Reactions of unsymmetrically substituted allyl radicals with methyl radicals

The Hg(6³P₁) photosensitized decompositions of 3-methyl-1-butene, 2-methyl-2-butene, 3,3-dimethyl-1-butene, and 2,3-dimethyl-1-butene have been used to generate 1-methylallyl, 1,2-dimethylallyl, 1,1-dimethylallyl, and 1,1,2-trimethylallyl radicals in the gas phase at 24 ± 1°C. From a study of the relative yields of the CH₃ combination products, the relative reactivities of the reaction centers in each of these unsymmetrically substituted ambident radicals have been determined. The more substituted centers are found to be the less reactive, and this is ascribed primarily to greater steric interaction at these centers during reaction. Measurement of the ratio of trans- to cis-2-pentene formed from the 1-methylallyl radical, combined with published values for this ratio at higher temperatures, enabled the differences in entropy and heat of formation of the trans- and cis-forms of this radical to be calculated as 0.62 ± 0.85 J mol^?1 K^?1 and - 0.63 ± 0.25 kJ mol^?1, respectively, at 298K. Approximate values of the disproportionation/combination ratios for reaction of CH₃ with 1,1-dimethylallyl and 1-methylallyl have been estimated and used to compute rate constants for the recombinations of tert-butyl and isopropyl radicals that are in agreement with recently published data.     

Combinations of alkyl radicals with alkyl and substituted allyl radicals

In contrast with expectations based upon simple collision theory, reactions of alkyl radicals with substituted allyl radicals yield cross-combination ratios of =2. Reactions of CH ₃ ^\ with some C₅ radicals give >2, in accordance with theory.

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, =2. CH₃ ^\ C₅ >2, .

     

Palladium-catalyzed direct allylation of fluorinated benzothiadiazoles with allyl chlorides

A palladium catalyzed cross-coupling of fluorinated benzothiadiazoles (FBTs) with allyl chlorides is reported. The significant feature of this method is synthetic simplicity, providing a straightforward access to unsymmetrical and symmetrical alkylated FBT derivatives that are of interest in organic electronic and optoelectronic materials.     

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.     

Competing sigmatropic shift rearrangements in excited allyl radicals

The competition between rearrangement of the excited allyl radical via a 1,3 sigmatropic shift versus sequential 1,2 shifts has been observed and characterized using isotopic substitution, laser excitation, and molecular beam techniques. Both rearrangements produce a 1-propenyl radical that subsequently dissociates to methyl plus acetylene. The 1,3 shift and 1,2 shift mechanisms are equally probable for CH(2)CHCH(2), whereas the 1,3 shift is favored by a factor of 1.6 in CH(2)CDCH(2). The translational energy distributions for the methyl and acetylene products of these two mechanisms are substantially different. Both of these allyl dissociation channels are minor pathways compared to hydrogen atom loss.     

The stability of allyl radicals following the photodissociation of allyl iodide at 193 nm

The photodissociation of allyl iodide (C3H5I) at 193 nm was investigated by using a combination of vacuum-ultraviolet photoionization of the allyl radical, resonant multiphoton ionization of the iodine atoms, and velocity map imaging. The data provide insight into the primary C-I bond fission process and into the dissociative ionization of the allyl radical to produce C3H3+. The experimental results are consistent with the earlier results of Szpunar et al. [J. Chem. Phys. 119, 5078 (2003)], in that some allyl radicals with internal energies higher than the secondary dissociation barrier are found to be stable. This stability results from the partitioning of available energy between the rotational and vibrational degrees of freedom of the radical, the effects of a centrifugal barrier along the reaction coordinate, and the effects of the kinetic shift in the secondary dissociation of the allyl radical. The present results suggest that the primary dissociation of allyl iodide to allyl radicals plus I*(2P(1/2)) is more important than previously suspected.     

Rates of reaction of allyl radicals: A reassessment

Values of the rate constants for allyl-radical abstraction from toluene and allyl-radical recombination have been recalculated in the light of recent data on butene-1 decomposition. The rate of recombination is in good agreement with that found by Van den Bergh and Callear [13] and the cross-combination ratio for the allyl + methyl system has a “normal” value of almost 4. It is concluded that allyl radicals behave like alkyl radicals in respect of their rates of reaction with other radicals.     

