Nickel metallacycles

A combination of Ni(cod)2 and R2Zn effects simultaneous nickel metallacycle formation and organozinc transmetallation, yielding a cyclic homoallyl alcohol (Scheme 16.87) [95],... 

The mechanism was proposed to involve the formation of a nickel metallacycle by the oxidative cyclization of Ni(0) with the aldehyde and alkyne, followed by conversion of the metallacycle to product by a transmetalation/reductive elimination sequence. If R possesses a P-hydrogen, then P-hydride elimination after the transmetalation step generates the product with R = H in some instances. The mechanism was shown to be ligand dependent, and the mechanism depicted below is undoubtedly oversimplified. ... 

Heterochalcogenides, with chromium, 5, 312 Heterocoupling reactions in olefin cross-metathesis, 11, 181 Pd-catalyzed, alkynes, 8, 274—275 Heterocubanes, reactions, 3, 8 Heterocumulenes in insertion reactions, 1, 107 nickel metallacycle reactions, 8, 103-104 Heterocyclic compounds... 

Nickel cyanide catalyzes the PTC carbonylation of vinyl halides to a,y9-unsat-urated acids [141]. A similar reaction involving 2-bromo-l-phenyl-l,3-butadiene gives the a-ketolactone (60%) and dienoic acid (15 %) (the former results from the double carbonylation reaction (eq. (11)) [142]), possibly proceeding via a nickel metallacycle [143]. 

The intermediacy of a nickel-metallacycle is likely involved in many classes of nickel-catalyzed couplings of 7t-systems. The highly ordered nature of the metal-... 

Evidence for the existence of a nickel-metallacycle intermediate has been demonstrated by the Ogoshi group [53]. Ogoshi and coworkers were able to obtain a crystal stmcture of oxametaUacycle complex 27 (Figure 8.2) generated from an... 

Scheme 55 Ligand Dependence of Nickel Metallacycle Decompositionl ...

The CO insertion into a nickel-acyl bond and formation of a nickel metallacyclic intermediate have been proposed to be responsible for the formation of a-keto compounds. ... 

During the past decade, the chemistry of nickel metallacycles has experienced a remarkable growth,which justifies a specific treatment. This view was already adopted in COMC (1995). 

Nickel metallacycles display a very rich reactivity. Complex processes consisting of several consecutive elemental steps are frequently found. 

In general, nickel metallacycles react readily with carbon monoxide. In most cases, this reaction is almost immediately followed by metal reduction (Scheme 66) and formation of a cyclic carbonylated organic compound (ketone,250.314,329,346 76,127,128,130,307,320,338 thiolactone, or related heterocycles25 )... 

As in the case of carbonylation, the reaction of alkynes with nickel metallacycles is often followed by reductive elimination. In some cases, the insertion product is stable and can be isolated (e.g., alkyne insertion in nickelalac-tones or fluorinated metallacycles, Equation (99)), but more frequently it is unstable and decomposes rapidly to afford substituted benzenes, dihydronaphthalenes,naphthalenes, or phenanthrenes. " The reaction of Ni(0) dippe complexes with biphenylene and alkynes in the presence of traces of O2 catalytically produces phenanthrenes, in a process that involves the intermediacy of a carbonickelacycle. " ... 

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