Base-promoted aryl-bromine bond cleavage with cobalt(ii) porphyrins via a halogen atom transfer mechanism

Article abstract of DOI:10.1039/c4dt00155a

Aryl-bromine bonds are successfully cleaved by cobalt(ii) porphyrins in basic media to give Co(por)Ar (por = porphyrin) in good yields. Mechanistic studies suggested that the aryl-bromine bond is cleaved through a halogen atom transfer mechanism, which is different from the aryl-halogen bond cleavage mechanism with other group 9 metalloporphyrins. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00155a

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Base-promoted aryl–bromine bond cleavage with
cobalt(II) porphyrins via a halogen atom transfer
mechanism†
Cite this: Dalton Trans., 2014, 43,
7771
Chun Ran Liu, Ying Ying Qian and Kin Shing Chan*
Received 16th January 2014,
Accepted 3rd March 2014
Aryl–bromine bonds are successfully cleaved by cobalt(II) porphyrins in basic media to give Co(por)Ar
(por = porphyrin) in good yields. Mechanistic studies suggested that the aryl–bromine bond is cleaved
through a halogen atom transfer mechanism, which is different from the aryl–halogen bond cleavage
mechanism with other group 9 metalloporphyrins.
DOI: 10.1039/c4dt00155a
www.rsc.org/dalton
1. Introduction
Aryl halides are commonly employed starting materials in tran-
sition metal-catalyzed cross-coupling reactions for the con-
struction of carbon–carbon bonds.1 Aryl halides are used in
Suzuki,2 Stille,3 Negishi,4 Kumada,5 and Hiyama6 cross-coup-
ling reactions to couple with aryl boronic acids, aryl tin com-
plexes, aryl zinc complexes, aryl magnesium halides and aryl
silanes to give biaryls. In these reactions, expensive palladium
catalysts are usually employed. Recently, first row transition
metals have been used as catalysts for cross-coupling reactions
as they are cheaper and less toxic. For example, Fe,7 Ni,8 Cu,9
and Co10 have been used as catalysts for cross-coupling reac-
tions. Detailed understanding of the aryl–halogen bond clea-
vage by first row transition metals are lacking. At the same
time, first row transition metal–alkyl/aryl intermediates are
difficult to isolate due to the weak metal–carbon bond.11 This
leads to the difficulty in conducting mechanistic studies.
Since the Ar–X bond cleavage has been reported to be the
rate determining step in cross-coupling reaction, it is impor-
tant to gain further understanding of the Ar–X bond cleavage
by transition metals, especially first row transition metals.12,13
There are mainly four types of mechanisms for the cleavage of
an aryl halogen bond (Ar–X): (1) oxidative addition, (2) nucleo-
philic aromatic substitution, (3) radical ipso-substitution, and
(4) halogen atom transfer (Scheme 1).
Scheme 1 Four types of aryl halogen bond cleavage by transition
metal complexes.
complex. Oxidative addition usually takes place with electron-
rich, low-valent middle-group transition metal complexes. For
example, oxidative addition of PhX (X = I, Br, Cl) to the
palladium(0) complex with the bidentate phosphine ligand
(PP) gives the palladium(II) phenyl halo complex (PP)Pd(Ph)
X.14 2-Tolyl-bromobenzene has been reported to react with the
nickel diaminocarbene complex (Ni0(timy)2) through oxidative
addition to form NiII(timy)(o-toyl)Br in 71% yield.15
Nucleophilic aromatic substitution usually occurs between
an electron rich, low valent transition metal complex and an
electron-deficient aryl halide. [Cp*RhI(PMe3)H]−, which is gen-
erated from the deprotonation of Cp*RhIII(PMe3)H2 by pyri-
dine, undergoes nucleophilic aromatic substitution with ArF–F
to give Cp*RhIII(PMe3)(ArF)H and pyridinium fluoride.16
Oxidative addition is the most reported mechanism for an
aryl halogen bond (Ar–X) cleavage with a transition metal
A radical ipso-substitution mechanism for Ar–X bond clea-
vage with transition metals was first reported by the Chan
group.17 Both iridium and rhodium porphyrin complexes
cleave the Ar–X bond (X = I, Br) to give IrIII(ttp)Ar (ttp =
5,10,15,20-tetratolylporphyrin) and RhIII(ttp)Ar in good to high
yields. Under basic conditions, IrIII(ttp)(CO)Cl and RhIII(ttp)Cl
Department of Chemistry, The Chinese University of Hong Kong, Shatin,
New Territories, Hong Kong, People’s Republic of China. E-mail: ksc@cuhk.edu.hk;
Fax: (+852) 3943-5057; Tel: (+852) 2609-6376
†Electronic supplementary information (ESI) available: Table, figure, and CIF
file giving crystallographic data for Co(ttp)(4-methoxyphenyl) and 1H and 13C
NMR spectra. CCDC: 981392. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt00155a
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undergo ligand substitution with hydroxide ions to give
IrIII(ttp)OH and RhIII(ttp)OH, which then form Ir2II(ttp)2 and
Rh2II(ttp)2, respectively, through reductive dimerization.18
IrII(ttp) and RhII(ttp), which exist in equilibria with the metal–
metal bonded dimers, attack the ipso-carbon of ArX to give the
MIII(por)-cyclohexadienyl (M = Ir, Rh) radical intermediate.
