Gas-phase thermochemical properties of some tri-substituted phenols: A density functional theory study

The study of the energetics of phenolic compounds has a considerable practical interest since this family of compounds includes numerous synthetic and naturally occurring antioxidants. In this work, density functional theory (DFT) has been used to investigate gas-phase thermochemical properties of the following tri-substituted phenols: 2,4,6-trimethylphenol, 2,6-dimethyl-4-tert-butylphenol, 2,6-dimethyl-4-methoxyphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-methoxyphenol, 2,4,6-trimethoxyphenol, 2,6-dimethoxy-4-methylphenol and 2,6-dimethoxy-4-tert-butylphenol. Molecular structures were computed with the B3LYP and the ωB97X-D functionals and the 6-31G(d) basis set. More accurate energies were obtained from single-point energy calculations with both functionals and the 6-311++G(2df,2pd) basis set. Standard enthalpies of formation of the phenolic molecules and phenoxyl radicals were derived using an appropriate homodesmotic reaction. The OH homolytic bond dissociation enthalpies, gas-phase acidities and adiabatic ionization enthalpies were also calculated. The general good agreement found between the calculated and the few existent experimental gas-phase thermochemical parameters gives confidence to the estimates concerning the phenolic compounds which were not yet experimentally studied.     

Synthesis and structural characterization of a highly substituted triazine ring comprising a sterically flexible methylene linker and coordinating substituents

The efficient multi-step, large-scale synthesis, spectroscopic characterization and solid-state molecular structure of a new type of three-fold functionalized, sterically demanding triazine is reported. The aromatic heterocycle 6,6′,6′'-((1,3,5-triazine-2,4,6-triyl)tris(methylene))tris(2,4-di-tert-butylphenol) possesses three 2,4-di-tert-butylphenol synthons bound to the 1,3,5-triazine ring via synthetically challenging methylene linkages in the 2,4,6 positions. The key to success was found in the generation of a highly reactive imidate hydrochloride salt, namely ethyl 2-(3,5-di-tert-butyl-2-methoxyphenyl)acetimidate hydrochloride, that readily undergoes cyclotrimerization. The reported preparation opens new perspectives in the design and synthesis of novel triazine molecules bearing flexible and sterically demanding functionalized groups for various applications.     

Reactions of perfluoro(2-methylpent-2-ene) and perfluoro(5-azanon-4-ene) with primary amines containing a 2,6-di-tert-butylphenol fragment

Reactions of perfluoro(2-methylpent-2-ene) and perfluoro(5-azanon-4-ene) with 4-(2-aminoethyl)-2,6-di-tert-butylphenol and 4-(3-aminopropyl)-2,6-di-tert-butylphenol in acetonitrile in the presence of triethylamine gave the corresponding azetidine, 1,2-dihydroazete, and 1,2-dihydro-1,3-diazete derivatives, respectively. The reaction mechanisms, role of triethylamine, and factors affecting the intramolecular nucleophilic cyclization process are discussed.     

β-Hydroxyalkylation of sterically hindered phenols with epoxides in acid medium

Reactions of 2,6-dialkylphenols with ethylene oxide, propylene oxide and epichlorohydrin in the presence of SnCl₄ at the temperature from ?5 to +5°C leads to the formation of respective phenols containing a hydroxy group in the β-position of the aliphatic chain of the para-substituent. The conditions for maximum selectivity of the reaction of 2,6-di-tert-butylphenol with ethylene oxide were determined. By HPLC-MS method the directions of the side reactions were explored. The method has been successfully tested in a pilot installation. With 2,6-dimethylphenol instead of 2,6-di-tert-butylphenol a sharp increase occurs in the content of ethers in the reaction product. With epichlorohydrin, 2,6-di-tert-butylphenol affords a product, which is easily converted into an epoxide containing a sterically hindered phenol in its structure.     

Mn(III) salen complexes-catalyzed enantioselective addition of trimethyl silylcyanide to N-benzylimines in the presence of 4-phenyl pyridine-N-oxide as an additive

Chiral monomeric and dimeric Mn(III) salen complexes viz., [(S,S)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato manganese(III) chloride and 5,5-methylene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N′-(3′,5′-di-tert-butyl salicylidene)}-1,2-cyclohexanediaminato manganese(III) chloride were used as catalysts for the highly enantioselective Strecker reaction of various imines (derived by the condensation of aldehydes and amine) with TMSCN as a source of cyanide in toluene. Excellent yield (95%) of α-amino nitrile with ee >99% was achieved when N-(2-methoxybenzylidine)-1-phenylmethanamine was used as a substrate and 4-phenyl pyridine-N-oxide (4-PPNO) as an additive at ?55 °C in 28 h. The dimeric catalyst was found to be more reactive and enantioselective than the monomeric catalyst. The chiral dimeric catalyst used in the present study was recoverable and recyclable several times with retention of its performance.     

