Showing papers on "Homolysis published in 2014"
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TL;DR: A C-P bond and a C-C bond are formed in the synthesis of 6-phosphorylated phenanthridines starting with readily prepared 2-isocyanobiphenyls and commercially available P-radical precursors.
223 citations
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TL;DR: In this paper, it was shown by means of density functional theory that oxides with polar M-O bonds might favor heterolytic dissociation, provided that metal ions are reducible.
Abstract: Mechanisms for H2 dissociation on metal oxides have been typically inferred from the infrared spectra of reaction products on the basis of the presence or lack of M–H fingerprints. Here, we demonstrate by means of density functional theory that oxides with polar M–O bonds might favor heterolytic dissociation. Moreover, we report that the resulting heterolytic product can further evolve to the homolytic one provided that metal ions are reducible. Hence, it follows that the redox capacity of the metal determines the reaction outcome. This finding sheds light on why both M–H and O–H bands appear in the infrared spectra of nonreducible oxides such as MgO or γ-Al2O3, while only O–H bands are observed for reducible oxides like CeO2. It results in a unified mechanism for polar oxides that can be generalized to other materials exhibiting significant charge separation. Importantly, we also show that the low activity of CeO2 toward H2 can be improved by enhancing the basicity of surface O atoms upon lattice expansi...
164 citations
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TL;DR: A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated.
Abstract: A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, (tBumPNP)CoX (X = H, CH3, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, (tBuPNP)CoCH3 and (tBuPNP)CoH, at 110 °C also resulted in pincer modification by H atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C–H bond strengths was corroborated by H atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, (tBumPNP)CoX were established by EPR spectroscopy ...
140 citations
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TL;DR: Evidence is provided for a homolytic aromatic substitution mechanism, in which a catalyticallygenerated α-amino radical undergoes direct addition to an electrophilic chloroarene.
Abstract: The direct α-heteroarylation of tertiary amines has been accomplished via photoredox catalysis to generate valuable benzylic amine pharmacophores. A variety of five- and six-membered chloroheteroarenes are shown to function as viable coupling partners for the α-arylation of a diverse range of cyclic and acyclic amines. Evidence is provided for a homolytic aromatic substitution mechanism, in which a catalytically-generated α-amino radical undergoes direct addition to an electrophilic chloroarene.
134 citations
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TL;DR: In this paper, the pyrolysis processes of β -O-4 type lignin dimer model compound 1 (1-phenyl-2-phenoxy-1,3-propanediol) were theoretically investigated by employing density functional theory (DFT) methods at the B3LYP/6-31G(d,p) level.
97 citations
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TL;DR: In this paper, five possible pyrolytic pathways of guaiacol were proposed with an emphasis on the reactivity of the methoxy group, and the standard thermodynamic and kinetic parameters of each reaction pathway were calculated at different temperatures based on density functional theory methods by using Gaussian 03 package at B3LYP/6-31G++(d,p) level.
83 citations
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TL;DR: In this paper, density functional theory calculations were applied to fully substituted lignin models to compare the activation energies for the concerted reactions and the bond dissociation energies of the homolysis reactions.
Abstract: Studies on the pyrolysis mechanisms of lignin model compounds have largely focused on initial homolytic cleavage reactions. It has been noted, however, that concerted mechanisms may also account for observed product formation. In the current work, the latter processes are examined and compared to the former, by the application of density functional theory calculations to fully substituted lignin models. Results show that activation energies for the concerted reactions are somewhat lower than the bond dissociation energies of the homolysis reactions. Kinetic analysis revealed that the concerted pathway is the retro-ene fragmentation mechanism.
78 citations
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TL;DR: A concise thiolation of C(sp3)-H bond of cycloalkanes with diaryl disulfides in the presence of oxidant of di-tert-butylperoxide (DTBP) has been developed, giving good to excellent chemical yields.
Abstract: The formation of C–S bonds represents an active research area in general organic chemistry, material science as well as biological and pharmaceutical chemistry.[1] In the past decade, a number of methodologies have been developed in this field. In particular, transition-metal-catalyzed cross-coupling reactions of thiols with aryl halides provided the most general strategies for constructing C–S bonds.[2] Recently, metal-catalyzed C–S bonds formation through C(sp2)–H bond activation has become an alternative and intriguing methods for preparation of sulfides due to its high atom-economy and efficiency.[3] However, the cleavage of C(sp3)–H bond leading to the C–S bond formation is less studied. Very recently, Xiang and co-workers reported a novel thiolation of C(sp3)–H bond adjacent to a nitrogen or oxygen atom (Scheme 1a and 1b).[4] To date, the thiolation of C(sp3)–H bond of unactivated alkanes still have not been developed.
