Cis-Unsaturated fatty acid products by hydrogenation with chromium hexacarbonyl

The use of Cr(CO)₆ was investigated to convert polyunsaturated fats intocis unsaturated products. With methyl sorbate, the same order of selectivity for the formation ofcis-3-hexenoate was demonstrated for Cr(CO)₆ as for the arene-Cr(CO)₃ complexes. With conjugated fatty esters, the stereoselectivity of Cr(CO)₆ toward thetrans, trans diene system was particularly high in acetone. However, this solvent was not suitable at elevated temperatures required to hydrogenatecis, trans- andcis, cis-conjugated dienes (175 C) and nonconjugated soybean oil (200 C). Reaction parameters were analyzed statistically to optimize hydrogenation of methyl sorbate and soybean oil. To achieve acceptable oxidative stability, it is necessary to reduce the linolenate constituent of soybean oil below 1–3%. When this is done commercially with conventional heterogenous catalysts, the hydrogenated products contain more than 15%trans unsaturation. By hydrogenating soybean oil with Cr(CO)₆ (200 C, 500 psi H₂, 1% catalyst in hexane solution), the product contains less than 3% each of linolenate andtrans unsaturation. Recycling of Cr(CO)₆ catalyst by sublimation was carried through three hydrogenations of soybean oil, but, about 10% of the chromium was lost in each cycle by decomposition. The hydrogenation mechanism of Cr(CO)₆ is compared with that of arene-Cr(CO)₃ complexes. Presented in part at Seventh Conference on Catalysis in Organic Syntheses, Chicago, Illinois, June 5–7, 1978.     

Homogeneous hydrogenation of dienes with rhodium complexes

Different Rh complex catalysts were compared for the hydrogenation of methyl sorbate and linoleate in the absence of solvents. At 100 C and 1 atm H₂ the following complexes, RhCl(Ph₃ P)₃ (Ph= phenyl), [RhClNBD]₂ (NBD=norbornadiene) and RhH(CO)(Ph₃P)₃, produced mainly methyltrans-2-hexenoate (34 to 56%). Their diene selectivity was not particularly high as they produced 14 to 41% methyl hexanoate. With RhCl(Ph₃ P)₃ constant ratios between rates of methyl sorbate disappearance and formation of methyltrans-2- andtrans-3-hexenoate indicate approximately the same activation energy for 1,2-addition of H₂ on the Δ4 double bond of methyl sorbate and for 1,4-addition to this substrate. In the hydrogenation of methyl linoleate with RhCl(Ph₃ P)₃, the kinetic curves were simulated by a scheme in which 1,2-reduction was more than twice as important as 1,4-addition of H₂ via conjugated diene intermediates. Although the complexes RhCl(CO)(Ph₃ P)₃ and [Rh(NBD)(diphos)]⁺PF₆ (diphos=diphosphine) were inactive in the hydrogenation of methyl sorbate, they catalyzed the hydrogenation of methyl linoleate at 100 C and 1 atm. Catalyst inhibition apparently was caused by stronger complex formation with methyl sorbate than with the conjugated dienes formed from methyl linoleate.     

Hydrogenation of methyl sorbate and soybean esters with polymer-bound metal catalysts

