Phenols, tert-butyl groups

Generally, oxepins have a tendency to contract to a six-membered carbocycle when treated with acid. The driving force is the aromaticity of the phenol formed. However, when the less stable cyclohexa-2,5-diene-1,4-diol with an appropriate substitution pattern is treated with acid, the oxepin system is obtained. The treatment of cyclohexadienediols that are substituted with tert-butyl groups in the 2- and 6-positions and aryl at Cl and C4 with trifluoroacetic acid produces oxepins 1 with elimination of water.186 187 This reaction, however, is restricted to certain aryl substituents with at least some electron-donating effect. Generally, cyclohexa-2,4-dienone derivatives 2 are formed.187,188... 

Tert-butyl calix[4]arene possessing four antennary amine groups (7) were initially synthesized from commercially available phenolic tert-butyl calix[4]arene according to a published procedure [13]. After derivatization of the phenolic group by alkylation with ethyl bromoacetate, hydrolysis, acid... 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

ButylatedPhenols and Cresols. Butylated phenols and cresols, used primarily as oxidation inhibitors and chain terminators, are manufactured by direct alkylation of the phenol using a wide variety of conditions and acid catalysts, including sulfuric acid, -toluenesulfonic acid, and sulfonic acid ion-exchange resins (110,111). By use of a small amount of catalyst and short residence times, the first-formed, ortho-alkylated products can be made to predominate. For the preparation of the 2,6-substituted products, aluminum phenoxides generated in situ from the phenol being alkylated are used as catalyst. Reaction conditions are controlled to minimize formation of the thermodynamically favored 4-substituted products (see Alkylphenols). The most commonly used is -/ -butylphenol [98-54-4] for manufacture of phenolic resins. The tert- butyl group leaves only two rather than three active sites for condensation with formaldehyde and thus modifies the characteristics of the resin. 

These heteropolyacids are superior to Nafion and give 4-R-phenol with 92-98% yields at 373-413 K by use of toluene or p-xylene for tert-butyl group acceptors. It is claimed that these catalysts can be readily separated from the reaction mixture and reused. 

The salen-Ni(II) complex 39a derived from (lR,2R)-[N,N -bis(2 -hydroxybenzyl-idene)]-l,2-diaminocyclohexane was also equally effective (Table 7.3, entry 4). In contrast to earlier reports on salen-metal complexes, where the introduction of a bulky tert- butyl substituent increased enantioselectivity [31], the use of complex 39b exhibited a significant decrease in enantioselectivity (entry 5). The presence of a bulky tert-butyl group obstructed the chelation of alkali metal ions by phenolic oxygen atoms. A dramatic increase in selectivity could be achieved when nickel was replaced with copper, and a salen-Cu(II) complex 39c afforded 85% ee (entry 6). Although screening of other bases or 50% NaOH were not advantageous, the use of 3 equiv. NaOH improved the enantiomeric excess to 92% (entry 9) and after recrystallization of a-methylphenylalanine optical purity was increased to 98% ee. 

Inclusion complexes were prepared by crystallization of Cl-MIT and host compounds from methanol. When alkyl phenol compounds (2-10) were used as hosts, a 2 1 complex (17) of 2,4-di-ferf-butylphenol (8) and Cl-MIT (1) was obtained (Table 2). However, other similarly structured phenol compounds (2-7,9,10) failed to yield inclusion complexes. For example, when a tert -butyl group from 8 was exchanged for a methyl group (6) or an ethyl group (7), no inclusion complex was obtained, so it is obvious that even a slight difference in structure influences whether or not an inclusion complex is formed. 

Oxidation of 2,6-di(tert-butyl)phenol (23), both ortfjo-positions of which are blocked by the bulky tert-butyl groups, was effected with 4 equivalents of methyl(trifluoromethyl)dioxirane (553) at 0°C for 1 min to afford three oxygenated products (554, 74 and 555) in 4, 24 and 70% yields, respectively. Dimethyldioxirane-promoted oxidation of 23 required much longer reaction time (48 h) to yield the... 

Cyanox 1790 and Lowinox WSP exhibited the next best stabilization efficiencies (of Irganox 1076). This can be related with the combination of substitution in both ortho positions of a methyl-phenyl group and a tert-butyl group, which increases the radical trapping rate constant [40]. These two products are similar from the point of view of their substitution in the para position of the phenolic nucleus [41] and they will give the same type of stable quinone methide. Lowinox TBP6 exhibited a lower... 

