Metallacycle stability

The metallacycles (4) generate the requisite titanium methylidene thermally. Consistent with this is the observation that reaction with a carbonyl compound is first order in the metallacycle and zero order in the carbonyl compound. Metallacycle stability depends on the alkene moiety that has replaced the aluminum-chlorine portion of (3) and thus could provide a series of reagents whose reactivity is temperature dependent. ... 

There is competition between cyclotrimer and alkylidene ligand formation. Depending upon the metallacycle stability, the initiation of polymerisation or the formation of an aromatic compound can be observed. 

Scheme 10.12 Relative metallacycle stability inferred from the results shown in...

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

Metallacyclocumulenes Five-membered metallacyclocumulenes 20 are very unusual. They are the smallest discrete cyclocumulenes to be isolated and structurally characterized. The structures of titana- and zirconacyclocumulenes show an almost planar arrangement of the metallacycle, containing three C—C double bonds, of which the central one is elongated. This elongation is ascribed to the intramolecular interaction of this bond with the metal center, which stabilizes this type of complex [21]. 

Homogeneous a-hydroxyalkyl complexes, L MCH(R)OH, have been proposed as catalytic intermediates in the conversion of CO-H2 gas mixtures to oxygen-containing organic molecules.1 However, isolable examples of metal a-hydroxyalkyl complexes are scarce.2 We have discovered that incorporation of the a-hydroxyalkyl moiety into a metallacycle provides a marked stability enhancement.3... 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

Palladacycle 5 reacts in various ways depending on ligands and reaction conditions. In particular it readily undergoes oxidative addition of alkyl halides to form a palladium(IV) complex 6, which has been isolated and characterized with stabilizing nitrogen ligands such as phenanthroline. This palladium(IV) metallacycle... 

The stability of metal alkylidene (carbene) complexes and the corresponding metallacycles can be dependent on various factors, but it is worth noting that the kind of metal, the metal oxidation state and the ligands surrounding the metal are considered to be of essential significance. Although stable metal carbene complexes are usually obtained from W and Mo compounds whereas metallacycles are obtained from Ti compounds, systems have been found in which both the metal alkylidene complex and its precursor metallacyclobutane can be detected at lowered temperature by NMR spectroscopy [45]. 

Three metal stabilized seven-membered rings containing 1,2,3-butatriene units have been prepared [Eq. (62)). In the metallacycle 367, independently synthesized by Hsu et al.153 and Burlakov et a/.114 by essentially the same method, zirconium serves as both a member of the seven-membered ring and as a cumulene-stabilizing metal. An X-ray crystal structure excluded the alternative structure 368, which was proposed as a possible intermediate in the reaction. The X-ray of 367 is consistent with the cumulene formulation in that bond lengths for C2-C3 [1.337(6) A], C3-C4 [1.298(6) A], and C4-C5 [1.337(6) A] indicate bonds of roughly similar bond order. The bond angles for C2-C3-C4 [148.8(5)°] and C3-C4-C5 [160.1(5)°] are also consistent with a severely distorted (from linearity) cumulene. 

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... 

Figure 6. X-ray crystal structure of the iron(II)-bis-N-alkylporphyrin complex incorporating twofive-membered metallacycles to stabilize the distorted porphyrin macrocycle (40).

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). 

These are pentane-soluble, volatile tetrahedral molecules. They have an extensive chemistry involving metallacycles which will be dealt with later (Section 13.6). Although MMe4 (M = Th, U) have not been isolated, this unit can be stabilized as eight-coordinate phosphine adducts, where the role of phosphines is to saturate the coordination sphere. The coordination of a soft phosphine ligand to a hard metal ion like uranium(iv) is noteworthy. 

A particular subset of these ligated dialkyls are those in which the ligand is tethered to the cyclopentadienyl ligand. Thus, chromacyclopentane and cycloheptane derivatives stabilized by the (7/ 7/ -Me4C5CH2CH2NMe2) ligand have been prepared and their reactivity supports the intermediacy of such metallacycles in the catalytic trimerization of ethylene. A variety of donor-ligand-substituted cyclopen-tadienylchromium(III) complexes with amino and phosphino substituents has been prepared and screened for an ethylene polymerization activity. ... 

Another route to heteroatom stabilized carbenes involves first the interaction of a terminal alkyne with an iridium complex to yield an iridium vinylidene compound. The vinylidene complex then reacts with nucleophiles, typically alcohols, to form oxa stabilized carbene complexes. O Connor extended this strategy to form a carbene ligand on the same iridium center as a metallacycle (76) and was also able to... 

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