Multifunctional Antioxidants with High Activity at Elevated Temperatures Based on Intramolecular Synergism

Article abstract of DOI:10.1002/ejoc.201701577

Multifunctional antioxidants with three active fragments were prepared through a simple two-step synthesis process. These novel compounds exhibited higher efficiency when inhibiting the oxidation of hydrocarbons compared to commercial products at 180 °C.

Full text of DOI:10.1002/ejoc.201701577

A Journal of
Accepted Article
Title: Multifunctional Antioxidants with High Activity at Elevated
Temperatures Based On the Intramolecular Synergism
Authors: Shasha Yu and Shenggao Liu
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European Journal of Organic Chemistry
10.1002/ejoc.201701577
COMMUNICATION
Multifunctional Antioxidants with High Activity at Elevated
Temperatures Based On the Intramolecular Synergism
[a]
[a]
Shasha Yu and Shenggao Liu*
Abstract: Multifunctional antioxidants with three active fragments
were prepared through a simple two-step synthesis process. These
novel compounds exhibited higher efficiency on inhibiting the
the rate constant for quenching of peroxyl radicals can be
increased by about four orders of magnitude after positioning
[
3]
[4]
chalcogen substituents (alkylseleno and alkyltelluro groups)
in phenols (4). Likewise, the introduction of alkyltelluro groups
into the arylamine scaffold (5) can also enhance the reactivity
o
oxidation of hydrocarbon than the commercial products at 180 C.
The rational framework gives rise to the unique intramolecular
synergism between arylamine moiety and sulfide moiety that can
scavenge radicals and decompose peroxides simultaneously.
[
5]
towards lipidperoxyl radicals with nearly 100-fold. Numbers of
[
6]
the vitamin derivatives functionalized with nitrogen atoms or
[
7]
chalcogen substituents
were successfully prepared and
performed excellent reactivity towards radicals under mild
conditions. However, the performance of the novel antioxidants
at elevated temperatures would make a difference especially for
the arylamine derivatives. The previous research indicated that
the stoichiometric factors of diphenylamine can sharply increase
Autoxidation is a destructive, inevitable and spontaneous
process happened to nearly all the living things and organic
materials through oxidating of hydrocarbon R-H to form the
hydroperoxides [Eq. (1) – (3)]. However, this reaction can be
largely inhibited or retarded by adding little amount of radical-
trapping antioxidants (RTAs) [Eq. (4)]. Substituted phenols and
arylamines, which represent the main family of RTAs, can
quickly donate their active hydrogen atoms to the radicals
formed during autoxidation to generate a relatively unreactive
species [Eq. (5)] due to their faster reaction rate than that of the
chain propagation step.
o
to 41 for the inhibited autoxidation of paraffin oil at 130 C due to
its catalytic mechanism (the stoichiometric factors detected at
[8]
room temperature is only 2). This superior behavior at elevated
temperatures enables arylamines preferentially to apply in a
range of materials such as rubbers, lubricants, fuels, and
plastics. In spite of this, the activity of the synthesized compound
is always restricted to its instability at high temperature. In this
regard, few compounds can still hold the remarkable activities as
[
9]
that under the mild condition.
As a consequence, the interest in developing highly active
RTAs is constantly increasing. For example, Pratt and co-
workers discovered that incorporation of nitrogen atoms into the
aromatic ring of phenols and substituting the para-position with a
strongly electron-donating tertiary amino group (1) can
In order to develop highly active antioxidants at elevated
temperatures, not long ago, we found another strategy to design
the new structures with two different active functional groups
[
10]
(phenol and diphenylamine).
Delightfully, the thus generated
compounds (SPDs) showed superior antioxidant efficiency at
[
1]
o
remarkably weaken the O-H bond. And the advanced di-3-
pyridyl amine (2) designed with the same principles was found to
be 200-times more reactive towards peroxyl radicals than the
180 C compared with the industrial standard phenol and
arylamine. Under the help of EPR experiment, an intramolecular
synergism mechanism was proposed involving amine moiety
quenching radicals to form aminic radicals followed by the
regeneration of amine moiety through rapidly abstracting active
hydrogen from intramolecular phenol moiety. Additionally,
extraordinary synergism was surprisingly discovered when
cooperating with organosulfur compounds. These pleasant
results implied us that combining the three active moieties
including phenol, diphenylamine and sulfide integrally may
probably further enhance the antioxidative efficiency in view of
the potential intramolecular synergism. Thus, in this
communication, we would like to report the synthesis of a series
[
2]
industrial standard arylamine (3). Engman’s group found that
[
a]
Dr. Shasha Yu, Prof. Dr. Shenggao Liu
Polymers and composites division
Ningbo Institute of Materials Technology and Engineering, Chinese
Academy of Sciences
No. 1219 Zhongguan West Road, Ningbo 315201, China
rukawayss@hotmail.com
Supporting information for this article is given via a link at the end of
the document.
