Catalytic hydrodebromination of aryl bromides by cobalt tetra-butyl porphyrin complexes with EtOH

Article abstract of DOI:10.1016/j.tet.2018.12.010

Hydrodebromination of aryl bromides catalyzed by electron rich and sterically unhindered cobalt 5,10,15,20-tetrabutylporphyrin was achieved at mild conditions in good yields employing EtOH as the hydrogen source. The catalytic efficiency was enhanced compared with previously reported by cobalt tetra-aryl porphyrin catalysts. A revised mechanism of single electron transfer was proposed.

Full text of DOI:10.1016/j.tet.2018.12.010

Accepted Manuscript
Catalytic hydrodebromination of aryl bromides by cobalt tetra-butyl porphyrin
complexes with EtOH
Chen Chen, Huiping Zuo, Kin Shing Chan
PII:
S0040-4020(18)31475-3
https://doi.org/10.1016/j.tet.2018.12.010
TET 29996
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 7 November 2018
Revised Date: 6 December 2018
Accepted Date: 7 December 2018
Please cite this article as: Chen C, Zuo H, Chan KS, Catalytic hydrodebromination of aryl bromides
by cobalt tetra-butyl porphyrin complexes with EtOH, Tetrahedron (2019), doi: https://doi.org/10.1016/
j.tet.2018.12.010.
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Catalytic Hydrodebromination of Aryl
Bromides by Cobalt Tetra-butyl Porphyrin
Complexes with EtOH
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Chen Chen, Huiping Zuo and Kin Shing Chan
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Catalytic Hydrodebromination of Aryl Bromides by Cobalt Tetra-butyl Porphyrin
Complexes with EtOH
Chen Chen, Huiping Zuo and Kin Shing Chan∗
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Hydrodebromination of aryl bromides catalyzed by electron rich and sterically unhindered
cobalt 5,10,15,20-tetrabutylporphyrin was achieved at mild conditions in good yields employing
EtOH as the hydrogen source. The catalytic efficiency was enhanced compared with previously
reported by cobalt tetra-aryl porphyrin catalysts. A revised mechanism of single electron transfer
was proposed.
Available online
2
018 Elsevier Ltd. All rights reserved.
Keywords:
Cobalt porphyrin
Hydrodebromination
Ethanol
Aryl bromides
1
. Introduction
the more attractive hydrogen donor ethanol can be used. Here we
II
report the electron rich Co (tbp) (tbp
=
5,10,15,20-
The reduction of aryl halides to the corresponding
tetrabutylporphyrinato dianion) catalyzed hydrodebromination of
aryl halides employing EtOH as the hydrogen source with high
efficiency. An updated reaction mechanism of single electron
transfer was proposed.
hydrocarbon is an important task not only in organic synthetic
transformations,
environmentally hazardous organic halides.
1
but also in the 2 detoxification of
3
Transfer hydrogenation (TH) for hydrodehalogenation has
been widely investigated as it is safer and more convenient than
4
hydrogenation with H gas. Reducing agents used in TH such as
2
5
6
1,7
1,8
sodium hydride, borohydride, silanes and tin hydrides are
widely used but quite expensive and sometimes not
environmentally friendly. Transfer hydrogenation with PrOH is
i
9
very common, but the use of EtOH as hydrogen source is
10
scarce. Use of ethanol in TH represents an attractive alternative
given its abundance, sustainability and low toxicity. However,
the difficulties associated with EtOH are: the larger α-C-H bond
dissociation energy of ethanol; the poisonous dehydrogenation
product acetaldehyde to many transition metal catalysts.
Fig. 1. Structure Illustration of Co(por) Catalysts
11
2. Reaction Conditions Optimization
12
13
Based on our reported work (eq. 1),
the catalytic
Our group has reported the Co(ttp) (ttp = 5,10,15,20-tetra-p-
hydrodebromination of 4-bromoanisole (1a) with more electron
tolylporphyrinato dianion) catalyzed hydrodebromination of aryl
II
i
i
13
rich Co (tbp) in PrOH solvent was examined. To our delight, the
catalytic transformation proceeded smoothly with 100% yield at
bromides in hydrogen donor solvents of both PrOH and THF.
The reaction mechanism operates through a halogen atom
abstraction of an aryl bromide with Co(II) porphyrin to give an
o
a lower temperature of 150 C in a shorter time of 3 h (eq. 2).
13,14
With this initial success in hand, the reaction conditions
aryl radical and Co(III) bromide,
subsequent hydrogen atom
(porphyrin ligand, solvent, catalyst loading, temperature, KOH
abstraction of the aryl radical from solvent yields the arenes. We
envisioned that a more electron rich porphyrin ligand would
loading and reaction atmosphere) were further optimized.
The catalytic efficiency of various Co porphyrin ligand was
then investigated. With the most electron rich and least sterically
II
15
facilitate the key oxidative addition of Co (por), thus possibly
enhancing the hydrodebromination rate. We also discovered that
—
——
∗
Corresponding author. Tel.: +852 39436376; fax: +852 26035057; e-mail: ksc@cuhk.edu.hk (K.S.Chan)

2
Tetrahedron
ACCEPTED MA(T
N
ab
U
le
S
3
CRI
ryPT2). Increasing catalyst loading to 10 or 20 %
, e
nt
enhanced the reaction rate slightly and required 6 h to finish the
reaction (Table 3, entries 4 and 5). Thus, a 5 mol% of catalyst
was chosen as the optimal catalyst loading.
a
Table 3 Catalyst Loading Effect
b
b
b
Entry
n
time
24 h
24 h
8 h
2a yield %
1a recovery % (1a+2a) yield %
1
2
3
4
5
0
8
80
5
88
II
hindered Co (tbp), the hydrodebromination process was
2.5
5
94
100
98
100
99
o
II
complete in 3 h at 150 C (Table 1, entry 1). While for Co (tap)
II
0
100
98
and Co (ttp), the catalytic reactions were not complete after 6 h
10
20
6 h
0
to give 82% and 17% yields of 2a, respectively (Table 1, entries
II
2
and 3). Thus, Co (tbp) was chosen as the optimal catalyst for
6 h
0
100
further investigation.
a
b
Co(tbp) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL EtOH;
GC-MS yield.
