- Chemoselective oxidative generation of ortho-quinone methides and tandem transformations
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ortho-Quinone methides are useful transient synthetic intermediates in organic synthesis. These species are most often generated in situ by the acid- or base-mediated transformation of phenols that have been pre-functionalized at a benzylic position, or by biomimetic oxidation of the corresponding ortho-alkylphenols with metal oxidants or transition-metal complexes. Here we describe a method for the transition-metal-free oxidative generation of o-QMs from ortho-alkylarenols, using hypoiodite catalysis under nearly neutral conditions, which can then be applied in one-pot tandem reactions. This method for the chemoselective oxidative generation of ortho-quinone methides may prove superior to previous methods with respect to environmental issues and scope, and can be applied to various tandem reactions such as inter- or intramolecular [4 + 2] cycloaddition, oxa-6π-electrocyclization, conjugate addition and spiroepoxidation. [Figure not available: see fulltext.].
- Ishihara, Kazuaki,Kondo, Ryutaro,Nishioka, Kohei,Uyanik, Muhammet
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p. 353 - 362
(2020/04/09)
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- Phenolic Oxidation Using H2O2 via in Situ Generated para-Quinone Methides for the Preparation of para-Spiroepoxydienones
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Phenols are attractive starting materials for the preparation of highly substituted cyclohexane rings via dearomative processes. Herein we report an efficient preparation of dearomatized 1-oxaspiro[2.5]octa-4,7-dien-6-ones (para-spiroepoxydienones) via the nucleophilic epoxidation of in situ generated para-quinone methides from 4-(hydroxymethyl)phenols using aqueous H2O2. The developed protocol bypasses the need for stoichiometric bismuth reagents or diazomethane, which are frequently deployed for p-spiroepoxydienone preparation. The p-spiroepoxydienones are further elaborated in numerous downstream complexity-building transformations.
- McLaughlin, Michael F.,Massolo, Elisabetta,Cope, Thomas A.,Johnson, Jeffrey S.
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p. 6504 - 6507
(2019/09/04)
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- Selective benzylic C-C coupling catalyzed by a bioinspired dicopper complex
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A highly preorganized bioinspired dicopper complex with imidazole ligation catalyzes the selective benzylic para-C-H activation of 2,4,6-trimethylphenol under aerobic conditions, yielding either the stilbenequinone or 4-methoxymethyl-2,6-dimethylphenol depending on the solvent used. The Royal Society of Chemistry.
- Prokofieva, Angelina,Prikhod'ko, Alexander I.,Dechert, Sebastian,Meyer, Franc
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p. 1005 - 1007
(2008/09/21)
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- Copper-mediated selective oxidation of a C-H bond
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(Chemical Equation Presented) An environmentally friendly procedure has been developed for the oxidation of 2,4,6-trimethylpheriol (TMP) at the para Csp3-H bond. Upon reaction with H2O2 in the presence of catalytic amounts of CuII and neocuproine in methanol at 65°C, 4-(methoxymethyl)-2,6-dimethylphenol (MDP) or 4-hydroxy-3,5- dimethylbenzaldehyde (HDB) is formed (see reaction).
- Boldron, Christophe,Gamez, Patrick,Tooke, Duncan M.,Spek, Anthony L.,Reedijk, Jan
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p. 3585 - 3587
(2007/10/03)
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- Selective copper(II)-mediated oxidative coupling of a nucleophilic reagent to the para-methyl group of 2,4,6-trimethylphenol
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A copper(II) neocuproine system has been developed for the efficient and very selective 1,6-addition of a nucleophile to the para-methyl group of 2,4,6-trimethylphenol. Crystallographic and spectroscopic data point towards the involvement of a μ-methoxo-μ-phenoxo-bridged copper species which appears to generate a highly reactive quinone methide intermediate that can be attacked by a nucleophilic reagent. The Royal Society of Chemistry 2005.
- Boldron, Christophe,Oezalp-Yaman, Seniz,Gamez, Patrick,Tooke, Duncan M.,Spek, Anthony L.,Reedijk, Jan
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p. 3535 - 3541
(2007/10/03)
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- The Oxidation of 2,4,6-Trimethylphenol with Molecular Oxygen Catalyzed by a Copper(II)-Oxime or Copper(II)-Amine System
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2,4,6-Trimethylphenol (1a) was selectively oxidized to 3,5-dimethyl-4-hydroxybenzaldehyde (2a) by molecular oxygen in the presence of a copper(II) chloride-oxime or copper(II) chloride-amine catalyst in alcohol at ambient temperature.When an acid was present in the catalyst system, the formation of both 2,6-dimethyl-p-benzoquinone (3) and a formaldehyde dialkyl acetal was accelerated.It is likely that 2a is produced by way of the formation of a quinonemethide intermediate, followed by a repeated 1,6-addition of alcohol on the intermediate to form a 4-(alkoxymethyl)-2,6-dimethylphenol (5a), and then 3,5-dimethyl-4-hydroxybenzaldehyde dialkyl acetal, which would be hydrolyzed to 2a.When the reaction time was shortened, 5a was isolated in good yield.Oxidations of the other p-methyl-substituted phenols were also examined using the present oxidation system.
