Metathesis in Oleochemistry

Journal of the Brazilian Chemical Society

Buffon, Regina¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Mol, Johannes C.¹  Universiteit van Amsterdam, Amsterdam, The Netherlands¹ 

关键词: metathesis;    catalysts;    oleochemicals;    esters;   

DOI  :  10.1590/S0103-50531998000100002

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Fats and oils In the next century the availability of fossil organic feedstocks - both as energy sources and for the production of organic chemical raw materials - will gradually decrease. Hence it is important to look for alternatives. These can be found in renewable resources both for energy purposes as well as for raw materials for the chemical industry. In the latter case, much attention has already been given to the use of natural fats and oils of vegetable and animal origin in the oleochemical industry. Oleochemicals are not only produced from renewable resources, but they also have the advantage of a good biodegradability and no net CO2 production. Fats and oils (composed predominantly of glyceryl esters of fatty acids) are important sources both for nutrition and as raw materials for the oleochemical industry. About 15% of the world production of fats and oils (81 million t/a in 1990, 105 million t/a expected in 2000) is used in the oleochemical industry as starting materials for a wide range of chemical products1. The most important group from the point of view of cost-effectiveness is that of the long-chain vegetable oils, such as soybean oil, sunflower oil and new rapeseed oil (all consisting mainly of unsaturated C18 fatty acid chains) and palm oil (containing both C16 and C18 chains). The oleochemical industry prefers a high content of oleic acid (mono-unsaturated C18 fatty acid) because this is an important starting material for many consecutive chemical reactions. Short- and medium-chain fatty oils, such as palm-kernel oil and coconut oil, consist mainly of lauric (C12) and myristic (C14) acid and are important sources for the production of detergents, soaps, emulsifiers, etc. Animal fats, such as tallow (containing 40-45% oleic acid), are also in demand as raw material, mainly because of their low price. Some other vegetable oils are the source of oleochemicals on a smaller scale. An example is castor oil, consisting mainly (85-95%) of ricinoleic acid, which has a wide range of industrial uses. The feedstocks for vegetable oils and fats come from different parts of the world. Soybeans are produced chiefly in the USA, Brazil, China and Argentina, rapeseed in China, Europe, Canada and India, and sunflowers in East & South Europe and Central & South America. Coconut and palm kernel oil come from Southeast Asian Countries. Fatty acid esters are generally obtained by transesterification of natural oils and fats with a lower alcohol, e.g. methanol. Although more than 95% of all chemical reactions (e.g. conversion into fatty alcohols and fatty amines) of fatty acid esters (or acids) are carried out at the carboxy function, transformation of unsaturated fatty acid esters by reactions of the carbon-carbon double bond, such as hydrogenation, epoxidation, ozonolysis, hydroformylation and dimerization, are becoming increasingly of industrial importance. Metathesis is another reaction of the carbon-carbon double bond.MetathesisOlefin metathesis is a catalytic exchange reaction between two alkene molecules during which two alkylidene groups are interchanged. For normal olefins this can be represented by reaction (1).A simple example is the metathesis of propene (R = H; R' = CH3) into ethene and but-2-ene. The forward reaction is often called self-metathesis, while the reverse reaction, i.e. a reaction between two different alkene molecules, is called cross-metathesis. In fact, many olefinic substrates can undergo metathesis in the presence of a suitable catalyst, resulting in a wide variety of possible products. These substrates include acyclic alkenes, dienes, polyenes, cyclic alkenes, and also functionally substituted alkenes, such as unsaturated esters, nitriles, halogens etc. At the moment there are various commercial applications of olefin metathesis: (i) the production of polymerisation-grade propene via cross-metathesis between ethene and but-2-ene; this is, in fact, the reverse process of the first industrial application of the olefin metathesis reaction, viz. the metathesis of propene into high-purity ethene and but-2-ene, called the Phillips Triolefin Process, which was in operation from 1966 till 1972; (ii) the production of neohexene (an important intermediate in the manufacture of synthetic musks) via cross-metathesis of di-isobutene with ethene; (iii) the Shell Higher Olefins Process (SHOP), a large-scale industrial process incorporating olefin metathesis, for converting ethene to detergent-range olefins; (iv) the production of several kinds of polymers produced via the metathesis of cyclic