6690
J. Am. Chem. SOC.1994,116, 6690-6696
A Mechanistic Study of Oleate Autoxidation: Competing Peroxyl H-Atom Abstraction and Rearrangement Ned A. Porter,' Karen A. Mills, and Randall L. Carter Contribution from the Department of Chemistry, Duke University. Durham, North Carolina 27708 Received February 22, 1994'
Abstract: The mechanism of methyl oleate autoxidation was investigated. HPLC techniques were developed to analyze the products of autoxidation (hydroperoxides and the corresponding alcohols). The alcohols could be completely resolved by normal-phase chromatography, six products being characterized having oxygen substitution and double position as follows: 1 l-OOH-trans-A+lo, 1 1-00H-cis-A9-lo,10-00H-trans-A~-9,9-00H-trans-A~0-~~,8-00H-trunsA9-10,8-00H-cis-A+loAs the hydrogen atom donor concentration of the medium of autoxidation is increased, increased 1 1-cis, 8-cis, 9-trans,and 10-trans hydroperoxides and decreased 1 1-trans and %trans hydroperoxides were obtained, consistent with a mechanism in which peroxyl H-atom abstraction and [2,3]allylperoxylrearrangement are in competition. An iterative computer kinetic analysis was developed which modeled the oleate autoxidation mechanism, and rearrangement rate constants were determined. Allylperoxyl radicals undergo rearrangement with different rates depending on the geometry of the allylperoxyl.
Autoxidation, the radical chain reaction of molecular oxygen and organic substrates,results in the formationof hydroperoxides.' This reaction has been of interest for many years since it leads to the degradation of natural and synthetic organic materials. Autoxidation of fatty acids and esters in foods leads to rancidity and spoilage, and in vivo autoxidation of these same compounds has been suggested in diverse pathologies.* Studies of the autoxidation of the methyl ester of oleic acid ((2)-9-octadecenoic acid) date back to 1943 in the workof Farmer and SuttonS3These workers suggested that oleate autoxidation products contain a mixture of CS and C11 oleate hydroperoxides in which the hydroperoxy group is attached to an allylic carbon atom. Later, a free radical chain mechanism was proposed: and it was postulated that there is equal probability for the hydroperoxy group to appear at positions 8, 9, 10, and 1 1 and for the olefin to remain at the original position (A9-10) or appear at the two adjacent C-C bonds (AS-9 and AIO-11). During the period of time 1977-1984, powerful analytical methods such as gas chromatography/mass spectrometry and NMR spectroscopy were applied to the problem of fatty acid autoxidation. By the use of these techniques, Frankel et al. reported that autoxidation of methyl oleate occurs symmetrically about the double bond.5 After autoxidation of methyl oleate at 25 O C , yields of the hydroperoxidesweredeterminedby a sequence of thin-layer chromatography, mass spectrometry, and carbon NMR. By this tedious protocol, product distribution was determined to be as follows: 11-trans, 12.9%; 11-cis, 13.7%; 10trans, 21.7%; 10-cis, 1.1%; 9-trans, 23.1%; 9-cis, 1.1%; &trans, 12.3%; &cis, 14.1%. Abstract published in Advance ACS Abstracts, June 1, 1994. (1) For reviews on autoxidation, see: (a) Porter, N. A. Acc. Chem. Res. 1986, 19, 262 (b) Huyser, E. S. Free Radical Chain Reactions; WileyInterscience: New York, 1970; pp 306312. (c) Mayo Acc. Chem. Res. 1968, 1, 193. (c) Porter, N. A. Membrane Lipid Oxidation; Vigo-Pelfrey, C., Ed.; CRC Press: Boca Raton, FL. (2) (a) Frankel, E. N. AutoxidationinFoodandBiologicalSyste~;Simic, M., Karel, M., Eds.; Plenum: New York, 1980; pp 141-183. (b) Niki, E. Organic Peroxides; Ando, W., Ed.;John Wiley & Sons: New York, 1992; pp 763-787. (3) Farmer, E. H.; Sutton, D. A. J. Chem. Soc. 1943, 119. (4) (a) Farmer, E. H.; Koch, H. P.; Sutton, D. A. J. Chem. Soc. 1949,541. (b) Bickford, W. G.; Fisher, G. S.;Kyame, L.; Swift, C. E. J. Am. Oil Chem. Soc. 1948, 24,254. (c) Ross, J.; Gebhart, A. I.; Gerecht, J. F. J. Am. Chem. SOC.1946,68, 1373. (5) (a) Frankel, E. N.; Neff, W. E.; Rohwedder, W. K. Lipids 1977,901. (b) Frankel, E. N.; Garwood, R. F.; Khambay, B. P.; Moss,G. P.; Weedon, B. C. J. Chem. Soc.. Perkin Trans I 1984, 2233.
