Oxidative coupling of phenols. Part 10. The role of steric effects in the formation of C–O coupled products

Armstrong, David R.; Cameron, Colin G.; Nonhebel, D. C.; Perkins, P. G.

doi:10.1039/p29830000587

Cited by 43 publications

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“…In both series 1 and 2 only three of the five possible products have been positively identified: A, C, and D; the p-C/ p-C and o-C/p-C condensation products, if present, were below 1% based on A. This contrasts with the results of Armstrong et al [15] on the peroxide-initiated liquid-phase reaction (140°C), wherein the overall site selectivities centred around O/o-C/p-C ഠ 10:60:30. This points at reversibility in the gas phase, at least with respect to the psite in phenoxy.…”

Section: Discussion (A) Radical-induced Lower-temperature Approachescontrasting

confidence: 79%

“…[14] The ortho-/para ratio of cresol is 1.2-1.5, essentially independent of temperature. Armstrong et al [15] investigated the products of the phenoxy ϩ phenoxy combination upon the decomposition of di-tert-butyl peroxide (dTBP) in neat phenol at 140°C over 24 h, and found all possible (enolised) condensation products; they are, in decreasing order: (Figure 1).…”

Section: Scheme 1 Formation Of Dibenzofuran Through the Dimerisationmentioning

confidence: 98%

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Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Wiater¹,

Born

Louw

2000

Eur. J. Org. Chem.

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Phenols are demonstrated precursors of “dioxins” ‐ polychlorinated dibenzo‐p‐dioxins (DDs) and dibenzofurans (DFs) ‐ in thermal processes, especially incineration. Heterogeneous catalysis, depending on conditions, can play an important role, but mere gas‐phase combination of phenolic entities to ultimately DD and/or DF is always possible. The present paper addresses the fundamental role of phenol itself. Phenol has long been known to give DF upon pyrolysis and in similar thermal reactions. In the liquid phase under oxidative conditions it yields five condensation products (A‐E); this clearly occurs through the dimerization of two phenoxy (PhO) radicals, followed by enolisation/rearomatisation. Our study shows that in the gas phase, at the lower T end, such dimers are also formed, but still with very little DF. That DF, indeed, is almost the only condensation product at elevated temperatures is substantiated by thermochemical‐kinetic analysis (favouring the pathway of ortho‐C/ortho‐C combination of two PhO radicals), as well as by results obtained with two plausible intermediates, viz. 2,2′‐dihydroxybiphenyl (A) and 2‐phenoxyphenol (C). Mechanisms for the requisite enolisation and dehydration steps leading to DF are discussed.

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Section: Discussion (A) Radical-induced Lower-temperature Approachescontrasting

confidence: 79%

Section: Scheme 1 Formation Of Dibenzofuran Through the Dimerisationmentioning

confidence: 98%

Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Wiater¹,

Born

Louw

2000

Eur. J. Org. Chem.

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“…Substrate ohne ortho-Substituenten bilden komplexe Mischungen aus ortho-und para-Kohlenstoff-Kohlenstoff-und -Kohlenstoff-Sauerstoff-Kupplungsprodukten sowie Chinon-artigen Produkten. [21][22][23] Ortho-Hydroxylierung wird häufig in Phenoloxidationen, die durch Cu/ O 2 /Amin-Systeme vermittelt werden, beobachtet, [24,25] was vermuten lässt, dass die Reaktivität vom Oxygenase-Typ mit Oxidase-Reaktionen (oxidativen Kupplungen) konkurrieren kann. Allgemein werden diese Beobachtungen durch Mechanismen erklärt, in denen die Bildung radikalischer Phenoxyl-Intermediate stattfindet, wobei die durch Einelektronen-Oxidation der Phenole gebildeten Kupfer(I)-Spezies durch molekularen Sauerstoff zu Kupfer(II) reoxidiert werden.…”

Section: Einleitung Und Historischer Kontextunclassified

Kupferkatalysierte aerobe oxidative C‐H‐Funktionalisierungen: Trends und Erkenntnisse zum Mechanismus

