Isomerization monoenes

Christie and Breckenridge (42) describe the application of this column to the isolation and determination of FAs containing trans double bonds in samples of natural and industrial origin. A column (250 X 4.6-mm ID) of NUCLEOSIL 5SA was flushed with 1% ammonium nitrate solution at a flow rate of 0.5 ml/min for 1 h, then with distilled water at 1 ml/min for 1 h. Silver nitrate (0.2 g) in water (1 ml) was injected onto column via the Rheodyne valve in 50-yu.l aliquots at 1-min intervals silver began to elute from the column after about 10 min, and 20 min after the last injection the column was washed with methanol for 1 h, then with 1,2-dichloroethane-dichloromethane (1 1 v/v) for 1 h. For most of the analytical work, the column temperature was maintained at 30°C in a thermostatted oven. 1,2-Dichloroethane-dichloromethane (1 1) (mixture A) at a flow rate of 1.5 ml/min was the mobile phase (detector operated at 242 nm) for the separation of isomeric monoenes, and the same solvents with the addition of 0.5% acetonitrile (mixture B) at a flow rate of 0.75 ml/min were employed for isomeric dienes and trienes. 

Figure 8.3. Formation of isomeric monoenes, and dienes by hydrogenation of linolenate. The conjugatable dienes are hydrogenated into isomeric monoenes. The nonconjugatable dienes or isolinoleate are not hydrogenated and accumulate in the product.

Tricarbonylchromium complexes are useful for 1,4-addition of hydrogen to 1,3-dienes to afford monoenes selectively (40,42,43,44). With 1,4-dienes, isomerization into conjugation precedes hydrogenation. Isolated double... 

To do so, one can take the enthalpy of formation of n -hexane from Pedley, and with the phase independence assumptions in Reference 7, employ the enthalpies of hydrogenation of 1-hexene and 1,5-hexadiene from References 11 and 12 respectively. Alternatively13, one can forget about the first quantity altogether and simply take the difference of the enthalpies of hydrogenation of the diene and twice that of the monoene. This reaction is endothermic by 1.1 1.8 kJ mol-1, a value statistically indistinguishable from the absence of any interolefin interaction in the diene. Relatedly, for the isomeric 1,4-hexadienes 14 and 15, equation 8 may be used. 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

Cyclooctadiene isomers (i.e., 1,5-cod or 1,3-cod) are selectively hydrogenated by [Ru(/74-cod)(/76-C8H1o)] (51) to produce exclusively cyclooctene in THF, under ambient temperature (20 °C) and 1 bar H2 pressure [64]. Again, cyclooctane is only detected when the diene substrate is completely transformed to the monoene. The rate of hydrogenation is higher in case of the conjugated 1,3-cycloocta-diene substrate, whereas isomerization of the non-conjugated 1,5-cyclooctadiene... 

The characteristics of the hydrogenation of norbornadiene, substituted butadienes and conjugated and cyclic dienes are all very similar. In the case of conjugated dienes, there appears to be hardly any isomerization activity, while in the case of 1,4-dienes an isomerization step to form the corresponding 1,3-diene is assumed prior to hydrogenation. The catalyst behavior changes after the diene has been completely converted to the monoene, whereupon the rhodium catalyst resumes its normaF monoene hydrogenation behavior. 

Hay and Morrison (1970) identified the monoenoic positional and geometric isomers in milk fat and determined the amounts of each total acid class and percentage of trans isomers. The geometric and positional isomers of the monoenes are primarily the result of biohydrogenation of polyunsaturated fatty acids in the rumen. Stearate is also produced, and cis-9-18 l accounts for most of the monoenes. The several positional isomers in trans 16 1 and 18 1 are due to the positional isomerization of double bonds which accompanies elaidinization. 

A mixture of palladium chloride and triphenylphosphine effectively catalyzes carboxylation of linoleic and linolenic acids and their methyl esters with water at 110°-140°C and carbon monoxide at 4000 psig. The main products are 1,3-and 1,4-dicarboxy acids from dienes and tricarboxy acids from trienes. Other products include unsaturated monocar-boxy and dicarboxy acids, carbomethoxy esters, and substituted a,J3-unsaturated cyclic ketones. The mechanism postulated for dicarboxylation involves cyclic unsaturated acylr-PdCl-PhsP complexes. These intermediates control double bond isomerization and the position of the second carboxyl group. This mechanism is consistent with our finding of double bond isomerization in polyenes and not in monoenes. A 1,3-hydrogen shift process for double bond isomerization in polyenes is also consistent with the data. 

Transfer hydrogenation of dienes to monoenes 1,5-Cyclooctadiene is selectively reduced to cyclooctene by transfer hydrogenation with isopropanol catalyzed by this metal carbonyl cluster. The first step is isomerization to conjugated diene isomers. 1,5-Hexadiene is reduced under these conditions to frms-3-hexene (19%), os-2-hexene (21%), trans-2-, and cw-3-hexene (56%). Ru3(CO)i2, Os3(CO)12, and Ir4(CO)i2 catalyze isomerization of 1,5-cyclooctadiene, but are less active than Rh6(CO)i6 for transfer hydrogenation. 

Proell, J.M., Mosley, E.E., Powell, G.L., Jenkins, T.C. 2002. Isomerization of stable isotopically labeled elaidic acid to cis and trans monoenes by ruminal microbes. J. Lipid Res. 43, 2072-2076. 

Stereoselective hydrogenation of 1,3-dienes. Hydrogenation of simple acyclic and cyclic 1,3-dienes catalyzed by (arene)Cr(CO), complexes results in highly stereoselective 1,4-addition of hydrogen to produce (Z)-monoenes. Under these conditions 1,4-dienes are isomerized to 1,3-dienes and then reduced to (Z)-monoenes, but 1,5-dienes are not reduced. ... 

The hydrogenation of conjugated dienes to monoalkenes poses questions of both regio- and stereoselectivity and is a function of the metal and the conditions.The distribution of products depends not only on the inherent selectivity of the initial addition process but also on the competition between the diene and the first-formed alkene(s) for the reactive sites. The latter may add hydrogen or be isomerized. If the structure of the diene allows the change, the diene itself may be isomerized. As for alkynes, palladium and nickel catalysts tend to be the most selective to the monoene. 

Figure 13. Partial hydrogenation. The partially hydrogenated intermediate (1) may iead to cis or trans unsaturated or saturated products. D—diene M—monoene S—saturate potentiaiiy isomerized. Formation of M is favored at a tow hydrogen concentration.

The bis-triphenylphosphine nickel halides are not activated by tin (II) halides. However, the iodide, (03P)2Nil2, is an effective catalyst for the hydrogenation of methyl linoleate to the monoene stage. The bromide is less effective, and the chloride has very little catalytic power. None of these nickel compounds has much ability to bring about isomerization. The nickel compounds are unstable in alcoholic solutions, and the experiments reported in Table III were carried out in either benzene, tetrahydro-furan, or toluene. 

As Tables I, II, and III show, isomerization of the esters from the cis forms to the trans forms takes place readily, especially with the platinum complexes. The results will be described in more detail elsewhere. It has also been shown (7) that movement of the double bonds along the chain takes place, and that the monoenate mentioned in the tables is a... 

A monoene such as oleate is not hydrogenated but is isomerized from the cis to the trans configuration. 

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