Scope and regioselectivity of iridium-catalyzed C-H borylation of aromatic main-chain polymers

Article abstract of DOI:10.1021/ma302588a

An efficient functionalization of aromatic main-chain polymers was established using a combination of iridium-catalyzed borylation of aromatic C-H bonds and the Suzuki-Miyaura coupling reaction. Comparative studies of various iridium catalysts and borylation reagents show that [Ir(OMe)(COD)]₂ is significantly more active than [IrCl(COD)]₂, and bis(pinacolato)diboron induces higher efficiency than pinacolborane. The regioselectivity of the borylation was investigated using model compounds that mimic the repeating unit structure of poly(arylene ether sulfone). The C-H bonds of the sulfone model compound were more reactive than those of the bisphenol model compound, and the borylation occurred preferentially at the meta position to the sulfone moiety owing to steric hindrance and electronic effects. The glass transition temperature of the borylated polymer increases with increasing concentration of pinacolboronic ester group. The pinacolboronic ester group could be conveniently converted to boronic acid [B(OH)₂] and potassium trifluoroborate (BF₃K), which could also serve as versatile reactive sites for the synthesis of a wide range of functionalized polymers via Suzuki-Miyaura couplings.

Full text of DOI:10.1021/ma302588a

Article
pubs.acs.org/Macromolecules
Scope and Regioselectivity of Iridium-Catalyzed C−H Borylation of
Aromatic Main-Chain Polymers
Ying Chang,†,‡ Hanniel H. Lee,‡ Se Hye Kim,‡ Tae Soo Jo,‡ and Chulsung Bae†,‡,*
†Department of Chemistry and Chemical Biology, New York State Center for Polymer Synthesis, Rensselaer Polytechnic Institute,
110 Eighth Street, Troy, New York 12180, United States
‡Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, Nevada 89154−4003
S
* Supporting Information
ABSTRACT: An efficient functionalization of aromatic main-
chain polymers was established using a combination of
iridium-catalyzed borylation of aromatic C−H bonds and the
Suzuki−Miyaura coupling reaction. Comparative studies of
various iridium catalysts and borylation reagents show that
[Ir(OMe)(COD)]2 is significantly more active than [IrCl-
(COD)]2, and bis(pinacolato)diboron induces higher effi-
ciency than pinacolborane. The regioselectivity of the
borylation was investigated using model compounds that mimic the repeating unit structure of poly(arylene ether sulfone).
The C−H bonds of the sulfone model compound were more reactive than those of the bisphenol model compound, and the
borylation occurred preferentially at the meta position to the sulfone moiety owing to steric hindrance and electronic effects. The
glass transition temperature of the borylated polymer increases with increasing concentration of pinacolboronic ester group. The
pinacolboronic ester group could be conveniently converted to boronic acid [B(OH)2] and potassium trifluoroborate (BF3K),
which could also serve as versatile reactive sites for the synthesis of a wide range of functionalized polymers via Suzuki−Miyaura
couplings.
weights can be used as starting materials, a controlled
postfunctionalization could, in principle, create a broad range
of functionalized PSUs with various molecular weights.
However, electrophilic functionalization of PSU involves the
formation of highly reactive carbocation intermediates and, as a
result, frequently causes undesirable side reactions such as chain
scission and gelation (i.e., extensive cross-linking of polymer
chains). These side reactions can alter the molecular weights
and physical properties of functionalized polymers. Thus,
delicate control of the reaction conditions is essential to avoid
or minimize undesirable side reactions along polymer chains.
Postfunctionalization of PSU through lithiation followed by
reaction with electrophiles has been investigated, but lithiation
also requires well-controlled, low-temperature conditions owing
to the extreme reactivity of lithiating reagents.13−15
Recently, functionalization of C−H bonds using transition
metal catalysts has emerged as a new versatile strategy in
organic synthesis16−21 and polymer functionalization.22−29 In
contrast to traditional polymer functionalizations via reactive
free radical or carbocation intermediates, which frequently
cause undesired side reactions, this new method induces
negligible changes in the molecular weights of functionalized
polymers. Of special interest, iridium-catalyzed borylation of
aromatic C−H bonds has afforded controlled functionalization
INTRODUCTION
■
Aromatic main-chain polymers are important high-performance
engineering plastics owing to excellent thermo-mechanical
stability and chemical resistance. For example, Udel poly-
(arylene ether sulfone) (PSU) has been used extensively as a
base material in membrane-based separation technology owing
to its robust stability and semipermeable structure.1,2
Incorporating specific functional groups into aromatic main-
chain polymers such as PSU expands the properties and
broadens the application of these polymers in material science.
Two approaches have been pursued to incorporate desired
functional groups into PSU: (1) copolymerization with
functionalized monomers, and (2) postfunctionalization. In
the copolymerization method, nucleophilic functional groups,
such as alcohol and amine, interfere with reactive sites during
nucleophilic aromatic substitution polymerization and form
undesired low-molecular-weight polymer products. In addition,
because the polycondensation is generally conducted at high
temperature (typically >150 °C), sensitive functional groups
cannot survive during the polymerization.3,4
Postfunctionalization allows the introduction of a wide range
of functional groups onto polymers, frequently that are not
compatible with the original polymerization methods. Among
postfunctionalization approaches, electrophilic aromatic sub-
stitutions (e.g., sulfonation,5,6 bromination,7,8 chloromethyla-
tion,9−11 and amidoalkylation12) are the most commonly used
strategies to introduce functional groups into the aromatic rings
of PSU. Because a variety of polymers with different molecular
Received: December 17, 2012
Revised: February 8, 2013
© XXXX American Chemical Society
A
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prepared by applying gentle heat to dissolve polymer in specific
deuterated solvents. The mol % of Bpin in borylated polysulfone
(PSU−Bpin) was estimated based on the relative intensity of
resonances of −C(CH3)2− in the polymer main chain (at 1.6−1.7
ppm) and the four methyl groups of the Bpin (at 1.0−1.2 ppm).
