Mixed anhydrides of carboxylic and trifluoroacetic acids as acylating agents: I. Reactions of 3,5-Di-tert-butylbenzoyl trifluoroacetate with tert-butylphenols

Article abstract of DOI:10.1023/A:1013915129700

The rate and pathway of reactions of 3,5-di-tert-butylbenzoyl trifluoroacetate with fert-butyl-phenols depend on the number and position of bulky substituents determining the extent of steric shielding of the phenolic hydroxyl.

Full text of DOI:10.1023/A:1013915129700

Russian Journal of General Chemistry, Vol. 71, No. 11, 2001, pp. 1799 1801. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 11, 2001,
pp. 1902 1904.
Original Russian Text Copyright
2001 by Khvoinova, Muslin, Khorshev.
Mixed Anhydrides of Carboxylic and Trifluoroacetic Acids
as Acylating Agents: I. Reactions of 3,5-Di-tert-butylbenzoyl
Trifluoroacetate with tert-Butylphenols
N. M. Khvoinova, D. V. Muslin, and S. Ya. Khorshev
Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhni Novgorod, Russia
Received August 17, 2000
Abstract The rate and pathway of reactions of 3,5-di-tert-butylbenzoyl trifluoroacetate with tert-butyl-
phenols depend on the number and position of bulky substituents determining the extent of steric shielding
of the phenolic hydroxyl.
Acylation with unsymmetrical (mixed) anhydrides
of carboxylic and trifluoroacetic acids is a simple and
convenient route to esters, ketones, amides, and sul-
fones [1]. When a mixture of a carboxylic acid with
the anhydride of another carboxylic acid is treated
with an alcohol, a mixture of esters of both acids is
formed. The ratio of the esters depends on the strength
and structure of the corresponding acids. With trifluo-
roacetic anhydride, as a rule, only the product contain-
ing no trifluoroacetyl fragment is formed. This syn-
thesis route is fairly simple; the reaction occurs under
mild conditions and usually with high yields of the
target products [2 6]. By this reaction, a series of
phenyl esters were prepared, and the role of the steric
factor in acylation was noted [7].
2,4-di-tert-butylphenyl 3,5-di-tert-butylbenzoate IIId,
and this was confirmed experimentally. Two m-tert-
butyl groups in isomeric 3,5-di-tert-butylphenol IId
do not prevent esterification either: 3,5-di-tert-butyl-
phenyl 3,5-di-tert-butylbenzoate IIIe is formed.
The IR spectra of products formed in reactions of
phenols IIa IIf with anhydride I contain no bands in
the range of phenolic hydroxyl absorption but contain
bands characteristic of esters: 1725 (C=O) and
1220 and 1090 cm 1 (C O C), which confirms forma-
tion of aryl benzoates IIIa IIIf. The analytical data
are also consistent with the composition of IIIa IIIf.
Thus, when the phenolic hydroxyl is not shielded
(IIa, IIb) or is shielded weakly (IIc IIe), the major
pathway of the reaction with mixed anhydride I is
To evaluate the effect of bulky substituents on the
rate and pathway (esterification or electrophilic substi-
tution) of the reactions of phenols with mixed anhy-
drides, we studied the reactions of 3,5-di-tert-butyl-
benzoyl trifluoroacetate (I) with mono- and polysub-
stituted tert-butylphenols differing in the extent of
steric shielding of the hydroxy group. Anhydride I is
formed by dissolving 3,5-di-tert-butylbenzoic acid in
trifluoroacetic anhydride [8].
esterification:
OH
R5
R4
R1
R2
3,5-(t-Bu)2C6H3C(O)OC(O)CF3
+
R3
IIa IIf
Bu-t
The series of the studied phenols IIa IIg includes
two groups of isomers: mono- (IIb, IIc) and di-tert-
butylphenols (IId, IIe, IIg). Reaction of 4-tert-butyl-
phenol IIb with anhydride I, as in the case of unsub-
stituted phenol IIa, yields the corresponding ester,
4-tert-butylphenyl 3,5-di-tert-butylbenzoate IIIb. The
presence in isomeric 2-tert-butylphenol IIc of one
o-substituent shielding the hydroxy group does not
alter the reaction pathway: the corresponding ester,
2-tert-butylphenyl 3,5-di-tert-butylbenzoate IIIc, is
formed.
R4
R5
O
R3
OC
Bu-t
R2
R1
IIIa IIIf
II, III, R1 = R2 = R3 = R4 = R5 = H (a); R1 = R2 = R4 =
R5 = H, R3 = Bu (b); R1 = Bu, R2 = R3 = R4 = R5 = H (c);
R1 = R3 = t-Bu, R2 = R4 = R5 = H (d); R1 = R3 = R5 = H,
R2 = R4 = t-Bu (e); R1 = R3 = R5 = t-Bu, R2 = R4 = H (f);
R1 = R5 = t-Bu, R2 = R3 = R4 = H (g).
These results suggest that the reaction of 2,4-di-
tert-butylphenol IId with anhydride I should yield
1070-3632/01/7111-1799$25.00 2001 MAIK Nauka/Interperiodica