a
Table 1 Porphyrin Ligand Effect
The loading of KOH was also examined. Without any base
added, the starting material 1a was recovered quantitatively after
heating for 24 h (Table 4, entry 1). Increasing KOH loading to 20
equiv resulted in incomplete reaction after 24 h with 68% of 2a
and 30% of 1a recovered (Table 4, entry 2). The reaction with 30
equiv of KOH required 16 h to give 98% yield (Table 4, entry 3).
b
b
b
Entry
por time
2a yield %
1a recovery % (1a+2a) yield %
1
2
3
tbp
tap
ttp
3 h
6 h
6 h
100
82
0
100
102
97
4
0
equiv of KOH was essential to ensure fast
20
80
hydrodebromination in 8 h (Table 4, entry 4). Reaction with 50
equiv of KOH still required 6 h to complete and the reaction rate
was not improved dramatically (Table 4, entry 5). Thus, 40 equiv
was found to be the optimal loading of KOH.
17
a
i
Co(por) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL PrOH;
GC-MS yield.
b
a
Table 4 KOH Loading Effect
II
With the more reactive Co (tbp) catalyst, we examined the
less reactive hydrogen donating solvent of EtOH and MeOH. The
i
reaction in EtOH was similar with that in PrOH and required 6 h
to give 100% yield of 2a (Table 2, entries 2 and 3). However, the
reaction in MeOH required 3 d to reach 69% yield of 2a with 5%
11
of 1a recovered (Table 2, entry 1). The stronger α-C-H bond
96 kcal/mol) of MeOH and poor solubility of catalyst in MeOH
b
b
b
Entry
m
time
24 h
24 h
16 h
8 h
2a yield %
1a recovery % (1a+2a) yield %
(
1
2
3
4
5
0
0
100
30
0
100
98
account for the slower reaction. Therefore, the cheaper EtOH was
11,12
chosen for further investigation.
20
30
40
50
68
98
100
96
a
Table 2 Solvent Effect
98
0
100
96
6 h
0
a
b
Co(tbp) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL EtOH;
GC-MS yield.
b
b
Entry
ROH
time
2a yield %
1a recovery %
(1a+2a)
yield %
o
b
The reaction was carried out at a lower temperature of 120 C
and afforded 21% of 2a together with 65% of recovered 1a after
1
2
3
MeOH
EtOH
72 h
6 h
69
5
0
0
74
o
2
5 h (Table 5, entry 1). Therefore, 150 C was chosen as the
100
100
100
100
optimal reaction temperature.
a
i
PrOH
3 h
Table 5 Temperature Effect
a
b
Co(tbp) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL alcohol;
GC-MS yield.
With the optimal solvent and catalyst in hand, the loading of
catalyst was also examined. Without any catalyst added, only 8%
yield of the target product 2a was formed with 80% yield of 1a
recovered after heating for 24 h, thus supporting the catalytic role
b
b
Entry
Temp.
(˚C)
time
2a yield %
1a recovery % (1a+2a)
b
yield %
II
II
of Co (tbp). In the presence of 5 mol% of Co (tbp) catalyst, the
reaction proceeded smoothly and finished in 8 h with complete
conversion (Table 3, entry 3). When the loading of catalyst was
reduced to 2.5 mol%, the reaction rate decreased a lot and
required 24 h to give 94% yield of 2a with 5% of recovered 1a
1
2
120
150
25 h
8 h
21
65
0
86
100
100
a
b
Co(tbp) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL EtOH;
GC-MS yield.

3
The hydrodebromination reaction was compa
atmosphere to give similar yield in slightly longer time of 10 h
Table 6, entry 1). The optimal reaction conditions were shown
A
tib
C
le
C
w
E
it
P
h
T
anEDair MAbr
N
om
U
o-
S
4-
C
(b
R
en
I
zyloxy)benzene (1m) was treated with the optimal
P
T
reaction conditions, 45% yield of benzyl phenyl ether was
achieved as the hydrodebromination product together with 60%
yield of toluene and 60% yield of 4-bromophenol as the C-O
bond hydrogenation product after heating for 17 h. For 4-
bromoacetophenone (1n), the reduction of carbonyl group to
hydroxy group occurred fast to afford 4-bromo-α-methylbenzyl
alcohol within 1 h, followed by further hydrodebromination to
generate α-methylbenzyl alcohol in 32% yield with 4-bromo-α-
(
below with 5 mol% of Co(tbp) catalyst and 40 equiv of KOH in
o
EtOH solvent at 150 C under N (eq. 3).
2
a
Table 6 Atmosphere Effect
methylbenzyl alcohol in 12% yield after
7
h. 4-
Bromobenzonitrile (1o) was consumed completely after heating
for 1 h without any formation of benzonitrile. After neutralization
of the reaction mixture with HCl, 86% of 4-bromocarboxylic acid
and 12% of benzoic acid were achieved. Alkaline hydrolysis of
nitrile occurred rapidly to generate the corresponding
carboxylate. Only 3% of hydrodechlorination product of 1p was
achieved with 95% of 1p recovered. The poor reactivity of C–Cl
bond in 1b, 1p and 1q suggests that aryl chlorides are inert
towards hydrodechlorination in the optimized reaction
conditions. 1r was reduced to biphenyl with 72% of isolated
yield. The efficient C–I bond reduction in 1c, 1q and 1r support
the high reactivity of aryl iodides in the cobalt porphyrin
b
b
Entry
N
2
/air
time
2a yield %
1a recovery % (1a+2a)
b
yield %
1
2
air
10 h
8 h
96
1
0
97
N
2
100
100
a
b
Co(tbp) (0.014 mmol) with 4-bromoanisole (0.072 mmol) in 1.0 mL EtOH;
GC-MS yield.
catalyzed
hydrodehalogenation
reactions.
The
hydrodebromination of heteroaromatic bromides were also
examined. 2-Bromopyridine (1s) was consumed completely
within 1 h with the formation of complex mixture. The
hydrodebromination of 1t went on smoothly to afford 2-
methylthiophene in 87% yield.
