Benzoyloxy Radical Research Articles

Stepwise generation of spin-polarized radicals and their reactions in the photolysis of an oxime ester compound as studied by pulsed EPR spectroscopy

To generate spin-polarized free radicals, the photolysis of 1,2-octane-dione, 1-[4-(phenylthio)-2-(o-benzoyloxime)] (OXE01) was investigated using time-resolved electron paramagnetic resonance (TR-EPR) and pulsed EPR. This photolysis proceeds through α-cleavage of the N-O bond of OXE01 to yield iminyl and benzoyloxy radicals, the latter of which is converted to a phenyl radical by decarboxylation. The phenyl radical promptly abstracts hydrogen atoms from the surrounding solvent molecules to produce a solvent dehydrogenated radical (solvent radical). The spin polarization created in the initial OXE01 triplet excited state was preserved during these series of reactions, which enhanced EPR sensitivity. This enabled us to observe the TR-EPR spectra of the intermediate free radicals, except phenyl. The decarboxylation rate of the benzoyloxy radical was measured in various solvents by probing the free induction decay signal of the product solvent radicals using pulsed EPR. This new method for preparing free radicals with spin-polarization was applied to determine the addition reaction rate constants of the cyclohexyl radical, a solvent radical, to olefins using pulsed EPR monitoring of the electron spin echo signals of the radical.

Effects of Lactobacillus plantarum TAR4, TAR7 and TAR8 Isolated From Tapai On The Growth Of Cutibacterium acnes

Acne is a skin disease involving clogged hair follicles with dead skin cells secreted from the skin. It is a common skin condition and at least 85% of the people in the United States suffered from this problem (1). Despite not being a life threatening disease, acne may form permanent scars on skin and leading to emotional problems such as decreased self- esteem, social withdrawal, depression, and frustration. Cutibacterium acnes (C. acnes) is Gram-positive bacteria, slow-growing aerotolerant bacteria and involve in the pathogenesis of acne. Medication with benzoyl peroxide is often used to treat acne. Benzoyl peroxide has bactericidal effect which kills surface bacteria by the cleavage of peroxide bond of benzoyl peroxide to form 2 benzoyloxy radicals. The interaction of the radicals with bacterial proteins would affect the bacteria function, viability and lead to decreased production of sebum around follicles. However, the use of benzoyl peroxide is associated with adverse effect such as skin irritation. Besides that, oral antibiotics are also prescribed to control the growth of C. acnes and inflammation. There are potential harm of long term oral antibiotics treatment. Hence, this study aims to investigate the effect of probiotic Lactobacillus plantarum (L. plantarum) TAR4, TAR7 and TAR8 isolated from a fermented food, Tapai on the growth of C. acnes using co-culture technique with the purpose to offer a safe alternative solution to the use of benzoyl peroxide medication and oral antibiotics for acne treatment. The results shown that the L. plantarum TAR4, TAR7 and TAR8 exhibited significant (P < 0.05) inhibitory effects on the growth of C. acnes.

Synthesis of uniform submicron poly(lactic acid)-based particles/capsules by radical precipitation polymerization

Poly(l-lactic acid) (PLLA) is a well-known biopolymer, usually synthesized via step-growth or ring-opening polymerization from lactic acid or a lactide monomer, respectively. PLLA microspherical particles are produced by dispersion polymerization with a ring-opening lactide monomer using a particular copolymer chain as a stabilizer. This is not easy to achieve when dehydration is needed. Here, a robust and simple synthesis of a nearly monodisperse, submicron PLLA-based particle/capsule was proposed via radical precipitation polymerization without the use of surfactant. A commercial PLLA was first glycolyzed with ethylene glycol to obtain a low molecular weight glycolyzed PLLA (GPLLA). Then, the GPLLA was copolymerized with methacrylic acid and ethylene glycol dimethacrylate monomers using a benzoyl peroxide initiator. Active sites on the GPLLA backbone were generated by hydrogen abstraction of benzoyloxy radicals that further copolymerized before self-assembly to form the polymer particles. Uniform particle size of about 580 nm with a low polydispersity index (PDI) of 0.012 was obtained. This method was also implemented to produce nearly monodisperse capsules containing linalool. The particle size of PLLA-based capsules was about 280 nm with narrow particle size distribution (PDI of 0.120). The PLLA-based capsules effectively inhibited microbial growth of Staphylococcus aureus, Escherichia coli and Candida albicans and were not toxic to human cells.

