Zero-valent nickel, 1,5-cyclooctadiene

The classical methodology of the Ullmann biaryl synthesis was significantly improved by Semmelhack et al. using zero-valent nickel complexes [28]. Aryl iodides were coupled in high yield using bis(1,5-cyclooctadiene)nickel(0) as catalyst. Yields were further improved by the application of tetrakis(triphenylphosph-ine)nickel(0) [29] (Scheme 40). 

Cycloaddition copolymerization of diynes (Equation 44, product 46) such as 3,11-tetradecadiyne, 3,9-dodecadiyne, 1,3- or 1,4-di(2-hexynyl)benzene with CO2 was successfully carried out in the presence of zero-valent nickel catalyst [NiCOD)2 2P( -C8Hi )3] prepared from bis(l,5-cyclooctadiene)nickel and tri-n-octylphosphine ligand Synthesis of soluble ladder-type polymers (47) was carried out by cycloaddition copolymerization of 1,7-cyclotridecadiyne (48) and CO2 (equation 45). 

Tetrachloropalladate(II) ion catalyzes the interconversion of 1- and 2-butenes in aqueous solutions containing chloride and hydronium ions. Sodium tetrachloropalladate(II) catalyzes the conversion of allylbenzene to propenyl-benzene in acetic acid solutions. Tetrakis(ethylene))Lt,/x -dichlororhodium(l) catalyzes butene isomerization in methanolic hydrogen chloride solutions . Cyclooctadienes isomerize in benzene-methanol solutions of dichlorobis-(triphenylphosphine)platinum(11) and stannous chloride. Chloroplatinic acid-stannous chloride catalyzes the isomerization of pentenes. Coordination complexes of zero-valent nickel with tris(2-biphenylyl)phosphite or triphenyl-phosphine catalyze the isomerization of cis-1,2-divinylcyclobutane to a mixture of c/5,m-l, 5-cyclooctadiene and 4-vinylcyclohexene . Detailed discussions of reaction kinetics and mechanisms appear in the papers cited. 

Since cyclooctadiene has no suitable low-lying unoccupied orbitals some of the 3d electrons of nickel are expected to have a relatively high antibonding character. It is therefore not surprising that the nickel complex is extremely reactive, air-sensitive, and very unstable in solutions even in the absence of oxygen. Carbon monoxide at room temperature completely displaces the cyclooctadiene molecules and yields nickel carbonyl (99). Acrylonitrile reacts with (LII) under similarly mild conditions, forming bis(acrylo-nitrile)-nickel 101), while duroquinone, well below room temperature, affords cyclooctadiene-duroquinone-nickel 101). These reactions uniquely demonstrate the close interrelationship between all complexes of zero-valent nickel. 

As zero-valent nickel complexes, mixtures of bis(l,5-cyclooctadiene)nickel and neutral ligands L have been employed. This polycondensation affords PT and PAT with relatively well-defined linkages between the monomer units in the polymer chains in high yields [92,266,271,272,571]. Vacuum deposition of PT gives crystalline films with their main chains essentially perpendicular to the surface of substrates such as carbon and gold [266]. The iodine-doped PT reaches an electrical conductivity of 8Scm and the FeCls-doped PT an electrical conductivity of 0.5 S cm [272]. 

Significant advances in organonickel chemistry followed the discovery of frtzws,fraws,fraws-(l,5,9-cyclododecatriene)nickel, Ni(cdt), and bis(l,5-cycloocta-diene)nickel Ni(cod)2 by Wilke et. al.1 In these and related compounds, in which only olefinic ligands are bonded to the nickel, the metal is especially reactive both in the synthesis of other compounds and in catalytic behavior. Extension of this chemistry to palladium and to platinum has hitherto been inhibited by the lack of convenient synthetic routes to zero-valent complexes of these metals in which mono- or diolefins are the only ligands. Here we described the synthesis of bis(l,5-cyclooctadiene)platinum, tris(ethylene)-platinum, and bis(ethylene)(tricyclohexylphosphine)platinum. The compound Pt(cod)2 (cod = 1,5-cyclooctadiene) was first reported by Muller and Goser,2 who prepared it by the following reaction sequence ... 

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