The relative reactivities of Ar-X (X = Cl, Br, I) towards
hydrodebromination were investigated in the optimized reaction
conditions using para halogen atom substituted anisoles (1a to
1
3
c). Rates increased in the order Cl < Br < I (Table 7, entries 1 to
). 4-Iodoanisole (1c) showed the highest reactivity and
Scheme 1 Substrate Scope of Co(tbp) Catalyzed
Hydrodehalogenation.
completed in 2 h with 92% yield (Table 7, entry 3). 4-
Chloroanisole (1b) was inert towards hydrodechlorination with
only 5% yield of 2a formed and 80% of 1b recovered after 24 h.
a
Table 7 Relative Reactivity of Ar-X
b
Entry
X
time
2a
yield %
1 recovery % (1+2a)
b
b
yield %
1
2
3
Cl (1b)
Br (1a)
I (1c)
24 h
8 h
5
80
0
85
100
92
100
92
2 h
0
a
b
Co(tbp) (0.014 mmol) with 4-haloanisole (0.072 mmol) in 1.0 mL EtOH;
GC-MS yield.
Substituted aryl bromides and aryl iodides were tolerated and
reacted smoothly. (Scheme 1). Dehalogenation of para and meta
bromoanisole (1a and 1d) were completed in 8 h to give anisole
in 100% and 90% yield, respectively. The ortho bromoanisole
(
9
1e) was more reactive and gave clean conversion in 4 h with
6% yield of 2a. When the phenyl ring was substituted with more
methoxy groups, the reaction times were shortened to 2 h for 1f
and for 1g. Hydrodehalogenation of 1-bromo-4-
4
h
chlorobenzene (1h) and 1-chloro-4-iodobenzene (1q) afforded
chlorobenzene quantitatively without further reduction to
benzene. Sterically hindered 2,4,6-triisopropylbromobenzene (1i)
was hydrogenated smoothly within 10 h with quantitative
conversion. Hydrodebromination of 1-bromo-4-(dimethylamino)
benzene (1k) was slow and generated 79% of N, N-
dimethylbenzenamine after 24 h with 20% of 1k recovered. 1-
Bromo-4-phenylbenzene (1j) was hydrogenated completely in 12
h. Hydrodebromination of 9-bromoanthracene (1l) was
accomplished in 4 h with slightly low yield of 77%. When 1-
a
b
c
d
chlorobenzene as sole product; isolated yield; 20% of 1k recovered; 50%
e
of toluene and 50% of 4-bromophenol were achieved; α-methylbenzyl
alcohol in 32% yield and 4-bromo-α-methylbenzyl alcohol in 12% yield were
achieved; 86% of 4-bromobenzoic acid and 12% of benzoic acid were
achieved after neutralization with HCl. 95% of 1p recovered after reaction;
isolated yield, biphenyl as sole product; complex mixture.
f
g
h
i

4
Tetrahedron
ACCEPTED MANUSCRIPT
3
. Mechanistic Investigation
Based on the established mechanism for Ar-X bond activation
with group 9 metalloporphyrins and cobalt porphyrin catalyzed
Co(tbp)aryl are important intermediates generated in the bond
activation process. To gain further mechanistic support and
understanding of the catalysis, the reaction (eq. 3) was closely
monitored by thin layer chromatography. The formation of
Co(tbp)Ph(p-OMe) in the reaction process was observed and
further confirmed by both TLC and HRMS through comparing
13-17
hydrodebromination of aryl bromides,
3
plausible
mechanistic pathways were proposed for the C-Br bond
activation step: (a) radical ipso substitution mechanism, (b)
halogen atom transfer mechanism, (c) single electron transfer
mechanism (Scheme 2).
14
with an authentic sample. The conversion of Co(tbp)Ph(p-OMe)
to anisole in the optimized reaction conditions was also
investigated. In the absence of base, Co(tbp)Ph(p-OMe) was
stable and did not react with ethanol to generate any anisole after
Pathway (a) (Scheme 2) involves a radical ipso substitution by
II
Co (tbp) based on the known mechanism of aryl carbon halogen
bond activation with rhodium and iridium porphyrin
16,17
II
o
complexes.
Co (tbp) attacks the ipso-carbon of Ar-Br to give
heating for 8 h at 150 C (eq. 4). When KOH was added,
III
the Co (tbp)-cyclohexadienyl radical intermediate. The radical
intermediate then eliminates a bromine atom to generate
Co(tbp)aryl. In benzene solvent, the bromine atom generated can
be trapped by benzene to afford bromobenzene through a leakage
reaction.
Co(tbp)Ph(p-OMe) was hydrolyzed smoothly to give anisole and
II
o
Co (tbp) quantitatively in 1.5 h at 150 C (eq. 5). KOH is
therefore needed in the hydrolysis of Co(tbp)aryl. The
experimental results strongly support the intermediacy of
Co(tbp)aryl in the formation of hydrodebromination product
under basic conditions and all 3 possible mechanisms are
reasonable.
Pathway (b) (Scheme 2) starts from a bromine atom
II
abstraction of aryl bromide by Co (tbp) to give an aryl radical
III
and Co (tbp)Br, analogous to the well-known halogen atom
II
3-
15(a)
no reaction
(4)
MeO
Co(tbp)
+
EtOH
transfer process between Co (CN) and an alkyl halide.
The
N2, 150 oC, 8 h
5
II
aryl radical formed can combine with Co (tbp) to generate
Co(tbp)aryl. When benzene solvent is employed, the aryl radical
intermediate can be trapped by benzene to afford 4-methoxy
biphenyl as a coupling product. In the presence of KOH,
3
.5 mM
4900 equiv
1
00% recovery
KOH (800 equiv)
N2, 150 oC, 1.5 h
III
Co (tbp)Br can be converted to Co(tbp)OH through ligand
Co(tbp)
+
EtOH
Co(tbp)
+
MeO
H
(5)
MeO
II
substitution, which regenerates Co (tbp) and H O through
2
2
3.5 mM
4900 equiv
Yield: 100%
100%
14,16
reductive elimination.
Pathway (c) (Scheme 2) goes through a single electron
-
transfer mechanism. In strongly basic media, [Co (tbp)OH]
complexes were formed from the coordination between OH and
Co (tbp), which subsequently reduce aryl bromides through
16
II
To determine whether any aryl radical intermediate exists in
the reaction process, benzene solvent was employed instead of
EtOH as a radical trapping reagent.
