Polyureas derived from CO2 and diamines: highly efficient catalysts for C-H arylation of benzene

Article abstract of DOI:10.1039/c6nj02785g

Polyureas derived from CO₂ and diamines were effective for C-H arylation of benzene in the presence of t-BuOK, producing biaryl products with various substituents in high yields up to 97%. Dual-activation of K⁺ and t-BuO^-, and π,π-stacking interaction between the substrate and the polymer backbone may account for the superior activity.

Full text of DOI:10.1039/c6nj02785g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Polyurea derived from CO and diamines: highly efficient catalysts
2
for C-H arylation of benzene
Received 00th January 20xx,
Accepted 00th January 20xx
1
,2,†
1,†
1
1,2
1
1
Zhenghui Liu,
Zhenzhen Yang, Leiduan Hao, Xinwei Liu, Hongye Zhang, Bo Yu, and Zhimin
1
,2,
Liu
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Polyureas derived from CO
2
and diamines were effective for C-H
Polyurea (PU) is a kind of polymer containing urea functional
arylation of benzene in the presence of t-BuOK, producing biaryl group, which shows high resistance to solvents, acids or bases,
products with various substituents in high yields up to 97%. Dual- and superior thermal stability, thus widely being applied in
+
-
activation of K and t-BuO , and π, π-stacking interaction between coatings and corrosion protection areas. Recently, PU has been
the substrate and the polymer backbone may account for the applied in catalysis due to their strong coordinating ability with
superior activity.
metal complex and nanoparticles, showing promising
2
4
application potentials. For example, PU-encapsulated Pd and
Pd-Cu bimetallic catalysts displayed excellent performances in
the hydrogenation of styrene oxide and reduction of aryl
The direct C-H arylation of benzene is a promising reaction to
1
-2
construct C-C bond within biaryl moieties, which can be
3
achieved via catalysis over Pd, Rh, Ru, Ir, and Fe
4
5
6
7-8
2
5-26
ketones.
synthesized via the reaction of CO
In our previous work, PU particles were
7
-16
complexes and organocatalysts.
Recently, a variety of N-
2
with 1,4-butanediamine,
9
-11
containing molecules (e.g., 1,10-phenanthroline
or its
MOF-253
amino-linked nitrogen
heterocyclic carbenes ) and O-containing compounds (e.g.,
and the PU supported Pd nanoparticles were found to have a
mean size <3.0 nm and show high efficiency for selective
1
2
13
derivatives,
N,N-dimethylethane-1,2-diamine,
1
4
2
7
involving bipyridine moieties,
hydrogenation of o-chloronitrobenzene to o-chloroaniline.
1
5
In our continuous work on PU, we discovered that PU
derived from CO and diamines could efficiently catalyze the
1
6
alcohols ) have been proved to effectively promote the C-H
arylation of benzene in the presence of a strong base (e.g. t-
BuOK). Notably, the molecules incorporating with both N- and
2
direct C-H arylation of benzene to biaryl compounds in the
presence of t-BuOK. Especially PU with aromatic ring in the
skeleton displayed high efficiency for benzene arylation with
aryl iodides containing both electron-donating and electron-
withdrawing groups, producing a series of biaryl products with
various substituents in high yields up to 97%. It was indicated
1
7
O-functionalities, such as proline, macrocyclic aromatic
20-21
1
pyridone pentamer, vasicine and 2-pyridyl carbinol,
8
19
exhibited synergistic effects from N- and O-functionalities.
Although these homogeneous catalysts showed efficient
catalytic activity, the heterogeneous catalysts are more
desirable for industrial applications owing to the convenience
+
-
that dual-activation of K and t-BuO by the urea unit in PU
together with π, π-stacking interaction between the substrate
and the polymer backbone accounted for the superior activity
of the PU catalysts. In addition, the PU catalysts had high
stability and easy recyclability.
2
2
of recovering and recycling of the catalysts. More recently,
Ma et al. reported a heterogeneous graphene oxide catalytic
system for the C-H bond arylation of benzene, in which both
the oxygen functional groups and the graphene
π system were
PUs were synthesized via the copolymerization of CO with
2
2
3
found to facilitate the overall reaction. Inspired by the above
progress, we envisaged that heterogeneous polymers
functionalized by both nitrogen- and oxygen- containing
groups may be favourable to catalyze the direct C-H arylation
of benzene to biaryl compounds in the presence of strong base.
