Pyrocatechol borates: Synthesis, reaction with formaldehyde, and solvent effect on polycondensation process

Article abstract of DOI:10.1016/j.polymer.2020.122162

Pyrocatechol (I) and dipyrocatechol (II) borates were synthesized by esterification between pyrocatechol (1,2-dihydroxybenzene) and boric acid in о-xylene with azeotropic distillation of water. The co-esterification between pyrocatechol and boric acid wit

Full text of DOI:10.1016/j.polymer.2020.122162

Journal Pre-proof
Pyrocatechol borates: Synthesis, reaction with formaldehyde, and solvent effect on
polycondensation process
Maxim A. Lenskiy, Elvira E. Shul'ts, Dmitriy V. Korabel'nikov, Andrey V. Ozhogin,
Alexey N. Novitskiy
PII:
S0032-3861(20)30007-0
DOI:
https://doi.org/10.1016/j.polymer.2020.122162
JPOL 122162
Reference:
To appear in: Polymer
Received Date: 11 September 2019
Revised Date: 28 December 2019
Accepted Date: 4 January 2020
Please cite this article as: Lenskiy MA, Shul'ts EE, Korabel'nikov DV, Ozhogin AV, Novitskiy AN,
Pyrocatechol borates: Synthesis, reaction with formaldehyde, and solvent effect on polycondensation
process, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2020.122162.
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Credit Author Statement
Maxim A. Lenskiy
Ideas; formulation or evolution of overarching research goals and
aims
Development or design of methodology; creation of models
Preparation, creation and/or presentation of the published work,
specifically writing the initial draft (including substantive
translation)
Conceptualization
Methodology
Writing - Original Draft
Preparation, creation and/or presentation of the published work by
Writing - Review & Editing those from the original research group, specifically critical review,
commentary or revision – including pre-or postpublication stages
Elvira E. Shul'ts
Methodology
Development or design of methodology; creation of models
Preparation, creation and/or presentation of the published work by
Writing - Review & Editing those from the original research group, specifically critical review,
commentary or revision – including pre-or postpublication stages
Dmitriy V. Korabel'nikov
Conducting a research and investigation process, specifically performing
the experiments, or data/evidence collection
Investigation
Andrey V. Ozhogin
Conducting a research and investigation process, specifically performing
the experiments, or data/evidence collection
Investigation
Alexey N. Novitskiy
Conducting a research and investigation process, specifically performing
the experiments, or data/evidence collection
Investigation

Pyrocatechol
borates:
synthesis,
reaction
with
formaldehyde, and solvent effect on polycondensation
process
Maxim A. Lenskiy1,2*, Elvira E. Shul'ts3, Dmitriy V. Korabel'nikov1, Andrey V.
Ozhogin1, Alexey N. Novitskiy1,2
1Biysk Technological Institute, Altay State Technical University (BTI AltSTU), Biysk
659305, Altai Krai, Russia
2Institute for Problems of Chemical and Energetic Technologies, Siberian Branch оf
the Russian Academy of Sciences (IPCET SB RAS), Biysk 659332, Altai Krai, Russia
3Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy
of Sciences (NIOCh SB RAS), Novosibirsk 630090, Russia
*Corresponding author. Tel.: +7(3854)43-22-85; fax: +7(3854)43-22-85.
E-mail address: lenskiy@bk.ru (M.A. Lenskiy).
ABSTRACT
Pyrocatechol (I) and dipyrocatechol (II) borates were synthesized by
esterification between pyrocatechol (1,2-dihydroxybenzene) and boric acid in о-
xylene with azeotropic distillation of water. The co-esterification between
pyrocatechol and boric acid with added equimolar quantities of phenol or 2,6-di-tert-
butylphenol was studied, and pyrocatechol phenyl (III) and pyrocatechol-(2,6-di-tert-
butyl)phenyl (IV) borates were prepared. The reaction of the resultant esters with
formaldehyde in the presence of boron trifluoride etherate as a catalyst was
examined. The coupling of the methylene moiety in several position of molecule III
was found to be dependent on the solvent used. The electrophilic substitution on the
phenol substituent took place in о-xylene, while the reaction in either benzene or
molten monomer proceeded to substitute the methylene moiety for the proton on the
catechol moiety. The molecular weights of the compounds were measured. Some
properties of the synthesized oligomers were determined and thermal and
thermooxidative destruction was examined.
Keywords: Esterification; Polycondensation; Boric acid
1

