Mechanism ruthenium-based catalysts

Using 1,5-hexadiene, it was shown that, depending upon whether molybdenum- or ruthenium-based catalysts are employed, a change in mechanism appears to occur. In the presence of Schrock s molybdenum catalyst, 1,5-hexadiene produces principally linear poly(l-butenylene) [scheme (24)] [33], but with Grubbs s ruthenium catalyst the primary product is the cyclic dimer 1,5-cyclooctadiene [scheme (25)] [25,33] ... 

The Kharasch addition reactions promoted by [RuCl2(PPh3)3] are believed to proceed through a redox chain mechanism (Eqs. 1-3) [ 16]. Their kinetics show a first-order dependence both on the ruthenium complex and on CC14. Whereas no clear-cut evidence for alkene coordination to the metal was found with catalyst precursor 1 (which readily loses one phosphine ligand), olefin coordination cannot be excluded because there is a saturation kinetic rate dependence on the alkene. This observation led to the proposal of a reversible step involving olefin coordination to the metal center [ 16,19,20]. Recent work with other ruthenium-based catalysts further supports olefin coordination (see later). 

These data in total have led to the revising of the ADMET mechanism with ruthenium-based catalysts to include the very important aspect of phosphine dissociation (Scheme 6.27). In this mechanism k, k4, ky, and kg are assumed to be very fast, such that the dissociative olefin-phosphine exchange is die rate-determining process [100]. 

Apart from copper-based complexes several other metals have been used as well. Fe (56-59), Ni (60), Ru (15), etc have been used to some extent. Especially noteworthy here is the work by Sawamoto and co-workers. As indicated in the Introduction, Sawamoto and Matyjaszewski simultaneously pioneered ATRR Matyjaszewski started off with the use of copper, whereas Sawamoto spent most of his efforts on ruthenium-based catalysts. Recent work from Sawamoto and co-workers shows that the Ru-based complexes can compete with the Cu-based systems on many fronts. Although not yet perfect it seems that a specific Fe-based catalyst is the first to pol5unerize vinyl acetate via an ATRP mechanism (61). 

TH of ketones by ruthenium-based catalysts represents the vast majority of protocols, following the seminal studies of the groups of Noyori and Zhang. " For TH of carbonyl compounds by i-PrOH several mechanisms have been proposed. The inner-sphere TH mechanism involves four steps (i) coordination... 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Synthesis, Structure, Mechanism and Activity of Ruthenium-Based Metathesis Catalysts... 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

In 2000, Yamaguchi et al. [116] synthesized a ruthenium-based hydroxyapatite catalyst, with the formula (RuCl)10(PO4)6(OH)2. This catalyst could also be recycled and displayed a reasonable substrate scope in the aerobic alcohol oxidations (Eq. 30). TOFs reported in this case were generally somewhat lower, on the order of 1 h 1 for 2-octanol to 12 h 1 for benzyl alcohol. The fact that distinct Ru-Cl species are present at the surface points in the direction of a hydridometal mechanism. 

Although this mechanism is based on known activation of the N-H bond of aniline by Ru3(CO)i2, a mechanism involving the activation of the carbon-carbon triple bond followed by a nucleophilic attack of the amine carmot be discarded. Indeed, typical Lewis acids such as Zn(II) or Cu(I) salts have been shown to be efficient catalysts for the intramolecular hydroamination of alkyne [93], However, contrary to ruthenium(II) complexes, mthenium(O) catalysts are not expected to electrophili-cally activate alkynes. 

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