Unsaturated fatty acids metathesis

Polymers Catalytic reactions involving C=C bonds are widely used for the conversion of unsaturated fatty compounds to prepare useful monomers for polymer synthesis. Catalytic C-C coupling reactions of unsaturated fatty compounds have been reviewed by Biermann and Metzger [51]. Metathesis reactions involving unsaturated fatty compounds to prepare co-unsaturated fatty acid esters have been applied by Warwel et al. [52], Ethenolysis of methyl oleate catalyzed by ruthenium carbenes developed by Grubb yields 1-decene and methyl 9-decenoate (Scheme 3.6), which can be very useful to prepare monomers for polyolefins, polyesters, polyethers and polyamide such as Nylon 11. 

Self-metathesis describes the reachon of an unsaturated fatty acid with itself. For example, methyl oleate gives a mixture of starting material (50%), unsaturated hydrocarbon (25%), and long-chain unsaturated diester (25%), aU as a mixture of T-and trani-isomers. (Figure 18). The diester can be converted to the musk component civetone, but a more efficient route is through self metathesis of the ketone oleon derived from methyl oleate by Claisen condensation (104) (Figure 18). 

Metathesis of olefins, through the use of transition metal catalysis, is an important application in the petrochemical as well as in the polymer chemical industry for the production of special olefins and polymers (cf. Section 2.3.3). This chemistry is also applicable to unsaturated fatty acid esters, such as acetic acid methyl ester. However, the high price and unsustainability of the catalysts, compared with the fatty acids as substrates, have made commercial utilization not yet possible nevertheless they are of great interest for researchers in this field. 

The metathesis of unsaturated fatty-acid esters has received considerable attention. Aspects of the metathesis of fatty-acid esters and related compounds were recently reviewed by Boelhouwer.With the WCl6/Me4Sn system, both self-metathesis and co-metathesis (with symmetric alkenes) of unsaturated esters occur, provided that the double bond and the ester group are separated by at least one methylene group.As an example, conversion of C8-C=C-C7-COOC of 50% for self-metathesis and 70% for co-metathesis... 

To prepare 9-decenoic and 13-tetradecenoic acid methylester, we used the transition-metal catalysed olefin metathesis [6, 7], actually applied in petro- and polymerchemistry in seven industrial processes [8], As stated first by C. Boelhouwer et al. [9, 10], this reaction is valid not only for olefins, but for unsaturated fatty acid esters, too. 

Other modifications of vegetable oils in polymer chemistry include the introduction of alkenyl functions, the study of novel polyesters and polyethers and the synthesis of semi-interpenetrating networks based on castor oil (the triglyceride of ricinoleic acid) [42], and also the production of sebacic acid and 10-undecenoic acid from castor oil [44]. Additionally, the recent application of metathesis reactions to unsaturated fatty acids has opened a novel avenue of exploitation leading to a variety of interesting monomers and polymers, including aliphatic polyesters and polyamides previously derived from petrochemical sources [42, 45]. 

Berezin, M. Y., Ignatov, V. M., Belov, P. S., Elev, I. V., Shelimov, B.N., Kazansky, V. B. (1991) Mechanism of Olefin Metathesis and Active Site Formation on Photoreduced Molybdenum Catalysts. 5. Metathesis of Unsaturated Fatty Acid Esters, Kinet. Ratal. 32, 379-389. 

The metathesis of triacylglycerols and unsaturated fatty acid derivatives has emerged as a powerful tool in synthetic organic chemistry. The metathesis approaches most commonly applied in oleochemistry are... 

The most frequently investigated cross-metathesis reaction in the field of oleochemistry is ethenolysis, the cross-metathesis of triglyceride of unsaturated fatty acids with ethylene. As an example, the metathesis of triolein... 

The double bonds that are present in some fatty acids may im-dergo olefin metathesis reactions. For example, the ethenolysis of fatty acid methyl esters in the presence of a Grubbs first generation catalyst was used to synthesize cD-unsaturated fatty acid methyl esters (68,69). 

Also, the cross metathesis reaction between oleyl alcohol and methyl acrylate has been studied (71). From these unsaturated fatty acid esters, several classes of pol nner can be S5mthesized, such as polyolefins, polyesters, and polyethers. 

As a tool for monomer synthesis, the metathesis reaction is applied in fatty-acid modifications. That is, exploitation of the reactions shown in Scheme 5.1 CM, SM, RCM and ROM. However, due to the inherent long, acyclic, aliphatic structure of vegetable oils and their derivatives, and in view of further applications in polymerisation (or any other modification reactions), most studies on application of metathesis systems to unsaturated fatty acids have been based on SM and CM mechanisms (Scheme 5.1a and b) to synthesise the desired ensuing molecules (usually monomers). 

Fatty acid esters are generally obtained from the transesterification of fats and oils with a lower alcohol, e.g. methanol, along with glycerol. More than 90% of all oleochemical reactions (conversion into fatty alcohols and fatty amines) of fatty acid esters (or acids) are carried out at the carboxy functionality. However, transformation of unsaturated fatty acid esters by reactions of the carbon-carbon double bond, such as hydrogenation, epoxidation, ozonolysis, and dimerization, are becoming increasingly of industrial importance. Here we will discuss another catalytic reaction of the carbon-carbon double bond, viz. the olefin metathesis reaction, in which olefins are converted into new products via the rupture and reformation of carbon-carbon double bonds [2]. Metathesis of unsaturated fatty acid esters provides a convenient route to various chemical products in only a few reaction steps. 

