Reaction of triphenyl borate with 1,3,5-trioxane

Article abstract of DOI:10.1134/S1070428009120045

Triphenyl borate was prepared by reaction of boric acid with phenol in xylene. Its reaction with 1,3,5-trioxane involved replacement of protons in the para positions of the benzene rings by methylene group.

Full text of DOI:10.1134/S1070428009120045

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 12, pp. 1772–1775. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.A. Lenskii, E.E. Shul’ts, A.A. Androshchuk, G.A. Tolstikov, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45,
No. 12, pp. 1780–1783.
Dedicated to the light memory of Prof. A.M. Belousov
Reaction of Triphenyl Borate with 1,3,5-Trioxane
a
b
a
b
M. A. Lenskii , E. E. Shul’ts , A. A. Androshchuk , and G. A. Tolstikov
a
Biisk Institute of Technology, a Division of the Polzunov Altai State Technical University,
ul. Trofimova 27, Biisk, Altai Region, 659305 Russia
e-mail: Lenskiy@bk.ru
b
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
Received January 10, 2009
Abstract—Triphenyl borate was prepared by reaction of boric acid with phenol in xylene. Its reaction with
,3,5-trioxane involved replacement of protons in the para positions of the benzene rings by methylene group.
1
DOI: 10.1134/S1070428009120045
Triphenyl borate (I) is used as a weak Lewis acid
and complexing agent toward nitrogen-containing
compounds. It also attracts interest as model com-
pound for the preparation of poly(methylenetriphenyl
borates). Published data on the properties of triphenyl
borate (I), in particular on its melting point, are
strongly different [1–3]. Several procedures for the
preparation of triphenyl borate (I) have been reported.
It can be obtained by esterification of boric acid with
excess phenol in methylene chloride, followed by
separation of phenol–water mixture by fractional distil-
lation. Compound I is also synthesized by exchange
reactions of boron trichloride with phenyl acetate at
phenol in o-xylene with simultaneous removal of water
as azeotrope [3].
The goal of the present work was to synthesize pure
triphenyl borate (I) and examine its reaction with
1,3,5-trioxane (II) as a model process in the prepara-
tion of heat-resistant polymeric boric acid methylene-
phenyl esters. Reactions of phenols with formaldehyde
are commonly carried out in a solvent. A classical
procedure implies polycondensation of phenols or
boric acid phenyl esters with 1,3,5-trioxane or para-
formaldehyde in xylene under an inert gas [3–6]. We
examined polycondensation of triphenyl borate (I)
with 1,3,5-trioxane (II) using boron trifluoride–ether
complex as catalyst and xylene as solvent (Scheme 1).
–
80°C or of boric acetic anhydride with phenol with
subsequent distillation of the resulting mixture at
60°C. We believe that the most convenient procedure
is based on the reaction of boric acid with 3 equiv of
Triphenyl borate (I) becomes liquid above 101°C.
We also performed its reaction with trioxane in melt,
following a procedure analogous to the synthesis in
3
Scheme 1.
(
)0.5
O
B
O
B
O
O
BF ·Et O
3 2
+
O
O
O
O
–
H O
2
O
n
I
II
III
1
772

