Homogeneous catalytic hydrogenation of unsaturated fats: Iron Pentacarbonyl

Abstract

Iron pentacarbonyl is an effective homogeneous catalyst for the reduction of polyunsaturated fats. Hydrogenation of soybean oil and its methyl esters has been achieved at 180C, hydrogen pressures of 100-1,000 psi, and 0.05-0.5 molar concentrations of catalyst. Analyses of partially reduced products show considerable isomerization of double bonds, reduction of linolenate and linoleate with little or no increase in stearate, and aecunmlation of cis, transand trans,trans-eonjugated dienes, and isolated tra~ts monoenes. The unreduced trienes include diene conjugated fatty esters. The noneonjugated dienes contain large amounts of trans and nonalkali conjugatable unsaturation. Considerable scattering of double bonds is evident in different fractions between the C4 and C~6 positions. Complex formation between iron earbonyl and unsaturated fats is also indicated.The course of the homogeneous hydrogenation catalyzed by iron pentacarbonyl appears similar to the heterogeneous catalytic reaction. Metal carbonyls are well known for their isomerizing effects and their ability to form stable complexes with olefins. These homogeneous complexes provide suitable model systems to study the mechanism of catalytic hydrogenation of fats.

Iron Catalysis in Reduction and Hydrometalation Reactions

“…Numerous pioneering contributions on the hydrogenation of alkenes and alkynes were reported in the early 1960s using iron species as catalysts, including [Fe­(CO) 5 ] ( C1 ) − and Fe­(acac) 3 ( C2 ) for the hydrogenation of polyunsaturated oils and fats. However, the main limitations of these reactions were often harsh reaction conditions, the lack of chemoselectivity, and the rather narrow scope of the applications. − In addition to hydrogenation reactions performed with gaseous hydrogen in the appropriate high-pressure apparatus, transfer hydrogenation reactions were conducted using easy-to-handle and cheap liquid hydrogen-donor substitutes such as alcohols and formic acid and are thus helpful alternative procedures for the reduction of unsaturated compounds …”

Use of Transition Metal Complexes as Selective Hydrogenation Catalysts

“…The hydrogenation of methyl lineolate was investigated in the presence of catalysts of the general type (R3Q)2MX2 (R = alkyl or aryl; Q = P, As, or Sb; M = Ni, Pd, or Pt) with or without addition of M'X2 or M'X4 (M' = Si, Ge, Sn, or Pb). In most cases M'X2 or M'X4 was converted to the -M'X:) ligand which could modify the M-H bond formed during hydrogenation, so that the reaction with an olefinic double bond was enhanced (Bailar and Itatani, 1967;Frankel et al, 1967).…”

“…A similar mechanism could be written for Ni(CO)4. Both Fe(CO), and Ni(CO)4 have been used to reduce nitriles to a mixture of primary and secondary amines (Bird, 1967c;Levering, 1964), and in the reduction of the methyl esters of unsaturated fatty acids to saturated and monosaturated esters (Bird, 1967c;Frankel et al, 1964Frankel et al, , 1965Frankel et al, , 1967Frankel et al, , 1968aOgata and Misono, 1964). A comparison of Fe(CO)5 with Co2(CO)8 for this latter reaction shows that, whereas the Fe(CO)5-catalyzed reactions yield both saturated and monoenic esters, Co2(CO)8 hyf-e < C 0) 5 Fe(C0)4…”