Methylidene ruthenium

The reaction rate of enyne 107j having a terminal alkyne is very slow, and the starting material is recovered [Eq. (6.80)]. ° Presumably, the terminal alkene of the product 108j should further react with ruthenium carbene complex Ih to form XVII, whose ruthenium carbene should be coordinated by the olefin in the pyrrolidine ring. Thus, the catalytic activity of Ih should be decreased. If complex XVII reacts with ethylene, 108j and methylidene ruthenium carbene complex Ih should be regenerated. On the basis of this idea, the reaction was carried out under ethylene... 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

In a formal sense, complexes 1 represent pre-catalysts that convert in the first turn of the catalytic cycle (vide infra) into ruthenium methylidene species of type 3 which are believed to be the actual propagating species in solution (Schemes 2,4). The ease of formation of 3 strongly depends on the electronic properties of the original carbene moiety in 1. In addition to complexes la-c with R1=CH=CPh2, ruthenium carbenes with Rx=aryl (e.g. Id, Scheme 3) constitute another class of excellent metathesis pre-catalysts, which afford the methylidene complex 3 after an even shorter induction period [5]. In contrast, any kind of electron-withdrawing (e.g. -COOR) or electron-donating substitu-... 

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

If ROM-RCM of cycloalkene-yne 123, which has a substituent at the 2-position of cycloalkene, is carried out under ethylene gas, what compound is formed In this reaction, ruthenium carbene XIX would be formed via [2-1-2] cycloaddition of ruthenium methylidene carbene and alkyne as shown in Eq. (6.91). If XIX reacts with an olefin intramolecularly or ethylene, bicyclic compound 124 or triene 125... 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

Such cases are not uncommon, but full quantitative treatments are rare, since often relatively large amounts of Y must be added to obtain measurable effects. Complications may then arise from the effects of the added Y on the nature of the medium (see Chapters 2 and 3). These are particularly notable when Y and I are charged, as is often the case. Under those circumstances, maintenance of the constant ionic strength of the medium with a known non-participating ionic species is essential. The classic case of common ion depression in solvolysis of benzhydryl chloride is dealt with in Chapter 2. A more recent example of this kind of treatment with neutral reactants occurs in the elucidation of the mechanism of olefin metathesis [20], catalysed by the ruthenium methylidene 9, Scheme 9.6. With ca. 5% of 9, disappearance of diene 10 was clearly not first order. However, reactions run in the presence of large excesses of phosphine 11 were much slower and showed first-order kinetics. The plot of kQ K against 1/ [ 11 ] was linear, consistent with dissociation of 9 to yield an active catalytic species prior to engagement with the diene, with k t [11] 3 > fc2[diene]. Because first-order kinetics were observed under these conditions, determination of order with respect to the catalytic species (as well as the diene) was simplified, and an outline for the mechanism could be constructed (see also Chapter 12 for more detailed consideration of catalysed olefin metathesis). 

For reaction of the various ruthenium carbenes with either the terminal olefin 1-hexene [103] or ethyl vinyl ether [101], the rate of reaction increases in the order methylidene < vinyl carbene < benzylidene < alkylidene. The differences in rates were quite significant, especially for the methylidene, which is quite slow to initiate. The data also show that internal olefins are slower to react, but only by about an order of magnitude the ds isomers being more readily metathesized than the... 

In an interesting process, the prochiral 2-substituted 5-methylidene-l,3-dioxane (88) was iso-merized to give a mixture of the enantiomeric dioxins using a chiral ruthenium catalyst. Enantiomeric excesses ranged from 13 to 38% (Equation (58)) <94SL517>. 

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