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European Journal of Organic Chemistry
10.1002/ejoc.201701577
COMMUNICATION
of sulfur-containing SPDs (SSPDs) and their antioxidative
capacity at elevated temperatures.
consistent with longer OIT. Without any antioxidant, the DSC
curve of the pure pao was quickly raised up after 2.8 minute.
Unfortunately, little improvement was observed after adding 5
μmol/g of the ortho-aminodiphenylamino substituted SSPDs
[a]
Table 1. Synthesis process of SSPDs.
(SSPD1 and SSPD4). Conversely, superior antioxidative
activities with a 10.6-fold to 19-fold increase in OIT were
furnished by the para-aminodiphenylamino substituted SSPDs,
extremely better than that of the industry standards (BHT,
Irganox 1076, DPA and ODA) and the sulfur-free compound
SPD1. Thereinto, SSPD5 exhibited the highest activity and was
found to be 3.45-fold more efficient than the phenolic ester
Irganox 1076 and 2.77-fold more efficient than the alkylated
diphenylamine ODA. Furthermore, it should be worth noting that
the thiomethyl substitution also impacted considerable effects on
the activity. Higher efficiency was provided by the
benzylthiomethyl substituted SSPDs than the corresponding n-
octylthiomethyl substituted ones regardless of the relative
position of the active moieties.
[
a]
Yields of isolated products of the second step are shown.
The SSPDs can be conveniently prepared in good-to-
excellent yields through
a
two-step sequence involving
thiomethyl substitution of the alkylated salicylaldehyde and the
condensation reaction of the resulting aldehyde with
aminodiphenylamine. This process would potentially enable the
large-scale production in industry since no complex isolated
operation were required during the whole synthesis process.
The different structures of SSPDs are shown in table 1. A strong
intramolecular hydrogen bond between phenol donor and imine
acceptor is present in all the SSPDs and identified by a high
chemical shift of the hydrogen above 13 ppm. The structures
can be further verified by the X-ray diffraction results of SSPD5
Figure 1. Oxidation induction time (OIT) of pao containing 5 μmol/g
antioxidants including the traditional ones and SSPDs.
Table 2. Splitting constants and g-factors of radicals by reaction of DPA,
SPD1 and SSPD1-6 with tert-butoxyl radicals in benzene at 298 K.
[
11]
and SSPD6. Unfortunately, attempts to obtain the crystals of
SSPD1 or SSPD4 were unsuccessful. But according to the
Sample
DPA
SPD1
SSPD1
SSPD2
SSPD3
SSPD4
SSPD5
SSPD6
a (N) [G]
10.29
10.28
-
9.59
9.96
-
g factor
2.0062
2.0061
-
2.0054
2.0054
-
[
12]
literature with the similar structures, the O-H group and the N-
H group are likely to be located at the same side of the imine.
The antioxidative activities of these compounds can be
rapidly and accurately evaluated based on measuring the time of
inhibiting the autoxidation of hydrocarbon (pao). Samples were
o
heated at 180 C under where the pressure of oxygen was kept
9.60
10.32
2.0054
2.0053
on 3.5 MPa during the test and the oxidation process can be
monitored by the exothermic situation through DSC. Once the
antioxidants were exhausted, hydrocarbon was quickly oxidized
accompanied by the drastic heat release and the oxidation
induction time (OIT) was obtained through measuring the time
from the start of the oxygen added in to the start of the
exothermic peak. Undoubtedly, higher antioxidative activity is
In order to figure out the reactive position of SSPDs towards
radicals, we treated them with photolytically generated tert-
butoxyl radicals from di-tert-butyl peroxide in deoxygenated
benzene solutions and detected the formed radicals using EPR
[
13-15]
spectrometer.
The results are shown in table 2. For the
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European Journal of Organic Chemistry
10.1002/ejoc.201701577
COMMUNICATION
para-aminodiphenylamino substituted SSPDs, only a single
paramagnetic species was obtained with spectral parameters
consistent with the aminyl radical derived from abstraction of
hydrogen atom from the NH group, similar to that of DPA and
SPD1. This highly persistent radical is characterized by a g-
activity of phenols has been blocked by the intramolecular
[
18]
bonding,
the arylamine can be responsible for the radical-
trapping action. And to our best knowledge, this is the first time
to discover the synergism between arylamine and sulfide
intramolecularly.
factor of nearly 2.0054 and a particular coupling of nitrogen (a
N
≈
From the aforementioned observations and results, two
possible mechanisms are proposed and shown in Scheme 1. In
Path A, diphenylamine moiety initially donates a hydrogen to the
1
0.0 G). Theoretically speaking, the hindered phenoxyl radicals
[
16]
are more stable than the aminyl radical.