-
II
14,16
35% of 1-phenyl-4-
single electron transfer to afford the corresponding radical
methoxybenzene was formed after 24 h as the biaryl coupling
product between benzene and 4-methoxyphenyl radical together
with 3% of anisole as the hydrodebromination product (eq. 6).
Therefore, the existence of an aryl radical in the reaction process
is supported. Halogen atom abstraction and single electron
transfer mechanisms are more reasonable (Scheme 2, pathways b
and c). Furthermore, no bromobenzene product was detected
from the leakage reaction between bromine atom and benzene
solvent, which rules out the presence of bromine atom
intermediate. The radical ipso substitution mechanism (Scheme
19
anion. The radical anion undergoes fast C-Br bond cleavage to
1
9(b)
afford an aryl radical and bromide anion.
The aryl radical can
II
II
combine with Co (tbp) to afford Co(tbp)aryl. Since Co (tbp) is
20
electron rich (Oxidation potential for Ni analouges: 0.6 eV for
tbp and 1.04 eV for tpp) with small steric hindrance, the single
electron transfer process is likely favored and faster than other
Co porphyrin complexes.
Scheme 2. Possible Mechanisms for Aryl C-Br Bond
II
Activation with Co (tbp).
14,16
2
, pathway a) is thus disfavored.
The catalysis is promoted by
electron withdrawing para-substituent (1h) and suppressed by
electron donating para-substituent (1k), which is not consistent
with a Co(tbp)-cyclohexadienyl radical intermediate stabilized by
both electron-rich and -poor para-substituents via resonance
17
interaction. Therefore, pathway (a) is ruled out.
Pathway (b) (Scheme 2) includes a C(aryl)-Br bond activation
II
through direct bromine atom abstraction by Co (tbp). However,
II
the absence of any stoichiometric product between Co (tbp) and
4
-bromoanisole (1a) without base (Table 4, entry 1) highly
disfavors this mechanism.
Pathway (c) (Scheme 2) is the most reasonable mechanism
based on the following experimental results. The increasing
reactivities of 1k, 1a and 1h towards hydrodebromination match
well with the decreased LUMO energy of each molecule (-0.09
21
eV for 1k. -0.35 eV for 1a and -0.76 eV for 1h), consistent with
the increasing ease of an aryl bromide towards single electron

5
2
2
reduction. 9-Bromoanthracene (1l) showed f
A
as
C
t r
C
ed
E
uc
P
tio
T
n
E
ra
D
te MApp
N
m
U
).
S
C
C
he
R
mi
I
c
P
al shifts (δ) are reported as parts per million
T
2
1
of 4 h due to the reduced LUMO energy (-1.93 eV for 1m)
(ppm) in δ scale downfield from TMS. Coupling constants (J) are
reported in Hertz (Hz). High-resolution mass spectrometry
(HRMS) was performed on a Bruker SolariX 9.4 Tesla FTICR
MS instrument in electrospray ionization (ESI) mode using
through extended conjugation. Rate enhancement by ortho
methoxy group substituent was achieved from 1a, 1e, 1f and 1g,
which can be explained by the stabilization of the corresponding
σ radical anion through inductive effect. Little steric effect was
observed in the reduction of hindered substrates 1g, 1i and 1l
taking 4, 10 and 4 h to complete, respectively. Thus, an outer
sphere electron transfer mechanism is likely to operate from
Co (tbp)(OH)] complexes to an aryl bromide. The inner sphere
reduction process of halogen atom transfer (Pathway b, Scheme
) is less likely to operate as the sterically hindered aryl bromides
1g, 1i and 1l) still yield high product yields.
Scheme depicts general catalytic cycle for the
hydrodebromination process. Co (tbp) coordinates with OH in
strongly basic conditions to afford [Co (tbp)(OH)] complexes,
which subsequently transfer one electron to an aryl bromide to
generate an aryl bromide radical anion. The radical anion
undergoes fast carbon bromide bond cleavage to generate an aryl
radical and a bromide anion. In ethanol solvent, the aryl radical
formed can directly abstract a hydrogen atom from EtOH to
afford the corresponding arene as the hydrogenation product. On
the other hand, Co(tbp)aryl intermediate can further be
23
MeOH/CH Cl2 (1/1) as the solvent. GC-MS analyses were
2
conducted on a GCMS-QP2010 Plus system using an Rtx-5MS
column (30 m × 0.25 mm). Details of the GC program are as
follows. The column oven temperature and injection temperature
II
-
o
[
were 100 and 250 C. Helium was used as the carrier gas. The
−1
flow control mode was chosen as linear velocity (36.3 cm s )
with a pressure of 68.8 kPa. The total flow, column flow, and
2
(
24
−1
purge flow were 13.5, 0.95, and 3.0 mL min , respectively. Split
mode injection with a split ratio of 10.0 was applied. After
injection, the column oven temperature was kept at 100 C for 2
min and was then elevated at a rate of 30 C min for 5 min until
250 C. The temperature of 250 C was kept for 4 min. The
retention time and mass spectrum of organic products obtained
were identical with those of commercially available authentic
samples.
3
a
II
-
o
II
-
o
−1
o
o
5
.1. Preparation of H (tbp).
2
2
5
H (tbp) was prepared according to literature method. Pyrrole
4.2 mL, 0.061 mol) and pentanal (3.5 mL, 0.033 mol) were
added to 300 mL of propionic acid containing 12 mL of H O and
mL of pyridine at 100-105 C and reflux for 30 min. Another
1.8 mL of pentanal (0.017 mol) was added and refluxed for
another 2 h. Chloroform (300 mL) was added to the cooled
reaction mixture, and the reaction mixture was washed with
water (300 mL × 3), 50 mM NaOH (300 mL × 3) and water (300
mL × 3) to remove propionic acid. The chloroform solution was
evaporated to dryness. The dark residue was recrystallized from
2
III
hydrolyzed to generate the corresponding arene and Co (tbp)OH
in basic conditions as reported in our previous work.
Co (tbp)OH then undergoes reductive dimerization to regenerate
Co (tbp) and H O , thus completing the catalytic cycle.
