Alkylation of 2,6-di-tert-butylphenol with methyl acrylate catalyzed by potassium 2,6-di-tert-butylphenoxide

Article abstract of DOI:10.1007/s11172-007-0318-3

The kinetics of catalytic alkylation of 2,6-di-tert-butylphenol (ArOH) with methyl acrylate (MA) in the presence of potassium 2,6-di-tert-butylphenoxide (ArOK) depends on the method for the preparation of ArOK. The reaction of ArOH with KOH at temperatures > 180 °C affords monomeric ArOK, whose properties differ from those in the case of potassium 2,6-di-tert-butylphenoxide synthesized by the earlier methods. The regularities of ArOH alkylation depend on the ArOK concentration, the ArOH: MA ratio, and the effect of microadditives of polar solvents.

Full text of DOI:10.1007/s11172-007-0318-3

2038
Russian Chemical Bulletin, International Edition, Vol. 56, No. 10, pp. 2038—2042, October, 2007
Alkylation of 2,6ꢀdiꢀtertꢀbutylphenol with methyl acrylate
catalyzed by potassium 2,6ꢀdiꢀtertꢀbutylphenoxide
A. A. Volod´kinꢀ and G. E. Zaikov
N. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences,
4
ul. Kosygina, 119991 Moscow, Russian Federation.
Fax: +7 (495) 137 4101. Eꢀmail: chembio@sky.chph.ras.ru
The kinetics of catalytic alkylation of 2,6ꢀdiꢀtertꢀbutylphenol (ArOH) with methyl acrylate
MA) in the presence of potassium 2,6ꢀdiꢀtertꢀbutylphenoxide (ArOK) depends on the method
(
for the preparation of ArOK. The reaction of ArOH with KOH at temperatures >180 °C affords
monomeric ArOK, whose properties differ from those in the case of potassium 2,6ꢀdiꢀtertꢀ
butylphenoxide synthesized by the earlier methods. The regularities of ArOH alkylation deꢀ
pend on the ArOK concentration, the ArOH : MA ratio, and the effect of microadditives of
polar solvents.
Key words: phenols, phenoxides, 2,6ꢀdiꢀtertꢀbutylphenol, methyl acrylate, Michael reacꢀ
tion, potassium hydroxide, kinetics, catalysis.
Alkali derivatives of sterically hindered phenols are
Experimental
reactants and catalysts in the synthesis and technology of
antioxidants. Most studies involve potassium 2,6ꢀdiꢀtertꢀ
butylphenoxide (ArOK) and potassium hydroxide (KOH),
and the effect of the ratio of alkaline components on the
kinetics of ArOH alkylation with methyl acrylate and the
yield of the alkylation product, viz., methyl 3ꢀ(4ꢀhydroxyꢀ
1Н NMR spectra were recorded on a Bruker WMꢀ400 inꢀ
strument (400 MHz) relative to the signal of residual protons
of the deuterated solvent. IR spectra were measured on a
Perkin—Elmer 1725ꢀX spectrometer (KBr pellets, resolution
–
1
2
cm ). Molecular weights were determined by the thermoꢀ
electric method and calculated using the formula
3
,5ꢀdiꢀtertꢀbutylphenyl)propionate (Ar´OH), was studꢀ
1
—4
ied.
The mechanism of ArOH alkylation with methyl
M = kgM /[100(t – t )],
s
1
2
acrylate in the presence of ArOK and KOH is interpreted
based on the structures of the intermediates and comꢀ
plexes as well as kinetic data. The effect of the tertꢀbutyl
groups in the orthoꢀposition of a phenol molecule results
in the appearance of specific properties related to the
negative charge delocalization in the sixꢀmembered cycle
where g is the weight of the sample in g (2—3% per 100 g of
solvent), k is the coefficient of the instrument with thermistors,
М is the molecular weight of the standard sample, and t – t is
the magnitude equivalent to the temperature change in the cell
with a solution of the sample on the potentiometer scale.