Scheme 1
C(sp3)–H bond functionalization.
Direct C(sp3)–H transformation of alkanes, which is a great challenge due to their low reactivity and the lack of a coordination site for the metal catalyst, has attracted many generations of organic chemists in recent years.[5] However, C(sp3)–H bond activation of cycloalkanes to form C–C,[6] C–O,[7] C–N[8] bonds still has not been well reported because C(sp3)–H bond activation of cycloalkanes is more difficult than C(sp3)–H bond activation adjacent to heteroatoms, double bonds, phenyl or electron withdrawing groups. Li and others have done elegant work in this field using a transition-metal catalyst (such as Ru, Sc, Fe, etc.) both for activating the C(sp3)–H bond of cycloalkanes and subsequent coupling to form C–Y (Y = O, N, C) bonds.[9] Recently, our group also developed a Fe-catalyzed decarboxylative alkenylation of cycloalkanes via a radical process.[10] The metal-free C(sp3)–H bond functionalization progress for C(sp2)–C(sp3) bond formation of heteroaromatics and cycloalkanes promoted by DTBP have also been reported (Scheme 1c).[11,12] To the best of our knowledge, no reports for the construction of C–S bond through C(sp3)–H bond functionalization of cycloalkanes are described. Herein, we would report the first realization on the thiolation of C(sp3)–H bond of cycloalkanes through DTBP-mediated oxidative C(sp3)–H bond functionalization without the aid of transition-metal catalyst (Scheme 1d).
Initially, we conducted our investigation by reacting 1,2-diphenyldisulfane 1 (0.5 mmol) with cyclohexane 2a (2 mL) in the presence 4.0 equiv of tert-butylhydroperoxide (TBHP) at 120 °C for 24 h. The reaction could happen, and afforded the expected product of cyclohexyl(phenyl)sulfane 3a in a poor yield (23%, Table 1, entry 1). Replacing TBHP with DTBP, the yield dramatically increased to 88% without the aid of any metal catalyst or other additives (Table 1, entry 2). The use of other oxidants such as DDQ, K2S2O8, H2O2 (30% aqueous solution) or TBPB did not provide better results (Table 1, entries 3–6). A decreased loading of DTBP to 2.0 equiv or a lower temperature to 100 °C, the lower yield would be obtained in 75% and 69%, respectively (Table 1, entries 7 and 8). No significant effect in yield of 3a was found with 6.0 equiv of DTBP or a higher temperature (Table 1, entries 9 and 10). Furthermore, the addition of metal-catalyst, Cu(OAc)2 (10 mol %), did not result any improvement on the yield (79%, Table 1, entry 11). Further optimization of the conditions showed that the reaction could not proceed without the use of oxidant DTBP (Table 1, entry 12).
Table 1
Optimization of reaction condition [a]
With the optimized reaction conditions in hand, this approach was then applied to the coupling of cyclohexane to a variety of diaryl disulfides (Scheme 2). As shown in Scheme 2, the process has a broad scope and high compatibility with functional groups, such as methyl, methoxy, halo and nitro substituent groups. Ortho-, meta- and para-substituted diaryl disulfides were well tolerated, and the reactions gave the corresponding products with good to excellent yields of 71–92% (3a-h). The lower yields observed in the case of ortho-substituted diaryl disulfides compared to its para- or meta- analogues, is possibly due to the steric hindrance (3d-f). The reaction with p-methyl and p-methoxy substituted diaryl disulfides afforded the expected product with excellent yields (3b and 3c). However, when p-fluoro and p-nitro substituted diaryl disulfides were used as the coupling partner the product yields dropped (85% for 3d and 75% for 3h). These results imply that the electronegativity of the substituents in the diaryl disulfides has an obvious effect on the chemical yields. In addition, the disubstituted diaryl disulfide is also well tolerated in this reaction to give the target product with a slightly lower yield of 80% (3i). Notably, the heterocyclic disulfides thienyl or pyridinyl disulfide can work well in the reaction to provide the corresponding alkyl heteroaryl sulfide (3j-k). It is worth to note that the reaction with complex heterocyclic disulfides 2,2’-dithiobis(benzothiazole) and 5,5’-dithiobis(1-phenyl-1H-tetrazole) also proceed successfully under the optimized condition (3l-m). Disappointingly, 1,2-dibenzyldisulfane was not a suitable substrate for the current thiolation system (3n).