Olefin metathesis is a catalytic reaction. A wide range of transition metal compounds will catalyse the reaction, the most important being based on W, Mo, Re and Ru. A problem in the case of functionalized olefins such as unsaturated fatty acid esters is the deactivation of catalytic sites, caused by the complexation of the polar functional group to the active site. This results in turnover numbers that are much lower than those obtained for the metathesis of analogous simple olefins. Relevant catalyst systems will be discussed in Section 4. 

Unsaturated fatty acid esters and oils are very promising and cheap feedstocks for metathesis, which makes the metathesis reaction of interest for the oleochemical industry. The first successful metathesis conversion in this area was the selective transformation of the methyl ester of oleic acid (methyl oleate, methyl cw-9-octadecenoate), a readily accessible unsaturated ester, into equimolar amounts of 9-octadecene and dimethyl 9-octadecene-l,18-dioate, equation (1), using 1-2 mol% of a WCl6/(CH3)4Sn catalyst system at 70°C [3]. 

Because the free enthalpy change in this type of reaction is virtually zero, the result at equilibrium is a random distribution of the alkylidene groups. Thus, starting with methyl oleate, the equilibrium mixture consists of 50 mol% of the starting material and 25 mol% of each of the products 9-octadecene and dimethyl 9-octadecene-1,18-dioate. The cis/trans ratio of the reaction products is also in accordance with thermodynamics. This demonstrates that - in the presence of a suitable catalyst - the metathesis of unsaturated fatty acid esters provides a convenient and highly selective route to unsaturated diesters. Unsaturated diesters are important intermediates for the production of useful chemical products such as macrocyclic compounds. For instance, the diester obtained by metathesis of ethyl oleate has been subjected to a two-step reaction sequence, i.e. a... 

Many other unsaturated fatty acid methyl esters of the general formula Me(CH2) CH=CH(CH2) COOMe have been shown to undergo metathesis with high selectivity, such as methyl palmitoleate (n = 5, m = 7), methyl erucate (n = 1, m = l) and methyl petroselenate ( = 10, m = 4) [8]. Another example is methyl 10-undecenoate, which can be obtained from castor oil via pyrolysis cleavage of the ricinoleic acid. The metathesis of methyl 10-undecenoate proceeds to completion when the by-product ethene is continuously removed during the reaction equation (3). 

Cross-metathesis of an olefinic compound with ethene is called ethenolysis. Ethenolysis of unsaturated fatty acid esters results in the synthesis of shorter-chain co-unsaturated esters, compounds with a broad range of application. Excess ethene can easily be applied (e.g. by use of ethene pressures of 30 bar) to suppress self-metathesis of the ester and to force the conversion to completion. Ethenolysis of methyl oleate produces methyl 9-decenoate and 1-decene [20,21] equation (9). 

Table 1 gives examples of catalyst systems that can withstand to a certain extent the negative effect of the polar group and are able to bring about the metathesis of unsaturated fatty acid esters. The values for the turnover number do not necessarily mean that these are the highest TON that can be reached other reaction conditions might give other values. 

TABLE 1. Examples of catalyst systems for the metathesis of unsaturated fatty acid esters. 

On the other hand, two-component catalyst systems, mainly WOCI4 and WCle in combination with a suitable cocatalyst, have been widely employed on a laboratory scale. These were the first catalysts developed for the metathesis of unsaturated fatty acid esters. Although they are more sensitive to moisture and air, they are cheaper and easier to handle than the alkylidene complexes. In this sense, the use of W(OAr)2Cl4 has... 

At the other hand, recent findings demonstrate that new well-defined homogeneous ruthenium complexes are also attractive as catalyst for the metathesis of unsaturated fatty acid esters and related compounds. Although they are rather expensive and decompose during the reaction, progress is being made to overcome these drawbacks. 

Mol, J.C. Metathesis of unsaturated fatty-acid esters and fatty oils. J. Mol. Catal. 1994, 90, 185-199. 

Matvey, B.B., du Plessis, J.A.K., Vosloo, H.C.M., and Mol, J.C. Metathesis of unsaturated fatty acid esters derived from South African sunflower oil in the presence of a 3 wt% Re207/Si02-Al203/SnBu4 catalyst. J. Mol. Cat. A Chem. 2003,... 

Long chain diacids can also be obtained by metathesis of (D-unsaturated fatty acids, e. g., 10-undecenoic acid (Figure 6). (o -Unsaturated fatty acids of different chain length can be produced by metathesis of e. g. oleic acid with ethylene (7). 

The successful metathesis of methyl oleate has led to the synthesis of many other unsaturated fatty acid esters with high selectivity. The development of homogeneous and heterogeneous catalyst systems and the metathesis of co-unsaturated fatty acid esters such as methyl 10-undecenoate, that proceeds to completion via the continuous removal of the volatile co-product, ethylene, is an example of an industrial process based on these reactions (Figure 25). 

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