Kinetic data were obtained during the analysis of the reacꢀ
tion mixtures by liquid chromatography on a Bruker LCꢀ31
chromatograph (IBM Cyano column, hexane—isopropyl alcoꢀ
s
1
2
(
ambidentate ion) and a possibility of aromatic strucꢀ
ture rearrangement to the cyclohexadienone structure.
As a consequence, alkali 2,6ꢀdiꢀtertꢀbutylphenoxide
molecules can form supramolecular structures (dimers)
hol—ethyl acetate (8 : 1 : 1, v/v/v) mixture as eluent, rate
–1
0
.4 mL min ).
5
and undergo Cꢀalkylation reactions. In continuation of
Potassium 2,6ꢀdiꢀtertꢀbutylphenoxide (ArOK). A. A flask was
these studies, it was of interest to study the catalytic propꢀ
erties of potassium 2,6ꢀdiꢀtertꢀbutylphenoxide in the alkyꢀ
lation of phenol ArOH with methyl acrylate. In this
work, we synthesized monomeric ArOK under the condiꢀ
tions of the highꢀtemperature reaction of phenol ArOH
with KOH and present the data concerning the properꢀ
ties of the monomer and earlier known ArOK dimers.
Taking into account the studies of the kinetic regulariꢀ
ties of ArOH alkylation with methyl acrylate, we proꢀ
pose a mechanism of catalytic alkylation involving monoꢀ
meric ArOK.
filled with ArOH (20.6 g, 0.1 mol), and the content was heated
in an argon flow to 190 °C. Granulated 85% KOH (1.5 g) was
added. After 10 min, crystals that formed were separated by
highꢀtemperature filtration under argon and washed with
nꢀoctane heated to 110—115 °C until the analysis to the presꢀ
ence of ArOH in the washings became negative. Solvent residues
were removed by evacuation at 120—130 °C. Found: Мmin = 260.
Calculated: МArOK = 244.28. The IR spectrum of the syntheꢀ
sized sample coincides with that published earlier.5
B. The flask was filled with ArOH (20.6 g, 0.1 mol), and the
content was heated in an argon flow to 190 °C. Granulated
85% KOH (1.5 g) was added. After 10 min, the reaction mixture
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1971—1974, October, 2007.
066ꢀ5285/07/5610ꢀ2038 © 2007 Springer Science+Business Media, Inc.
1

CꢀAlkylation of 2,6ꢀdiꢀtertꢀbutylphenol
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
2039
as a suspension was cooled to ~20 °C, and heptane (50 mL) was
added. The precipitate was filtered off and washed on the filter
with heptane until the absence of ArOH in the washings
was achieved. The yield of ArOK was 5.1 g (90%). Found:
alkylation and no methyl acrylate were found in the mother
liquor. The precipitate is insoluble in acetone, toluene, and
ether, which is characteristic of a methyl acrylate polymer.
Methyl acrylate oligomer. Potassium phenoxide ArOK
(2.44 g, 0.01 mol) was added to methyl acrylate (33 g, 0.39 mol),
and the mixture was kept for 8 days at ~20 °C. Then the mixture
was neutralized with 10% НС1, extracted, and washed with
heptane until the absence of ArOH in the washings was achieved.
The heptaneꢀinsoluble mixture as a viscous liquid was dissolved
in ether and dried over lithium hydride. After the solvent was
distilled off, a thick viscous liquid soluble in acetone was formed.
1
Мmin = 480. Calculated: МArOK = 244.28. The Н NMR and
IR spectra of the synthesized sample coincide with those deꢀ
5
scribed earlier.
On cooling of the ArOK sample synthesized by method A to
1
~
20 °C, the intensity of the IR bands at 1700—1300 cm– changes
and the spectrum becomes identical to that of ArOK synthesized
by method B.