1. Introduction
Esters of organoboron compounds finding increased use in modern organic
synthesis and in catalysis [1-3], components for the synthesis of modified phenolic
and epoxy resins [4-6] and as effective ablative thermal protection materials [7].
Nowadays, boron-containing phenolic resins are preferred materials for thermo-
structural composites with respect to their high flame retardancy, thermo-oxidative
resistance and anti-neutron radiation compared to the traditional phenolic resins.
They are interest as components of synthetic transmission oils [8], and as
complexing agents for antibodies immobilized on heteroaromatic compounds [9].
Boron-containing phenolic resins and ways to synthesize them have been
reported in the literature before [4-6, 10, 11]. Those compounds are usually
synthesized by the Formaldehyde method, in which phenol borate synthesized from
phenol and boric acid is followed by the reaction with poly-formaldehyde [4-6]. An
another approach to the synthesis of boron-containing phenolic resins has been done
using the Formalin method [10, 11], that consists in the initial formation of alcohols
(reaction of phenols with formaldehyde) that followed the reaction with boric acid.
However, the reaction of phenols with formaldehyde usually produced a mixture of
several compounds in the first step.
Our research interests include the design of convenient ways to access esters
and polyesters based on phenol, resorcinol, hydroquinone, bisphenol A, and boric
acid, the assessment of their interaction with formaldehyde under conditions of water
azeotropic distillation, and the characterization of the obtained derivatives [12-14].
The aim of this work was to development ideas about boron-containing
phenolic resins based on diatomic phenol - pyrocatechol and boric acid. Thus, we
describe the co-esterification between pyrocatechol and boric acid with added
equimolar quantities of phenols, synthesis of pyrocatechol phenyl and pyrocatechol-
(2,6-di-tert-butyl)phenyl borates, reactions with formaldehyde and also the study of
the properties of the so formed oligomers.
2. Experimental section
2.1. General
NMR spectra were recorded on Bruker AV-300 (300.13 MHz for 1Н), DRX-500
(500.13 for 1Н, 125.76 for 13С, 160.46 MHz for 11В), and AV-600 (600.13 MHz for 1H,
2

150.90 MHz 13С and 192.55 MHz for 11В) spectrometers. Chemical shifts were
measured relative to residual solvent signals: СDСl3 (δН 7.24 ppm and δС 76.90
ppm), CD3OD (δН 3.34 ppm and δС 47.66 ppm) or DMSO-d6 (δН 2.50 ppm, and δС
39.50 ppm).
IR spectra were measured on a Bruker Vector 22 spectrometer in KBr pellets.
Elemental analysis was performed on a Сarlo Erba 1106 СНN analyzer (Italy).
The ash content was measured after a sample was incinerated in oxygen.
The Vicat softening temperature was determined by the V50 method (50 N
load, 50 oC/h temperature increment rate).
The number-average molecular weight was measured by acetylating the
terminal hydroxyls with acetic anhydride [15].
The thermal destruction was studied by differential thermogravimetric analysis
on a Mettler Toledo TGA SDTA 851E instrument. Samples were heated to 900°С
under dry nitrogen and to 600°С under dry air: 30 mL/min purge gas flow rate,
10 /min heating rate, an Al2O3 crucible.
2.2. Synthesis
2.2.1. Pyrocatechol borate (benzo[d][1,3,2]dioxaborol-2-ol) (I)
Into a three-neck round-bottom flask fitted with a mechanical stirrer, a
thermometer, a Dean-Stark trap, a reflux condenser and a chlorocalcium tube were
put boric acid (17.16 g, 0.278 mol), catechol (30.56 g, 0.278 mol) and о-xylene (77
mL). The whole was heated to a boil and boiled for 13 min until an estimated amount
of water was liberated. о-Xylene was withdrawn in vacuo. The reaction mixture was
recrystallized from dry benzene in inert atmosphere. Yield 34.64 g (92%). Mp = 74-
79°С (Lit. m. p. 70-75 oC [16]). Found (%): С 53.34; Н 4.14; ash (B2O3) 21.09. Calcd.
(%): С 53.02; Н 3.72; ash (B2O3) 25.61. 1Н NMR (δH, ppm): 6.68 m (1Н, Н3,6); 6.81 m
(1Н, Н4,5). 11В NMR (δB, ppm): 18.49 s. 13С NMR (δC, ppm): 117.35 (C3,6); 119.96
(C4,5); 151.53 (C1,2). IR (λ, cm-1): 742; 1281; 1365; 1470; 1514; 1602; 2963; 3453.
2.2.2. Dipyrocatechol borate (2,2'-oxydibenzo[d][1,3,2]dioxaborol, bis(benzo-1,3,2-
dioxaborolanyl)oxid) (II)
A three-necked round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser capping with a drying tube was
changed with boric acid (13.74 g, 0.222 mol), catechol (24.47 g, 0.222 mol) and о-
xylene (77 mL). The mixture was heated for 5 h under reflux until the water was
3

