Esters unsaturated, metathesis

Hybrid catalysts prepared were also successfully used in metathesis of unsaturated esters. Thus, metathesis of 5-hexenyl acetate proceeded with Mof/MCM-41 in benzene at room temperature with 76% conversion and 100% selectivity. RCM of diethyl diaUyhnalonate under the same conditions gave 60% conversion and 100% selectivity. 

A different task was pursued by the CM of CsA with various maleates 339 [ 148]. The CM demanded in this case the highly active Hoveyda catalyst D, that exhibits potency not reached by the phosphine-containing catalysts C and E. Under the conditions given in Scheme 65, metathesis with maleates 339 led (E)-selectively to the a,/J-unsaturated ester derivatives 340 in high yield. Compounds 340 still demonstrated activity comparable to that of CsA and are thus potential soft drugs via esterase-mediated biotransformation to the corresponding inactive carboxylic acids 341. 

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. 

Tungsten aryloxo complexes have been shown to catalyze the intramolecular metathesis reactions of di- and tri-substituted co-unsaturated glucose and glucosamine derivatives to yield bicyclic carbohydrate-based compounds containing 12- and 14-membered rings [108,214,215]. An example is shown in Eq. 37. The tolerance for amides and esters is noteworthy, as are the yields and the size of the rings that are formed. 

Particularly noteworthy was the rhenium catalysed cross-metathesis of trans-hex-3-ene with vinyl acetate or a,p-unsaturated esters [4]. For example, crossmetathesis of methyl frans-crotonate with frans-hex-3-ene gave the desired cross-coupled product without any self-metathesis of the crotonate (Eq. 2). 

Unsaturated esters and silanes are not the only functionalised alkenes to have been employed as cross-metathesis substrates unsaturated alkyl chlorides [9], silylethers [10] and nitriles have all participated in metathesis reactions utilising... 

The success of the cross-metathesis reactions involving styrene and acrylonitrile led to an investigation into the reactivity of other Ji-substituted terminal alkenes [27]. Vinylboranes, enones, dienes, enynes and a,p-unsaturated esters were tested, but all of these substrates failed to undergo the desired cross-metathesis reaction using the molybdenum catalyst. 

The first published report on the use of this catalyst for the cross-metathesis of functionalised acyclic alkenes was by Blechert and co-workers towards the end of 1996 [37]. This report was also noteworthy for its use of polymer-bound alkenes in the cross-metathesis reaction. Tritylpolystyrene-bound AT-Boc N-al-lylglycinol 18 was successfully cross-metathesised with both unfunctionalised alkenes and unsaturated esters (Eq. 17) (Table 1). 

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. 

The CM of olefins bearing electron-withdrawing functionalities, such as a,/ -unsaturated aldehydes, ketones, amides, and esters, allows for the direct installment of olefin functionality, which can either be retained or utilized as a synthetic handle for further elaboration. The poor nucleophilicity of electron-deficient olefins makes them challenging substrates for olefin CM. As a result, these substrates must generally be paired with more electron-rich crosspartners to proceed. In one of the initial reports in this area, Crowe and Goldberg found that acrylonitrile could participate in CM reactions with various terminal olefins using catalyst 1 (Equation (2))." Acrylonitrile was found not to be active in secondary metathesis isomerization, and no homodimer formation was observed, making it a type III olefin. In addition, as mentioned in Section 11.06.3.2, this reaction represents one of the few examples of Z-selectivity in CM. Subsequent to this report, ruthenium complexes 6 and 7a were also observed to function as competent catalysts for acrylonitrile... 

Starting from simple precursors via stereodefined alkenic esters, and ringclosing metathesis using Grubbs catalyst produced p,y-unsaturated lactones, which could be converted into 2,6-dideoxyhexopyranoses.162... 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

The metathesis of oleochemicals in the presence of ethylene, also called ethenolysis, provides an efficient way to a-oleftns and 0)-unsaturated esters, which are useful intermediates for the synthesis of polymers, fragrances, surfactants, lubricants, and others [51, 52], The ethenolysis of methyl oleate was demonstrated in 1994 by Grubbs et al. using C3 [32], They could reach productive turnovers of 130-140. In 2001, Warwel et al. carried out the ethenolysis of the methyl esters of oleic, erucic, 5-eicosenoic, and petroselinic acids also in the presence of C3 [53]. The reactions were performed at 50°C and 10 bar using 0.025 mol% of catalyst and gave conversions from 58% to 74%. 

The complementary approach to ADMET for the synthesis of plant oil-based polyesters is the SM of fatty acids, esters, or alcohols, followed by classic polycondensation of the generated ot,co-difunctional compounds. In 2001, Warwel and coworkers showed the self-metathesis of different co-unsaturated fatty esters and their subsequent polycondensation in the presence of diols and Ti(OBu)4 or Ca... 

As already noted by Verkuijlen and Boelhouwer in 1974 [29], the SM of highly unsaturated fatty esters produces, among other compounds, considerable amounts of 1,4-cyclohexadiene (1,4-CHD). This fact has been exploited by Mathers et al. for the production of 1,3-cyclohexadiene (1,3-CHD) via metathesis and isomerization reactions of plant oils [141]. For instance, 1,4-CHD was obtained by treatment of soybean oil with C4 and was subsequently isomerized with RuHCl(CO)(PPh3)3. Then, the produced 1,3-CHD was polymerized with nickel(II)acetylacetonate/ methaluminoxane. Interestingly, the polymerizations could be carried out in bulk and using hydrogenated D-limonene as renewable solvent. The polymers thus obtained presented / m around 300°C. 

This cross-reaction is general for unsaturated esters, ketones, aldehydes and amides. In these cases, the dominant product is the cross-product even when the reactions are run with a 1 1 stoichiometry. In general, if one of the cross partners is slow to homodimerize but will take part in metathesis, the reaction is driven to the cross product. This observation holds for a wide variety of electron-deficient (and sterically hindered) olefins. For example, a,/3-unsaturated ketones, aldehydes and amides all undergo clean and efficient cross metathesis reactions, [41] with the dominant product in all cases being the E isomer (Eqs. 6.22 and 6.23). 

On the other hand, our recent study on the highly efficient cross-metathesis of vinyltrialkoxy-and vinyltrisiloxy-silanes with various olefins, for example, with styrene [12] allyl eth [13] and esters [14] as well as octavinylsilsesquioxane [IS] with several olefins have opened a new opportunity for the use of alkene-cross-metathesis in the synthesis of unsaturated organosilicon compounds (see also Refs. [5] and [6]). In this p r new examples of the two reactions involving hetero(N,S,B)organic olefins have been overviewed. 

Despite the fact that polar entities are catalyst poisons, a variety of acyclic olefins containing a heteroatom functional group can undergo metathesis in the presence of a suitable catalyst, although at a high catalyst level. These include unsaturated esters, ethers, ketones, amines, nitriles, halogens, etc. [14]. In particular metathesis reactions - including ethenolysis - of unsaturated fatty esters and fatty oils are of interest, as they have perspectives for the oleochemical industry [15]. 

Mo catalysts like 1 and 2 are also active in the metathesis of functionalized alkenes, such as unsaturated carboxylic esters. 

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