Intramolecular reorientations and the effects of thermal history and hydrogen bonding in four closely related organic molecular solids

We have measured proton spin—lattice relaxation rates at 30 MHz from 77 to 330 K in para-di-t-butylbenzene, 2,5-di-t-butylphenol, 2,6-di-t-butylphenol, 3,5-di-t-butylphenol and modifications of the latter two in which the hydroxyl proton has been replaced with a deuteron.The observed maxima in the relaxation rates are interpreted in terms of the reorientations of the t-butyl groups and their constituent methyl groups. The observed relaxation rate depends, to some degree, on the thermal history of the samples and in the phenols this is attributed, in part, to the formation of dimeric and polymeric forms via intermolecular hydrogen bonding forming OH... O bridges. Infrared spectra are run with the three phenols in both the dilute liquid and solid state and the results interpreted in terms of hydrogen bonding. Nuclear spin relaxation due to the flip-flop of the hydroxyl proton is observed in 2,6-di-t-butylphenol but not in 3,5-di-t-butylphenol as expected on the basis of the IR spectra.     

Development of a one-stage synthesis of 2,6-di-<Emphasis Type="Italic">tert</Emphasis>-4-ethylbutylphenol from 2,6-di-<Emphasis Type="Italic">tert</Emphasis>-butylphenol

Investigation of the catalyzed reaction of 2,6-di-tert-butylphenol with ethanol, ethylene glycol, oligomeric glycols, and paraldehyde in a strongly basic medium permitted to develop a technologically suitable procedure for manufacture of 2,6-di-tert-4-ethyl-butylphenol, used in the synthesis of Antioxidant-425.     

Catalytic activity of CuI/CuII cyanide based phenanthroline-bicarbonate system for enhancing aerobic oxidation of 2,6-di-tert-butylphenol

The metal organic framework {[Cu₂(CN)₃(phen)₃]5H₂O} MOF1- bicarbonate system was investigated as an efficient catalyst for aerobic oxidation of 2, 6-di-tert-butylphenol (2,6- DTBP). The catalytic system showed very efficient catalytic behavior for the oxidation of selective coupling of 2,6- DTBP to 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) in excellent yield. The influence of reaction parameters on the selective oxidation of 2, 6-DTBP to DPQ had been investigated. Photoluminescence probing technology of Disodium salt of terephthalic acid as well as scavenging experiments revealed the creation of the hydroxyl radicals as the main active oxidation radicals produced by the MOF1/O₂/basic bicarbonate system. The oxidation reaction mechanism was also discussed. The recycled catalytic system retained its activity for eight successive runs.     

Synthesis and ESR study of Co and Ni hydrazides and hydroxamates containing 2,6-di-tert-butylphenol moiety in the ligands

A series of Co and Ni hydrazides and hydroxamates containing the 2,6-di-tert-butylphenol moiety as a potential free radical precursor in the ligand environment of the complexes were synthesized. ESR spectroscopy was used to study the dependence of the stability of metal complexes incorporating a paramagnetic center in the ligand on the type of the coordination environment of the metal and the nature of the ligand. The complexes, in which a saturated aliphatic bridging group separates the 2,6-di-tert-butylphenol moiety and the coordination environment of the metal, can undergo oxidation to produce stable paramagnetic species with an unpaired electron in the ligand environment. The interaction of the free radical center in the ligand with the coordination environment of the metal is practically absent (excepting the weak spin-spin interaction). Metal complexes incorporating a hydrazine moiety conjugated with the 2,6-di-tert-butylphenol substituent do not produce detectable stable paramagnetic forms with a free radical in the ligand.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1794–1797, September, 1995.     

Mononuclear tungsten(VI) complexes with bis(phenolato) ligands: syntheses,characterisations and activity in ROMP reaction

The reaction of bulky ligand precursor 2,2′-methylenebis(4-methyl-6-tert-butylphenol) (H₂mbp) or 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (H₂ebp) with trisdiolatotungsten(VI) complex [W(eg)₃] 1 (eg=ethanediolate dianion) provides heteroleptic complexes [W(mbp)(eg)₂] 2 or [W(ebp)(eg)₂] 3, respectively. Sterically less hindered 2,2′-dihydroxy-1,1′-dinaphtylmethane (H₂dinap) forms heteroleptic disubstituted complex [W(dinap)₂(eg)] 4. The X-ray crystal structure determinations confirmed that the isolated compounds are made of monomeric tris(diolato)tungsten(VI) molecules in which the central tungsten atom is bonded to six oxygen atoms forming a distorted octahedral coordination sphere around the metal ion. Complexes 2 and 3 catalyse the ring opening metathesis reaction of norbornene when activated by Et₂AlCl.     

