Hydrolysis of aryl halides

Of limited use is the hydrolysis of aryl halides containing strongly electron-withdrawing groups ortho and para to the halogen (Sec. 25.9) 2,4-dinitrophenol and 2,4,6-trinitrophenol (picric acid) are produced in this way on a large scale ... 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

The susceptibility of alkyl halides to hydrolysis is a function of the halide (generally, I > Br > Cl > F). In contrast, the rates of aryl halide hydrolysis are generally F > Cl > Br > I. The reversal of the order of rates is because aryl halides hydrolyze via an addition-elimination reaction (March, (129)). The nucleophilic addition step, which is typically the ratedetermining step, is facilitated by the strongly electronegative fluorine. 

Hydrolysis of chlorobenzene and the influence of silica gel catalysts on this reaction have been studied by Freidlin and co-workers (109). Pure silica gel gave up to 45% phenol from chlorobenzene at 600°C. When the silica gel was promoted with 2% cupric chloride, up to 75% phenol was obtained (381). A number of other salts were tested by Freidlin and co-workers as promoters, but they exerted an adverse effect on the activity or selectivity of the catalyst. With 0.2% cupric chloride and 6% metallic copper, the activity of silica-gel was doubled (389). At 500° under the above conditions, the halides were hydrolyzed at rates decreasing in the following order chloride, bromide, iodide, fluoride (110). The specific activation of aryl halides by cupric chloride was demonstrated by conversion of chlorobenzene to benzene and of naphthyl chloride to naphthalene when this catalyst was supported on oxides of titanium or tin (111). The silica promoted with cupric chloride was also found to be suitable for hydrolysis of chlorophenols and dichlorobenzenes however, side reactions were too prominent in these cases (112). 

Most reactions such as halogenation, nitration, sulphonation etc. are reactions with a positive ion, with an electrophilic reagent therefore, in which the aromatic molecule reacts nu-cleophilically. In hydrolysis, alcoholysis and aminolysis of aryl halides the reagents are nucleophilic. Radical reactions are also possible, especially in the gas phase at higher temperatures. 

If halide ion is present during hydrolysis of benzenediazonium ion or i -nitro-benzenediazonium ion, there is obtained not only the phenol, but also the aryl halide the higher the halide ion concentration, the greater the proportion of aryl halide obtained. The presence of halide ion has no effect on the rate of decomposition of benzenediazonium ion, but speeds up decomposition of the p-nitrobenzenediazonium ion. 

Generally, the reaction rates of aryl halides follow the order iodides > bromides > chlorides > fluorides. This fact can be used for the selective substimtion in polyhalogenated systems. For instance, 2-bromo -chlorotoluene gives 76% of 5-chloro-2-methylphenol by treatment with sodium hydroxide at 200 °C. Nevertheless, polyhalogenated systems which contain fluorides have a variable behaviour depending on the reaction temperature. At lower temperatures preferential hydrolysis of the fluoride takes place and at >200 °C the usual reactivity order iodides > bromides > chlorides > fluorides is observed. For instance, l,2-dibromo-3,4,5,6-tetrafluorobenzene affords 2,3-dibromo-4,5,6-trifluorophenol in 87% yield by treatment with potassium hydroxide at 85 °C. Under the same conditions, 1,4-dibromo-2,3,5,6-tetrafluorobenzene produces a 78% yield of 2,5-dibromo-3,4,6-trifluorophenol. However, 4-fluorobromobenzene with NaOH at 200 °C gives 4-fluorophenol in 70-79% yield. ... 

Homogeneous, aqueous tv o-phase catalysis is also of industrial interest for the production of the important intermediate phenylacetic acid (PAA), -which is used in perfume and pesticides syntheses. The previous process (benzyl chloride to benzyl cyanide -with hydrolysis of the latter) suffered from the formation of large amounts of salt (1400 kg per kg of PAA). The new carbonylation method reduces the amount of salt by 60% and makes use of the great cost difference between —CN and —CO [79-81]. Finally, the Suzuki coupling of aryl halides and arylboronic acids, substituting Pd/TPPMS with Pd/TPPTS catalysts, should be also mentioned. 

Several important homogeneous catalytic reactions (e.g. hydroformylations) have been accomplished in water by use of water-soluble catalysts in some instances water can act as a solvent and as a reactant for hydroformylation. In addition, formation of aluminoxanes by partial hydrolysis of alkylaluminum halides results in very high activity bimetallic Al/Ti or Al/Zr metallocene catalysts for ethene polymerization which would be otherwise inactive. Polymerization of aryl diiodides and acetylene gas has recently been achieved in water with palladium catalysts. Finally, nickel-containing enzymes, such as carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase, operate in water with reaction mechanisms comparable with those of the WGSR or of the Monsanto methanol-to-acetic-acid process. ... 