REACTION OF TRIPHENYL BORATE WITH 1,3,5-TRIOXANE
1773
1
1
o-xylene as solvent. In this case, smaller amount of the
catalyst was necessary. Exothermic reaction started
after addition of even 33% of the catalyst. The reaction
was carried out with simultaneous removal of water by
distillation, so that the reaction mixture became more
viscous. Nevertheless, there was no need of changing
the experimental setup, for the required amount of
reduced pressure. The B NMR spectrum of III con-
tained a singlet at δ 18.45 ppm, which is typical
of boric acid esters, as well as a minor singlet at
0.34 ppm, which belonged to residual boron tri-
B
δ
B
fluoride–ether complex.
1
11
Thus the IR, H and B NMR, and analytical data
indicate that the reaction of triphenyl borate (I) with
,3,5-trioxane (II) gives a polycondensation product
consisting of p-methylene-substituted triphenyl borate
1
,3,5-trioxane was added before termination of stir-
1
ring. The spectral parameters of the products obtained
by the two methods were identical.
units.
The IR spectrum of dry product III contained ab-
Our results unambiguously show that the solvent-
free procedure is more advantageous, for it does not
require removal of solvent and its regeneration. In ad-
dition, the amount of the catalyst necessary to initiate
the process is approximately three times as small,
which is important taking into account its cost and
toxicity. The polymeric product obtained under sol-
vent-free conditions had a higher characteristic vis-
cosity by a factor of 1.75; i.e., its molecular weight is
greater than that of a sample obtained in o-xylene.
sorption bands typical of 1,4-disubstituted benzene
–
1
ring (δCH 818 cm (cf. [7]). A strong absorption band
–
1
at 1224 cm was assigned to stretching vibrations of
the C–O bonds. Stretching vibrations of the B–O
bonds gave rise to a strong broadened absorption band
–
1
at 1348 cm [8], while bending vibrations of the
Csp –H bonds (methylene groups) were characterized
by a strong band at 1454 cm . Absorption bands at
3
–
1
–
1
1
503 and 1599 cm are typical of stretching vibrations
of aromatic C=C bonds, and the strong band at
–
1
From the mechanistic viewpoint, the reaction of tri-
phenyl borate (I) with 1,3,5-trioxane (II) is analogous
to the formation of phenol–formaldehyde resins in the
presence of acid catalyst [9]. In the first step, cleavage
of the trioxane ring by the action of boron trifluoride–
ether complex gives rise to oxymethylene cation which
then acts as hydroxymethylating agent (Scheme 2).
2
918 cm belongs to stretching vibrations of aromatic
C–H bonds. In addition, a strong diffuse band was
–
1
observed in the region 3013–3600 cm with its maxi-
–
1
mum at 3365 cm ; this band arises from intermolec-
ular H-bonds between the polymer molecules [7].
The structure of polymeric product III followed
1
from its H NMR spectrum, as well as from the spec-
1
trum of its hydrolysis product. The H NMR spectrum
Scheme 2.
of III contained doublet signals at δ 6.71 and 6.81 ppm
from protons in the ortho and meta positions of ben-
zene rings, respectively, and a singlet at δ 3.84 ppm
from the methylene protons. Compound III was sub-
jected to hydrolysis; the hydrolysis product was ex-
tracted into benzene, while the aqueous phase con-
O
BF
·Et
O
δ–
δ+
3
2
3
H C—O···BF ···OEt
2 3 2
O
O
The next step is relatively slow, and its rate deter-
mines the overall rate of the process. It involves elec-
trophilic attack by the carbocation at the para-position
of the benzene ring in triphenyl borate (Scheme 3).
Hydroxymethyl group in acid medium readily loses
hydroxide ion to give stable electrophilic benzyl cation
which rapidly replaces hydrogen atom in the para
position of benzene ring in another triphenyl borate
molecule to produce diphenylmethane derivative as
shown in Scheme 4.
1
tained mainly boric acid. In the H NMR spectrum
of the hydrolysis product we observed a singlet at
δ 3.98 ppm due to methylene protons and two doublets
centered at δ 6.91 and 7.11 ppm from protons in the
ortho and meta positions, respectively. The intensity
ratio for the aromatic and methylene protons was 24:6.
The intensity ratio for the ortho- and meta-proton
signals in the spectrum of I was 1:1.
Elemental analysis revealed larger than theoretical
percentage of carbon in the polymeric product, which
may be due to partial hydrolysis of the boric acid ester
moiety with water liberated during the polycondensa-
tion process. This is confirmed by separation of some
amount of boric acid (1–2 wt %) upon evaporation of
an alcoholic solution of the reaction mixture and
liberation of phenol while drying the product under
According to published data, hydroxymethylation
of phenol in alkaline medium gives hydroxymethyl-
phenols as the major products, whereas such com-
pounds cannot be isolated in acid medium. Electro-
philic substitution by both hydroxymethyl and methyl-
ene groups in acid medium occurs mainly at the para
position, but the formation of ortho- and ortho,para-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 12 2009

