Polyethylene glycol (PEG) promoted hydrodehalogenation of aryl halides

Article abstract of DOI:10.1016/j.tetlet.2017.03.016

Transition-metal-free hydrodehalogenation of aryl halides (eg. bromides) can take place efficiently in the presence of green polyethylene glycol (PEG-800) and t-BuOK without adding any extra solvents and additives. A radical mechanism is proposed for this transition-metal-free dehalogenation of aryl halides and the role of polyethylene glycol is both reaction promotor and activated alkyl C[sbnd]H donor.

Full text of DOI:10.1016/j.tetlet.2017.03.016

Accepted Manuscript
Polyethylene glycol (PEG) promoted hydrodehalogenation of aryl halides
Wei Liu, Fang Wang
PII:
S0040-4039(17)30302-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.03.016
TETL 48717
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 January 2017
4 March 2017
7 March 2017
Please cite this article as: Liu, W., Wang, F., Polyethylene glycol (PEG) promoted hydrodehalogenation of aryl
halides, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.03.016
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Polyethylene glycol (PEG) promoted hydrodehalogenation of
aryl halides
Wei Liu* and Fang Wang
Lipid Chemistry, College of Food Science and Technology, Henan University of
Technology, 100 Lianhua Street, Zhengzhou 450001, P. R. China
*Corresponding author.
E-mail address: liuwei307@hotmail.com (Wei Liu)
ABSTRACT: Transition-metal-free hydrodehalogenation of aryl halides (eg.
bromides) can take place efficiently in the presence of green polyethylene glycol
(PEG-800) and t-BuOK without adding any extra solvents and additives. A radical
mechanism is proposed for this transition-metal-free dehalogenation of aryl halides
and the role of polyethylene glycol is both reaction promotor and activated alkyl C-H
donor.
Keywords:
Dehalogenation; aryl halides; transition-metal-free; radical; polyethylene glycol
1. Introduction
Cross-coupling reactions catalyzed by noble metal catalysts (eg. Pd, Rh, Ru) or
cheap metal catalysts (eg. Ni, Cu, Fe, Co) have become one of the most important
topics in organic chemistry during the past decades.1 With the recent development of
green and sustainable chemistry, chemists are inspired to develop various transition-
metal-free coupling reaction processes.2
In 2010, several groups independently developed transition-metal-free cross-
coupling between unactivated arenes and haloarenes through base-promoted
homolytic aromatic substitution.3-5 In these processes, a catalytic amount of diamine
1

ligands (eg. phenanthroline4a,c, DMEDA4b) could efficiently catalyze the cross-
coupling of aryl iodides/bromides with unactivated arenes (eg. benzene) in the
presence of t-butoxide base (eg. t-BuOK). Since then, much attentions have been
focused on developing new catalysts6 toward this novel methodology. Meanwhile, this
transition-metal-free strategy has been successfully applied in other type of organic
reactions, such as intramolecular arylation7, Heck-type coupling8, carbonylation9 and
so on.
Recently, great efforts have been devoted to develop new methods achieving the
hydrodehalogenation of aromatic halides from the synthetic10 and environmental11
points of view. Although many hydrodehalogenation methods based on transition-
metal-catalysis (eg. Pd12, Rh13, Ni14 and Fe15) have been successfully developed,
examples on transition-metal-free version of hydrodehalogenation of aryl halides is
still rare.16
Utilization of polyethylene glycols (PEG) as reaction media has attracted much
attentions in chemical and pharmaceutical industries.17 Because PEG are cheap,
nontoxic, and their properties can be tuned by changing molecular weight,18 different
reactions in PEGs have been studied, such as polymerization19, substitution reaction20,
oxidation reactions21, and cross-coupling reactions22, etc.
In continuation with our ongoing work on transition-metal free approach in cross-
coupling reactions,6g, q we proposed that cheap and green PEG could also role as an
efficient promotor for cross-coupling of benzene with aryl halides. Herein, we report
our results on polyethylene glycol (PEG-800) promoted transition-metal-free
hydrodehalogenation of aryl halides (Scheme 1).
2