Scheme 2
Thiolation of cyclohexane with disulfides. Reaction conditions: 1 (0.5 mmol), cyclohexane (2 mL), DTBP (4.0 equiv), 120 °C, 24 h. Isolated yields based on 1.
Subsequently, other cycloalkanes, including cyclopentane, cycloheptane and cyclooctane were employed as substrates for this reaction to further examination of the reaction scope (Scheme 3). Fortunately, they work well in the system under the optimized conditions, and can react with different diaryl disulfides 1, giving the corresponding products 4a–j in 67–89% chemical yield. Comparing with the results shown in Scheme 2 for cyclohexane as a substrate, the reaction with cyclopentane showed a lower efficiency, and obvious lower yields were found (67–82%, 4a–g). However, for the cases of larger cycloalkanes, such as cycloheptane and cyclooctane, comparative chemical yields were obtained (4h–j).
Scheme 3
Thiolation of cyclopentane with disulfides. Reaction conditions: 1 (0.5 mmol), cycloalkanes (2 mL), DTBP (4.0 equiv), 120 °C, 24 h. Isolated yields based on 1.
Then intramolecular and intermolecular competition experiments were carried out. Firstly, heteroaryl aryl disulfide 1o was used for the investigation of the intramolecular competition reaction (Scheme 4). We found both of these two ArS• could react with cyclohexane well, giving the cross-coupling products 3l and 3b in 85% and 86% chemical yields respectively.
Scheme 4
Investigation of the intramolecular competition reaction. Reaction conditions: 1o (0.5 mmol), cyclohexane (2 mL), DTBP (4.0 equiv), 120 °C, 24 h.
Then, the examination of the intramolecular competition reaction was performed with the using of disulfides 1l and 1b as substrates at the same time (Scheme 5). The reaction with both of these two disulfides proceeded very well, and good chemical yields were obtained (3l and 3b in 82% and 84% respectively).
Scheme 5
Investigation of the intermolecular competition reaction. Reaction conditions: 1l (0.25 mmol), 1b (0.25 mol), cyclohexane (2 mL), DTBP (8.0 equiv), 120 °C, 24 h.
To gain insight into the reaction mechanism, several control experiments were carried out (Scheme 6). Addition of the radical-trapping reagents 2,2,6,6 tetramethylpiperidine N-oxide (TEMPO) or azobisisobutyronitrile (AIBN) can completely inhibit the reaction and almost no desired product was observed. These results indicate that the transformation may proceed via a radical course.
Scheme 6
Insights into the mechanism.
Based on the above results and literature reports,[4, 10, 13] a possible mechanism for the cycloalkane thiolation reaction is illustrated in Scheme 7, which includes a key radical oxidative coupling step. At the beginning, homolysis of DTBP give tert-butoxy radical intermediate A under the condition of heating. Then, cyclohexane radical intermediate B is generated via reaction of intermediate A and cyclohexane 2a through a C–H bond cleavage. The following step is the reaction between intermediate B and 1,2-diphenyldisulfane 1, affording the final target product 3a, along with the formation of PhS• free radical intermediate C. Finally, the free radical C couples with cyclohexane readical B, giving another molecular of product 3a.
Scheme 7
A possible reaction mechanism.
In conclusion, we have presented a novel and highly efficient method for C–S cross-coupling through direct C(sp3)–H bond functionalization of cycloalkanes with diaryl sulfides using DTBP as the oxidant without use of any metal catalyst. Varieties of substituted diphenyl disulfides and heterocyclic disulfides could be tolerated and coupled with cycloalkanes, giving the cycloalkyl aryl sulfides in good to excellent yields. Moreover, this synthetic strategy for direct C–S bond formation might be very valuable and attractive in radical chemistry.
75 citations
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TL;DR: The radical addition of the Cl-S σ-bond in sulfenyl chlorides to various C-C triple bonds has been achieved with excellent regio- and stereoselectivity in the presence of a catalytic amount of a common iron salt.