New polymer-bound hydrogenation catalysts were made by complexing PdCl₂, RhCl₃·3H₂O, or NiCl₂ with anthranilic acid anchored to chloromethylated polystyrene. The Pd(II) and Ni(II) polymers were reduced to the corresponding Pd(O) and Ni(O) catalysts with NaBH₄. In the hydrogenation of methyl sorbate, these polymer catalysts were highly selective for the formation of methyl 2-hexenoate. The diene to monoene selectivity decreased in the order: Pd(II), Pd(O), Rh(I), Ni(II), Ni(O). Kinetic studies support 1,2-reduction of the Δ4 double bond of sorbate as the main path of hydrogenation. In the hydrogenation of soybean esters, the Pd(II) polymer catalysts proved superior because they were more active than the Ni(II) polymers and produced lesstrans unsaturation than the Rh(I) polymers. Hydrogenation with Pd(II) polymers at 50~100 C and 50 to 100 psi H₂ decreased the linolenate content below 3% and increasedtrans unsaturation to 10~26%. The linolenate to linoleate selectivity ranged from 1.6 to 3.2. Reaction parameters were analyzed statistically to optimize hydrogenation. Recycling through 2 or 3 hydrogenations of soybean esters was demonstrated with the Pd(II) polymers. In comparison with commercial Pd-on-alumina, the Pd(II) polymers were less active and as selective in the hydrogenation of soybean esters but more selective in the hydrogenation of methyl sorbate. Presented at ISF-AOCS Meeting, New York, April 1980.     

Homogeneous catalytic hydrogenation of unsaturated fats: Cobalt carbonyl

Homogeneous hydrogenation of unsaturated fats by cobalt carbonyl has been compared with the previously reported catalysis by iron carbonyl. Soybean methyl esters, methyl linoleate and linolenate have been hydrogenated at 75–180C, 250–3,000 psi H₂ and 0.02 molar concn of catalyst. The cobalt carbonyl catalyst is more active at lower temp than iron carbonyl. The partially reduced products are similar to those observed with iron carbonyl, but the reaction differs in showing much less accumulation of conjugated dienes, no selectivity toward linolenate, almost complete absence of monoene hydrogenation to saturates, less double bond migration and moretrans isomerization. No evidence was found for a stable complex between cobalt carbonyl and unsaturated fats as previously observed with iron carbonyl. The rates of hydrogenation/double bond were the same for linoleate and linolenate on one hand, and for alkali-conjugated linoleate and nonconjugated linoleate on the other. Presented at AOCS Meeting in Minneapolis, 1963. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, USDA.     

Photo-activated metal carbonyl complexes as stereoselective catalysts for the homogeneous hydrogenation of dienes

Significantly increased activity of Cr(CO)₆ was achieved for the stereoselective homogeneous hydrogenation of methyl sorbate andtrans,trans-conjugated fatty esters at ambient temperature and pressure by exposing the catalyst to UV irradiation (3500 Å) in a solvent mixture of cyclohexane-acetonitrile (20:1). In this solvent mixture, methyl sorbate was converted quantitatively at ambient conditions into methylcis-3-hexenoate, and methyltrans-9,trans-11-octadecadienoate into methylcis-10-octadecenoate (99.9%). These products are expected by 1,4-addition of hydrogen. Under these conditions no hydrogenation of methyl linoleate occurred. Under the same conditions, cycloheptatriene-Cr(CO)₃ showed lower activity than Cr(CO)₆, and Mo(CO)₆ and mesitylene-Mo(CO)₃ showed no significant activity toward conjugated substrates. When Cr(CO)₆ and Mo(CO)₆ were irradiated at 2537 Å they caused the geometric isomerization of methyl sorbate without hydrogenation, but had no effect on methyl linoleate. A hydrogenation mechanism is proposed for Cr(CO)₆ that involves CH₃CN- and H₂-Cr(CO)₃ complexes as intermediates for the stereoselective 1,4-addition of hydrogen totrans,trans-conjugated dienes.     

Selective conjugation of soybean esters to increase hydrogenation selectivity

Polyunsaturated fatty acid methyl esters of soybean oil (MeSBO) were selectively conjugated as a means of increasing the linolenate selectivity of various homogeneous and heterogeneous hydrogenation catalysts. Kinetics of the conjugation reaction in various solvents indicated that linolenate conjugated 5–8 times faster than linoleate. Selective conjugation of MeSBO with potassiumt-butoxide in dipolar solvents resulted in an increase in linolenate hydrogenation selectivity to 7–8 with Ni and Pd heterogeneous catalysts, and to 7–10 with homogeneous and heterogeneous chromium carbonyl catalysts.Trans-unsaturation in the hydrogenated products was only 1–3% with the chromium carbonyl catalysts, in contrast to 30–39% with the heterogeneous metal catalysts. Triglycerides were readily converted to partial glycerides andt-butyl esters with the potassiumt-butoxide reagent. Presented at the AOCS North Central Section Symposium, March 1980.     