separated. The solvent was removed and dipyrocatechol borate was distilled in
vacuo (1 mm Hg) at 198°С in a argon stream. Yield 25.33 g (90%). Mp = 150–155°С.
(Lit. b. p. 160°/0.1 mm [16]; 146-147 oC [17]). Found (%): С 56.34; Н 3.2; ash (B2O3)
1
22.12. Calcd. (%): С 56.79; Н 3.18; ash (B2O3) 27.43. Н NMR (δH, ppm): 6.68 m
(1Н, Н3,6); 6.80 m (1Н, Н4,5). 11В NMR (δB, ppm): 19.04 s. 13С NMR (δC, ppm): 117.44
(С3,6); 120.01 (С4,5); 151.60 (С1,2). IR (λ, cm-1): 742; 1280; 1364; 1470; 1514; 1602;
1620; 2965.
2.2.3. Pyrocatechol phenyl borate (2-phenoxybenzo[d][1,3,2]dioxaborol) (III)
A three-necked round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser capped with a drying tube was
charged with boric acid (34.32 g, 0.555 mol), catechol (61.12 g, 0.555 mol), phenol
(52.21 g, 0.555 mol) and о-xylene (200 mL). The mixture was heated for 3 h under
reflux until the water was separated. о-Xylene was removed in vacuo. The
pyrocatechol phenyl borate was distilled in vacuo (1 mm Hg) at 150°С in a argon
stream. Yield 95.56 g (81%). Mp = 62–65°С. [Lit m. p. 44оС [16]). Found (%): C
1
68.31; H 4.57; ash (B2O3) 19.61. Calcd. (%): C 67.98; H 4.29; ash (B2O3) 16.42. Н
NMR (δH, ppm): 6.68 m (2H, H3,6, J3-4, 6-5 7.2 Hz, J3-5, 6-4 2.4 Hz); 6.78 t (1H, H4', J4'-3',
4'-5' 7.2 Hz); 6.85 m (2H, H2',6', J2'-3', 6'-5' 6.0 Hz); 6.85 m (2H, H4,5, J4-3, 4-5, 5-6 7.8 Hz, J4-
2.4 Hz); 7.13 t (2H, H3',5', J3'-2', 3'-4', 5'-4', 5'-6' 7.8 Hz). 11В NMR (δB, ppm): 18.52 s.
6, 5-3
13С NMR (δC, ppm): 115.87 (С2',6'); 116.21 (C3,6); 120.6 (C4'); 121.08 (C4,5); 130.15
(C3',5'); 145.4 (C1,2); 157.23 (C1'). IR (λ, cm-1): 648; 742; 1242; 1364; 1470; 1514;
1601; 1620; 2959.
2.2.4.
Pyrocatechol-2,6-(di-tert-butyl)phenyl
borate
{2-(2,6-di-tert-
butylphenoxy)benzo[d][1,3,2]dioxaborol} (IV)
A three-necked round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser capped with a drying tube was
charged with boric acid (34.32 g, 0.555 mol), catechol (61.12 g, 0.555 mol), 2,6-di-
tert-butylphenol (114.53 g, 0.555 mol) and о-xylene (200 mL). The mixture was
heated for 5.5 h until the water was separated. о-Xylene was removed under reduced
pressure. The pyrocatechol-2,6-(di-tert-butyl)phenyl borate was distilled in vacuo (1
mm Hg) at 180°С. Yield 152.51 g (85%). Mp = 143–155°С. Found (%): С 74.42; Н
8.15; ash (B2O3) 12.31. Calcd. (%): С 74.09; Н 7.79; ash (B2O3) 10.74. 1Н NMR (δH,
ppm): 1.42 s (18Н, СН3); 6.69 m (2Н, Н3,6, J3-4, 6-5 7.5 Hz, J3-5, 6-4 2.1 Hz); 6.76 t (1Н,
4