(
14
2
III
o
1
II
2
2
II
Scheme 3 Catalytic Cycle for Co (tbp) Catalyzed
Hydrodebromination
Ar-Br
[Co (ttp)(OH)]-
II
+
-
EtOH
CH3CHOH
+ OH-
Ar
Br-
-
Ar-H
DCM and MeOH and washed with MeOH repeatedly to give fine
purple crystal as H (tbp) (291 mg, 0.545 mmol) in 4% yield. H
NMR (CDCl , 400 MHz) δ -2.63 (s, 2 H), 1.14 (t, J = 5.9 Hz, 12
H), 1.83 (m, 8 H), 2.51 (m, 8 H), 4.95 (t, J = 6.5 Hz, 8 H), 9.47
III
1
+
Co (tbp)OH
Co(ttp)Ar
Hydrogen Atom
Transfer
2
2
II
Co (tbp)
3
2
-
+
-
OH , EtOH
CH3CHOH
III
(s, 8 H).
Hydrolysis
Ar-H
2
- 0.5 H O2
Co (ttp)(OH)
5
.2. Preparation of Co(tbp).
2
6
Co(tbp) was prepared according to literature method. H (tbp)
2
4
. Conclusion
In conclusion, the Co (tbp) catalyzed hydrodebromination of
(
150 mg, 0.28 mmol), Co(OAc) ·4H O (300.1 mg, 1.2 mmol)
2 2
II
was added to 20 mL DMF and heated to reflux for 3 h. 60 mL
water was added to the cooled reaction mixture and filtrated to
get the brown solid as product. The product was further
recrystallized from MeOH and DCM to get Co(tbp) (140 mg,
0
aryl bromides using EtOH as the hydrogen source was achieved
with high yields. Mechanistic investigation shows the single
electron transfer is the most plausible pathway for aryl carbon
bromine bond cleavage.
1
.24 mmol) in 84% yield as purple solid. H NMR (CDCl , 400
3
MHz) δ 1.93 (brs, 12 H), 3.47 (brs, 8 H), 6.69 (brs, 8 H), 12.65
5
. Experimental Section
(brs, 8 H), 15.93 (brs, 8 H).
5
.3. Catalytic Hydrodebromination of Aryl Bromides at Various
Unless otherwise noted, all reagents were purchased from
Reaction Conditions
commercial suppliers and directly used without further
purification. Hexane was distilled from anhydrous calcium
chloride. Benzene was distilled over sodium under nitrogen. All
reactions were protected from light by wrapping with alumina
foils. The reaction in Teflon screw capped pressure tubes were
heated in heat blocks on heaters, monitored by TLC and GC-MS.
Thin-layer chromatography was performed on precoated silica
5
.3.1. Porphyrin Ligand Effect
With Co(tbp). 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
Co(tbp) (8.3 mg, 0.014 mmol) and KOH (155.1 mg, 2.8 mmol)
were added in PrOH (1.0 mL). The mixture was degassed for
three freeze-pump-thaw cycles, purged with N , and heated to
150 C for 3 h. 100% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
With Co(tap). 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
Co(tap) (11.3 mg, 0.014 mmol) and KOH (154.5 mg, 0.11 moL)
were added in PrOH (1.0 mL). The mixture was degassed for
three freeze-pump-thaw cycles, purged with N , and heated to
i
2
o
25
26
gel 60 F254 plates. H (tbp)
and Co(tbp)
have been
2
characterized and were prepared according to the literature
process. Silica gel (Merck, 70-230 and 230-400 mesh) was used
in column chromatography to isolate. Neutral alumina (Merck,
i
9
0 active neutral, 70-230 mesh)/H O (~10:1 v/v) was used in
2
2
column chromatography to isolate.
o
1
150 C for 6 h. 80% yield of anisole (2a) and 20% of recovered
4
decalin as internal standard.
H NMR spectra was recorded on a Bruker AV-400
-bromoanisole (1a) were detected by GC-MS analysis using
instrument at 400 MHz. Chemical shifts were referenced with the
residual solvent protons in CDCl (δ 7.26 ppm), C D (δ 7.15
3
6
6

6
Tetrahedron
l), MA
With Co(ttp). 4-Bromoanisole (1a) (9 µ
A
L,
C
0
C
.0
E
72
P
m
T
m
E
o
D
m
N
mo
U
l)
S
w
C
ere
R
a
I
dded in EtOH (1.0 mL). The mixture was degassed
P
T
Co(ttp) (10.3 mg, 0.014 mmol) and KOH (157.5 mg, 0.11 moL)
were added in PrOH (1.0 mL). The mixture was degassed for
for three freeze-pump-thaw cycles, purged with N , and heated to
2
i
o
150 C for 16 h. 98% yield of anisole (2a) was detected by GC-
three freeze-pump-thaw cycles, purged with N , and heated to
MS analysis using decalin as internal standard.
2
o
1
4
50 C for 6 h. 17% yield of anisole (2a) and 80% of recovered
-bromoanisole (1a) were detected by GC-MS analysis using
With 50 equiv of base. 4-Bromoanisole (1a) (9 µL, 0.072
mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (199.8 mg, 3.6
mmol) were added in EtOH (1.0 mL). The mixture was degassed
decalin as internal standard.
for three freeze-pump-thaw cycles, purged with N , and heated to
2
5
.3.2. Solvent Effect
With MeOH. 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
o
1
50 C for 6 h. 96% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
Co(tbp) (8.4 mg, 0.014 mmol) and KOH (153.7 mg, 2.7 mmol)
were added in MeOH (1.0 mL). The mixture was degassed for
5
.3.5. Temperature Effect
three freeze-pump-thaw cycles, purged with N , and heated to
At 120 ˚C. 4-Bromoanisole (1a) (9 µL, 0.072 mmol), Co(tbp)
(2.1 mg, 0.0035 mmol) and KOH (160 mg, 2.8 mmol) were
2
o
1
4
50 C for 72 h. 69% yield of anisole (2a) and 5% of recovered
-bromoanisole (1a) were detected by GC-MS analysis using
added in EtOH (1.0 mL). The mixture was degassed for three
o
decalin as internal standard.
freeze-pump-thaw cycles, purged with N , and heated to 120 C
2
for 25 h. 21% yield of anisole (2a) and 65% of recovered 4-
bromoanisole (1a) were detected by GC-MS analysis using
decalin as internal standard.
