Nickel aryl coupling scheme

An elegant synthesis method which is specific to sulfone polymers containing phenyl—phenyl linkages (such as PPSF) is the nickel-catalysed coupling of aryl dihahdes. The scheme for this synthesis involves a two-step process. First, an aromatic dihaUde intermediate is formed which carries the backbone features of the desired polymer. This aromatic dihahde intermediate is then self-coupled in the presence of sero-valent nickel, triphenylphosphine, and excess sine to form the biphenyl- or terphenyl-containing polymer. AppHcation of this two-step scheme to PPSF can be depicted as follows ... 

Scheme 72 Nickel-catalyzed coupling of aryl halides and alkenes.

Because of the extraordinary strength of the carbon-fluorine bond, transition metal-mediated activation of fluoroalkanes and arenes is not easy to achieve. Nevertheless, activation of the C-F bond in highly electron-deficient compounds such as 2,4,6-trifluoropyrimidine, pentafluoropyridine, or hexafluorobenzene is possible with stoichiometric amounts of bis(triethylphosphano) nickel(O) [101] (Scheme 2.45). More recently Herrmann and coworkers [102] have described a variant of the Kumada-Corriu cross-coupling reaction [103] between fluorobenzene and aryl Grignard compounds which uses catalytic amounts of nickel carbene complexes. Hammett analysis of the relative kinetic rate constants indicated that the reaction proceeds via initial oxidative addition of the fluoroaromatic reactant to the nickel(O) species. 

The classical methodology of the Ullmann biaryl synthesis was significantly improved by Semmelhack et al. using zero-valent nickel complexes [28]. Aryl iodides were coupled in high yield using bis(1,5-cyclooctadiene)nickel(0) as catalyst. Yields were further improved by the application of tetrakis(triphenylphosph-ine)nickel(0) [29] (Scheme 40). 

The introduction of a t-butyl group a - to a carbonyl group has been achieved by Friedel-Crafts alkylation of the appropriate trimethylsilyl enol ether. In the presence of a nickel(ii) catalyst, u/c-bromotrimethylsilyloxyalkenes couple with Grignard reagents to produce alkylated and arylated silyl enol ethers, which, after hydrolysis, furnish the corresponding a-alkylated or a-arylated ketones (Scheme 36). ... 

The low reactivity of aryl chlorides is usually attributed to the strength of the C—Cl bond (bond dissociation energies for Ph-X Cl, %kcalmor Br, SlkcalmoT I, 65kcalmoT ), which leads to a reluctance by aryl chlorides to oxidatively add to either Pd(0) or Ni(0), which is a critical initial step in palladium- and nickel-catalyzed coupling reactions (Scheme 2.1) [6, 7]. 

Nickel plays a prominent role in C—N bond formation. The first nickel-catalyzed C—N coupling has been reported using Ni(cod)2-dppf and Ni(cod)2-l,10-phenanthroline for the synthesis of aryl amine (Scheme 20.29) [65]. The use of Ni/bipyridine [66], Ni/C-dppf [67], and Ni(II)-(NHC) [68, 69] has been subsequently described for the coupling of aryl halides with amines in the presence of sodium alkoxide. Ackermann and coworkers used Ni(cod)2-dppf for... 

In 2011, Joshi-Pangu etal. [176] reported a very successful nickel-catalyzed Negishi- Baba coupling of secondary alkylzinc halides and aryl iodides (Scheme 1.54). This group has presented the first process that overcomes the isomerization and p-hydride elimination problems that are associated with the use of secondary nucleophiles, and that have limited the equivalent Pd-catalyzed reactions. 

SCHEME 4.243 Nickel-catalyzed coupling of secondary phosphites with aryl halides [336]. 

Typical aryl-aryl coupling reactions that utilize catalytic amounts of Ni(II) salts are outlined in Scheme 12.24. Zinc metal was used to reduce Ni(II) to the cataly tically active Ni(0) species in the homocoupling of dimethoxypyridine 108 and the efficient synthesis of dimer 109 (Eq. 12.24-1) [88]. Alternatively, nickel salts can also efficiently be reduced by means of electrochemical methods. As shown in Equation 12.24-2, Troupel and coworkers were able to recover Ni(0) on a steel electrode and isolated the coupling products in excellent yield [89]. Comparable studies were carried out with slightly more complex substrates and confirmed that... 

Nickel-promoted or nickel-catalyzed reactions are well suited for the preparation of imsymmetrical biaryls via cross coupling. A variety of different protocols have been developed, and a small selection of reactions is outlined in Scheme 12.25. The aryl coupling shown in Equation 12.25-1 is a nice example of an electrochemical protocol where the coupling chemoselectively takes place at the more active bromide while the chloride in halide 112 does not participate in the coupling reaction [90]. 

Kim and Webster first prepared hb polyphenylenes from 3,5-dibromophenyl 4-boronic acid and dihalophenyl Grignard reagents by palladium-catalyzed and nickel-catalyzed aryl-aryl coupling reactions, respectively [76]. The structures of 3,5-dibromophenyl 4-boronic acid (1-8) and dihalophenyl Grignard reagents (1-9) are shown in Scheme 6. 

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). 

Scheme 6.44 Nickel-catalyzed cross-coupling of zirconocenes and aryl halides.

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