Solvolysis of 3,5-di-tert-butyl-4-hydroxybenzyl acetate in alcohol solutions

Article abstract of DOI:10.1023/A:1020490525414

Solvolysis of 3,5-di-tert-butyl-4-hydroxybenzyl acetate in alcohol solutions involves intermediate formation of 2,6-di-tert-butyl-4-methylene-1-benzoquinone that further takes up a molecule of the alcohol.

Full text of DOI:10.1023/A:1020490525414

Russian Journal of General Chemistry, Vol. 72, No. 6, 2002, pp. 924 927. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 6, 2002,
pp. 988 991.
Original Russian Text Copyright
2002 by Bukharov, Syakaev, Pod’’yachev, Nugumanova, Mukmeneva.
Solvolysis of 3,5-Di-tert-butyl-4-hydroxybenzyl Acetate
in Alcohol Solutions
S. V. Bukharov, V. V. Syakaev, S. N. Pod’’yachev,
G. N. Nugumanova, and N. A. Mukmeneva
Kazan State Technological University, Kazan, Tatarstan, Russia
Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Received August 3, 2000
Abstract Solvolysis of 3,5-di-tert-butyl-4-hydroxybenzyl acetate in alcohol solutions involves intermediate
formation of 2,6-di-tert-butyl-4-methylene-1-benzoquinone that further takes up a molecule of the alcohol.
It is known that 3,5-di-tert-butyl-4-hydroxybenzyl-
benzyl acetate (I) is a highly reactive precursor of 3,5-
di-tert-butyl-4-hydroxybenzyl cation (II) and 2,6-di-
tert-butyl-4-methylene-1-benzoquinone (III). For this
reason, compound I is active in reactions with aro-
matic hydrocarbons and certain nucleophiles, which
allow synthesis of compounds with sterically con-
gested benzyl fragments under mild conditions and in
high yields [1 5].
cause of the low nucleophilicity of the latter, are not
so smooth. Alcohols are insufficiently active with
benzyl cation II and methylenequinone III, which
results in formation of appreciable amouns of by-
products IV and V [6].
The formation of compounds IV and V can be
suppressed by using an excess of alcohols. Under
these conditions, 3,5-di-tert-butyl-4-hydroxybenzyl-
substituted alcohols VI (for instance, 3,5-di-tert-butyl-
4-hydroxybenzylmethanol) are formed in high yields
[7].
OH
Bu-t
t-Bu
We studied conversions of benzyl acetate I in me-
thanol-d4 and ethanol-d6 by H NMR spectroscopy.
CH2+
1
OH
As seen from Fig. 1, the spectra of freshly prepared
solutions of compound I in methanol-d4 show two
groups of signals (subspectra A and B), between
which there is a rapidly established reversible con-
centration and temperature equilibrium. Increasing
concentration of benzyl acetate I and lowering
temperature render subspectrum A stronger and sub-
spectrum B weaker. The same pattern is characteristic
of the spectra of benzyl acetate I in ethanol-d6. The
concentration and temperature variations of the spectra
of compound I in deuterated alcohols can be ex-
plained in terms of equilibrium (1).
II
Bu-t
t-Bu
+H+
H+
CH2OC(O)Me
O
Bu-t
t-Bu
I
CH2
III
Reactions of benzyl acetate I with alcohols, be-
t-Bu
t-Bu
t-Bu
III
1/4
+ 1/4
CH
O
CH2
HO
t-Bu
t-Bu
CH2OR
HO
t-Bu
2
2
IV
V
VI
1070-3632/02/7206-0924$27.00 2002 MAIK Nauka/Interperiodica