External-oxidant-free amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines.

A new copper-catalyzed two-component amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines as the benzoyloxy sources is developed. Chemoselectivity of this method toward amino-benzoyloxylation or oxy-benzoyloxylation of alkenyl ketoximes relies on the position of the tethered olefins, and provides an external-oxidant-free alkene difunctionalization route that directly utilizes O-benzoylhydroxylamines as the benzoyloxy radical precursors and internal oxidants for the divergent synthesis of cyclic nitrones and isoxazolines.

Low-Temperature Photolysis of Benzoyl Peroxide

The photolysis of dry benzoyl peroxide (BP) at 77 K in the 480–236 nm range of wavelengths and an ethanol solution is studied via EPR. It is determined that the main photochemical process in irradiating BP at λ = 480–365 nm is the direct photodissociation of the O–O peroxide bonds and C–H bonds of benzene rings to form benzoyloxy radicals and atomic hydrogen, which likely forms via a two-quantum mechanism. The quantum yield of primary intermediates is φ ≈ 2.7 × 10−5. Upon subsequent irradiation of a sample using light with λ = 300–236 nm, benzoyloxy radicals release carbon dioxide and transform into phenyl radicals with a quantum yield of φ ≈ 3 × 10−4. At 120–145 K, phenyl radicals react with double bonds of aromatic rings of PB to form phenyl-substituted cyclohexadienyl radicals vanishing at 273 K. Quantum chemical calculations of the radical structures of photolyzed peroxide are made with the density functional approach (B3LYP/6-311g), and good agreement between the experimental and theoretical parameters is noted. Calculations show that in the structure of benzoyloxy radicals, the spin density is distributed almost equally between the two oxygen atoms of the radical, due to the conjugation of the unpaired electron to the lone p electron pair on the oxygen atom of the C=O group. In the photolysis of BP solutions in ethanol at λ ≥ 365 nm, solvent radicals form via sensitized photolysis through the reaction between primary benzoyloxy radicals and solvent molecules.

Investigation of the formation of benzoyl peroxide, benzoic anhydride, and other potential aerosol products from gas–phase reactions of benzoylperoxy radicals

The secondary organic aerosol (SOA) products of the reaction of benzaldehyde with Cl atoms and with OH radicals in air in the absence of NOx were investigated in an environmental chamber in order to better understand the possible role of organic peroxy radical self-reactions in SOA formation. SOA products and authentic standards were analyzed using mass spectrometry and liquid chromatography, and results show that the yields of benzoyl peroxide (C6H5C(O)OO(O)CC6H5) and benzoic anhydride (C6H5C(O)O(O)CC6H5), two potential products from the gas-phase self-reaction of benzoylperoxy radicals (C6H5C(O)OO·), were less than 0.1%. This is in contrast to results of recent studies that have shown that the gas-phase self-reactions of β-nitrooxyperoxy radicals formed from reactions of isoprene with NO3 radicals form dialkyl peroxides that contribute significantly to gas-phase and SOA products. Such reactions have also been proposed to explain the gas-phase formation of extremely low volatility dimers from autooxidation of terpenes. The results obtained here indicate that, at least for benzoylperoxy radicals, the self-reactions form only benzoyloxy radicals. Analyses of SOA composition and volatility were inconclusive, but it appears that the SOA may consist primarily of oligomers formed through heterogeneous/multiphase reactions possibly involving some combination of phenol, benzaldehyde, benzoic acid, and peroxybenzoic acid.

Light-induced Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines

A variety of 4-substituted 5-acetyl- and 5-carboethoxy-2-oxo-1,2,3,4-tetrahydropyrimidines were oxidized under UV irradiation in the presence or absence of benzoyl peroxide. The nature of the substituents on the 4- and 5-positions of the heterocyclic ring affects the rate of photo-oxidation, and irradiation of these compounds in the presence of benzoyl peroxide decreases the time of reaction drastically. Removal of 4-H by a benzoyloxy radical under formation of a trihydropyrimidinoyl radical intermediate occurs in the rate-determining step. The stability of this benzylic and allylic radical intermediate is affected by the nature and the position of the additional substituent on the phenyl group located at C-4.

