Alkene Metathesis Catalysis: A Key for Transformations of Unsaturated Plant Oils and Renewable Derivatives
Catalyse de métathèse d’oléfines : une clé pour des transformations d’huiles végétales insaturées et de substances renouvelables
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes, “Organometallics: Materials and Catalysis”, Centre for Catalysis and Green Chemistry, Campus de Beaulieu, 35042
Rennes – France
* Corresponding author
This account presents the importance of ruthenium-catalysed alkene cross-metathesis for the catalytic transformations of biomass derivatives into useful intermediates, especially those developed by the authors in the Rennes (France) catalysis team in cooperation with chemical industry. The cross-metathesis of a variety of functional alkenes arising from plant oils, with acrylonitrile and fumaronitrile and followed by catalytic tandem hydrogenation, will be shown to afford linear amino acid derivatives, the precursors of polyamides. The exploration of cross-metathesis of bio-sourced unsaturated nitriles with acrylate with further catalytic hydrogenation has led to offer an excellent route to α,ω-amino acid derivatives. That of fatty aldehydes has led to bifunctional long chain aldehydes and saturated diols. Two ways of access to functional dienes by ruthenium-catalyzed ene-yne cross-metathesis of plant oil alkene derivatives with alkynes and by cross-metathesis of bio-sourced alkenes with allylic chloride followed by catalytic dehydrohalogenation, are reported. Ricinoleate derivatives offer a direct access to chiral dihydropyrans and tetrahydropyrans via ring closing metathesis. Cross-metathesis giving value to terpenes and eugenol for the straightforward synthesis of artificial terpenes and functional eugenol derivatives without C=C bond isomerization are described.
Cet article présente l’importance de la réaction de métathèse croisée des oléfines par catalyse au ruthénium pour la transformation de dérivés de la biomasse en produits utiles pour l’industrie. Il constitue une revue des principaux travaux réalisés dans ce domaine par les auteurs dans leur équipe de catalyse à Rennes (Frence) en coopération avec l’industrie chimique. La métathèse croisée d’une variété d’oléfines fonctionnelles issues des huiles végétales avec l’acrylonitrile et le fumaronitrile, suivie d’une hydrogénation tandem conduit à des dérivés d’amino acides linéaires, précurseurs de polyamides par polycondensation. L’exploration de la métathèse croisée de nitriles insaturés bio-sourcés avec des acrylates a conduit à des dérivés d’α,ω-amino acides, tandis que les aldéhydes gras ont permis un accès rapide à des aldéhydes bifonctionnalisés à longue chaîne et à des diols saturés.
Deux voies d’accès à des diènes conjugués fonctionnels impliquant des réactions de métathèse catalysées par le ruthénium à partir d’oléfines issues de la biomasse sont décrites : l’une par métathèse croisée avec un alcyne, l’autre par métathèse croisée avec un chlorure allylique suivie d’une réaction de déhydrohalogénation catalytique. Les dérivés de l’acide ricinoléïque ont permis un accès direct à des dihydropyrannes et tétrahydropyrannes chiraux par fermeture de cycle par métathèse. Des métathèses croisées pour valoriser des dérivés de terpènes et de l’eugénol en offrant des méthodes directes de synthèse de terpènes non naturels et de dérivés d’eugénol fonctionnels sans migration de double liaisons sont aussi présentées.
© P.H. Dixneuf et al., published by IFP Energies nouvelles, 2016
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The production of chemical industry intermediates from renewable resources is a topic of current interest to offer alternatives to petrochemicals, energy savings, and to contribute to sustainable development [1-10]. Within this trend, the use of renewable unsaturated vegetable oils is attractive [1-4]. Catalytic olefin metathesis has a strong potential for selective biomass transformations. Vegetable oils by transesterification easily release unsaturated fatty esters which offer, via cross-metathesis with alkenes, selective access to terminal alkenes or bifunctional internal olefins that are crucial intermediates for industry. Thus catalytic cross-metathesis has become a key to reveal the potential of renewable plant oils or natural terpenes [11-18].
The objective of this review is to show several applications of plant oils and terpenes discovered in Rennes with ruthenium catalysts during the last 10 years such as the ethenolysis of renewable unsaturated fatty esters to produce terminal functional alkenes, their cross-metathesis with acrylonitrile in the route to produce by sequential catalyses linear amino acid derivatives that are key intermediates for polyamide production, the cross-metathesis of renewable fatty nitriles to generate linear amino esters or diamines, and selective terpene modifications. Tandem and sequential alkene metathesis/hydrogenation catalytic reactions will be presented illustrating transformations of renewable materials with atom economy and offering new contribution to green and sustainable chemistry.
Catalytic ethenolysis of methyl oleate 1 has the potential to produce 1-decene 2 and methyl 9-decenoate 3, that are two useful intermediates for the preparation of lubricants  or polyesters  (Scheme 1). Initial catalysts used for this ethenolysis were WCl6/SnMe4 , and the heterogeneous Re2O7/Al2O3/SnMe4 [22, 23] catalysts, as well as the first well-defined ruthenium catalysts such as I, II, IV (Fig. 1) [24-27]. These studies revealed that double bond isomerization easily takes place, is favoured by high temperature, and produces many products. However, the formation of homometathesis products 9-octadecene 4 and symmetrical diester 5 remained low.
Ruthenium catalysts for alkene metathesis.
Ionic alkylidene-ruthenium catalysts.
Alkylidene-ruthenium catalysts for alkene metathesis and hydrogenation.
Ionic Liquids (IL) as solvents have shown their ability to favour several catalytic processes [28-32] and alkene metathesis can be performed in Room Temperature Ionic Liquids (RTIL) since 1995 [33-41]. We have shown the efficient ethenolysis of methyl oleate 1 under mild conditions with several ruthenium alkylidene catalysts in organic solvents and, for the first time, in imidazolium-type ionic liquids at room temperature without C=C bond isomerization .