The radical intermediates then eliminate a halogen atom to
give IrIII(ttp)Ar and RhIII(ttp)Ar.
Table 2 Porphyrin electronic effects on the Ar–Br bond cleavage with
CoII(por)
Entry
por
Time/h
Yield 3/%
Halogen atom transfer commonly occurs between ArX (X =
I, Br, Cl) and metalloradicals. LCo0(N2), generated from
1
2
3
4
tap (1b)
ttp (1a)
tpp (1c)
1.5
3
2
69 (3b)
79 (3a)
55 (3c)
73 (3d)
LCoICH2SiMe3 (L
= 2,6-bis[2,6-dimethylphenyliminoethyl]-
pydidine) and H2, undergoes binuclear oxidative addition with
ArCl to yield LCoIAr and LCoICl through the halogen atom
transfer mechanism.19 [CoII(CN)5]3− has also been proposed to
abstract an iodine atom from the fairly reactive 2-iodopyridine
t
4-Clpp (1d)
5
tpp
=
5,10,15,20-tetraphenylporphyrin;
t
4-Clpp
=
5,10,15,20-tetra-
(p-chlorophenyl)porphyrin.
to give [ICoIII(CN)5]3− 20 Another [CoII(CN)5]3− then combines
.
PhBr to give 73% yield of CoIII(ttp)Ph (Table 1, entry 2). tBuOH
was then added to increase the solubility of KOH in benzene
so as to increase the reaction rate. When 30 equivalents of
tBuOH were added, the reaction time was significantly shor-
tened to 6 h with similar yield of CoIII(ttp)Ph obtained
(Table 1, entries 2 and 3). Higher base loading slowed down
the reaction significantly (Table 1, entries 3–5). Optimization
with the released pyridyl radical to generate [2-pyridyl-
CoIII(CN)5]3−
.
The Chan group has reported that iridium and rhodium
porphyrins react with ArX (X = I, Br, Cl) to give iridium and
rhodium porphyrin aryls, respectively, through a radical ipso-
substitution mechanism.17 However, the halogen atom trans-
fer mechanism has been reported for most examples of the
Ar–X bond cleavage by cobalt complexes.19,20 The effects of
metal in the Ar–X bond cleavage are thus interesting and intri-
guing, and we therefore examined the reaction scope as well as
the mechanistic features.
t
on increasing BuOH loading to 50 equivalents reduced the
reaction times from 10 h to 3 h (Table 1, entries 3, 6 and 8).
t
Further increasing to 100 equivalents of BuOH reduced the
reaction time to 1.5 h, however, the yield of CoIII(ttp)Ph
decreased from 79% to 67% (Table 2, entries 8 and 9). The
temperature effect was then examined. At 120 °C, the reaction
was very slow and required two days to complete (Table 1,
entry 10). At 200 °C, although the reaction only required 1.5 h,
the yield of CoIII(ttp)Ph was lower (Table 1, entry 11). Reaction
in THF resulted in only 5% yield of Co(ttp)Ph after 4 days,
which indicates that THF may coordinate to CoII(ttp) and
block the vacant sites (Table 1, entry 7). Therefore, 10 equi-
2. Results and discussion
2.1 Optimization of reaction conditions
Initially, CoII(ttp) did not react with PhBr in benzene at 150 °C
for 2 days with 92% yield of CoII(ttp) recovered (Table 1, entry
1). Based on our previous success of base-promoted Ar–X clea-
vage reaction, when KOH was added, CoII(ttp) reacted with
t
valents of KOH and 50 equivalents of BuOH were used for the
Ar–Br bond cleavage with CoII(ttp) at 150 °C in benzene for
further studies.