Coupling of aryl halides and trifluoroacetamide occurred at 45-75 °C catalyzed by CuI/DMEDA (Lll), followed by in situ hydrolysis to provide the corresponding primary arylamines (entry 2) [32]. The Cul/l,10-phen (L13) catalytic system was found to be able to promote the selective coupling of N-Boc hydrazine [33] (entry 3) and the coupling of N- and O- functionalized hydroxylamines with aryl iodides (entry 4) [34]. Hosseinzadeh and coworkers reported the coupling reaction of aryl iodides with amides catalyzed by CuI/l,10-Phen (L13) by using KF/AI2O3 as the... 

Examples of the three mechansims are, respectively (a) hydrolysis of aryl diazonium salts to phenols (b) reaction of aryl diazonium ions with Ns to give the aryl azides " and (c) the Sandmeyer reaction, involving cuprous chloride or bromide for synthesis of aryl halides. Specific synthetically important substitution processes are considered in the succeeding sections. 

There has been a review of the use of transition-metal catalysts in the formation of C—S, C—Se, and C—Te bonds." Copper catalysis enables the formation of unsymmetrical diaryl thioethers from two differently substituted aryl iodides using ethylxanthogenate as the source of sulfur. Initial formation of an aryl xanthate, such as (17), is followed by hydrolysis to the arenethiolate, which couples with the second aryl iodide." Copper catalysis has also been used in the methylthiolation of aryl halides by DMSO. The method requires the presence of a source of fluoride ions, such as zinc fluoride." In the presence of a palladium catalyst, the reaction of aryl and heteroaryl bromides with AgSCp3 gives the corresponding trifluoromethylsulfldes." ... 

Synthesis of 0-Arylhydro iylamines. Reaction of tricarbo-nylchromium complexes of aryl halides (1) with TYf-butoxycarbo-nylhydroxylamine (2) in DMSO and powdered KOH under N2 at rt resulted in the nucleophilic substitution of the Cl to give the corresponding tricarbonyl[(/-butoxycarbonylaminoxy)arene]chro-mium complexes (3) (eq 3). Consecutive I2 treatment and acid hydrolysis gave the O-arylhydroxylamines in high overall yields. ... 

Figure 2.19 A cross-section of results for the copper-catalyzed amination of aryl halides with nitriles to give amides and benzoxazoles via in situ hydrolysis, by Xiang et al. [89].

Owing to the extreme difBculty of nucleophilic substitution of aryl halides, hydrolysis will not likely occur in natural waters. Nitro compounds also resist hydrolytic scission. Thus, in general, hydrolysis under ambient environmental conditions is highly improbable. 

A new type of catalyst, a cobalt carbonyl complex, has been found for low-temperature (viz. 50 °C) homogeneous hydroformylation of alkenes. Nafion-H (a superacidic perfluorinated resin sulphonic acid) impregnated with mercury is recommended as a catalyst for the hydration of alkynes R C=CR (R = H or aryl, R = H, alkyl, or aryl) to form ketones R CH2C0R. Two mild methods for the hydrolysis of vinyl halides to ketones have been described one utilizes Bp3,Et20 and mercury(ii) acetate in acetic acid and the second mercury(ii) acetate in trifluoroacetic acid/ ... 

A change of the sulfide loading from stoichiometric to catalytic amounts (20 mol%) reduced yields and increased reaction times. The diastereoselectivity was very poor, enantioselectivities for the chiral organocatalyst 228 were not reported. Attempts to extend the reaction to other bromides were unsuccessful. Saito et al. utilized the same one pot protocol with another chiral catalyst (289) in the presence of an excess of aryl halides and base. He achieved moderate to excellent conversions. Use of dry acetonitrile prevented imine hydrolysis and enantioselectivities became good to excellent (Scheme 7.47) [172]. 

Many organic compounds are also prepared by hydrolysis of the appropriate halides. Alcohols can be prepared by hydrolysis of alkyl halides (though yields are often poor due to the formation of considerable amounts of olefins and polyolefins) phenols are prepared from aryl halides under pressure, in the presence of concentrated base (Samuel and Scheinmann, private communication), aromatic aldehydes and ketones from gem-dihalides (Doering and Dorfman, 1953), and aromatic acids from trihalides (Ponticorvo and Rittenberg, 1954). 

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