Abstract: The radical addition of the ClS σ-bond in sulfenyl chlorides to various CC triple bonds has been achieved with excellent regio- and stereoselectivity in the presence of a catalytic amount of a common iron salt. The reaction is compatible with a variety of functional groups and can be scaled up to the gram-scale with no loss in yield. As well as terminal alkynes, internal alkynes underwent stereodefined chlorothiolation to provide tetrasubstituted alkynes. Preliminary mechanistic investigations revealed a plausible radical process involving a sulfur-centered radical intermediate via iron-mediated homolysis of the ClS bond. The resulting chlorothiolation adducts can be readily transformed to the structurally complex alkenyl sulfides by cross-coupling reactions. The present reaction can also be applied to the complementary synthesis of the potentially useful bis-sulfoxide ligands for transition-metal-catalyzed reactions.
67 citations
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TL;DR: In this paper, the pyrolysis of methoxy substituted α-O-4 dimeric phenolic compounds at 500°C using a Frontier Lab micro-pyrolyzer was investigated.
65 citations
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TL;DR: The mechanism for the formation of Au-PBI dimer is revealed with scanning tunnelling microscopy studies and density functional theory calculations and a PBI radical generated from the homolytic C-Cl bond dissociation can covalently bind a surface gold atom and partially pull it out of the surface to form stable PBI-Au hybrid species.
Abstract: The surface-assisted synthesis of gold-organic hybrids on Au (111) and Au (100) surfaces is repotred by thermally initiated dehalogenation of chloro-substituted perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs). Structures and surface-directed alignment of the Au-PBI chains are investigated by scanning tunnelling microscopy in ultra high vacuum conditions. Using dichloro-PBI as a model system, the mechanism for the formation of Au-PBI dimer is revealed with scanning tunnelling microscopy studies and density functional theory calculations. A PBI radical generated from the homolytic C-Cl bond dissociation can covalently bind a surface gold atom and partially pull it out of the surface to form stable PBI-Au hybrid species, which also gives rise to the surface-directed alignment of the Au-PBI chains on reconstructed Au (100) surfaces.
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TL;DR: In this paper, the authors used IEF-PCM B3LYP/6-311++G ∗∗ method in benzene and water to identify the thermodynamically preferred mechanism and OH group in the two solvents and describe the solvent effect on the homolytic and heterolytic cleavage of OH groups in studied flavonoids.
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TL;DR: The low-energy CID homolytic fragmentation was diagnostic for structural identification of flavonol 3-O-glycosides, and new counterexamples were found for previously reported fragmentation rules.
Abstract: RATIONALE
Negative ESI-QIT-MS of several subtypes of flavonoid O-glycosides is known to produce deprotonated molecular ions which undergo homolytic fragmentation at the glycosidic bond upon collision-induced dissociation (CID). However, these subtypes have never been simultaneously compared under unified MS conditions.
METHODS
The (−)-ESI-MSn fragmentations of 69 flavonoid O-glycosides, involving eight subtypes, were analyzed using a quadrupole ion-trap mass spectrometer with collision energies varying from 18–44%. Factors influencing the homolytic glycosidic bond fragmentation, such as collision energy, hydroxylation of aglycone, and glycosylation pattern, were comprehensively studied.
RESULTS
Under the unified CID-QIT-MS2 conditions, the precursor deprotonated molecular ions [M–H]– for 3-O-glycosyl, 3,7-di-O-glycosyl and 3,6,7-tri-O-glycosyl flavonols experienced homolytic fragmentation at the glycosidic bond and generated the radical aglycone ion [Y0–H]–•. This gas-phase CID fragmentation behavior was not observed for the other subtypes. A general trend was found that hydroxyl substitution at C-6, glycosylation at C-6/C-7, and acetylation of the saccharide moiety remarkably suppressed this fragmentation. In addition, flavonol 3-O-diglycosides (disaccharides) possessing a 1 → 2 glycosidic bond generated more abundant [Y0–H]–• product ions than those with a 1 → 3 or 1 → 6 bond. The terminal sugar triggered the homolytic fragmentation in the order Rha > Xyl > Glc. Moreover, new counterexamples were found for previously reported fragmentation rules.
CONCLUSIONS
The low-energy CID homolytic fragmentation was diagnostic for structural identification of flavonol 3-O-glycosides. We have summarized key factors affecting this fragmentation. The results could be useful for rapid characterization of flavonoid O-glycosides in complicated herbal extracts. Copyright © 2014 John Wiley & Sons, Ltd.
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TL;DR: The presence of the tricin bound to wheat straw lignin was confirmed, and it was shown to participate in lign in reactions during the SE treatment, and the preferred homolytic β-O-4 cleavage reaction was calculated to follow bond dissociation energies.