2011

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Die selektive Oxidation von C‐H‐Bindungen und der Einsatz von O2 als stöchiometrisches Oxidationsmittel stellen zwei wesentliche Herausforderungen in der organischen Chemie dar. Kupfer(II) ist ein vielseitiges Oxidationsmittel für verschiedene oxidative Kupplungen, die durch Einelektronen‐Transfer (SET) von elektronenreichen organischen Molekülen ausgelöst werden. Viele dieser Reaktionen können mit Kupfer katalytisch durchgeführt werden, wenn molekularer Sauerstoff als Oxidationsmittel eingesetzt wird, um den aktiven Kupfer(II)‐Katalysator zu regenerieren. Daneben wurden auch zahlreiche neue Cu‐katalysierte C‐H‐Oxidationen mit elektronenarmen Substraten beschrieben, von denen ein SET zum Kupfer(II) unwahrscheinlich scheint. In einigen dieser Fälle konnte die Beteiligung von Organokupfer(III)‐Intermediaten im Reaktionsmechanismus nachgewiesen werden. Metallorganische C‐H‐Oxidationen dieser Art eröffnen neue bedeutende Möglichkeiten für das Gebiet der Cu‐katalysierten aeroben Oxidationen.

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“…The mixture was separated by column chromatography; one of the fractions (18 mg) consisted of two compounds in a 1 : 2 ratio, both a molecular ion peak at m/z 248 and a fragmentat ion peak at m/z 166 [24]. The 1 H NMR spectrum of this fraction contained signals of aromatic (6.737.0 ppm) and hydroxyl (5.58 and 5.65 ppm) protons and an OCH multiplet (4.20 3 4.27 ppm).…”

Section: äääääääääääämentioning

confidence: 99%

Reaction of o-Quinones in Cyclohexane Solutions with Alkyl Radicals Generated by Solution γ-Irradiation

Maslovskaya

Savchenko

2005

Russ J Gen Chem

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The structures of products formed in reactions of tert-butylated o-quinones with alkyl radicals were determined by 1 H and 13 C NMR and two-dimensional 1 H3 1 H NOESY spectroscopy, and also by gas chromatography3mass spectrometry. The major products formed upon g-irradiation of deaerated solutions of 4-tert-butyl-1,2-benzoquinone and 3,5-di-tert-butyl-1,2-benzoquinone in cyclohexane are monoalkyl ethers and products of addition to the C=C bond. In the case of 4-tert-butyl-1,2-benzoquinone, these are products of mixed O3C and C3C alkylation; the adduct formed by addition of the cyclohexyl radical to the C=C bond in 3,5-di-tert-butyl-1,2-benzoquinone gives an unsymmetrical dimer whose structure was proved by single-crystal X-ray diffraction.Quinone fragments are present in molecules of many natural compounds, e.g., flavonoids, vitamins, and drugs; they participate in such biologically important processes as photosynthesis and electron transfer [13 4]. Quinones are formed by two-electron oxidation of the corresponding dihydroxybenzenes [5], by inhibited oxidation in the presence of phenolic antioxidants as a result of disproportionation of hydroxysubstituted phenoxyl radicals, by decomposition of quinone methide peroxides, and by b-cleavage of phenoxyls containing alkoxy substituents [6,7]. Accumulation of quinones and quinoid compounds is believed to be responsible both for the toxicity of cer-ÄÄÄÄÄÄÄÄÄÄÄÄ tain drugs [8,9] and for enhancement of the performance of certain antioxidants (e.g., 2,6-di-tert-butyl-4-methylphenol) in stabilization of polyolefins and polystyrenes [10 312].

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Oxidative coupling of phenols. Part 10. The role of steric effects in the formation of C–O coupled products

Cited by 43 publications

References 0 publications

Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Kupferkatalysierte aerobe oxidative C‐H‐Funktionalisierungen: Trends und Erkenntnisse zum Mechanismus

Reaction of o-Quinones in Cyclohexane Solutions with Alkyl Radicals Generated by Solution γ-Irradiation

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