The ethenolysis of methyl oleate was investigated with several ruthenium catalysts I, III, IV, and V, and the in situ prepared catalyst VI [43, 44] (Fig. 1) at 70°C in toluene under 1 bar of ethylene  (Scheme 1). These experiments demonstrated the superiority of catalyst III, which showed a better efficiency and selectivity. The best results were obtained under the mildest conditions at 20°C and under 1 bar pressure of ethylene leading to 93% conversion in 3.5 h, with a total selectivity to produce 1-decene and methyl 9-decenoate in 93% yield. The effect of the ethylene pressure is less pronounced but a higher pressure appears to improve the conversion of the reaction. The Hoveyda catalyst IV was found to allow an excellent selectivity at 20°C and at 1 bar but with lower conversion (80% instead of 93 for III).
Ethenolysis of methyl oleate 1.
Catalyst I was further evaluated in the ethenolysis of methyl oleate at 20°C and 10 bar of ethylene but in IL. Low conversion took place in [bmim][OTf] ([bmim] = 1-butyl-3-methylimidazolium), but in [bmim][NTf2] and in [bdmim][NTf2], [bdmim] = 1-butyl-2,3-dimethylimidazolium], 79% and 83% conversion were respectively obtained after only 2 hours with excellent selectivity .
Two Hoveyda-type catalysts VII and VIII , bearing an ionic tag (Fig. 2), were evaluated in the ethenolysis of methyl oleate in the ionic liquid [bdmim] [NTf2] at 20°C and 10 bar of ethylene for 2 h. Catalyst VIII only displayed suitable catalytic activity leading to 89% conversion with high selectivity. However it was not possible to recycle this catalyst .
By contrast the neutral catalysts I, III, and IV (Fig. 1) in ionic liquid [bdmim] [NTf2] at 20°C and 10 bar of ethylene for 2 h were preferable and thus subjected to consecutive runs after product extraction in hexane. We showed that catalysts I and III led to good conversion for the first run but more importantly that catalyst III could be recycled 3 times without loss of activity and with an excellent selectivity (conversion: 1st run 95%, 2nd run 95%, 3rd run 85%) . This demonstrates that it is not necessary to introduce an ionic tag in the catalyst to get excellent activity, selectivity and recyclability and that catalyst III in [bdmim] [NTf2] can offer these targets for ethenolysis of methyl oleate.
2 Cross-metathesis of acrylonitrile with unsaturated acids or esters: a key step toward linear amino acid derivatives
The transformation of unsaturated acids and esters arising from plant oils into linear amino acid intermediates constitutes a challenge to generate useful polyamides from renewables. Cross-metathesis of acrylonitrile and unsaturated acids or esters, associated with the further hydrogenation of both the C=C and CN bonds, constitutes a potential strategic way for the production of polyamides [14, 15] (Scheme 2).
From cross metathesis of fatty ester with acrylonitrile to linear aminoester.
The first example of cross-metathesis of terminal olefins with acrylonitrile was described with the Schrock catalyst Mo(=CHCMe2Ph)(NAr)(OCMe(CF3)2), Ar = 2,6-diisopropylphenyl, affording moderate yields . Then ruthenium catalysts, especially containing a heterocyclic carbene ligand, improved the cross-metathesis of acrylonitrile with terminal alkenes [46-50], with an example of internal alkene in the presence of copper salt .
We have first attempted the cross-metathesis with ruthenium catalysts of acrylonitrile with two unsaturated acids and their esters arising from renewable resources: the C11 ester 6 obtained from castor oil and its related carboxylic acid 7, and the C18 diester 8 and diacid 9 obtained via biotransformation or self-metathesis of oleic acid  (Scheme 3).
Cross metathesis of fatty derivatives 6-9 with acrylonitrile.
The cross-metathesis of the unsaturated monoester 6 with 2 equivalents of acrylonitrile was performed with the Grubbs II and Hoveyda IV catalysts in toluene at 100°C (Scheme 3). The Hoveyda catalyst IV was the most efficient. With low catalyst loading (1 mol%) it allowed better conversion (95% after 1 h) giving the C12 nitrile ester 10 as a mixture of Z (major) and E isomers (3/1). Increasing the reagent concentration resulted in lower conversion (best concentration of 6 = 0.05 M)  (Scheme 3).
The similar cross-metathesis reaction was performed with acid 7. Catalyst IV (1 mol%, toluene) showed the best performance giving 90% conversion in 1 h at 100°C, the Z isomer being the major one (Z/E = 3.2/1) of nitrile acid 11 . These experiments show that cross-metathesis is not significantly affected by the presence of an acidic functionality when catalyst IV is used.
However, it was observed that the catalyst IV progressively decomposed. Thus step by step addition of the catalyst allowed to reach better conversions and this effect was improved by dropwise addition. Lower catalyst loadings were then attempted at 100°C using a syringe pump equipment (dropwise addition) which offered a real breakthrough. Thus it was possible to decrease the catalyst IV loading down to 0.05 mol% still leading to a high conversion (96%) and to a TON of 1 900. The TON could be slightly increased to 2 280 using a lower catalyst loading but the conversion dropped to 55% .
A large excess of acrylonitrile was observed to inhibit the transformation. Simultaneous slow addition of catalyst and acrylonitrile still improved the process. It was shown that addition with two syringes, one containing a mixture of ester 6 and 1 equivalent of acrylonitrile and the other one a solution of catalyst IV (0.025 mol%) and the second equivalent of acrylonitrile, improved the process, which furnished 75% conversion of 6 and led to the highest TON (3 120) to produce the nitrile ester 10 .