1
774
LENSKII et al.
Scheme 3.
OB(OPh)2
OB(OPh)2
OB(OPh)2
δ–
δ+
+
3H C—O···BF ···OEt
+
BF ·Et O
3 2
2
3
2
δ–
δ+
H
CH O···BF ···OEt
2 3
2
CH OH
2
Scheme 4.
OB(OPh)2
OB(OPh)2
BF ·Et O
3
2
δ–
δ+
+
HO···BF ···OEt
3 2
CH OH
CH2
2
OB(OPh)2
OB(OPh)2
(
PhO) BO
OB(OPh)2
2
S Ar
E
+
+
H+
CH2
δ–
δ+
H+
+
HO ···BF ···OEt2
H O
2
+ BF ·Et O
3 2
3
substituted products cannot be excluded [9]. For ex-
ample, the reaction of triphenyl borate with paraform-
aldehyde at 90–130°C was reported [10] to give both
para and ortho isomers, but the o/p ratio was not
given. In the examined reactions, despite the presence
of excess 1,3,5-trioxane, only the corresponding para-
substituted derivatives were formed. Presumably, the
reason is steric factor. Figure shows the structure of
triphenyl borate molecule according to PM3 semiem-
pirical quantum-chemical calculations performed with
the use of HyperChem 7.1 software. It is seen that
ortho positions in the benzene rings of molecule I are
sterically shielded by both oxygen atoms and neigh-
boring phenyl groups. Therefore, electrophilic attack
occurs exclusively at more accessible para positions.
EXPERIMENTAL
The NMR spectra were recorded on Bruker AM-
1
4
(
00 (400.13 MHz for H) and Bruker DRX-500
11
1
500.13 MHz (1H, B) spectrometers; the H chemical
shifts were determined relative to residual proton
signals of the solvent (CDCl , δ 7.24 ppm; CD OD,
3
3
δ 3.34 ppm). The IR spectra were measured in KBr on
a Bruker Vector-22 instrument. Elemental analysis was
performed on a Carlo Erba 1106 CHN analyzer. The
ash content was determined after burning a sample in
oxygen. The characteristic viscosities of solutions of
the polymers in acetone were determined at 25°C
using a VPZh-2 glass capillary viscometer.
Commercially available o-xylene of pure grade,
distilled phenol of analytical grade, boric acid of pure
grade, freshly distilled 1,3,5-trioxane of pure grade,
and freshly distilled boron trifluoride–ether complex of
pure grade were used.
Structure of the molecule of triphenyl borate (I) according to
PM3 calculations.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 12 2009