Н4', J4'-3', 4'-5' 7.8 Hz); 6.83 m (2Н, Н4,5, J4-3, 4-5, 5-6 7.2 Hz, J4-6, 5-3 2.4 Hz); 7.13 d (2Н,
Н3',5', J3'-4', 5'-4' 7.8 Hz). 11В NMR (δB, ppm) 22.38 s. 13С NMR (δC, ppm): 29.73 (CH3);
34.20 (CMe3); 115.30 (С3,6); 119.92 (С4'); 124.51 (С4,5); 126.28 (C3',5'); 137.41 (C2',6');
144.97 (С1,2); 153.94 (C1'). IR (λ, cm-1): 741; 770; 1280; 1365; 1470; 1514; 1601;
1619; 2917.
2.2.5. Polymethylene pyrocatechol borate (V)
Pyrocatechol borate I (34.64 g, 0.255 mol) and о-xylene (77 mL) were placed
into a four-neck round-bottom flask equipped with a mechanical stirrer, thermometer,
Dean-Stark trap, and reflux condenser capped with a drying tube, under an argon
flow. The mixture was heated under stirring to 80°С, boron trifluoride etherate
(BF3•Et2O) (3.09 g, 0.022 mol) was added and the stirring was continued for 15 min.
Paraformaldehyde (6.38 g, 0.213 mol) was then added portionwise (0.5–1.0 g each)
at such a rate that the temperature of the mixture did not exceed 110°С. After all the
paraformaldehyde had been added, the reaction mixture was stirred at 90°С for 1 h.
The solvent was removed in vacuo, and the residue was dried at 90°С for 2 h under
an argon flow. The resultant oligomer was dissolved in a fivefold volume of ethanol
and filtered. Ethanol was evaporated under reduced pressure and the residue was
dried in vacuo at 150°С for 1 h. Yield 29.88 g (79%). Found (%): С 58.04; Н 4.16;
ash (B2O3) 19.66. Calcd. (%): С 56.84; Н 3.38; ash (B2O3) 23.54. 1Н NMR (δH, ppm):
3.81 m (2H, CH2); 6.68 m (2Н, Н3,6, J3-4, 6-5 7.2 Hz, J3-5, 6-4 2.4 Hz); 6.78 m (1Н, Н4,5,
J4-3, 4-5, 5-6 7.2 Hz, J4-6, 5-3 2.4 Hz). 11В NMR (δB, ppm): 18.46 s. IR (λ, cm-1): 854; 1277;
1354; 1443; 1473; 1600; 2965; 3000–3638.
2.2.6. Polymethylene dipyrocatechol borate (VI)
Dipyrocatechol borate II (72.0 g, 0.284 mol) and о-xylene (250 mL) were
placed into a four-neck round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser capped with a drying tube,
under a nitrogen flow. The mixture was heated under stirring to 80°С, BF3•Et2O (5.7
g, 0.04 mol) was added and the stirring was continued for 15 min. Paraformaldehyde
(14.2 g, 0.473 mol) was then added portionwise (0.5–1.0 g each) at such a rate that
the temperature of the mixture did not exceed 110°С. After all the paraformaldehyde
had been added, the reaction mixture was stirred at 90°С for 1 h. The solvent was
then removed in vacuo, and the residue was dried at 90°С for 2 h under a nitrogen
flow. The resultant oligomer was dissolved in a fivefold volume of ethanol and
5

filtered. Ethanol was evaporated under reduced pressure and the residue was dried
in vacuo at 150°С for 1 h. Yield 26.54 g (78%). Found (%): С 59.42; Н 3.16; ash
(B2O3) 22.96. Calcd. (%): С 60.53; Н 2.91; ash (B2O3) 25.06. 1Н NMR (δH, ppm): 3.80
m (2H, CH2); 6.66 m (2Н, Н3,6); 6.78 m (1Н, Н4,5). 11В NMR (δB, ppm): 18.43 s. IR (λ,
cm-1): 853; 1277; 1358; 1473; 1508; 1600; 2965; 3000–3662.
2.2.7. Polymethylene pyrocatechol phenyl borate (VIIa)
Pyrocatechol phenyl borate III (78.94 g, 0.378 mol) and о-xylene (250 mL)
were placed into a four-neck round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and a reflux condenser capped with a drying tube,
under a nitrogen flow. The mixture was heated under stirring to 80°С, BF3•Et2O (2.28
g, 0.016 mol) was added and the stirring was continued for 15 min.
Paraformaldehyde (9.5 g, 0.317 mol) was then added portionwise (0.5–1.0 g each) at
such a rate that the temperature of the mixture did not exceed 110°С. After all the
paraformaldehyde was added, the reaction mixture was stirred at 90°С for 1 h. The
solvent was then removed in vacuo, and the residue was dried at 90°С for 2 h under
a nitrogen flow. The resultant oligomer was dissolved in a fivefold volume of ethanol
and filtered. Ethanol was evaporated and the residue dried in vacuo at 150°С for 1 h.
Yield 68.8 g (83%). Found (%): С 70.52; Н 4.06; ash (B2O3) 18.41. Calcd. (%): С
1
69.70; Н 4.06; ash (B2O3) 15.54. Н NMR (δH, ppm): 3.84 m (1Н, СН2); 6.69 m (1Н,
Н3',6', J3'-4', 6'-5' 6.9 Hz, J3'-5', 6'-4' 2.4 Hz); 6.81 m (1Н, Н4',5', J4'-3', 4'-5', 5'-6' 7.2 Hz, J5'-3', 4'-6'
2.4 Hz); 7.05 m (1Н, H3,5). 11В NMR (δB, ppm): 18.41 s. 13С NMR (δC, ppm): 29.59
(СН2); 34.41 (CH2); 115.01 (С3',6'); 119.68 (С4',5'); 126.92 (С3,5); 129.39 (C2,6); 133.60
(C4); 144.56 (С1',2'); 153.80 (C1). IR (λ, cm-1): 747; 853; 1270; 1357; 1441; 1509;
1569; 2965; 3000–3613.
2.2.8. Polymethylene pyrocatechol phenyl borate (VIIb in molten monomer)
Pyrocatechol phenyl borate III (98.56 g, 0.465 mol) was placed into a four-
neck round-bottom flask equipped with a mechanical stirrer, thermometer, Dean-
Stark trap, and a reflux condenser capped with a drying tube. After stirring for 2 min
at 80°С borate III was fully dissolved and BF3•Et2O (2.28 g, 0.016 mol) was added
under argon flow. The mixture was stirred for 15 min, then paraformaldehyde (11.69
g, 0.39 mol) was added portionwise (0.5–1.0 g each) at such a rate that the tempera-
ture of the mixture did not exceed 110°С. After all the paraformaldehyde was added,
the reaction mixture was stirred at 90°С for 1 h under a nitrogen flow. The resultant
6