The influence of tertiary aromatic amines on the BPO initiated cure of unsaturated epoxy polyesters with styrene studied by non-isothermal DSC

The influence of tertiary aromatic amines on the course of BPO initiated cure reaction of unsaturated epoxy polyesters with different styrene content has been studied by non-isothermal differential scanning calorimetry. Unsaturated epoxy polyesters prepared from cyclohex-4-ene-1,2-dicarboxylic anhydride, maleic anhydride and suitable aliphatic glycol: ethylene glycol or 1,4-butanediol or 1,6-hexanediol were dissolved in vinyl monomer (styrene) resulting in a styrene content of 20–80% by weight. The styrene solutions of polyesters were subjected to the cure reaction with suitable curing agent: benzoyl peroxide (BPO) used in various concentration (0.5–3.0 wt%) or the mixture of BPO/stoichiometric ratio of chosen tertiary aromatic amines: (N,N-dimethylbenzylamine (BDMA) or 2,4,6-tri(dimethylaminomethyl) phenol (DMP-30). The curing characteristic such as: temperature of the cure initiation (Tonset), peak maximum temperature (Tmax), final cure temperature (Tend), heat generated during the cure reaction (ΔH) were evaluated. It has been found that the course of the cure reaction depended on the styrene content in prepared compositions and the initiating system used. The performed investigations confirmed that one of the applied tertiary aromatic amine: BDMA was an effective promoter for BPO decomposition process, causing a decrease in characteristic curing temperatures of unsaturated epoxy polyesters with styrene. The organic peroxide-amine interactions caused the promotion of BPO decomposition to benzoyloxy radicals at lower temperatures and thus accelerated the copolymerization process. However, DMP-30 was a very sluggish promoter for BPO decomposition, probably due to the presence of both hydroxyl group, their ortho-position to two of three amine groups and their branched structure. The redox reaction between BPO and DMP-30 probably resulted in non-radical products or radical formation which was incapable of initiating the polymerization reaction.

ChemInform Abstract: The Generation and Reactivity of Oxygen Centered Radicals from the Photolysis of Derivatives of N-Hydroxy-2-thiopyridone.

Abstract Oxygen centered radicals, formed by the photolysis of derivatives of N -hydroxy-2-thiopyridone, have been trapped by vinyl ethers. In the case of the benzoyloxy radical, smooth deoxygenation is effected by III P compounds. Evidence for the formation of the dimethyl t butylsilyloxy and of the hydroxy radical is also presented.

A Quantum-chemical study of the mechanism of formation of styrene polymerization centers under initiation by the ferrocene—benzoyl peroxide system

The mechanism of formation of free polystyrene growth radicals and radicals bound to complexes with ferrocene when polymerization was performed in the presence of the ferrocene—benzoyl peroxide initiating system was studied by quantum-chemical calculations with the use of the PRIRODA program and the PBE/3z method. The calculated structure of the charge transfer complex between ferrocene and benzoyl peroxide was in agreement with the experimental data. The calculated heat effects of possible complex decomposition reactions showed that radical formation easily occurred in the presence of the monomer only. The most favorable energetically scheme of the formation of the active centers of the complex-radical polymerization of styrene, including the intermediate formation of the ferrocene—benzoyloxy radical complex, was suggested.

Unravelling the ultrafast photodecomposition mechanism of dibenzoyl peroxide in solution by time-resolved IR spectroscopy

The ultrafast photo-fragmentation of dibenzoyl peroxide (DBPO) is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidate the dissociation mechanism. With the help of (13)C-labeled DBPO it was possible to unambiguously assign transient IR bands in the fingerprint region to the benzoyloxy radical. Our experiments show that the fragmentation is controlled by the S(1)-lifetime of DBPO and within 0.4 +/- 0.2 ps leads to a benzoyloxy/phenyl radical pair plus CO(2)via concerted bond breakage of the O-O and the phenyl-C(carbonyl) bond. 20% of the radical pairs geminately recombine to phenyl benzoate on a timescale of 70 ps.