Photooxidation of copolymers of fluorinated olefins and allyl or vinyl ethers

This article reports the results on the photooxidation mechanisms of two copolymers of fluorinated olefins and allyl or vinyl ethers. It is shown that the presence of the fluorine atoms influences strongly both the orientation of the reaction and the photooxidation kinetics. Due to the neighbouring fluorine atoms, the methylene groups in α-position of the oxygen of the ether groups are not equivalent regarding oxidation and the secondary carbon becomes more oxidable than the tertiary one. Because these unexpected results were obtained, the study has been extended to non fluorinated polyethers. On the basis of the results obtained, a general mechanism of the primary oxidation of polyethers is given, and the role played by the fluorine atoms on the orientation of the reactions is discussed.     

The power of trichlorosilylation: isolable trisilylated allyl anions,allyl radicals,and allenyl anions

Treatment of hexachloropropene (Cl₂C C(Cl)–CCl₃) with Si₂Cl₆ and [nBu₄N]Cl (1 : 4 : 1) in CH₂Cl₂ results in a quantitative conversion to the trisilylated, dichlorinated allyl anion salt [nBu₄N][Cl₂C C(SiCl₃)–C(SiCl₃)₂] ([nBu₄N][1]). Tetrachloroallene Cl₂C C CCl₂ was identified as the first intermediate of the reaction cascade. In the solid state, [1]⁻ adopts approximate C_s symmetry with a dihedral angle between the planes running through the olefinic and carbanionic fragments of [1]⁻ of C C–Si//Si–C–Si = 78.3(1)°. One-electron oxidation of [nBu₄N][1] with SbCl₅ furnishes the distillable blue radical 1˙. The neutral propene Cl₂C C(SiCl₃)–C(SiCl₃)₂H (2) was obtained by (i) protonation of [1]⁻ with HOSO₂CF₃ (HOTf) or (ii) H-atom transfer to 1˙ from 1,4-cyclohexadiene. Quantitative transformation of all three SiCl₃ substituents in 2 to Si(OMe)₃ (2^OMe) or SiMe₃ (2^Me) substituents was achieved by using MeOH/NMe₂Et or MeMgBr in CH₂Cl₂ or THF, respectively. Upon addition of 2 equiv. of tBuLi, 2^Me underwent deprotonation with subsequent LiCl elimination, 1,2-SiMe₃ migration and Cl/Li exchange to afford the allenyl lithium compound Me₃Si(Li)C C C(SiMe₃)₂ (Li[4]), which is an efficient building block for the introduction of Me, SiMe₃, or SnMe₃ (5) groups. The trisilylated, monochlorinated allene Cl₃Si(Cl)C C C(SiCl₃)₂ (6), was obtained from [nBu₄N][1] through Cl⁻-ion abstraction with AlCl₃ and rearrangement in CH₂Cl₂ (1˙ forms as a minor side product, likely because the system AlCl₃/CH₂Cl₂ can also act as a one-electron oxidant).

Treatment of hexachloropropene (Cl₂C C(Cl)–CCl₃) with Si₂Cl₆ and [nBu₄N]Cl (1 : 4 : 1) in CH₂Cl₂ results in a quantitative conversion to the trisilylated, dichlorinated allyl anion salt [nBu₄N][Cl₂C C(SiCl₃)–C(SiCl₃)₂] ([nBu₄N][1]).     

Kinetics of the reaction between H atoms and allyl radicals

The reaction between H and C₃H₅ has been studied at 291 K. Exciplex laser flash photolysis at 193.3 nm of hexa-1,5-diene-He mixtures generated both H and C₃H₅ ([H] ? [C₃H₅]), which were detected in time-resolved mode by resonance fluorescence and absorption spectroscopy, respectively. Rate coefficients are presented at four pressures in the range 98 ? P/torr ? 400; no clear pressure-dependence is found in this range of pressures and the mean rate coefficient is (2.8 ± 1.0) × 10^?10 cm³ molecule^?1 s^?1. Calculations based on the Troe factorization method confirm that this reaction is near its high-pressure limit under the experimental conditions.     