oligomer was dissolved in a fivefold volume of ethanol and filtered. Ethanol was
evaporated under reduced pressure and the residue was dried in vacuo at 150°С for
1
1 h. Yield 85.27 g (82 %). Н NMR (δH, ppm): 3.67 m (2H, CH2); 6.60 m (2H, H3,6);
6.66 m (1H, H4’); 6.73 m (1H, H4,5); 6.78 m (2H, H2’,6’), 6.98 m (2H, H3’,5’). IR (λ, cm-1):
640, 754, 817, 1243, 1358, 1454, 1512, 1608, 2916, 3000–3750.
2.2.9. Polymethylene pyrocatechol phenyl borate (VIIb in benzene)
Pyrocatechol phenyl borate III (77.17 g, 0.364 mol) and benzene (250 mL)
were placed into a four-neck round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser capped with a drying tube. The
mixture was heated to 78°С, BF3•Et2O (2.28 g, 0.016 mol) was added under argon
flow. The mixture was stirred at that temperature for 15 min, then paraformaldehyde
(11.69 g, 0.39 mol) was added portionwise (0.5–1.0 g each). After all the
paraformaldehyde was added, the reaction mixture was stirred at 78°С for 1 h. The
solvent was then removed in vacuo, and the reaction mixture was dried at 90°С for 2
h under a argon flow. The resultant oligomer was dissolved in a fivefold volume of
ethanol and filtered. Ethanol was evaporated under reduced pressure and the
residue was dried in vacuo at 150°С for 1 h. Yield 66.85 g (82 %). Found (%): С
1
70.15; Н 4.21. Calcd. (%): С 69.70; Н 4.02. Н NMR (δH, ppm): 3.67 m (2H, CH2);
6.61 m (2H, H3,6); 6.67 m (1H, H4'); 6.71 m (1H, H4,5); 6.79 m (2H, H2’,6’); 6.99 m (2H,
H3’,5’). 11В NMR (δB, ppm): 18.04 s. IR (λ, cm-1): 647, 753, 814, 1231, 1358, 1454,
1511, 1600, 2921, 2958–3733.
2.2.10. Polymethylene pyrocatechol-2,6-(di-tert-butyl)phenyl borate (VIII)
Pyrocatechol aryl borate IV (78.0 g, 0.241 mol) and о-xylene (250 mL) were
placed into a four-neck round-bottom flask equipped with a mechanical stirrer,
thermometer, Dean-Stark trap, and reflux condenser, capped with a drying tube. The
mixture was heated to 80°С, BF3•Et2O (7.14 g, 0.5 mol) was added under argon flow.
The mixture was additionally stirred at that temperature for 25 min and
paraformaldehyde (6.02 g, 0.201 mol) was added portionwise (0.5–1.0 g each) at
such a rate that the temperature of the mixture did not exceed 110°С. After all the
paraformaldehyde was added, the reaction mixture was stirred at 90°С for 1 h. The
solvent was then removed in vacuo, and the residue was dried at 90°С for 2 h. The
resultant oligomer was dissolved in a fivefold volume of ethanol and filtered. Ethanol
was evaporated under reduced pressure and the residue was dried in vacuo at
7