α-Tocopherol amplifies benzoyl peroxide free radical decomposition in a chemical system

Benzoyl peroxide is commonly used in the treatment of acne, even though some adverse effects have been reported, probably mediated by the formation of peroxide-derived free radicals and the depletion of antioxidants. In the present work we have studied, in a chemical system, the effect of α-tocopherol on benzoyl peroxide radical decomposition to analyse the presence of an interaction between these two compounds, leading to an enhanced peroxide-cytotoxicity, as we have previously reported. Under our experimental conditions α-tocopherol strongly amplified the peroxide free radical decomposition occurring either in the presence or in the absence of UV irradiation, and lead to the formation of an unknown radical species in addition to benzoyloxy, phenyl and tocopheroxyl free radicals. The results of this study show that the enhancement of benzoyl peroxide toxicity in cells exposed simultaneously to this peroxide and α-tocopherol, is likely due to the generation of the detected radical species.

Chemical modification of styrene–butadiene–styrene co-polymer by grafting of N-carbamyl maleamic acid

A styrene–butadiene–styrene (SBS) block co-polymer was functionalized using different amounts of N-carbamyl maleamic acid (NCMA) and benzoyl peroxide as initiator. NCMA, which is a bifunctional monomer, was synthesized in our laboratories. The concentration of NCMA used in the functionalization of SBS ranged from 0.5 to 3% (w/w) based on the co-polymer mass. Benzoyloxy radicals generated from the thermal decomposition of benzoyl peroxide initiated the grafting reaction. The concentration of the initiator was kept constant at 0.076% (w/w). FT-IR spectroscopy was used to determine the amount of NCMA effectively grafted onto the SBS. The maximum amount of monomer grafted was about 0.3% (w/w) when the SBS was modified with 1% (w/w) NCMA. The effect of grafting on the surface properties and the adhesion to polyurethane adhesive of the modified SBS were evaluated. Contact angle values were obtained using liquid droplets. When the concentration of the NCMA used in the grafting reaction was 1% (w/w), the contact angles with water on original and modified SBS were 95° and 77°, respectively. Adhesion properties were evaluated by standard peel tests employing a commercial polyurethane adhesive. The modified SBS having the largest amount of NCMA displayed a T-peel strength value 5-times higher than the corresponding value measured with the original SBS.

Ultrafast Decarboxylation of Carbonyloxy Radicals: Influence of Molecular Structure

Experimental and theoretical investigations on the ultrafast photoinduced decomposition of three tert-butyl peroxides of general structure R−C(O)O−O−tert-butyl with R = phenyloxy, benzyl, or naphthyloxy in solution are presented. Photoinduced O−O bond scission occurs within the time resolution (200 fs) of the pump−probe experiment. The subsequent dissociation of photochemically excited carbonyloxy radicals, R−CO2, has been monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R−CO2 is simulated via statistical unimolecular rate theory using molecular energies, geometries, and vibrational frequencies obtained from density functional theory (DFT) calculations. The results are compared with recent data for tert-butyl peroxybenzoate (R = phenyl). While benzoyloxy radicals exhibit nanosecond to microsecond lifetimes at ambient temperature, insertion of an oxygen atom or a methylene group between the phenyl or naphthyl chromophore and the CO2 ...

Experimental and Theoretical Investigations of the Ultrafast Photoinduced Decomposition of Organic Peroxides in Solution: Formation and Decarboxylation of Benzoyloxy Radicals

The light-induced (266 nm) ultrafast decarboxylation of two peroxides R1−C(O)O−OR2, with R1 = phenyl and R2 = benzoyl or tert-butyl, in solution has been studied on the picosecond time scale by absorption spectroscopy with a time resolution typically of 100 to 200 fs. The reaction was investigated in various solvents of different polarity and viscosity to elucidate the influence of the solvent environment on the decarboxylation rate. Transient intermediate benzoyloxy radicals, R1−CO2, were monitored at wavelengths between 300 and 1000 nm. While the primary dissociation of the peroxide is too fast to be resolved, the dissociation of intermediate benzoyloxy radicals is clearly detected on the picosecond time scale. The mechanism of light-induced two-step dissociation is discussed, as is the dependence of reaction dynamics on the type of substituent R2 as well as the branching ratio between prompt and delayed CO2 formation. A model for the decarboxylation process is presented that is based on molecular struc...