Molecular weight measurement was performed using a VISCOTEK
SEC equipped with three Jordi−HPLC Series columns and tetra
detector array (UV/vis, low and right angle light scattering, refractive
index, viscometer) at 40 °C. THF was the mobile phase and the flow
rate was set at 0.3 mL/min. The instrument was calibrated using
polystyrene standards.
of commercial polystyrenes with various tacticities. The
pinacolboronic ester (Bpin) group has been introduced into
the aromatic rings of polymer chains without negatively
affecting chain length or the tacticity of the starting
polymer.26,27 We have also discovered that aromatic main-
chain polymers such as PSU can be functionalized in a
controlled manner using an iridium-catalyzed C−H borylation.
Bpin-functionalized PSU is a versatile precursor that can
generate a variety of polymers containing specific functional
groups with targeted concentrations.28,29 This new transition
metal-catalyzed synthetic method of functionalized aromatic
polymers is unique in a sense that it can convert aromatic C−H
bonds of polymers directly to a variety of functional groups (a)
without requiring to have a precursor group in polymerization
and (b) without restriction on the molecular weight of
polymers. For example, Grignard metathesis polymerization
(GRIM) has been a widely used synthetic method for
conjugated aromatic polymers. However, due to incompatibility
with Grignard reagents only a limited examples of functional
groups (alkyl and alkoxy in most cases) can be attached to the
aromatic main-chain polymers.30 Although ruthenium-catalyzed
ring-opening metathesis polymerization (ROMP) allows
incorporation of more diverse ranges of functional groups
into of polyolefins,31 it cannot be applied for the synthesis of
aromatic main-chain polymers. While both GRIM and ROMP
require preinstallation of a functional (or precursor) group in
monomer before the polymerization, the iridium-catalyzed C−
H borylation of aromatic polymers does not.
Unless polymer substrate has a preferentially reactive C−H
bond, the iridium-catalyzed C−H borylation of an aromatic
polymer affords a mixture of randomly functionalized isomeric
polymer products, which is a major difference in comparison
with GRIM and ROMP. Thus, we investigated the scope and
regioselectivity of the iridium-catalyzed C−H borylation using
PSU as parent polymer in this study. The PSU repeating unit
has four different aromatic C−H bonds that can be borylated,
each with a different reactivity toward the borylation. Because
the direct analysis of regioisomers of functionalized polymers
was difficult, we examined the regioselectivity of the PSU
borylation by studying the reaction of corresponding model
compounds.
Table 1. Iridium-Catalyzed Arene C−H Borylation of
Poly(arylene ether sulfone) (PSU) with
a
Bis(pinacolato)diboron (B2pin2).
PSU−Bpin
c
B2pin2
/PSU
PDI (Mw/
Mn)
b
c
d
e
entry
cat.
Mn
Bpin (%) eff. (%)
1
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.8
1.0
1.0
1.0
1.0
1.2
1.2
1.4
1.4
1.6
1.6
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
22.4
23.3
23.9
29.0
28.8
30.6
30.3
32.0
32.9
35.6
22.3
32.8
36.5
40.5
33.5
42.1
37.6
45.8
1.97
1.82
1.97
1.66
1.78
1.76
1.86
1.78
1.77
1.66
1.83
1.83
1.80
1.57
1.88
1.47
1.81
1.42
23
57
69
80
90
79
84
79
85
74
83
13
79
73
77
70
74
65
71
2
27
3
64
4
72
5
95
6
101
126
135
147
165
26
7
8
9
10
f
11
f
12
158
175
184
196
208
206
227
13
14
15
16
17
18
a
Bpin = pinacolboronate ester. PSU (Mn = 22.0 kg/mol, PDI = 1.65).
Unless otherwise specified, C−H borylation was conducted on 0.20 g
of polymer with 3 mol % of iridium and 3 mol % of ligand relative to
b
B2pin2 in 1 mL THF at 80 °C for 12 h. A: [IrCl(COD)]2. B:
c
[Ir(OMe)(COD)]2. Mn in kg/mol and PDI were measured with size-
exclusion chromatography using THF as the mobile phase at 40 °C
d
EXPERIMENTAL SECTION
relative to polystyrene standards. Molar percentage of Bpin group per
■
1
repeating unit of PSU calculated based on H NMR spectrum. 100%
General Comments. Udel poly(arylene ether sulfone) [PSU; Mn
= 22.0 kg/mol, polydispersity index (PDI) = 1.65 measured from size
exclusion chromatography (SEC) using THF as eluent] and
pinacolborane (HBpin) were purchased from Sigma-Aldrich. 4,4′-Di-
tert-butyl-2,2′-dipyridyl (dtbpy), di-μ-methoxobis(1,5-cyclooctadiene)-
diiridium(I) ([Ir(OMe)(COD)]2), and tetrakis(triphenylphosphine)-
palladium(0) [Pd(PPh3)4] were obtained from Strem Chemicals Inc.