REACTION OF TRIPHENYL BORATE WITH 1,3,5-TRIOXANE
1775
Triphenyl borate (I). A three-necked flask
equipped with a mechanical stirrer, thermometer,
Dean–Stark trap, and reflux condenser capped with
a drying tube was charged with 11.45 g (0.185 mol) of
boric acid, 57.52 g (0.611 mol) of phenol, and 77 ml of
o-xylene. The mixture was heated to the boiling point
1.0-g portions. The mixture was then treated as
described in a. Yield of III 50.49 g (94%). Character-
1
istic viscosity 0.098 dl/g. H NMR spectrum (CD OD),
3
δ, ppm: 3.84 s (6H, CH ), 6.74 d.d (12H, o-H, J = 8.2,
2
1.7 Hz), 6.83 d.d (12H, m-H, J = 8.2, 2.4 Hz).
Hydrolysis of polymer III. The hydrolysis was
performed with water at 100°C (reaction time 24 h).
The mixture was extracted with benzene (5×100 ml),
the extract was evaporated on a rotary evaporator, and
(
129.5°C) and heated for 3 h under reflux until 9.5 ml
of water separated. The solvent and unreacted phenol
were distilled off under reduced pressure (1 mm), and
the residue was distilled in a vacuum. Yield 49.90 g
the residue was dried for 1 h at 150°C under reduced
1
(
92%), bp 230–240°C (1 mm), mp 98–101°C. IR spec-
pressure. H NMR spectrum (CD OD), δ, ppm: 3.98 s
3
–
1
trum, ν, cm : 2839, 1599, 1450, 1377, 1304, 899, 721.
(
6H, CH ), 6.91 d.d (12H, o-H, J = 8.4, 1.7 Hz),
2
1
H NMR spectrum (CDCl ), δ, ppm: 7.15 t (3H, p-H,
3
7.11 d.d (12H, m-H, J = 8.4, 2.8 Hz).
J = 7.4 Hz), 7.20 d (6H, o-H, J = 8.4 Hz), 7.37 t (6H,
This study was performed under financial support
by the Foundation for Support of Small-Scale
Research and Technical Enterprises (Federal program
START 07, state contract no. 4927r/7343, Moscow).
1
1
m-H, J = 8.4, 7.4, 1.6 Hz). B NMR spectrum
CDCl ): δ 16.39 ppm, s.
(
3
B
Reaction of triphenyl borate (I) with 1,3,5-tri-
oxane (II). a. A four-necked flask equipped with
a mechanical stirrer, thermometer, Dean–Stark trap,
and reflux condenser capped with a drying tube was
charged with 49.90 g (0.172 mol) of triphenyl borate
REFERENCES:
1
. Nesmeyanov, A.N. and Sokolik, R.A., Metody ele-
mentoorganicheskoi khimii. Bor, alyuminii, gallii, indii,
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Aluminum, Gallium, Indium, Thallium), Kochesh-
kov, K.A., Ed., Moscow: Nauka, 1964, p. 246.
(
I) and 50.0 ml of o-xylene. The reaction was carried
out in a stream of dry nitrogen. The mixture was
thoroughly stirred to obtain a uniform suspension and
heated to 80°C, 2.5 g (0.022 mol) of boron trifluoride–
ether complex was added, the mixture was stirred for
2
. Mikhailov, B.M., Khimiya borovodorodov (Chemistry
of Boron Hydrides), Moscow: Nauka, 1967.
2
5 min, and 12.84 g (0.143 mol) of 1,3,5-trioxane (II)
3
. Müller-Bore, G., Manitz, G., and Deufel, P., FRG
Patent no. 1816241, 1968; Chem. Abstr., 1969,
no. 71468k.
. Huster, F.J., FRG Patent no. 1233606, 1960; Chem.
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no. 84153q.
7. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion, New York: Wiley, 1972. Translated under the
title Sputnik khimika, Moscow: Mir, 1976, p. 202.
8. Pretsch, E., Bühlmann, P., and Affolter, C., Structure
Determination of Organic Compounds: Tables of
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slated under the title Opredelenie stroeniya organiche-
skikh soedinenii. Tablitsy spektral’nykh dannykh,
Moscow: Mir, 2006, p. 314.
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was added in 0.5–1.0-g portions at such a rate that the
temperature of the mixture did not exceed 110°C.
When the addition was complete, the mixture was
stirred for 1 h at 90°C, the solvent was distilled off
under reduced pressure (1 mm), and the residue was
dried for 2 h at 90°C. The polymeric product was
dissolved in five volumes of ethanol, the solution was
filtered, the filtrate was evaporated under reduced
pressure (1 mm) until alcohol no longer liberated, and
the residue was dried for 1 h at 150°C under reduced
pressure (1 mm). Yield of polymer III 50.23 g (94%).
Characteristic viscosity 0.056 dl/g. IR spectrum, ν,
4
5
6
–
1
cm : 3600–3013, 2918, 1599, 1503, 1454, 1348, 1224,
1
8
18. H NMR spectrum (CD OD), δ, ppm: 3.84 s (6H,
3
CH ), 6.71 d.d (12H, o-H, J = 8.4, 1.7 Hz), 6.81 d.d
2
1
1
(
(
12H, m-H, J = 8.4, 2.8 Hz). B NMR spectrum
CD OD), δ , ppm: 0.34 s (BF ·OEt ), 18.45 s (BO ).
3
B
3
2
3
Found, %: C 78.00; H 4.95. [C19.5H BO ] . Calculat-
14
3 n
ed, %: C 76.00; H 4.92.
9
b. The reaction of triphenyl borate (I) with 1,3,5-tri-
oxane (II) was carried out in melt in a stream of dry
nitrogen. A flask was charged with 49.90 g (0.172 mol)
of compound I and heated to 105°C, 0.83 g
1
(
0.007 mol) of BF ·Et O was added, the mixture was
3 2
stirred for 25 min, and trioxane II was added in 0.5–
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 12 2009