T-butoxy Radicals Research Articles

Proton-Coupled Electron Transfer Enables Tandem Radical Relay for Asymmetric Copper-Catalyzed Phosphinoylcyanation of Styrenes.

A tandem radical relay strategy was realized for the first Cu(I)-catalyzed enantioselective phosphinocyanation of styrenes. In this reaction, tBuOOSiMe3 generated in situ from tBuOOH serves as a radical initiator to trigger t-butoxy radical production upon oxidization of L*Cu(I) species via proton-coupled-electron transfer (PCET) pathway, which leads to sequential phosphinoyl radical and benzyl radical formations. The resultant β-cyanodiarylphosphine oxides could be easily converted to a series of chiral γ-amino phosphine ligands.

Mechanistic insights into light-driven graphene-induced peroxide decomposition: radical generation and disproportionation.

Interaction between adsorbed t-butyl peroxybenzoate and photoexcited graphene rendered trapped phenyl and t-butoxy radicals. Post-irradiation thermal desorption showed benzene, t-butanol, and isobutylene oxide as the end products. The required hydrogen atoms were obtained via the radical disproportionation. Graphene enabled radical species to be captured and their on-surface chemistry to be revealed.

Laser-Induced Fluorescence Spectroscopy of Jet-Cooled t-Butoxy.

The vibrational structures of the Ã(2)A1 and X̃(2)E states of t-butoxy were obtained in jet-cooled laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectroscopic measurements. The observed transitions are assigned based on vibrational frequencies calculated using the complete active space self-consistent field (CASSCF) method and the predicted Franck-Condon factors. The spin-orbit splitting was measured to be 36(5) cm(-1) for the lowest vibrational level of the ground (X̃(2)E) state, which is significantly smaller than that of methoxy, and increases with increasing vibrational quantum number of the CO stretch mode. Vibronic analysis of the DF spectra suggests that Jahn-Teller active modes of the ground-state t-butoxy radical are similar to those of methoxy and would be the same if methyl groups were replaced by hydrogen atoms. The rotational and fine structure of the LIF transition to the first CO stretch overtone level of the Ã(2)A1 state has been simulated using a spectroscopic model first proposed for methoxy, yielding an accurate determination of the rotational constants of both à and X̃ states.

ChemInform Abstract: Reaction of t-Butoxy Radicals with Cyclic Alkenes Studied by the Nitroxide Radical-Trapping Technique.

The pattern of reactions occurring between t- butoxy radicals and a number of cyclic alkenes has been investigated by the nitroxide radical-trapping technique. The major reaction pathway is allylic abstraction unless this position is at a bridgehead as in norbornene where the major pathway is radical addition. The technique is sufficiently sensitive to identify minor reaction pathways of non-allylic substitution and radical addition when in the presence of extensive allylic substitution. Some stereoselective effects are also detected.

Chain extension and block copolymer synthesis using silane radical atom abstraction coupled with nitroxide mediated polymerization

Silane radicals were used to abstract bromine termini from monobrominated polystyrene (PStBr) in the presence of excess monomer and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), generating polymer radicals that underwent chain extension. Typically, 70–85% of the PStBr precursors were activated by silane radical atom abstraction (SRAA) and were elongated by nitroxide mediated polymerization (NMP), shifting to higher number-average molecular weight (Mn) values as observed by gel permeation chromatography (GPC). Chain extension did not occur until the temperature was elevated to 130°C, with no increase in Mn values observed when the reaction was held at 80°C, which is the temperature of the SRAA phase. The NMP phase of the reaction showed a linear correlation between Mn values and monomer consumption, along with first order kinetics with respect to styrene. SRAA/NMP was then applied to the synthesis of polystyrene-b-poly(n-butyl acrylate) and polystyrene-b-poly(p-methylstyrene), with analysis by GPC indicating the formation of block copolymers with a similar amount of unreacted PStBr remaining. Quantitative activation and elongation of the polymer precursors were prevented due to the ability of both the t-butoxy radicals and tris(trimethylsilyl)silane radicals to add across monomeric double bonds, competing with atom abstraction. Reactions were thus performed in which the monomer was added only after the transition to the higher temperature, which resulted in improved activation of the PStBr.