1
,4-butanediamine,
xylylenediamine, respectively, catalyzed by [n-Bu N] WO
4
1,4-cyclohexanediamine
and
p-
4
2
using N-methyl pyrrolidone (NMP) as the solvent (for
experimental details, see ESI), and their chemical structures
are shown at the footnote of Table 1. The formation of PUs
was revealed by Fourier transform infrared (FTIR) spectroscopy,
elemental analysis (Table S1, ESI) and cross-polarization magic-
1
3
angle spinning (CP/MAS) C NMR. In the FTIR spectra of PU
Figure S1, ESI), the urea units was confirmed by the
(
-1
-1
characteristic bands at 1566 cm (CO-NH), 1625 cm (C=O),
-1
3
324 cm (N-H), and the respective bands for the methylene
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-
1 27
a
group around 2950 cm . The presence of chemical shifts Table 1 PU-catalyzed C-H arylation of benzene.
1
3
around 159 ppm in CP/MAS C NMR spectra of PU belonged
to the carbonyl carbon among the backbone (Figure S2, ESI),
and those at 28-48 ppm to methylene carbon, together with
1
31-138 ppm to the aromatic carbons in PU-Benz
.
b
Entry
Catalyst
-
Yield/%
Thermogravimetric analysis (TGA) indicated that the resulting
o
PUs were stable up to 300 C in an air environment (Figure S3,
1
2
3
4
<1
87
55
46
<1
0
PU-Benz
c
ESI), which met the demands for potential applications in
heterogeneous catalysis at high temperatures. Scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM) images of PU showed morphologies of
lamellar structure (Figure S4, ESI), which may be owing to the
intramolecular and intermolecular hydrogen bonding
PU- Hex
n
PU- Bu
c
5
PU-Benz
PU-Benz
PU-Benz
PU-Benz
U-Benz
c
6
c
7
0
c
8
0
d
2
8
9
65
25
55
61
formation between urea groups.
d
1
1
1
1
0
1
2
3
N-methylbenzylamine
Benzylamine
The reaction of iodobenzene (1a) with benzene was taken as
d
d
d
e
a model system to identify and optimize the key reaction
o
parameters in the presence of PU and t-BuOK at 130 C. As
4-phenylbutan-2-one
4-phenylbutan-2-one + N-methylbenzylamine
88
0
shown in Table 1, almost no biphenyl product (2a) was formed
when the reaction was conducted in the absence of the PU
catalysts (entry 1). Excitingly, all the resultant PUs were
effective for the reaction, and especially, 87% yield of biphenyl
was obtained in the presence of PU-Benz (entry 2), indicating
that PU was indeed a promising catalyst for the arylation of
benzene. This result was comparable to the reported
14
PU-Benz
a
Reaction conditions: PhI 0.4 mmol, benzene 4 mL, t-BuOK 1.2 mmol, PU-
c
Benz 12.5mg/PU- Hex 10.7 mg/PU- Bu 9.0 mg (containing 0.07 mmol urea
n
o
units based on the elemental analysis results in Table S1, ESI), 130 C, 24 h;
b
c
Determined by GC using dodecane as an internal standard; 1.2 mmol t-
BuONa (entry 5), t-BuOLi (entry 6), KOH (entry 7) and K PO (entry 8),
3
4
d
respectively, was added instead of t-BuOK; 4-phenylbutan-2-one 0.07
e
mmol, amine, 0.14 mmol for entry 10~13. 0.4 mmol TEMPO was added to
o
23
heterogeneous graphene oxide (82% yield of 2a at 120 C). In
contrast, PU derived from aliphatic amines showed lower
catalytic activities, with 55% and 46% yields of 2a being
the reaction mixture.
c
n
obtained in the presence of PU- Hex and PU- Bu, respectively
entries 2 vs. 3, 4), indicating that the chemical structures of
PUs influenced the activities of the catalysts considerably. The
(
2
9
π
,
π
-stacking interaction between the aromatic ring of PU-
Benz and the arene substrate together with the ion-
interaction of PU with -BuOK may be favourable to
π
t
9
, 30
facilitating the reaction
cation was essential, while other bases with tert-butoxide
anion such as -BuONa, -BuOLi, as well as weaker potassium
bases (KOH and K PO ) (entries 5~8) were ineffective. For
.
