- Efficient synthesis of 2,5-di-t-butyl-4-fluorophenol
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When 4-fluorophenol was refluxed with excess of t-butyl chloride in the presence of various catalysts, e.g. Envirocat EPZG, EPZ10, EPIC, sulfated zirconia, natural kaolinitic clay, zirconium nitrate, zinc chloride and bismuth nitrate, the product obtained was 2,5,di-t-butyl 4-fluorophenol in excellent yield.
- Bandgar,Kasture,Dudhmal, Chaya
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Read Online
- Practical, mild and efficient electrophilic bromination of phenols by a new I(iii)-based reagent: The PIDA-AlBr3 system
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A practical electrophilic bromination procedure for phenols and phenol-ethers was developed under efficient and very mild reaction conditions. A broad scope of arenes was investigated, including the benzimidazole and carbazole core as well as analgesics such as naproxen and paracetamol. The new I(iii)-based brominating reagent PhIOAcBr is operationally easy to prepare by mixing PIDA and AlBr3. Our DFT calculations suggest that this is likely the brominating active species, which is prepared in situ or isolated after centrifugation. Its stability at 4 °C after preparation was confirmed over a period of one month and no significant loss of its reactivity was observed. Additionally, the gram-scale bromination of 2-naphthol proceeds with excellent yields. Even for sterically hindered substrates, a moderately good reactivity is observed.
- Satkar, Yuvraj,Ramadoss, Velayudham,Nahide, Pradip D.,García-Medina, Ernesto,Juárez-Ornelas, Kevin A.,Alonso-Castro, Angel J.,Chávez-Rivera, Ruben,Jiménez-Halla, J. Oscar C.,Solorio-Alvarado, César R.
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p. 17806 - 17812
(2018/05/28)
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- Efficient, rapid, and regioselective bromination of phenols and anilines with N-bromosaccharin using tungstophosphoric acid as a heterogeneous recyclable catalyst
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A simple, efficient, and rapid method for high-yielding regioselective mono bromination of phenols and anilines has been achieved by treatment with N-bromasaccharin in the presence of a catalytic amount of tungstophosphoric acid. Copyright Taylor & Francis Group, LLC.
- Alinezhad, Heshmatollah,Tavakkoli, Sahar Mohseni,Salehian, Fatemeh
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experimental part
p. 3226 - 3232
(2010/12/24)
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- Tyrosine analogues for probing proton-coupled electron transfer processes in peptides and proteins
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A series of amino acids analogous to tyrosine, but differing in the physicochemical properties of the aryl alcohol side chain, have been prepared and characterized. These compounds are expected to be useful in understanding the relationships between structure, thermodynamics, and kinetics in long-range proton-coupled electron transfer processes in peptides and proteins. Systematic changes in the acidity, redox potential, and O-H bond strength of the tyrosine side chain could be induced upon substituting the phenol for pyridinol and pyrimidinol moieties. Further modulation was possible by introducing methyl and t-butyl substitution in the position ortho to the phenolic hydroxyl. The unnatural amino acids were prepared by Pd-catalyzed cross-coupling of the corresponding halogenated aryl alcohol protected as their benzyl ethers with an organozinc reagent derived from N-Boc L-serine carboxymethyl ester. Subsequent debenzylation by catalytic hydrogenation yielded the tyrosine analogues in good yield. Spectrophotometric titrations revealed a decrease in tyrosine pK a of ca. 1.5 log units per included nitrogen atom, along with a corresponding increase in the oxidation (peak) potentials of ca. 200 mV, respectively. All told, the six novel amino acids described here have phenol-like side chains with pKa's that span a range of 7.0 to greater than 10, and an oxidation (peak) potential range of greater than 600 mV at and around physiological pH. Radical equilibration EPR experiments were carried out to reveal that the O-H bond strengths increase systematically upon nitrogen incorporation (by ca. 0.5-1.0 kcal/mol), and radical stability and persistence increase systematically upon introduction of alkyl substitution in the ortho positions. The EPR spectra of the aryloxyl radicals derived from tyrosine and each of the analogues could be determined at room temperature, and each featured distinct spectral properties. The uniqueness of their spectra will be helpful in discerning one type of aryloxyl in the presence of other possible aryloxyl radicals in peptides and proteins with multiple tyrosine residues between which electrons and protons can be transferred.
- Nara, Susheel J.,Valgimigli, Luca,Pedulli, Gian Franco,Pratt, Derek A.