Other groups' works
t-BuOOt-Bu/Cs2CO3
X
H
H
2-Propanol
R
R
R
Ref. 16a
NaH/LiI
X
THF
Ref. 16b
R
This work
PEG
H
X
?
R
R
The roles of PEG
Cataly st
Reducing agent
Reaction medium
Scheme 1. Transition-metal free hydrodehalogenation of aryl halides.
2. Results and discussion
We embarked on our research by testing the feasibility of the cross-coupling
between 1-iodo-4-methoxybenzene (1a) and benzene using PEG as promotor (Table
1). In the presence of t-BuOK base, introducing PEG-200 (0.02g) as promotor led to
29% of anisole (2a) as hydrodehalogenation product and 30% of 4-methoxybiphenyl
(3a) as aryl-aryl coupling product at 80 oC after 12 h (Table 1, entry 1). And the use of
PEG-400 and PEG-800 could also produce 31-37% yields of hydrodehalogenation
product (2a) and 32-50% yields of arylation product (3a), respectively (Table 1,
entries 2-3). When the amount of PEG-800 were increased from 0.02 g to 0.05g, the
yields of hydrodehalogenation product (2a) were increased from 37% to 45%, and the
yields of aryl-aryl coupling product (3a) were reduced from 50% to 38% (Table 1,
entries 3-4). Continuing increasing the amount of PEG-800 (0.1-0.5g) led to increased
yields of hydrodehalogenation product 2a (58-79%) and decreased yields of aryl-aryl
coupling product 3a (31-12%) (Table 1, entries 5-7). And the reaction using same
amount of PEG-400 or PEG-200 afforded much lower conversions (Table 1, entries 8-
9). Control experiment indicated that hydrodehalogenation could not take place in the
absence of PEG (Table 1, entry 10). Based on the these observations, we can conclude
3

that polyethylene glycols (PEG) has the potential to promote the transition-metal-free
hydrodehalogenation of aryl halides without the aid of any other reducing agents.
Table 1. Cross-coupling between Ar-I and benzene in the presence of PEG.a
Ph
I
PEG/t-BuOK
+
+
o
80 C, 12 h
OMe
2a
OMe
3a
OMe
1a
Entry
PEG
Yield of 2a (%)
Yield of 3a (%)
1
2
PEG-200 (0.02g)
PEG-400 (0.02g)
PEG-800 (0.02g)
PEG-800 (0.05g)
PEG-800 (0.10g)
PEG-800 (0.20g)
PEG-800 (0.50g)
PEG-400 (0.50g)
PEG-200 (0.50g)
none
29
31
37
45
58
62
79
16
8
30
32
50
38
31
27
12
6
3
4
5
6
7
8
9
<5
0
10
<5
o
a Conditions: Ar-I (0.5 mmol), t-BuOK (2.0 mmol), benzene (4.0 mL), 80 C, 12h. Yields are
determined by GC. PEG-200, 0.02g, 0.1 mmol; PEG-400, 0.02g, 0.05 mmol; PEG-800, 0.02g,
0.025 mmol; PEG-800, 0.50g, 0.625 mmol; PEG-400, 0.50g, 1.25 mmol; PEG-200, 0.50g, 2.50
mmol.
Encouraged by the above results, we conducted on our research by testing 1-iodo-
4-methoxybenzene (1a) as a model compound in hydrodehalogenation in PEGs solely
(Table 2). Considering the tunable molecular weight of PEGs, a set of PEGs, such as
PEG-4000, PEG-2000, PEG-1000, PEG-800, PEG-600, PEG-400 and PEG-200 were
examined for this transition-metal-free hydrodehalogenation, and PEG-800 showed
o
better yield of hydrodehalogenation (67%) than other PEGs at 100 C when the
4

reactions were conducted in 1.0 g of PEGs (Table S1, entries 1-7). Increasing the
amount of PEG-800 (2.0-3.0g) led to reduced conversions and yields (Table 2, entries
2-3). And decreasing the amount of PEG-800 (0.75-0.5g) led to increased yields of
hydrodehalogenation (81-90%) (Table 2, entries 4-5). Then the amount of t-BuOK
was optimized. 3.5 equiv of t-BuOK showed similar results with 4.0 equiv of t-BuOK
(90-91% yields of hydrodehalogenation) (Table 2, entries 5-6). Continuing decreasing
the amount of t-BuOK from 3.0 equiv to 1.0 equiv led to both reduced conversions
and yields sharply (Table 2, entries 7-9). Moreover, the reaction temperature was also
inverstigated and the results showed that the hydrodehalogenation could also take
o
place efficiently at 80 C and afford the desired hydrodehalogenation product 2a in
o
90% yield (Table 2, entry 10). Much lower temperature (60 C) showed nearly no
effect on the hydrodehalogenation reaction (Table 2, entry 11). It was worth noting
that other bases, such as t-BuONa, t-BuOLi and KOH showed nearly no effect on this
transition-metal-free hydrodehalogenation reaction (Table 2, entries 12-14). Based on
the above results, we can conclude that PEG (PEG-800) can not only role as a suitable
reaction medium, but also role as an efficient catalyst and reducing agent for this
transition-metal-free hydrodehalogenation.
Table 2. Optimization of reaction conditions of hydrodehalogenation.a
PEG/t-BuOK
MeO
I
MeO
T, 12 h, N2
1a
2a
Entry
PEG/Base
T (oC)
100
100
100
100
100
100
100
100
100
80
Conv. (%)b
Yield (%)b
1
2
PEG800 (1.0g)/t-BuOK
PEG800 (2.0g)/t-BuOK
PEG800 (3.0g)/t-BuOK
PEG800 (0.75g)/t-BuOK
PEG800 (0.5g)/t-BuOK
PEG800 (0.5g)/t-BuOKc
PEG800 (0.5g)/t-BuOKd
PEG800 (0.5g)/t-BuOKe
PEG800 (0.5g)/t-BuOKf
PEG800 (0.5g)/t-BuOKc
86
45
67
29
12
81
90
91
83
65
7
3
27
4
95
5
100
100
97
6
7
8
82
9
21
10
100
90
5

