Formation, metallacycle

An important reaction in many homogeneous catalj4ic systems is the formation of a metallacycle. As the name suggests, a metallacycle is a cyclic structure where one of the constituents of the cyclic framework is the metal atom. A few such complexes have been isolated. In many cases, their structures have been inferred and proven on the basis of spectroscopic and other evidences. 

Three types of metallacycles are of importance in homogeneous catalytic reactions. First, there are metallacycles formed by the reactions of alkylidenes with alkenes. As shown by reaction 2.3.3.1, this reaction leads to a four-membered metallacycle and is of pivotal importance in metathesis reactions (Section 7.3.2). 

Second, a five-membered metallacycle may be formed by pseudo-OA involving the metal atom and two olefinic double bonds. This is shown by reaction 2.3.3.2. As shown by reaction 2.3.3.3, the five-membered metallacycle can also become a seven-membered one by the insertion of an ethylene in the metal carbon bond. This type of reaction is encountered in the Cr-catalyzed tri- and tetramerization of ethylene (Section 7.2.4). 

Finally, as shown by reaction 2.3.3.4, two butadiene molecules can form two fj -allyl bonds to a nickel atom to give a nine-membered metallacycle. This type of reaction is of importance in the catalytic di-and trimerization of butadiene (see Section 7.2.5). 

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The initially proposed mechanism [14], and one that continues to be considered as the likely pathway for most variants, involves the oxidative cyclization of a Ni(0) complex of an aldehyde and alkyne to a metallacycle (Scheme 18). Metallacycle formation could proceed independently of the reducing agent via metallacycle 19, or alternatively, metallacycle 20a or 20b could be formed via promotion of the oxidative cyclization transformation by the reducing agent. Cleavage of the nickel-oxygen bond in a o-bond metathesis process generates an alkenyl nickel intermediate 21. In the variants involv-... 

Metallacycle Formation Involving Electrophilic Carbene Addition to a Benzene Ring of the PPh3 Ligand... 

An example occurs in the attempted synthesis of a CHC1 complex which immediately undergoes metallacycle formation (99) ... 

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

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],... 

Treating the sterically encumbered zirconocene ZrCFfn-CslliiBi F with "BuLi in the presence of ethene also provides a structurally characterised zirconocyclo-pentane Zr(C4H8)(r -C5H3Bu2)2 18. The reversibility of metallacycle formation was demonstrated by its reaction with diphenylacetylene to provide the zirconacyclo-pentene 19 incorporating the alkyne and one equivalent of ethene 22... 

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). 

Scheme 2. Palladium(IV) metallacycle formation and subsequent reductive elimination. Isolated species with L—L=phen R = Me, CH2=CHCH2, 4-N02C6H4 X= Br, Cl.

Metallacycles, formation from carbene complexes of transition metals 91UK169. 

Similar stability and reactivity have also been observed for bridged-Cp systems. The catalytically active (CH3)2Si(C5(CH3)4)2ThR2, where R = Cl [89597-06-8 ], alkyl, CH2C6H5 [89597-10-4], aryls, or H [89597-11-5]. Similar to Group 4 transition-metal Zeigler-Natta catalysts, stable cationic Th(TV) species, eg, [Cp 2ThCH3]+ [108834-69-1], have been isolated with a host of noncoordinating/nonreactive anions. Metallacycle formation has also been... 

The effect of the pyrrolyl ligand may be due to a type of hemilability (see Box 8) arising from an to t coordination shift, the former facilitating metallacycle formation and the latter favouring further ethylene insertion and formation of the chromium-bonded 5-hexenyl species. 

In this general area quite efficient intramolecular examples have been described as well (Scheme 6). A thorough examination of the intermediates and likely mechanisms associated with this system has been published. In particular, it has been found that the type of metallacycle formed depends on the length of the chain separating the two alkyne units in the diyne. Efficient intramolecular cycloaddition to a bicy-clic metallacycle occurs only from 1,6- and 1,7-diynes. The poor results on attempted cyclization of 1,5-diynes to give highly strained bicyclo[3.2.0]hepta-l,4-dien-3-ones result from resistance towards closure of the strained four-membered ring upon metallacycle formation. ... 

C0)4Fe(C02Me)[C(0)C(0)Me] 45 to afford two isomers of anionic trifunctionalized metallalactones 46, whose formation resulted from an addition of the nucleophile to the /3-carbonyl of the pyruvoyl ligand <20050M1709>. In this reaction, the metallacycle formation occured via a further addition of the oxygen of the same /3-carbonyl onto a terminal carbon monoxide ligand (Scheme 3). 

A series of aryl-substituted titanocene diazoalkane complexes, ( 5-CsMes)2Ti(N2CHAr), has also been synthesized 183.111 Dinitrogen loss affords the transient alkylidene complexes 184 that can be trapped with excess styrene to yield the titanacyclobutane complexes, (if -C5Me5)2Ti(CHArCHAr C112) 185. Measuring the rate of metallacycle formation as a function of /wra-substituent on the diazoalkane ligand has produced little effect. This observation has... 

Two distinct pathways have been proposed for the formation of the metallacyclobutane that differ in the orientation of the metallacycle with respect to the other ligands around Ru (Figure 2). In the bottom-bound pathway, metallacycle formation takes place with an olefin bound trans to the NHC, leading to a metallacycle on the opposite face to the NHC and the two anionic ligands (X) being trans to each other. Alternatively, in the side-bound pathway, metallacycle formation takes place with an olefin bound cis... 

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

Although the equilibrium lies to the right, VI can be regenerated under vacuum. The propylene analog of VI similarly forms VIII with excess propylene. In the case of neopentylethylene, NMR reveals the presence of both the olefin complex and the metallacycle in detectable amounts in the equilibrium. An additional indication that metallacycle formation is reversible arises by submitting VI to reaction with propylene ... 

Dienes and alkenes can function formally as monoenes in metallacycle formation, e.g., (CODjPt in reaction with butadiene in hexane at — 30°C affords XV for which the transconfiguration is established by X-ray diffraction. Treatment of XV with t-BuNC... 

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