Bromobenzene catalyst

Halogenation Bromine reacts with benzene in the presence of iron(lll) bro mide as a catalyst to give bromobenzene Chlorine reacts similarly in the presence of iron(lll) chloride to give chlorobenzene... 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

The arylation of the methoxyethanol with the bromobenzene was investigated changing the nature of the base (Fig. 11), but using the bipyridylnickel catalyst, a shorter time, and keeping the same temperature. 

In the same way, the correlation of the nature of the copper catalyst with the rate of the hydrolysis of bromobenzene exhibits in all cases an induction time of about 1 hour, and a transformation time of 10 to 40 minutes (Fig. 18). 

Several optimization studies have been carried out under these phosphine-free conditions. The reaction of bromobenzene and styrene was studied using Pd(OAc)2 as the catalyst, and potassium phosphate and (V,(V-dimethylacetamide (DMA) were found to be the best base and solvent. Under these conditions, the Pd content can be reduced to as low as 0.025 mol %.142 The reaction of substituted bromobenzenes with methyl a-acetamidoacrylate has also been studied carefully, since the products are potential precursors of modified amino acids. Good results were obtained using either N, (V-diisopropylethylamine or NaOAc as the base. 

The arylation of heteroaromatic compounds is also achieved by aryl-aryl coupling reaction. The arylation of A-methylimidazole with bromobenzene occurs under palladium catalysis (Equation (62)).72 The arylation of thiazole with aryl iodide occurs at the 2-position under PdCl2(PPh3)2/CuI catalysis.73 In this case, tetrabutylammonium fluoride improves the activity of the catalyst. Alternatively, thiazoles and benzothiazole are efficiently arylated... 

Previous investigations of heterogeneous sonochemistry have involved ultrasonic extraction of pollutants from sediments and ultrasound assisted reactions employing solid catalysts. However, more extensive quantitative results are needed concerning sonochemistry in environmentally relevant systems. We report results of a preliminary set of experiments, involving the ultrasonic irradiation of bromobenzene, trichloroacetonitrile, and chloropicrin in the presence of silica solids (15 im and 10 nm). 

Bromobenzenes are converted into the corresponding chloro compounds on reaction with aqueous sodium hypochlorite in the presence of tetra-n-buty lammoni um hydrogen sulphate [40]. The reaction is pH dependent. At pH > 10, the bromobenzenes are effectively inert, but over the pH range 7.5-9, conversion occurs into the chlorobenzenes without any side reactions and the reaction appears to be light-induced. At more acidic levels (pH 4-5), bromobenzene is converted quantitatively into chlorobenzene within one hour. No reaction occurs in the absence of the catalyst and yields from light and dark reactions are comparable. Side reactions are observed, however, with substituted bromobenzenes under these low pH conditions. 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

For comparison, fluorous-phase-soluble Pd complexes are only 74-98% selective towards the trans product [168-170]. The isolated yields of the product approached 70% when a threefold excess of olefin to iodobenzene was used (Table 3) however, the percent yield decreased with the use of bromobenzene as expected since activation of bromine-carbon bonds is less favorable than iodo-carbon bonds. It was also possible to catalyze the reaction in the absence of additional triethylamine base (Table 3). In this case, the tertiary amines of the den-drimer most likely act as the base. The catalysts, in general, were fully recover-... 

An instructive example for a positive dendritic effect was reported by Reetz et al. [71]. The authors described a poly(propylenimine) dendrimer, with diphenylphos-phine groups in the periphery (Fig. 7.20). A dendritic [PdMe2]-complex was tested as an efficient catalyst in the Heck reaction of bromobenzene and styrene to yield stilbene (85-90% conversion). The separation technique originally investigated for... 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

Carbonylation of bromobenzene (Scheme 5.7) with [Pd(TPPTS)3] required still higher temperatures (150 T). The possible acyl intermediates of such reactions [PdBr(C6H5CO) Ph3)2] and [PdBr(C6H5CO)(TPPTS)2] were synthetized and characterized [26]. Bromobenzene was also carbonylated to benzoic acid in water/toluene using a catalyst prepared from [PdCl2(COD)j and 27 in the presence of NEt3 [21]. 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

In the presence of Priedel-Crafts catalysts (BF3, FeClj, and others), comparatively less reactive compounds, such as benzene (50°C, 2 hr) and toluene (100°C, 4 hr), can be reacted in an autoclave with GFsSCl to give CjHs—SCFs or a mixture of m- andp-CFsS—CgHaCHs, respectively. Ghloro- and bromobenzene react under more vigorous conditions (200°G, 2 hr) with the formation of a mixture of ortho-, meta-, and para- THF, tetrahydrofuran. 