Abstract: Chemical changes of lignin induced by the steam explosion (SE) process were elucidated. Wheat straw was studied as the raw material, and lignins were isolated by the enzymatic mild acidolysis lignin (EMAL) procedure before and after the SE treatment for analyses mainly by two-dimensional (2D) [heteronuclear single-quantum coherence (HSQC) and heteronuclear multiple-bond correlation (HMBC)] and 31P nuclear magnetic resonance (NMR). The β-O-4 structures were found to be homolytically cleaved, followed by recoupling to β-5 linkages. The homolytic cleavage/recoupling reactions were also studied by computational methods, which verified their thermodynamic feasibility. The presence of the tricin bound to wheat straw lignin was confirmed, and it was shown to participate in lignin reactions during the SE treatment. The preferred homolytic β-O-4 cleavage reaction was calculated to follow bond dissociation energies: G–O–G (guaiacyl) (69.7 kcal/mol) > G–O–S (syringyl) (68.4 kcal/mol) > G–O–T (tricin) (67.0 kcal/mol).
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TL;DR: In this paper, the catalytic activity of aqua complexes toward the radical decomposition of H 2 O 2 and generation of the HO radicals was investigated in detail by theoretical (DFT) methods.
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TL;DR: An experimental study of three coal-based model compound (anisole, phenyl ethyl ether, and p-methyl anisole) pyrolysis was carried out at low pressure (below 50 Pa) within the temperature range from 573 to 1323 K as discussed by the authors.
Abstract: An experimental study of three coal-based model compound (anisole, phenyl ethyl ether, and p-methyl anisole) pyrolysis was carried out at low pressure (below 50 Pa) within the temperature range from 573 to 1323 K. The pyrolysis process was investigated by detecting the reactants, radicals, and products using vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry. The similarities and differences of three model compounds in the pyrolysis process were discussed. The results suggested that the radical reactions were dominant in the pyrolysis process at higher temperatures, whereas the intermolecular reactions were significant at lower temperatures. β–H was a key factor for the non-radical reactions. The PhO–C homolytic bond scission was the first step for the radical reaction. Substituents on the benzene ring play an important role in the pyrolysis process of phenyl ethers, which can directly form conjugated stable structure compounds. These observations were supported by our theoretica...
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TL;DR: The recent findings concerning the chemical triggering of the C-ON bond homolysis in alkoxyamines, affording the controlled generation of alkyl radicals at room temperature are described.
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TL;DR: In this article, a silver-catalyzed 2-isocyanobiaryls insertion/cyclization with phosphine oxides was described for the construction of the 6-phosphorylated phenanthridines through radical addition of in situ formed P-centered radical to 2-IsOCyanobiphenyls and homolytic aromatic substitution process.
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TL;DR: P-Toluenesulfonohydrazide was shown to promote the highly efficient direct arylation of unactivated arenes with aryl iodides, bromides, or chlorides in the presence of potassium tert-butoxide without the assistance of any transition metals.
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TL;DR: Chemistry that enables excitation energy transfer (EET) to be accurately measured via action spectroscopy on gaseous ions in an ion trap is reported and it is demonstrated that EET between tryptophan or tyrosine and a disulfide bond leads to excited state, homolytic fragmentation of the disulfides bond.
Abstract: Herein, we report chemistry that enables excitation energy transfer (EET) to be accurately measured via action spectroscopy on gaseous ions in an ion trap. It is demonstrated that EET between tryptophan or tyrosine and a disulfide bond leads to excited state, homolytic fragmentation of the disulfide bond. This phenomenon exhibits a tight distance dependence, which is consistent with Dexter exchange transfer. The extent of fragmentation of the disulfide bond can be used to determine the distance between the chromophore and disulfide bond. The chemistry is well suited for the examination of protein structure in the gas phase because native amino acids can serve as the donor/acceptor moieties. Furthermore, both tyrosine and tryptophan exhibit unique action spectra, meaning that the identity of the donating chromophore can be easily determined in addition to the distance between donor/acceptor. Application of the method to the Trpcage miniprotein reveals distance constraints that are consistent with a native-...
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TL;DR: The results presented here have significant implications in the future design of antioxidant additives: diarylamines designed to yield intermediate alkoxyamines that undergo the retro-carbonyl-ene reaction are likely to be much more effective than existing compounds and will display catalytic radical-trapping activities at lower temperatures due to lower barriers to regeneration.