Reaction of Single-Walled Carbon Nanotubes with Organic Peroxides

Single-walled carbon nanotubes (SWNTs) induce the decomposition of four diacyl peroxides by single electron transfer (SET). Phthaloyl peroxide functionalizes SWNTs to the greatest extent of the four. It was also found that t-butoxy radicals add to SWNTs but that SWNTs fail to inhibit cumene autoxidation. Thus, SWNTs are reactive to alkoxy radicals but not to alkylperoxy radicals.

The reactivity of antioxidants with peroxy and alkoxy radicals in the presence of transition metals. II. Radical processes in multicomponent antioxidant systems

In nonpolar solvents the mechanism of bimolecular substitution reactions between t-butyl-peroxy and t-butoxy radicals with different phenols and diarylamines was quantitatively studied by means of electron paramagnetic resonance (EPR) spectroscopy in the temperature range of +70 to −140°C. The selective reactivity, in the ligand field, of cobalt complex-bonded radicals [Co] BuO2, [Co] BuO., and the free continuously generated noncomplex-bonded BuO2 and BuO radicals to antioxidants are compared. The antioxidant ArH forms with cobalt (II)-acetylacetonate a complex [Co(acac)2]2 [Arh]n = 1-4 and prevents the subsequent reactions of the primarily generated RO. radicals with the transition metal (RO. + Co(II) RO′ + Co(III)) and with the excess of hydroperoxides (RO. + ROOH ROH + RO2). The concentration and the ratio of RO. and RO.2 radicals in a chain oxidation process in the presence of phenyl β- or α-napthylamine (Ar1HNAr2) can be determined from the different EPR spectra of fixed [Co] [Ar1 NAr2]. and free stable nitroxy (Ar1HNAr2) radicals.

FTIR and computational studies of gas-phase hydrogen atom abstraction kinetics by t-butoxy radical

By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10 −16 to 10 −14 cm 3 molecule −1 s −1 have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic compounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for selected reactions have been measured in the temperature range 299–318 K. Transition states and activation barriers for such reactions have been computed with the help of G aussian 03 software and found to match very well with the experimental values.

Absolute Kinetic Characterization of 17-β-Estradiol as a Radical-Scavenging, Antioxidant Synergist

We directly measured the absolute reactivity of 17-β-estradiol (E2) and several phenolic model compounds for E2 toward t-butoxy radical (t-BuO·) by nanosecond time-resolved optical spectroscopy. Compared to other phenols, E2 is a moderate, but not strong deactivator of oxyradicals. The absolute bimolecular rate constant for H-atom transfer from E2 to t-BuO· is 1.3 ± 0.3 × 109 M−1 s−1 (23°C, benzene). We estimate the O–H bond strength of 17-β-estradiol to be approximately 85 ± 2 kcal/mol and calculate the reaction rate constant of E2 toward peroxy radical to be 105 M−1 s−1 at 37°C. The conjugate phenoxy radical of 17-β-estradiol, E2O·, is unusually reactive toward α-tocopherol and ascorbate by H-atom transfer in homogeneous solution (108–109 M−1 s−1). Our findings suggest that E2 functions in vivo as a highly localized, synergistic biological antioxidant. This may partly explain the clinical effectiveness of ovarian steroids in delaying the manifestations of Alzheimer's Disease as well as in protecting against cardiovascular pathologies. In the absence of complementary antioxidant synergists, E2O· is expected to be a pro-oxidant.

Graft copolymerization studies Part II. Models related to polyethylene terephthalate

The chemistry of free radical graft copolymerization, initiated with t-butoxy radicals, has been investigated using ethylene glycol dibenzoate and diethylene glycol dibenzoate as models for polyethylene terephthalate (PET). Although diethylene glycol dibenzoate is more reactive than ethylene glycol dibenzoate the site of grafting from the polymer is determined by the concentration of ethylene glycol and diethylene glycol residues and the relative reactivity at these positions. Hence grafting occurs most frequently at the ethylene glycol positions. This study also suggests that PET is less reactive than the polyolefins LLDPE and polypropylene and that PET chain scission is possible using high t-butoxy radical concentrations in the presence of adventitious oxygen at relatively low temperatures.