- Shimizu, Masao,Watanabe, Yoshihito,Orita, Hideo,Hayakawa, Takashi,Takehira, Katsuomi
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p. 251 - 257
(2007/10/02)
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- A Facile Synthesis of 4-Alkoxymethylphenols by a Copper(II)-Acetoxime Catalyst/O2 System
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4-Alkoxymethyl-2,6-dimethylphenols were synthesized from 2,4,6-trimethylphenol in good yields by a copper(II) chloride-acetoxime catalyst/molecular oxygen system in alcohols at ambient temperature.The oxidation of the other p-methylsubstituted phenols were also examined.
- Shimizu, Masao,Watanabe, Yoshihito,Orita, Hideo,Hayakawa, Takashi,Takehira, Katsuomi
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p. 2053 - 2056
(2007/10/02)
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- Oxidation products of paramethyl-substituted hindered phenols
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Paramethyl-substituted hindered phenols are selectively oxidized by contacting with an oxidizing agent at elevated temperatures in the presence of a heterogeneous oxidative coupling catalyst. In the absence of an optional nucleophile the products comprise carbon-carbon oxidative coupling products. In the presence of a nucleophile, addition products result that may be further oxidized by continued exposure to the oxidizing agent to yield substituted p-hydroxybenzaldehydes.
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- THE CHEMISTRY OF PENTAVALENT ORGANOBISMUTH REAGENTS. PART XI. REACTIONS WITH STERICALLY HINDERED PHENOLS.
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The reactivity of BiV reagents towards very hindered phenols with tert.-butyl groups at 2 and 6 under basic conditions has been studied.Unexpected phenylation at the 4-position and, in several cases, replacement of a tert.-butyl group by phenyl have been observed.The mechanism of these unexpected reactions has been discussed.
- Barton, Derek H. R.,Finet, Jean-Pierre,Gianotti, Charles,Halley, Frank
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p. 4483 - 4494
(2007/10/02)
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- THE CHEMISTRY OF PENTAVALENT ORGANOBISMUTH REAGENTS. PART X. STUDIES ON THE PHENYLATION AND OXIDATION OF PHENOLS
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The influence of the substituents on the phenol on the regiochemistry of the arylation reactions with Ph3BiCl2 and other bismuth reagents has been studied.O-Phenylation occurs with phenols substituted with electron-withdrawing groups.Electron-donating substituted phenols undergo ortho C-phenylation.Oxidative dimerisation has been observed with 2,6-dialkyl phenols.
- Barton, Derek H. R.,Yadav-Bhatnagar, Neerja,Finet, Jean-Pierre,Khamsi, Jamal,Motherwell, William B.,Stanforth, Stephen P.
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p. 323 - 332
(2007/10/02)
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- Migrations in Oxidations of Mesidine
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The oxidation of mesidine in methanolic media by ferricyanide, dichromate, and persulfate afforded an anil 4 containing a shifted methoxymethyl group in addition to the principal anil 3 formed by oxidative dealkylation.Possible intermediates 6, 7, and 8 were prepared and oxidized to the product anils.Oxidations of related anilines 9, 10, and 13 did not parallel those of mesidine but afforded analogues of 3.There is significant spectral evidence for anils with alkyl shifts but little for anils analogous to 4.
- Goldstein, Stephen L.,McNelis, Edward
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p. 1613 - 1620
(2007/10/02)
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- Oxidation of Phenols with Iodine in Alkaline Methanol
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The use of iodine as an oxidizing agent for phenolic compounds has been explored.The reaction has been conducted in methanol containing such alkali as potassium hydroxide and, depending on the nature of the substituents and on the amount of iodine employed, leads to iodination, oxidation to give a stable phenoxy radical, oxidative dimerization, or benzylic oxidation.In general the reaction proceeds smoothly at room temperature, and under appropriate conditions yields of products are good to excellent.Oxidative dimerization of 2,4- and 2,6-di-tert-butylphenols invol-ves iodination followed by iodine-catalyzed dimerization.The oxidation of 4-methylphenols with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in methanol has been carried out for comparison.
- Omura, Kanji
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p. 3046 - 3050
(2007/10/02)
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- Kinetic Studies of the Methanolysis Reaction of Chloromethylated Phenols
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The solvolysis of 25 differently substituted chloromethylated phenols was studied kinetically in methanol at 25 deg C.A sharp decrease of the initial reaction rate with increasing concentrations of added acids can be explained by a very fast solvolysis of the phenolate anions in comparison with the undissociated compounds.The latter show strictly first order kinetics up to high conversions and the rate constants can be partly correlated with the Jaffe relation.Highly negative values for the reaction constants ρ=-5.4 and ρ=-6.2 for ortho- and para-chloromethylated compounds show, that the undissociated phenols react according to the SN1-mechanism.However, deviations are found for compounds with strongly electron attracting substituents, which may be partly caused by an intramolecular catalytic effect of the phenolic hydroxy group in the case of the ortho-isomers. - Keywords: Methanolysis, Chloromethylated Phenols, Anchimeric Assistance
- Stein, Guenter,Boehmer, Volker,Lotz, Werner,Kaemmerer, Hermann
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p. 231 - 241
(2007/10/02)
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