Table 1 Base-promoted Ar–Br bond cleavage with CoII(ttp)
Various cobalt(II) porphyrin complexes were then used to
study the electronic effects of porphyrin ligand on the Ar–Br
bond cleavage reactions. Cobalt(II) porphyrins with both the
electron rich and electron deficient substituents cleaved the
Ar–Br bond successfully (Table 2). Although the more electron
rich CoII(tap) (tap = 5,10,15,20-tetra-4-anisylporphyrin) gave
the fastest reaction rate, CoII(ttp) gave the highest product
yield (Table 2, entries 1 and 2). Therefore, CoII(ttp) was used
for further studies.
Entry
n (KOH)
m (tBuOH)
Temp/°C
Time/h
Yield 3a/%
1
2
3
4
—
10
10
20
50
10
10
10
10
10
10
—
—
150
150
150
150
150
150
150
150
150
120
200
48
48
6
0
73
77
75
69
73
5
79
67
73
67
30
30
30
10
10
50
100
50
50
2.2 Substrate scopes
8
ArBr bearing both electron donating and withdrawing func-
tional groups reacted with CoII(ttp) smoothly to give moderate
to good yields of CoIII(ttp)Ar within 1 day (Table 3). Functional
groups such as NO2 and C(O)Me were compatible under the
reaction conditions (Table 3, entries 7 and 8).
5
18
10
96
3
1.5
48
1.5
6
7a
8
9
10
11
These base-promoted Ar–Br cleavage reactions of ArBr with
CoII(por) provide a more convenient method for the synthesis
a The reaction was carried out in THF solvent.
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Table 3 Substrate scopes for the Ar–Br bond cleavage with CoII(ttp)
Entry
FG
Time/h
Yield 3/%
Scheme 2 Possible mechanisms for aryl-bromine bond cleavage by
1
2
3
4
5
6
7
8
OMe (2e)
Me (2f)
tBu (2g)
H (2a)
Cl (2h)
F (2i)
7
2.5
24
3
2.5
3
75 (3e)
89 (3f)
72 (3g)
79 (3a)
61 (3h)
74 (3i)
79 (3j)
57 (3k)
CoII(por).
ents to CoII(por) is very sterically hindered due to the planar
porphyrin ligand. Moreover, the cobalt(IV) oxidation state is
very uncommon and generated as transients in chemical and
electrochemical redox processes.22 Therefore, the oxidative
addition mechanism is unlikely since it involves the gene-
ration of unstable cobalt(IV) porphyrin intermediates.
NO2 (2j)
C(O)Me (2k)
18
3
For the nucleophilic substitution mechanism, the dis-
proportionation of CoII(por) to form CoIII(por)OH and
CoI(por)− can occur under basic conditions (Scheme 2,
pathway II).23 However, CoI(por)− (pKa of Co(por)H is 10.5 in
aqueous methanol), being a salt, is a poor nucleophile and is
poorly soluble in benzene.24 Therefore, it does not likely attack
the Ar–Br bond.
The electron transfer mechanism is possible from energetic
considerations but inconsistent with reactivity patterns. First,
electron transfer from CoII(ttp) to PhBr to give CoI(ttp)− and
PhBr radical anions is uphill by about 1 V,25 in line with the
reaction rate at 150 °C. However, both more electron rich and
poor FG–C6H4–Br (FG = OMe (2e), NO2 (2j)) react slower than
Fig. 1 ORTEP presentation of Co(ttp)(4-methoxyphenyl) (3e) (30% PhBr. The rate determining electron transfer is inconsistent
with the substituent electronic effect.26 Second, in polar
solvent THF, which facilitates electron transfer, resulted in low
probability displacement ellipsoids). Hydrogen atoms are omitted for
clarity.
yield of Co(ttp)Ar (Table 1, entry 7). Therefore, the electron
transfer mechanism is unlikely.
