Grubbs mechanism

Figure 17.11 The Chauvin Grubbs Mechanism for the Grubbs Metathesis Reaction...

Grubbs and Brunck (86) have recently reported experimental evidence supporting this mechanism. They have made an attempt to synthesize the proposed metallocyclic intermediate for the metathesis of ethene. Starting from the assumption that a mixture of WC1 and two equivalents of (C4H9)Li forms an active metathesis catalyst (49), they treated WC1 with 1,4-dilithio-2,3-dideuterobutane. One may expect that the following reaction would take place ... 

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

For a recent report on the mechanism of the Ru-catalyzed ring-dosing metathesis, see Dias EL, Nguyen ST, Grubbs RH (1997) J Am Chem Soc. 119 3887... 

The mechanism involves a [2 + 2] cycloaddition reaction between an alkene and a transition metal carbene (Scheme 10.13). In the absence of a transition metal carbene catalyst, the reaction between two alkenes is symmetry forbidden and only takes place photochemically. However, the d-orbitals on the metal catalyst (typically Grubbs s catalyst as shown in Scheme 10.13), break the symmetry and the reaction is facile. 

Recent mechanistic work has shown that 16 e Ru methylene complexes (such as bisphosphine 11) are slow to re-enter the catalytic cycle. Their reluctance to initiate can result in competitive decomposition see Mechanism and Activity of Ruthenium Olefin Metathesis Catalysts, M.S. Sanford, J.A. Love, R.H. Grubbs,/. 

Many applications of surface modified maferials (such as in molecular electronics, separation science or continuous flow catalysis) require the use of mechanically and pressure-stable carriers. Grubbs et al. and later Nuzzo et al. reported on the surface modification of Si(lll). Conversion of surface Si - H into Si-allyl groups allowed them to pursue the grafting-from approach shown in Schemes [36,37]. The thickness of the polymer layer could be... 

This short analysis supports (a) the statement by Herrmann that from the work in numerous academic laboratories and in industry, a revolutionary turning point in organometahc catalysis is emerging thanks to NHC (b) the conclusion by Grubbs that the exact mechanism of these powerful new catalysts remains... 

Sanford, M. S. Love, J. A. Mechanism of Ruthenium-catalyzed Olefin Metathesis Reactions. In Handbook of Metathesis-, Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2003 Vol. 1, pp 112-131. 

There thenfollowed reports by Katz [13] and Grubbs [14] and their co-workers on studies that aimed to simplify and confirm the analysis. The key remaining issue was whether a modified pairwise mechanism, in which another alkene can coordinate to the metal and equilibrate with the product prior to product displacement, would also explain the appearance of the anomalous cross-over products early in the reaction evolution. However, a statistical kinetic analysis showed that for a 1 1 mixture of equally reactive alkenes, the kinetic ratio of cross-metathesis should be 1 1.6 1 for the pairwise mechanism and 1 2 1 for the Chauvin mechanism. Any equilibration (substrate or product) would, of course, cause an approach towards a statistical distribution (1 2 1) and thus allow no distinction between the mechanisms. 

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] ... 

Isotope labeling studies by Grubbs [20] and Katz [21] presented further proof for the metal carbene mechanism. 

Isomerization of allylic alcohol to ketone has been extensively studied [13], and two different pathways have been established, including tt-allyl metal hydride and the metal hydride addition-elimination mechanisms [5,14]. McGrath and Grubbs [ 15] investigated the ruthenium-catalyzed isomerization of allyl alcohol in water and proposed a modified metal hydride addition-elimination mechanism through an oxygen-functionality-directed Markovnikov addition to the double bond. 

Several mechanisms were proposed to explain the catalytic metathesis reactions, but the mechanism published by Yves Chauvin in 1971 has come to be accepted as correct. We can think of an alkene as two alkylidene groups bonded together. Similarly, the Schrock and Grubbs catalysts are like a metal atom bonded to one alkylidene group. 

---