Coupling reactions tetrakis nickel

Alkenyl-alkenyl cross-coupling. Baba and Negishi have prepared a catalyst from this Pd(II) complex and 2 equiv. of diisobutylaluminum hydride that promotes this coupling reaction. Tetrakis(triphenylphosphine)palladium(0) is inactive, as is material prepared in situ from palladium chloride, triphenylphos-phine and HAKr-CtHg) . A nickel catalyst prepared from Ni(acac)2, PfCnHsja, and diisobutylaluminum hydride is somewhat less efficient. The coupling Involves (E)-alkenylalanes (4, 158, 159) and alkenyl halides. The products are (E,E)- and (E,Z)-dienes. 

Although the copper mediated Ullmann reaction is a well known method for biaryl synthesis, drastic conditions in the range of 150-280 °C are required. Zerovalent nickel complexes such as bis(l,5-cyclooctadiene)nickel or tetrakis(triphenylphosphine)nickel have been shown to be acceptable coupling reagents under mild conditions however, the complexes are unstable and not easy to prepare. The method using activated metallic nickel eliminates most of these problems and provides an attractive alternative for carrying out aryl coupling reactions(36,38). 

Biphenyl bond formation has been achieved in the diiodobiaryl derivative 41 by an intramolecular aryl halide coupling reaction promoted by tetrakis(tri-phenylphosphine) nickel. This short and efficient route gave the dibenzazonine 42 in good yield (42) (Scheme 9). In a similar way, tetrasubstituted derivative 43 was successfully coupled with the more easily prepared tris(triphenylphos-phine)nickel, giving 44 in 62% yield. Subsequent debenzylation led to the most efficient synthesis of dibenzazonine 35a reported to date (43). 

In conclusion, oxidative addition of aryl halides to metallic nickel proceeded smoothly under mild conditions, and the corresponding biaryls were obtained in good yields. The present reaction is superior to the Ullmann synthesis in the scope of the reaction conditions [125] and to the coupling reaction of Grignard reagents catalyzed by transition metals [133]. Finally, because of the simple and easy procedure for the preparation of nickel powder, it is far more convenient to work with than zerovalent nickel complexes such as tetrakis(triphenylphosphine)nickel(0) or bis(l,5-cyclooc-tadiene)nickel(O) [128]. 

Fig. 3 Common reactions and cross-couplings in oligofluorene synthesis shown for example structures, where PdCl2(dppf) is [l,l -bis(diphenylphospino)ferrocene] dichloropaUadium(II), Pd(PPh3)4 is tetrakis(triphenylphosphino)palladium(0), and Ni(COD) is bis(l,5-cyclooctadiene)nickel(0)... 

Ullmann reaction Semmelhack (4, 33) has shown that bis( 1,5-cyclo-octadiene)nickel(O) and tetrakis(triphenylphospine)nickel(0) are useful reagents for coupling of aryl halides (Ullmann reaction). However, these zerovalent nickel complexes are not easy to prepare and are extremely sensitive to moisture. Kende reasoned that the coordinatively unsaturated Ni(TPP)3 is actually the species involved and indeed found that aryl halides couple to biaryls in the presence of 1 eq. of this complex. Aryl halides with two orr/io-substituents do not couple. Otherwise, yields of biaryls are in the range of 60-80%. 

Dienes. The (E)-l-alkcnylzirconium compounds, obtained by hydrozirconation of 1-alkynes (6, 177-178), undergo cross coupling with alkenyl halides to form conjugated dienes (cf. the similar reaction of alkcnylalanes with alkenyl halides, 7, 95-96). In the present synthesis tetrakis(triphenylphosphine)-palladium (6, 571-573) can usually serve as catalyst. Nickel catalysts are less effective than Pd catalysts in this case. ... 

Tetrakis(triphenylphoshphine)nickel(0) is also an effective reagent for coupling aryl halides. Medium-sized rings can be formed in intramolecular reactions. 

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