of cobalt porphyrin aryls with reasonable rates and yields
without using aryllithiums or Grignard reagents in their reac-
tions with cobalt porphyrin halides.21
Scheme 3 shows two possible mechanisms for the Ar–Br
cleavage with CoII(por). When a radical ipso-substitution oper-
ates (Scheme 3, pathway IV), CoII(por) first attacks the ipso-
carbon of ArBr to generate CoIII(por)–cyclohexadienyl radical
intermediate. Then a bromine atom is eliminated to generate
CoIII(por)Ar. The bromine atom can either combine with
another CoII(por) to give CoIII(por)Br or attack benzene to give
PhBr, which can further react with CoII(por) to give CoIII(por)-
Ph. Alternately, when the halogen atom transfer operates
(Scheme 3, pathway V), CoII(por) abstracts the bromine atom
from ArBr to generate CoIII(por)Br and an aryl radical. The aryl
radical can either combine with another CoII(por) to give
CoIII(por)Ar (Scheme 3, pathway (i)), or add to benzene solvent
to give biaryl (Scheme 3, pathway (ii)). CoIII(por)Br further
undergoes ligand substitution with KOH to produce CoIII(por)-
OH (Scheme 3, pathway (iii)), which undergoes reductive elim-
ination to give back CoII(por) and H2O2.18
2.3 X-ray crystallographic details
The structure of CoIII(ttp)(4-methoxyphenyl) (3e) was shown in
Fig. 1.‡ This complex crystallized in the monoclinic space
group P21/c. It is a five coordinated cobalt complex in a
square–pyramidal geometry with the cobalt–carbon bond
length of 1.903 Å.
2.4 Mechanistic studies
The Ar–Br bond cleavage by CoII(por) may occur through one
of the four possible mechanisms mentioned in Scheme 1. For
the oxidative addition mechanism, the Ar–Br bond is oxida-
tively added to the cobalt(II) center and gives an aryl-CoIV(por)–
Br intermediate (Scheme 2, pathway I). This intermediate is
highly unstable since the syn-addition of two bulky substitu-
In order to differentiate pathway IV from pathway V, the
substituent electronic effect by the Hammett study is an
effective strategy as we have successfully applied in the studies
with iridium and rhodium porphyrin chemistry.17 However,
‡Single crystal of Co(ttp)(4-methoxyphenyl) (3e) was recrystallised from dichloro-
methane and methanol. CCDC: 981392.
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Scheme 3 Possible radical ipso-substitution and halogen atom transfer mechanism for Ar–Br bond cleavage of ArBr with CoII(por).
Co(ttp)Ar was not stable under the reaction conditions and
exchange of Co(ttp)Ar also occurred (eqn (1) and (2)). There-
fore, the Hammett study is not informative in mechanistic
studies.
ð1Þ
Scheme 4 Proposed mechanism for the Ar–Br bond cleavage with
Co(ttp).
ð2Þ
ð4Þ
To figure out the reaction mechanism, the organic co-pro-
ducts were thus analyzed. In the reaction of CoII(ttp) with
4-bromoanisole, apart from 73% yield of CoIII(ttp)(4-methoxy-
phenyl), 54% yield of anisole was detected by GC-MS analysis
From the above findings, the halogen atom transfer mech-
and 187% yield of 4-methoxylbiphenyl was isolated, which we
anism was concluded to be the most probable reaction mech-
have been previously reported as a product in the Co(por)-
anism (Scheme 4). First, CoII(por) abstracts the bromine atom
catalyzed biaryl synthesis from Ar–Br in benzene (eqn (3)).13
from ArBr through halogen atom transfer to give an aryl
Neither bromobenzene nor biphenyl was observed by GC-MS
radical and CoIII(por)Br (Scheme 4, pathway 1). The aryl
or 1H NMR analysis. These observations are consistent with
radical then combines with another CoII(por) to form CoIII-
the halogen atom transfer mechanism.