With EtOH. 4-Bromoanisole (1a) (9 µL, 0.072 mmol), Co(tbp)
8.3 mg, 0.014 mmol) and KOH (156 mg, 2.8 mmol) were added
(
in EtOH (1.0 mL). The mixture was degassed for three freeze-
o
pump-thaw cycles, purged with N , and heated to 150 C for 6 h.
5.3.6. Atmosphere Effect
2
1
00% yield of anisole (2a) was detected by GC-MS analysis
4
-Bromoanisole (1a) (9 µL, 0.072 mmol), Co(tbp) (2.1 mg,
using decalin as internal standard.
0.0035 mmol) and KOH (159.4 mg, 2.8 mmol) were added in
EtOH (1.0 mL). The mixture was heated to 150 C for 10 h. 96%
yield of anisole (2a) was detected by GC-MS analysis using
decalin as internal standard.
o
5
.3.3. Catalyst Loading Effect
With 0 mol%. 4-Bromoanisole (1a) (9 µL, 0.072 mmol) and
KOH (154 mg, 2.7 mmol) were added in EtOH (1.0 mL). The
mixture was degassed for three freeze-pump-thaw cycles, purged
5.4. Substrate Scope
o
with N , and heated to 150 C for 24 h. 8% yield of anisole (2a)
2
4
-Chloroanisole. 4-Chloroanisole (1b) (9 µL, 0.073 mmol),
and 80% of recovered 4-bromoanisole (1a) were detected by GC-
MS analysis using decalin as internal standard.
With 2.5 mol%. 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
Co(tbp) (1.1 mg, 0.0018 mmol) and KOH (160 mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (160 mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
1
4
50 C for 24 h. 5% yield of anisole (2a) and 80% of recovered
-chloroanisole (1b) were detected by GC-MS analysis using
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
decalin as internal standard.
-Iodoanisole. 4-Iodoanisole (1c) (16.7 mg, 0.071 mmol),
1
4
50 C for 24 h. 94% yield of anisole (2a) and 5% of recovered
-bromoanisole (1a) were detected by GC-MS analysis using
4
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (158.7 mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
decalin as internal standard.
With 5 mol%. 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (156.7mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
1
50 C for 2 h. 92% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
2-Bromoanisole. 2-Bromoanisole (1e) (9 µL, 0.072 mmol),
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (160.0 mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
1
50 C for 8 h. 100% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
With 10 mol%. 4-Bromoanisole (1a) (9 µL, 0.072 mmol),
Co(tbp) (4.2 mg, 0.007 mmol) and KOH (157.9mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
1
50 C for 4 h. 96% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
3-Bromoanisole. 3-Bromoanisole (1d) (9 µL, 0.071 mmol),
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (157.2 mg, 2.8 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
1
50 C for 6 h. 98% yield of anisole (2a) was detected by GC-
MS analysis using decalin as internal standard.
5
.3.4. KOH Loading Effect
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
Without base. 4-Bromoanisole (1a) (9 µL, 0.072 mmol) and
150 C for 8 h. 90% yield of anisole (2a) was detected by GC-
Co(tbp) (2.1 mg, 0.0035 mmol) were added in EtOH (1.0 mL).
MS analysis using decalin as internal standard.
The mixture was degassed for three freeze-pump-thaw cycles,
1-Bromo-2,5-dimethoxybenzene.
dimethoxybenzene (1f) (11 µL, 0.073 mmol), Co(tbp) (2.1 mg,
1-Bromo-2,5-
o
purged with N , and heated to 150 C for 24 h. 0% yield of
2
anisole (2a) and 100% of recovered 4-bromoanisole (1a) were
detected by GC-MS analysis using decalin as internal standard.
With 20 equiv of base. 4-Bromoanisole (1a) (9 µL, 0.072
mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (80.9 mg, 1.4
mmol) were added in EtOH (1.0 mL). The mixture was degassed
0.0035 mmol) and KOH (161.8 mg, 2.9 mmol) were added in
EtOH (1.0 mL). The mixture was degassed for three freeze-
o
pump-thaw cycles, purged with N , and heated to 150 C for 2 h.
2
100% yield of 1,4-dimethoxybenzene was detected by GC-MS
analysis using decalin as internal standard.
for three freeze-pump-thaw cycles, purged with N , and heated to
Bromo-2,4,6-trimethoxybenzene.
trimethoxybenzene (1g) (17.6 mg, 0.071 mmol), Co(tbp) (2.1 mg,
0.0035 mmol) and KOH (157.5 mg, 2.8 mmol) were added in
1-Bromo-2,4,6-
2
o
1
4
50 C for 24 h. 68% yield of anisole (2a) and 30% of recovered
-bromoanisole (1a) were detected by GC-MS analysis using
decalin as internal standard.
With 30 equiv of base. 4-Bromoanisole (1a) (9 µL, 0.072
mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (120.1 mg, 2.1
EtOH (1.0 mL). The mixture was degassed for three freeze-
o
pump-thaw cycles, purged with N , and heated to 150 C for 4 h.
2

7
1
9
9% yield of 1,3,5-trimethoxybenzene was de
analysis using decalin as internal standard.
-Bromo-4-chlorobenzene. 1-Bromo-4-chlorobenzene (1h)
13.7 mg, 0.071 mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and
A
tec
C
te
C
d b
E
y
P
G
T
C
E
-M
D
S
MA12
N
%
U
of
S
b
C
en
R
zo
IP
ic acid were detected by H NMR using 10 µL
T
CHCl CHCl as internal standard.
2
2
1
4-Bromobenzaldehyde. 4-Bromobenzaldehyde (1n) (14.1 mg,
0.07 mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (156.3
mg, 2.8 mmol) were added in EtOH (1.0 mL). The mixture was
(
KOH (156.2 mg, 2.78 mmol) were added in EtOH (1.0 mL). The
mixture was degassed for three freeze-pump-thaw cycles, purged
degassed for three freeze-pump-thaw cycles, purged with N , and
2
o
o
with N , and heated to 150 C for 6 h. 100% yield of
heated to 150 C for 7 h. 32% of α-methylbenzyl alcohol and
2
1
chlorobenzene was detected by GC-MS analysis using decalin as
internal standard.
