Formation of metallacycles

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

Substitution of cyclopropane rings with the alkenyl group permits unique ring transformations based on metal coordination interaction with four -electrons. The transition-metal-induced ring-opening rearrangement also results in the formation of metallacycles. Further elaboration is attained by insertion and reductive elimination. 

Examples of heavier alkali metal complexes include [ GH(SiMe3)(SiMe2OMe) M] (M = Na 59, K 60) as well as the polymeric etherate [CH(SiMe3)(SiMe2OMe)K(OEt2)]oo 61.69 All these examples demonstrate the potency of intramolecular coordination, since methoxide-metal interactions under formation of metallacycles are observed in all cases. 

Two essentially different mechanisms, (i) oxidative cyclization of two 7T-components (formation of metallacycle) and (ii) oxidative addition of reducing or alkylating agents followed by insertion of 7t-components, can operate in these three-component reactions.426 However, the aforementioned phenomena such as the reversal of regiochemistry and the crossover from reductive to alkylative manifolds remain unsolved. 

A nickel(O) complex catalyzes insertion of alkynes into cyclobutanones (Equation (79)).437 Formation of metalla-cycle 194 via oxidative cyclization of an alkyne with the carbonyl group of a cyclobutanone followed by /3-carbon elimination (formation of metallacycle 195) and reductive elimination are postulated for the mechanism (Scheme 92). 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

Dihydroazepines have been synthesized by the first rhodium-catalyzed hetero-[5+2] cycloaddition of cyclopropylimines and alkynes (Scheme 8.62) [138]. The reaction proceeds via formation of metallacycle 147 which undergoes migratory insertion of dimethyl acetylenedicarboxylate (DMAD) to form 148. Finally, dihydroa-zepine 149 is obtained via reductive elimination. 

The Ni-catalyzed cyclizations of butadiene and acetylene opened a fruitful field of cycloaddition of various unsaturated compounds to afford various cyclic compounds. These cyclizations are now understood by the formation of metallacycles as intermediates (eq. 1.8). 

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). 

The main stereochemical requirement for ligands of type 369 is the arrangement of donor centers in postions which are convenient for the formation of metallacyclic structures 370 [689]. Behaving as ligands of the type discussed, most of the C,N-donors take part to form complexes of amines 371 [679,689], azomethines 372 [13,679,686,690], azocompounds 373 [13,679,689,691], aldazines 374... 

Sixteen-electron ruthenium(O) species of type (rj6-arene)(L)Ru(0) and containing two-electron ligands are probable intermediates for C—H bond activation and formation of metallacyclic complexes (Section II,A,3,c). A variety of 18-electron complexes of general formula (arene)(L1)(L2)Ru(0) have been prepared by H. Werner and co-workers either by deprotonation of hydride ruthenium(II) complexes or by reduction of cations RuX(L)2-(arene)+. Some of these Ru(0) complexes have already been discussed with the formation of alkyl or hydridoruthenium complexes (Sections... 

In addition to a- and p-C-H activation, the possibility occasionally arises for y- or even 8-functionalization. This is particularly common for aryl phosphine and phosphite ligands that may undergo metallation of the ortho-C-H bond of an aryl substituent. This process may be reversible however, if a suitable co-ligand is present which can undergo subsequent reductive elimination of the hydride, stable metallacyclic organyls are obtained (Figure 4.31). The formation of metallacyclic alkyls may confer some stability, as does the possibility of increased hapticity, e.g. in the case of xylyene ligands (see also Chapter 6). 

Insertion into C—H bonds of the ligands as well as formation of metallacycles (i.e., metallaoxiranes and 1,2,4-metalladioxolanes) according to Eqs. (2)-(4), depending on the coordination sphere and reaction conditions. All metallaoxiranes and 1,2,4-metalladioxolanes are listed in Tables VII and VIII. Three-membered rings have been synthesized by reaction of metal... 

In addition to insertion products into M—CH3 of types (21-XXXI) to (21-XXXIII), rearrangement to the 1-azaallyl (21-XXXIV) has also been observed, as has the involvement of iminoacyls in the formation of metallacycles (21-XXXV) if the insertion is conducted in polar solvents such as acetone or MeCN.164... 

Pt(0), NVE 16/Pt(II), NVE 16). Oxidative additions generally occur most readily for low valent complexes, and for metals in the order 5d > Ad 3d. In addition to those that formally cleave C-X, H-X, and X-X (X = halide) bonds, oxidative addition reactions are also known where the metal is inserted into C-0, C-H, and some strained or activated C-C bonds. Another reaction which is effectively an oxidative addition is the formation of metallacycles from a low valent metal and an olefin (Equation 7). 