Electrochemical method in determination of antioxidative activity using ferrocene derivatives as examples

Electrochemical method for the evaluation of antioxidative activity of compounds based on their reaction with the stable radical, 2,2-diphenyl-1-picrylhydrazyl, was suggested with monitoring of the reaction by cyclic voltammetry (CVA). Antioxidative properties of new ferrocene derivatives Fc(L)R (where Fc is the ferrocenyl, R is the fragment of 2,6-di-tert-butylphenol or its aromatic analog, L is the spacer) were studied. Anodic oxidation of the compounds Fc(L)R, which contain azomethine and 2,6-di-tert-butylphenol moities, proceeds in three steps, that suggests a possibility of intramolecular proton-coupled electron transfer process. Conjugates of ferrocene and 2,6-di-tert-butylphenol are efficient antioxidants.     

Potassium and sodium 2,6-di-tert-butylphenoxides and their properties

The crucial factor of the reaction of 2,6-di-tert-butylphenol with alkali hydroxides is temperature, depending on which two types of potassium or sodium 2,6-di-tert-butylphenoxides are formed. These types exhibit different catalytic activity in the alkylation of 2,6-di-tert-butylphenol with methyl acrylate. More active forms of 2,6-Bu^t ₂C₆H₃OK or 2,6-Bu^t ₂C₆H₃ONa are synthesized at temperatures higher than 160 °C and are predominantly the monomers, which dimerize on cooling. The data of ¹H NMR, electronic, and IR spectra for the corresponding forms of 2,6-Bu^t ₂C₆H₃OK and 2,6-Bu^t ₂C₆H₃ONa isolated in the individual state are in agreement with cyclohexadienone structure. In DMSO or DMF, the dimeric forms of 2,6-di-tert-butylphenoxides react with methyl acrylate to form methyl 3-(4-hydroxy-3,5-di-tert-butylphenyl)propionate in 64–92% yield. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2138–2143, December, 2006.     

Cobalt and manganese complexes with redox-active ligands in polymerization of acrylonitrile and methyl methacrylate

The influence of bis[4,6-di-tert-butyl-N-(2,6-dimethylphenyl)-o-iminobenzosemiquinonato]cobalt(ii) and 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzosemiquinonato]-[4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-amidophenolate]manganese(iii) on polymerization of methyl methacrylate and acrylonitrile in the presence of azobisisobutyronitrile (as a traditional radical initiator) and alkyl halides (used for initiation of controlled atom transfer radical polymerization process) was studied. The effect of the nature of the activating agents (amines) and the temperature conditions on the overall polymerization rate of the indicated monomers, as well as molecular weight characteristics of the synthesized polymers, were analyzed. The optimal conditions for the synthesis of polyacrylonitrile and poly(methyl methacrylate) with a relatively narrow molecular weight distribution were selected.     

Determination of antioxidants in new and used lubricant oils by headspace-programmed temperature vaporization–gas chromatography–mass spectrometry

A sensitive method is presented to determine antioxidants (2-, 3-, and 4-tert-butylphenol, 2,6-di-tert-butylphenol, 3-tert-butyl-4-hydroxyanisol, 2,6-di-tert-butyl-4-methylphenol, 1-naphthol, and diphenylamine) in new and used lubricant oil samples. Research was carried out on a GC device equipped with a headspace sampler, a programmed temperature vaporizer, and an MS detector unit. The proposed method does not require sample treatment prior to analyses, hence eliminating possible errors occurring in this step. Sample preparation is reduced when placing the oil sample (2.0 g) in the vial and adding propyl acetate (20 μL). Solvent vent injection mode permits a pre-concentration of the compounds of interest in the liner filled with Tenax-TA®, while venting other species present in the headspace. Thereby, both the life of the liner and the capillary column is prolonged, and unnecessary contamination of the detector unit is avoided. Calibration was performed by adding different concentrations of analytes to a new oil which did not contain any of the studied compounds. Limits of detection as low as 0.57 μg/L (2-tert-butylphenol) with a precision lower or equal to 5.3% were achieved. Prediction of the antioxidants in new oil samples of different viscosities (5W40, 10W40, and 15W40) was accomplished with the previous calibration, and the results were highly satisfactory. To determine antioxidants in used engine oils, standard addition method was used due to the matrix effect.     