and used as received. Hexane, methanol, and chloroform were reagent
grade and used without further purification. Anhydrous tetrahydrofur-
an (THF) was obtained from Acros Organics and stored in a nitrogen-
filled glovebox. Bis(pinacolato)diboron (B2pin2) and chloro-1,5-
cyclooctadiene iridium(I) dimer ([IrCl(COD)]2) were obtained
from Frontier Scientific Co. and Sinocompound Technology Co.,
respectively, and used without further purification. Synthetic
procedures of model compounds and their NMR spectra, including
S, B, P, S-o-Bpin, S-m-Bpin, B-m-Bpin, S-o-BF3K, S-m-BF3K, and B-
m-BF3K, were given in the Supporting Information.
indicates an average of 1.00 Bpin group is attached to the PSU
e
repeating unit. Efficiency of C−H borylation: amount of Bpin
attached to the polymer divided by the number of boron atoms added.
f
C−H borylation was conducted at room temperature for 18 h.
Preparation of PSU−Bpin Using B2pin2 (Entry 1 of Table 1). In a
nitrogen-filled glovebox, PSU (0.20 g, 0.45 mmol of polymer repeating
unit), B2pin2 (23 mg, 0.09 mmol), [IrCl(COD)]2 (0.9 mg, 1.5 mol %
based on the amount of B2pin2), dtbpy (0.7 mg, 3 mol % based on the
amount of B2pin2), THF (1 mL) and a magnetic stirring bar were
placed into a vial and the vial was capped with a Teflon-lined septum.
The vial was removed from the glovebox and stirred in an oil bath at
80 °C for 12 h. After cooling to room temperature, the reaction
solution was diluted with chloroform and filtered through a short plug
of silica gel to remove the catalyst. The filtrate was concentrated using
a rotary evaporator and poured into hexane to precipitate polymer.
The dissolution and precipitation process was repeated one more time
to ensure complete removal of any impurities trapped in polymer. The
borylated polymer was isolated as a white solid and was dried under
1H, 13C, 11B, and 19F NMR spectra were obtained at 400, 100, 128,
and 376 MHz, respectively. H and 13C NMR chemical shifts were
1
referenced to solvent residue peak, and 19F NMR chemical shifts were
referenced to CFCl3. 11B NMR chemical shifts were reported relative
to an external reference of BF3−Et2O. The NMR samples were
1
vacuum at 60 °C for 12 h (0.20 g). H NMR (CDCl3): δ 8.26−8.34
(Harom), 7.72−7.92 (Harom), 7.14−7.30 (Harom), 6.80−7.04 (Harom),
B
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1.60−1.74 [C(CH3)2 of PSU main chain], 1.22 and 1.01 [BOCCH3].
13C NMR (CDCl3): δ 165.4, 161.9, 154.7, 152.8, 147.3, 147.1, 145.9,
136.3, 135.6, 135.3, 131.0, 129.7, 129.5, 129.4, 128.4, 128.1, 119.8,
119.7, 118.7, 118.3, 117.6, 116.5, 113.3 (all from Carom), 83.9
[BOC(CH3)], 42.6 [C(CH3)2 of PSU main chain], 30.9 [C(CH3)2
of PSU main chain], 24.5 [BOC(CH3)]. 11B NMR (CDCl3): δ 30.4
(broad).
mL) and 30% H2O2 (3 mL) were slowly added in sequence to the
stirring polymer solution and the reaction mixture was stirred for 1 h
at 50 °C. The polymer was precipitated as a white solid during the
reaction due to the change of solubility. After removal of liquid from
the solution residual polymer was washed with water and methanol,
filtered and dried at 80 °C under vacuum for 12 h (0.31 g). 1H NMR
(DMSO-d6): δ 7.60−7.90, 7.25, 6.80−7.10, 6.30−6.70 (all from
Harom), 1.40−1.70 [C(CH3)2].
Preparation of PSU−B(OH)2. 46% PSU−Bpin (0.50 g, 0.46 mmol
Bpin) was dissolved in dioxane (60 mL) and NaIO4 (1.47 g, 6.89
mmol, 15 equiv) was added. Water (3 mL) was added dropwise to the
polymer solution and the reaction mixture was stirred at 90 °C for 8 h.
After evaporation of the solvent using a rotary evaporator, the residual
polymer was washed with a mixture of methanol and water, filtered
and dried at 80 °C under vacuum for 12 h (0.50 g). 1H NMR (DMSO-
d6): δ 8.26, 7.80−8.05, 7.25, 6.70−7.10 (all from Harom), 1.61
[C(CH3)2].
Preparation of PSU−BF3K. 46% PSU−Bpin (0.50 g, 0.46 mmol
Bpin) was dissolved in dioxane (60 mL). An aqueous solution of KHF2
[0.54 g (6.89 mmol, 15 equiv) dissolved in 6 mL of water] was added
dropwise to the polymer solution and the reaction mixture was stirred
at 90 °C for 8 h. The reaction solution was concentrated using a rotary
evaporator and poured into methanol to precipitate polymer. The
obtained polymer was washed with water and methanol, filtered and
dried at 80 °C under vacuum for 12 h (0.48 g). 1H NMR (DMSO-d6):
δ 8.27, 7.80−8.05, 7.26, 6.70−7.12 (all from Harom), 1.62 [C(CH3)2].
19F NMR (DMSO-d6): δ −137.6 (BF3K).
