Nickel-containing heterocycles

Scheme l.545 The cobalt and nickel complexes M(S2N2H)2 (M = Co or Ni) may also take part in condensation reactions with chlorosilanes to give cobalt- and nickel-containing heterocyclic systems (Scheme 2).546 The... 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

Crude oil consists mainly of a mixture of paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of metals-containing heterocyclic compounds. The most abundant metals found in oils are those contained in porphyrin or porphyrin-like complexes (nickel, copper, iron, and vanadium). These... 

Sulfur-containing heterocyclic rings are readily desulfurized by treatment with Raney nickel. Thus the 1,2,4-thiadiazine (146) is fully desulfurized and cleaved to give (147) (67CB2159). Similarly, Raney nickel treatment of (148) gives RO2CNHNHCO2R... 

A number of methods are available for desulfurization of sulfur-containing heterocycles. The use of Raney nickel is a common method for reductive desulfurization which has been employed in the cleavage reaction of the fused uracil (123) <76CPB979>. 

SCHEME 4.78 Nickel-catalyzed preparation of phosphorus-containing heterocycles [130]. 

Thiophthalic anhydride 15 also reacted with various alkynes in the presence of a nickel catalyst to afford sulfur-containing heterocyclic compounds (Scheme 12.8). The use of Ni(0)/PPr3 catalyst in combination with MAD afforded thioisocoumarin 16... 

The IR bands in a number of nickel complexes of triaryl formazans have been assigned by Arnold and Schiele.415 A similar assignment of the electronic bands has been carried out.414 LCAO-MO calculations correlate well with these assignments417 and have been extended to include both inner ligand transitions as well as charge transfer bands and d—d transitions.418 EPR spectra have been used to study the nature of bonding in copper complexes of heterocyclic-containing formazans.419 Metal formazan complexes have also been studied by electrochemistry.283,398 420-422... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Some of the ring expansion reactions discussed in Section 2.03.3.3.1 can be extended to five-membered heterocycles containing two or more heteroatoms. Reaction of imidazoles and pyrazoles with dichlorocarbene, for example, gives chloropyrimidines together with small amounts of chloro-pyrazines or -pyridazines, and oxidative ring expansion of 1-aminopyrazole with nickel peroxide gives 1,2,3-triazine (this, in fact, constitutes the only known synthesis of the unsubstituted triazine). There are, however, a number of interesting and useful transformations which are unique to five-membered polyheteroatom systems. 

The ligands tris(2-pyridylmethyl)amine (tpma), tris(3,5-dimethyl-l-pyrazolylmethyl)amine (metpyma) and tris(3,5-dimethyl-l-pyrazolylethyl)amine (metpyea) are examples of tetraden-tate tripod-shaped ligands containing three heterocyclic nitrogen donors connected to an apical tertiary nitrogn donor by.aliphatic chains. Selected nickel(II) complexes with these ligands are reported in Table 50. 

Both molecules contain the same donor atom configuration. Diacetyldioxime is highly specific for nickel and palladium. If the same donor atom configuration is incorporated into a heterocyclic system, as in the phenanthrolines, the ligands become specific for copper and iron. This specificity can be directed by choice of the substituents R in the molecule if R = H, the ligand is specific for iron with R = CH3 and/or C 6H5. the ligand is copper-specific. 

---