Abstract: Diarylamine radical-trapping antioxidants are important industrial additives, finding widespread use in petroleum-derived products. They are uniquely effective at elevated temperatures due to their ability to trap multiple radicals per molecule of diarylamine. Herein we report the results of computational and experimental studies designed to elucidate the mechanism of this remarkable activity. We find that the key step in the proposed catalytic cycle–decomposition of the alkoxyamine derived from capture of a substrate-derived alkyl radical with a diarylamine-derived nitroxide–proceeds by different mechanisms depending on the structure of both the substrate and the diarylamine. N,N-Diarylalkoxyamines derived from saturated substrates and diphenylamines decompose by N–O homolysis followed by disproportionation. Alternatively, those derived from unsaturated substrates and diphenylamines, or saturated substrates and N-phenyl-β-naphthylamine, decompose by an unprecedented concerted retro-carbonyl-ene reaction....
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TL;DR: The unimolecular thermal decomposition mechanisms of o, m, and p-dimethoxybenzene (CH3O-C6H4-OCH3) have been studied using a high temperature, microtubular (μtubular) SiC reactor with a residence time of 100 μs to confirm mechanisms and comment on kinetics.
Abstract: The unimolecular thermal decomposition mechanisms of o-, m-, and p-dimethoxybenzene (CH3O-C6H4-OCH3) have been studied using a high temperature, microtubular (μtubular) SiC reactor with a residence time of 100 μs. Product detection was carried out using single photon ionization (SPI, 10.487 eV) and resonance enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry and matrix infrared absorption spectroscopy from 400 K to 1600 K. The initial pyrolytic step for each isomer is methoxy bond homolysis to eliminate methyl radical. Subsequent thermolysis is unique for each isomer. In the case of o-CH3O-C6H4-OCH3, intramolecular H-transfer dominates leading to the formation of o-hydroxybenzaldehyde (o-HO-C6H4-CHO) and phenol (C6H5OH). Para-CH3O-C6H4-OCH3 immediately breaks the second methoxy bond to form p-benzoquinone, which decomposes further to cyclopentadienone (C5H4=O). Finally, the m-CH3O-C6H4-OCH3 isomer will predominantly follow a ring-reduction/CO-elimination mechanism to form C5H4=O. Electronic structure calculations and transition state theory are used to confirm mechanisms and comment on kinetics. Implications for lignin pyrolysis are discussed.
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TL;DR: The role of axial ligand binding to a high-spin iron(III)−alkylperoxo complex and its axial effect on the alkylperoxideo O−O bond cleavage was investigated in this article.
Abstract: The mechanism of the alkylperoxo O–O bond cleavage of low-spin iron(III)–alkylperoxo species has been well established in nonheme iron models. In contrast, the alkylperoxo O–O bond cleavage in nonheme high-spin iron(III)–alkylperoxo species binding an axial ligand has yet to be elucidated. Herein, we report the synthesis and characterization of mononuclear nonheme high-spin iron(III)–alkylperoxo complexes each bearing an N-tetramethylated 13-membered macrocyclic ligand (13-TMC), [FeIII(OOC(CH3)3)(13-TMC)]2+ and [FeIII(OOC(CH3)2C6H5)(13-TMC)]2+. The high-spin iron(III)–alkylperoxo complexes were converted to an iron(IV)–oxo complex at a fast rate upon addition of thiocyanate (NCS−) via the formation of a short-lived intermediate. This intermediate was identified as a high-spin iron(III)–alkylperoxo complex binding a thiocyanate ion as an axial ligand by characterizing it with various spectroscopic methods and density functional theory (DFT) calculations. We have also provided strong evidence that conversion of the high-spin iron(III)–alkylperoxo complex to its corresponding iron(IV)–oxo complex occurs via O–O bond homolysis. Thus, we have concluded that the role of the axial ligand binding to a high-spin iron(III)–alkylperoxo complex is to facilitate the alkylperoxo O–O bond cleavage via the “push effect”, which has been well established in heme enzymes. To the best of our knowledge, the present study reports the first clear example showing the O–O bond homolysis of a high-spin iron(III)–alkylperoxo complex and the axial ligand effect on the alkylperoxo O–O bond cleavage in nonheme iron models.
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TL;DR: The mechanism of the ortho-hydroxylation of aromatic compounds is explored to reason out the importance of ligand design in fine-tuning the reactivity of such catalytic transformations and the concept can be used to stabilize high-valent intermediates which can be trapped and thoroughly characterized.