Whereas the internal double bond required a higher catalyst loading of 5 mol% , the octadec-9-enedioic diester 8, derived from bioconversion of sunflower oil in the presence of Candida tropicalis M 25  or by self-metathesis of oleic acid derivatives [55-57], was evaluated for cross-metathesis with acrylonitrile. Diester 8 and acrylonitrile in the presence of catalysts II and IV lead to the C11 nitrile-ester 12 (Scheme 3). Catalyst IV was also the most efficient for the reaction of 8 ( = 0.1 M) with 4 equivalents of acrylonitrile in toluene at 100°C for 1 h. With 5 mol% of catalyst the nitrile ester 12 was obtained (86%) as a mixture of Z (major) and E isomers (2.9/1) with a small amount of C10 ester with a terminal double bond . Thus the cross-metathesis with acrylonitrile of alkenes containing an internal double bond can be efficiently achieved, as it was also possible with acrylic esters [16, 17]. Using syringe pump addition of catalyst IV at 100°C for 5 h, the unsaturated nitrile ester 10 was obtained in 88% yield, with high TON of 1960 .
The direct cross-metathesis of the diacid 9 with acrylonitrile was performed using 5 mol% of catalyst IV in toluene at 100°C for 1 h to give 90% conversion of 9 and the formation of the diacid 13 (Z/E = 2.4) (Scheme 3).
The cross-metathesis of fumaronitrile (2 equivalents) instead of acrylonitrile with the diester 8 (Scheme 4) can be performed using ruthenium catalyst IV (5 mol%) and the nitrile ester 12 was obtained in 92% isolated yield . These experiments show that fumaronitrile can be used for cross-metathesis, although it is more profitable to use the cheapest and readily available acrylonitrile.
Cross metathesis of diester 8 with fumaronitrile.
Analogously, methyl oleate 1 with an internal double bond, was reacted with an excess of acrylonitrile in toluene at 100°C for 2 h with 5 mol% of ruthenium catalyst IV and led to full conversion of methyl oleate and to a mixture of the expected nitrile ester 12 and unsaturated nitrile 15 in equal amounts  (Scheme 5). Cross-metathesis of methyl oleate with fumaronitrile in the presence of 5 mol% of catalyst IV in toluene at 100°C for 2 h also provided 12 and 15 as a mixture of E and Z isomers, in equal proportions  (Scheme 5).
Cross metathesis of methyl oleate with acrylonitrile and fumaronitrile.
The possibility of synthesizing saturated nitrile-esters from renewable oils by tandem cross-metathesis/hydrogenation catalysis was investigated. After cross-metathesis of ester 6 and diester 8 with catalyst IV, and formation of the nitrile-esters 10 and 12, the crude reaction mixture containing residual ruthenium catalyst was concentrated, transferred to an autoclave, and hydrogenated without additional catalyst under 10 bar of H2 to quantitatively give the single compounds 16 and 17, respectively (Scheme 6) . This set of reactions constitutes an example of sequential alkene metathesis/hydrogenation catalytic reactions. However the hydrogenation catalyst is not powerful enough yet to reduce the nitrile group into terminal amine.
Tandem catalytic cross metathesis of fatty esters with acrylonitrile and hydrogenation.
4 Tandem alkene metathesis / hydrogenation catalyses: from plant oil unsaturated esters to linear amino esters
The hydrogenation of aliphatic nitriles into primary amines requires more efficient catalysts operating in more drastic conditions. Takemoto et al.  and Li et al.  reported examples of homogeneous hydrogenation of nitriles into primary amines when alkoxide were added to ruthenium catalyst. Das et al.  and Enthaler et al. [61, 62] also showed that addition of tBuOK and phosphine to simple ruthenium complex led to the fast hydrogenation of various nitriles and by replacing the phosphine ligand by a N-heterocyclic carbene, milder conditions could be operating .
We have now shown that, after performance of the ruthenium alkylidene-catalyzed cross-metathesis of alkenes with acrylonitrile, when the catalytic hydrogenation of the resulting nitrile ester with the residual ruthenium catalyst was carried out in the presence of tBuOK or even KOH, the hydrogenation of the nitrile group into primary amine could be obtained, without formation of secondary amine, and that the α,ω-linear amino ester, a precursor of polyamides, could be obtained in excellent yield [64, 65] (Scheme 7).
Tandem catalysis cross metathesis/hydrogenation of ester 6.
Thus, after the cross-metathesis of methyl oleate with acrylonitrile in the presence of the best catalyst: the Hoveyda catalyst IV (3 mol%) at 100°C for 1 h in toluene, the reaction mixture containing the residual ruthenium was transferred into an autoclave. Addition of 30 mol% of tBuOK under 20 bar of hydrogen for 44 h led to the formation of 92% of methyl 12-aminododecanoate 18. No remaining nitrile ester 16 was observed . It is noteworthy that this reaction takes place without formation of secondary amine as observed in many hydrogenation of nitriles into amines.
The evaluation of other basic additives such as potassium hydroxide, sodium hydroxide, or cesium carbonate showed that KOH could be efficiently used but in 60 mol%. Using only 30 mol% of KOH led to 56% yield of 18 together with 40% yield of nitrile ester 16  (Scheme 7).
We also evaluated the influence of the initial ruthenium alkylidene catalyst and we showed that the Umicore M51 catalyst VIII provided similar results as catalyst IV but with the advantage of requiring only 15 mol% of tBuOK  (Scheme 7).
We explored the sequential cross-metathesis with acrylonitrile and hydrogenation of the renewable fatty diester 8  (Scheme 8). The reaction of 8 with 4 equivalents of acrylonitrile with 6 mol% of catalyst IV was first performed in toluene for 2 h at 100°C and then the reaction mixture was transferred into an autoclave charged with 60 mol% of tBuOK, 30 bar of H2. After 48 h at 80°C, the C11 aminoester 19 was isolated in 78% yield.
