Nucleophilic attack nickel complexes

Allylic substitution using hard nucleophiles proceeds through a different mechanism. Instead of attacking the allyl group of the 71 allyl-metal complex, hard nucleophiles attack the metal first and the product is subsequently formed by reductive elimination. Nickel(O) complexes have often been used for this purpose. Reports of good enantioselectivities in this type of reaction are limited. 

The oxidation of the metal complexes of l,10-phenanthroline-5,6-quinone is thought to proceed in a similar manner, with the first step being a benzilic acid rearrangement. Rearrangements of this type may also be followed directly in nickel(u) and cobalt(m) complexes of 2,2 -pyridil. The first step of the reaction involves nucleophilic attack on an O-bonded carbonyl group to form a hydrate, followed by a benzilic acid rearrangement. In this case, the benzilic acid rearrangement products may be isolated as metal complexes (Fig. 8-43). 

From a study of the enzyme kinetics with a range of substrates and inhibitors, and the chemistry of related metal-ion complexes, Dixon et al. [39] proposed a model of the environment of the active site and reaction cycle. Although differing in details from the structure as now determined, this mechanism provided a basis for understanding the functions of the two metal ions. One nickel ion (Ni-1) binds the urea, and the other nickel ion (Ni-2) binds a hydroxide ion that makes a nucleophilic attack on the urea, leading to the formation of a tetrahedral intermediate. 

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],... 

Numerous dinickel complexes have been reported as models for urease. As depicted in Scheme 5, a dinickel complex [Ni2( 4-0H)( j,-H20)(bdptz)(H20)2][0Ts]3 is capable of hydrolyzing a bound amide substrate by intramolecular nucleophilic attack of a coordinated hydroxide ion, either from a bridging position or from a transient terminally bound form. This amide hydrolysis mimics the hydrolysis of urea by urease in that a hydroxide nucleophile is generated by the dinickel center, and the coordination of the substrate to the dinickel center as well as a nickel-bound hydroxide ion serving as the nucleophile are crucial to hydrolysis. Protonation of the amine group by an acidic residue in the active-site results in loss of ammonia. 

Vinyl bromides are directly aminocarbonylated by nickel carbonyl and amines. Very similarly, Rh (CO)i6 and BU4NCI as cat yst convert allylphosphates to. -y-unsaturated amides via rr-allylrhodium complexes (equation 43). Although palladium(O) complexes are more reactive than rhodium(I) complexes, palladium(O) complexes undergo side reactions, like reductive elimination in the presence of carbon monoxide, and direct nucleophilic attack by amines. 

In contrast, nucleophilic attack at an sp -carbon center with anionic 18-electron species like [Co(CO)4] and [Fe(CO)4] usually fails to activate aromatic or vinylic halides. Thus, catalytic carbonylations of aryl halides are initiated by oxidative addition of a C-X bond to an electronically unsaturated metal complex, normally a palladium [7], cobalt [8] or nickel complex [9]. The rate of this oxidative addition decreases along the sequence... 

In Nolte and Drenth s nickel catalyzed system, the polymerization was believed to be initiated by a nucleophilic attack by the alcohol used as a solvent or the halide on the starting complex on the coordinated isocyanides. Successful asymmetric polymerization was achieved using a dicationic tetrakis(isocyanide)nickel(II) complex 46 with enantiopure primary amines, which served as a chiral nucleophile in the initial step (Scheme 35) [58, 59]. In a typical experiment, a catalyst prepared from (f-BuNC)4Ni(II)(C104)2 (46a) (1 mol%) and an optically active amine (1 mol%), was used for polymerization of isocyanides with, or without a solvent, such as n-hexane, in... 

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>. 

Eisch et al. (24) performed a mechanistic study of the desulfurization of dibenzothiophene by a nickel(0)-bipyridyl complex and reported that a radical anion of the thiophene nucleus was formed and underwent C-S bond cleavage into S and an aromatic radical. In addition, they suggested that the oxidative reaction of the nickel(0)-bipyridyl complex toward dibenzothiophene had the characteristics of stepwise electron transfer rather than nucleophilic attack. However, no correlations occurred between the desulfurization rate and the reaction indexes of Fr(E), Fr(N), and Fr(R), as shown in Table II. The results suggested no evidence for either electron transfer or nucleophilic attack in this study. Moreover, the radical reaction was not... 

Most metals are electron-attracting with respect to isocyanide ligands and the coordinated ligand is sensitive to nucleophilic attack. This reversal in behaviour is called Umpolung , a term introduced by Seebach . Umpolung is an important feature in organometallic chemistry. An example of such a nucleophilic attack is the initiation step in the polymerization of isocyanides by nickel(II) complexes ... 

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