SOLVOLYSIS OF 3,5-Di-tert-BUTYL-4-HYDROXYBENZYL ACETATE
925
D
(a)
0.3
0.1
1
B
2
A
250
300
350 , nm
B
A
B
B
A
A
Fig. 2. Fragments of the UV spectra of solutions of
benzyl acetate I in (1) methanol and (2) carbon tetra-
A
5
chloride (c
2
10 M).
(b)
I
Ke = [cIII][cVII]/[cI],
(2)
(3)
(4)
G0
G0
=
RTln Ke,
H0 T S0.
=
B
3
In methanol-d4, Ke 6.6 10 (20 C), H0 22.2
1.3 kJ/mol, S0 34.9 4.3 J deg 1 mol . In ethanol-d6,
Kr 2.1 10 (20 C), H0 28.8 1.9 kJ/mol, S0
1
A
3
A
A
B
B
B
72.1 6.2 kJ/mol.
The endothermicity of a direct reaction (1) is not
surprising, since in its course the six-membered ring
loses aromaticity, while the heat release associated
with formation of new bonds is scarcely sufficient to
compensate for the energy consumption for cleavage
of old bonds.
7.0
6.0
5.0
4.0
3.0
2.0 , ppm
1
Fig. 1. H NMR spectra of solutions of benzyl acetate I
in methanol-d . c , M: (a) 3.50 10 and (b) 4.72 10
3
2
.
4
I
I
III + MeCOOH + Q.
(1)
It is known that methylenequinone III is readily
generated from benzyl acetate I under the action of
bases [5].
VII
In this case, subspectrum A contains signals of tert-
butyl, acetyl, methylene, and aryl protons of benzyl
acetate I, and subspectrum B, signals of tert-butyl,
vinyl, and cyclohexadienyl protons of methylenequi-
none III and methyl protons of acetic acid. The lack
of signal of the hydroxy group of benzyl acetate I is
explained by fast proton exchange with the solvent.
According to [8], the 1H NMR spectrum of methylene-
quinone III in tetrachloromethane contains four
signals ( , ppm, 80 MHz): 1.27 s (CMe3), 5.62 s
(CH2), 6.82 s (CH), and the UV absorption maximum
of this compound in hexane is at 286 nm. The same
absorption maximum we observe in the UV spectrum
of benzyl acetate I in methanol (Fig. 2).
t-Bu
I + B
CH2OC(O)Me
O
t-Bu
III + MeCO2 +BH
However, the formation of methylenequinone III
is alcohol solutions of benzyl acetate I cannot be ex-
plained by basic properties of these solvents. Me-
1
thylenequinone III is not detected by H NMR spec-
troscopy in solutions of benzylacetate I in acetone and
dimethyl sulfoxide. The latter is much more basic
than alcohols. Moreover, the formation of methylene-
quinone III in chloroform from compound I under
the action of a strong base, triethylamine, is much
slower than in alcohol solutions of benzyl acetate I.
In this connection we suggest that in the latter solu-
tions methylenequinone III is formed by acid dis-
From the temperature dependences of the concen-
trations of compounds I and III in the temperature
ranges 20 50 (methanol-d3) and 20 60 C (ethanol-d6)
by Eqs. (2) (4) we calculated thermodynamic para-
meters of equilibrium (1) in alcohol solutions of
benzyl acetate I.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002

926
c, M
BUKHAROV et al.
3,5-di-tert-butyl-4-hydroxybenzyl methyl ether (VIII)
appear at 7.28 (Ar H) and 4.49 ppm (CH2 O); the
chemical shifts of tert-butyl protons of compounds I
and VIII are coincident. Figure 3 shows the plots the
concentrations of benzyl acetate I, methylenequinone
III, and compound VIII vs. time of methanolysis of
0.01
0.008
1
1
benzyl acetate I. After 1 day, the H NMR spectrum
2
3
of the solution contained no other signals than those
of benzyl ether VIII.
0.006
0.004
0.002
According to scheme (5), solvolysis of benzyl
acetate I with a fast reversible stage is a pseudo-first-
order reaction. At the same time, the kinetics of this
process exert effect on the position of the benzyl
acetate I methylenequinone III equilibrium, which
depends on the initial concentration of benzyl acetate
I (Fig. 1) and changes as the concentration varies in
the course of reaction because of the different number
of molecules in the left- and right-hand parts of
scheme (1). For this reason, the dependence of the
negative logarithm of the current concentration of
benzyl acetate I on time is a straight line only at con-
versions no higher than 40 50%. Therewith, the slopes
of these dependences for different initial concentra-
tions of benzyl acetate I are different (see table).
0
100
200
300
400 , min
Fig. 3. Concentrations of (1) benzyl acetate I, (2) me-
thylenequinone III, and (3) ether III vs. time of me-
thanolysis of benzyl acetate I (c 0.0173 M).
0
I
sociation of benzyl cation II which, in its turn, is
formed as an ion pair by alcoholysis of benzyl acetate
I [scheme (5)].
The table lists the yields of ether VIII for different
reaction times, and the apparent first-order rate
constants for solvolysis of benzyl acetate I in solu-
tions with different I:III ratios, depending on the
concentration of the initial reagent and the composi-
tion of the solvent. The rate constants were deter-
mined from the semilog anamophoses of the initial
parts of the kinetic curves of variation of the con-
centration of benzyl acetate I. As seen from the table,
the solvolysis rate increases with increasing concentra-
tion of methylenequinone III in the solution, which
is consistent with scheme (5).
t-Bu
CH+2 OC(O)Me
I
HO
t-Bu
III + MeCO2H
CD3OD
(5)
t-Bu
CH2OCD3
HO
t-Bu
Since the benzyl acetate I methylenequinone III
equilibrium is established almost immediately after
dissolution of benzyl acetate I and methylenequinone
III slowly reacts with alcohols, the latter can be in-
volved in generation of methylenequinone III from
benzyl acetate I in reactions with weak nucleophiles.
Thus, for instance, aromatic amines in the absence of
stronger bases fail to react with benzyl acetate I,
because they are incapable of generating methylene-
quinone III from the latter. We found that in me-
thanol, unlike carbon tetrachloride, chloroform, and
benzene, benzyl acetate I rapidly reacts with aniline
VIII
1
In the H NMR spectrum of a solution of benzyl
acetate I in methanol-d4, 10 15 min after its prepara-
tion, the signal of methyl protons of acetic acid begins
to grow (compared with equilibrium) and signals of
Parameters of solvolysis of benzyl acetate I in methanol-d4
20
k
,
cI,
M
cD O
,
Yield of ether
app
2
c
III/cI
1
M
VIII, %a
s
1
5
5
5
0.0730
0.0173
0.0180
0.22
0.62
0.75
12
26
33
1.6 10
3.1 10
4.1 10
already at room temperature. In the H NMR spectrum
of a solution of benzyl acetate I in methanol-d4 (c 5
3
0.5
10 M) 20 min after addition of excess aniline we
observed disappearance of the signals of tert-butyl
protons of methylenequinone III at 1.43 ppm and of
a
Reaction time 4 h.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002

