Nucleophilic reactions nickel complexes

A different example of this general type involves the ds-dihydrazone complex (64). This complex is prepared by condensation of the corresponding dialdehyde with hydrazine in the presence of nickel or copper ion (Curtis, Einstein Willis, 1984). The two uncoordinated -NH2 groups react with a ketone or a diketone such that cyclization occurs in each case [2.8], In these systems, the pendant -NH2 groups are expected to be reactive since their non-coordination results in the respective nitrogen lone pairs being readily available for nucleophilic reaction. 

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

Sulfonium ylides R2S=CR 2 [672,673] and metallated sulfones [674-676] can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,... 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

Reactions of organomagnesium compounds with dialkyl sulfates or alkyl sulfonates often give satisfactory yields of the products of displacement of sulfate or sulfonate. Side-reactions have been observed, but they can often be avoided for example, an excess of the sulfate or sulfonate should be used with Grignard reagents, as some is consumed by nucleophilic attack by halide ion [A]. The dialkyl sulfates are reactive, but hazardous. Toluenesulfonates (tosylates) are less reactive, but their reactions are catalysed by copper complexes the reactions of trifluoromethanesulfonates (triflates) are catalysed by nickel complexes. Reactions of Grignard reagents with secondary tosylates appear to follow an Sn2 mechanism, with inversion of configuration [43],... 

It also appears that the metal can act as a nucleophile in reactions of certain nickel complexes with polyfluoro-aromatic compounds [145-147]. Surprisingly, with penta-fluoropyridine, insertion occurs at the 2-position [145], which is in direct contrast with reactions of most other nucleophiles with this system (see Chapter 9), where... 

It is interesting that in essence complex (XX) is the result of cyclopentadienyl transfer from nickel to the iron atom. In Maitlis s opinion (180) the reaction mechanism resembles the tetraphenylcyclobutadiene transfer from palladium to the iron atom. In terms of CO substitution in the Fe and Ni carbonyls these are nucleophilic reactions in which nickelocene plays the role of a nucleophile. 

Nucleophilic reactions of the spin-paired tris(o-phenanthroline) iron(II) ion are bimolecular 70-72). The tris complex is close to the spin-free complex in energy since dithiocyanatobis(o-phenanthroline) iron(II) exists in a spin-free = spin-paired equilibrium 53). The corresponding tris(o-phenanthroline)nickel(II) ion is unaffected by the same nucleophile, which probably rules out Sat2 attack on the organic ring as the predominant factor. 

Cyclocodimerization and cyclooligomerization reactions of cyclopropenes catalyzed by nickel complexes require alkenes with electron-withdrawing substituents as reaction partners. In this respect, these reactions are complementary to the copper-catalyzed additions discussed in the previous section which do not proceed with electron-poor alkenes due to the low nucleophilicity of the copper-carbene complex. 

Sulfur nucleophiles give reduced products consistent with either nucleophilic attack at a sulfur atom or electron transfer reactions <90PS(53)425>. The dithiolate anions formed by these reactions can be characterized as nickel complexes <89JAP(K)63222188>. Thiocyanate ion attacks at S(2) in 1,2-dithiole-3-thiones <88jhci223>. 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

In 1998, Guitian, Perez and co-workers reported that the cyclotrimerization of benzyne (1) to triphenylene (56) is efficiently catalyzed by palladium(O) complexes (Scheme 11) [39]. This was the first time an aryne had been the substrate in a metal-catalyzed process. The success of the reaction was based on a judicious choice of the reaction conditions, particularly with regard to the catalytic system and the method of generating the aryne. Some significant results of this seminal study are shown in Table 1. After some preliminary experiments in which nickel complexes were used as catalysts (entry 1), it was found that nucleophilic palla-dium(O) complexes efficiently promote the desired [2+2+2] cycloaddition of the strongly electrophilic substrate 1 (entries 2-6) [40]. At the same time, a systematic study of benzyne generation conditions showed that Kobayashi s method... 

Tetraphenylcyclobutadienepalladium and -nickel complexes and tetra-methylcyclobutadienenickel chloride react readily with nucleophilic reagents to give 7r-cyclobutenyl complexes 12, 30, 31, 65, 91), a reaction reminiscent of those described by Chatt et al. for diene-palladium and diene-platinum halide complexes (Section VI, F). Non-halogen-containing cyclobutadiene complexes, however, appear inert under similar conditions so that this reaction is very dependent on the other ligands present. Some similarity between cyclobutadiene-metal and diene-metal complexes appears to exist but how far the parallel can be drawn remains to be seen. The reactions are fully discussed in Section VI. 

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