Organometallic derivatives of 2,6-di-tert-butylphenols as specific inhibitors of methyl oleate oxidation

Article abstract of DOI:10.1023/A:1011398109034

2,6-Di-tert-butylphenols containing the Pt-SnCl₃ and Pt-GeCl₃ groups in the para position exert a dual effect on the oxidation of methyl oleate by molecular oxygen. Initially, these compounds act as antioxidants producing the corresponding phenoxyl radicals whose decomposition is accompanied by elimination of SnCl₂ and GeCl₂, which are oxidation promoters.

Full text of DOI:10.1023/A:1011398109034

Russian Chemical Bulletin, International Edition, Vol. 50, No. 6, pp. 1097—1100, June, 2001
1097
Organometallic derivatives of 2,6-di-tert-butylphenols
as specific inhibitors of methyl oleate oxidation
E. R. Milaeva,a« D. B. Shpakovsky,a Yu. A. Gracheva,a N. T. Berberova,b and M. P. Egorovc
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: +7 (095) 939 5546. E-mail: milaeva@org.chem.msu.su
bAstrakhan´ State Technical University,
16 ul.Tatishcheva, 414025 Astrakhan´, Russian Federation.
Fax: +7 (851 2) 25 6427. E-mail: berberova@astu.astranet.ru
cN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. E-mail: mpe@cacr.ac.ru
2,6-Di-tert-butylphenols containing the Pt—SnCl3 and Pt—GeCl3 groups in the para
position exert a dual effect on the oxidation of methyl oleate by molecular oxygen. Initially,
these compounds act as antioxidants producing the corresponding phenoxyl radicals whose
decomposition is accompanied by elimination of SnCl2 and GeCl2, which are oxidation
promoters.
Key words: 2,6-di-tert-butylphenols, phenoxyl radicals, organometallic compounds, oxi-
dation of olefins, antioxidants, methyl oleate, hydroperoxides.
Organic derivatives of sterically hindered phenols are
widely used in industry as antioxidants.1 The efficiency
of 2,6-di-tert-butylphenols as inhibitors of oxidative de-
struction of hydrocarbons is determined by the nature of
the group located in the para position with respect to
the OH group.2 On the other hand, metal salts and
complexes are known to catalyze oxidation of organic
substrates.3 Previously, we have demonstrated4—8 that
the systems obtained by introduction of sterically hin-
dered phenol into the ligand environment of metal
complexes, which exhibit catalytic activity in oxidation
of different organic compounds by molecular oxygen,
possess polyfunctional properties. For example, metallo-
phthalocyanines containing the 2,6-di-tert-butylphenol
fragments in the macroring act as either catalysts or
inhibitors of oxidation depending on the nature of the
metal atom, the solvent, pH, and the concentration of
the catalyst. At the same time, their synthetic precur-
sors, viz., (3,5-di-tert-butyl-4-hydroxyphenyl)phthalo-
nitriles, are efficient antioxidants in thermooxidative
destruction of polyethylene and an oligohexene oil.
Properties of 2,6-di-tert-butylphenols, which contain
metal atoms in the para
tylphenol of general formula 1 on the oxidation of
methyl oleate by molecular oxygen and compared them
with 2,6-di-tert-butylphenol.
We chose methyl oleate as the substrate for, at least,
two reasons. First, this compound, being a representa-
tive of unsaturated fatty acids, serves as a model in
studies of in vitro oxidative destruction of food oils and
pharmacological preparations. Second, methyl oleate is
used in studies of the oxidative stress caused by in vivo
lipid peroxidation.9,10 In this connection, a search for
stabilizers (antioxidants) with nontrivial molecular struc-
tures possessing a combination of inhibiting and pro-
moting properties could serve as the basis for the con-
struction of systems with the variable controlled action
effect.
Experimental
Methyl oleate (Sigma, 99%), 2,6-di-tert-butylphenol
(Merck), and SnCl2 (Aldrich) were used without additional
purification. Compounds 1a—c 11,12 and GeCl2 (the complex
with dioxane)13 were prepared according to procedures de-
scribed previously. Methyl oleate was oxidized by oxygen at
50 °C with continuous air supply using a temperature-con-
trolled apparatus protected against light. Since the reaction was
carried out as autooxidation without the addition of initiators,
air was bubbled through methyl oleate placed in the tempera-
ture-controlled apparatus for 2 h prior to the addition of
compounds and the mixtures under investigation. In all cases,
position, as antioxidants
(or as catalysts of oxi-
dation) are as yet virtu-
ally unknown.
In the present work,
we studied the effect of
organometallic deriva-
tives of 2,6-di-tert-bu-
But
HO
But
PPh3
Pt
X
PPh3
–3
the concentrations of the additives were 1•10 mol L–1. The
1a—c
concentrations of hydroperoxides were determined (the accu-
racy was ±0.01 mmol L–1) by iodometric titration according to
X = Cl (a), SnCl3 (b), GeCl3 (c)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1049—1052, June, 2001.
1066-5285/01/5006-1097 $25.00 © 2001 Plenum Publishing Corporation