Cr(CO)3-complexed,silica-bonded polyphenylsiloxane as a stereoselective catalyst for hydrogenation of sorbate and soybean methyl esters

A silica-bonded complex was prepared by reacting polyphenylsiloxane with silylated Chromosorb and then with Cr(CO)₆. This complex catalyzed stereoselective hydrogenation of sorbate tocis-3-hexenoate. Soybean methyl esters were hydrogenated at 210 C in cyclohexane to form products high incis unsaturation. The recovered catalyst could be recycled once with methyl sorbate. IR showed decreased Cr(CO)₃ in the recovered catalysts, and the hydrogenation products contained inactive Cr.     

Homogenous catalytic conjugation of polyunsaturated fats by chromium carbonyls

Various arene-Cr (CO)₃ complexes and Cr(CO)₆ are effective soluble catalysts for the conjugation of polyunsaturated fats. Methyl benzoate-Cr(CO)₃ is one of the most active catalysts. The following conjugation levels were obtained: methyl linoleate, 65%; methyl linolenate, 45%; the polyunsaturates in soybean and safflower oils, 73%; and in linseed oil 48%. Conjugated dienes from linoleate were predominantlycis,trans in configuration. Their double bonds were distributed between C₅ and C₁₆ of the fatty acid chain. Hydrogenation and dehydrogenation are side reactions, which seem to limit the yield of conjugated dienes from methyl linoleate. A conjugation mechanism is proposed that involves allyl-HCr(CO)₃ complexes as intermediates undergoing 1,3- and 1,5-hydrogen shifts. Presented at the AOCS Meeting, San Francisco, April 1969. No, Utiliz. Res. Dev. Div., ARS, USDA.     

cis-bond-producing hydrogenation of polyunsaturates catalyzed by polymer-complexed Cr(CO)3 catalysts

cis-Bond-producing chromium carbonyl catalysts were prepared by complexing conventional or macroreticular, styrene-divinylbenzene copolymers or cross-linked poly (vinyl benzoate) with Cr(CO)₆. With one exception, these polymer-Cr(CO)₃ catalysts were as selective as the corresponding homogeneous arene-Cr(CO)₃ complexes for the formation ofcis-monoenes from methyl sorbate and from conjugated, polyunsaturated fatty esters in cyclohexane. Although several of the polymer catalysts were very active when fresh, they all lost activity on recycling. They could not be recycled more than two times before a marked decrease in activity occurred due to loss of Cr, as shown by elemental analysis and infrared absorption in the recovered catalyst. Thermal analysis indicated instability of the polymer complexes at hydrogenation temperatures.     

Selective homogeneous hydrogenation of triunsaturated fats catalyzed by tricarbonyl chromium complexes

The need for a selective catalyst to hydrogenate linolenate in soybean oil has prompted our continuing study of various model triunsaturated fats. Hydrogenation of methylβ-eleostearate (methyltrans,trans,trans-9,11,13-octadecatrienoate) with Cr(CO)₃ complexes yielded diene products expected from 1,4-addition (trans-9,cis-12- andcis-10,trans-13-octadecadienoates). Withα-eleostearate (cis,trans,trans-9,11,13-octadecatrienoate), stereoselective 1,4-reduction of thetrans,trans-diene portion yielded linoleate (cis,cis-9,12-octadecadienoate). However,cis,trans-1,4-dienes were also formed from the apparent isomerization ofα- toβ-eleostearate. Hydrogenation of methyl linolenate (methylcis,cis,cis-9,12,15-octadecatrienoate) produced a mixture of isomeric dienes and monoenes attributed to conjugation occurring as an intermediate step. The hydrogenation ofα-eleostearin in tung oil was more stereoselective in forming thecis,cis-diene than the corresponding methyl ester. Hydrogenation of linseed oil yielded a mixture of dienes and monoenes containing 7%trans unsaturation. We have suggested how the mechanism of stereoselective hydrogenation with Cr(CO)₃ catalysts can be applied to the problem of selective hydrogenation of linolenate in soybean oil. No. Market. Nutr. Res. Div., ARS, USDA.     