11
12
13
14
PEG800 (0.5g)/t-BuOKc
PEG800 (0.5g)/t-BuONac
PEG800 (0.5g)/t-BuOLic
PEG800 (0.5g)/KOHc
60
80
80
80
<5
12
<5
<5
<5
7
<5
<5
a
Standard reactions conditions: 1a (0.5 mmol), base (4.0 equiv.), 12 h, N2. PEG-800, 0.50g,
0.625 mmol. b Calibrated GC yields were reported using hexadecane as the internal standard. c 3.5
f
equiv of base was used. d 3.0 equiv of base was used. e 2.0 equiv of base was used. 1.0 equiv of
base was used.
With the optimal reaction conditions (0.5 g of PEG800, 3.5 equiv of t-BuOK, 80
oC, 12h) in hand, we investigated the hydrodehalogenation reaction of various aryl
iodides (Scheme 2). In general, electron-rich aryl iodides (1a-1f) took place
hydrodehalogenation reaction smoothly and afforded the corresponding
dehalogenated products in excellent yields (84-90%). Sterically hindered aryl iodides,
such as 1-iodo-2,4-dimethoxybenzene (1b), 1-iodo-2-methoxybenzene (1c) and 1-
iodo-2-methylbenzene (1f) were also suitable under the same reaction conditions and
afforded the corresponding dehalogenated products in good yields (84-87%),
respectively. And 4-iodobiphenyl (1g) underwent hydrodehalogenation reaction and
afforded 91% isolated yield of biphenyl as dehalogenated product. 1-fluoro-4-
iodobenzene (1h), 3-fluoroiodobenzene (1i), 4-chloroiodobenzene (1j), and 3-
chloroiodobenzene (1k) could all undergo chemoselective hydrodehalogenation n and
produced the mono-dehalogenated products in 88-90% yields, respectively. And ethyl
4-iodobenzoate (1l) could produce the dehalogenated product ethyl benzoate in 78%
yield.
6

t-BuOK (3.5 equiv)
I
H
PEG-800, 80 oC, 12 h
R
R
1
2
MeO
I
MeO
I
I
OMe
OMe
1a
1b
1c
, 87%
, 90%
, 87%*
I
I
I
Me
Me
Me
Me
1e
1d
1f
, 90%
, 91%
, 84%
I
I
I
F
F
1g
, 91%*
1h, 88%
1i, 87%
I
I
I
Cl
EtO2C
Cl
1k, 88%
1j, 90%
1l, 78%*
Scheme 2. Hydrodehalogenation of aryl iodides. Conditions: Ar-I (0.5 mmol),
PEG-800 (0.5g, 0.625 mmol), t-BuOK (1.8 mmol), 80 oC, 12 h, N2. Yields are
determined by GC. *The yields reported are for products isolated and purified by
flash chromatography.
Next, other unactive aryl halides, such as aryl bromides were examined as well. 4-
bromoanisole (4a) was less reactive at 80 oC. Considering the more unactive aromatic
o
C-Br bond in aromatic compounds, increasing the reaction temperature from 80 C to
o
120 C was conducted to promote the cleavage of aromatic C-Br bond, and good
conversion of 4-bromoanisole (4a) was observed (Scheme 3). Encouraged by the
above results, various aryl bromides were then examined and the results were listed in
Scheme 3. In general, electron-rich aryl bromides (4a-4f) took place
hydrodehalogenation smoothly and afforded the corresponding dehalogenated
products in 84-87% yields, respectively. Dihalide substrates, such as 1-bromo-4-
chlorobenzene (4g) and 1-bromo-3-chlorobenzene (4h) could both undergo
7

