Anisole, bromination

Bromination has been shown not to exhibit a primary kinetic isotope effect in the case of benzene, bromobenzene, toluene, or methoxybenzene. There are several examples of substrates which do show significant isotope effects, including substituted anisoles, JV,iV-dimethylanilines, and 1,3,5-trialkylbenzenes. The observation of isotope effects in highly substituted systems seems to be the result of steric factors that can operate in two ways. There may be resistance to the bromine taking up a position coplanar with adjacent substituents in the aromatization step. This would favor return of the ff-complex to reactants. In addition, the steric bulk of several substituents may hinder solvent or other base from assisting in the proton removal. Either factor would allow deprotonation to become rate-controlling. 

The selective bromination of a ketone in the presence of another susceptible functional group was achieved in a diterpene synthesis 240). A competing bromination of an anisole ring could be avoided here through the use of a pyrrolidine enamine derivative for activation of the methylene group adjacent to the carbonyl function. 

Positive bromination was first observed by Shilov and Kaniaev189 who found that the bromination of sodium anisole-m-sulphonate by bromine-free hypo-bromous acid was accelerated by the addition of nitric or sulphuric acids, and was governed by the kinetic equation... 

The most extensive study of the effect of conditions upon the kinetics of bromination was made by Robertson et al.23l 279, who measured the rates of bromination of alkylbenzenes, acetanilide, aceto-p-toluidide, mesitylene, anisole and p-tolyl methyl ether in acetic acid at 24 °C. They found that at relatively high concentrations of bromine (A//40-M/100) the reaction is second-order in bromine, i.e. the rate equation is... 

Robertson et al.261 measured rates of bromination of some aromatic hydrocarbons in acetic acid containing sodium acetate (to eliminate protonation of the aromatic by liberated hydrogen bromide) and lithium bromide (to reduce the rate to a measurable velocity ) at 25 °C, the second-order rate coefficients for 3-nitro-N,N-dimethylaniline and anisole being 14.2 and 0.016 respectively the former compound was thus stated to be about 1012 times as reactive as benzene (though no measurement of the latter rate coefficient, inferred to be 1.33 xlO-11, could be found in the literature) and this large rate spread gives one further indication of the unreactive nature of the electrophile. Other rates relative to benzene were ... 

Bromine acetate has also been proposed310 as an intermediate species in the bromination of anisole, phenetole, and methyl p-tolyl ether by 2,4,6-tribromo-N-bromoacetanilide in acetic acid at 25 °C, since this latter compound was stable to both the ethers and to acetic acid, but in the presence of both, bromination of the ethers occurred, presumably via bromine acetate formed as in the equilibrium... 

The kinetics of bromination with the complex formed between bromine and dioxan have been examined using benzene (which is unattacked) as solvent311, and it is probably appropriate to regard this as a catalysed bromination in view of the effect of dioxan upon the polarity of the bromine-bromine bond. With anisole, phenetole, and isopropoxybenzene, third-order kinetics are obtained, viz. 

Bromodesilylation T-methoxy-l-indanones. Cyclization contrary to the normal para-selectivity of anisole derivatives can be effected by temporary use of an ort/jo-trimethylsilyl group introduced by directed orf/io-metallation (11,75). Thus the anisole derivative 1 undergoes bromodesilylation and hydrolysis to provide 2. This product undergoes cyclization to 3 in good yield on conversion to the lithio salt followed by bromine-lithium exchange (8,65-66). 

Contrary to the above expectations, the bromination of anisole (Tee and Bennett, 1984) and of phenols (Tee and Bennett, 1988a) in the presence of a-CD is not strongly retarded, so that some form of catalysis must occur. In some cases, actual rate increases are observed in spite of the several complexations that reduce the free reactant concentrations. Analysis of the effects of substituents on the kinetics leads to the conclusion that the catalysis by a-CD most probably results from reaction of CD-bound bromine with free substrate (12a) and that the a-CD-Br2 complex is 3-31 times more reactive than free Br2 towards phenols and phenoxide ions (cf. Tee et al., 1989). For the kinetically equivalent reaction of the substrate CD complex with free bromine (12b), the rate constants (A 2 ) for phenols do not correlate sensibly with the nature and position of the substituents, and for three of the phenoxide ions they have unrealistically high values, greater than 10u m 1 s . 

A combination of bromide ions and methyl octyl sulphide is able to oxidise secondary alcohols at the potential necessary to fonn bromine. Conversion of the alcohol to the ketone follows the Scheme 8.2 and uses an undivided cell with benzo-nitrile as the solvent containing 2,6-lutidine as base and tetraethylamnionium bromide. The reaction occurs using a platinum anode at 1.1 V vs-, see [28], Thio-anisole alone, in absence of bromide, will function as a catalyst for the oxidation of secondary alcohols but in these cases a more positive anode potential of 1.5 V vs. see is needed to oxidise the thioether [29]. 

Farrell TJ. 1980. Glass capillary gas chromatography of chlorinated dibenzofurans, chlorinated anisoles, and brominated biphenyls. J Chromatogr Sci 18 10-17. 

Digital simulation by Bard and Prater agrees well with the numerical solution provided by Albery and the analytical solution [229], Experimental verification using the bromination of anisole and Fe(II)/V(V) reaction was given [230]. The range of measurable rate constants is 0.2 < K < 5 which corresponds to... 

Although the NBS-silica system has useful potential for bromination of electron rich heterocycles, it has limited application to non-acdvated aromatics and sometimes meets problems with polybromination even for the activated heterocycles. Thus, it was of interest to investigate the potential of different brominating agents and different solids. Bromination of anisole with... 

Figure 2.2, in which a constants are plotted against log klk0, for the bromina-tion of monosubstituted benzenes, shows an example of the usefulness of these new parameters. As can be seen from Structures 12 and 13—which are representations of the intermediates in the ortho and para bromination of anisole—substituents electron-donating by resonance ortho or para to the entering bromine can stabilize the positive charge in the intermediate and therefore also in the transition state by through-resonance. 

At 0.3 mm/Hg the fraction boiling at 95-105 °C was a yellow liquid weighing 28.5 g which was largely 3-ethoxy-2-(ethylthio)anisole which seemed to be reasonably pure chromatographically. It was used as such in the bromination step below. 

Flodin C, Whitfield FB (2000) Brominated Anisoles and Cresols in the Red Alga Polysiphonia sphaerocarpa. Phytochemistry 53 77... 

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