Table 2. Iridium-Catalyzed Arene C−H Borylation of
Poly(arylene ether sulfone) (PSU) with Pinacolborane
a
(HBpin).
PSU−Bpin
b
c
c
d
e
entry HBpin/PSU cat.
Mn
PDI Mw/Mn Bpin (%)
eff. (%)
1
2
3
4
5
6
0.6
0.6
1.0
1.0
1.4
1.4
A
B
A
B
A
B
22.8
25.2
29.8
28.9
29.8
31.8
1.96
1.82
1.81
1.73
1.75
1.68
33
39
55
66
66
93
55
65
55
66
47
66
a
Bpin = pinacolboronate ester. PSU (Mn = 22.0 kg/mol; PDI = 1.65).
Unless otherwise specified, C−H borylation was conducted on 0.20 g
of polymer with 3 mol % of iridium and 3 mol % of ligand relative to
b
HBpin in 1 mL THF at 80 °C for 24 h. A: [IrCl(COD)]2. B:
c
[Ir(OMe)(COD)]2. Mn in kg/mol and PDI were measured with size-
Competitive C−H Borylation of Model Compounds S and B.
Model compounds S (53 mg, 0.19 mmol) and B (49 mg, 0.19 mmol),
B2pin2 (77 mg, 0.3 mmol), [IrCl(COD)]2 (3.0 mg, 1.5 mol % of
B2pin2), dtbpy (2.5 mg, 3 mol % of B2pin2), THF (0.25 mL) and a
magnetic stirring bar were added to a 4 mL vial which was capped with
a Teflon-lined septum in a nitrogen-filled glovebox. The reaction
mixture was stirred at 80 °C for 12 h. After cooling to room
temperature, the reaction solution was diluted with ethyl acetate (30
mL), washed with water (15 mL × 3) and brine (15 mL), and dried
with magnesium sulfate. Evaporation of volatile solvent using a rotary
evaporator gave 0.16 g of borylated product as a brown syrup which
exclusion chromatography using THF as mobile phase at 40 °C
d
relative to polystyrene standards. Molar percentage of Bpin group per
1
repeating unit of PSU calculated based on H NMR spectrum. 100%
indicates an average of 1.00 Bpin group is attached to the PSU
e
repeating unit. Efficiency of C−H borylation: amount of Bpin
attached to the polymer divided by the number of boron atoms added.
Preparation of PSU−Bpin Using HBpin (Entry 1 of Table 2). In a
nitrogen-filled glovebox, PSU (0.20 g, 0.45 mol of polymer repeating
unit), HBpin (35 mg, 0.27 mmol), [IrCl(COD)]2 (2.7 mg, 1.5 mol %
based on the amount of HBpin), dtbpy (2.2 mg, 3 mol % based on the
amount of HBpin), THF (1 mL) and a magnetic stirring bar were
placed into a vial and the vial was capped with a Teflon-lined septum.
The vial was removed from the glovebox and stirred in an oil bath at
80 °C for 24 h. After cooling to room temperature, the reaction
solution was diluted with chloroform and filtered through a short plug
of silica gel to remove the catalyst. The filtrate was concentrated using
a rotary evaporator and poured into hexane to precipitate polymer.
The dissolution and precipitation process was repeated one more time
to ensure complete removal of any trapped impurities in the polymer.
The polymer was isolated as a white solid and dried under vacuum at
60 °C for 12 h (0.21 g). NMR data (1H, 13C, 11B) were identical to
those prepared with B2pin2.
1
was analyzed by H NMR spectroscopy (DMSO-d6): δ 8.08−8.10,
7.90−8.00, 7.80−7.86, 7.40−7.42, 7.06−7.24, 6.78−6.87 (all from
Harom), 3.81, 3.71, 1.57, 1.28, 1.24, 1.16.
Conversion of a Mixture of S-Bpin and B-Bpin to S-BF3K and B-
BF3K. The above crude product (a mixture of S-Bpin and B-Bpin, 0.16
g), KHF2 (0.24 g, 3.04 mmol), THF (1.5 mL), water (0.5 mL) and a
magnetic stirring bar were added to a 4 mL vial which was capped with
a Teflon-lined septum. The reaction mixture was stirred for 12 h at
room temperature. After evaporation of the solvent using a rotary
evaporator, the residue was dissolved in acetone and filtered to remove
insoluble solid. Evaporation of the filtrate using a rotary evaporator
gave 92 mg of product which was analyzed by 19F NMR spectroscopy
(DMSO-d6): δ −135.9, −137.5 (an integral ratio of 1:6).
General Procedure for Preparation of PSU−FG via Suzuki−
Miyaura Coupling Reaction of PSU−Bpin. In a nitrogen-filled
glovebox, 46% PSU−Bpin (0.15 g, 0.14 mmol of Bpin), functionalized
phenyl bromide (1.38 mmol, 10 equiv based on the amount of boryl
group of PSU−Bpin), Pd(PPh3)4 (4.8 mg, 3 mol % based on the
amount of boryl group of PSU−Bpin), K2CO3 (57 mg, 3 equiv based
on the amount of boryl group of PSU−Bpin), THF (2 mL) and a
magnetic stirring bar were placed into a 4 mL vial. The vial was capped
with a Teflon-lined septum and was removed from the glovebox.