olefins, such as cyclooctene, norbornene, and dicyclopentadiene. A promising synthetic application for the metathesis reaction concerns the metathesis of acyclic olefins containing heteroatom functional groups. This would allow single-step syntheses of various mono- and difunctional derivatives of hydrocarbons with well-defined structures. Considering the extensive possibilities offered by the metathesis of unsaturated hydrocarbons within chemical synthesis, it is not surprising that much research is being carried out into the development of active and selective catalysts for this reaction. The metathesis reaction can be catalysed by both heterogeneous and homogeneous catalysts. A wide variety of transition metal compounds will catalyse the reaction, the most important ones being based on W, Mo, Re and Ru. Heterogeneous catalysts generally consist of a transition metal oxide, or an organometallic precursor, deposited on a high-surface-area support (an inorganic oxide). Examples are Re2O7/Al2O3, MoO3/SiO2, WO3/SiO2, and (p-C4H7)4W/SiO2. In particular a supported Re2O7 catalyst shows pronounced activity and high selectivity even at room temperature; when promoted with R4Sn (R = Me, Et, Bu) it becomes also active for the metathesis of unsaturated esters and other functionalized olefins. Homogeneous catalysts mainly consist of (i) a combination of the transition metal compound (usually a chloride, such as WCl6, WOCl4 and ReCl5) and an organometallic compound, e.g. R4Sn (R = alkyl) or EtAlCl2, as cocatalyst, or (ii) a well-defined alkylidene (carbene) complex of a transition metal, e.g. Ru(=CHCH=CPh2(Cl)2 (PCy3)2. Only a relatively small number of the many metathesis catalyst systems are able to bring about the metathesis of functionalized olefins; these will be discussed in Catalysts section. Metathesis is a transalkylidenation reaction and it is generally accepted that the reaction proceeds via the so-called metal-carbene mechanism. The propagation reaction involves a transition-metal carbene as the active species with a vacant co-ordination site at the transition metal. The alkene co-ordinates at this vacant site, and subsequently a metallacyclobutane intermediate is formed. The metallacycle is unstable and cleaves to form a new metal carbene complex and a new alkene, reaction (2). The initial metal carbene can be formed by a reaction between the catalyst and the cocatalyst, if present, or by interaction of the substrate alkene with the transition metal centre. When using an actual metal carbene as catalyst a metal-carbene forming step is, of course, not necessary. A detailed monograph dealing with all aspects of olefin metathesis has recently appeared2.  Metathesis of Unsaturated Fatty Acid EstersSelf-metathesisUnsaturated fatty acid esters and fatty oils are very promising and cheap feedstocks for metathesis. For this reason, the metathesis reaction is of interest to the oleochemical industry. The first successful metathesis conversion in this area was the selective transformation of methyl oleate (methyl cis-octadec-9-enoate), a readily available ester, into equimolar amounts of octadec-9-ene and dimethyl octadec-9-enoate in the presence of a WCl6/Me4Sn catalyst system3, reaction (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 octadec-9-ene and dimethyl octadec-9-enoate. 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 can be used 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 Dieckmann condensation followed by hydrolysis-decarboxylation to give 9-cycloheptadecen-1-one, whose cis form, civetone, is an important base material in the perfume industry4,5, reaction (4). Moreover, unsaturated dicarboxylic esters and acids are interesting starting materials for the manufacture of polyesters and polyamides6.On the other hand, the product octadec-9-ene can be dimerized and hydrogenated to 10,11-dioctyleicosane, a lube-oil range hydrocarbon intermediate7.Many other unsaturated fatty acid methyl esters of the general formula Me(CH2)nCH=CH(CH2)mCOOMe have been shown to undergo metathesis, such as methyl palmitoleate (n = 5, m = 7), methyl erucate (n = 7, m = 11) and methyl petroselenate (n = 10, m = 4). Another example is methyl undec-10-enoate, which can be obtained from castor oil via pyrolysis cleavage of the ricinoleic acid. This reaction proceeds to completion when the by-product ethene is continuously removed during the reaction, reaction (5). It should be noted that olefins with an OH-containing functional group, such as COOH and CHO, deactivate most catalysts which are active for the metathesis of esters8.For the metathesis of methyl oleate a very pure substrate is required. In an alternative process for the synthesis of civetone, methyl oleate is first converted to the doubly-unsaturated ketone pentatriaconta-9,26-dien-18-one, oleon (1), which can be separated in pure form from the reaction mixture. Oleon is then converted into 9-cycloheptadecen-1-one (2) via an intramolecular metathesis reaction (see Scheme 1), at room temperature in the presence of a Re2O7 catalyst supported on SiO2-Al2O3 and promoted with Bu4Sn. To reduce the possibility of intermolecular metathesis between two oleon molecules it is necessary to carry out the reaction under high dilution conditions9.    