Reduction 2-bromonitrobenzene

A related Heck reaction of substituted o-bromoacetanilides with styrenes followed by selenium-induced cyclization of the resulting o-styiylacetanilides gives 2-arylindoles [378], Substituted o-bromonitrobenzenes react with ethyl vinyl ether under the influence of Pd(OAc)2 to give the corresponding o-ethoxyethenylnitrobenzenes. Zinc reduction then yields indoles [379]. The one-step Pd-catalyzed conversion of o-bromoanilines to indoles 302 with enamines (or with A/-vinyl-2-pyrrolidone) has been reported [380]. 

Studies have also been conducted on electrode processes where homogeneous chemical reactions are coupled to heterogeneous electron transfer(s). In this example, the reductive dehalogenations of 3- and 4-bromobenzophenone and of o-bromonitrobenzene (denoted as ArBr) dissolved in N,N-dimethylformamide solution were studied (Compton et al., 1996b). The one-electron reductions of these compounds result in the formation of the corresponding chemically reactive radical anions as shown in (96), where HS denotes the solvent/supporting electrolyte system. 

The asymmetric reaction of cyclic ketones can be performed with chiral bi-naphthylphosphines (Eq. 8) [47-50]. The reaction of acetophenones with ortho-bromonitrobenzenes followed by reduction affords indole derivatives (Eq. 9)... 

Several indoles with carbocyclic ring substituents have been synthesized by palladium-catalyzed cyclization of A-(o-vinylphenyl)-A-arenesulfonamides. The starting materials are prepared by coupling 2-bromonitrobenzene with vinyl-tri-n-butylstannane, followed by reduction and sulfonylation (Scheme 28) <88JOC1170>. 

An optimized design employing a tubular electrode in a cylindrical cavity has been described [638]. The mechanism and kinetics of the electrooxidation of several para-haloanilines and the follow-up reactions in acetonitrile have been investigated with this cell [639]. A similar design that is suitable for low temperature measurements (233 K) has been reported [640]. It has been employed in a study of the temperature dependence of the reduction of bromonitrobenzene in acetonitrile solution. The electroreduction of perinaphthenone in a single electron process has been investigated with this cell [641]. The lifetime of the neutral radical formed by deprotonation of the radical anion has been estimated to be around 1 min. A similar electrochemical behavior of benzanthrone was observed. 

A great many model systems that have been studied by electroanalytical methods are available for comparison with sono-voltammetric measurements. The reduction of halogenated aromatic compounds is known to cause in many cases the cleavage of the carbon halide bond with a first-order rate constant determined by the properties of the molecule. From the known range of accessible diffusion layer thicknesses in sonovoltammetry, ca. 1-15 xm, unimolecular rate constants ranging from 10 to 10" s are accessible. The reduction of 3-bromobenzophenone and ortho-bromonitrobenzene in DMF [66] may be described by the ECE type mechanism given in Eqs. 7(a-d). 

In this equation is the purely transport controlled limiting current, D is the diffusion coefficient assumed to be identical for all species, and k is the first-order rate constant for the chemical reaction step. A plot of the observed limiting currents versus 5 determined from the first one-electrode process is shown in Fig. 10. A theoretical line can be fitted to the experimental points and a rate constant of k = 600 100 s-i is in good agreement with the value reported in the literature, k = 740 200 s [67]. For the reduction of ortho-bromonitrobenzene, a rate constant ofk = 200 50 s was determined, which is also in close agreement with the literature value of k = 250 s [71]. The implication is that ultrasound facilitates the measurement of fast rate constants under steady state conditions at... 

Cooper JA, Compton RG (1998) Photoelectrochemical reduction of p-bromonitrobenzene mechanistic discrimination via channel microband array voltammetry. Electroanalysis 10 (17) 1182-1187. doi 10.1002/(sici)1521-4109(199811)10 17<1182 aid-elanll82>3.0.co 2-k... 

Here the dehalogenation rate constant (600 s ) is similar within experimental error to that reported in silent conditions (800 s"i). Likewise for the mechanistically related reduction of 2-bromonitrobenzene (200 s " vs 250 s i), suggesting that ultrasound did not promote the chemical cleavage step in either case. Electrodes of millimeter dimensions were employed here, whereas normally such fast kinetics under steady-state conditions are probed using electrodes of micron dimensions. Thus, ultrasound allows the study of fast reactions using the simpler millielectrode system. The above reactions are termed "ece" by electrochemists, in which chemical step(s) are sandwiched between electron transfers. 

The first problem will prepare 4-bromoaniline (168) from benzene. Syntheses that transform one aromatic compound into another aromatic compound, such as this one, do not lend themselves to the retrosynthetic analysis approach presented in Chapter 25. Most of these syntheses involve functional group transformations. For the sake of continuity, a retrosynthesis is shown in which the amino group is removed to generate bromobenzene, which is obtained directly from benzene. The NH2 unit probably comes from reduction of a nitro group (Section 21.6.2), so the first precursor is 4-bromonitrobenzene (59) and disconnection of the C-N bond leads to the preparation of 59 by reaction of bromobenzene (35) with nitric acid/sulfuric acid. Bromobenzene is prepared directly from benzene as shown in the illustration. 

The deposition of PdCl2 on interlamellar silylamine afforded a catalyst that catalyzed the selective hydrogenation of only one nitro group of dinitrobenzene 170 to nitroaniline 171 (Scheme Exclusive hydrodebromination was observed with p-bromonitrobenzene to give nitrobenzene, but only nitro reduction was observed with dichloronitrobenzene 176. The formyl group of 174 was stable under the reaction conditions. The catalyst was only... 

For an example of applying this capability in synthetic strategy, consider the preparation of 3-bromobenzenamine. Direct bromination of benzenamine (aniline) leads to complete ortho and para substitution (Section 16-3) and is therefore useless. However, bromination of nitrobenzene allows preparation of 3-bromonitrobenzene, which can be converted into the required target molecule by reduction. The outcome is a benzene derivative in which two ortho, para directors emerge positioned meta to each other. 

Activation of Aryl Halides. Ullmann coupling of o-bromonitrobenzene is accomplished under exceptionally mild conditions and in homogeneous solution by reaction with cop-per(I) triflate in the presence of aqueous NH3 (eq 41). Yields are enhanced by the presence of a small quantity of copper(Il) triflate. That the reaction is diverted to reductive dehalogenation hy Ammonium Tetrafluoroborate is presumptive evidence for an organocopper intermediate that can be captured by protonation. 

Activation of Vinyl Halides. Under the optimum conditions for reductive coupling of o-bromonitrobenzene (eq 41), diethyl... 

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