Cohen and Jones (36) have shown that ortho substitution of two tert-butyl groups causes a large increase in the basicity of basic phenoxide ions. However, this effect becomes smaller with decreasing pK of the phenol and essentially disappears for phenols of pK < 7. This evidence indicates that the importance of solvation, and presumbly the number of solvating water molecules, decreases sharply for weakly basic substituted phenoxide ions. 

Ferric chloride can be applied with advantage for introduction of two acyl groups into polyhydric phenols,518 and has been recommended for introduction of a tert-butyl group into 0-xylene (73% yield).519... 

Properties of homologues of phenols with two tert. butyl groups in an ortho-positions are bound to influence of the next atoms of hydrogen tert. butyl groups on reactivity of phenolic hydroxyl and geometric parameters of an aromatic cycle. This influence causes appearance of antioxidative properties which are characterized by value k [15]. Operator Hartrii-Fo-cks with an odd number of electrons considers a spin component that is used at calculation energy formations phenoxy radicals. Calculations of structure nonsubstituted phenol (1) are carried out and 2-10 4-Z-replased -2,6-di-reA r.butylphenols in approach PM3 and PM6 too (Eq. (1)). 

Takacs [85] has separated the o- and p-benzoquinol acetates and o-quinone diacetate resulting from oxidation with lead tetraacetate of phenols containing an o-isopropyl, sec-butyl or tert.-butyl group. The o-derivatives were separated and purified on silica gel G. A 1 % solution of p-dimethylaminobenzaldehyde in concentrated sulphuric acid was... 

In 2009, Fu et al. developed highly 4-phenoxy-substituted prolinamide phenols, which could promote the asymmetric aldolisation of cyclohexanone with a range of aldehydes with a high degree of diastereo- and enantioselectivity in a large amount of water (Scheme 2.12). The best enantioselectivities of up to 97% ee were obtained with the most steric hindered catalyst that bore a tert-butyl group on the phenol moiety. The scope of the reaction could be extended to aliphatic ketones with enantioselectivities of up to 94% ee, albeit with low to moderate diastereoselectivities of up to 50% de. 

These results may be contrasted to the reaction of phenol with tert-butyl alcohol where the second tert-butyl group went to the 4-position of the phenol, probably due to steric effects. In the case of isopropyl alcohol, steric effects are less severe and the second isopropyl group went to the 6-position. Reaction of phenol with 1-propanol proceeded even more slowly than the isopropyl alcohol. The yield of products was less than 5% yield over a time period of 144 h, with the major product 2-isopropylphenol plus very small amounts of 2,6-diisopropylphenol and 2-n-propylphenol. Clearly, the incipient primary carbonium ion rearranged to the more stable secondary carbonium ion. 

Their effectiveness depends on the number of phenol groups and their steric hindering. Phenols are sterically hindered by substituents such as tert-butyl-groups in position 2,4 and/or 6. Large substituents prevent the reaction of the phenoxyl radicals with the polymer chain and the dimerization of two phenoxyl radicals however, the rate of hydrogen abstraction increases with a decrease in steric hindering. 

Selective Functional Group Conversion of Phenols or Aromatic Ethers. The selective conversion of para-oriented functional groups of substituted phenols and aromatic ethers to ni-tro groups has been accomplished using nitric acid. For example, 2,4,6-tribromophenol can be converted to 2,6-dibromo-4-nitrophenol upon reaction with 1 equiv of nitric acid in ether (eq 26). The selective conversion of tert-butyl groups into ni-tro groups has also been extensively developed and used in the preparation of aromatic substrates and in calixarene chemistry (eq 27). 3- ... 

Schiff base catalyst was also active in TMC polymerization. Moreover, chromium(III) salen derivatives in the presence of anionic initiators have been shown to be very effective catalytic systems for the alternating copolymerization of oxetane and carbon dioxide to provide the corresponding PC with a minimal amount of ether linkages. The best results were achieved for the salen ligand with tert-butyl groups in the 3,5-positions of the phenolate rings and a cyclohexylene backbone for the diimine along with an azide ion initiator (Scheme 57). 

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