Alkylation of 2,6ꢀdiꢀtertꢀbutylphenol with methyl acrylate in
the presence of ArOK. A. Granulated 85% KOH (0.13 g,
1
t
Н NMR (acetoneꢀd ), δ: 1.41 (s, 18 H, Bu ); 1.82 (m, n (3 H),
6
CH CH); 2.27 (m, n (4 H), CH CH ); 3.62 (s, n (3 H), OMe);
2
2
2
0
.002 mol) was added to ArOH (20.6 g, 0.1 mol) at 190 °C in an
argon flow. After 10 min, the reaction mixture was cooled to
35 °C, and methyl acrylate (11.2 g, 0.13 mol) was added. After
3.64 (s, n (3 H), OMe); 6.32 (m, 3 Н, СН =СН). М = 12 000;
2 exp
Мcalc = 12 500 (n = 143). The calculation was based on the ratio
1
1
of integral intensities of the signals in the Н NMR spectrum
t
mixing of the reactants, the temperature of the reaction mixture
was 116 °C, and this temperature was maintained during the
experiment. After 40 min, the content of Ar´OH in the reaction
mixture reached 98 mol.%.
from the protons of the MeO and Bu groups.
Results and Discussion
B. Granulated 85% KOH (0.13 g, 0.002 mol) was added to
ArOH (20.6 g, 0.1 mol) at 190 °C in an argon flow. After 10 min,
the reaction mixture was cooled to 143 °C, and methyl acrylate
The quantitative data characterizing the dependence
of the properties of potassium 2,6ꢀdiꢀtertꢀbutylphenoxide
ArOK) on the method of its synthesis indicate the forꢀ
(
11.2 g, 0.13 mol) was added. After mixing of the reactants, the
(
temperature of the reaction mixture was 125 °C, and this temꢀ
perature was maintained during the experiment. After 40 min,
the content of Ar´OH in the reaction mixture reached 97 mol.%.
C. Granulated 85% KOH (0.13 g, 0.002 mol) was added to
ArOH (20.6 g, 0.1 mol) at 190 °C in an argon flow. After 10 min,
the reaction mixture was cooled to 120 °C, and methyl acrylate
mation of two species: monomeric and dimeric. The inꢀ
teraction of 2,6ꢀdiꢀtertꢀbutylphenol with KOH at temꢀ
peratures >180 °C results in dimerization. The data of
measurements of the molecular weight of ArOK in DMF
indicate that the results depend on the method of syntheꢀ
sis of phenoxide samples. The minimum value of the moꢀ
lecular weight is close to that of the ArOK monomer, and
the maximum value is close to the molecular weight of the
(
11.2 g, 0.13 mol) was added. After mixing of the reactants, the
temperature of the reaction mixture was 105 °C, and this temꢀ
perature was maintained during the experiment. After 40 min,
the content of Ar´OH in the reaction mixture reached 96 mol.%.
D. Methyl acrylate (11 g, 0.13 mol) was added in an argon
flow at 116 °C to a mixture of ArOH (20.6 g, 0.1 mol) and ArOK
dimer. The formation of dimeric associate has been found
earlier6 upon the reaction of sodium 2,6ꢀdiꢀtertꢀbutylꢀ
phenoxide with DMSO. The structure of this associate
was confirmed by Xꢀray diffraction analysis. The IR specꢀ
trum of ArOK synthesized by the highꢀtemperature treatꢀ
ment of the reaction mixture contains bands of the conjuꢀ
(
0.488 g, 0.002 mol) synthesized according to method B. After
2
1
5 min, the content of Ar´OH in the reaction mixture was
5 mol.%. After 180 min of the reaction at 116 °C, the yield of
Ar´OH reached 87%.
t
–1
E. ArOH (20.6 g, 0.1 mol) was added to a solution of Bu OK
gated С=О group at 1653 and 1633 cm . After the ArOK
t
(
0.248 g, 0.002 mol) in Bu OH (20 mL). The solvent was disꢀ
sample is cooled to ~20 °C, the intensity of these bands
tilled off in vacuo at 100 °C, and methyl acrylate (11.2 g,
.13 mol) was added to the resulting suspension of ArOK in
ArOH. The mixture was heated for 180 min at 116 °C with
sampling and analysis of the samples to the content of АrOH
and Ar´OH.
decreases with the simultaneous increase in the intensity
of the band at 1601 cm , which is characteristic of the
0
–1
dimeric molecular associates with the cyclohexadiene
7
structure.