12% of 4-bromo-α-methylbenzyl alcohol were detected by H
NMR using 10 µL CHCl CHCl as internal standard.
2
2
1
-Bromo-2,4,6-triisopropylbenzene.
triisopropylbenzene (1i) (36 µL, 0.142 mmol), Co(tbp) (4.2 mg,
.007 mmol) and KOH (319.5 mg, 5.7 mmol) were added in
1-Bromo-2,4,6-
1-tert-Butyl-4-chlorobenzene. 1-tert-Butyl-4-chlorobenzene
(1p) (12 µL, 0.07 mmol), Co(tbp) (2.2 mg, 0.0037 mmol) and
KOH (155.7 mg, 2.8 mmol) were added in EtOH (1.0 mL). The
0
EtOH (2.0 mL). The mixture was degassed for three freeze-
mixture was degassed for three freeze-pump-thaw cycles, purged
o
o
pump-thaw cycles, purged with N , and heated to 150 C for 10
with N , and heated to 150 C for 24 h. 3% of tert-butylbenzene
2
2
h. 100% yield of 1,3,5-triisopropylbenzene was detected by GC-
MS analysis using decalin as internal standard. The reaction
mixture was purified with silica gel column chromatography
and 95% of recovered 1-tert-Butyl-4-chlorobenzene were
detected by GC-MS analysis using decalin as internal standard.
1-Chloro-4-iodobenzene. 1-Chloro-4-iodobenzene (1q) (16.9
mg, 0.07 mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH
(153.3 mg, 2.8 mmol) were added in EtOH (1.0 mL). The
using hexane as eluent to get the first fast moving fraction as
1
1
,3,5-triisopropylbenzene (27.4 mg, 0.13 mmol) in 94% yield. H
2
NMR (CDCl , 400 MHz) δ 1.28 (d, 18H, J = 6.9 Hz), 2.85-2.95
mixture was degassed for three freeze-pump-thaw cycles, purged
3
o
(m, 3 H), 6.94 (s, 3 H).
with N , and heated to 150 C for 1 h. 100% of chlorobenzene
2
9
-Bromoanthracene. 9-Bromoanthracene (1l) (35.4 mg, 0.14
was detected by GC-MS analysis using decalin as internal
standard.
4,4’-Diiodobiphenyl. 4,4’-Diiodobiphenyl (1r) (28.5 mg, 0.07
mmol), Co(tbp) (2.2 mg, 0.0037 mmol) and KOH (157.0 mg, 2.8
mmol) were added in EtOH (1.0 mL). The mixture was degassed
mmol), Co(tbp) (4.1 mg, 0.007 mmol) and KOH (316.3 mg, 5.6
mmol) were added in EtOH (2.0 mL). The mixture was degassed
for three freeze-pump-thaw cycles, purged with N , and heated to
2
o
1
50 C for 4 h. The reaction mixture was purified with silica gel
column chromatography using hexane as eluent to get the first
for three freeze-pump-thaw cycles, purged with N , and heated to
150 C for 2 h. The reaction mixture was purified with silica gel
column chromatography using hexane as eluent to get the first
fast moving fraction as biphenyl (7.8 mg, 0.05 mmol) in 72%
yield.
2
o
fast moving fraction as product of anthracene (18.8 mg, 0.11
1
mmol) in 77% yield. H NMR (CDCl , 400 MHz) δ 7.46-7.49
3
(m, 4 H), 8.00-8.03 (m, 4 H), 8.44 (s, 2 H).
4
-Bromobiphenyl. 4-Bromobiphenyl (1j) (32.7 mg, 0.14
mmol), Co(tbp) (4.1 mg, 0.007 mmol) and KOH (318.5 mg, 5.7
mmol) were added in EtOH (2.0 mL). The mixture was degassed
2-Bromopyridine. 2-Bromopyridine (1s) (7 µL, 0.073 mmol),
Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (150.3 mg, 2.7 mmol)
were added in EtOH (1.0 mL). The mixture was degassed for
for three freeze-pump-thaw cycles, purged with N , and heated to
2
o
1
50 C for 12 h. 98% yield of biphenyl was detected by GC-MS
three freeze-pump-thaw cycles, purged with N , and heated to
2
o
analysis using decalin as internal standard. The reaction mixture
was purified with silica gel column chromatography using
hexane as eluent to get the first fast moving fraction as biphenyl
150 C for 1 h. Complex mixture was achieved after reaction
with 2-bromopyridine consumed completely.
2-Bromo-5-methylpyridine. 2-Bromo-5-methylpyridine (1t) (8
µL, 0.07 mmol), Co(tbp) (2.2 mg, 0.0037 mmol) and KOH
(154.7 mg, 2.8 mmol) were added in EtOH (1.0 mL). The
(18.7 mg, 0.12 mmol) in 87% yield.
4
-Bromo-(N, N-dimethyl)aminoanisole. 4-Bromo-(N, N-
dimethyl)aminoanisole (1k) (14.3 mg, 0.072 mmol), Co(tbp) (2.1
mg, 0.0035 mmol) and KOH (156.8 mg, 2.8 mmol) were added
mixture was degassed for three freeze-pump-thaw cycles, purged
o
with N , and heated to 150 C for 1 h. 87% of 2-methylthiophene
2
in EtOH (1.0 mL). The mixture was degassed for three freeze-
was detected by GC-MS analysis using decalin as internal
standard.
o
pump-thaw cycles, purged with N , and heated to 150 C for 24
2
h. 79% yield of (N, N-dimethyl)aminoanisole and 20% of
recovered 4-Bromo-(N, N-dimethyl)aminoanisole (1k) were
detected by GC-MS analysis using decalin as internal standard.