Cage effects in recombination of allyl radicals in irradiated polyethylene

A cage model has been presented to describe the kinetics of recombination of radicals in solid polymer. The theory includes Torrey's treatment for jump diffusion and radiative boundary condition in the diffusion equation to account for the hindrance to the diffusion of macroradicals and the finite cage process of recombination reaction, respectively. The result has been applied to the interpretation of data on the decay of allyl radical in irradiated polyethylene.     

Dynamics of collisions of hydroxyl radicals with fluorinated self-assembled monolayers

We present a classical trajectory study of the dynamics of collisions between OH radicals and fluorinated self-assembled monolayers (F-SAMs). The gas/surface interaction potential required in the simulations has been derived from high-level ab initio calculations (focal-point-CCSD(T)/aug-cc-pVQZ) of various approaches of OH to a model fluorinated alkane. The two lowest-energy doublet potential energy surfaces considered in the electronic structure calculations have been averaged to produce a pairwise analytic potential. This analytic potential has been subsequently employed to propagate classical trajectories of collisions between OH and F-SAMs at initial conditions relevant to recent experiments on related systems. The calculated rotational distributions of the inelastically scattered OH agree well with the experiment, which serves to validate the accuracy of the simulations. Investigation of the dynamics of energy transfer for different initial rotational states of OH indicates that an increase in the initial rotation of OH results in increases in both the final average OH rotational and translational energy and in a slight decrease in the amount of energy transferred to the surface. Analysis of the dynamics as a function of the desorption angle of OH from the surface shows that while there is a correlation between the final scattering angle and OH’s amount of final translational energy, the amount of rotational energy in OH is largely independent of the desorption angle. The mechanism of the collisions is found to be mostly direct; in about 90% of most trajectories, OH only collides with the surface once before desorbing, which exemplifies the rigidity of fluorinated monolayer surfaces and their inability to efficiently accommodate gas species.     

Shock wave study on the thermal unimolecular decomposition of allyl radicals

We present the first direct study on the thermal unimolecular decomposition of allyl radicals. Experiments have been performed behind shock waves, and the experimental conditions covered temperatures ranging from 1125 K up to 1570 K and pressures between 0.25 and 4.5 bar. Allyl radicals have been generated by thermal decomposition of allyl iodide, and H-atom resonance absorption spectroscopy has been used to monitor the reaction progress. A marked pressure dependence of the rate constant has been observed which is in agreement with the results from a master equation analysis. However, our experimental results as well as our Rice-Ramsperger-Kassel-Marcus calculations seem to contradict the results of Deyerl et al. (J. Chem. Phys. 1999, 110, 1450) who investigated the unimolecular decomposition of allyl radicals upon photoexcitation and tried to deduce specific rate constants for the unimolecular dissociation in the electronic ground state. At pressures around 1 bar we extracted the following rate equation: k(T) = 5.3 x 10(79)(T/K)(-19.29) exp[(-398.9 kJ/mol)/RT] s(-1). The uncertainty of the rate constant calculated from this equation is estimated to be 30%.     

Formation of allyl radicals in the adsorption of propylene on zinc oxide

It has been found by the ESR technique that the adsorption of propylene on zinc oxide leads to the formation of a -allyl radical. It is suggested that the radical species of the surface allyl compound on this catalyst is responsible for its high selectivity in the dehydroaromatization of propylene to benzene.

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, - . , .

     

X-ray structural and dynamic behaviour study of allyl palladium compounds with fluorinated benzenethiolate bridges

In solution, allyl palladium homobimetallic complexes bridged by fluorinated benzenethiolates, [{Pd(μ-SR)(η³-C₃H₅)}₂] where R=C₆F₅, 1; C₆HF₄-4, 2; C₆H₄F-2, 3; C₆H₄F-3, 4 and C₆H₄F-4, 5, are found as a mixture of syn/anti and cis/trans isomers. The variable temperature ¹H and ¹⁹F NMR study of these compounds show that the four isomers undergo interconversion through two probable mechanisms, allyl rotation assisted by the solvent and inversion of the configuration at the sulphur atoms. The X-ray crystal structure determination of [{Pd(μ-SC₆F₅)(η³-C₃H₅)}₂] and [{Pd(μ-S C₆HF₄-4)(η³-C₃H₅)}₂] shown both complexes to be bimetallic with the metal centres found in a slightly distorted square planar environments.     