150°С for 1 h. Yield 62.58 g (77%). Found (%): С 75.61; Н 7.66; ash (B2O3) 12.78.
Calcd. (%): С 75.02; Н 7.51; ash (B2O3) 10.35. 1Н NMR (δH, ppm): 1.19 s (18Н, CH3);
3.69 m (2H, СН2); 6.65 m (2Н, Н3,6); 6.80 m (1Н, Н4,5); 6.99 m (2Н, Н3',5'); 7.18 m
(1Н, Н4'). 11В NMR (δB, ppm): 18.41 s. IR (λ, cm-1): 752; 817; 1265; 1363; 1436;
1506; 1604; 2963; 3013–3675.
3. Results and discussion
The reaction between 1,2-aromatic diols and boric acid is known to furnish
esters [18]. Different methods for the formation of the oxoborolane ring have been
reported, for example, the exchange reaction of boron halides with pyrocatechol in
methylene chloride [16], the reaction between various boron hydrides and catechol
[17, 19], the transesterification of catechol borates [20], and the reaction of catechol
with boron anhydride [21, 22], boric acid [17, 23] or boronic acid [24]. The most
convenient method is the reaction of pyrocatechol with boric acid in xylene with
azeotropic distillation of water. The reaction between stoichiometric quantities of
catechol and boric acid yielded pyrocatechol borate I. Borate I underwent
degradation by further heating to give bis(benzo-1,3,2-dioxaborolanyl)oxid II which
was isolated in the yield about 90% by distillation in vacuo in inert atmosphere
(Scheme 1). None formation of B2cat3 (2-(o-hydroxyphenyloxy)-benzo-1,3,2-
dioxaborolan) or (cat)B(catH) [17] in this conditions was observed.
The formation of I was complete after 13 min, with the loss of water in the
amount close to the estimated. Compound I was refined by recrystallization from
absolute benzene under dry argon. Distillation of I also furnished dipyrocatechol
borate II.
OH
+ H3BO3
O
T
T
B
OH
- H2O
- H2O
O
OH
I
O
O
O
B
B
O
O
II
Scheme 1. Esterification of pyrocatechol and boric acid in o-xylene
8

Compounds I and II are well known compounds and its physico-chemical
characteristics were compared with the literature date [16, 17].
The reaction of boric acid (1 mol) with catechol (1 mol) afforded dipyrocatechol
II, which was refined by vacuum distillation under argon. The formation reaction of
the ester was complete in 5 hrs. The NMR spectra of I and II are identical.
The addition of equimolar quantities of phenol or 2,6-di-tert-butylphenol to the
reaction between pyrocatechol and boric acid delivered the respective mixed esters:
pyrocatechol phenyl borate III or pyrocatechol-2,6-(di-tert-butyl)phenyl borate IV,
respectively (Scheme 2). The resultant esters were refined by vacuum disttillation.
OH
3'
2'
6'
OH
OH
O
O
T
1'
+
+ H3BO3
B
O
4'
- 3 H2O
5'
III
OH
t-Bu
2' 3'
OH
OH
t-Bu
t-Bu
O
O
T
1'
+
+ H3BO3
4'
B
O
- 3 H2O
6'
5'
IV
t-Bu
Scheme 2. Co-esterification of catechol, boric acid and phenol or 2,6-di-tert-
butylphenol in o-xylene
1
The Н NMR spectrum of III is made up of the multiplet centered at 6.68 ppm
typical of H3,6 atoms of the catechol ring. The triplet with a peak at 6.78 ppm is
characteristic of the phenolic H4' atom. The complex multiplet centered at 6.85 ppm is
attributed to catecholic H4,5 atoms and phenolic H2',6' atoms. The triplet with a peak at
7.13 ppm is representative of phenolic H3',5' atoms.
The 1Н NMR spectrum of IV shows the singlet at 1.42 ppm typical of methyl H
atoms of the tert-butyl group. The multiplet centered at 6.69 ppm pertains to H3,6
atoms. The triplet centered at 6.76 ppm is characteristic of H4' atoms, and the
multiplet centered at 6.83 ppm is typical of H4,5 atoms. The doublet centered at 7.13
ppm is representative of H3'5' atoms.
All the synthesized monomers (I–IV) entered into a reaction with
paraformaldehyde under conditions as described [12-14] using o-xylene as the
solvent and boron trifluoride etherate as the catalyst to furnish oligomeric
compounds.
9

The reaction between I and formaldehyde afforded polymethylene
pyrocatechol borate V (Scheme 3).
HO
B
OH
B
O
C
O
O
O
O
H
H
T, Kat
- H2O
V
I
H2C
n
Scheme 3. Polycondensation of I with formaldehyde in o-xylene
The 1Н NMR spectrum of V shows the multiplet centered at 6.68 ppm typical of
H3,6 atoms and the multiplet centered at 6.78 ppm typical of H4,5 protons. The
multiplet signal at 3.81 ppm relates to the methylene protons. The signal intensity of
H4,5 protons is much lower than that of H3,6 protons, suggesting the electrophilic
substitution of the catecholic H4,5 protons by the methylene group.
The reaction of II with formaldehyde resulted in polymethylene pyrocatechol
borate VI (Scheme 4). The 1Н NMR spectra of VI and V are alike.
H2
C
O
O
O
O
O
O
O
O
O
C
B
O
B
B
O
B
H
H
T, Kat
- H2O
II
VI
C
H2
n
Scheme 4. Polycondensation of II with formaldehyde in o-xylene
The reaction of III with formaldehyde proceeded to yield polymethylene
pyrocatechol phenyl borate VII (Scheme 5).
10