Isomerization of unsaturated fatty esters by iron pentacarbonyl. Preparation of iron tricarbonyl complexes of polyunsaturated fats

Iron pentacarbonyl is a powerful isomerization agent of unsaturated fatty esters. Highly conjugated fats are obtained when polyunsaturated fatty esters are treated with an excess Fe(CO)₅ to form complexes followed by decomposition of the complexes with FeCl₃. Iron tricarbonyl complexes were prepared in 80 to 95% yields from methyl linoleate, linolenate and polyunsaturated fatty esters of soybean, linseed and safflower oils by heating at 180–185C with 2 moles Fe(CO)₅ per mole ester under nitrogen pressure. Decomposition of these complexes with FeCl₃ resulted in 90 to 97% conjugation of the polyunsaturated fatty esters mainly in the alltrans configuration. Isolatedtrans unsaturation reached levels of 18 to 30%. Methyl oleate yielded 74%trans unsaturation but no complex of iron carbonyl was obtained. Presented in part at AOCS meeting in Houston, 1965. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Homogeneous catalytic hydrogenation of unsaturated fats: Metal acetylacetonates

Hydrogenation of linseed and soybean methyl esters was achieved at 100–180C, 100–1000 psi H₂ and 0.05–0.25 moles catalyst per mole of ester. The relative activity of metal acetylacetonates in decreasing order was: nickel (III), cobalt (III), copper (II) and iron (III). Reduction occurred readily in methanol solution but only slowly in dimethylformamide and acetic acid. No reduction occurred in the absence of solvents. Soybean oil was also hydrogenated rapidly with nickel (III) acetylacetonate in methanol, but in this system the triglycerides were converted to methyl esters. Nickel (III) acetylacetonate was the most selective catalyst toward linolenate hydrogenation. Methyl linoleate and linolenate hydrogenated with nickel(III) acetylacetonate were fractionated into monoenes, dienes and trienes. Thecis monoenes separated in 62 to 68% yield had double bonds in the original position. The remainingtrans monoenes had extensively scattered unsaturation. The dienes and trienes showed no conjugation, but some of the double bonds in the dienes were not conjugatable with alkali. Little stearate was formed. Presented at AOCS meeting in Chicago, 1964 No. Util. Res. and Dev. Div. ARS, USDA     

Selektivhydrierung von Fettstoffen mit metallorganischen Mischkatalysatoren nach Ziegler-Sloan-Lapporte IV: Isomerenverteilung während der Hydrierung mehrfach ungesättigter Fettsäuremethylester

Selective Hydrogenation of Fats and Derivatives Using Ziegler-Type Organometallic Catalysts IV: Distribution of Isomers during Hydrogenation of Polyunsaturated Fatty Acid Methylesters Hydrogenation of methyl linoleate using a Ziegler-type catalyst, containing nickel stearate and triethyl aluminium, proceeds mainly without previous conjugation or trans-isomerization. Both olefinic double bonds are hydrogenated with equal probability. As long as the reaction mixture contains double unsaturated esters, these compounds are inhibiting hydrogenation and isomerization of single unsaturated esters. During hydrogenation of methyl linolenate there is only less selectivity to formation of methyl linoleate. Intermediate product is a mixture of single and double unsaturated fatty acid methylesters. In the latter compounds after consumption of triple unsaturated esters both double bonds are separated by two or more methylene groups. Polyenic compounds with 1,4-position of olefinic double bonds are preferably hydrogenated than polyenic compounds with greater distance between the double bonds.     

