Arylzinc reagent coupling

Arylzinc reagents couple with ethyl bromoacetate and with bromoacetonitrile in the presence of Ni complexes (equation 181). 

Commercially available Pd(PtBu3)2 is a unique, air-stable 14e Pd° complex, an excellent catalyst for cross-coupling reactions of aryl chlorides. The ability of P Bu3 to stabilize such a coordin-atively unsaturated, extremely reactive, and yet easily manageable form of Pd° is one of the most amazing and fruitful recent findings in Pd-based catalysis. The cross-coupling of arylzinc reagents with aryl or vinyl chlorides can be readily accomplished with as little as 0.03% of this catalyst. Both electron-rich and sterically hindered substrates are welcome in this protocol.404... 

Fu s group showed more recently that ot-bromo ketones 6b can be coupled with arylzinc halides catalyzed by 5 mol% NiCl2 DME and 6.5 mol% of PyBOX ligand 5e with good asymmetric induction providing benzylic ketones in 71-93% yield and 72-96% ee (entry 12) [54]. The lower ee values were obtained with orf/io-substituted aryl bromo ketones, while orf/to-substituted arylzinc reagents as well as a-branched bromo ketones did not react. 

Radical addition/cross-coupling products 61 were obtained in 60-91% yield when Ni(dppf)Cl2 was applied as a catalyst in reactions of alkyl halides 60 with 2,3-disubstituted dienes 59 and aryl Grignard or arylzinc reagents (Fig. 12). Competition experiments of n-, sec-, and ferf-butyl bromide with 2,3-dimethylbutadiene... 

Additionally, the Negishi cross-coupling of arylzinc reagents and aryl iodides using Pd(dba)2 and the cationic phosphine ligand 4 was performed in a biphasic system of an imidazolium salt 1 and toluene [34]. 

Iron Catalysts Iron catalysts have been used in various reactions of sp3 C—H bond functionalization as shown above however, there are few examples of direct sp2 C—H bonds functionalization. Significantly, the first cross-coupling of an arylzinc reagent and 2-arylpyridine has been reported recently (Equation 11.29) [67]. This reaction represents an excellent example of synthetically viable iron-catalyzed C—C bond formation through C—H bond activation. The combination of iron, zinc, magnesium, 1,10-phenanthroline, TMEDA, and l,2-dichloro-2-methylpropane is important for the success of the reaction. It has been speculated that the phenanthroline coordinates to the iron and TMEDA coordinates to the zinc. The reactions proceed via a redox cycle of iron with the l,2-dichloro-2-methylpropane acting as an electron acceptor. 

Teraryls are readily synthesized from l,4-his(iodozincio)benzene by consecutive Negishi coupling reactions. Such is feasible because of the different reactivity of the two types of arylzinc reagents, products of the first coupling are less reactive toward the Pd catalyst. ... 

The reaction was exploited very recently in a solid phase synthesis of biaryls. Aryl zinc bromides undergo palladium catalyzed coupling reactions with aryl bromides bound to a polystyrene resin. The product can be released from the resin by transesterification [44]. Ni(0) catalysed homocoupling of arylzinc reagents could also be realised using aryl triflates [45], as well as aryl tosylates and mesylates [46]. 

Nakamura s group reported the first efficient cross-coupling reaction between primary and secondary allq l sulfonates using arylzinc reagents achieved under iron catalysis (Scheme 13.23). This contribution displays... 

Scheme 14.24 Ni-catalysed cross-coupling of aryl/alkenyl pivalates with arylzinc reagents.

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