Deionized water (0.2 mL) was added to the vial using a micro syringe,
and the reaction mixture was stirred at 80 °C oil bath for 6 h. After
cooling to room temperature, the reaction solution was diluted with
chloroform and filtered through a short plug of silica gel, concentrated
using a rotary evaporator and poured into hexane to precipitate
polymer. The dissolution and precipitation process was repeated one
more time. The filtered polymer was dried under vacuum at 80 °C for
12 h.
C−H Borylation of Model Compound P. Model compound P (60
mg, 0.12 mmol), B2pin2 (24 mg, 0.09 mmol, 0.8 equiv), [IrCl(COD)]2
(0.9 mg, 1.5 mol % of B2pin2), dtbpy (0.7 mg, 3 mol % of B2pin2),
THF (0.2 mL) and a magnetic stirring bar were added to a 4 mL vial
which was capped with a Teflon-lined septum in a glovebox. The
reaction mixture was stirred at 80 °C for 12 h. After cooling to room
temperature, the reaction solution was diluted with ethyl acetate (30
mL), washed with water (15 mL × 3) and brine (15 mL), and dried
with magnesium sulfate. Evaporation of volatile solvent using a rotary
evaporator gave borylated product as a brown syrup (76 mg). The
borylated P, KHF2 (73 mg, 0.93 mmol), THF (1.0 mL), water (0.3
mL), and a magnetic stirring bar were added to a 4 mL vial which was
capped with a Teflon-lined septum. After stirring for 12 h at room
temperature, the reaction solution was evaporated using a rotary
evaporator. The residue was dissolved in acetone and filtered to
remove insoluble solid. Evaporation of the filtrate gave 92 mg of
yellowish solid which was analyzed by 19F NMR spectroscopy
(DMSO-d6): δ −136.4, −137.6 (an integral ratio of 1:7).
Preparation of PSU−OH. 46% PSU−Bpin (0.50 g, 0.46 mmol
Bpin) was dissolved in dioxane (60 mL). Two M NaOH solution (3
C
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C−H bonds of sp3 carbon is known to be unreactive in the
iridium-catalyzed C−H borylation of arenes,32,33 we selected
RESULTS AND DISCUSSION
■
A. C−H Borylation of Poly(arylene ether sulfone). The
iridium-catalyzed C−H borylation of PSU with various molar
ratios of B2pin2 to polymer repeating unit in THF produced
borylated polymers (PSU−Bpin) containing various concen-
trations of Bpin group (Scheme 1). PSU repeating unit contains
1
the H NMR resonance of the isopropylidene group as a
reference and the mol % of attached Bpin group per PSU
repeating unit was calculated by comparing the resonances
intensities of −C(CH3)2− in the polymer backbone (at 1.6−1.7
ppm) and the methyl groups of Bpin (at 1.0−1.2 ppm). These
results are summarized in Table 1. The Bpin concentration
could be controlled from 20 to 225 mol % by adjusting the ratio
of B2pin2 relative to PSU repeating unit. The efficiency of C−H
borylation, which is defined as the percentage of incorporated
Bpin relative to added boron atoms, was consistently high (60−
85% in most cases) when a precursor catalyst mixture
composed of 1.5% [IrCl(COD)]2 (COD = 1,5-cyclooctadiene)
and 3% dtbpy was used. Thus, this approach allows convenient
control of the degree of functionalization via stoichiometric
tuning of the amount of added B2pin2. We also discovered that
the quality of THF affects the efficiency of C−H borylation:
anhydrous THF stored in a nitrogen-filled glovebox consis-
tently demonstrated an efficiency 15−20% higher than that of
ACS grade THF.
Scheme 1. Iridium-Catalyzed C−H Borylation of
Poly(arylene ether sulfone)
Since our initial report of the aromatic C−H borylation of
PSU catalyzed by [IrCl(COD)]2 and dtbpy,28 we have
investigated the scope of the catalyst system of the borylation.
Miyaura and co-workers32,33 have reported that the catalyst
activity of arene C−H borylation depends on the basic strength
of the anionic ligands of the iridium complex. Compared with
[IrCl(COD)]2, Ir(I) complexes with alkoxy ligands were
reported to catalyze the C−H borylation of arenes at a faster
rate owing to the more expeditious formation of (boryl)IrI
intermediates. Thus, we compared activity of two iridium
precursor catalysts, [IrCl(COD)]2 and [Ir(OMe)(COD)]2, in
THF at 80 °C and room temperature (see Table 1 and Figure
2). Although both catalysts had comparable activity at 80 °C,
the former was significantly less active at room temperature
(see Table 1, entries 9 and 11). By contrast, [Ir(OMe)-
(COD)]2 displayed a similar activity at both 80 °C and room
temperature (see Table 1, entries 10 and 12). Both catalysts
gave consistent results of borylation over a wide range of the
ratios of B2pin2 and PSU (see Figures S28 and S29 of the
Supporting Information).
one isopropylidene group and four aromatic rings with multiple
C−H bonds that can be borylated. As shown in Figure 1, the
1H NMR spectra of xx-PSU−Bpin (where xx indicates the
molar percentage of Bpin group attached to the repeating unit)
showed distinctive new resonances at 1.0−1.2 ppm from the
methyl groups of Bpin attached to the polymer chain. Since the
C−H Borylation using HBpin as the borylating reagent was
also investigated (Table 2). Because only one boron atom in
HBpin participates in the reaction, the degrees of borylation in
this reaction were significantly lower than those using B2pin2.