The differences in the properties of two forms of ArOK
are most pronounced when they are used as catalysts
for the alkylation of phenol ArOH with methyl acrylate.
The ArOK monomer was obtained by the reaction of
KOH with ArOH at 180—190 °C and the molar ratio
КОН : ArOH = 0.03—1. After the reaction mixture was
cooled to 120—143 °C, methyl acrylate was added to the
mixture. Under these conditions, alkylation ceases within
Influence of solvent additives on the catalytic alkylation of
ArOH with methyl acrylate in the presence of ArOK. Granulated
8
0
5% KOH (0.13 g, 0.002 mol) was added to ArOH (20.6 g,
.1 mol) at 190 °C in an argon flow. After 10 min, the reaction
mixture was cooled to 135 °C, and methyl acrylate (11.2 g,
.13 mol) and DMSO (0.2 mL) were added. The reactions with
0
additives of HMPA, DMF, DME, and MeCN were carried out
similarly.
Reaction of potassium 2,6ꢀdiꢀtertꢀbutylphenoxide with meꢀ
thyl acrylate in the presence of methanol. Methanol (1 mL) was
added at ~20 °C to a mixture of ArOK (0.25 g, 0.001 mol) in
methyl acrylate (5 mL). The reaction resulted in the spontaneꢀ
ous heating of the reaction mixture and precipitation. According
to the data of liquid chromatography, no products of Ar´OH
2
0—40 min to form Ar´OH in yields up to 98%. The data
on the kinetics of this reaction in a temperature interval of
05—125 °C showed that the shape of the kinetic curves is
1
temperatureꢀindependent (Fig. 1). The rate of the reacꢀ
tion of ArOH with methyl acrylate in the presence of the

2040
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
Volod´kin and Zaikov
[
ArOH] (mol.%)
microamounts. Additives of the polar solvents were introꢀ
duced into methyl acrylate, and these mixtures were used
as reactants in the alkylation of ArOH in the presence of
the ArOK monomer. We studied the kinetics of the reacꢀ
tion in the presence of DMSO, DMF, HMPA, DME,
and MeCN. The rate of the reaction of ArOH with methyl
acrylate under the catalytic alkylation conditions in the
presence of these additives decreases, and the inhibition
effect of the polar solvent additives increases in the folꢀ
lowing order: DMSO < HMPA < DMF < DME < МеCN.
It can be assumed that these results are related to the
interaction of ArOK with the polar solvent additives at a
level of ion pairs or a complex, and this interaction comꢀ
petes with the step of formation of a transition state
of molecules of phenoxide ArOK, ArOH, and methyl
acrylate.
1
8
6
4
2
0
0
0
0
2
3
0
1000
2000
3000
τ/s
Fig. 1. Kinetic curves of the reaction of ArOH with methyl
acrylate (MA) in the presence of the ArOK monomer at 125 (1),
16 (2), and 105 °C (3); [ArOH] = 3.29 mol L , [MA]0 =
Taking into account the known data and present reꢀ
sults, we can suggest the kinetic scheme for the inhibition
of alkylation of phenol ArOH with methyl acrylate in the
presence of the ArOK monomer and a polar solvent
–
1
1
3
0
–
1
–1
.75 mol L , and [ArOK] = 0.08 mol L
.
0
(
Scheme 1).
