Olefin metathesis catalyst decomposition

Mechanisms of Olefin Metathesis Catalyst Decomposition and Methods of Catalyst Reactivation... 

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,/. 

Although this process has shown much promise, decomposition of the olefin metathesis catalyst appears to limit the conversion nonetheless, it is expected that a more robust and compatible olefin metathesis catalyst will yield higher TONs. 

All these catalytic results, however, were usually achieved at very low (2-3%) conversions. The only exception is a paper reporting up to 80% selectivity at 20% conversion over a M0CI5—R4Sn-on-silica olefin metathesis catalyst (700°C, 1 atm, CH4 air = l).42 In general, higher temperature and lower—about ambient— pressure compared to homogeneous oxidation, and high excess of methane are required for the selective formation of formaldehyde in catalytic oxidations.43 The selectivity, however, decreases dramatically at conversions above 1%, which is attributed to the decomposition and secondary oxidation of formaldehyde.43,44 It is a common observation that about 30% selectivity can be achieved at about 1% conversion. 

XV. Both the first and second generation Grubbs olefin metathesis catalysts have been shown to isomerize allylic ethers to vinyl ethers that are readily hydrolyzed.It is a decomposition product of the catalyst that was shown to be the isomerization catalyst. ... 

The deactivation of this tandem, dual-catalytic system has been investigated. It was found that the low TONs obtained under these reaction conditions were mainly the result of the decomposition of the olefin metathesis catalyst. It was found that adding an additional amount of olefin metathesis catalyst at the end of a catalytic run reinitiated the process [18]. Therefore, to improve the catalytic efficiency, olefin metathesis catalysts as robust as the Ir-pincer dehydrogenation catalyst and more active at temperatures above 125 °C needed to be developed. A collaboration with Schrock and coworkers allowed for the synthesis of 40 different... 

Hong SH, Wenzel AG, Salguero TT, Day MW, Grubbs RH. Decomposition of Ruthenium Olefin Metathesis Catalysts. /Chem Soc. 2007 129(25) 7961—7968. Vougioukalakis GC, Grubbs RH. Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Unsymmetrical N-HeterocycHc Carbene Ligands Synthesis, Structure, and Catalytic Activity. Chem EurJ. 2008 14(25) 7545—7556. 

In this study we focused on the ethylene-promoted Hoveyda-Grubbs catalyst decomposition. The formation of large amount of propylene, butenes, and ethane was observed as well as different isopropoxystyrene derivatives. Future investigations of substrate role in olefin metathesis catalj decomposition and the mechanism of etiiane formation are in progress. 

Initial attempts to introduce other cyclometalated substituents or bulkier orthosubstituents on the A-aryl group only led to decomposition of the catalyst [48, 73]. Fortunately, replacement of silver pivalate with sodium pivalate allowed for a milder protocol to prepare previously inaccessible catalysts (e.g., 10 and 11) [52, 55]. With the exception of catalysts lacking ort/jo-substitution on the A-aryl ring (e.g., 13) [73], a variety of A-aryl and cyclometalated substituents were accommodated (Fig. 2). When these new catalysts were assayed in homo-CM reactions, a dramatic improvement was noted for catalyst 10, which exhibited TONs approaching 7,400 and near perfect Z-selectivity (>95%) [52]. This represents the highest catalytic efficiency exhibited by a Z-selective olefin metathesis catalyst reported to date. Catalyst 10 maintained remarkable activity and Z-selectivity in a variety of homodimerization reactions, as well as a selection of more complicated RCM and CM reactions (cf. Sect. 3). 

W. Janse van Rensburg, P J. Steynberg, M. M. Kirk, W. H. Meyer and G. S. Forman. Mechanistic comparison of ruthenium olefin metathesis catalysts DFT insight into relative reactivity and decomposition behavior. J. Organomet. Chem. 691, 2006, 5312-5325. 

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. 

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

The application of olefin metathesis to the synthesis of piperidines continues to be widely employed. The use of ring closing metathesis (RCM) in the synthesis of fluorovinyl-containing a,P-unsaturated lactams 148 and cyclic amino acid derivatives 149 is shown below. A key improvement in these reactions is the addition of the Grubbs 2nd generation catalyst (G2) in small portions during the reaction to compensate for catalyst decomposition that occurs at elevated reaction temperatures <06EJOl 166>. 

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