In this coupling system, potassium
1
3). However, 4- phenylbutan-2-one was very unstable in the
t
t
presence of strong base, e.g. t-BuOK, which reacted through
3
4
nucleophilic attack on the carbonyl carbon and hydrogen
13
comparison, U-Benz as the corresponding urea monomer of
PU-Benz was examined, and it exhibited inferior catalytic
activity (2a yield: 65%) compared with the polymer PU-Benz
1
transfer of the methylene group, as detected by H and
C
NMR spectra (Figure S5, ESI), implying impossibility for its
21
recyclability. Nonetheless, carbonyl group of the urea unit in
(
entry 9 vs. 2). This implies that the cross-linked hydrogen
bonding and π -stacking interaction of aromatic ring within
the backbone of PU-Benz might facilitate the
PUs was stable, confirmed by the fact that addition of t-BuOK
to the suspension of U-Benz or PU-Benz did not result in any
products detectable by NMR and FTIR (Figure S6 and S7, ESI).
Subsequently, influences of various factors (e.g., catalyst
loading, temperature, volume of benzene, amount of base and
reaction time) on the reaction outcome were investigated, and
the optimal reaction conditions involved the use of 12.5 mg of
,
π
-
t
-BuO
activation and further catalyse the reaction. Both the carbonyl
and amino groups within the PU skeleton may show catalytic
activity for C-H arylation of benzene in the presence of t-BuOK.
To confirm this, several molecules with separate carbonyl and
amino groups, respectively, were examined for the reaction. It
was demonstrated that 25% yield of 2a was attained catalyzed
by equimolar N-methylbenzylamine (Table 1, entry 10), while
much higher catalytic activity was observed taking
benzylamine as the catalyst (2a yield: 55%) (Table 1, entry 11).
o
PU-Benz and 3 equivalents of t-BuOK at 130 C for 24 h (Figure
S8, ESI).
The generality of this protocol for other benzene arylation
reactions was then examined and the results are summarized
in Table 2. Generally, aryl iodides with electron-donating
groups were more reactive than electron-neutral ones and
those with electron-withdrawing groups. Aryl iodides with
para-substituents were more reactive than those with meta-
or ortho-ones owing to steric hindrance. For example, 97%
4-Phenylbutan-2-one catalyzed C-H arylation to give 2a in 61%
yield, demonstrating the superior catalytic activity for oxygen-
2
3
containing groups. A physical mixture of homogeneous 4-
phenylbutan-2-one and N-methylbenzylamine showed
identical activity (2a yield: 88%) towards PU-Benz (entry 2 vs.
2
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a
Table 2 PU-Benz catalyzed C-H arylation of benzene with benzene halides.
in 37% yield (entry 10). Aryl bromide was tested as well, but
only 7% yield of biphenyl was obtained (entry 11).
b
Entry
1
Substrate
Product
Yield/%
87
The reusability of the catalyst PU-Benz was examined, and it
was indicated that 86% yield of bipenyl 2a was still obtained
after the catalyst was reused for five times (Figure 1) (for
experimental details, see ESI), suggesting good reusability and
stability. The structure of the recycled PU-Benz was kept well
2
3
97
85
1
3
as detected by FTIR and CP/MAS C NMR (Figure S9, ESI).
8
6
0
0
4
74
c
5
6
58(72)
c
40
20
0
63(76)
d
7
15(83)
d
1
2
3
4
5
8
9
10(81)
28
Cycle
Figure 1 Recyclability test of PU-Benz. Reaction conditions: PhI 0.4 mmol, t-BuOK 1.2
o
mmol, PU-Benz 12.5 mg, benzene 4 mL, 130 C, 24 h. Yield of 2a was determined by GC
using dodecane as an internal standard.
1
1
0
1
37
7
Further investigation was performed to gain a deep insight
into the reaction mechanism. Firstly, consistent with the
involvement of aryl radicals in the arylation reactions reported
1
0, 21
e
before,
addition of one equivalent of free radical
1
2
22/21/42
scavengers of TEMPO ceased the reaction and no product was
o-/m-/p-
obtained (Table 1, entry 14) in the presence of PU-Benz. The
a
Reaction conditions:
1 0.4 mmol, benzene 4 mL, t-BuOK 1.2 mmol, PU-
o
Benz 12.5 mg, 130 C, 24 h; Determined by GC using dodecane as an
b
ESI-MS characterization of the CH
3
OH solution of U-Benz and t-
c
internal standard. Yield in the bracket was obtained within 48 h. Yield in
d
BuOK with 1:1 molar ratio showed a peak at 279, ascribing to
+
[U-Benz + K] species from the coordination of K with the urea
+
e
the bracket was obtained within 72 h. 4 mL toluene was added instead of
+
group. This implies that K may coordinate with the urea unit
benzene.
within the backbone of PU-Benz during the reaction process.