However, the efficiency, defined as the ratio of incorporated
boron atoms relative to added boron atoms, was only slightly
lower than that with B2pin2. Again, between the two iridium
catalysts, [Ir(OMe)(COD)]2 displayed degrees of borylation
and efficiency that were consistently higher than those of
[IrCl(COD)]2.
To investigate the possibility of side reactions affecting Mn
and polydispersity index (PDI; Mw /Mn] during C−H
borylation, we conducted SEC before and after the reaction
(Figure S30 in the Supporting Information). The unfunction-
alized PSU had a Mn of 22.0 kg/mol and a PDI of 1.65. As
shown in Table 1, an increase in the ratio of B2pin2 to polymer
repeating unit in the polymer functionalization reaction
increased the Mn. A Different solubility of polymer after
postfunctionalization may affect the size of hydrodynamic
volume and average molecular weights of SEC measurement.
However, since both PSU and borylated PSUs have a good
solubility in THF, we attributed the increase of molecular
1
Figure 1. H NMR spectra of (a) PSU, (b) 27-PSU−Bpin (Table 1,
entry 2), and (c) 101-PSU−Bpin (Table 1, entry 6).
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Scheme 2. Suzuki−Miyaura Cross-Coupling Reactions with
PSU−Bpin
Figure 2. (a) Mol % of pinacolboronic ester (Bpin) in the
poly(arylene ether sulfone) (PSU) repeating unit, and (b) efficiency
of C−H borylation: (A) [IrCl(COD)]2; (B) [Ir(OMe)(COD)]2.
weights of PSU−Bpin to the incorporation of Bpin group into
the polymer chain and the corresponding increase in
hydrodynamic volume rather than solubility difference. PDI
values always stayed within the range of 1.6−1.9. Similar results
were obtained from borylation with HBpin: the Mn of the
borylated PSU gradually increased with increasing amounts of
HBpin, and PDIs consistently ranged from 1.7 to 2.0 (see Table
2). These results clearly indicate that the iridium-catalyzed
borylation of the aromatic C−H bonds of PSU induces no
deleterious side reactions (i.e., chain degradation or cross-
linking reactions) on the polymer chains.
B. Transformation of Bpin in PSU to Other Functional
Groups via Suzuki−Miyaura Cross-Coupling Reactions.
Palladium-catalyzed cross-coupling of aryl halide and aryl boron
(the Suzuki−Miyaura reaction) is widely used to construct
biaryl C−C bonds in organic synthesis because of its excellent
tolerance for a broad range of functional groups.34−37
Accordingly, Bpin group in an aromatic polymer can be
converted to various functional groups via Suzuki−Miyaura
reactions with functionalized aryl halides. To investigate this
possibility, we selected 46-PSU−Bpin as a precursor polymer
and conducted the Suzuki reaction with phenyl bromides that
contained a ketone, an amine, a tert-butyl carbamate- (Boc-)
protected amine, an alcohol, and an aldehyde (Scheme 2). As
shown in Figure 3, the coupling reactions of 46-PSU−Bpin
afforded the corresponding functionalized PSUs (PSU−FG,
where FG is COCH3 for ketone, NMe2 for amine, NHBoc for
Boc-protected amine, CH2OH for benzyl alcohol, and CHO for
aldehyde) with almost identical functional degrees. Molecular
Figure 3. 1H NMR spectra of functionalized poly(arylene ether
sulfone)s from Suzuki−Miyaura cross-coupling reactions: (a) 47%
PSU−COCH3, (b) 48% PSU−NMe2, (c) 46% PSU−NHBoc, (d) 46%
PSU−CH2OH, and (e) 47% PSU−CHO.
weight analysis using SEC revealed no detectable polymer chain
scission or cross-linking.
The thermal properties of PSU, PSU−Bpin, and PSU−FG
were investigated using differential scanning calorimetry. As
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shown in Figure 4, the glass transition temperature (Tg) of
PSU−Bpin was higher than that of PSU, and it increased with
oxidation, the resulting polymer (PSU−OH) became insoluble
in CHCl3 and THF but soluble in polar aprotic solvents such as
dimethyl sulfoxide and dimethylacetamide. This dramatic
solubility change is due to the transformation of the less
polar Bpin group to the more polar hydroxyl group. The Bpin
resonances disappeared in the 1H NMR spectrum of PSU−OH
(Figure 6b), and a strong O−H stretching band around 3400
cm−1 appeared in the FT-IR spectrum (Figure S26 of
Supporting Information).
We also converted the Bpin moiety of PSU−Bpin to boronic
acid [PSU−B(OH)2] and potassium trifluoroborate (PSU−
BF3K) using NaIO4 and KHF2, respectively (Scheme 3). These
new boron functional groups induced significantly higher
polarity in the resulting polymers and made them poorly
soluble in chloroform and THF but readily soluble in dimethyl
sulfoxide. The disappearance of methyl resonances at 1.0−1.3
ppm in the 1H NMR spectrum (Figure 6c) and the appearance
of a broad O−H stretching band around 3500 cm−1 in the FT-
IR spectrum confirmed the conversion of Bpin to B(OH)2 in
the polymer (see Figure S26). Transformation of PSU−Bpin to
PSU−BF3K was also confirmed by a characteristic resonance at
−137.6 ppm in the 19F NMR spectrum (Figure S27).