1098
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 6, June, 2001
Milaeva et al.
–1
a standard procedure.14 The reactions were carried out for 5 h,
and the samples were withdrawn every 1 h. The rate constants
of hydroperoxide accumulation were determined from the loga-
rithmic anamorphoses of the kinetic curves of product accumu-
lation constructed by the least-squares method. The approxi-
mation coefficients of the linear dependences were 0.978—0.998.
C•10–3/mol L
25
1
a
20
Results and Discussion
15
10
5
Oxidation of oleic acid and its methyl ester by
molecular oxygen follows the general regularities of
liquid-phase oxidation of hydrocarbons15 and belongs to
chain radical processes.
2
The mechanism of oxidation of methyl oleate (RH)
by molecular oxygen has been studied in sufficient
detail.16 It involves generation of the corresponding
substituted allylic radical followed by the formation of
cis and trans isomers of hydroperoxides (ROOH) con-
taining the hydroperoxide groups at positions 8, 9, 10,
and 11, respectively. The addition of efficient proton
donors, for example, of 2,6-di-tert-butylphenols, can
lead to a change in the ratio between isomeric prod-
ucts,17 but does not prevent determination of their
overall concentrations.
In the present work, the activities of compounds
1a—c as antioxidants, which differ substantially from
the known inhibitors based on 2,6-di-tert-butylphenol
in that the former contain metal atoms, were studied for
the first time using oxidation of methyl oleate as an
example. The concentration of methyl oleate hydroper-
oxides was used as the criterion for the efficiency of the
compounds under study.
3
0
5
10
15
τ•10–3/s
C•10–3/mol L
–1
4
40
b
30
20
10
1
3´
2´
0
5
10
15
τ•10–3/s
The kinetic curves of ROOH accumulation over 5 h
in the absence of additives (curve 1) and in the presence
of compound 1a and 2,6-di-tert-butylphenol (curves 2
and 3, respectively) are shown in Fig. 1, a. It can be
seen that the activity of 1a virtually coincides with that
of 2,6-di-tert-butylphenol. This fact is attributable to
the characteristics of the inhibitors InH responsible for
their efficiency among which are, in the general case,
the energy of the In—H bond and the stability of the
C•10–3/mol L
–1
50
4´
c
40
30
20
10
1
•
resulting In radicals.1,2 The dissociation energies of
the O—H bonds in 4-substituted 2,6-di-tert-butylphenols
3″
2″
–1
are in the range 330—360 kJ mol and increase in the
presence of electron-withdrawing substituents.2 The
Pt(PPh3)2Cl group exhibits weak π-acceptor proper-
ties,18 which are comparable with the effect of the phenyl
ring (the energy of the O—H bond in 2,6-di-tert-butyl-
0
5
10
15
τ•10–3/s
Fig. 1. Kinetic curves of accumulation of methyl oleate hydro-
peroxides without additives (1) and in the presence of: a, 1a (2)
and 2,6-di-tert-butylphenol (3); b, 1b (2´) and equimolar mix-
tures of 1a with SnCl2 (3´) and SnCl2 (4); c, 1c (2″) and
equimolar mixtures of 1a with GeCl2 (3″) and GeCl2 (4´).
–1
4-phenylphenol2 is 337.7 kJ mol ). Consequently, the
Scheme 1
But
PPh3
extremely high stability of the corresponding phenoxyl
radical 2a generated from compound 1a (Scheme 1) is
the significant factor.11
•
1a
O
Pt
Cl
•
–H
PPh3
But
Unlike radical 2a formed upon H abstraction from
phenols 1b,c, phenoxyl radicals 2b,c are unstable and,
2a