-
Moreover, the interaction of the anion t-BuO with the urea
1
unit was also witnessed by H NMR spectra of U-Benz with t-
yield of 2b was obtained taking 4-iodotulene (1b) as an
arylation reagent, while the reactivities of iodobenzene (1a
)
and 1-fluoro-4-iodobenzene (1f) were much lower, giving 87%
yield of 2a and 63% yield of 2f, respectively (entries 1, 2 vs. 6).
In addition, the yield of 2f increased to 76% by prolonging the
reaction time to 48 h. The coupling of benzene with 1-tert-
butyl-4-iodobenzene (1e) was less efficient owing to steric
BuOK (Figure S6). The signal of methylene group in U-Benz was
located and split as dual peaks at 4.35 and 4.37 ppm by the
adjacent N-H, which was merged as a single peak at 4.36 ppm
after addition of equimolar t-BuOK, indicating the hydrogen
-
bonding formation between N-H and t-BuO . This interaction
effect of the tert-butyl substituent, affording 58% yield of 2e
,
was further evidenced by the signal of N-H in U-Benz located
at 5.69 ppm, which became a broad one in the presence of t-
BuOK. In the FTIR spectrum of PU-Benz, the sharp peak
which increased to 72% within 48 h. Comparatively, only 28%
yield of product (2i) was observed due to a combination of
electronic and steric effect of the trifluoromethyl group of 1i
-1
centred around 3324 cm for N-H vibration was obviously
(
entries 5 vs. 9). The position of the substituents had great
receded in the spectrum of the physical mixture of PU-Benz
and t-BuOK (Figure S7).
effect on the reactivity. For methyl or fluoro-substituted
iodobenzenes, the activity was decreased in the order of para- >
meta- > ortho-substitution, that is, the yields decreased as 2b
Based on the above findings and related results reported by
10-11, 21, 23
others,
we speculated that interactions of the
(
97%) > 2c (85%) > 2d (74%), and 2f (63%) > 2g (15%) > 2h
potassium cation and tert-butoxide anion with the urea unites
and aromtic ring in the backbone of PU-Benz acted
synergistically to catalyze the C-C bond formation reaction. A
radical-mediated catalytic mechanism was proposed (Scheme
(10%), owing to steric effect (entries 2~4 and 6~8). 83% yield
of 2g and 81% yield of 2h were achieved by prolonging the
reaction time to 72 h. Notably, less reactive 2-iodothiophene
(
1j) was also performed to arylate with benzene, producing 2j
1
). The potassium cation and tert-butoxide anion was activated
by the urea unit among the skeleton of PU-Benz, which
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subsequently transferred a single electron to aryl iodide
1,
Notes and references
affording the intermediate aryl radical anion I. And then, aryl
1
T. L. Chan, Y. Wu, P. Y. Choy and F. Y. Kwong, Chem. Eur. J.,
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radical II was formed by departure of iodide anion, and then
reacted with benzene to generate a biary radical III. Oxidation
of III by the previously formed radical cation of t-BuOK
produced the biaryl cation IV. Finally, deprotonation of IV by a
2
2
3
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tert-butoxide anion furnished the biary product
2. Notably, π,
4
5
6
7
8
9
,
π
-stacking interaction between aryl ring within the polymer
3
chains with the arene substrate played an important role in
attracting the substrate to the vicinity of the catalytic center,
and stabilizing the resultant intermediates.
2
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Scheme 1 Proposed mechanism for the PU-Benz/t-BuOK catalyzed C-H arylation.
1
1
1
1
2
2
2
In conclusion, PUs derived from CO
2
and diamines,
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was proved to be an efficient heterogeneous catalytic material
for C-H arylation of benzene in the presence of t-BuOK,
together with broad substrate scope, high stability and easy
recyclability. Both urea and benzene functionalities among the
polymer backbone were important for the catalytic process,
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-
realizing dual-activation of K and t-BuO , together with π, π-
stacking interaction between the substrate and the polymer
backbone, and thus resulting in good catalytic activity. Based
on the efficient coordination of urea units with alkali metal
cations, applications of polyurea materials in hydrogen-
transfer reactions were under investigation.
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Heterogeneous polyureas were effective for C-H arylation of benzene in the presence of t-BuOK,
producing biaryl products in high yields up to 97%.