5.5. Mechanistic Investigation
Independent Synthesis of Co(tbp)Ph(p-OMe). Co(tbp)Ph(p-
OMe) was prepared according to literature method. Co(tbp) (9.0
14
4
-Bromo-benzylphenyl ether. 4-Bromo-benzylphenyl ether
mg, 0.015 mmol), 4-bromoanisole (175 µL, 1.4 mmol), KOH
(1m) (37.7 mg, 0.143 mmol), Co(tbp) (4.2 mg, 0.007 mmol) and
t
(
8.9 mg, 0.15 mmol) and BuOH (65 µL, 0.70 mmol) were added
KOH (312 mg, 5.56 mmol) were added in EtOH (2.0 mL). The
in benzene (1.0 mL). The mixture was degassed for three freeze-
pump-thaw cycles, purged with N , and heated to 150 C for 4 h.
mixture was degassed for three freeze-pump-thaw cycles, purged
o
o
2
with N , and heated to 150 C for 17 h. The reaction mixture was
2
Benzene solvent was removed with evaporation. The residue was
purified with column chromatography on silica gel and using
hexane/DCM (v:v/3:1) as eluent to get the second red fraction as
product of Co(tbp)Ph(p-OMe) (12.5 mg, 0.018 mmol) in 56%
neutralized with HCl (3 M) before GC-MS analysis. 45% yield of
benzyl phenyl ether, 50% of toluene and 50% of 4-bromophenol
were detected by GC-MS analysis using decalin as internal
standard.
1
2
yield. H NMR (CDCl , 400 MHz) δ 0.14 (d, J = 8.9 Hz, 2 H),
3
4
-Bromobenzonitrile. 4-Bromobenzonitrile (1o) (12.6 mg,
2
0
(
.86 (t, J = 7.3Hz, 12 H), 1.24 (m, 8 H). 2.16-2.23 (m, 8 H), 2.74
s, 3 H), 4.31 (d, J = 8.9 Hz, 2 H), 4.58 (t, J = 7.6Hz, 8 H), 9.37
s, 8 H). C NMR (CDCl , 176 MHz) δ 14.16, 23.01, 34.23,
0
.07 mmol), Co(tbp) (2.1 mg, 0.0035 mmol) and KOH (161.6
2
2
mg, 2.9 mmol) were added in EtOH (1.0 mL). The mixture was
13
(
3
degassed for three freeze-pump-thaw cycles, purged with N , and
2
o
38.90, 54.07, 108.65, 119.63, 130.35, 132.35, 143.91, 154.96.
HRMS (ESI-MS): calcd for C H CoN O [M] m/z 698.3389;
found m/z 698.3392.
Trap experiment. 4-Bromobenzene (1a) (9 µL, 0.072 mmol),
Co(tbp) (2.1 mg, 0.035 mmol) and KOH (161 mg, 2.9 mmol)
were added in benzene (1.0 mL). The mixture was degassed for
heated to 150 C for 1 h. The reaction mixture was diluted with
+
43
51
4
water and extracted with dichloromethane for 3 times to get rid
of Co(tbp). The water layer was acidified with HCl (3M) and
extracted with Et O for 3 times. The organic layer was combined
2
and dried with rotary evaporator. 86% 4-bromobenzoic acid and

8
Tetrahedron
TED
Yasutomo, Y. Makoto, N. Kyoko, Angew. Chem. Int. Ed. 51
(2012) 6956.
3. K.S. Chan, C.R. Liu, K.L.Wong, Tetrahedron Lett. 56 (2015)
three freeze-pump-thaw cycles, purged with
A
N
C
, a
C
nd
E
h
P
e
ate
d
to MANUSCRIPT
2
o
1
50 C for 24 h. 3% of anisole (2a) was detected by GC-MS
1
1
analysis using decalin as internal standard. 35% of 4-
methoxybiphenyl and 35% of recovered 4-bromoanisole (1a)
were detected by H NMR analysis using CHCl CHCl2 as
internal standard.
Reaction between Co(tbp)Ph(p-OMe) and EtOH in neutral
conditions. Co(tbp)Ph(p-OMe) (2.5 mg, 0.0035 mmol) was
added in EtOH (1.0 mL). The mixture was degassed for three
freeze-pump-thaw cycles, purged with N , and heated to 150 C
for 8 h. Both TLC and GC-MS analysis showed no reaction
occurred.
2
728.
4. C.R. Liu, Y.Y. Qian, K.S. Chan, Dalton Trans 43 (2014) 7771.
1
2
15. (a) J. Halpern, J.P. Maher, J. Am. Chem. Soc. 87 (1965) 5361; (b)
D. Zhu, I. Korobkov, P.H.M. Budzelaar, Organometallics 31
(
(
2012) 3958; (c) V. Lyaskovskyy, B. de Bruin, ACS Catal. 2
2012) 270.
1
1
6. K.L. Wong, C. Chen, K.S. Chan, Organometallics 35 (2016) 1847.
7. (a) C.W. Cheung, K.S. Chan, Organometallics 30 (2011) 1768; (b)
C.W. Cheung, K.S. Chan, Organometallics 30 (2011) 4269; (c)
C.W. Cheung, K.S. Chan, Organometallics 30 (2011) 4999; (d)
Y.Y. Qian, K.S. Chan, Organometallics 31 (2012) 5452; (e) B.Z.
Li, Y.Y. Qian, J. Liu, K.S. Chan, Organometallics 33 (2014) 7059;
o
2
Reaction between Co(tbp)Ph(p-OMe) and EtOH in basic
conditions. Co(tbp)Ph(p-OMe) (2.5 mg, 0.0035 mmol), KOH
(f) Y.Y. Qian, M.H. Lee, W. Yang, K.S. Chan, J. Organomet.
Chem. 791 (2015) 82; (g) W. Yang, H.P. Zuo, W.Y. Lai, S.Y.
Feng, Y.S. Pang, K.E. Hung, C.Y. Yu, Y.F. Lau, H.Y. Tsoi, K.S.
Chan, Organometallics 34 (2015) 4051; (h) H. Zuo, Z. Liu, W.
Yang, Z. Zhou, K.S. Chan, Dalton Trans. 44 (2015) 20618.
(154.9 mg, 2.8 mmol) were added in EtOH (1.0 mL). The
mixture was degassed for three freeze-pump-thaw cycles, purged
with N , and heated to 150 C for 1.5 h. 100% of anisole (2a) was
detected by GC-MS analysis with quantitative formation of
Co(tbp).
o
2
1
1
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We thank the Research Grants Council (GRF No. 14300115)
of Hong Kong SAR and the Direct Grant of CUHK for financial
support.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
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