ArOK dimer at 110 °C decreases, which is confirmed by
the kinetics of ArOH consumption under the catalysis
conditions in the presence of the ArOK monomer and
dimer (Fig. 2). The shape of the kinetic curves for the
alkylation of ArOH with methyl acrylate in the presence
of the catalyst obtained by the reaction of 2,6ꢀdiꢀtertꢀ
butylphenol with potassium tertꢀbutoxide was found to be
identical to that detected for catalysis by the ArOK dimer.
It is known that the polar solvents (DMSO, DMF) have
earlier8 been applied to the alkylation of phenol ArOH
with methyl acrylate in the presence of the alkaline cataꢀ
lyst, and the yield of Ar´OH in this case was 80—85%.
According to our data, the presence of polar solvents
induces the inhibition of the process, and the negative
influence of the solvents emerges when they are used in
Under the assumption that k1 > k–1, k2 > k–2
,
k > k , k > k , and [Solv] = 0, the results of calculation
of the kinetic scheme coincide with the experimental data
at k1 = 3.5•10 and k2 = 1.5•10 L mol
kinetics of inhibition of alkylation in the presence of solꢀ
vent additives is shown in Fig. 3. The k rate constants
(
3
1
3
2
0
–
2
–2
–1 –1
s . The
4
k4 > k–4) are given below.
Reaction
k •10–2/L mol–1 s–1
4
ArOK + DMSO
ArOK + HMPA
ArOK + DMF
ArOK + DME
ArOK + MeCN
2.1
2.9
3.5
4.2
9.5
Mathematical simulation was performed using the proꢀ
[
ArOH] (mol.%)
gram for reaction kinetic calculation on the basis of the
solution of a "rigid system" of differential equations. These
regularities are characteristic of the reactions with solvent
additives in the concentration comparable with that of
ArOK phenoxide. According to this scheme, the formaꢀ
tion of the Ar´OH reaction product is related to complex
formation involving molecules of the ArOK monomer,
phenol ArOH, and methyl acrylate and the migration of
1
2
3
8
6
4
2
0
0
0
0
the metal cation. However, it cannot be excluded that the
+
K
cation escapes to the bulk to form a kinetically indeꢀ
pendent species. In this case, the catalytic alkylation of
,6ꢀdiꢀtertꢀbutylphenol with methyl acrylate proceeds via
2
the chain ion mechanism.
0
10
20
30
40
50
τ/min
An increase in the ArOK concentration in the reacꢀ
tion mixture higher than the optimum value (5 mol.%)
changes the character of the interaction between the reꢀ
actants. The changes are associated with the activation of
side reactions and the formation of methyl acrylate oligoꢀ
Fig. 2. Kinetic curves of the reaction of ArOH with methyl
acrylate in the presence of the ArOK using methods A (1), D (2),
and E (3); 116 °C, [ArOH] = 3.29 mol L , [MA]0
–
1
=
0
–
1
–1
3
.75 mol L , and [ArOK] = 0.08 mol L .
0

CꢀAlkylation of 2,6ꢀdiꢀtertꢀbutylphenol
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
2041
Scheme 1
[ArOH]/mol L–1
3.0
2.5
2.0
1.5
1.0
0.5
5
4
3
2
1
0
500
1000
1500
2000
2500
τ/s
Fig. 3. Kinetic curves of ArOH consumption in the catalytic
reaction with methyl acrylate in the presence of the ArOK monoꢀ
mer in DMSO (1), DMF (2), HMPA (3), DME (4), and
–
1
МеCN (5) at 116 °C; [ArOH] = 3.29 mol L , [MA]0
=
=
=
=
0
.75 mol L–
1
1
1
,
;
;
[ArOK]0
[DMF]0
[DME]0
=
=
=
0.08 mol L
0.22 mol L
–1
–1
,
;
;
[DMSO]0
[HMPA]0
[MeCN]0
3
0
0
0
.21 mol L–
.19 mol L
.24 mol L–1.