Sequential catalytic synthesis of aminoester 19 from diester 8.
A tandem metathesis/hydrogenation process in the presence of ruthenium alkylidene catalyst has already been used to prepare saturated polymers [66-68]. Thus, we have explored whether ruthenium–benzylidene and ruthenium–indenylidene olefin metathesis catalysts could also perform hydrogenation of fatty alkyl nitriles and could be more efficient than the residual ruthenium arising from cross-metathesis.
The hydrogenation of methyl 11-cyano undec-10-enoate 10 (Scheme 9), was first performed in the presence of catalyst IV (3 mol%) and 30 mol% of tBuOK, under hydrogen pressure of 20 bar at 80°C for 39 h. It quantitatively provided the saturated C12 α,ω–amino ester 18, which was isolated in 89% yield  (Scheme 9). The presence of tBuOK not only created the hydrogenation catalyst but was shown to inhibit the formation of secondary amines.
Catalytic hydrogenation of nitrile esters into linear amino ester 18.
The catalyst activity of ruthenium-alkylidenes I, II, and VIII-X (Fig. 3) was evaluated on the hydrogenation of 10, in the presence of 30 mol% of tBuOK. The best results were obtained with catalyst X and II requiring the presence of only 15 mol% of tBuOK to get quantitative formation of 18 after 16 h. More importantly these two catalysts could perform the quantitative formation of 18 by hydrogenation of nitrile ester 10 at 80°C for 16 h  (Scheme 9).
Another approach for the preparation of the same C12 α,ω-amino ester 18 can arise from the hydrogenation of methyl (E)-11-cyanoundec-2-enoate 20, which is arising from cross-metathesis of 10-undecenenitrile with methyl acrylate (see Sect. 6). The catalytic hydrogenation of nitrile ester 20 using 3 mol% of catalyst IV under an initial hydrogen pressure of 20 bar at 80°C for 40 h in the presence of 30 mol% of tBuOK, led to complete conversion of compound 20 and 75% of compound 18 and 22% of compound 16 were obtained  (Scheme 9). The hydrogenation of 20 with ruthenium catalysts I, II, and VIII–X (3 mol%) in the presence of 30 mol% of tBuOK provided excellent yields of amino ester 18, except for catalyst I. On lowering the catalyst loading to 1 mol%, and on shortening the reaction time to 16 h, ruthenium catalysts II and X offered the best efficiency for the production of 18 from nitrile ester 20  (Scheme 9).
6 Cross-metathesis of renewable 10-undecenenitrile and methyl acrylate: new synthesis of polyamides precursors
In some examples it was shown that acrylonitrile inhibits the ruthenium-catalysed cross-metathesis of functional alkenes, whereas methyl acrylate does not inhibit this reaction as some cross-metathesis can be performed in bulk in methyl acrylate [16, 17, 70-72]. Thus, it appeared that the linear amino ester 18 could also arise from the cross-metathesis of the 10-undecenenitrile 21 with methyl acrylate followed by hydrogenation. 10-Undecenenitrile 21 is a bio-sourced derivative arising from ammoxidation of 10-undecenoic acid . 10-Undecenoic acid is produced by thermolysis of ricinoleic acid arising from castor oil followed by hydrolysis and it has already been used for the industrial production of polyamide PA11 in the Rilsan process .
The cross-metathesis of 10-undecenenitrile 21 with methyl acrylate (2 equivalents) in toluene was attempted with (NHC)Ru alkylidenes II, IV, VIII, and XI (0.5 mol%) (Fig. 1, 3). The catalysts IV, VIII and XI bearing a chelating benzylidene ligand were very efficient, at temperature of 50°C or even room temperature, to quantitatively produce nitrile ester 20, with high percentage of the E isomer  (Scheme 10). This cross-metathesis transformation is not so easily performed than that of oleate with methyl acrylate but it occurs much more easily than the cross-metathesis of methyl oleate with acrylonitrile, thus showing the inhibition by the nitrile function especially when it is conjugated with the double bond.
Sequential catalytic synthesis of amino ester 18 from unsaturated nitrile 21.
To improve the TON values, catalyst IV was slowly injected with a syringe pump apparatus. The catalyst solution was added within 2 h 40 min and the reaction was prolonged for further 5 h. With a catalyst loading of 0.05 mol%, the temperature could be decreased down to 50°C to ensure a TON close to 2 000 with almost complete conversion.
With lower catalyst loadings, a higher temperature of 100°C was required to reach good conversion. Thus with 0.01 mol% of catalyst IV, 92% conversion of 21 was obtained and led to 92% of nitrile ester 20, thus providing a high TON of 9200. The increase of the concentration of the nitrile 21 to 0.5 M improved the conversion and catalyst IV loading could be decreased to 0.005 mol%. At 100°C a TON of 17 200 could be reached.
Similarly to the transformation of oleate into linear amino ester (Scheme 7) we have investigated the tandem cross-metathesis/hydrogenation in the presence of tBuOK starting with cross-metathesis of nitrile 21 with methyl acrylate, followed by hydrogenation of the formed nitrile ester, taking profit of the ruthenium residue as hydrogenation catalyst. The nitrile hydrogenation could not be achieved with the very low loading of 0.05 mol% of catalyst IV used for the cross-metathesis reaction. Thus the metathesis was performed during the first step, with 3 mol% of catalyst loading, and hydrogenation with the corresponding residual ruthenium was performed under 20 bar of hydrogen at 100°C to give complete conversion into the saturated amino ester 18 which was obtained only in the presence of 30 mol% of tBuOK with 96% yield  (Scheme 10).