1800
KHVOINOVA et al.
Yields, melting points, and elemental analyses of aryl benzoates IIIa IIIf
Found, %
Calculated, %
Comp. Reaction
Yield,
%
mp, C (solvent
for crystallization)
Formula
no.
time
C
H
C
H
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
1 h
87
86
84
83
85
76
88 90
(heptane)
108 109
(ethanol)
47 49
(ethanol)
133 134
(hexane)
83 84
80.90
81.67
81.60
82.08
82.16
82.44
8.14
9.10
9.05
9.76
9.21
10.18
C21H26O2
C25H34O2
C25H34O2
C29H42O2
C29H42O2
C33H50O2
81.25
81.92
81.92
82.41
82.41
82.79
8.44
1 h
9.35
2 days
2 days
2 days
1 month
9.35
10.06
10.06
10.53
(hexane)
130 132
(ethanol cyclohexane)
The high extent of steric shielding of the hydroxy
group in 2,6-di-tert-butylphenol IIg, which is an iso-
mer of IId and IIe, affects the pathway of the reaction
with I. In the IR spectrum of the reaction product, the
carbonyl absorption band characteristic of esters IIIa
stitution at the free p-position, followed by isomeriza-
tion. The occurrence of the alternative esterification
pathway is favored by a decrease in the esterification
rate in the order IIa IIb > IIc IId IIe >> IIf
(see table), in accordance with the number and posi-
tion of tert-butyl groups sterically shielding the phe-
nolic hydroxyl.
1
IIIe (1725 cm ) is lacking, but a broad absorption
band is observed in the range of OH stretching vibra-
1
tions (3380 cm ). Such a spectrum is inconsistent
with the structure of 2,6-di-tert-butylphenyl 3,5-di-
EXPERIMENTAL
tert-butylbenzoate. At the same time, the absorption
1
band at 3380 cm does not correspond to vibrations
The IR spectra were taken on a Perkin Elmer spec-
of sterically shielded phenolic hydroxyl, and the car- trophotometer from mulls in mineral oil.
1
bonyl absorption at 1630 cm is untypical of diaryl
Phenyl 3,5-di-tert-butyl benzoate IIIa. To a solu-
ketones. Thus, the structure of 4-hydroxy-3,5,3 ,5 -tet-
tion of 5 mmol of 3,5-di-tert-butylbenzoic acid [9] in
ra-tert-butyldiphenyl ketone should also be excluded
5 ml of CH2Cl2 we added with stirring 0.85 ml of
from the consideration. The IR spectrum is consistent
trifluoroacetic anhydride [10] and, 10 min later, a
with the structure of the tautomer, 4-[3,5-di-tert-butyl-
solution of 5 mmol of IIa in 2 ml of CH2Cl2. The
phenyl(hydroxy)methylene]-2,6-di-tert-butyl-2,5-cy-
mixture was stirred for 1 h at room temperature, 10 ml
clohexadien-1-one IV:
of benzene was added, and the mixture was succes-
sively washed with 10% aqueous KOH and water. The
OH
t-Bu
Bu-t
O
organic layer was separated and dried over Na2SO4.
The solvent was removed in a vacuum, and the re-
sidue was recrystallized from heptane. Esters IIIb
IIIf were prepared similarly.
OH
C
t-Bu
Bu-t
I +
t-Bu
Bu-t
Reaction of I with IIg. The reaction mixture pre-
pared as described above from 1.2 g of 3,5-di-tert-
butylbenzoic acid, 0.85 ml of trifluoroacetic anhy-
dride, and 1.03 g of IIg was kept for 1 month at room
temperature and then worked up as described above.
Compound IV was obtained; yield 1.8 g (80%),
IIg
IV
However, with 2,4,6-tri-tert-butylphenol IIf, in
which the hydroxy group is sterically shielded to the
same extent as in IIg, we obtained aryl benzoate IIIf.
Thus, the steric factors do not forbid formation of
2,6-di-tert-butylphenyl 3,5-di-tert-butyl benzoate from
IIg. The reaction pathway actually observed with IIg
is due to preferableness of electrophilic aromatic sub-
1
mp 163 164 C (from heptane). IR spectrum, , cm :
3380 (OH), 1630 (C=O). Found, %: C 82.16; H 9.71.
C29H42O2. Calculated, %: C 82.41; H 10.06.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 11 2001

MIXED ANHYDRIDES OF CARBOXYLIC AND TRIFLUOROACETIC ACIDS ... : I.
1801
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 11 2001