Metathesis Product Purification

Handbook of Metathesis VoL 1 Catalyst Development and Mechanism, Second Edition. 

Ring-closing metathesis reaction typically used to assay ruthenium-removal 

Unsurprisingly, in view of the considerable number of advances made in the area during this period, such an important reaction as olefin metathesis has been the subject of a number of reviews since the publication of the first volume of the Handbook of Metathesis [2,7]. The original Nobel Prize presentations by Chauvin, Schrock, and Grubbs [8] also make entertaining and highly informative reading. 

In some instances, the subsequent chemistry can actually assist ruthenium removal. An example of this phenomenon was observed during a later hydrogenation step of an initial metathesis product using a palladium-carbon catalyst 

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While the stoichiometry of the reaction appears simple, the reaction mechanism is not well understood, despite numerous studies,77 and the reaction tipically gives rise to a number of by-products. Purification of Herz salts is best achieved via metathesis to one of a number of more soluble salts (A1C14, BF4 , SbCl6-), which can be recrystallised to a high degree of purity.78... 

Factors which often make the silylamide route superior to traditional salt metathesis reactions are (i) the reaction in non-coordinating solvents due to the high solubility of the monomeric metal amides, (ii) mild reaction conditions often at ambient temperature, (iii) avoidance of halide contamination, (iv) ease of product purification [removal of the released amine along with the solvent under vacuum (bp HN(SiMe3)2 125 °C)], (v) base-free products (coordination of the sterically demanding, released amine is disfavored), (vi) quantitative yield , and (vii) the facile availability of mono- and heterobimetallic amide precursors. 

The thallous derivative so formed was found to undergo metathesis with methyl iodide and to yield thallous iodide and methyl cellulose. Analyses of the latter product for methoxyl content after suitable purification yielded a measure of the accessible hydroxyl groups. The reactions were all conducted in non-aqueous systems. 

As a model metathesis reaction, diethyl diallyhnalonate was cychzed in the presence of 5 mol% (41) within forty minutes in methylene chloride at 40 °C the product was isolated as a colorless oil after filtration and removal of volatiles. An analogous reaction with the soluble catalyst (29) was complete in less than five minutes, suggesting that metathesis reactions catalyzed by (41) are diffusion-controlled. Nonetheless, a reaction time of forty minutes is reasonable, and the validation for this chemistry lies in the simplified purification procedure and minimization of waste streams. 

The primary advantage in the first step of the method described here (using 1-chlorobutane diluted in MeCN) is that it eliminates long reaction periods and allows the use of secondary alkyl halides without competitive elimination reactions. For example, the reaction of sec-butyl bromide with N-methylimidazole using the classical method (in neat alkyl halide) produces, along with the desired product, 20-30% of butenes and 1-methylimidazole hydrobromide. In the second step, the use of water as solvent allows the anion metathesis reaction to be quantitative in a very short time and allows the easy purification of the ionic liquids. Moreover, employing the potassium salt avoids the use of corrosive and difficult to handle hexafluorophosphoric add and the expensive silver tetrafluoroborate. ... 

A polymer-supported TosMIC reagent 196 has been developed in which the isocyanide is attached to a ringopening metathesis polymer (ROMP gel) (Scheme 57). The use of this reagent significantly simplified the purification of the oxazole products <20010L271>. 

Efficient chiral molybdenum catalysts (Structure 21) which are, at the same time, easy to handle were generated in. situ and used without further purification in asymmetric olefin metathesis. For example, the RCM following eq. (17) yields > 80% of the desired product at >88% stereoselectivity [118]. 

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