Scheme 1. Proposed Methyl Oleate Autoxidation Mechanism (1 984) II
10
vR' = CHy(CHzk = (CHI)ICOzCHy
8
CHJ-(CH~)~.W
(CH*)&O~CHJ
Bi R
R'
9
J
Methyl Oleate R
V
R
'
-t
b; R E (CHi)&OzC4; R' I CH,(CHZ)s \R
la & l b
2a&2b
.) O2
.) O2 11-cis
_RH
& (I-c~s
R
c
-R
R'
EL 11-brmr
.
&ahw
R'
These results were explained upon the basis of the mechanism in Scheme 1 . Hydrogen atom abstraction from the oleate 1 1 position was proposed to lead to delocalized radical la, which isomerizes, particularly at elevated temperatures, to give the isomeric allyl radical 2a. Subsequent reaction of the allyl radical la with molecular oxygen was suggested to give the 1 1-cis and 9-trans oleate hydroperoxides, and reaction of 2a was proposed to generate the 1 1-trans and 9-trans oleate hydroperoxides. According to this mechanism, hydrogen atom abstraction from the 8 position would give delocalized radical lb, which would isomerize to allyl radical 2b. Addition of molecular oxygen to lb and 2b generates the 8-cis, 8-trans, and 10-trans oleate hydroperoxides. It would appear that the mechanism shown in Scheme 1 is not correct since, under normal oxygen pressures, the allyl radical is trapped with molecular oxygen at or near the diffusion controlled rate, making the allylperoxylradical the chain carrying Allyl radical isomerizationEthus seems an unlikely competitor with reaction of allyl with oxygen. Indeed, the oleate products ~
~~
(6) Howard, J. A. Advances in Free Radical Chemistry, Academic: New York, 1972; Vol. IV, pp 75-138. (7) We have previouslyreportedin diene autoxidation that dienyl peroxyls, rather the pentadienyl radicals, play a dominant role in determining product distribution, see ref la. (8) Korth, H.-G.; Heinrich, T.; Sustmann, R. J. Am. Chem. SOC.1981, 103,4483.
0002-7863/94/1516-6690$04.50/0 Q 1994 American Chemical Society
J. Am. Chem. SOC.,Vol. 116, No. 15, 1994 6691
Figure 1. Methyl oleate hydroperoxides formed by autoxidation.
7 199 OOH
169
216
m 30 35 40 45
25
186
F i e 2. Diagnostic fragmentation ions for oleate hydroperoxides (conditions: CIMS with CH4/NH3 (96:4)as the reagent gas).
could be accounted for upon the basis of the anticinated chemistry of intermediate peroxyl radicals,1aand we thertLareundertook an investigation of oleate autoxidation to address this point. We report here the results of the study. Results and LHsclrSsion
HPLCAoalysis of Oleate Hydroperoxides. Methyl oleate was autoxidized in air with 5 mol % of the free radical initiator ditert-butyl hyponitrite (DTBN) at 30 OC. After 4 h, the reaction was quenched with the radical inhibitor tri-tert butylphenol (‘M’BP). The extent of oxidation did not exceed 5% in the cases reported here. Six regioisomerichydroperoxidesthat are partially separated by HPLC (UV detection) (Figure 1) are formed in methyl oleate autoxidation. The hydroperoxideswere characterized by HPLC-CIMS (chemical ionization mass spectrometry). By this technique, the hydroperoxides could be analyzed directly, avoiding tedious derivatization stepsS6*The regioisomeric oleate hydroperoxides were identified by diagnostic fragmentation ions that occur via scission a to the hydroperoxy group with Ioss of water (Figure 2). The technique demonstrated sensitivity at microgram to nanogram quantities of hydroperoxide. A typical HPLC chromatogram is presented in the supplementarymaterial. Chromatography was carried out on 5-pm silica with 0.6% isopropyl alcohol in hexane solvent. The site of hydroperoxy group substitution on the chain was identified by the diagnostic mass spectral fragmentation ions (Figure 2), and olefin geometry was readily assigned by ‘H NMR (vinylic vicinal coupling constants J,,,,,,., = 15.4 Hz and Jcis = 11.4 Hz). The first and second eluting HPLC fractions were identified as the 11-cisand 11-trans oleate hydroperoxides. The third peak contains both the 10-trans and 9-trans compounds, and the fifth and sixth eluting fractions were identified as the 8-oleate hydroperoxides, with the 8-cis compound eluting before the 8-trans. Separation of the hydroperoxides is therefore not complete since the 9- and 10-trans compounds coelute on silica. The methyl oleate used in these experiments contained
