Alkene metathesis intermediate carbene complex

Treatment of an alkyne/alkene mixture with ruthenium carbene complexes results in the formation of diene derivatives without the evolution of byproducts this process is known as enyne cross-metathesis (Scheme 22). An intramolecular version of this reaction has also been demonstrated, sometimes referred to as enyne RCM. The yield of this reaction is frequently higher when ethylene is added to the reaction mixture. The preferred regiochemistry is opposite for enyne cross-metathesis and enyne RCM. The complex mechanistic pathways of Scheme 22 have been employed to account for the observed products of the enyne RCM reaction. Several experiments have shown that initial reaction is at the alkene and not the alkyne. The regiochemistry of enyne RCM can be attributed to the inability to form highly strained intermediate B from intermediate carbene complex A in the alkene-first mechanism. Enyne metathesis is a thermodynamically favorable process, and thus is not a subject to the equilibrium constraints facing alkene cross-metathesis and RCM. In a simple bond energy analysis, the 7r-bond of an alkyne is... 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

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. 

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

Metal aUcylidene complexes (see Schrock-type Carbene Complexes) have been proposed as intermediates in many catalytic reactions, including alkene metathesis (see Organic Synthesis Using Metal-mediated Metathesis Reactions), alkene and aUcyne polymerization, methylenation of carbonyl compounds, and cyclopropanation of alkenes. ... 

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

Schrock carbene complexes undergo reaction with multiple bonds via four-center metallacyclic intermediates (51). Chapter 11 will consider what occurs when alkylidenes react with alkenes, a reaction known as alkene metathesis. Below are examples of Schrock carbenes reacting with polar multiple bonds such as C-N and C=0. 

Mechanism 3 shows a pathway that was strongly influenced by the results of Herisson and Chauvin and is outlined in Scheme 11.2. Two key intermediates in this pathway are an alkene-metal carbene complex (5) and a metallacyclobu-tane (6), formed through concerted cycloaddition of the M=C and C=C bonds. A highly significant feature of the mechanism, caused by the unsymmetrical structure of 6, is its explanation of randomization early in the course of reaction. The Herisson-Chauvin mechanism does not require a specific pair of alkenes to interact directly for metathesis to occur, hence the name non-pairwise mechanism. 

Alkene metathesis catalysis involves intermediates in which a transition metal is multiply bonded to carbon. These species are often referred to as nucleophilic carbenes when the carbon atom is negatively polarized. A more functional description is to name these compounds as alkylidene complexes, since they react to transfer an alkylidene moiety from a transition meUd to a substrate carbon atom. Previous sections of this chapter have focused on a common example of this chemistry the process of metathesis that involves transition metal mediated interaction of carbon-carbon multiple bonds. 

Historically one of the first asymmetric methods to be explored, cyclopropanation came of age32 with box and salen ligands on Cu(I). Diazo compounds, particularly diazoesters 138, react with Cu(I) to give carbene complexes 140 that add to alkenes, particularly electron-rich alkenes to give cyclopropanes 141. The reaction is stereospecific with respect to the alkene -1runs alkenes giving trans cyclopropanes - and reasonably stereoselective as far as the third centre is concerned. Any enantioselectivity comes from the chiral ligand L. You have already seen the Ru carbene complexes are intermediates in olefin metathesis (chapter 15). 

Olefin metathesis is a catalytic process in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. The key step in this process is the 2 + 2 reaction between an olefin and a transition metal alkylidene (carbene) complex, generating an unstable metallacyclobutane intermediate. This intermediate can either revert to the starting material, or open productively to regenerate a metal carbene and produce a new olefin (Eq. 1). 

Herisson and Chanvin proposed that metathesis reactions are catalyzed by carbene (alkyl-idene) complexes that react with alkenes via the formation of a cyclic intermediate, a metallacyclobutane, as shown in Figure 14.24c. In this mechanism, a metal carbene complex first reacts with an alkene to form the metallacyclobutane. This intermediate can either revert to reactants or form new products because all steps in the process are in equilibria, an equilibrium mixture of alkenes results. This non-pairwise mechanism would enable the statistical mixture of products to form from the start by the action of catalytic amounts of the necessary carbene complexes, with both R and R groups, as shown in Figure 14.24c. 

A variety of experimental evidence showed results consistent with a non-pairwise mechanism involving carbene complexes and a metallacyclobutadiene intermediate. For example, in the Figure 14.28 reaction, the gaseous products H2C=CH2, D2C=CD2, and H2C=CD2 formed in the expected statistical ratio early in the reaction, with no indication that the mixed product H2C=CD2, the first product by the pairwise mechanism, forms before the others." The first direct demonstration that a carbene complex could engage in metathesis involved the carbene complex in Figure 14.29, which was able to replace its carbene ligand with a terminal alkene in a reaction consistent with the non-pairwise mechanism. ... 

In this example, and in many before it, the formation of the carbene is initiated by a metal— often copper, rhodium, or silver. The carbene intermediates in these reactions are formed as reactive complexes with those metals, but in other cases the complexes are extremely stable. For example, decomposition of phenyldiazomethane in the presence of a ruthenium(II) complex gives a carbene complex stable enough to be isolated and stored for months. This complex, and a family of related Ru complexes, are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or olefin) metathesis. 

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

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