–
0.21 mol L–1
characteristic of the protons of the tertꢀbutyl groups, which
possibly indicate that an oligomer molecule contains the
9
1
terminal phenol moiety. It is known that the H NMR
spectrum of an analog of the oligomer, being the condenꢀ
sation product of 2,6ꢀdiꢀtertꢀbutylphenol with three meꢀ
thyl acrylate molecules, contains signals of protons in the
region characteristic of an oligomer with a higher content
of the methyl acrylate fragments (n = 143).
The reaction of phenoxide ArOK with methyl acryꢀ
late in the presence of MeOH at room temperature afꢀ
fords a methyl acrylate polymer insoluble in organic
solvents.
To summarize, depending on the reactant ratio and
reaction conditions, potassium 2,6ꢀdiꢀtertꢀbutylphenoxide
can manifest the properties of the catalyst of both the
alkylation of 2,6ꢀdiꢀtertꢀbutylphenol (ArOH) with meꢀ
thyl acrylate and oligomerization. An alternative for the
mechanism via this scheme is the chain ion mechanism
involving the nonꢀassociated cation. The latter is favored
by the temperatureꢀdependent reaction kinetics and
the inhibition of the alkylation by microadditives of
polar solvents and the formation of methyl acrylate oliꢀ
gomers.
Solv is solvent.
mers. At the ratios ArOH : methyl acrylate > 3 and the
ArOK content higher than 10 mol.%, the yield of the
alkylation product (Ar´OH) is at most 25%, and the
yield of methyl acrylate oligomers increases simultaꢀ
neously.
References
Methyl acrylate oligomers were characterized by an
1
average molecular weight of ~12 000. The H NMR specꢀ
1
. A. A. Volod´kin, V. I. Paramonov, F. M. Egidis, and L. K.
Popov, Khim. Promꢀst´ [Chemical Industry], 1988, No. 12, 7
(in Russian).
trum of the oligomer exhibits a narrow signal at δ 3.62
from the protons of the MeO group and a signal at δ 1.3

2042
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
Volod´kin and Zaikov
2
3
. V. V. Ershov, G. A. Nikiforov, and A. A. Volod´kin,
Prostranstvennoꢀzatrudnennye fenoly [Sterically Hindered
Phenols], Khimiya, Moscow, 1972, 73 (in Russian).
. A. A. Volod´kin, A. S. Zaitsev, V. L. Rubailo, V. A. Belyakov,
and G. E. Zaikov, Izv. Akad. Nauk SSSR, Ser. Khim., 1989,
7. N. A. Malysheva, F. I. Prokof´ev, N. N. Bubnov, S. P.
Solodovnikov, T. I. Prokof´eva, A. A. Volod´kin, and
V. V. Ershov, Izv. Akad. Nauk SSSR, Ser. Khim., 1977, 1522
[Bull. Acad. Sci. USSR, Div. Chem. Sci., 1977, 36 (Engl.
Transl.)].
1
829 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1989, 38, 1677
8. US Pat. 4659863; Chem. Abstr., 1988, 107, 77439.
9. T. F. Titova, A. P. Krysin, V. A. Bulgakov, and V. I.
Mamatyuk, Zh. Org. Khim., 1984, 20, 1899 [J. Org. Chem.
USSR, 1984, 20 (Engl. Transl.)].
(
Engl. Transl.)].
4
5
6
. A. A. Volod´kin, Izv. Akad. Nauk, Ser. Khim., 1994, 827 [Russ.
Chem. Bull., 1994, 43, 769 (Engl. Transl.)].
. A. A. Volod´kin and G. E. Zaikov, Izv. Akad. Nauk, Ser.
Khim., 2006, 2138 [Russ. Chem. Bull., Int. Ed., 2006, 55, 2212].
. L. Matilainen, M. Leskela, and M. Klinga, J. Chem. Soc.,
Chem. Commun., 1995, 421.
Received March 13, 2006;
in revised form May 7, 2007