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experimental part
p. 863 - 872
(2010/03/25)
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- Mapping the active site in a chemzyme: Diversity in the N-substituent in the catalytic asymmetric aziridination of imines
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(Chemical Equation Presented) The active site of the aziridination catalyst derived from either the VANOL or VAPOL ligand and B(OPh)3 is larger than expected and can accommodate not only significant substitution on the diarylmethyl unit of the imine but also that alkyl (but not perfluorylalkyl) substituents on the aryl groups lead to enhanced rates and enantioselection. The screen of diarylmethyl N-substituents on the imine revealed that the 3,5-di-tert-butyldianisylmethyl group (BUDAM) gave exceptionally high asymmetric inductions for imines of aryl aldehydes.
- Zhang, Yu,Lu, Zhenjie,Desai, Aman,Wulff, William D.
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supporting information; experimental part
p. 5429 - 5432
(2009/06/20)
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- Electron transfer between protonated and unprotonated phenoxyl radicals
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(Chemical Equation Presented) The reaction of phenoxyl radicals with acids is investigated. 2,4,6-Tri-tert-butylphenoxyl radical (13), a persistent radical, deteriorates in MeOH/PhH in the presence of an acid yielding 4-methoxycyclohexa-2,5-dienone 18a and the parent phenol (14). The reaction is facilitated by a strong acid. Treatment of 2,6-di-tert-butyl-4-methylphenoxyl radical (2), a short-lived radical, generated by dissociation of its dimer, with an acid in MeOH provides 4-methoxycyclohexa-2,5-dienone 4 and the products from disproportionation of 2 including the parent phenol (3). A strong acid in a high concentration favors the formation of 4 while the yield of 3 is always kept high. Oxidation of the parent phenol (33) with PbO2 to generate transient 2,6-di-tert-butylphenoxyl radical (35) in AcOH/H2O containing an added acid provides eventually p-benzoquinone 39 and 4,4′-diphenoquinone 42, the product from dimerization of 35. A strong acid in a high concentration favors the formation of 39. These results suggest that a phenoxyl radical is protonated by an acid and electron transfer takes place from another phenoxyl radical to the protonated phenoxyl radical, thus generating the phenoxyl cation, which can add an oxygen nucleophile, and the phenol (eq 5). The electron transfer is a fast reaction.
- Omura, Kanji
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p. 858 - 867
(2008/09/19)
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- Ligand designed with pending phenol group
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The synthesis of a salicylaldehyde derivative facing an encumbered phenol group on a naphthalene block as a molecular shaft is reported. This molecular unit has been designed to elaborate coordinating ligands holding non-coordinating phenol group for the generation of phenoxyl radical in the close proximity of a metal complex.
- Sabater, Laurent,Lachaud, Fabien,Hureau, Christelle,Aukauloo, Ally
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p. 569 - 571
(2007/10/03)
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- Synthesis of 2,5- and 2,6-di-t-butyl-4-halo- or -4-methoxy-phenols using silica, lithium perchlorate and lithium bromide as neutral catalysts
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When a mixture of 4-halo- or 4-methoxy-phenol and excess of t-butyl chloride in the presence of neutral catalyst such as silica or lithium perchlorate or lithium bromide was refluxed, 2,5-and 2,6-di-t-butyl-4-halo or 4-methoxy phenols were obtained in good yields.
- Bandgar,Uppalla,Sadavarte
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p. 582 - 583
(2007/10/03)
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- Asymmetric synthesis and Lewis acid mediated type II carbonyl ene cyclisations of (R)-2-isopropyl-5-methylhex-5-enal
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The asymmetric synthesis of (R)-2-isopropyl-5-methylhex-5-enal in 98% ee is described. It was discovered that the key alkylation step employing an Evans chiral auxiliary and 3-methylbut-3-en-1-yl trifluoromethanesulphonate as the alkylating agent led to significant competing O-alkylation, a phenomenon not previously reported. Type II carbonyl ene cyclisation of the aldehyde with a range of Lewis acids led to either the (R,R)- or (R,S)-5-methylidenecyclohexanols without concurrent racemisation of the alpha stereogenic centre of the aldehyde. Conditions for effecting the easy racemisation of a model enantiomerically pure aldehyde, (S)-2-methylbutanal, were developed. In an effort to secure a dynamic kinetic resolution procedure, these conditions were applied to (R)-2-isopropyl-5-methylhex-5-enal. However, a competing and dominant Prins cyclisation occurred instead leading to a mixture of all possible cycloadducts, all of which were obtained in 98% ee. Any unreacted aldehyde was found to be enantiomerically pure. Copyright (C) 2000 Elsevier Science Ltd.
- Braddock, D. Christopher,Brown, John M.
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p. 3591 - 3607
(2007/10/03)
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- A novel protecting group for hindered phenols
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Boc2O and DMAP were used to protect hindered phenols as their Boc derivatives under mild conditions. Deprotection conditions were developed to suppress loss of a tert-butyl group from the aromatic ring, or alkylation of an additional tert-butyl group at an unsubstituted ortho or para position.