H2C
O
C
(CH2)0.5
H
H
O
B
O
B
T, Kat
- H2O
O
O
O
O
2'
o-xylene
1'
6'
3'
5'
4'
III
VIIa
n
Scheme 5. Polycondensation of III with formaldehyde in о-xylene
The 1Н NMR spectrum of VIIa shows the multiplet centered at 6.69 ppm typical
of H3',6' atoms. The multiplet centered 6.78 ppm refers to H4',5' protons. The signal
centered at 7.05 ppm pertains to phenolic H3,5 protons. The multiplet signal at 3.84
ppm corresponds to methylene protons.
It follows from the comparison between 1Н NMR spectra of III and VIIa that the
electrophilic substitution took place on the phenolic ring. Along with that, the
catecholic ring had a higher electron density than the phenolic one. The reason
behind the electrophilic substitution on a ring with a lower electron density could
probably be steric hindrances and the solvent effect.
The spatial structure of III consists of two planes in one of which there lies the
sp2-hybrid residue of boric acid and the catecholic ring, and the flat phenolic ring is
attached to this plane at 120° [25]. Thus, the spatial structure of III cannot explain the
presence of steric hindrances of the electrophilic attack on the catechol aromatic ring.
Fukin et al. [25] noted the possible reaction of similar boron-containing esters,
in particular the pyrocatechol phenyl boronic ester, with methyl protons through
means of hydrogen-bonding of the latter with oxygen atoms of the dioxaborolane
fragment. Thus, o-xylene having two methyl groups may generate hydrogen bonds
with the oxoborolane ring oxygen of III, shielding the catechol ring.
To experimentally confirm the ability of o-xylene methyl protons to form a
hydrogen bond with the oxygen atom of the oxaborolan ring of compound III, we
studied the changes in the 1H NMR spectra of compound III in C6D6 recorded with o-
11

xylene additives (0.052 equiv) relatively pure xylene signals. We found, that the
singlet signal of xylene methyls is shifted to the low magnetic field (∆δ 0.01 ppm).
Also the multiplisity of the aromatic protons have some changers including the
integral intencity and the form of the multiplet at 7.11-7.17 ppm. This data are
confirmed the propose of the posibiility of o-xylene hydrogen bonding to the oxygen
atoms of the compound III.
To corroborate the above assumption, the polycondensation of III using
benzene as the solvent and reaction without the solvent (in molten monomer) was
examined herein (Scheme 6). We found, that the result of interaction of III with
formaldehyde strongly depends on the solvent used. If VIIa can be obtained from III
and formaldehyde in xylene, the contact of neat III with formaldehyde caused
vigorous reaction and formation of VIIb. In addition, formation of VIIb occurred when
formaldehyde was added to molten III. The obtained picture reveals the signifcant
diference of reactivity of compounds on the reaction conditions.
Scheme 6. Polycondensation of III with formaldehyde in benzene or molten
monomer
The 1Н NMR spectrum of VIIb has a signal centered at 6.60 ppm typical of H3,6
atoms. The signal at 6.73 ppm pertains to phenolic H4,5 and H2',6' protons. The signal
centered at 6.98 ppm refers to phenolic H3',5' protons. The signal at 6.66 ppm is
attributed to phenolic H4' protons. The multiplet signal at 3.67 ppm corresponds to
methylene protons. It follows from the comparison between 1Н NMR spectra of III and
12