About selective methods of synthesis of 6-<Emphasis Type="Italic">tert</Emphasis>-butyl-2-methylphenol and 6-<Emphasis Type="Italic">tert</Emphasis>-butyl-2,4-dimethylphenol

Vapor phase catalytic methylation with methanol of 2-tert-butylphenol at the temperature 280–300°C proceeds selectively with formation of 6-tert-butyl-2-methylphenol. Elevating reaction temperature above 300°C leads to formation of 2,6-dimethylphenol. Reaction of 2-tert-butylphenol with methanol in alkaline medium in the presence of zinc oxide is shown to lead initially to formation of a mixture of calixarenes and methylenebisphenols that at elevated temperature exert splitting leading in future to 6-tert-butyl-2,4-dimethylphenol. Obtaining it in this reaction from 2,2′-methylenebis-(6-tert-butyl-4-methylphenol) proceeds selectively. Pathways of the reductive methylation of methylenebisphenols with methanol in alkaline medium is considered.     

Synthesis of 3,3′,5,5′-Tetra-<Emphasis Type="Italic">tert</Emphasis>-butyl-4,4′-stilbenequinone and Its Catalytic Activity in the Liquid-Phase Oxidation of Inorganic Sulfides

The oxidation of 2,6-di-tert-butyl-4-methylphenol with hydrogen peroxide in the presence of potassium iodide gave 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone which catalyzed liquid-phase oxidation of sodium sulfide with oxygen more efficiently than did 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone.     

Design of postmetallocene Schiff base-like catalytic systems for polymerization of olefins: XII. Synthesis of tetradentate bis-salicylaldehyde imine ligands

Reactions of salicylaldehyde, 3-tert-butylsalicylaldehyde, and 3,5-di-tert-butylsalicylaldehyde with 1,4-diaminobutane, 1,6-diaminohexane, 4,4′-diaminodiphenylmethane, 4,4′-diamino-3,3′,5,5′-tetramethyldiphenylmethane, 4,4′-diamino-5,5′-dicyclopentyl-3,3′-dimethyldiphenylmethane, 4,4′-diamino-5,5′-dicyclohexyl-3,3′-dimethyldiphenylmethane, bis(4-aminophenyl) sulfone, o,o′- and p,p′-diaminodiphenyl ethers, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 4,4″-diamino-p-terphenyl gave a series of the corresponding Schiff bases which can be used as tetradentate ligands for the synthesis of titanium and zirconium complexes.     

Reactions of stabilizer compounds. III. Oxidative reactions of some 2′-hydroxy-2-phenylbenzotriazole light stabilizers in the AIBN-initiated autoxidation of cumene

Oxidative decay of number of commercially important 2′-hydroxy-2-phenylbenzotriazole light stabilizers during the AIBN-initiated autoxidation of cumene at 65°C is described. The reactivity of the hydroxyphenylbenzotriazoles studied increased in the order 2-(2′-hydroxy-5′-methyl)phenylbenzotriazole (Ia) < 2-(2′-hydroxy-3′,5′ -di-tert-pentyl)phenylbenzotriazole (Ie) < 5-chloro-2-(3′,5′-di-tert-butyl-2′-hydroxy)phenylbenzotriazole (Ic) < 5-chloro-2-(3′-tert-butyl-2′-hydroxy-5′-methyl)phenylbenzotriazole (Ib). The major product from the reaction of Ib was identified as the cumylperoxycyclohexa-2,5-dienone (III). The possible occurrence of these reactions during the degradation of stabilized polymers is discussed.     

Molecular Complexes of Phenols with Dicyclohexylamine

The Mannich reaction of 2,4-di-tert-butylphenol with dicyclohexylamine was found to involve formation of a stable 1:1 molecular complex between the phenol and amine. The complex does not change on melting, sublimation, and chromatography on silica gel. Its structure was determined by X-ray analysis. Stable 1:1 complexes of dicyclohexylamine with a series of substituted phenols were synthesized; exceptions were 2,6-disubstituted phenols with bulky substituents.     

Photolysis of 2,6 di-tert-butyl-4-tert-butylperoxy-4-methyl-2,5-cyclohexadienone

The photolysis of a heptane solution of 2,6-di-tert-butyl-4-tert-butylperoxy-4-methyl- 2,5-cyclohexadienone (1) with light of 360–480 nm in the presence of 2,6-di-tert-butyl-4-methylphenol or 2,4,6-tri-tert-butylphenol gives rise to their corresponding phenoxyls in a reaction with the tert-butoxyl arising in the photolysis. Direct evidence of the formation of t-BuO^. by the photolysis of 1 was provided by the ESR characterization of spin-adducts of this radical with nitrosobenzene and nitrosodurene. Evidence of the photolysis of the peroxide bond in 1 is important for the mechanism of reaction of phenolic chain-breaking antioxidants.