Abstract: There is a growing interest in probing the mechanism of catalytic transformations effected by non-heme iron-oxo complexes as these reactions set a platform for understanding the relevant enzymatic reactions The ortho-hydroxylation of aromatic compounds is one such reaction catalysed by iron-oxo complexes Experimentally [FeII(BPMEN)(CH3CN)2]2+ (1) and [FeII(TPA)(CH3CN)2]2+ (2) (where TPA = tris(2-pyridylmethyl)amine and BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) complexes containing amino pyridine ligands along with H2O2 are employed to carry out these transformations where complex 1 is found to be more reactive than complex 2 Herein, using density functional methods employing B3LYP and dispersion corrected B3LYP (B3LYP-D) functionals, we have explored the mechanism of this reaction to reason out the importance of ligand design in fine-tuning the reactivity of such catalytic transformations Dispersion corrected B3LYP is found to be superior to B3LYP in predicting the correct ground state of these species and also yields lower barrier heights than the B3LYP functional Starting the reaction from the FeIII–OOH species, both homolytic and heterolytic cleavage of the O⋯O bond is explored leading to the formation of the transient FeIVO and FeVO species For both the ligand systems, heterolytic cleavage was energetically preferable and our calculations suggest that both the reactions are catalyzed by an elusive high-valent FeVO species The FeVO species undergoes the reaction via an electrophilic attack of the benzene ring to effect the ortho-hydroxylation reaction The reactivity pattern observed for 1 and 2 are reflected in the computed barrier heights for the ortho-hydroxylation reaction Electronic structure analysis reveals that the difference in reactivity between the ligand architectures described in complex 1 and 2 arise due to orientation of the pyridine ring(s) parallel or perpendicular to the FeVO bond The parallel orientation of the pyridine ring is found to mix with the (πFe(dyz)–O(py))* orbital of the Fe-oxo bond leading to a reduction in the electrophilicity of the ferryl oxygen atom Our calculations highlight the importance of ligand design in this chemistry and suggest that this concept can be used to (i) stabilize high-valent intermediates which can be trapped and thoroughly characterized (ii) enhance the reactivity and efficiency of the oxidants by increasing the electrophilicity of the ferryl oxygen containing FeVO species Our computed results are in general agreement with the experimental results
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TL;DR: In this paper, the relationship between the structure and the antioxidant activity of trans-resveratrol (RSV) and its phenantrene analogs in the gas phase, benzene and water was investigated.
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TL;DR: The authors' simulations suggest a new decomposition mechanism for the organic polyazido initial explosive at the atomistic level that is inhibited due to low mobility, long distance from each other, and strong carbon-nitrogen bonds.
Abstract: Ab initio molecular dynamics simulations were performed to study the thermal decomposition of isolated and crystal 3,6-di(azido)-1,2,4,5-tetrazine (DiAT). During unimolecular decomposition, the three different initiation mechanisms were observed to be N–N2 cleavage, ring opening, and isomerization, respectively. The preferential initial decomposition step is the homolysis of the N–N2 bond in the azido group. The release mechanisms of nitrogen gas are found to be very different in the early and later decomposition stages of crystal DiAT. In the early decomposition, DiAT decomposes very fast and drastically without forming any stable long-chains or heterocyclic clusters, and most of the nitrogen gases are released through rapid rupture of nitrogen–nitrogen and carbon–nitrogen bonds. But in the later decomposition stage, the release of nitrogen gas is inhibited due to low mobility, long distance from each other, and strong carbon–nitrogen bonds. To overcome the obstacles, the nitrogen gases are released through slow formation and disintegration of polycyclic networks. Our simulations suggest a new decomposition mechanism for the organic polyazido initial explosive at the atomistic level.
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TL;DR: In this paper, a triarylsulfonium salt (TAS) was used as a mechanophore for the selective homolysis of chemical bonds in poly(methyl acrylate) molecules.
