Metallacycles metathesis

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

CM has, in most cases, a good to excellent ii-selectivity. This is primarily due to steric reasons in the metallacycle intermediate of the metathesis. The high ii-selectivity of the CM makes it an ideal method for the stereocontrolled synthesis of stilbenes [152], while there is still no highly Z-selective Ru-based catalyst known [153]. 

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-... 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Catalytic cycles involving both alkyne hydrorhodation and silylrhodation are proposed.613 However, mechanistic studies performed on related hydrogen-mediated enyne reductive cyclizations (vide supra) suggest oxidative cyclization of the enyne followed by hydrosilylytic cleavage of the resulting metallacycle via cr-bond metathesis is also plausible (Scheme 27). 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

Initially alkynes were polymerised by trial and error with the use of Ziegler type recipes and the mechanism for these reactions may well be an insertion type mechanism. Undefined metathesis catalysts of ETM complexes were known to give poly-acetylene in their reaction with alkynes (acetylene) [45] and metallacycles were proposed as intermediates. Since the introduction of well-defined catalysts far better results have been obtained. The mechanism for this reaction is shown in Figure 16.24 [46], The conductive polymers obtained are soluble materials that can be treated and deposited as solutions on a surface. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

More recently, the doubly lithiated derivatives of [4.1.1]- and [3.1.1]propellasilanes underwent bridging metathesis at the bicyclobutane ends with dihalides of germanium, tin and transition metal titanium, to give the corresponding highly strained metallacyclic propellanes (equation 7)29. 

Heterochalcogenides, with chromium, 5, 312 Heterocoupling reactions in olefin cross-metathesis, 11, 181 Pd-catalyzed, alkynes, 8, 274—275 Heterocubanes, reactions, 3, 8 Heterocumulenes in insertion reactions, 1, 107 nickel metallacycle reactions, 8, 103-104 Heterocyclic compounds... 

The synthesis of four-membered ring metallacycles was initially driven by the recognition that low-valent transition metals could mediate cyclopropane carbon-carbon bond cleavage and by the extensive attention devoted to reactivity patterns thought to be relevant to olefin metathesis. More recently, however, organometallic and organic researchers... 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

A characteristic feature of cycloolefin ring-opening metathesis polymerisation is alteration of the metal-carbon active bonds from the metal carbene a, n bond into metallacycle a bonds, and vice versa, as polymerisation progresses. It is worth mentioning, in this connection, that metallacyclobutanes can be successfully used as catalysts for this polymerisation [36,37]. 

An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

Note that the transformation of metal carbene vice versa, during polymerisation occurs as in the case of cycloalk-ene ring-opening metathesis polymerisation. Considering the mechanism of metathesis polymerisation of acetylenic monomers, it is worth noting that catalysts containing a transition metal carbyne bond (Mt=C) can induce polymerisation only when this bond is transformed into the respective metal carbene bond (Mt=C) [39],... 

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

When heated at 65 °C in the presence of excess monomer, the formed metallacycle promotes ring-opening metathesis polymerisation ... 

As regards metal alkylidene and metallacycle active sites participating in metathesis polymerisation, it should be emphasised that either the alkylidene or the metallacyclobutane can be the resting state of the catalyst, depending on the catalyst used for the polymerisation [99]. 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

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