Thus the
•
disappearance of the phenoxyl radicals can be explained by the
firmly restraining of the hydrogen of the hydroxyl group by the
imine nitrogen atom. It can also be concluded that higher
alkyl peroxy radical (ROO ) to afford diphenylaminyl radical. The
released ROOH is quickly decomposed by the intramolecular
sulfide to generate stable alcohol and the sulfoxide intermediate
•
•
reactivity was observed for SSPD5 than others since it was
1. For Path B, the ROO firstly interacts with the sulfide moiety to
found to transform to other species more quickly when
prolonging the irradiation time. In contrast, regardless of the
length of the reaction time, no signals were observed in the case
of SSPD1 or SSPD4. Considering their terrible antioxidative
activities tested by PDSC, the disappearance of signals can be
ascribed to the extreme inert reactivity of N-H and O-H in the
molecule. This is probably because the large steric hindrance
and the intramolecular hydrogen bond restrained both of the
phenolic moiety and the aminic moiety to interact with the
radicals.
form alkoxyl radical which is then captured by the intramolecular
[
5]
diphenylamine moiety. The formed diarylaminyl radical moiety
in 1 can react with peroxyl radicals to generate nitroxides which
can further trap the alkyl radicals to form alkoxyamines. And the
elevated temperatures can facilitate the regeneration of the
[
19]
diphenylamine through retro-carbonyl-ene (RCE) process.
While the sulfoxide moiety can be further oxidized to form
sulfone, sulfinic acid or sulfuric acid. The remarkable activity of
SSPDs can be summarized for four points: first, the large
conjugated system provided by the phenolic Schiff base moiety
can greatly stabilize the formed diphenylaminyl radical and
facilitate their reaction towards the radicals; second, ROOH can
be in-time decomposed intramolecularly without escaping from
the “cage” so as to reduce the possibility to degrade to more
radicals before being captured by the added peroxide
[
20]
decomposers;
third, all of the generated sulfur-bearing
derivatives have the distinctive capacity to decompose more
[
21]
peroxides;
fourth, the diphenylamine can be quickly
regenerated, resulting in the large stoichiometric efficiency for
[
8]
per molecule. However, it must be pointed out that each single
moiety can play an efficient role only when their relative position
is reasonable. For the radical-trapping moiety, the active group
must be “free” thus can play the intramolecular synergism with
the peroxide decomposers.
Figure 2. Comparison of SSPDs (5 μmol/g) and the mixtures of SPD1 (5
o
μmol/g) and sulfide (5 μmol/g) in PAO using isothermal PDSC (190 C).
With the intention of exploring the intramolecular synergism
of the SSPDs, the antioxidative activities of the corresponding
co-antioxidant systems (SPD1 + sulfides) were also tested,
shown in Figure 2. It clearly indicated that the OITs of the
formulas were all worse than that of the corresponding para-
aminodiphenylamino substituted SSPDs. Our previous work
indicated that the intermolecular synergism between the SPD as
the radical scavengers and sulfide as the peroxide decomposer
Scheme 1. Proposed mechanism.
In conclusion, a serious of sulfur-containing Schiff base
phenolic diphenylamines (SSPD) have been conveniently
[
10]
are present in the mixture. Thus, the better performance of the
novel antioxidants can be ascribed to the intramolecular
synergism between the two active moieties that lead to a faster
synthesized.
The
para-aminodiphenylamine
derivatives
achieved a remarkable increase in the reactivity towards peroxyl
radicals at elevated temperatures. The EPR experiments
[
17]
reaction rate towards the radicals and the peroxide. Since the
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European Journal of Organic Chemistry
10.1002/ejoc.201701577
COMMUNICATION
suggested that diphenylamine moiety is responsible for the
reaction with radicals while the phenol lose the activity due to
the strong intramolecular hydrogen bonding. The comparison
with the corresponding co-antioxidant systems indicated that a
unique intramolecular synergism is present in the molecule
which can be the explanation for the improved activities. Thus,
two mechanisms were proposed involving the radicals
scavenging by the diphenylamine moiety and the peroxide
decomposing by the sulfide moiety. Alcohol is the finally product
rather than a hydroperoxide since the possibility to degrade to
more radicals at elevated temperatures can be largely reduced.
Moreover, the regeneration of diphenylamine moiety and the
constantly decomposing peroxide of sulfide as well as its
derivatives enable SSPDs to trap multiple radicals or peroxides
per molecule.
(or from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax (+44) 1223-336-033; or
deposit@ccdc.cam.ac.uk.
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Keywords: antioxidant
• phenol • arylamine • sulfide •
multifunction
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obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
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European Journal of Organic Chemistry
10.1002/ejoc.201701577
COMMUNICATION
COMMUNICATION
Multifunctional antioxidants with
Shasha Yu*, Shenggao Liu*
three
active
fragments
exhibit
remarkable activity on inhibiting the
oxidation of hydrocarbon at elevated
temperatures owing to the unique
intramolecular synergism.
Page No. – Page No.
Multifunctional Antioxidants with
High Activity at Elevated
Temperatures Based On the
Intramolecular Synergism
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