Secondary products from high pressure hydrogenation of crambe oil

Hydrogenation of crambe oil, mainly an α,α’-dierucoyl triglyceride, in the presence of cadmium promoted copper-chromite provides long chain waxes being sought as sperm whale oil replacements. Gas liquid chromatography and gas chromatography-mass spectrometry analyses of secondary products indicate, however, that reduction proceeds rather differently from Adkins-type hydrogenations of triglycerides over copper-chromium oxide catalysts. Monoand diunsaturated alkenes ranging from C₁₆-C₂₄, odd chain lengths included, constitute ca. 1% of the product. Esters of crambe acids with methyl, ethyl, n-propyl, and isopropyl alcohols account for less than 5%. These alcohols and traces of 1,2-propanediol from the hydrogenolysis of glycerol occur in either the head gas or the reaction mixture or both. In contrast to published results for Adkins-type hydrogenations of triglycerides, n-propyl alcohol is far more abundant than isopropyl alcohol or 1,2-propanediol. Isopropyl esters of C-18 acids are not present, and those of C-22 acids constitute no more than 2% of the total esters. Low proportion of isopropyl esters and enrichment of C-22 acids in the secondary products compared with C-18 acids suggest that the acyl substituent at the β-position of glycerol is eliminated during hydrogenation of crambe oil with a Cd−Cu−Cr catalyst. Presented at the AOCS Meeting, Mexico City, April 1974. ARS, USDA.     

Partial separation of polyunsaturated fatty acid esters from FAMEs mixtures by adsorption on silver nitrate-impregnated silica gel

Mixtures of FAMEs derived from soybean and canola oils were fractionated by contacting their hexane solutions with AgNO₃/SiO₂ adsorbents. Methyl linolenate (18∶3) adsorbed most strongly, followed by methyl linoleate (18∶2), on the AgNO₃/SiO₂. Conditions of the extractions (AgNO₃ loading, amount of adsorbent, methyl soyate/hexane solution concentration, use of successive extractions, and methods of adsorbent regeneration) were varied. Under optimal conditions, the 7.0% of 18∶3 in methyl soyate could be reduced to 0.1%. The described process is a simple method for separating a FAMEs mixture into a fraction that is depleted in polyunsaturated FAMEs and one that is enriched.     

Comparison of homogeneous and heterogeneous palladium hydrogenation catalysts

Mechanistic and kinetic studies of Pd-catalyzed hydrogenation at atmospheric pressure and 30–100 C were carried out with methyl sorbate, methyl linoleate and conjugated linoleate. Homogeneous Pd catalysts and particularly Pd-acetylacetonate [Pd(acac)₂] were significantly more selective than Pd/C in the hydrogenation of sorbate to hexanoates, mainlytrans-2-hexenoate. Relative rate constants for the different parallel and consecutive reactions, determined by computer simulation, indicated that the low diene selectivity of Pd/C can be dattributed to a significant direct reduction of sorbate to hexanoate. The similar behavior of PdCl₂ to that of Pd/C suggests that Pd(II) was initially reduced to Pd(O). Valence stabilization of PbCl₂ by adding DMF or a mixture of Ph₃P and SnCl₂ increased the diene selectivity but decreased the activity. Stabilization of Pd(acac)₂ with triethylaluminum (Ziegler catalyst) resulted in increased activity but decreased selectivity. The kinetics of methyl linoleate hydrogenation showed that although Pd(acac)₂ was only half as active as Pd/C, their respective diene selectivity was similar (10.4 and 9.6). The much greater reactivity of conjugated compared with unconjugated linoleate toward Pd(acac)₂ suggests the possible formation of conjugated dienes as intermediates that are rapidly reduced and not detected in the lipid phase during hydrogenation.     

