Metathesis 9-octadecene

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

Polar-substituted alkenes where the functionality is not attached to a strained ring are considerably more discriminating in their compatibility with metathesis catalysts and as a rule require relatively high catalyst charges. In the aliphatic series, unsaturated esters have received the most attention. Boelhouwer reported in 1972 the metathesis of the ester methyl oleate and its trans isomer, methyl elaidate, with a homogeneous catalyst based on a 1/1.4 molar combination of WCl6/(CH3)4Sn (23). At 70°C and an ester/W molar ratio of 33, near-thermodynamic equilibrium was attained, and 49 and 52% of the respective esters were converted to equal amounts of 9-octadecene and the dimethyl ester of 9-octadecene-1,18-dioic acid. 

These reactions, which are believed to occur predominantly inter-molecularly, are capable of producing intermediates which hold some potential as precursors for important chemical products. For example, metathesis of olive oil, which consists chiefly of triglycerides of oleic acid, produces the glyceride of 9-octadecene-l,l8-dioic acid from which can be obtained, after saponification, acidification, and low-temperature crystallization, the free acid, which can be transformed by intramolecular condensation to civetone. 

Recently, additional catalyst systems which are effective for the metathesis of olefins bearing polar functional groups have been revealed. Nakamura and co-workers found that either WC16 or (C2H50)2MoCl3 in combination with triethylborane were capable of converting c/s-9-octa-decenyl acetate to 1,18-diacetoxy-9-octadecene and 9-octadecene at the rather high temperature of 178°C (89). 

The application of olefin metathesis to fatty acids and related compounds has its starting point in 1972 with the selective transformation of methyl oleate into equimolar amounts of 9-octadecene and dimethyl 9-octadecene-l,18-dioate by Van Dam, Mittelmeijer, and Boelhouwer (Scheme 1) [29]. In this early work, 1-2 mol% of a... 

Since a number of insect pheromones are nonfiinctionalized alkenes, they are potential target molecules for synthesis from petrochemicals by the metathesis reaction. Cross metathesis between unsymme-trical internal alkenes can lead to a complex mixture of products however, in some cases, this reaction provides the easiest method for production of such alkenes. For example, the pheromone for Musca do-mestica, tricosene, has been prepared from 2-hexadecene and 9-octadecene (equation 6). Both of these alkenes are readily available. 

Two approaches to the formation of large rings by metathesis have been attempted. The first approach is demonstrated by the synthesis of 9-octadecen-18-olide by Tsuji by the closure of an a,(o-diene... 

Metathesis has been applied in oleochemistry for many years, but only fairly recently technical realization comes within reach [33, 34]. As typical catalysts, ruthenium carbene complexes of the Grubbs type are applied because of their very high activity (turnover numbers up to 200 000). In principle, oleochemical metathesis can be divided into two different types in self-metathesis the same fatty substrate reacts with itself and in cross-metathesis a fatty substrate reacts with, for example, a petrochemical alkene. The simplest case, the self-metathesis of methyl oleate forms 9-octadecene and dimethyl 9-octadecenedioate. The resulting diester can be used along with diols for the production of special, comparatively hydrophobic, polyesters. An interesting example of cross-metathesis is the reaction of methyl oleate with an excess of ethene, so-called ethenolysis. This provides two produds, each with a terminal double bond, 1-decene and methyl 9-decenoate (Scheme 3.3). 

Olefin metathesis has been used by a number of workers to produce macrocyclic musks. One of the earliest examples is Mol s synthesis of civetone from methyl oleate. This takes advantage of the ideally placed double bond in the starting oleic acid. Unfortunately, the ds-geometry of the olefin is lost on metathesis and a mixture of isomers results. Furthermore, almost half of the weight of the starting material is lost as the unwanted 9-octadecene. Dieckmann cyclisation of the metathetical diester, followed by hydrolysis and decarboxylation, gives a mixture of E- and Z-civetone. The synthesis is shown in Figure 4.59. 

Interesting results have been obtained in the synthesis of biologically active compounds such as insect pheromones. Conventional synthetic routes to these pheromones are often multistep sequences, which make many pheromones too expensive for widespread use [23]. Metathesis offers a shorter, alternative route to pheromone synthesis, generating these compounds in a few steps only. The use of insect sex pheromones is an environmentally friendly, effective, and selective method of pest control. Kiipper and Streck [24] synthesized insect sex pheromones by cross-metathesis reactions between linear olefins. In the presence of the catalyst Re207/Al203, 9-tricosene was synthesized by cross-metathesis of the readily available aUcenes 2-hexadecene and 9-octadecene (Eq. 8). 

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... 

From a synthetic point of view, cross-metathesis reactions are very useful for the production of fine chemicals, which often can hardly be obtained by other means. An example is the synthesis of 1-triacontanol, CH3(CH2)28CH20H, a plant growth stimulant. This synthesis was performed in a relatively simple two-step process by cross-metathesis of methyl erucate with 1-octadecene in the presence of a WCl6/Me4Sn catalyst, equation (7), followed by hydrogenation over a Cu/Zn catalyst of the ester thus obtained [14]. 

Due to the fact that the conversion of oleochemical feedstocks into more valuable chemicals appears especially attractive, an economical metathesis of fats and oils and their derivatives should be considered seriously by the oleochemical industry. It has perspectives especially for production of products with a high added value. As an example, Figure 1 presents a process scheme for the (batch-wise) production of dimethyl octadecene-l,18-dioate from methyl oleate via metathesis in the presence of a supported Re207 catalyst [68]. 

For example, 1 can convert selectively, in 60 min at 25 C, ca. 50% of 500 equiv of ethyl oleate into 9-octadecene and diethyl-9-octadecenedioate (with an initial turnover rate for the conversion of ethyl oleate higher than 800 h" ) (Figure 3). To our knowledge, this is the highest activity reported so far with homogeneous catalysts in the metathesis of Ais type of substrate [4, 9]. 

Figure 3. Yield of 9-octadecene vs. reaction time in the metathesis of ethyl oleate with 1 (ethyl oleate/1 molar ratio = 500, T = 25 C, solvent chlorobenzene (5 mL)).

Further investigation on the metathesis of 1-decene (an expected olefin during the alkane metathesis of -decane) and the hydrogenation of the products formed clearly demonstrated that the distribution resulting from alkane and olefin metathesis completely differs with the same catalyst. If there is no double-bond migration, 9-octadecene and ethylene are expected to be the major primary products. Indeed, these primary products are observed, as the temperature reaches 150°C. However, after just 15 min, Cj to C12 and C13 to C20 olefins are also observed, clearly indicating that some isomerization of double bond occurs leading to several competitive metatheses (Fig. 5). 

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