Olefin Metathesis: Catalyst Inhibition as a Result of Isomerization

Abstract

Ring-closing metathesis of N,N-diallyl-tosylamide and allyl-N-10-undecenenoyl-l-proline by using low levels of Ru-catalyst loading were investigated. The undesirable olefin isomerization, promoted by ruthenium speciesf ormed during the decomposition of the catalyst, was observed. The volume of isomerization products depends on the catalystl oading. Accumulation of isomerst hat can form inactiveF ischer-type alkylidene complexes,l eads to the full inhibition of the catalytic reaction.Catalyst deactivationi safundamentalp roblem in catalysis and is of extreme importancee specially for industrial catalytic processes. [1] In particular, catalystd ecomposition is one of the major limiting factors for the use of ruthenium alkylidene catalysts in olefin metathesis reactions. [2] Different ways of Ru catalyst decomposition in the metathesis reactions have been discussed in the literature. [3, 4] Although thermald ecomposition is ac ommonw ay for catalysts to deactivate, the inhibition of the catalystd iffers from case to case, depending mostly on impurities presentedi nt he solvent and starting olefin. [5] Particularly, alcohols and basic amines lead to irreversible catalystd ecomposition. [6] Identifying the catalysts inhibitors and catalyst poisons is generally not trivial in catalysis. Especially in industrially relevant applications that use very low catalystl oading, small quantities of catalystp oisons have substantial impact.T here are only few reports dealing with identifying the impurities that have an egative impact on metathesis reactions. [7] In this survey it is demonstrated how the otherwise insignificant catalyst decomposition leads to the significant accumulation of ac ompound that acts as ac atalyst inhibitor.T he double-bond isomerization, as ac ommon side-reaction in olefin metathesis, has been at opic of numerous reports. [8,9] It was recently proposed that undesired double-bond isomerization can lead to accumulation of isomerst hat act as catalyst poison. [10] Herein, cleare videncei sp rovided to support this proposal.The ring-closing metathesis (RCM) of N,N-diallyl-tosylamide 1,a so ne of the most popularb enchmarking substrates, has been the subject of numerous reports. Theb est results, with conversion of 88-97%,w ere obtaineda tn on-elevated temperatures and 100-250 ppm catalyst loading. [11] Conversion drops to 21-62 %b yl owering the catalystl oading to 60 ppm. [12] Recently,i tw as shown that 2,5-dihydropyrrole 2 can be obtained in 54-60 %y ield after 20-40min at 80 8Cb y using catalysts (A, B,o rC)w ith catalystl oadings as low as 50 ppm. [10] Using ac atalystl oading of 100 ppm increases the yield of 2 to 88-90 % (Schemes 1, 2). Figure 1s hows the reaction profiles for catalyst loadings of 50-200 ppm. Doubling of the catalyst loading from 50 to 100 ppm leads to an improvement in the conversionb ya bout half. Further increasing the catalystl oadingh ardly contributes to an improvement of the transformation. The most surprising findingi st hat increasing the loading from 150 to 200 ppm...

p‐Cymene as Solvent for Olefin Metathesis: Matching Efficiency and Sustainability

“…In toluene with 2mol %o fH-II ( Figure 1) at 80 8C, the yield of isomerization products reached 54 %. [37] To choose the initial conditions for this work, some reactions employing 1,2-dichloroethane (DCE) as solvent were examined. The addition of 1,4-benzoquinone( 5mol %r elative to the catalyst) resulted in ac onsiderable reduction in isomerization activity (9 %) and allowed up to 78 %i solated yield of the cross-metathesis product.…”

“…[2] Besides the selectivityl oss, in some cases the isomerization products result in Fischer-type carbenes that inhibit the metathesis activity. [37] To choose the initial conditions for this work, some reactions employing 1,2-dichloroethane (DCE) as solvent were examined. Considering that 1 is prone to undergo self-metathesis and that for acrylates this tendency is much smaller, [38] the latter was used in af our-fold molar excess to maximize the crossmetathesis of 1.T he increasei nc atalystc oncentration seems to accelerate the decomposition process of the catalyst, with ac oncomitant increase in the rate of isomerization ( Table 1, entries 1-9).…”

Unprecedented Multifunctionality of Grubbs and Hoveyda–Grubbs Catalysts: Competitive Isomerization, Hydrogenation, Silylation and Metathesis Occurring in Solution and on Solid Phase

“…Also proved was that for other 2nd generation Ru metathesis catalysts, the contribution of the isomerization pathway hinges on the catalyst loading [28]. Nevertheless, non-metathetic reactions initiated by ruthenium alkylidene catalysts can provide an expedient entry into otherwise inaccessible interesting compounds.…”

“…Olefin isomerization has been decreased by the addition of electron-deficient benzoquinones (e.g., 1,4-benzoquinones, 10 mol %), tricyclohexylphosphine oxide (0.5 mol %) or phenylphosphoric acid (5 mol %), as well as quinone-containing Hoveyda-type complexes [23][24][25][26][27]. Also proved was that for other 2nd generation Ru metathesis catalysts, the contribution of the isomerization pathway hinges on the catalyst loading [28].…”

Hoveyda‐Type Quinone‐Containing Complexes – Catalysts to Prevent Migration of the Double Bond under Metathesis Conditions

“…Additionally, the side‐products resulting from unwanted isomerisation are frequently difficult to remove by standard purification techniques. Moreover, evidence has been provided that undesired double‐bond isomerisation can lead to the accumulation of isomers that can act as catalyst inhibitors …”