Graft copolymerisation studies Part 1. Models related to polyolefins

The chemistry of free radical graft copolymerisation, initiated with t-butoxy radicals, has been investigated using 3-methylpentane and 2,4-dimethylpentane as models for LLDPE and PP, respectively. The tertiary C–H reaction site of 3-methylpentane is twice as reactive as the secondary position. However, the site of grafting from the polymer is determined by the concentration of tertiary and secondary reaction sites and the relative reactivity at these positions and hence grafting occurs most frequently from the secondary C–H reaction sites. Abstraction from 2,4-dimethylpentane is solvent dependent and occurs most readily at the tertiary position. Therefore, grafting will be predominantly concentrated at the tertiary C–H site. The models also indicate that PP is less reactive than LLDPE. Competition reactions between 3-methylpentane and the monomers methyl methacrylate, styrene or 4-vinylpyridine suggest that the nucleophilicity of the monomer influences the competing abstraction–monomer initiation reactions. Evidence for grafting methyl methacrylate from 3-methylpentane is also presented. This study also provides evidence of the incorporation of peroxide linkages into the graft even when standard deoxygenating techniques are used.

The β C–C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323–383 K. The low and the high pressure limiting rate constants as well as the broadening factor Fc have been extracted from a complete falloff analysis of the experimental results: k0=[He]×1.5×10−8 exp(−38.5 kJ mol−1/RT) cm3 s−1, k∞=1.0×1014 exp(−60.5 kJ mol−1/RT) s−1, and Fc=0.87−T/870 K. We anticipate an uncertainty for these rate constants of ±30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C–C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C–C bond scission rate constants of all alkoxy radicals of A=1014±0.3 s−1. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k∞=1.0×1014 exp(−78.2 kJ mol−1/RT) and 1.0×1014 exp(−63.1 kJ mol−1/RT) s−1, respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)2CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol−1, respectively.

Déplacements homolytiques intramoléculaires: 27. Décompositions induites de peroxydes allyliques àdouble liaison activée

Thermolysis of 2,2-dimethylethyl peracetate in cyclohexane solutions of 2-(1,1-dimethylethylperoxy)methylpropenenitrile, 2-(1,1-dimethylethylperoxy)methylbut-3-enone and 3-(1,1-dimethylethylperoxy)-2-phenylpropene showed a similar behaviour of these compounds. Two induced decompositions of these unsaturated peroxidic derivatives were identified (1) free radical addition to the double bond followed by an homolytic intramolecular substitution on the peroxidic bond; (2) allylic hydrogen abstraction followed by a t-butoxy radical elimination.

An Efficient Aerobic Oxidation of Isobutane to t-Butyl Alcohol by N-Hydroxyphthalimide Combined with Co(II) Species

Abstract Highly selective aerobic oxidation of isobutane to t-butyl alcohol was successfully achieved by the use of a radical catalyst, N-hydroxyphthalimide (NHPI) in the presence of Co(II) salt under relatively mild conditions. The oxidation of isobutane by NHPI combined with Co(acac)2 under a pressure of air (10 atm) in benzonitrile at 100 °C gave t-butyl alcohol in high yield (84%) along with acetone (13%). The reaction is thought to proceed via hydrogen abstraction from isobutane by the phthalimidooxyl radical (PINO), which seems to be a key active species. The formation of acetone can be explained by a partial β-scission of the t-butoxy radical, generated from the redox decomposition of t-butyl hydroperoxide by cobalt ion. Alkyl-substituted butanes and pentanes were difficult to be oxidized selectively under these conditions because of easy degradation to smaller fragments of the resulting alkoxyl radical intermediates.

Reaction of Acyclic Hydrocarbons Towards t-Butoxy Radicals. A Study of Hydrogen Atom Abstraction by Using the Radical Trapping Technique

The reaction of 3-methylpentane and 2,4-dimethylpentane toward t-butoxy radicals has been investigated, in neat and benzene solutions, by using the radical trapping technique. Abstraction occurs principally from the tertiary and secondary C-H reaction sites of 3-methylpentane and the tertiary position of 2,4-dimethylpentane. The tertiary and in particular secondary C-H reaction sites of 2,4-dimethylpentane are shown to be considerably less susceptible towards t-butoxy radical facilitated abstraction compared with the equivalent reaction sites of 3-methylpentane. As a result, the latter is three times as reactive as 2,4-dimethylpentane as a neat hydrocarbon solution and seven times as reactive in a diluted mixture of benzene. Diferences in selectivity and rate of hydrogen abstraction, between the substrates, are interpreted in terms of non-bonding interactions retarding t-butoxy radicals from approaching sterically demanding C-H reaction sites. The selectivity from 3-methylpentane is solvent-insensitive whereas abstraction from 2,4-dimethylpentane is modified in benzene. Further, the rate of hydrogen abstraction, from either substrate, to t-butoxy radical β-scission is considerably smaller in benzene. Both observations are interpreted in terms of t-butoxy radical solvation by the aromatic solvent.

