considered solvents, which can be linked with the solvophobic effects observed in polyhydric alcohols. However, the solvophobic acceleration was not as strong as the hydrophobic acceleration in water. Hydrogen bonding of the reactants and transition state with solvent also influences the reaction rate. The studied reaction is slightly promoted in hydrocarbon solvents in comparison with aprotic polar solvents. This was explained by hydrogen bonding of the hydroxyl groups of diene with dienophile in transition state, which requires prior breaking of the hydrogen bonds of these groups with polar solvent molecules.     

Kinetics of the HBr-catalyzed photoisomerization of the allyl halides

The kinetics of the gas phase bond isomerization of allyl fluoride, allyl chloride and allyl bromide, catalyzed by HBr and ultraviolet light, has been studied in the temperature range of 150–250° and at pressures of 3.5 to 50 mm. The reactions are very clean, first order in allyl halide and HBr, and have a light intensity exponent of unity. A quantum yield for allyl chloride of 3200 indicates a chain reaction. Dilution with inert gases is almost without effect, indicating that excited state intermediates are not involved. A small wall effect is observed. The evidence indicates a free radical reaction, involving hydrogen abstractions by bromine atoms, with replacement at the other end of the allylic radical.     

The Transition State of Free Radical SH2′ Reaction of Allyl Chloride with Organomercurial Study by Inverse Secondary α‐Deuterium Kinetic Isotope Effect

The concerted mechanism of free radical S_H2′ reaction of 2‐substituted allyl chloride was suggested again by inverse secondary α‐deuterium isotope effect. The transition state of free radical S_H2′ reaction of allyl chlorides seems to be symmetrical and is not as early as that of a free radical addition reaction.     

Acid-catalyzed nucleophilic aromatic substitution: experimental and theoretical exploration of a multistep mechanism

The mechanism for the acid-mediated substitution of a phenolic hydroxyl group with a sulfur nucleophile has been investigated by a combination of experimental and theoretical methods. We conclude that the mechanism is distinctively different in nonpolar solvents (i.e., toluene) compared with polar solvents. The cationic mechanism, proposed for the reaction in polar solvents, is not feasible and the reaction instead proceeds through a multistep mechanism in which the acid (pTsOH) mediates the proton shuffling. From DFT calculations, we found a rate-determining transition state with protonation of the hydroxyl group to generate free water and a tight ion pair between a cationic protonated naphthalene species and a tosylate anion. Kinetic experiments support this mechanism and show that, at moderate concentrations, the reaction is first order with respect to 2-naphthol, n-propanethiol, and p-toluenesulfonic acid (pTsOH). Experimentally determined activation parameters are similar to the calculated values (Delta H exp not equal =105+/-9, Delta H calcd not equal =118 kJ mol(-1); Delta G exp not equal =112+/-18, Delta G calcd not equal =142 kJ mol(-1)).     

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