Arly halides mechanism

The reaction between an alkoxide ion and an aryl halide can be used to prepare alkyl aryl ethers only when the ar yl halide is one that reacts rapidly by the addition-elimination mechanism of nucleophilic aromatic substitution (Section 23.6). 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The present reaction may be reasonably explained by the smooth oxidative addition of aryl halides to metallic nickel to give aryl nickel halides, followed by disproportionation to bisarylnickels, which upon reductive elimination afford the dehalogenative coupled products. Providing strong support for this mechanism, the intermediates, arylnickel halide and bisarylnickel (Ar=C F ), were isolated as the phosphine complexes. 

Exposure of (Me3Ge)2Hg to visible fight in the presence of alkyl halides results in the rapid formation of Me3GeHal and MesGel IgR104. This reaction is thought to occur via a radical chain mechanism (Scheme 26). When the reaction is instead carried out in the presence of aryl halides, the products are R3GeI Ial and Ar—Hg—Ar this reaction has a... 

The proposed mechanism involves the usual oxidative addition of the aryl halide to the Pd(0) complex affording a Pd(II) intermediate (Ar-Pd-Hal), subsequent coordination of allene 8 and migratory insertion of the allene into the Pd-C bond to form the jt-allylpalladium(II) species 123. A remarkable C-C bond cleavage of 123 leads by decarbopalladation to 1,3-diene 120 and a-hydroxyalkylpalladium species 124. /8-H elimination of 124 affords aldehyde 121 and the H-Pd-Hal species, which delivers Pd(0) again by reaction with base (Scheme 14.29). The originally expected cyclization of intermediate 123 by employment of the internal nucleophilic hydroxyl group to form a pyran derivative 122 was observed in a single case only (Scheme 14.29). 

Halides are displaced by a variety of nucleophiles including alkoxides, phenoxides, thiolates, cyanide, and amines. 4-Halogenobenzofurazans yield either 4-or 5-substituted products resulting from normal ipso or cine reactions, and 5-halogeno derivatives react similarly. An addition lim-ination AE mechanism has been invoked to explain the cine products, whereas ipso substitution can result from both AE and Sj.,Ar pathways (Scheme 14) <81MI 405-02). 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

The same mechanism can be applied in the preparation of isotopically labeled halides. Na36Cl reached complete equilibrium with -octyl chloride after 5 hours ar reflux temperature177. The similar iodide/radioiodide exchange is completed within 5 minutes. 

The S 2 Ar mechanism is characterized by direct interaction between the aromatic halide in its excited state (in many cases a triplet state) and the nucleophile, leading to a o-complex. In its most simple form, the mechanism may be summarized as in equations 154 and 155. The photosubstitution of bromide by chloride in 3-bromonitrobenzene in... 

In the S l Ar and the S l Ar mechanisms, the aryl halide becomes reactive towards nucleophiles by loss of an electron or a halide anion. Another means of activation is protonation, followed by excitation. This mechanistic pathway has been proposed for the photohydroxydehalogenation of 1-chloroanthraquinones in 97% sulphuric acid734, which leads to high yields (60-70%) of 1-hydroxyanthraquinones. The yields of the photochemical replacement of chlorine by pyridine in 1- and 2-chloroanthraquinone are much lower (3-6%)735. The reactions were performed with pyridine as solvent and the products were converted into aminoanthraquinones by treatment with alkali. Pyridine as nucleophilic... 

The amine-ligated aryl halide complexes react with alkoxide or silylamide bases to form arylamine products (Eq. (48)) [279]. The reaction of Pd[P(o-C6H4Me)3](HNEt2)(p-Bu, )(Br) and LiN(SiMe3)2 occurred immediately at room temperature to form the arylamine in greater than 90 % yield. Low-temperature reactions conducted in the NMR spectrometer probe allowed direct observation of the anionic halo amido complex Pd[P(o-C6H4Me)3](NEt2)(Ar)(Br) [279]. Thus, one experimentally supported mechanism for gen-... 

Bakkenist AR, de Boer JE, Plat H, Wever R (1980) The halide complexes of myeloperoxidase and the mechanism of the halogenation reactions. Biochim Biophys Acta 613 337-348... 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

The mechanism of the above reactions was studied by Tsou and Kochi (Scheme 5.10). The aryl nickel halides A, formed by the oxidative addition of Ar-Br to NiL4, on reaction with aryl halides give biaryls. 

The fact that the order of halide reactivity is Br > I > Cl > F (when the reaction is performed with KNH2 in liquid NH3) shows that the S Ar mechanism is not operating here. ... 

Copper(I) salts such as CuCN and ROCu undergo aromatic substitution reactions very readily with ordinary aryl halides. The mechanism has not been established with certainty. One reasonable possibility is an SrnI mechanism. Another reasonable possibility involves oxidative addition of Ar X to N=C Cu(I) to give a Cu(III) complex, followed by reductive elimination of Ar-CN to give CuX. 

The mechanism was discussed recently [15]. The zero-valent metal (produced at the interface by reduction of the metal cation) induces an oxidant addition, in principle followed by nucleophilic substitution and a reducing elimination. The scheme below exhibits a simplified catalytic mechanism with aromatic halides. Electrochemistry is involved in the activation phase (formation of zero-valent metal [16-18]). The nucleophile is obviously produced by the oxidant insertion, and in the catalytic cycle hereafter Ar—Nu has to be considered of course as the Ar—Ar dimer. 

By far the most important of all these mechanisms is addition-elimination. The carbanion intermediate must be stabilised by a group Z such as Ar, COR, CN, RSO, or RS02. The reaction then becomes a conjugate substitution as 10 to 12. It appears there that, if the intermediate 11 has a lifetime longer than bond rotation, only the more stable of the two products (E in this case) will be formed. However, there are quite a few cases where the first step is rate-determining and the intermediate has a very short lifetime indeed. These reactions go with retention of configuration. An important example is the replacement of halides by thiolate nucleophiles3 in 2-bromo styrenes 19. 

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