Nickel catalysis aryl coupling

Recently, this reaction has been extensively studied since it is currently the only method to couple aryl Grignard reagents with secondary alkyl halides Indeed, secondary aUtyl halides do not react under palladium or nickel catalysis . On the other hand, let us recall that the coupling of secondary alkyl Grignard reagents with aryl halides leads to poor results (see above). 

A. Nickel Catalysis in the Cross Coupling of Aryl Halides with Alkylmetals. The Role of Arylalkylmckel(II) Species as Intermediated For the study of nickel catalysis in the formation of aralkanes, we employed the system consisting of aryl bromides and methyllithium or methylmagnesiiun bromide. 

Kappe and Stadler have developed an MW procedure for rapid production of triaryl phosphines by coupling diphenylphospine with aryl halides and triflates [134]. Taking into account the importance of phosphine ligands in a variety of transition metal-catalyzed reactions, convenient procedures for their production is valuable. Both homogeneous Pd-Ni and heterogeneous Pd catalysts were explored and the more unusual substrate phenyl triflate could also be coupled swiftly by use of nickel catalysis (Scheme 15.68). Couplings with other aryl halides proceeded in 26-85% yield after 3-30 min microwave irradiation at 180-200 °C. 

The tremendous popularity of palladium catalysis has overshadowed the utility of nickel catalysis in most processes involving the union of sp -sp and sp-sp carbon-carbon bonds. However, the development of nickel-catalyzed cross-couplings emerged as useful in couplings of aryl chloride substrates. The high reactivity of nickel catalysts toward aryl chlorides, coupled with the well-defined mechanistic pathways of aryl chlorides compared with the corresponding aryl iodides and bromides, renders the aryl chloride-based processes especially useful. " ... 

In 1998, Knochel reported that, in the presence of 4-trifluoromethylstyrene, [Ni(acac)2] efficiently catalyzed cross-couplings between polyfunctional arylzinc derivatives and alkyl halides possessing P-hydrogens (Equation 5.15). While the alkyl halides were limited to primary alkyl iodides, the scope of nickel catalysis was significantly expanded. The role of the electron-deficient olefin, 4-trifluoromethylstyrene, was proposed to accelerate the reductive elimination step by decreasing the electron density at the nickel center of an (alkyl) (aryl)nickel intermediate [18]. 

Under nickel catalysis, it was foimd that both azoles (Figure 13.13) [36] and tertiary ammonium salts (Figure 13.14) [37] can serve as effective coupling partners. Carbohydrate-based azoles, as well as aryl-azole substrates, were effective coupling partners. The use of ammonium salts was restricted to aromatic systems however, the use of a common structural motif is a significant improvement to the coupling methodology. 

Liu, N., Wang, Z.-X. (2012). Nickel-catalyzed cross-coupling of arene- or heteroarenecarbonitriles with aryl- or heteroarylmanganese reagents through C-CN bond activation. Advanced Synthesis and Catalysis, 354, 1641-1645. 

Changing the enolate counter ion can also be beneficial. Reformatsky reagents 2.494 are zinc enolates, generated indirectly. These reagents can couple with aryl halides using either palladium or nickel catalysis (Scheme 2.141). This may also be considered as a variant of Negishi coupling (Section 2.3). 

The availability of newer, more effective methods for aryl aryl coupling has been an important driving force for the development of new synthetic strategies for PPPs and other polyarylenes. Transition metal catalysis, such as the Pd(0)-catalyzed aryl-aryl coupling developed by Suzuki [63] and nickel(0)-catalyzed or -mediated coupling... 

The nickel-catalyzed Suzuki-Miyaura-type C-O bond arylation has been successfully applied to steroidal architecture (Scheme 4). A hydroxyl moiety in estrone can readily be substituted by an array of aryl groups under nickel catalysis via conversion into pivalate instead of the typically used triflate [38]. The carbonyl moiety in estrone also serves as a suitable precursor for an alkenyl C-O electrophile. Treatment of estrone with 2-propenyl acetate affords the compounds bearing two acetate groups, both of which are potentially reactive toward nickel-catalyzed cross-coupling. However, selective arylation took place at the alkenyl position, and... 

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