VIIb that the integral intensity of catecholic H4,5 protons declined, that is, the
substitution of catecholic aromatic protons occurred.
Thus, the solvent had a determining influence on the direction of electrophilic
substitution in the polycondensation reaction between pyrocatechol phenyl borate III
and formaldehyde.
The reaction of IV with formaldehyde led to the formation of VIII (Scheme 7).
4'
5'
6'
3'
2'
O
C
1'
H
H
O
B
O
T, Kat
- H2O
B
O
O
O
O
C
VIII
IV
H2
n
Scheme 7. Polycondensation of IV with formaldehyde in o-xylene
1
The Н NMR spectrum of VIII shows the multiplet centered at 6.65 ppm
representative of H3,6 atoms. The signal at 6.99 ppm pertains to phenolic H3',5'
protons. The signal at 7.18 ppm is attributed to the phenolic H4' proton. The multiplet
signal at 3.69 ppm corresponds to methylene protons. The singlet at 1.19 ppm refers
to the methylene protons of 2,6-di-tert-butylphenol. It is seen from the spectral data
that the integral intensity of the H4,5 protons is much lower than that of the H3,6
protons, suggesting the electrophilic substitution of the catecholic H4,5 protons by the
methylene group.
The basic properties of the synthesized compounds with a polymeric structure
are summarized in Table 1 wherein it is seen that the molecular weight of the listed
compounds is not high, that is, compounds V–VIII are oligomers themselves and
have a relatively moderate Vicat softening temperature. All the compounds are well
soluble in polar organic solvents. The number-average molecular weight is not high,
and is equivalent approximately to 8−14 units.
13

Table 1
Some properties of synthesized oligomers
Solubility (g/dL)
Acetone
Entry
V
Vicat temp (°С)
Mn
Alcohol
26.2
DMF
readily
soluble
readily
soluble
readily
soluble
readily
soluble
readily
soluble
103
105
85
1780
2210
2550
3190
4090
36.6
34.9
20.0
16.7
13.9
VI
28.4
15.7
11.3
11.1
VIIa
VIIb
VIII
90
92
3.1. Thermal destruction and stability
The thermal destruction curves of the oligomers (V, VI, VIIa, VIIb, VIII) are
displayed in Fig. 1. It was shown, that the thermal destruction of the oligomers has
the same nature.
Fig. 1. Weight loss during thermal destruction of oligomers under nitrogen
14

The degradation occurred in two stages. The first stage was high-speed and
ranged from 100 to 370°C, the destruction being 37 to 47.5% for all the oligomers.
The second stage started at 370°C and the destruction rate declined. The residual
weights were 38% for V, 44.5% for VI, 42.5% for VIIa, 39% for VIIb, and 20% for VIII.
The thermal oxidation graphs of the oligomers are given in Fig. 2. It is seen
that the stability of the oligomers in oxidizing medium was nearly the same. The
stages of thermal oxidation nearly coincided those of thermal destruction. The
residual weights at 600°С were 49% for V, 44% for VI, 49% for VIIa, 38 % for VIIb,
and 39.5% for VIII.
Fig. 2. Weight loss during thermooxidative destruction of oligomers under air
As can be seen from the presented data, the kinetics of destruction both in an
inert atmosphere and during thermal oxidation is almost the same. In this case, the
mass of the residue during thermal oxidation is much less.
Compound VI is showed the maximum heat resistance, probably due to the
fact that both pyrocatechol residues are fixed by methylene bounds, which reduces
their mobility with increasing temperature. Compound V during the destruction after
100°C with the loss of water is converted to compound VI. The lowest heat
resistance, both in an oxidizing medium and in an inert atmosphere, is shown by
compound VIII. It is evidently, that this fact is due to the large number of tert-butyl
radicals, which are less resistant to temperature.
15

When studying the thermal and thermooxidative degradation of compounds
VIIa and VIIb, it can be seen that, at the initial stages of destruction from 100 to
300°С, compound VIIb is showed higher heat resistance than VIIa. The stability of
both compounds during thermal degradation is almost the same. In this case, VIIb is
less stable during thermal oxidation. Accordingly, additional binding of the phenyl ring
in the structure of the oligomer reduces its mobility (ring) and leads to an increase in
heat resistance.
4. Conclusions
In summary, mixed boric acid esters were synthesized on the basis of
pyrocatechol in this study. The interaction between the resultant esters and
formaldehyde using boron trifluoride etherate as the catalyst was examined. The
reaction of 2-phenoxybenzo[d][1,3,2]dioxaborol III with formaldehyde in o-xylene was
found to occur on the catechol ring, while the reaction proceeded on the phenolic ring
when benzene or molten monomer was used. The structures of all the synthesized
1
compounds were resolved by IR, Н-, 13С- and 11В spectroscopies and elemental
analysis. The thermal stability of the oligomers in both inert atmosphere and oxidizing
medium was studied by the TGA technique. Results from the studies shown that the
thermal and thermooxidative destruction of the oligomers, apparently, are of the
same nature. The obtained data allow us to consider new oligomers as promising
additives to polymer composite materials based on rubbers and epoxy resins.
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was financially supported by the Russian Foundation for Basic Re-
search and by Altai Krai [project No.18-43-220002].
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18

The polycondensation of 2-
phenoxybenzo[d][1,3,2]dioxaborol is effect by the solvent.
The method for the preparation and purification of
oxaborolans based on catechol.
Thermostability of catechol resins depends on phenyl
rings fixed by methylene bonds.