Metallacycles of nickel

The oxidative addition of methylene- or allylidenecyclopropane to nickel(O) or palladium(O) can also lead to the formation of six-, seven-, and nine-membered metallacycles of nickeF " and palladium. Various substituted platinacyclobutanes (30) were obtained by oxidative addition of cyclopropanes to the complexes [CljPtLJj (Scheme... 

Similar to the nickel-catalyzed reactions, metallacycles of palladium are assumed to be intermediates of these conversions. They were isolable in some cases. Thus, 3,3-dimethylcyclopropene with -allyl( ) -cyclopentadienyl)palladium, on cyclizing oxidative addition, formed metallacyclopentanes 20 and metallacyclononanes 21, depending on the reaction conditions. Thermal decomplexation via reductive elimination gave cyclopropane systems and products thereof. ... 

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]. 

There are several possible mechanisms that have been proposed since the first report of nickel-catalyzed coupling of alkynes and a,P-unsaturated carbonyls. This section will briefly cover the metallacycle-based mechanism as it is generally considered in both the alkylative and reductive coupling manifolds. However, it should be emphasized that different variants may proceed via differing pathways. 

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-... 

The enormous scope and utility of nickel-catalyzed cycloadditions can be traced to pioneering studies of Reppe, who reported in 1948 that nickel catalysts promote the cyclooligomerization of acetylene to cyclooctatetraene. Not only was this unprecedented transformation described, but also a remarkable level of mechanistic insight was articulated in this report. In particular, this report depicted the formation of a 9-membered metallacycle en route to formation of cyclooctatetraene (Scheme 3-19). 

The scope of catalytic ring-opening [3+2] cycloadditions was expanded to include simple cyclopropyl ketones, which combine with vinyl ketones to provide substituted cyclopentane derivatives (Scheme 3-32). Cyclopropyl imines were also found to be effective cycloaddition substrates under similar conditions, as illustrated in the sample procedure below. " Oxidative additions of nickel(O) to cyclopropyl ketones were found to produce six-membered metallacyclic nickel (9-enolates, which were competent species in the [3+2] cycloaddition process. " ... 

Other classes of nickel-catalyzed [3+2] cycloadditions include the addition of 2-haloacetophenone derivatives to alkynes to produce indenol derivatives (Scheme 3-36). This process likely involves initial oxidative addition to the haloaromatic, followed by alkyne insertion and carbonyl addition. In this case, zinc powder serves as reducing agent to regenerate the active nickel(0) catalyst. A mechanistically intriguing cycloaddition that proceeds without the action of reducing agents is the direct formation of bicyclic products from the addition of unsaturated ketones with alkynes. This process likely involves initial metallacycle formation followed by unusual rearrangement steps unique to the requisite doubly unsaturated carbonyl component. 

Nickel complexes of alkenes are involved in many catal)dic transformations. Many of the reaction classes of alkenes involve migratoiy insertion of an alkylnickel or a hydridonick-el species. Alternatively, some transformations are initiated by the oxidative cyclization of nickel-alkene complexes with a second unsaturated component to produce five-mem-bered metallacycles. Several examples of nickel-bis(alkene) complexes and nickel metallacyclopentanes are known, and the interconversion of these two structural classes has been studied (Scheme SS). " ... 

Scheme 55 Ligand Dependence of Nickel Metallacycle Decompositionl ...

Many crystal structures of nickel-alkene complexes have been reported. As demonstrated in Scheme 55, bis(alkene) complexes may exist in equilibrium with the corresponding metallacyclopentane complex. However, several alkene complexes which have the potential to undergo oxidative cyclization to a metallacycle have been fuUy characterized. The substitution chemistry of bis(iY -cycloocta-l,5-diene)nickel(0) (2) is representative of most nickel(0)-alkene complexes, which are readily substituted by a variety of ligands. Bis(q -ethene)(tricyclohexylphosphine)nickel(0) has been prepared and fully character-ized,l " l and a variety of complexes of electron-deficient alkenes such as 69 have been prepared which tend to be more stable than the complexes of ethene (Scheme 56).l" " The alkene complexes may be prepared directly from bis(q -cycloocta-l,5-diene)nickel(0) (2)l" l or from nickel(ll) chloride " that is reduced by zinc metal. 

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