Ricinoleic derivatives are useful renewable materials that have a hydroxy functionality which offers an extra-value for further derivatization. However, the homoallylic hydroxyl group may also prevent alkene metathesis [76-79]. We thus studied the cross-metathesis of methyl ricinoleate 22 with acrylonitrile using both the single dose addition and the dropwise slow addition of catalyst IV (Scheme 11). This transformation led us to the formation of nitrile ester 12 obtained also from the cross-metathesis of diester 8 with acrylonitrile, and to the new compound 23. Single dose addition of 1 mol% of Hoveyda catalyst IV at 80°C allowed full conversion of 22 providing 12 in 82% and 23 in 76% isolated yields. By using the dropwise slow addition of catalyst a higher TON (920) was reached. Using 0.1 mol% of catalyst 12 and 23 were obtained in 73% and 66% yields, respectively (Scheme 11). These first results demonstrate that the transformation of methyl ricinoleate is more difficult to perform than cross-metathesis of diester 8, thus illustrating the inhibition presented by the homoallylic alcohol functionality with respect to a simple internal alkene [14, 53].
Cross metathesis of methyl ricinolate 22 with acrylonitrile and methyl acrylate.
An alternative route for the production of the fatty diesters was explored by the cross-metathesis of methyl ricinoleate 22 with methyl acrylate, for the production of the diester 24 (89%) with concomitant formation of hydroxyl ester 25 (85%), which is a potential precursor of the fatty diol by hydrogenation (Scheme 11). The slow addition of catalyst IV afforded 90% conversion of 22 using 0.1 mol% of catalyst IV and a high TON (900) was reached .
The aldehyde functionality is often reached in industry via hydroformylation of alkenes, and allows the access to a large variety of functional compounds by selective catalytic transformations. Alkene metathesis catalysis, especially promoted by well-defined homogeneous ruthenium alkylidenes and indenylidenes, is offering functionalization by double bond exchange and with the advantage of tolerating a large variety of functional groups including the formyl group. The previous successful results of cross-metathesis motivated our exploration of the cross-metathesis of unsaturated aldehydes arising from derivatives of plant oils, especially via cross-metathesis with acrylonitrile or via self-metathesis for the production of linear diol precursors or amino alcohols.
The 10-undecylenic aldehyde 26 is a renewable derivative obtained from thermal treatment of castor oil [76, 77]. Its self-metathesis was attempted to produce the C20 dialdehyde a key intermediate for the production of symmetrical diol or diamine and the formation of polyesters and polyamides. The self-metathesis of aldehyde 26 was attempted at 40°C in toluene first with catalysts II and IV, containing a N-heterocyclic carbene ligand. They showed high catalytic activity but also yielded isomerization by-products. The catalysts I and III containing the PCy3 ligand offered better production of dialdehyde 27. The catalyst III afforded 79% conversion of 26 with only 9% of self-metathesis by-product  (Scheme 12).
Homometathesis of 10-undecylenic aldehyde 26 and hydrogenation of dialdehyde 27.
The direct synthesis of eicosane-1,20-diol 28 was then directly performed from the mono aldehyde 26, using tandem self-metathesis/hydrogenation catalysis. The aldehyde 26 was first transformed via self-metathesis with catalyst III or XII, and the crude products containing the ruthenium residue were transferred into an autoclave and hydrogenated under 10 bar of H2 at 50°C for 24 h. The diol 28 was isolated in 70% yield.  (Scheme 12).
The cross-metathesis of unsaturated aldehyde 26 with various acrylic derivatives was attempted as a route to shorter bifunctional compounds  (Scheme 13). The reaction of 26 with acrolein (2 equivalents) with 1 mol% of catalyst IV in toluene at 100°C led to the formation of the unsaturated dialdehyde 29, which was isolated in 78% yield as a pure E isomer. The corresponding C12 terminal diol was obtained directly from 26 using tandem cross-metathesis with acrolein and catalyst IV, followed by hydrogenation with 10 bar of H2 at 50°C for 24 h. The saturated C12 diol was isolated in 72% yield .
Cross-metathesis of 10-undecylenic aldehyde 26 with acrylic derivatives.
ω-Nitrile-aldehydes have the potential to produce linear amino alcohols and amino acids as a route to polyamides and mixed polyamide-esters. The cross-metathesis of aldehyde 26 with acrylonitrile (2 equivalents) in the presence of catalyst IV was performed at 80°C in toluene for 5 h and led to 30 in 94% isolated yield  (Scheme 13).
The cross-metathesis of 26 with methyl acrylate (2 equivalents) led to the isolation of the E isomer of the formyl ester 31 in 92% yield. The formation of 31 was achieved with catalyst IV (1 mol%) in toluene at 80°C for 16 h. It is noteworthy that the cross-metathesis of 26 with acrylic acid can be performed under similar conditions and after only 14 h the corresponding formyl carboxylic acid was obtained in 84% yield .
In order to evaluate the influence of a nitrile group at the terminal position of a fatty acid derivative on the cross-metathesis with acrylonitrile, 10-undecenenitrile 21 was subjected to cross metathesis with acrylonitrile to produce the dinitrile 32 (Scheme 14) . We found that this reaction was efficiently achieved in the presence of Hoveyda IV and Umicore M51 VIII catalysts. The best results were obtained with the dropwise addition of the catalyst to the reaction mixture and a turnover number of 12 800 was reached at 80% conversion with a catalyst loading of 0.00625 mol% in diethyl carbonate at 120°C for 5 h.
Cross-metathesis of 10-undecenenitrile 21 with acrylonitrile.
These excellent results confirm that suitable experimental conditions make possible the selective cross-metathesis of acrylonitrile without isomerization side reactions in the presence of another nitrile functionality.
