Methoxybenzenes, substitution reactions

The present method of preparation of 4,4 -dimethyl-l,l -biphenyl is that described by McKillop, Elsom, and Taylor 15 It has the particular advantages of high yield and manipulative simplicity and is, moreover, applicable to the synthesis of a variety of symmetrically substituted biaryls 3,3 - and 4,4 -Disubstituted and 3,3, 4,4 -tetrasubstituted 1,1 -biphenyls are readily piepared, but the reaction fails when applied to the synthesis of 2,2 -disubstituted-l,T biphenyls The submitters have effected the following conversions by the above procedure (starting aromatic bromide, product biphenyl, % yield) bromobenzene, biphenyl, 85,1 -bromo-4-methoxybenzene, 4,4 -dimethoxy-l, 1 -biphenyl, 99, 1 bromo 3 methylbenzene, 3,3 dimethyl-1,l -biphenyl, 85 4-bromo-l,2-dimethylbenzene, 3,3, 4,4 -tetramethyl-l,l -biphenyl, 76, l-bromo-4-chlorobenzene, 4,4 -dichloro-l,l -biphenyl, 73, l-bromo-4-fluorobenzene, 4,4 -difluoro-l,l -biphenyl, 73... 

In 1999, Ikeda et al. reported a new type of sulfur-oxazoline ligands with an axis-fixed or -unfixed biphenyl backbone prepared in good yields by coupling reactions of methoxybenzene derivatives substituted with a chiral oxazoline and a sulfur-containing Grignard reagent. These ligands were subsequently evaluated for the test palladium-catalysed asymmetric allylic alkylation... 

Iodination versus chlorination. Three distinct classes of halogenation reactions are observed with various substituted methoxybenzenes and IC1 when carried out under an identical set of conditions, i.e., exclusive iodination, exclusive chlorination, and mixed chlorination and iodination. For example, equimolar mixtures of anisole, 2,5-dimethyl-1,4-dimethoxybenzene and 1,4-dimethoxybenzene and iodine monochloride (kept in the dark) yield p-iodoanisole, chloro-2,5-dimethyl- 1,4-dimethoxybenzene, and a (4 6) mixture of chloro- and iododimethoxybenzene, respectively, in nearly quantitative yields,225 i.e.,... 

Anodic nuclear substitution by methoxide or cyanide ions gives acceptable yields only for methoxybenzenes and methoxynaphthalenes. The nucleophile is attached to the point of highest positive charge density in the radical-cation and for many examples this leads to ipio-substitution. Oxidation of 1,4-dimethoxybenzene in methanol containing potassium hydroxide leads to the quinone diketal 8 [69]. The reaction is a general one for 1,4-dimethoxybenzenes [70, 71] and 1,4-... 

Anodic oxidation of methoxybenzenes in aqueous sulphuric acid also leads to loss of the methoxy substituent, this time through jju o-substitution on the radical-cation by water. Anisole and 4-niethoxyphenol are both converted to quinone [81]. The elimination of methanol is catalysed by protons by the mechanism illustrated in Scheme 6.8. Diphenyl derivatives have also been isolated from oxidation of some methoxybenzenes. They arise through the competitive reaction involving a... 

Explain the nature of this reaction. What is likely to be the substituting agent What products would you expect from trifluoroperoxyethanoic acid and fluorobenzene Would fluorobenzene be more, or less, reactive than methoxybenzene ... 

Ohashi, M., Tanaka, Y., and Yamada, S. (1977) The [2 + 2]cycloaddition vs substitution in photochemical reactions of methoxybenzene-acrylonitrile systems. Tetrahedron Letters, 18, 3629-3632. 

Similar reactions with 1,4-dihydroxybenzene and para-substituted methoxybenzenes did not produce the expected diaryl tellurium dichlorides1. 

We have seen in Chapter 2 that electrophilic attack of methoxybenzene gives a mixture of the 2- and 4-isomers. The significance of the orthometallation process is the ease with which the 2-substituted compound can be obtained exclusively through reaction of the organolithi-um product with a variety of electrophiles (Scheme 10.4). 

B-J Zhao, MC Depew, NA Weir, and JK-S Wan. Some Mechanistic Aspects of the Light-Induced Yellowing of Lignin A Further Study of Photochemical Reactions of a-Guaiacoxy-(3-hydroxyproprioveratrone, and Some Substituted Methoxybenzenes. Res Chem Intermed 19 449 61, 1993. 

The scope of this reaction is limited to electron-rich arenes and heteroarenes such as thiophenes, pyrroles, furans, indoles, and alkoxybenzenes as nucleophilic partners, corresponding to a Mayr ir-nucleophilicily parameter N>-1 [75-78], Electron-neutral to electron-deficient iodo(hetero)arenes are suitable electrophilic partners. Aryl halides or pseudohalides that are less reactive towards oxidative addition (Br, Cl, OTf) are not sufficiently reactive partners in this reaction. The reactivity of sterically hindered and/or ortho substituted iodoarenes has not been demonstrated. However biaryls bearing one ortho substituent of relatively small steric demand (e.g., from methoxybenzene or /V-mcthylindole) have been prepared. 

Because the Mannich reaction involves relatively weak electrophiles there are few other substituted benzenes that are sufficiently nucleophilic to react. Thus, anisole is not nucleophilic enough but 1,3-di-methoxybenzene is. Even with the latter compound the classical aqueous reaction conditions cannot be used. The reactions of 1,3-dimethoxybenzene and 1,3,5-trimethoxybenzene with preformed methyl-eneiminium chlorides in aprotic solvents were the first examples that used this procedure, which has subsequently been shown to be one of the most important recent advances in Mannich technology... 

Electrophilic aromatic substitution is a situation in which it is useful to discuss TS structure in terms of a reaction intermediate. The ortho, para, and meta directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups activate aromatic rings toward substitution and direct the entering electrophile to the ortho and para positions, whereas others are deactivating and lead to substitution in the meta position. The bromination of methoxybenzene (anisole), benzene, and nitrobenzene can serve as examples for discussion. 

A look at the general mechanism of substitution shows how different groups influence the position of subsequent reaction. We can also see how these orientation (regiochemical) effects are connected to the rates of the reactions. Let s look first at methoxybenzene (anisole). Methoxy (CH3O) is one... 

Hypervalent diaryliodonium salts are important arylating agents in organic synthesis [375, 376]. The arylation of thienyUodonium bromides 160 with methoxybenzenes as aromatic nucleophiles 161 is possible under mild reaction conditions in hexafluoroisopropanol (HFIP) and without the necessity for a metal catalyst (Scheme 63, Table 40). TMSOTf is a suitable additive for the direct ipso substitution. 

The cationic monomeric complexes Re(CO)4(diimine) formed in the reaction of Eq. (6,39) when L=CO show excited state redox reactions. Reductive quenching experiments of the excited state with substituted methoxybenzenes and with chloride ion leads to an sq)proximate value of 1.5 V being found for the Re(CO)4(di-imine) /Re(CO)4(diimine) excited state potential. Photosubstitution is also observed with these complexes, although a definitive assignment of the photoactive states that lead to these different reactions has not yet been achieved. 

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