Nickel complexes alkyl phosphines

Alkylnickel amido complexes ligated by bipyridine have been prepared that undergo reductive elimination of V-alkyl amines (Equation (54)).207,208 Unlike the phosphine-ligated palladium arylamides, these complexes underwent reductive elimination only after oxidation to nickel(III). Thermally induced reductive elimination of alkylamines from phosphine-ligated nickel complexes appears to occur after consumption of phosphine by arylazides 209... 

Biaryl synthesis from aryl halides is a more interesting reaction due to the value of these molecules and their difficult access by chemical methods. The first electrosyntheses were simultaneously done in 1979-80 by three groups [21-23] who used NiCljPPha (1-20%) as catalyst precursor in the presence of excess PPhs. Later, several groups investigated the use of bidentate phosphines like dppe associated with nickel in the synthesis of various biaryls, and notably 2,2 -bipyridine and of 2,2 -biquinoline from respectively 2-chloropyridine and 2-chloroquinoline [24], More recently new nickel complexes with l,2-bis(di-2-alkyl-phosphino)benzene have been studied from both fundamental and synthetic points of view [25]. They have been applied to the coupling of aryl halides. 

The effect of tin compounds, especially tetra-alkyl and tetra-aryl tin compounds, is similar to that of phosphine, though lower temperature and pressure are required for the catalyst s optimum activity. Tin can promote the activity of the nickel catalyst to a level that matches that of rhodium under mild conditions of system pressure and temperature e.g. 400 psig at 160 C. The tin-nickel complex is less stable than the phosphine containing catalyst. In the absence of carbon monoxide and at high temperature, as in carbonyl-ation effluent processing, the tin catalyst did not demonstrate the high stability of the phosphine complex. As in the case of phosphine, addition of tin in amounts larger than required to maintain catalyst stability has no effect on reaction activity. 

In the case of phosphine, the active catalyst is presumably either bisphosphine dicarbonyl or the phosphine tricarbonyl complex. Kinet-ically the bis-phosphine nickel complex cannot be the predominant species. However, in the presence of very high phosphine concentration it may have an important role in the catalyst cycle. After ligand loss and methyl iodide oxidative addition, both complexes presumably give the same 5 coordinate alkyl species. 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

Tetrakis(alkyl isocyanide) complexes of nickel(O), Ni(CNR)4, and the mixed isocyanide complexes with phosphines and unsaturated molecules are strictly analogous to the corresponding carbonyl complexes.23,24 They are generally more stable than [Ni(CN)4]4-. Mixed isocyanide complexes have been prepared by the reaction of Ni(cod)2 and CNBu followed by reaction with the appropriate phosphine or unsaturated molecules (alkenes, arylnitroso compounds, azo compounds, etc.) as outlined in equations (7) and (S).25... 

Thiocarbonyl derivatives of 1,3-dioxolanes and 1,3-oxathiolanes are readily isomerized to the 2-carbonyl compounds as shown in Scheme 20. Alkylation of the sulfur atom with alkyl halides usually leads to ring-opened products (Scheme 21) (69JOC3011). Most of the other chemistry of the sulfur derivatives has focused on desulfurization and subsequent generation of alkenes. The reaction is shown in equation (20) and proceeds with cis elimination via carbene intermediate (see Section 4.30.2.2.5) and is usually carried out with a phosphine (73JA7161) or a zero-valent nickel complex (73TL2667). 

But Wilke, Bogdanovic and co-workers have shown that, in the case of the catalytic system r-allyl-nickelhalide/aluminum alkyl, the addition of bulky phosphines such as tricyclohexylphosphine, triisopropylphosphine or di(t-butyl)ethylphosphine, directs the dimerization reaction to 2,3-dimethylbutene ab route), with a selectivity of 60—80%, An X-ray structural analysis of the active species has revealed a mononuclear, approximately square-planar nickel complex with a PR3, and a... 

Numerous complexes of nickel(II) and nickel(O) catalyze the addition of the Si—H bond to olefins. Among such catalysts are nickel-phosphine complexes, such as [Ni(PR3)2X2] (where X = Cl, I, and NO3 R = alkyl and aryl), [Ni(PPh3)4], and [Ni(CO)2(PPh3)2], as well as bidentate complexes [NiCl2-(chelate)], [Ni(acac)2L] (L = phosphine), and [Ni(cod)2(PR3)2l (3,6,10,64). Ni(0)-phosphine complexes were used for the hydrosilylation of alkenes and butadienes (65,66). Cationic nickel complexes, such as [(indenyl)Ni(PPh3)]+ (67) and [Ni( r-allyl)PR2(CH2CH=CH2)]+ (68), were reported as novel effective catalysts of regioselective hydrosilylation of styrene with PhSiHa. 

The addition reaction of hydrosilanes to alkenes leads to alkyl derivatives of silicon. Thus, 1-alkenes react with hydrosilanes in the presence of most catalysts (except for some Pt(ll) and Pd(II) chiral phosphine complexes and some nickel complexes) to give 1-silylalkanes ... 

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