Nickel aryl chlorides

This is a problem that has been reported by several researchers in other cya-nation methods on heteroaromatic halides. (Hetero)aryl chlorides have also been tackled via in situ halogen exchange to (hetero)aryl bromides followed by sequential cyanation (Scheme 71). For this microwave-assisted process an equimolar amount of NiBr2 and a two-fold excess of NaCN were used. The only heteroaromatic chloride tested was 2-chloropyridine. Although the procedures described involve the use of significant amounts of nickel salts, a clear advantage is that the reactions can be performed in air. Moreover, the cyanat-ing reagents are easily removed since they are water soluble. 

Scheme 6.23 Nickel-catalysed Negishi coupling between unactivated aryl chlorides and oiganozinc reagents...

However, the Buchwald-Hartwig reaction with NHCs as hgands is not limited to palladium. Nickel has also been successfully employed in this catalytic amination. In situ procedures have been described for the coupling of aryl chlorides [163] and tosylates [164] and, more interestingly, anisoles [165]. The use of well-defined Ni(0) catalysts has also been studied [166] (Scheme 6.49). 

Pretreatment with nickel bromide causes normally unreactive aryl chlorides to undergo Pd-catalyzed substitution,139 and aryl and vinyl triflates have been found to be excellent substrates for Pd-catalyzed alkenylations.140... 

Activated aryl chlorides, which are close in reactivity to unactivated aryl bromides, underwent reaction with the original P(o-tol)3-ligated catalyst.58 Nickel complexes, which catalyze classic C—C bond-forming cross-couplings of aryl chlorides, 9-64 also catalyzed aminations of aryl chlorides under mild conditions.65,66 However, the nickel-catalyzed chemistry generally occurred with lower turnover numbers and with a narrower substrate scope than the most efficient palladium-catalyzed reactions. 

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

Finally, polymer 594 has been used as an arene-catalyst to activate nickel from nickel(II) chloride and lithium, in order to perform hydrogenation of different organic substrates such as afkenes, afkynes, carbonyl compounds and their imines, alkyl and aryl halides (chlorides, bromides and iodides), aromatic and heteroaromatic compounds as well as nitrogen-containing systems such as hydrazines, azoxy compounds or Af-amino oxides, giving comparable results to those obtained in the corresponding reaction in solution . 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

Aryl chlorides Aryl chlorides will substitute alkenes only under very special conditions, and then catalyst turnover numbers are generally not very high. Palladium on charcoal in the presence of triethylphos-phine catalyzes the reaction of chlorobenzene with styrene,58 but the catalyst becomes inactive after one use.59 Examples employing an activated aryl chloride and highly reactive alkenes, such as acrylonitrile, with a palladium acetate-triphenylphosphine catalyst in DMF solution at ISO C with sodium acetate as base react to the extent of only 51% or less.60 Similar results have been reported for the combination of chlorobenzene with styrene in DMF-water at 130 C, using sodium acetate as the base and palladium acetate-diphos as a catalyst.61 Most recently, a method for reacting chlorobenzene with activated alkenes has been claimed where, in addition to the usual palladium dibenzilideneacetone-tri-o-tolylphosphine catalyst, nickel bromide and sodium iodide are added. It is proposed that an equilibrium concentration of iodobenzene is formed from the chlorobenzene-sodium iodide-nickel bromide catalyst and the iodoben-zene then reacts in the palladium-catalyzed alkene substitution. Moderate to good yields were reported from reactions carried out in DMF solution at 140 C 62... 

Prochiral acyclic olefins have been used in the reaction with an aryl Grignard reagent. With the complex of nickel(II) chloride and chiraphos (2), a high enantioselectivity could be obtained, albeit with poor rcgiosclecdvity (Eq. 8E.26) [204]. When the sterically more bulky 1,3-diphenylallyl... 

In the event of a first substrate screening, aryl iodides and bromides yielded mainly the reduced compound B with n-C6H13MgBr in presence of catalytic amounts of Fe(acac)3 in THF-NMP at 0°C (Table 5.4, entries 1 and 2). In contrast, the corresponding aryl chloride furnished the desired product in quantitative yield (entry 3) the triflate and even the tosylate, which turned out to be a difficult substrate in nickel- and palladium-catalyzed reactions [24], were converted in high yields (entries 4 and 5). 

In aryl and vinyl bromides and iodides, silylation has been performed by reaction with tris(trimethylsilyl)aluminum etherate in the presence of a nickel(II) chloride-triphenylphosphine complex381. In vinyl iodides the alkene stereochemistry is retained but it is lost in bromides. 

Lipshutz has reported nickel on carbon catalysts for the amination of aryl chlorides [173]. The reactions were conducted with added DPPF as the ligand. The scope of the process is similar to that seen with homogeneous nickel species. Secondary amines provide good yields with electron-poor or electron-rich aryl chlorides, and anilines are suitable for coupling with a range of aryl chlorides. 

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