Aryl boronic acid and aryl potassium trifluoroborate are
commonly used boron substrates in Suzuki−Miyaura coupling
reactions.45 To confirm that the transformations of Bpin to the
new boryl groups caused no deborylation in the polymer, we
conducted Suzuki−Miyaura reactions of 46-PSU−B(OH)2 and
46-PSU−BF3K with 4′-bromoacetophenone (Scheme 4). The
resulting polymer, PSU−COCH3, maintained a similar func-
tional degree, demonstrating that both PSU−B(OH)2 and
PSU−BF3K kept the same level of boryl group as the parent 46-
PSU−Bpin.
Figure 4. Differential scanning calorimetry scans of (a) PSU, (b) 46-
PSU−Bpin, (c) 138-PSU−Bpin, and (d) 176-PSU−Bpin.
increasing degrees of borylation owing to restricted polymer
chain mobility with the bulky Bpin group. The functionalized
PSUs containing ketone, amine, Boc-protected amine, benzyl
alcohol, and aldehyde showed Tgs ranging from 190 to 206 °C
(Figure 5). Under identical concentrations of functional groups,
the polymer functionalized with the bulky group (i.e., PSU−
NHBoc) showed the highest Tg (206 °C; Figure 5c).
The present functionalization method allows an efficient
introduction a broad range of functional groups, including
boronic ester, boronic acid, potassium trifluoroborate, and
functionalized arenes, to the aromatic polymer chains without
negative impact on the polymer chain length. Since aromatic
C−H bonds are abundant in polymer materials, the present
functionalization method involving the C−H borylation and
Suzuki−Miyaura coupling is highly versatile and offers a viable
alternative approach to polymer functionalization by electro-
philic aromatic substitutions. However, considering the cost of
metal catalysts (iridium and palladium) this method is more
suitable for functionalization processes of commodity plastics
targeting relatively a low degree of functionalization. For
preparation of functionalized specialty polymers, the issue of
catalyst cost could be considerably alleviated. This is of
particular importance for the preparation of those key polymer
materials with critical applications, which are synthetically
challenging to approach by other routes.27,29
C. Regioselectivity Study of C−H Borylation via Model
Compounds. Four types of aromatic C−H bonds in the PSU
repeating unit can undergo borylation: two in the sulfone unit
and two in the bisphenol unit. Owing to unfavorable steric
hindrance,46,47 the C−H bond at the ortho position to the
Figure 5. Differential scanning calorimetry scans of (a) 47% PSU−
COCH3, (b) 48% PSU−NMe2, (c) 46% PSU−NHBoc, (d) 46%
PSU−CH2OH, and (e) 47% PSU−CHO.
In addition to conversion via Suzuki coupling, Bpin group
can be converted into a variety of polar functional groups
including hydroxyl,38 boronic acid,39−42 and potassium
trifluoroborate.43,44 The oxidation of boronic ester to hydroxyl
group using hydrogen peroxide in the presence of a base is one
of the most widely used transformations of organoboranes. We
prepared hydroxyl-functionalized poly(arylene ether sulfone)
(PSU−OH) by treating PSU−Bpin with sodium hydroxide and
hydrogen peroxide in aqueous dioxane (Scheme 3). Upon
1
isopropylidene would be unreactive in the borylation. The H
NMR spectrum of PSU−Bpin with borylation levels up to
100% showed two relatively clean proton resonances of Bpin
group at 1.22 and 1.01 ppm (see Figure 1), indicating the
formation of two regioisomers. Unfortunately, identifying the
regioisomers of PSU−Bpin directly with NMR spectroscopy
was difficult owing to their structural similarities. Therefore,
model compounds that closely mimic the chemical structures of
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Scheme 3. Transformations of PSU−Bpin
Scheme 4. Suzuki−Miyaura Cross-Coupling Reactions with
PSU−B(OH)2 and PSU−BF3K
1
Figure 6. H NMR spectra of (a) 46-PSU−Bpin, (b) 46-PSU−OH,
counterparts (S-o-BF3K, S-m-BF3K, and B-m-BF3K) were
prepared via the independent synthetic routes shown in
Scheme 5 and fully characterized using NMR spectroscopies
(see Supporting Information). A regioselectivity study was then
conducted via a competitive C−H borylation reaction with a
1:1 mixture of S and B (Scheme 6). The resulting product
mixtures containing S-Bpin and B-Bpin were converted to the
potassium trifluoroborate counterparts via treatment with
(c) 46-PSU−B(OH)2, and (d) 46-PSU−BF3K.
the PSU repeating unit and their borylated reference
compounds were prepared and studied instead: sulfone unit
model compound (S) and bisphenol unit model compound
(B). Their Bpin-functionalized reference products (S-o-Bpin,
S-m-Bpin, and B-m-Bpin) and potassium trifluoroborate
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Scheme 5. Preparation of Borylated Reference Model Compounds
Scheme 6. Competitive C−H Borylation of Model
Compounds (S and B) with B2pin2
Scheme 7. Preparation of Model Compound P and its C−H
Borylation with B2pin2
KHF2. We also prepared a third model compound, P, that
resembles the repeating unit of PSU and incorporated the Bpin
and BF3K groups using a similar synthetic method (P-Bpin and
P-BF3K in Scheme 7).