Decarboxylation of carbonyloxy radicals: a density functional study

We present a thorough theoretical investigation on the decarboxylation process of carbonyloxy radicals of general structure RC(O)O which play a key role in the decomposition of organic peroxides that are widely used as initiators in free-radical polymerizations. The effect of variation in the molecular structure (R = PhO, PhCH2 and NphO) (Nph = naphthyl) is studied systematically. Geometries, energies, and vibrational frequencies of the carbonyloxy radicals, the dissociation products, and the first-order saddle points pertinent to decarboxylation have been calculated employing density functional theory. The results are compared with recent data for the benzoyloxy radical (R = Ph). Insertion of an oxygen atom or a CH2 group between the chromophore and the CO2 moiety yields a significant reduction in the stability of the carbonyloxy radicals. Despite the structural similarity of the three compounds, the reaction coordinates are markedly different. The structural effects are discussed in terms of changes in thermochemistry, barrier heights and transition modes. The calculated data serve as essential input parameters for statistical unimolecular rate theory calculations and are crucial for the modeling of recent femtosecond pump/probe spectroscopy experiments on the carbonyloxy radical decomposition mechanism.

Reactivity of ubiquinones and ubiquinols with free radicals

The reactivity of quinones 1–4 and of the corresponding quinols 5–8 towards carbon- and oxygen-centred radicals were studied. All quinones bearing at least one nuclear position free, readily react with alkyl and phenyl radicals to afford the alkylated quinones 12–24; however, quinones 1 and 3 reacted with 2-cyano-2-propyl radical to yield products (the mono- and di-ethers 9–11) derived from the attack on the carbonylic oxygen. The reactions carried out on quinones with the benzoyloxy radical led to no reaction products and in the case of Q10, the isoprenic chain also remained unchanged. Quinols 5–8 reacted only with oxygencentred radicals (benzoyloxy and 2-cyano-2-propylperoxy radicals) to give the corresponding quinones. The isoprenic chain of Q10 did not undergo attack even with peroxy radicals. Carbon-centred radicals resulted unable to abstract hydrogen from the studied quinols.

Polymers with furyl groups at chain-ends: the use of 2-styrylfuran as an additive in radical polymerizations

2-Styrylfuran (STF) resembles stilbene and certain other 1,2-substituted ethylenes, e.g. 2-styrylthiophene, in being very effective in capturing benzoyloxy radicals. When STF is used as an additive during polymerization of a monomer such as methyl methacrylate with benzoyl peroxide as initiator, it is incorporated preferentially at sites adjacent to benzoate end-groups to give macromolecules with furyl groups very close to chain-ends.

Reactions of vinyl ethers with 2-cyano-2-propyl and benzoyloxy radicals

tert-Butyl, cyclohexyl, n-propyl, and n-dodecyl vinyl ethers have been used as comonomers with styrene and methyl methacrylate using 13 C-enriched samples of azobis(isobutyronitrile) and benzoyl peroxide as initiators at 60°C. Examination by 13 C-NMR spectroscopy of either ( 13 CH 3 ) 2 C(CN)- or Ph 13 COO- end-groups in the products has shown that the vinyl ethers have low reactivities toward the 2-cyano-2-propyl radical but high reactivities toward the benzoyloxy radical.

Further studies of end groups derived from benzoyl peroxide

There has been a suggestion that, for polymerizations initiated by benzoyl peroxide, the use of benzene as diluent may cause uncertainties about the conclusions reached from studies of the end groups in the resulting polymers. It is now shown that no substantial changes are caused by the replacement of benzene by other solvents. This finding contributes to the conclusion that any capture of benzoyloxy radicals by benzene is only transitory and does not invalidate the deductions made from the results of analyses of polymers for benzoate and phenyl end groups derived from the peroxide.