yamamoto coupling 山本耦合反应

1.2.4.2 Yamamoto coupling reaction

Diorganonickel (II) complexes, NiR2Lm, undergo coupling reactions to give R-R

and has been used by Yamamoto et. al. for coupling reactions between organic compounds [96, 103]. They have applied this reaction and used it for the21

polycondensation of dihaloaromatic compounds [104]. General representations of the coupling reactions are shown below.

where Lm is a neutral ligand.

Various solvents can be used for the coupling reaction and it proceeds under relatively mild conditions. It can be applied to a wide range of aromatic compounds. It is simple and presents a very versatile means for the synthesis of electrically conducting polymers from dihaloaromatic compounds.

The halo Ni(II) complex, LmNiX(RX) is formed as the polymerization proceeds through the oxidative addition of the organic halide to the Ni(0) complex. LmNi(RX)2 is formed via the disproportionation of the complex and this yields X-Ar-Ar-X on reductive elimination. The Ni(0) catalyst is formed from the reduction of Ni(II) in NiCl2 by Zn. A general representation of the cycle is illustrated in scheme 1.5.

Trans-Ni(II) intermediate complexes are usually formed when tertiary phosphine ligand such as PPh3 is the neutral ligand [105]. Cis-complexes are formed when bidentate

2,2’-bipyridine is the neutral ligand. As reductive elimination is more favoured by cistype complexes, a certain amount of 2,2’-bipyridine is normally used as a promoter of the polymerization process [106].

Scheme 1.5 General catalytic cycle for Yamamoto coupling

Yamamoto coupling reactions must be conducted in a dry and inert environment

as moisture acts as a chain terminator and deactivates the catalyst. Oxygen also poisons the catalyst and so the reaction is normally carried out in nitrogen. An example of its use in polymer synthesis is shown below in scheme 1.6.

Scheme 1.6 (i) NiCl2, Zn, PPh3, bpy, DMF, reflux [95]

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