Phenol organometallics as antioxidants
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 6, June, 2001
1099
as we have demonstrated previously,12 eliminate SnCl2
and GeCl2 to give radical 2a (Scheme 2).
was also confirmed by the data obtained with the use of
equimolar 1a + SnCl2 or 1a + GeCl2•C4H4O2 mixtures
because in these cases the catalytically active salts were
added at the beginning of the reaction, which led to the
concurrent action of inhibitor 1a and the catalysts and
to the increase in the concentration of hydroperoxides
(see Figs. 1, b and c).
Scheme 2
But
PPh3
Previously, we have demonstrated12 that the rate of
decomposition of radicals 2b,c was ∼103 s–1 and, hence,
this was several orders of magnitude higher than the rate
of formation of hydroperoxides. In this case, no inser-
tion of MCl2 at the Pt—Cl bond in radical species 1a
was also observed.12 Consequently, the high rate of
decomposition of radicals 2b,c is responsible for the
rapid appearance of the antioxidant and the catalyst in
the reaction mixture. In this case, the effects of com-
pounds 1b,c would be expected to be comparable with
those of mixtures of 1a with SnCl2 and GeCl2•C4H4O2,
respectively. The data presented in Figs. 1, b and c and
in Table 1 demonstrate that the rate constants of forma-
tion of hydroperoxides (i.e., the efficiency of the inhib-
iting activity) are somewhat different for the above-
mentioned pairs of the additives. Thus compounds 1b,c
exhibit higher antioxidating activity than the above-
mentioned mixtures. Apparently, this difference can be
explained by the competition of two processes, viz., the
promotion of peroxidation of methyl oleate in the pres-
ence of MCl2 (M = Sn or Ge) and the establishment of
the equilibrium between the antioxidant and the corre-
sponding phenoxyl radical.
•
1b,c
O
Pt MCl3
•
–H
PPh3
But
2b,c
2a + MCl2
M = Sn (b), Ge (c)
In this case, the presence of metal compounds,
which promote oxidation of the substrate,3 in the reac-
tion medium is the critical factor.
The kinetic curves of accumulation of methyl oleate
hydroperoxides in the presence of compounds 1b,c in
comparison with SnCl2 and GeCl2•C4H4O2 as additives
are shown in Figs. 1, b and c, respectively. It can be
seen that these additives actually catalyze oxidation of
methyl oleate, which is manifested in the increase in the
concentration of ROOH compared to that obtained in
the control experiment without additives.
In all cases, the kinetic curves of accumulation of
primary oxidation products of methyl oleate (see
Figs. 1, a—c) follow the exponential law, whereas their
logarithmic anamorphoses are well described by linear
functions, which made it possible to calculate the rate
constants of the first-order total accumulation of hydro-
peroxides (Table 1). Based on these data, compounds
1b,c containing the Pt—Sn and Pt—Ge bonds can be
assigned to inhibitors whose activity is lower than that
of compound 1a. In our opinion, a decrease in the
inhibiting activity of 1b,c can be explained not only by
the lower stability of radicals 2b,c, but also by accumu-
lation of SnCl2 and GeCl2 in the medium as the anti-
oxidants are consumed (see Scheme 2). This conclusion
In the case of compounds 1b,c, the additives change
the effect from the inhibitory to catalytic one, which is
clearly seen from comparison of the concentrations of
the hydroperoxides within 1 and 5 h after their addition
(Fig. 2). In the case of compounds 1a—c, the inhibiting
properties were manifested during the first hour due to
the fact the H atom was readily transferred to the
–1
C•10–3/mol L
30
20
10
0
Table 1. Rate constants of accumulation of me-
thyl oleate hydroperoxides in the presence of
different additives (50 °C, the concentration of
–3
–1
the additives was 1•10 mol L
)
–1
Additive
k•10–4/s
—
1.25±0.12
0.25±0.06
0.37±0.09
0.68±0.03
0.83±0.03
0.81±0.04
0.85±0.04
1.53±0.11
1.59±0.14
2,6-di-tert-Butylphenol
1a
1b
1c
1
2
3
4
5
6
7
Fig. 2. Accumulation of methyl oleate hydroperoxides without
additives (1) and in the presence of 2,6-di-tert-butylphenol (2),
compounds 1a (3), 1b (4), 1c (5), SnCl2 (6), and GeCl2 (7);
the concentrations of ROOH after 1 h are shown by hatching,
the concentrations after 5 h are not hatched (the initial
concentrations of hydroperoxides were in the range of
1a + SnCl2*
1a + GeCl2•C4H4O2*
SnCl2
GeCl2•C4H4O2
–3
* Equimolar mixtures were used.
3.07—3.12•10 mol L–1).

1100
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 6, June, 2001
Milaeva et al.
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peroxide ROO radicals resulting in chain termination.
However, further consumption of the functionally active
inhibitor (1b,c) was accompanied by an increase in the
concentration of the catalyst (SnCl2 or GeCl2), which
led to a change in the mechanism of action of species
involved in this process, and, as a consequence, to a
change in the overall effect of the above-mentioned
compounds. Due to the inhibitor—catalyst transforma-
tion, the concentrations of hydroperoxides in the pres-
ence of inhibitors 1b,c were 1.5—2 times higher than
those in the presence of 1a.
Thus, the results of the present study provide evi-
dence that organometallic derivatives of 2,6-di-tert-bu-
tylphenols are promising nontrivial agents in oxidation
of organic substrates. These compounds are character-
ized by the inversion of the inhibitor—catalyst effect.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
33052), by INTAS (Grant 97-30344), and by the Rus-
sian State Program (the Program "Fundamental Prob-
lems of Modern Chemistry," Project No. 9.3.03).
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Received December 28, 2000