Metathesis of polyunsaturated fatty acid esters, e.g. methyl linoleate and methyl linolenate, leads to a variety of reaction products, including polyenes, monoesters, diesters and cyclopolyenes10.Cross-metathesisCross-metathesis of unsaturated fatty acid esters with a normal alkene is an elegant way of synthesising homologues of these esters, and greatly extends the versatility of the metathesis reaction in the field of oleochemistry. From most industrial vegetable oil crops fatty acid esters are obtained with a predominant chain length of 18 carbon atoms. Shortening these esters to medium-chain fatty acid esters (C10-C14) is possible via cross-metathesis with lower olefins, see e.g. reaction (6)11-13.A large excess of the normal alkene can force the reaction to the product side. Fatty acid derivatives of medium chain length - especially C12 - are in high demand for the industrial production of surfactants. With regard to their chain length these esters are quite similar to those of the fatty acids derived from palm kernel oil and coconut oil. The by-product alkenes, with the double bond near the end of the chain, can be used for example for the production of C12-C14 alcohols by hydroformylation. On the other hand, instead of shortening the carbon chain of unsaturated esters, it is possible to lengthen it, as illustrated for the cross-metathesis between methyl undec-10-enoate and hex-3-ene in Eq. 7.From a synthetic point of view, cross-metathesis reactions are very promising in opening new routes for the synthesis of derivatives that often can hardly be obtained by other means. An example is the is the synthesis of 1-triacontanol, CH3(CH2)28CH2OH, a plant growth stimulant, in a relatively simple two-step process by cross-metathesis between methyl erucate and octadec-1-ene in the presence of a WCl6/Me4Sn catalyst, reaction (8), followed by hydrogenation of the ester thus obtained14.Another example of organic synthesis via cross-metathesis is the synthesis of biologically active unsaturated compounds such as insect pheromones. Use of such pheromones offers an effective and selective pest control method. Thus, cross-metathesis of ethyl oleate with dec-5-ene results in a cis-trans mixture of ethyl tetradec-9-enoate, a pheromone precursor15. Ethyl tetradec-9-enoate is also obtained by cross-metathesis of mixtures of unsaturated C18 ethyl esters (oleic, linoleic, linolenic) derived from olive, sunflower or linseed oil with excess dec-5-ene16. Other examples are summarised elsewhere8,17. Cross-metathesis of an unsaturated ester with a cyclic olefin leads to long-chain linear di-unsaturated esters. Thus, 1-triacontanol is also obtained by cross-metathesis between methyl oleate and cyclododecene, reaction (9), followed by hydrogenation of the unsaturated diester product18.EthenolysisCross-metathesis of an olefinic compound with ethene is called ethenolysis. Ethenolysis of unsaturated fatty acid esters allows the synthesis of shorter-chain w-unsaturated esters which have a broad range of applications. An excess of ethene can easily be applied (e.g. by using an ethene pressures of 30- 50 bar) to suppress self-metathesis of the ester and to force the conversion to completion. The ethenolysis of methyl oleate will lead to methyl dec-9-enoate, together with dec-1-ene19,20, reaction (10).Methyl dec-9-enoate is the hypothetical source of many polymers and copolymers; it can be converted e.g. into the w-amino acid, and then used for the production of nylon-10. It is an interesting chemical building block for the synthesis of relevant chemical products; after hydrolysis and hydrogenation it yields decanoic acid or decanol, substances used in lubricants and plasticizers. In addition, fragrances can be obtained (such as 9-dec-1-enol and civetone), as can pheromones, prostaglandins etc.8,17, which can easily be isolated in pure form. Dec-1-ene, like other alk-1-enes, is an important intermediate in organic syntheses, and has a variety of end uses in polymers, surfactants and lubricants. Ethenolysis of methyl erucate gives another w-unsaturated ester, methyl tetradec-13-enoate, which could have applications analogous to those of methyl dec-9-enoate. For efficient production of the diester of methyl oleate a two-step process can be considered21. First, methyl oleate undergoes ethenolysis to dec-1-ene and methyl dec-9-enoate; high conversions can be obtained by using a high ethene pressure. After product separation, methyl dec-9-enoate undergoes self-metathesis to ethene and dimethyl octadec-9-enoate. In the latter

【 授权许可】

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