Metallacycles metallacyclopentane

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

A crystal structure has also been reported for the CpRe(CO)2(C4H8) metallacycle by Yang and Bergamn53 showing the commonly encountered 8/X. disorder at the (I carbon atoms of the metallacyclopentane ring (Fig. 17). 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

As was seen in the previous section, addition of excess olefin to formally divalent zirconium or hafnium alkene complexes usually induces coupling to form the corresponding metallacyclopentane. Interestingly, metallacycle... 

Metallacycles bearing cyclopropane units can be obtained by various methods. They are available via oxidative addition-type metallacyclopentane formation, using alkenes or alkynes with attached cyclopropane rings (such as vinylcyclopropanes, e.g. formation of 1). Methylenecyclopropanes and cyclopropenes can also be used vide infra). 

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. ... 

The interest1 in transition-metal metallacycles has led to an intensive study of a number of such complexes from the structural and chemical point of view. In this context, the availability of metallacyclopentane derivatives of the cobalt-triad metals, which are implicated in so many catalytic and stoichiometric reactions, enables one to extend the range of studies of the chemistry of this class of organometallics. 

Two ethylene ligands coordinate to a low valent transition metal complex and couple with each other to form a hve-membered ring called a metallacycle, in this case a metallacyclopentane. The metallacycles sometimes show particular... 

Recently the oxidative coupling of olefins and alkynes, not only with late transition metal complexes, but also with low valent early transition metal complexes to give metallacyclopentane or metallacyclopentadiene complexes is attracting increasing attention. For example, titanium(II) and zirconium(II) complexes react with olefins and form metallacycles. Various stoichiometric processes to produce useful compounds that are otherwise unavailable can be produced by exploit-... 

Metallacycles have been suggested as intermediates in many transition-metal catalyzed reactions of olefins, acetylenes, and cyclopropanes. Metallacyclobutane complexes are invoked in olefin and cyclopropane isomerization schemes (Scheme as well as in olefin metathesis schemes. Metallacyclopentane, -pentene, and -pentadiene complexes can all be invoked in olefin and acetylene dimerization and polymerization (Scheme Many of these involve early transition metals and do not include phosphine ligands. 

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). " ... 

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. 

The involvement of metallacycles has been proposed for the [3+2] cycloaddition of meth-ylenecyclopropanes with alkenes to produce methylenecyclopentanes.l Oxidative cyclization of a methylenecyclopropane and an electron-deficient alkene produces a spi-rocyclic metallacyclopentane 86. Cyclopropane ring opening followed by reductive elimination affords the observed methylenecyclopentane products 87 and 88 (Scheme 65). Another report describes the novel use of nanostructured nickel clusters as catalysts. ]... 

In many respects, the synthetic methods and reactivity of metallacyclic complexes parallel those of analogous dialltyl complexes. Metallacyclobutanes and metallacyclopentanes, on the other hand, display unique features. They may easily transform into, or be pre-... 

This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

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