chemoselective hydrodehalogenation and produced the corresponding cleavage of
aromatic C-Br bond products in 83-85% yields, respectively. However, aryl chlorides,
such as 4-chloroanisole was unactive at 120 oC under the same conditions.
t-BuOK (3.5 equiv)
Br
H
o
PEG-800, 120 C, 24 h
R
R
4
2
Br
Br
Br
MeO
Me
Cl
MeO
OMe
4c, 85%
4a, 86%
4d, 87%
4b, 84%
Br
Br
Br
Me
Me
4e, 84%
4f, 84%
Br
Br EtO2C
Br
Cl
4g
4h, 85%
4i
, 75%*
, 83%
Scheme 3. Hydrodehalogenation of aryl bromides. Conditions: Ar-Br (0.5 mmol),
o
PEG-800 (0.5g, 0.625 mmol), t-BuOK (1.8 mmol), 120 C, 24 h, N2. Yields are
determined by GC. *The yields reported are for products isolated and purified by
flash chromatography.
To gain some preliminary insights into the mechanism of such transition-metal-free
hydrodehalogenation of aryl halides, we conducted a set of control experiments
(Scheme 4). When the reaction was conducted under air atmosphere, nearly no
conversion of 4-bromoanisole (4a) was detected, which suggested that this
hydrodehalogenation was oxygen sensitive. Moreover, the hydrodehalogenation
reaction was almost shut down when 100 mol% of 2,2,6,6-tetramethyl-1-
piperidinoxyl (TEMPO) or butylated hydroxytoluene (BHT) was added as radical
inhibitors. The above control experiments suggested that this transition-metal-free
hydrodehalogenation transformation involved a radical reaction mechanism.5
8

t-BuOK (3.5 equiv)
MeO
Br
H
MeO
o
PEG-800, 120 C, 24 h
4a
2a
"Conditions"
N2
Air
86%
<5%
<5%
<5%
TEMPO (100 mol%)
BHT (100 mol%)
Scheme 4. Control experiments for this hydrodehalogenation reaction.
On the basis of above results and previous studies on t-BuOK-mediated transition-
metal-free coupling reactions6s,23, a plausible mechanism is proposed as shown in
Scheme 5. An aryl halide radical anion (A) is generated from aryl halides (1) by SET
process in the presence of PEG-800/t-BuOK and converted to an aryl radical (B) upon
elimination of X-. Aryl radical B reacts with PEG to afford dehalogenation product 2
and radical (C). From radical C, two possible pathways (Path A and Path B) are
presented in Scheme 5. Path A is a deprotonation of radical C with the assistance of t-
BuOK, followed by another SET with aryl halides to form aryl halide radical anion
(A) and di-dehydrogenated PEG (5).24 Path B is a SET process between radical C and
aryl halides (1) to produce cation E, followed by a deprotonation by t-BuOK to give
di-dehydrogenated PEG (5). Based on the above proposed mechanism, we can
conclude that PEG (PEG-800) can not only role as a suitable reaction medium, but
also role as an efficient radical initiator and reducing agent for this transition-metal-
free hydrodehalogenation of aryl halides.
9

Initiation
PEG/t-BuOK
Ar-X
Ar-X
Ar
B
SET
-X
1
A
Propagation
H
O
O
Ar
O
O
Ar H
-
+
B
2
C
Path A
t-BuO-
Ar-X
O
O
O
O
O
O
Ar-X
+
PT
ET
C
D
5
A
Path B
O
O
O
O
Ar-X
t-BuO-
O
O
O
O
ET
PT
5
C
E
Scheme 5. Possible mechanism of this hydrodehalogenation.
Conclusion
In conclusion, we have developed a transition-metal-free hydrodehalogenation
reaction of aryl halides in the presence of PEG-800 and t-BuOK under mild
conditions. PEG (PEG-800) can not only role as a suitable reaction medium, but also
role as an efficient radical promotor and reducing agent for this transition-metal-free
hydrodehalogenation of aryl halides. Both aryl iodides and bromides can be employed
as substrate to afford the hydrodehalogenation products in moderate to excellent
yields. Control experiments indicated that radical process occurred during the
transition-metal-free hydrodehalogenation reaction. Further studies on the mechanism
and the application to other reactions are underway in our laboratory, and will be
reported in due course.
10

Acknowledgments
This work was supported by the National Natural Science Foundation of China (No.
21102036), the Young-aged Backbone Teacher Funds of Henan Province of China
(No. 2014GGJS-058) and Basic Research Funds in Henan Universities of Henan
University of Technology (No. 2015RCJH01).
A. Supplementary material
Supplementary data associated with this article can be found, in the online version
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[24] We have tried to analyse the reaction mixture by NMR to comfirm the
unsaturated PEG (5) after quenching the reaction system. However, no direct evidence
of the formation of double bond was observed from the HNMR spectrum.
14

Graphical abstract
t-BuOK
X
H
PEG-800, 80-120 oC
R
R
X = I, Br
Radical process
21 examples,
75-91% yields
15

Highlights
1) Transition-metal-free hydrodehalogenation of aryl halides has been developed.
2) Polyethylene glycol and t-BuOK are used without extra solvents and additives.
3) PEG role as an efficient radical promotor and reducing agent.
16