Aryl halides reductive

Indeed, it is now considered that carbon-halide bond cleavage is concerted with electron uptake (see Ref. 14 and Chapter 8), that is, that there is no intermediacy of an anion radical, such as [RR R"C — X] , during the electron transfer [15]. These cases must be contrasted with those, such as the aryl halide reduction in Eq. (18),... 

However, this does not mean that biaryls cannot be obtained via aryl halide reduction but only that this strategy is not experimentally valid as soon as Ar is more easily reducible than the parent halide. Yet other approaches to the synthetic problem may be found. An obvious one consists in suppressing the facile reductions of Ar by using an electrophore as the leaving group X , which will be the site of the initial electron uptake [Eq. (48)]. Ar may not then be reducible at the reduction potential of the parent pseudohalide, as in reaction (54), which is shown in benzylic series [22]. 

Reduction of alkyl or aryl halides. Reduction of organic halogen compounds by sodium borohydride proceeds more rapidly in DMSO than in anhydrous diglyme.8... 

Scheme 9 Directly observed aryl-halide reductive elimination from sterically hindered Pd(II) complexes...

The first example of Aryl-Halide reductive elimination from a Pt complex was reported by Ettorre in 1969 [69]. Eollowing the reaction progress by UV-vis spectroscopy provided evidence for the iodide dissociation taking place prior to the reductive elimination step (1). 

The initial step is oxidative addition of the acid chloride to the dissociated form of the complex (LVI) to form the acyl complex (LXV). Aryl migration (reverse insertion) affords a supposed intermediate (LXVI) which can eliminate CO to form (LXVII), which in turn loses aryl halide to regenerate the catalyst. Alternatively (LXVI) can lose aryl halide (reductive elimination) to form a rhodium carbonyl (LXVIIl). It is thought that this path is... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

The 2-substituted 3-acylindoles 579 are prepared by carbonylative cycliza-tion of the 2-alkynyltrifluoroacetanilides 576 with aryl halides or alkenyl tri-flates. The reaction can be understood by the aminopalladation of the alkyne with the acylpalladium intermediate as shown by 577 to generate 578, followed by reductive elimination to give 579[425]. 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

In the arylations of enamines with very reactive aryl halides (352,370) such as 2,4-dinitrochlorobenzene, the closely related mechanistic pathway of addition of the enamine to the aromatic system, followed by elimination of halide ion, can be assumed. The use of n-nitroarylhalides furnishes compounds which can be converted to indolic products by reductive cycliza-tion. Less reactive aryl halides, such as p-nitrochlorobenzene, lead only to N-arylation or oxidation products of the enamines under more vigorous conditions. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

The possible mechanism for the reactions involving stoichiometric amount of preformed Ni(0) complexes is shown in Fig. 9.8. The first step of the mechanism involves the oxidative addition of aryl halides to Ni(0) to form aryl Ni(II) halides. Disproportion of two aryl Ni(II) species leads to a diaryl Ni(II) species and a Ni(II) halide. This diaryl Ni(II) species undergoes rapid reductive elimination to form the biaryl product. The generated Ni(0) species can reenter the catalytic cycle. 

The rate-determining step in the homo-coupling reaction of aryl halides could be the oxidative step or the reduction of Ni(II) to Ni(I) step. 

This cycle involves, first, a monoelectronic transfer from the nickel (0) complex to the aryl halide affording a Ni(I) complex and then an oxidative addition affording a 16 electron-nickel (II) which undergoes a nucleophilic substitution of Nu-, then a monoelectronic transfer occurs once again with a second aryl halide, and, last, a reductive elimination of the arylated nucleophile regenerates the active Ni(I) species. 

The reduction of aryl halide can be probably explained according to the following catalytic cycle (Fig. 6). 

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

If, instead of a palladium catalyst, a nickel catalyst, such as the bipyridylnickel(II) bromide, is used for the arylation of amines (Fig. 7), the reduction of the aryl halide into the corresponding aromatic hydrocarbon is still present for the primary or secondary benzylamines but, the arylation into substituted anilines is the main reaction even most often the only one, for the other types of amines. 

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