Ruthenium-methylidenes

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-... 

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... 

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). 

In conclusion, the disappearance of the benzylidene fragment during the ATRP of methyl methacrylate could be explained by the reaction of the ruthenium benzylidene with the monomer, giving rise to highly unstable ruthenium ester-carbene complexes, and it is possible that these species then quickly decompose. In addition, the absence of [Ru=CH2] is also most probably indicative of the decomposition of these ruthenium carbene species, since [Ru=CH2] are presumed to be the propagating species in RCM and related ruthenium methylidene derivatives have a quite long lifetime in olefin metathesis. Until now, the exact nature of the inorganic decomposition products is not known. 

Stoichiometric reactions of the Grubbs catalyst with vinylsilanes give predominantly silylstyrene and ruthenium methylidene but traces of styrene derived from the opposite regioselectivity was also detected. Unfortunately, ruthenium silylcarbene complex was not detected, equations 19a and 19b. 

The catalysis is based on the commonly accepted metathesis mechanism in which ruthenium-methylidene intermediate is generated in the following preliminary step (equation 21) which initiates the catalytic cycle [49]. 

Gibbs free energies were computed at 298 K with M06-L/SDD(f )-6-31 IG and the CPCM solvation model in dlchloromethane. Energies are in respect to the ruthenium methylidene and diene. 

Cavallo and coworkers [56] have explored the decomposition of the second-generation ruthenium methylidene and benzylidene catalysts induced by the coordination of % acids. Carbon monoxide (CO) was used as a model it-acid ligand in these computations, although it is not normaUy added during metathesis. The DFT calculations indicated that the coordination of CO trans to the Ru-alkylidene bond was highly exothermic and promoted a cascade of reactions with very low energy barriers (Scheme 7.11) [57]. The coordination of the % acid reduced the electron density on the alkylidene and thus promoted the... 

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