Physical properties of fatty acid methyl esters. II. Refractive index and molar refraction

The refractive indices of methyl oleate, linoleate, linolenate, erucate, and the saturated fatty acid methyl esters from acetate to nonadecanoate have been measured at 20C and 40C for the Na_d, H _α , H _β , H _γ lines. The values for the saturated series have been correlated with the Smittenberg relation. Molar refractions have been computed and checked for additivity. The limiting refractive indices obtained from the Smittenberg relation are compared to those obtained from the molar refraction.     

Analyses of lipids and oxidation products by partition chromatography: Hydroxy fatty acids and esters

A liquid-partition chromatographic procedure was used to separate hydroxy fatty acids, their methyl esters, and reduced fatty ester hydroperoxides. Mixtures of methyl stearate, mono- and dihydroxystearate, and mixtures of the corresponding free fatty acids were easily separated. Chromatographic determinations for ricinoleate in castor oils compared favorably with the chemical and infrared analyses. The chromatographic procedure was used to separate hydroxy fatty acids inDimorphotheca andStrophanthus seed oils. The methyl ester of dimorphecolic acid, the principal hydroxy fatty ester ofDimorphotheca oil, behaved like reduced methyl linoleate hydroperoxide and showed a polarity intermediate between methyl 12-hydroxystearate and methyl 9,10-dihydroxystearate. The 9-hydroxy-12-octadecenoic ester ofStrophanthus oil had a larger retention volume than methyl ous hydroxy fatty esters isolated chromatographically. The diene content of the reduced hydroperoxides agrees well with values reported in the literature (1,5,16). The diene content of the chromatographed methyl dimorphecolate is higher than reported by Smithet al. (20) for their preparations but agrees well with the value reported by Chipault and Hawkins (6) for puretrans-trans conjugated methyl linoleate. The extinction coefficient of methyl 12-hydroxystearate at 2.8 μ is higher than that reported for ricinoleate and the absorption band is much sharper. Because of these two conditions no association of the hydroxyl groups is indicated. These results also confirm the purity of the hydroxy fatty esters obtained by LPC. This method has been a valuable adjunct to the study of various oxygen-containing fatty acid and esters and was used to characterize the hydroxy esters obtained from the hydrogenation of methyl linolenate hydroperoxides (9). This work offers a basis for the development of analytical methods to determine the hydroxy and other polar acid content of fatty glycerides and their derivatives.     

Catalytic isomerization of safflower oil with rhodium complexes

Cationic rhodium (I) complexes of the type [(NBD)RhL₂]⁺ ClO₄ ⁻ (NBD, norbornadiene; L, triphenyl phosphine or diphenyl phosphino ethane) have been studied as catalysts for the isomerization of methyl linoleate and safflower oil. The catalysts gave very good yields of conjugated products with both oil and methyl linoleate. Isomerization could be carried out under very mild conditions (55–65 C, 1 atm N₂). Although the catalyst undergoes transformation in the course of the reaction, it maintains its catalytic activity. In fact, the catalysts isolated from the reaction with safflower oil were recycled with practically no loss of activity.     

Biphasic Hydrogenation of α,β-unsaturated Aldehydes with Hydrosoluble Rhodium and Ruthenium Complexes

Abstract  

The hydrosoluble complexes [Rh(CO)(Pz)(L)]_2, and [RuH(CO)(CH₃CN)(L)₃][BF₄] [L = TPPMS (triphenylphosphinemonosulfonated) and TPPTS (triphenylphosphine-trisulfonated)] were evaluated in the selective hydrogenation of α,β-unsaturated aldhydes in biphasic media The Rh complexes produced the saturated aldehydes while the Ru complexes generated the alcohols with high chemoselectivity. The catalytic phase can be recycled up to five times.