Study on Initiation Mechanisms of Peresters Using α-Methylstyrene Dimer (MSD) Trapping Technique

The thermal decomposition of t-butyl peroxybenzoate (2a) and t-butyl peroxylaurate (2b) was carried out in 2,4-diphenyl-4-methyl-1-pentene (α-methylstyrene dimer, MSD) at 140°C. Free radicals generated from the peresters were efficiently trapped by MSD through the addition-fragmentation reaction. Benzoyloxy radicals generated from 2a underwent two competitive reactions, i.e., addition to MSD (71%) and decarboxylation (29%). On the other hand, decarboxylation was the sole reaction for lauroyloxy radicals from 2b. For t-butoxy radicals, both results from 2a and 2b were identical, the ratio of addition to MSD/β-scission being 61/39. In the case of 2b, the efficiency of free radical production was significantly lowered by cage reactions.

Biomimetic oxidation studies. 10. Cyclohexane oxidation reactions with active site methane monooxygenase enzyme models and t-butyl hydroperoxide in aqueous micelles: Mechanistic insights and the role of t-butoxy radicals in the CH functionalization reaction

The oxidation of cyclohexane (CyH) in an aqueous micelle system with t-butyl hydroperoxide (TBHP) in the presence of biomimetic methane monooxygenase enzyme complexes, [Fe 2O( η 1-H 2O)( η 1-OAc)(TPA) 2] 3+, 1, [Fe 2O( η 1-H 2O)( η 1-OAc)(BPIA) 2] 3+, 2, and O 2, was studied and found to provide cyclohexanol (CyOH), cyclohexanone (CyONE), and cyclohexyl- t-butyl peroxide (CyOO t-Bu). The mechanistic aspects of this oxidation reaction in aqueous micelles were studied and included the effects of the surfactant concentration, cetyltrimethylammonium hydrosulfate; concentration of CyH and TBHP; and a trapping reagent, CCl 4. Several factors allowed us to conclude that a t-butoxy radical ( t-BuO) was generated from the favorable redox chemistry of the biomimetic complexes with TBHP, and was responsible for the free radical initiation process with CyH in the aqueous micelle system.

Reaction of t-Butoxy Radicals With Cyclic Alkenes Studied by the Nitroxide Radical-Trapping Technique

The pattern of reactions occurring between t- butoxy radicals and a number of cyclic alkenes has been investigated by the nitroxide radical-trapping technique. The major reaction pathway is allylic abstraction unless this position is at a bridgehead as in norbornene where the major pathway is radical addition. The technique is sufficiently sensitive to identify minor reaction pathways of non-allylic substitution and radical addition when in the presence of extensive allylic substitution. Some stereoselective effects are also detected.

Strained-ring silicon-centred radicals: silacyclobut-1-yl and related radicals

Silacyclobut-1-yl, 1-methylsilacyclobut-1-yl and 3-methylsilacyclobut-1-yl radicals have been prepared by γ-irradiation of the parent silane in adamantane matrices. Electron paramagnetic resonance studies show that the radicals have non-planar rings; the SOMO on silicon is pseudo-equatorial. There is a substantial long range coupling of 16.4 G to the pseudo-equatorial γ-proton in the silacyclobut-1-yl radical and a smaller δ-coupling is shown to the pseudo-equatorial methyl protons in the 3-methylsilacyclobut-1-yl radical. In fluid solution at −40°C t-butoxy radicals react with silacyclobutane by an S H2 reaction at silicon to give 4-t-butoxy-4-silabut-1-yl rdicals, rather than by the expected hydrogen abstraction to give silacyclobut-1-yl radicals.

Initiation mechanisms in radical polymerization: reaction of t-butoxy radicals with allyl acrylate and with diallyl ether

The radical trapping technique employing 1,1,3,3-tetramethyl-1,3-dihydroisoindol-2-yloxyl (1) as a scavenger has been used to study the reaction of t-butoxy radicals with allyl acrylate and with diallyl ether. With allyl acrylate, extensive H-abstraction as well as addition to both allyl and acryloyl double bonds was observed whereas with diallyl ether, the only products obtained were derived exclusively from H-abstraction.