Halogens reductive elimination from

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

Reductive Elimination from Pt" Halogen Complexes [Pt(CH=CH2)2X2l (X=CI, Br, I)... 

In the complexes, the C-C bond formation takes place rather easily with very low barriers of 12.9 and 11.4 kcal/mol for 11 (X=Br) and 12 (X=I), respectively (Table 2). These are about 14 kcal/mol lower than for the corresponding Pt" complexes, while the exothermicity of the reactions is increased by as much as 26 kcal/mol. The high exothermicity suggests very early transition state structures. The results (see Table 2) show that reductive elimination from Pd complexes would proceed most easily among the M and halogen derivatives (M=Pt, Pd). 

Based on the extensive proposals of carbon-halogen bond-forming reductive elimination from Pd in catalysis, many groups have pursued model studies to investigate the viability and mechanism of such transformations. These studies are detailed in the following lines, and are arranged on the basis of the type of C-X bond that is being constructed (C-1, C-Br, C-Cl, and C-F, respectively). 

Palladium.— Halogenation of the enyl complexes (5) proceeds with retention of configuration at the C—Pd bond (Scheme 9). Reductive elimination from the... 

Ni(n) compounds are used usually in this Ni catalyzed reaction. Kochi disproved already in 1979 (25) the obvious assumption that the catalytic cycle is dominated by a change of Ni(0)/Ni(H). The shortened reaction scheme shows that the oxidative addition involves Ni(I) i(III). In the reductive elimination the product fragments are involved. The halogen is eliminated from the reaction cycle as a Ni(II) compound. Methyl substituted polyphenylenes can be obtained according to tfiis reaction scheme. The GPC of the THF soluble part is shown in figure 2. In Ais GPC the peaks of bi-, quarter-, sexi- and octiphenylene derivatives are included. 

Although homonuclear cyclosilanes have been known for a long time, it is only recently that the smallest members of this series, the cyclotrisilanes, have become available [1], Cyclotrisilanes 1 are obtained by reductive halogen elimination from ort/zo-disubstituted diaryldichlorosilanes [2, 3] or, in particular cases, from hexaalkyl-l,3-dichlorotrisilanes [4]. 

Fig. 2 Reductive elimination of halogens from chalcogenopyrylium dyes 6-11 containing tellurium(IV) dihalide groups. 

The reductive elimination reactions of halogen from 6-11 illustrate several examples of ligand loss from trigonal bipyramidal species B to generate onium species A. Activation parameters in these processes were of 73 to 100 kJ mol and Eyring activation parameters of 70-97 kJ molfor and —4 to... 

Another arylation method, in the case of nitrogen heterocycles, does not need a halogenated derivative but a heterocycle activated by triflic anhydride260,261 (reaction 22). Simple aryl halides usually do not react with phosphines and special methods therefore have to be used for their arylation. The most widely used is the complex salt method , in which an aryl halide is heated with a phosphine in the presence of a transition metal such as nickel (II)2e (reaction 23). The catalytic cycle probably takes place by means of a reduced nickel(I) complex, generated in situ from the starting nickel(II) salt this nickel(I) species could undergo an oxidative addition of the aryl halide to yield a transient nickel(III) adduct, which after the reductive elimination of the aryphosphonium affords the recovery of the first active-nickel(I) complex (reaction 24). 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive -elimination. The major pathway for reductive transformation of lindane involves vicinal dehalogenation, which can proceed by steps all the way to benzene (28). Recently, data has shown that this pathway not only can convert alkanes to alkenes, but can produce alkynes from dihaloalkenes (29). 

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