Nickel dehalogenative coupling

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. 

Active Nickel-Mediated Dehalogenative Coupling of Aryl and Benzylic Halides... 

The results of the homocoupling reaction of iodobenzenes by activated nickel powder are summarized in Table 7.11. Metallic nickel worked well for the dehalogenative coupling of unsubstituted and 4-substituted iodobenzenes. For instance, the reaction of 4-iodomethoxybenzene reached completion in 2 h at 80 C to afford 4,4 -dimethoxybiphenyl in 85% yield (via GLC) along with the reduction product anisole (15%). The usual woricup of the reaction mixture gave the biphenyl in 68% isolated yield. Previous workers obtained 4,4 -dimeth-... 

Active Nickel-Mediated Dehalogenative Coupling of Aryl and Benzylic Halides 291 Table 7.13 Reaction of benzyl halides with metallic nickel powders. 

Active Nickel-Mediated Dehalogenative Coupling ofAryl and Benzylic Halides j 295... 

Yamamoto et al prepared polythiophene by the dehalogenative coupling of 2,5-dibromothiophene with magnesium catalyzed by nickel(II) chloride and bipyridine [86]. Lin and Dudek also prepared polythiophene in a similar way [87]. The conductivity of a polythiophene pellet was 10 "Scm and was enhanced to 0.1 S cm after doping with iodine. 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

Transition metal-catalyzed cross-coupling reactions of halogenopyrazines is an efficient synthetic method for alkyl-, alkenyl-, and alkynylpyrazines (Section 6.03.5.4.2). Palladium and nickel complexes are particularly effective as catalysts in this reaction. The Wittig reaction of halogenomethyl-pyrazines likewise leads to the formation of alkenylpyrazines (Section 6.03.8.1). Dehalogenation of halogeno pyrazines is a very practical synthetic method for alkyl- or aryl-substituted pyrazines. For example, phenylpyrazine has been prepared by catalytic hydrogenolysis of the 2-chloro-3-phenyl compound in the presence of triethylamine. This product is also obtained by decarboxylation of... 

Ong and co-workers reported an alternative synthesis of poly(3,6-dialkylthieno[3,2- ]thiophene-co-bithiophene) (Scheme 17.7) [50]. In this case, a symmetrical monomer was constructed from the Stille cross-coupling of 2,5-dibromo-3,6-dialkylthieno[3,2- ]thiophene with 2-tributylstannylthiophene followed by bromination in the free a-positions with iV-bromosuccinimide. Dehalogenative polymerization (often termed Yamamoto polymerization [86]) was performed by refluxing in toluene in the presence of excess bis(cyclooctadiene)nickel(0) and 2,2 -bipyridyl afforded the product in 92 % yield. The number-average molecular weight was 12400 gmoP with a polydispersity of 1.6. 

For the preparation of PAT (n = 1,6, 8,12) and of poIy(3-cyanothiophene) with high yields, a zero-valent nickel complex is also used [266]. Hexyl substituted oligo(thiophene)s (number of thiophene rings 6,9,12,15) can be prepared by the Ni(0)-catalyzed coupling reaction of a 5,5"-dibromo-3,3"-dihexyl-2,2 5, 2"-terthiophene) [572, 573]. PT derivatives having alkyloxy substituents at 3 positions are synthesized by dehalogenation polycondensation of the corresponding 2,5-dibromothiophene derivatives with zero-valent nickel complexes [247],... 

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