Abstract: High mechanical forces applied to polymeric materials typically induce unselective chain scission. For the last decade, mechanoresponsive molecules, mechanophores, have been designed to harness the mechanical energy applied to polymers and provide a productive chemical response. The selective homolysis of chemical bonds was achieved by incorporating peroxide and azo mechanophores into polymer backbones. However, selective heterolysis in polymer mechanochemistry is still mostly unachieved. We hypothesized that highly polarized bonds in ionic species are likely to undergo heterolytic bond scission. To test this, we examined a triarylsulfonium salt (TAS) as a mechanophore. Poly(methyl acrylate) possessing TAS at the center of the chain (PMA-TAS) is synthesized by a single electron transfer living radical polymerization (SET-LRP) method. Computational and experimental studies in solution reveal the mechanochemical production of phenyl cations from PMA-TAS. Interestingly, the generated phenyl cation reacts with its counter-anion (trifluoromethanesulfonate) to produce a terminal trifluoromethyl benzene structure that, to the best of our knowledge, is not observed in the photolysis of TAS. Moreover, the phenyl cation can be trapped by the addition of a nucleophile. These findings emphasize the interesting reaction pathways that become available by mechanical activation.
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TL;DR: Results point to a dual role for glutamate mutase residues: they both stabilize the homolytic state through electrostatic interactions between the protein and the dissociated coenzyme and correctly position the adenosyl radical to facilitate the abstraction of hydrogen from the substrate.
Abstract: Adenosylcobalamin (AdoCbl) serves as a source of reactive free radicals that are generated by homolytic scission of the coenzyme’s cobalt–carbon bond. AdoCbl-dependent enzymes accelerate AdoCbl homolysis by ∼1012-fold, but the mechanism by which this is accomplished remains unclear. We have combined experimental and computational approaches to gain molecular-level insight into this process for glutamate mutase. Two residues, glutamate 330 and lysine 326, form hydrogen bonds with the adenosyl group of the coenzyme. A series of mutations that impair the enzyme’s ability to catalyze coenzyme homolysis and tritium exchange with the substrate by 2–4 orders of magnitude were introduced at these positions. These mutations, together with the wild-type enzyme, were also characterized in silico by molecular dynamics simulations of the enzyme–AdoCbl–substrate complex with AdoCbl modeled in the associated (Co–C bond formed) or dissociated [adenosyl radical with cob(II)alamin] state. The simulations reveal that the nu...
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TL;DR: Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os(•) and (η(5)-(i)Pr4C5H)(CO) 2Os( •) radicals, formed by photoinduced homolysis of the corresponding osmium dimers.
Abstract: We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C–H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os• and (η5-iPr4C5H)(CO)2Os• radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 105 M–1 s–1 to 1.7 × 107 M–1 s–1 at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η5-iPr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 °C temperature range were carried out to obtain th...
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TL;DR: The revelation of the domination of homolysis in Cld indicates not only the high efficiency of enzyme, but also the sensitivity of a heme and the significance of the enzymatic active-site surroundings (the His170 and Arg183 residues in the present case), which gives more insights into heme chemistry.
Abstract: Chlorite dismutase (Cld) is a heme-dependent enzyme that catalyzes the decomposition of toxic chlorite (ClO2−) into innocuous chloride and O2. In this paper, using the hybrid B3LYP density functional theory (DFT) method including dispersion interactions, the Cld reaction mechanism has been studied with a chemical model constructed on the X-ray crystal structure. The calculations indicate that the reaction proceeds along a stepwise pathway in the doublet state, i.e. a homolytic O–Cl bond cleavage of the substrate leading to an O–Fe(heme) species and a ClO˙ radical, followed by a rebinding O–O bond formation between them. The O–Fe(heme) species is demonstrated to be a low-spin triplet-state Fe(IV)O diradicaloid. A low-spin singlet-state Fe(IV)O is much less stable than the former, with an energy difference of 9.2 kcal mol−1. The O–Cl bond cleavage is rate-limiting with a barrier of 10.6 kcal mol−1, in good agreement with the experimental reaction rate of 2.0 × 105 s−1. Furthermore, a heterolytic O–Cl bond dissociation in the initial step is shown to be unreachable, which ensures the high efficiency of the Cld enzyme by avoiding the generation of chlorate byproduct observed in the reactions of synthetic Fe porphyrins. Also, the pathways in the quartet and sextet states are unfavorable for the Cld reaction. The present results reveal a detailed mechanism III (defined in the text) including an interesting di-radical intermediate composed of a low-spin triplet-state Fe(IV)O and a ClO˙ radical. Compared to a competitive heterolytic Cl–O cleavage in synthetic Fe porphyrins, the revelation of the domination of homolysis in Cld indicates not only the high efficiency of enzyme, but also the sensitivity of a heme and the significance of the enzymatic active-site surroundings (the His170 and Arg183 residues in the present case), which gives more insights into heme chemistry.