A novel photoredox-active group for the generation of fluorinated radicals from difluorostyrenes

A 4-tetrafluoropyridinylthio group was suggested as a new photoredox-active moiety. The group can be directly installed on difluorostyrenes in a single step by the thiolene click reaction. It proceeds upon visible light catalysis with 9-phenylacridine providing various difluorinated sulfides as radical precursors. Single electron reduction of the C–S bond with the formation of fluoroalkyl radicals is enabled by the electron-poor azine ring. The intermediate difluorinated sulfides were involved in a series of photoredox reactions with silyl enol ethers, alkenes, nitrones and an alkenyl trifluoroborate.

A new photoredox-active group was applied for the generation of fluorinated radicals from difluorostyrenes under blue light irradiation.

Organofluorine compounds have gained increasing attention due to their utility in medicinal chemistry and agrochemistry in the last few decades.¹ Among various methods of fluorine incorporation, major attention in recent years has been devoted to radical pathways of fluoroalkylation by visible light photoredox catalysis.^2,3 This approach has attracted much attention because of the exceptionally mild reaction conditions and functional group tolerance. Known reagents, which are suitable for efficient radical fluoroalkylation such as halides, sulfonyl chlorides, sulfones, sulfinates, and Umemoto and Togni reagents (Scheme 1a),³ suffer from limited structural diversity and complicated preparation. Indeed, besides a significant amount of CF₃ and CF₂H derivatives, most of the other R_f radical precursors require multistep preparation under harsh conditions.^3c,4 As a result, operationally simple methods are still needed.Open in a separate windowScheme 1Generation of fluorinated radicals.Herein we report a simple and diversity-oriented strategy based on the direct introduction of a photoredox-active group into the fluorinated substrate. Our approach involves the addition of tetrafluoropyridine-4-thiol (2, PyfSH) to readily accessible difluorostyrenes followed by photocatalytic reduction with fluoroalkyl radical formation (Scheme 1b). As is well known, the thiol-ene reaction is an excellent instrument for difluorostyrene functionalization leading to sulfides.⁵ However, C–S bond reduction in common sulfides is challenging owing to their unfavorable redox potential compared to their S-oxygenated counterparts.^6,7 To overcome this obstacle, we propose to use an electron-withdrawing fluorinated pyridine moiety, which would be susceptible to SET reduction (Scheme 1a).Thus, we report the application of sulfides as a new class of readily available, bench-stable and easy-to-handle reagents for radical fluoroalkylation under visible-light photoredox catalysis. It is worth mentioning that our concept allows the synthesis of precursors of various R_f radicals in a single step from easily accessible compounds, which is often hard in practice for other photoredox-active groups.^3c,4 Thiol 2 can be easily prepared from commercially available pentafluoropyridine⁸ and the initial difluorostyrenes come from aldehydes by the Wittig-type reaction.⁹Styrenes 1 serve as a basis for a number of ionic synthones^9b,10,11 and recent developments in visible light photoredox catalysis allowed us to replace most of them with complementary radical synthones (Scheme 1c).^12,13 Herein, we disclose the last “blind spot” in the map of radical analogues of ionic difluorostyrene synthones. It should be noted that due to facile elimination of the fluorine atom in polar reactions, only a few examples of difluoroalkyl cation synthons have been described previously.¹¹To perform the addition of thiol 2 to styrenes 1, we applied a protocol involving the activation of thiols based on proton coupled electron transfer recently developed by our group.^5,14 Thus, screening of reaction conditions allowed us to identify the optimal system: 9-phenylacridine (PC-I) as the photocatalyst under blue light irradiation (see the ESI for details^†). The reaction is performed in cyclohexane and requires a virtually stoichiometric amount (1.1 equiv.) of the thiol 2. A series of styrenes 1 were reacted with thiols 2 leading to difluorinated sulfides 3 (Table 1). Aromatic and heteroaromatic substrates provided sulfides 3 in good to excellent yields. The only exception was the furan substituted product 3j, which had a low yield due to instability of 1j.¹⁵ The product 3k derived from α-pentyl-substituted difluorostyrene was also obtained. In contrast to the reactions with styrenes, only traces of sulfides were obtained with aliphatic gem-difluoroalkenes (see the ESI for details^†). It should also be pointed out that all reactions shown in Table 1 were performed on a 5 mmol scale.Addition of thiol 2 to gem-difluorostyrenes 1^a