Nickel heteroarylation

As described in Section in.B.4.d, a palladium catalyst was required to couple ArZnX with AFX. Indeed, the remaining nickel(II) complex in the medium was unable to catalyze this coupling reaction even if it was reduced to the corresponding zero-valent nickel complex. In contrast, with 2-chloropyridine as the heteroaryl halide added in the second stage, the coupling reaction was efficiently catalyzed by Ni°(bpy). 2-Arylpyridine compounds were obtained in good yields. Results are reported in Table 5. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

An extensive series of neutral macrocyclic complexes, mainly of nickel(II), copper(II), platinum(II) and palladium(II), has been developed by Dziomko and coworkers. The cyclization step in the template reaction is a nucleophilic aromatic substitution of an arylamine on to a haloaryl azo compound. A variety of aryl and heteroaryl rings can be incorporated in different combinations. For instance, a diaminoazo compound can be combined with a dihaloazo compound (Scheme 58).246 247 Another synthetic strategy involves the dimerization of an aminohaloazo compound and leads to more symmetrical macrocyclic complexes (Scheme 59).248 249 Most recently, dihalodiazo compounds have been synthesized from dihydrazines and pyrazolinediones and undergo template reactions with simple 1,2-diamines (Scheme 60).249 250... 

The copper-catalysed, Ullman-type coupling of aryl, heteroaryl and alkenyl halides may be achieved at ambient temperature using copper(I) thiophene-2-carboxylate as catalyst.60 A new semiconducting poly(anthraquinone-l,5-diyl) with nitro groups at the 4- and 8-positions has been prepared by Ullman-type coupling using metallic copper or a zerovalent nickel complex as catalyst.61... 

The cross-coupling of organozinc compounds has been accomplished using a number of metal enolate complexes, namely Rh(acac)(H2C=CH2)2 (6) , Co(acac)3 (30) °, Ni(acac)2 (47) , Li(acac) (107) and Cr(acac)3 (108) . For instance, complex 47 proved to be an extremely effective catalyst system for the Negishi cross-coupling of arylzinc halides (109) with aryl (110), heteroaryl and alkenyl halides, triflates and nonaflates to give the corresponding biaryl compounds (111) (equation 30). The solvent played an important role in these reactions and optimal conditions were found with 8 1 mixtures of THF and A-ethylpyrrolidinone (nep). Only 0.05 mol% of the nickel complex... 

Other coupling partners to organozinc reagents include heterocyles such as 2-methylthiobenzothiazole, alkenyl aryl iodonium triflates (alkenyl group transfer for synthesis of trisubstituted alkenes), and aryl heteroaryl ethers. " Improved nickel-catalyzed cross-coupling conditions between oz-rto-substituted aryl iodides-nonaflates and alkyl-zinc iodides in. solution and in the solid phase have been defined. ... 

The use of transition-metal catalysts in the substitution of aryl halides by amines has been reviewed. There has also been a summary of the use of palladium catalysts in the reaction of aryl and heteroaryl halides with primary and secondary amines. It has been shown that the amination of aryl sulfamates by aliphatic amines may be achieved using a nickel catalyst with an A-heterocyclic carbene ligand. Suitable ligands for nickel and palladium in the catalysed amination reactions of aryl sulfamates and imidazolylsulfonates have been identified. " The palladium-catalysed reaction of aryl nonafluorobutanesulfonates with primary sulfonamides may yield substituted products such as (22). Kinetic data suggest that reductive elimination from the palladium intermediate is likely to be rate limiting. A-Arylmethanesulfonamides may also be formed from aryl bromides and chlorides using a palladium catalyst... 

Nickel catalysis has also been used in the formation of biaryls, such as (51), by substitution of the methoxy group in 1-methoxynaphthalene by tolylmagnesium bromide. It is also reported that the reaction of aryl or heteroaryl tosylates with phenylmag-nesium bromide to give biaryl derivatives is catalysed by palladium complexed with heteroatom-substituted secondary phosphine oxide ligands. 

Cobalt has attracted attention in recent years as a cost-effective and sustainable alternative to palladium or nickel. As indicated earlier, functionalized arylzinc reagents can be prepared from aryl halides and sulfonates by zinc insertion in the presence of catalytic amounts of CoBtj [50, 51, 210). Interestingly, the presence of this cobalt salt suffices to catalyze in situ the cross-coupling of the arylzincs thus produced with aryl, alkyl, and heteroaryl electrophiles such as organic halides [211-214] or thioethers, as exemplified with the formation of 286 [214] (Scheme 4.63). 

The metal-catalyzed cross-coupling of aryl-, heteroaryl-, and alkenyhnagnesium derivatives is a broad-scope transformation that has found many synthetic applications C(sp )-C(sp ) couplings are by far the most common. Catalysts by nickel, palladium, and iron complexes is most widespread, but the emerging fields of cobalt and manganese catalysis can also provide useful alternatives. 

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