The mechanistic scheme proposed by Dry (38) that we mentioned earlier to explain branching can be considered a modification of the Maitlis mechanism as well as of the Gaube mechanism. In the Gaube mechanism, CH2 is proposed to insert also at the internal secondary carbon atom. Dry proposed the formation of metallacycle-type intermediates as chain-propagating species, as illustrated in Scheme 6. 

The synthesis and characterization of a family of mono-Gp dichloro complexes with disubstituted aryloxo ligands has been reported, and their molecular structures provide some means of quantifying the number of electrons donated to the metal center by an aryloxide ligand. These complexes can be reduced by Grignard reagents or LiBu11 in the presence of enynes. The formation of metallacyclic derivatives (Scheme 348) was observed for the Cp but not for the Cp complexes, as deduced by NMR spectroscopy. The complexes have been investigated as catalysts... 

Syntheses of heterocycles by transition metal-catalyzed cycloaddition with the formation of metallacycles as intermediates 02YGK26. 

Hydrogenation of alkynes is catalyzed by alkyne cluster complexes Fe3(GO)9(RG2R ). In addition to hydrogenation, formation of metallacyclic byproducts such as Fe3(GO)8 (RG2R )(RG2R ) was observed, resulting in decreased catalytic activity. 

N-Substituents in hetarylazomethine ligands are represented by three-and five-membered rings with one heteroatom as well as azole and azine heterocycles. They may not always participate in the formation of metallacycles and thus play the role of coordination-active or inactive constituents of the hetarylazomethine ligands. A great majority of coordination compounds includes mononuclear complexes, although di- and oligonuclear structures are also known. 

Since the mechanism of ethene polymerisation commonly suggested for transition-metal catalysts involves the formation of metallacycle, development of this type of catalyst was performed in order to elucidate the type of intermediate involved in the reaction. Binuclear Cr(ii) metallacycles showed little production of 1-hexene when reacted with ethene at room temperature, even after reacting for over 24 h. However, mononuclear Cr(m) metallacycles were considered possible intermediates as these complexes are able to trimerise ethene. Thus, Monillas et al. concluded this to be the likely intermediate involved in the reaction. Nonetheless, the use of the Cr(i) dinitrogen complex yields the desired product, 1-hexene, in a mechanism... 

Oxanickelacycles of type la have also been synthesized by an alternative procedure based on inserting of carbon dioxide into the Ni-C bond of certain reactive nickelacycles. Thus, reaction of complex 13 with carbon dioxide leads to die formation of metallacycle 14 resulting from an insertion into the Ni-C(aryl) bond of 13 (Eq. 7). Complex 13 is... 

The individual steps in the cycloaddition are not known but several proposals have been put forward. Our studies build on these proposals. Following the complexation of NBD and the acetylene to the low-valent cobalt, formation of metallacycle IV is likely to occur (Scheme 5). Isolation of 5-10% of side product N (when R = TMS) provides further support for the formation of such a metallacycle, but insertion into the... 

Direct application of Ru3(CO)i2 in photochemical synthesis has been described in detail [120]. Thermal reactions of this cluster in presence of two-electron donors L affords [Ru3(CO)9L3]. The discovery in 1974 that irradiation of the cluster under those conditions produces mononuclear products instead of the substituted clusters initiated a wealth of research in Ru-clusters as precursors in photochemical synthesis [121]. Much research has been devoted to the preparation of mononuclear f/ -olefin complexes, as well as alkyne complexes. For example, [Ru(CO)3(PPh3)2] has been reported as an active catalyst for olefin polymerisation, and as such, many investigations have dealt with the reactivity of this compound. Other directions of research include formation of metallacycles, generation of new cluster species, and mixed transition metal/non-metal clusters. 

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

Mechanistically, the formation of metallacycle (Scheme 5.1a) might result from a three-center concerted reaction involving the metal, the substrate, and CO2, but it is unlikely that the three reactants in Scheme 5.1 might react in one step. More likely, the oxidative coupling reaction may proceed through a stepwise pathway, involving either the coordination of CO2 to the metal center, followed by the reaction of the CO2 adduct with the substrate (pathway (i) 5.2a, b). 

The reaction of alkynes with carbon dioxide in the presence of nickel (o) catalysts leads to the formation of metallacyclic 1 1 and 2 1 complexes. For example, the reaction of dimethylacetylene with carbon dioxide, in the presence of 1,5,9-cyclododecatrienenickel and Af,Af,A, Af-tetramethylenediamine, affords the five-membered ring metallacycle 20 in 65 % yield Hydrolysis of the metallacycle affords 2-methylcrotonic acid. 

Scheme 3.49 Formation of metallacycles or r -vinylketene complexes from cyclobutenones.

In metaUocyclopentene pathway, simultaneous activation of alkyne and alkene moieties leads preferentiaUy to the formation of metallacycle intermediate (32). Almost aU transition metals could react to generate such intermediates however their reactivity can be quite different as will be... 

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