SOLVOLYSIS OF 3,5-Di-tert-BUTYL-4-HYDROXYBENZYL ACETATE
927
acetyl protons of benzyl acetate I at 2.19 ppm and
appearance of a signal at 4.19 ppm due to methylene
protons of benzylaniline IX.
A solution of 2.78 g of benzyl acetate I and 23.25 g
of aniline in 55 ml of methanol was allowed to stand
at room temperature for 1 day and then poured into
400 ml of water. The precipitate was filtered off,
washed with water, and dried in air. Recrystallization
from ethanol gave colorless crystals, mp 104 105 C,
yield 79%. 1H NMR spectrum (CDCl3), , ppm:
1.44 s (18H, CMe3), 4.19 s (2H, CH2N), 5.21 s (1H,
OH), 6.65 6.85 m (3H, Hb, Hd), 7.18 (2H, Ha), 7.21 t
(2H, Hc).
I + PhNH2
t-Bu
H c
H b
Ha
MeOH
H d + MeCO2H
CH2NH
HO
t-Bu
IX
REFERENCES
At a higher concentration of compound I, the reac-
tion with aniline, like methanolysis, slows down
because of decreasing concentration of methylene-
quinone III in the reaction mixture.
1. Bukharov, S.V., Nugumanova, G.N., and Mukmene-
va, N.A., Zh. Obshch. Khim., 1998, vol. 68, no. 10,
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and Nugumanova, G.N., Zh. Obshch. Khim., 1996,
vol. 66, no. 10, pp. 1725 1727.
Thus, the reactions of benzyl acetate I with al-
cohols in the absence of acidic and basic catalysis
involve ionic dissociation of compound I with inter-
mediate formation of methylenequinone III which
then slowly takes up a molecule of the alcohol.
3. Liakumovich, A.G., Gurvich, Ya.A., Kumok, S.T., Sa-
muilov, Ya.D., and Bukharov, S.V., RU Patent
2036893, 1995, Byull. Isobret., 1995, no. 16.
The use of alcohols as solvents is an effective way
of generation of methylenequinone III from benzyl
acetate I, which allows one to avoid application of
stong bases in reactions of the latter with weak
nucleophiles.
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pp. 1977 1981.
EXPERIMENTAL
6. Nugumanova, G.N., Cand. (Chem.) Dissertation, Kazan,
1
The H NMR spectra were obtained on a Varian
1999.
Gemini-200 spectrometer (200 MHz) using signals of
residual protons of deuterated solvents as internal
references. The electronic spectra were measured on a
Specord UV-Vis spectrophotometer. Solvents were
preliminarily dried over Zeolite NaX.
7. Nikiforov, G.A., Bannikov, G.F., Romero Mal’dona-
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3,5-Di-tert-butyl-4-hydroxybenzylaniline (IX).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002