Reetz et al. 16) were the first to recover and recycle a dendritic catalyst through a precipitation procedure. The dimethylpalladium complex of the phosphine-functionalized DAB-dendr-[N(CH2PPh2)2]i6 dendrimer (la) is an active catalyst for the Heck reaction of bromobenzene and styrene to give trara-stilbene (89% trans-stilbene and 11% 1,1-diphenylethylene, at a conversion of 85—90%, Scheme 8). 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

When another palladium complex, diiodobis(l, 3-dimethylimidazolium-2-ylidene)palladium(II), was used as a catalyst (257), it resulted in a large improvement in catalyst stability in the same ionic liquid. The Heck reaction performed better in the ionic liquid than in organic solvents such as dimethylfuran (DMF). In the reaction of bromobenzene with styrene, the yield of stilbene was increased from 20% in DMF to 99% in [NBu4][Br]. The ionic liquid showed excellent solubility for all the reacting molecules. 

As the last example of C-C bond-formation reactions catalyzed by alkaline earth hydroxides, we mention the recently reported a-arylation of diethyl malonate in the presence of a palladium catalyst and a base in a separate phase 299). The arylation of carbonyl compounds is a carbon-carbon coupling reaction between an aryl halide and an enolate, which is usually catalyzed by palladium salts in the presence of an appropriate base (300,301). The arylation of diethyl malonate with bromobenzene (Scheme 48) was performed with tetrachloropalladate as the... 

The carboxylation reaction shown in reaction (11) is catalyzed by both nickel and palladium phosphine complexes. For example, Ni(dppe)Cl2 (where dppe is l,2-bis(diphenylphosphino)ethane) and Pd(PPh3)2Cl2 both catalyze reaction (11) [84-86]. Mechanistic studies have been carried out on these two systems, and the results indicate that two different mechanisms are involved. In the case of the Ni complex, the first step is the reduction of Ni(dppe)Cl2 to a transient Ni(dppe) species [85]. This process occurs in two one-electron steps (reaction 12). Bromobenzene then oxidatively adds to Ni(dppe) to form Ni(dppe)(Br)(Ph), reaction (13). The resulting Ni(II) aryl species is reduced in a one-electron process to form Ni(dppe)(Ph), which reacts rapidly with CO2 to form a Ni—CO2 intermediate as shown in reaction (14). The rate-determining step for the overall catalytic reaction is the insertion of CO2 into the Ni-aryl bond, reaction (15) step 1. This reaction is followed by a final one-electron reduction to regenerate Ni(dppe), the true catalyst in the cycle (reaction 15, step 2). 

The reaction of 1-phenylethyl-, 2-octyl-, and 2-butyl-magnesium chloride (36a, b, c) with vinyl bromide (37a), (E)-p-bromostyrene (37b), 2-bromopropene (37c), and bromobenzene (37d) was carried out in the presence of 0.5 mol- % of a nickel catalyst prepared in situ from nickel chloride and the chiral ligand (35). 

V-Methylinudazole, when heated in DMF with bromobenzene in the presence of a palladium-triphenylphosphine catalyst and potassium carbonate furnished two products (6.87.) the 5-phenyl and the 2,5-diphenyl derivative. The product distribution suggests that the preferential site of the arylation is the more electron-rich 5-position.118 Prolonged heating in a polar, high boiling solvent in the presence of base is characteristic of such transformations. 

Attempted reductive carbonylation of bromobenzene with CO/H2 or with CO/HCOONa in the presence of a Pd/tppts catalyst failed to afford any benzaldehyde 464... 

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