Additive effects in olefin metathesis

No additive 23% conversion with phenol (1000 equiv) 65% conversion 

The Meyer report demonstrates that the activity of Ru catalyst 1 can be enhanced by the simple addition of phenol. In some instances, the addition of phenol has led to a 10-fold increase in catalyst lifetime and significantly higher turnover numbers. The phenol additive has also exhibited an effect on metathesis reactions catalyzed by Ru catalyst 3. Mechanistic studies (NMR, DFT calculations) suggest that phenol influences the kinetics of phosphine association and dissociation, and can activate the carbene carbon for reaction with an olefinic substrate. 

The authors additionally proposed an alternative explanation for the observed increase in conversion and selectivity that involves the reduction of Ru(III) byproducts. For instance, the added tin and iron salts have well-described redox couples (Sn(II)/Sn(IV) and Fe(ll)/Fe(lll)) that could be acting as single-electron reductants to convert the Ru(lll) decomposition products of catalyst 1 back to the necessary Ru(II) oxidation state. Some evidence for this theory is that the addition of FeClg, which cannot act as a reductant, did not improve any of the metathesis reactions studied. 

The observation that the isolated yields of macrocyclization products in RCM reactions can be influenced by the proximity of a Lewis-basic carbonyl to the site of metathesis has been made by other groups as well. Grubbs [29] and Fiirstner [30] have each reported similar problematic macrocyclizations due to the position of the olefin in a metathesis precursor. In each case, the formation of cyclic chelates between the Lewis-acidic Ru atom in the catalyst and a carbonyl moiety in the substrate was responsible for the low yields. 

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The effect of free hydroxyl groups in proximity to reactive olefins and their effects on olefin metathesis reactions has been an area of ongoing study [25]. For example, the use of alcohol additives, particularly phenols, to improve catalyst activities and hfetimes in olefin metathesis has been reported by Meyer and coworkers [26]. Their study evaluated the use of phenol as an additive in selfmetathesis, ethenolysis, and CM reactions. When the CM of styrene with terminal olefins 37 and 38 was investigated using Ru-based catalyst 1, the conversions were typically low (< 50%) (Scheme 12.13). Upon the addition ofphenol as an additive,a significant improvement in the conversion of the CM reactions was observed, and olefins 40 and 41 were obtained in 76% and 65% conversion, respectively. Similarly, a CM reaction between 3,3-dimethylbutene and 1-undecene showed none of the desired CM until phenol was added, whereby cross product 42 was observed in 57% conversion. 

Halide salts are another class of additives that can be exploited in olefin metathesis reactions employing Ru-based catalysts. The addition of NaBr and Nal, usually in excess, with a Ru catalyst of the type L2Cl2Ru=CHR can promote halide - ligand exchange, effectively substituting Cl for Br or 1. Halide additives have been exploited primarily in asymmetric olefin metathesis, where they can effectively modulate the reactivity and selectivity of a given catalyst. 

Recently, additional catalyst systems which are effective for the metathesis of olefins bearing polar functional groups have been revealed. Nakamura and co-workers found that either WC16 or (C2H50)2MoCl3 in combination with triethylborane were capable of converting c/s-9-octa-decenyl acetate to 1,18-diacetoxy-9-octadecene and 9-octadecene at the rather high temperature of 178°C (89). 

When the ruthenium Schiff base olefin metathesis catalyst was used to polymerize cyclooctadiene Mw s > 100,000 Da were obtained. Although only 12 Schiff bases were identified as effective in these polymerizations, additional analogs are anticipated to be reported from the University of Ghent group. 

In some instances removal of ruthenium from the desired product has proved difficult. To overcome this problem, there are several reported ways to remove the ruthenium catalyst after the olefin metathesis reaction is complete. The first and most common is addition of a modifier or adsorbent followed by chromatography. The additives include dimethyl sulfoxide (DMSO), triphenylphos-phine oxide,74 or activated carbon.75 Although this is generally not suitable for commercial production, it can be quite effective. A convenient alternative is the use of Brockmann I basic alumina, followed by simple filtration through a bed of filter agent such as activated carbon. This method is capable of reducing ruthenium levels down to less than 20 ppm.76... 

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