GC/MS analysis of the product mixture of S-Bpin and B-
Bpin revealed only monoborylated products and unreacted
starting materials. The structures of the monoborylated
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products were then investigated using NMR spectroscopy. As
1
shown in Figure 7d, the H NMR spectrum of the crude
1
Figure 7. H NMR spectra of (a) S-o-Bpin, (b) S-m-Bpin, (c) B-m-
Bpin, and (d) crude mixture of S-Bpin and B-Bpin from Scheme 6.
mixture of S-Bpin and B-Bpin showed two proton resonances
of the Bpin groups at 1.28 (major) and 1.24 (minor) ppm in a
ratio of 7:1 in addition to the methyl peak of remaining B2pin2
(1.16 ppm). Comparing these results with the chemical shifts of
the borylated reference model compounds prepared using the
independent routes shown in Scheme 5 (Figure 7a−c), we
tentatively concluded that S-o-Bpinpredicted to have a
resonance around 1.39 ppmwas not formed in the
borylation. Accordingly, this result indicates that the C−H
bond at the ortho position to the sulfone group of S is
unreactive under these conditions. Owing to the proximity of
the two proton resonances of S-m-Bpin and B-m-Bpin, the
distinction of two isomers with 1H NMR data alone was
inconclusive.
Thus, we converted the Bpin group of the model compounds
to the BF3K group and compared their 19F NMR resonances.
The potassium trifluoroborate groups of the reference model
compounds appeared as distinct peaks at −134.8, −137.3, and
−135.8 ppm for S-o-BF3K, S-m-BF3K, and B-m-BF3K,
respectively (Figure 8a−c). The product mixture from the
C−H borylation of equimolar amounts of S and B showed two
19F NMR resonances that match well with those of S-m-BF3K
and B-m-BF3K, and their integral ratio was 6:1. Similar to the
conclusion drawn from 1H NMR analysis, no 19F NMR
resonance that matched S-o-BF3K was observed. The 19F NMR
spectrum of P-BF3K also displayed two peaks (−137.6 and
−136.4 ppm) with an integral ratio of 7:1, and the proximity of
their chemical shifts allowed us to assign them to the meta
position to sulfone (−137.6 ppm) and the meta position to tert-
Figure 8. 19F NMR spectra of (a) S-o-BF3K, (b) S-m-BF3K, (c) B-m-
BF3K, (d) crude mixture of S-BF3K and B-BF3K from Scheme 6, and
(e) P-BF3K of Scheme 7.
1
butyl (−136.4 ppm). Thus, data obtained from H and 19F
NMR analyses suggested that (1) the C−H bond ortho to the
sulfone group is unreactive (i.e., no formation of S-o-Bpin), and
(2) the C−H bond meta to the sulfone group in S is more
reactive than the C−H bond meta to the isopropylidene group
in B under these borylation conditions. These results are
consistent with those of previous kinetic studies of the C−H
borylation of monosubstituted benzenes: while an electron-
withdrawing group activates the aromatic C−H bond in the
borylation, an electro-donating group deactivates it.33,47,48
Given the NMR results from the model compounds study,
we believe that the aromatic C−H bonds of the sulfone
segment are more reactive than those of the bisphenol segment
in the PSU repeating unit. Thus, the two proton resonances of
27-PSU−Bpin at 1.22 and 1.01 ppm (see Figure 1b)
correspond to the two borylated polymer isomers at which
C−H borylation occurs meta to the sulfone (major isomer) and
meta to the isopropylidene (minor isomer) groups, respec-
tively.
CONCLUSIONS
We studied the catalyst activity, scope, and regioselectivity of
the iridium-catalyzed C−H borylation of aromatic PSU. B2pin2
■
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̇
(6) Johnson, B. C.; Yilgor, I.; Tran, C.; Iqbal, M.; Wightman, J. P.;
̈
displayed a borylation efficiency higher than that of HBpin
under comparable conditions. Comparison of the catalysts
showed that although similar borylation reactivity could be
achieved at elevated temperature, [Ir(OMe)(COD)]2 afforded
a significantly higher activity than [IrCl(COD)]2 at room
temperature. Regioselectivity of the borylation of PSU was
studied using model compounds, and the results suggest that
(a) the aromatic C−H bonds of the sulfone unit are more
reactive than those of the bisphenol unit and (b) the borylation
takes place preferentially at the meta position of the sulfone
than at the meta position of the isopropylidene group. The
incorporation of functional groups and their concentrations
affects the glass transition temperature of the resulting polymer.
Bpin group along the polymer chain could be conveniently
transformed to more polar boronic acid and potassium
trifluoroborate forms, all of which are versatile precursors for
the synthesis of a wide variety of functionalized PSUs via
Suzuki−Miyaura coupling reactions. All chemical conversion
processes described herein proceeded efficiently without
causing polymer chain degradation or cross-linking reactions,
which are common side reactions in conventional polymer
functionalizations. Our results are of great importance in the
design and synthesis of functionalized aromatic main-chain
polymers and may broaden their potential applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Supplementary experimental details of the synthesis and NMR
characterization of model compounds, FT-IR spectra of 46-
PSU−Bpin, 46-PSU−OH and 46-PSU−B(OH)2, degree of C−
H borylation and borylation efficiency with error bars, and size
exclusion chromatography of PSU and PSU−Bpin.This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
́
ez, P. J. Macro-
AUTHOR INFORMATION
Corresponding Author
*E-mail: baec@rpi.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.B. thanks the NSF for generous support (CAREER DMR-
0747667). The gifts of B2pin2 from Frontier Scientific Co. and
[IrCl(COD)]2 from Sinocompound Technology Co. are
acknowledged. The authors thank Dr. Jongwon Park for his
help in the SEC measurements and Dr. James Selsser at the
University of Nevada Las Vegas for allowing access to his
instrument.
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