The synthesis of conjugated dienes by ene-yne cross-metathesis  is another interesting pathway for the transformation of Fatty Acid Methyl Esters (FAME). In 2009, we reported the first example of such transformation applied to methyl oleate 1 and to the diester 8 . However, our initial attempts to apply this transformation directly to methyl oleate failed whatever the cross-partner, i.e. terminal or internal alkyne. This result was likely due to the lower reactivity of vinylcarbene catalyst species generated in this reaction. To overcome this issue, we set up a sequential reaction involving an initial ethenolysis of methyl oleate followed by ene-yne cross-metathesis of the resulting terminal alkynes (Scheme 15).
Sequential ethenolysis and ene-yne cross-metathesis of methyl oleate.
One-pot ethenolysis/ene-yne cross-metathesis of methyl oleate with propargyl carbonate.
This sequential reaction was performed in a one-pot fashion employing the Hoveyda first generation catalyst III for the initial ethenolysis step and the second generation Grubbs catalyst II for the ene-yne cross-metathesis step (Scheme 15). The choice of a first generation catalyst for the ethenolysis step was dictated by previous results demonstrating the better performances of this catalyst in ethenolysis of methyl oleate in particular since no C=C double-bond migration and low amounts of self-metathesis products were observed with this catalyst [41, 42]. As depicted in Scheme 16, the initial ethenolysis of methyl oleate proceeded with high conversion furnishing the desired ethenolysis product and very low amounts of self-metathesis products. Addition of 1 equivalent of propargyl carbonate 33 and 1 mol% catalyst II produced the expected dienes 34, 35 in high yields as a mixture of stereoisomers. It must be underlined that these reactions were conducted in dimethyl carbonate, a solvent earlier found to be fully compatible with olefin metatheses transformations including ethenolysis .
Similarly to cross-metatheses of olefins, ene-yne cross-metathesis requires an excess of one reagent (usually 2 equivalents of olefinic partner) to proceed efficiently. This constitutes a major impediment regarding sustainability in particular for the transformation of a bio-sourced compound. This issue was overcome by the implementation of a slow addition of the alkyne reagent in order to maintain an excess of olefin all along the transformation . As shown in Scheme 17 the diester 8 was first submitted to ethenolysis delivering 1.8 equivalent of methyl decenoate. The catalyst II and 0.9 equivalent of alkyne were then added and the ene-yne cross-metathesis reaction started. The second 0.9 equivalent of alkyne was then slowly added using a syringe pump apparatus. This reaction led to a higher yield in diene 35 than the previous protocol employing an excess of olefin hence improving the sustainability of this reaction sequence.
One-pot ethenolysis/ene-yne cross-metathesis using continuous alkyne addition.
Conjugated dienes are valuable building blocks in a number of organic transformations. In particular, dienes arising from ene-yne metathesis have received attention in Diels-Alder transformations . To extend the scope of the transformations of these conjugated dienes, we turned our attention to the nucleophilic allylic substitution reaction. The choice of the alkyne involved in the initial ene-yne cross-metathesis is crucial to allow for further allylic activation. Hence, n-decene and methyl decenoate were reacted with the propargylic carbonate 33 to furnish the corresponding dienes in high yields . These dienes were then transformed by nucleophilic allylic substitution catalyzed by a ruthenium complex previously disclosed in our group . The corresponding products 36 were obtained in high yields and regioselectivity. Of note, this transformation could be performed in high yields in a one-pot manner without isolation of the diene intermediate (Scheme 18).
Sequential eneyne metathesis and allylic substitution from conjutated dienes.
As exemplified in this article the transformation of FAME is almost exclusively dedicated to the synthesis of polymer precursors. Only rare examples deal with the transformation of FAME into fine chemicals . In 2012, we reported the transformation of methyl ricinoleate into a variety of pyran and lactone derivatives of interest as fragrance ingredients. The synthesis strategy required a preliminary transformation of the hydroxyl functional group of methyl ricinoleate 37 with various olefinic reagents followed by Ring Closing Metathesis (RCM) of the resulting dienes . Hence, following a Ruthenium-Catalyzed (RC) allylation of methyl ricinoleate, RCM delivered a variety of dihydropyrans 38-40 along with methyl 9-decenoate or its self-metathesis diester as co-products of the reaction (Scheme 19). The ratio of these two valuable polymer precursors could be easily tuned by a judicious selection of the RCM catalyst. An unsaturated lactone could also be prepared following the same strategy and preliminary esterification of methyl ricinoleate.
Methyl ricinoleate-based dihydropyrans catalytic synthesis.
The scope of this reaction was further extended by implementation of tandem RCM/hydrogenation or domino RCM/CM transformations (Scheme 20). As depicted in Scheme 20b, the tetrahydropyran derivative 41 was obtained in good yield via RCM reaction in a single reaction, in the presence or methyl acrylate or acrylonitrile, which concomitantly delivered the bifunctional co-products, suitable as monomer precursors, upon cascade cross-metathesis without hampering the efficiency of the RCM reaction.
Tandem and domino transformation of methyl ricinoleate derivatives.
In a continuous effort toward the implementation of environmentally friendly processes, the potential of ThermoMorphic Solvent systems (TMS)  in olefin metathesis was explored with an emphasis on FAME and methyl undecenoate arising from castor oil .
It was demonstrated that a TMS system comprising of Propylene Carbonate (PC) as the polar solvent, Ethyl Acetate (EA) as the medium polarity solvent and CycloHexane (CH) as the apolar phase was suitable for the synthesis of a polyester precursor (Scheme 21). Interestingly, a simple phase separation and extraction of the polar phase with CH furnished the desired product with a ruthenium content of 62 ppm (Th ~ 2 500 ppm). A rapid filtration on a short plug of silica was sufficient to decrease the ruthenium content below 1 ppm. Further studies demonstrated that ethyl acetate could be replaced by methyl undecenoate resulting in a more efficient reaction.