- Hansen, Marvin M.,Riggs, Jennifer R.
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p. 2705 - 2706
(2007/10/03)
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- Dienone tautomers of 4-alkoxy-2,6-di-terf-butylphenols
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Generation and isolation of 4-alkoxycyclohexa-2,5-dienones 9, the tautomeric forms of the title phenols (10), is described. They are generated efficiently by the Ag ion mediated reaction of 4-bromocyclohexa-2,5-dienone 3b with simple alcohols, although they can be irreversibly isomerized into 10 under the reaction conditions. Crude materials with high amounts of 9 can be obtained by conducting the reaction with AgClO4 in the presence of Na2CO3 or with AgOCOCF3 and by interrupting the reaction shortly after the formation of 9 is complete. The AgOCOCF3 reaction produces labile 4-(trifluoroacetoxy)cyclohexa-2,5-dienone 11 also, the formation of which becomes significant as the alcohol becomes bulky. All of 9 prove to be very much susceptible to the prototropic rearrangement into 10 by catalysis with base, acid, or SiO2. Crude dienones 9 can be conveniently prepared directly from phenol 6 by treatment for a short time with Br2 in alcohols containing AgClO4 and Na2CO3. A one-pot synthesis from 6 of 4-oxyfunctionalized 2,6-di-tert-butylphenols, including 10, is also described.
- Omura, Kanji
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p. 7156 - 7161
(2007/10/03)
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- A mechanistic approach to the reaction of 2,6-di-tert-butylphenol with an iodinating agent in methanol: Electrophilically assisted solvolysis of intermediary 4-iodocyclohexa-2,5-dienones
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Reactions of the title phenol (1) and of 4-iodophenol 2 with an iodinating agent, I2 and H2O2, are conducted in MeOH for varying times with varying amounts of I2, and the results are compared. The reaction of 1 gives 2, 4,4′-biphenol 3, 4,4′- diphenoquinone 4, 4-methoxyphenol 5, and p-benzoquinone 6, exclusively. The yields of the phenolic products (2, 3, and 5) vary with reaction time, but they disappear or almost disappear eventually, to make 4 and 6 the almost exclusive products. The reaction of 2 always gives 4 and 6 alone. In both of the reactions of 1 and of 2, employment of a higher initial I2 concentration not only completes the formation of 4 and 6 faster but also makes the final proportion of 6 higher. However, the ultimate yield of 6 from the reaction of 1 is significantly higher than that from the reaction of 2, irrespective of the initial I2 concentration. These results are interpreted as follows. 4-Iodocyclohexa-2,5-dienone 12, the primary product of electrophilic iodination of 1, undergoes solvolysis (methanolysis), which is electrophilically assisted by I2. The solvolysis of 12 can be so fast as to overwhelm its prototropic rearrangement to give 2. 4-Methoxycyclohexa-2,5-dienone 13, which is the primary product of the methanolysis of 12 and is suggested to be detectable by 1H NMR spectroscopy, is converted into 6 via 5. Benzoquinone 6 can also arise from 4,4-diiodocyclohexa-2,5-dienone 7, the product of iodination of 2, by an analogous mechanism. The selectivity of the formation of 6 from 7 is low because the competing reaction, homolytic scission of the C-I bond in 7, predominates. The mechanism of the formation of 3 and 4 is also discussed.
- Omura, Kanji
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p. 2006 - 2012
(2007/10/03)
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- Formation of Diphenyl Ethers from Cyclohexa-2,5-dienones via 4-phenoxy-4-(1-alkoxy)cyclohexa-2,5-dienones as Probable Intermediates
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Acid-catalysed reactions of the cyclohexa-2,5-dienones (3) and (14) with phenol afford diphenyl ethers.Quinol ethers are considered to be intermediates in these reactions, with aromatisation of the cyclohexa-2,5-dienone ring by loss of the 4-(1-alkoxy) side-chain as an aldehyde constituting the driving force.
- Karhu, Matti
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p. 303 - 306
(2007/10/02)
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- Reactions of 4-Bromo-4-(1-hydroxypropyl)-2,6-di-t-butylcyclohexa-2,5-dienone in Methanolic Sulphuric Acid
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Reactions of the title compound (1) in methanolic sulphuric acid are described.At high acid concentrations the main reaction is the cleavage of the side chain as propionic aldehyde.At low acid concentrations the products are mainly formed by hydrolysis of (1) and subsequent reduction and oxidation reactions involving the bromide ion and bromine.
- Karhu, Matti
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p. 1595 - 1598
(2007/10/02)
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