Cross-metathesis of methyl undecenoate with methyl acrylate in a TMS system.
Olefin metathesis was then applied to other types of unsaturated natural products including monoterpenes, and eugenol substrates derived from essential oils from cloves, cinnamon, etc. or lignin degradation. Most terpenes contain at least one carbon-carbon double bond either included in a cyclic structure or in a linear isoprenoid structure, which is prone to give olefin metathesis transformation. However, due to the presence of other double bonds or oxygenated functional groups (alcohol or aldehyde), metathesis with these substrates has not been studied extensively [93-95]. There are more examples for which terpene derivatives have been modified on purpose by introduction of an olefinic or alkynyl group prior to a metathesis step. However, direct cross-metathesis from natural terpenes represents a straightforward route to produce new functionalized derivatives, and provides more direct and greener strategies to synthesize known products previously prepared using multistep procedures.
We have investigated the cross-metathesis reaction of citronellal, citronellol and citral with methyl acrylate and methacrylate (Scheme 22) . It is noteworthy that if the cross-metathesis with acrylate destroys the C10 isoprenoid structure of the initial terpenes, cross-metathesis with methacrylate gives birth to new products that belong to the terpene family.
Cross metahesis of 3 monoterpenoids with methyl acrylate and methacrylate.
From our previous experience in cross-metathesis involving electron-deficient olefins, we selected a second generation phosphine-free ruthenium catalyst, namely the Hoveyda type catalyst IV.
Starting from the monoolefins citronellal and citronellol, and methyl acrylate as metathesis partner, the best conditions were obtained with 2 equivalents of acrylate in dimethyl carbonate as solvent with 2 mol% of the Hoveyda second generation catalyst IV at 60°C for 15 h. Under these conditions, complete conversion of the terpene was observed and the products were isolated in 68% yield (Scheme 22). GC and 1H NMR analyses revealed that the sole E-stereoisomer was formed. In the case of the commercially available citral, which is a mixture of (Z)- and (E)-isomers (neral and geranial), full conversion also took place at 80°C in toluene or dimethyl carbonate, but the reaction was not selective and only 44% isolated yield of the desired product was obtained.
It turned out that the conditions used for cross-metathesis with methyl acrylate were not suitable for cross-metathesis with methyl methacrylate as full conversion could not be obtained even at 100°C in toluene or dimethyl carbonate. The best results were obtained when the reactions were carried out in an excess of methyl methacrylate (29 equivalents with respect to the terpene) without additional solvent in the presence of 2 mol% of catalyst IV at 90°C during 8 h. Selective and complete cross-metathesis reactions were observed with citronellal and citronellol in these conditions. Side polymerization of the excess of methyl methacrylate required an additional precipitation step and the desired products were isolated in 70-75% yields (Scheme 22). In accordance with the results obtained from citral with acrylate, the cross-metathesis of this dienic substrate proved to be less selective and the corresponding terpenoid was isolated in only 40% yield after reaction under neat conditions. The products arising from cross-metathesis with methyl methacrylate also presented a E-configuration as shown by NOE NMR experiments.
This study on cross-metathesis of terpenes with acrylates highlights that the trisubstituted isoprenoid double bond is reactive with this type of electron deficient olefins and that the reaction is E-stereoselective. This is not the case with acrylonitrile, which is reluctant for this type of cross-metathesis with terpenes. This strategy is much more direct and atom economic than previous multistep procedures that have been proposed to synthesize the products resulting from citronellol/methyl acrylate  citronellol/methyl methacrylate [97, 98] and citral/methyl methacrylate [99, 100] cross-metathesis.
We then turned our attention to natural products featuring an allylbenzene motif. These are typically eugenol derivatives present in some essential oils or arising from hydrolytic or thermal degradation of lignocellulosic biomass. We first investigated the cross-metathesis of eugenol with methyl acrylate and then extended the study to other electron deficient olefins such as methyl methacrylate, acrylonitrile and amides.
The metathesis of eugenol with 2 equivalents of acrylate in the presence of the second generation Hoveyda catalyst IV (2 mol%) in dimethyl carbonate at 80°C led to complete conversion of eugenol within 3 h. However the reaction was not selective and led to 3 major products: one resulting from the expected cross-metathesis, one arising from tandem metathesis/double bond migration featuring the same number of carbon atoms, and a shorter third one resulting from tandem double bond migration/metathesis (Scheme 23) .
Cross-metathesis of eugenol with methyl acrylate.
The possibility of isomerization via carbon-carbon double bond migration is well known in olefin metathesis with second generation ruthenium catalysts. This type of reaction is favoured in the present case because the resulting styrene products present a double bond conjugated with the phenyl ring. In order to avoid this side reaction, the use of a hydride scavenger inhibiting the formation of metal hydride species responsible for catalytic isomerization is recommended. 1,4-Benzoquinone has been extensively used to prevent this reaction, which often takes place in the presence of olefin metathesis ruthenium catalysts. Using 5 mol% of benzoquinone under the previous reaction conditions, the formation of the expected methyl (4-hydroxy-3-methoxyphenyl)-4-but-2-enoate became selective with a ratio of this product to its isomerized isomer of 91:9 leading to 78% isolated yield.
Eugenol products arising from cross-metathesis with various electron-deficient olefins.
They can be summarized as follows:
for methyl acrylate: 2 mol% of Hoveyda catalyst IV in dimethyl carbonate at 80°C with 5 mol% of 1,4-benzoquinone;
for methyl methacrylate: 2 mol% of Hoveyda catalyst IV without solvent (30 equivalents of methacrylate) at 90°C with 5 mol% of 1,4-benzoquinone;
for acrylonitrile: 2 mol% of Hoveyda catalyst IV in diethyl carbonate at 100°C with slow addition of the catalyst in the presence of 5 mol% of 1,4-benzoquinone;
for amides: 2 mol% of Hoveyda catalyst IV in dimethyl carbonate at 80°C with slow addition of the catalyst in the presence of 5 mol% of 1,4-benzoquinone.
The new disubstituted olefins resulting from these cross-metathesis reactions exhibited the expected stereochemistry. They were all E-isomers except the nitrile derivatives, which contained both Z- and E-isomers with a Z/E ratio of 2:1.
Following our works on cross-metathesis from natural products, we explored the possibility of associating two RL transformations in a sequential manner to produce conjugated bio-sourced dienes. Indeed, we have recently shown that [Cp*Ru(MeCN)3][PF6] catalyzed elimination reactions from allylic carbonates to generate dienes via an allylic activation/elimination mechanism . Thus we applied a 2 step strategy involving first cross-metathesis with allylic chloride followed by HCl elimination as shown in Scheme 25.
Sequential procedure to produce 1,3-dienes.
The cross-metathesis step was achieved using 6 equivalents of allylic chloride in refluxing dichloromethane in the presence of 2 mol% of the Umicore M51 catalyst VIII . Under these conditions, no isomerization of fatty aliphatic chain and only trace amounts of self-metathesis of the natural substrate were observed. Five natural products and two allylic chlorides were used to exemplify the generality of the transformation. 1-Decene, methyl 10-undecenoate and 10-undecenenitrile as fatty acid derivatives, citronellal as a terpene and eugenyl acetate were cross-metathesized with allyl and 2-methylprop-2-enyl chlorides. The corresponding internal olefins were isolated in good yields as mixtures of Z- and E-isomers with the E-isomer as the major product (Scheme 26).
Allylic products from cross-metathesis of allylic chlorides with natural products.
The elimination reaction was then studied in details from methyl 12-chlorododec-10-enoate and we showed that the reaction could be performed at room temperature in acetonitrile in the presence of 5 mol% of [Cp*Ru(MeCN)3][PF6] as catalyst and 1.2 equivalent of Cs2CO3 as a base. The terminal dienes were isolated in good yields (Scheme 27). For the dienes resulting from elimination from unsubstituted allylic chlorides, the internal double bond exhibited both E and Z-configuration with E/Z ratio of 8 to 1, except in the case of the eugenol derivative where the internal double bond conjugated with the phenyl ring presented only the E-configuration. For the other dienes obtained from 2-methylprop-2-enyl chloride in the first metathesis step, only the E-isomers were obtained in a highly stereoselective manner .
Formation of terminal dienes from allylic chlorides.
This catalytic sequence requires the elimination of the solvent of the metathesis step which is not suitable for the elimination reaction, where the presence of the coordinating acetonitrile is required.
However, this protocol appears to be quite general as far as the substrate contains a removable hydrogen atom at the α-carbon of the initial olefin, and it can be applied as a green solution liberating only HCl trapped by the carbonate base as by product to produce conjugated terminal dienes.
The above results show that several commercially available alkylidene-ruthenium catalysts allow selective, efficient transformations of unsaturated renewables, arising from plant oils, via cross-metathesis with electron deficient olefins: acrylates, methacrylates, acrylonitrile, acrylamides. They provide access directly to bifunctional linear products without C=C bond isomerization, especially they allow the access to a variety of nitrile esters via cross-metathesis with acrylonitrile and, after hydrogenation with the ruthenium catalyst, to linear amino acid derivatives, useful as polymer precursors. The introduction of catalyst solution via syringe pump allows to reach high TON. Several routes from alkenes to functional conjugated dienes have been established via enyne metathesis of renewable alkenes with alkyne, and via cross-metathesis with simple allylic chlorides followed by ruthenium-catalyzed dehydrohalogenation. Olefin metathesis opens new opportunities allowing access to cyclic molecules with olfactory properties from natural products and giving value to natural terpenes and eugenol via cross-metathesis without isomerization.
There is no doubt that further investigation in cascade, tandem and sequential catalytic transformations of renewables involving at least one metathesis step will provide chemical industry with new atom economic and sustainable processes.
The authors thank IFP Energies nouvelles, Arkema company, the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement No. 241718 EuroBioRef, ADEME, The Region Bretagne, the bilateral France-Tunisia PHC-UTIQUE No. 09G1203, CNRS, the French Ministry of Research, the University of Rennes 1, for support. Thanks also go to Umicore Precious Metals Chemistry, Hanau (Germany) for loan of metathesis catalysts.
The authors wish to acknowledge all the students and researchers from academy and industry, whose names appear in the Rennes Catalysis team references for their contribution to the development of this topic.
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Cite this article as: P.H. Dixneuf, C. Bruneau and C. Fischmeister (2016). Alkene Metathesis Catalysis: A Key for Transformations of Unsaturated Plant Oils and Renewable Derivatives, Oil Gas Sci. Technol 71, 19.
Tandem catalytic cross metathesis of fatty esters with acrylonitrile and hydrogenation.
|In the text|
Cross metathesis of methyl ricinolate 22 with acrylonitrile and methyl acrylate.
|In the text|
Homometathesis of 10-undecylenic aldehyde 26 and hydrogenation of dialdehyde 27.
|In the text|
One-pot ethenolysis/ene-yne cross-metathesis of methyl oleate with propargyl carbonate.
|In the text|
Eugenol products arising from cross-metathesis with various electron-deficient olefins.
|In the text|
Allylic products from cross-metathesis of allylic chlorides with natural products.
|In the text|