(por)Ar (Scheme 4, pathway 2). CoIII(por)Br undergoes ligand
substitution with KOH to give CoIII(por)OH, which undergoes
reductive elimination to generate CoII(por) and H2O2
(Scheme 4, pathways 3 and 4).18 H2O2 quickly decomposes
ð3Þ
to water and oxygen under basic conditions (Scheme 4,
pathway 5).28
2.5 Comparison of Ar–X bond cleavage mechanism
However, an aryl radical can also form from the homolysis
of CoIII(por)Ar if the ipso-substitution mechanism occurs. It is interesting to find that the mechanism for Ar–Br cleavage
Thus, the control experiment was carried out to see if anisole by cobalt porphyrins goes through a halogen atom transfer
and 4-methoxylbiphenyl formed from Co(ttp)(4-methoxy- mechanism while that of iridium and rhodium porphyrins
phenyl). The results showed that only 50% of anisole was operate by the ipso-substitution mechanism. For a radical ipso-
obtained without any 4-methoxylbiphenyl (eqn (4)). This con- substitution mechanism, the cyclohexadienyl radical forms
firmed that the 4-methoxylbiphenyl is formed from the aryl first (Scheme 5, pathway A). For rhodium and iridium porphyr-
radical generated from halogen atom transfer. Anisole was prob- ins, the Rh–C and Ir–C bonds are stronger than that of Co–C
ably formed from the hydrolysis of CoIII(ttp)(4-methoxyphenyl) bond.11,29 When the weaker π bond of an aryl halide is broken
with tBuOH or residual water in KOH (Scheme 3, pathway iv).27
and forms the M–C (M = Co, Rh, Ir) bond, the rhodium and
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4.2. Experimental instrumentation
1H and 13C NMR spectra were recorded on a Bruker Avance III
400 at 400 MHz). Chemical shifts were referenced with the
residual solvent protons in chloroform-d (δ = 7.26 ppm) or
tetramethylsilane (δ
= 0.00 ppm) and benzene-d6 (δ =
7.15 ppm) in 1H NMR spectra, CDCl3 (δ = 77.16 ppm) in
13C NMR spectra as the internal standards. Chemical shifts (δ)
were reported as part per million (ppm) in (δ) scale downfield
from TMS. Coupling constants (J) were reported in hertz (Hz).
High resolution mass spectra (HRMS) were recorded on a
ThermoFinnigan MAT 95 XL mass spectrometer. Fast atom
bombardment spectra were recorded with 3-nitrobenzyl
alcohol (NBA) as the matrix.
Scheme 5 Ar–Br
9 metalloporphyrins.
bond
cleavage
of
ArBr
with
group
iridium intermediates are much more stabilized than the
cobalt one.30 On the other hand, in the halogen atom transfer
mechanism, the partial formation of a Co–Br bond and partial
breaking of an Ar–Br bond in the transition state can decrease
the activation barrier of the Ar–Br bond cleavage by cobalt por-
phyrins (Scheme 5, pathway B). Thus, the strength of M–C
bond plays an important role in controlling the Ar–Br bond
cleavage mechanism.
GC/MS analysis was conducted on
a
Shimadzu
GCMS-QP2010 Plus system using an Rtx-5MS column (30 m ×
0.25 mm). The details of the GC program are as follows. The
column oven temperature and injection temperature are
50.0 °C and 270.0 °C, respectively. Split injection mode was
applied. The carrier gas used was helium with primary
pressure of 500–900 kPa. Linear velocity was chosen as the
flow control mode. The pressure, total flow, column flow,
linear velocity, purge flow and split ratio were 53.5 kPa,
24.0 mL min−1, 1.00 mL min−1, 36.3 cm s−1, 3.0 mL min−1 and
20.0, respectively. The column oven temperature was kept at
50 °C for 5 minutes and then elevated at a rate of 20 °C min−1
until 270 °C and the temperature was kept for 4 minutes. The
ion source temperature and interface temperature were 230 °C
and 270 °C, respectively.
3. Conclusions
In conclusion, the base-promoted Ar–Br bond cleavage with
CoII(por) has been achieved in basic media to give CoIII(por)Ar
in moderate to high yields. Mechanistic studies showed that
the Ar–Br bond cleavage step follows a halogen atom transfer
mechanism.
4. Experimental section
4.1 General procedure
4.3. Optimization of base-promoted Ph–Br bond cleavage by
CoII(por)
Unless otherwise noted, all reagents were purchased from
(a) Reaction between CoII(ttp) and PhBr without a base.
commercial suppliers and directly used without further purifi- CoII(ttp) (1a, 10.0 mg, 0.014 mmol), PhBr (2a, 145.0 μL,
cation. Hexane for chromatography was distilled from anhy- 1.400 mmol) were added to benzene (1.0 mL) and the mixture
drous calcium chloride. Benzene and benzene-d6 were distilled was degassed for three freeze–pump–thaw cycles in a Teflon
from sodium and were stored in a Teflon-capped tube under screw capped tube and filled with N2. The reaction was then
nitrogen gas prior to use. Thin layer chromatography was per- heated at 150 °C for 2 days. The solvent was then removed
formed on aluminum oxide 150 F254 neutral. All reactions under vacuum and the crude product was purified by column
carried out in a Teflon screw capped tube under N2 with the chromatography over alumina eluted with CH2Cl2–hexane
mixture degassed for three freeze–thaw–pump cycles were (1 : 2). No product was obtained.
wrapped with aluminum foil to prevent undesired photo-
(b) Reaction between CoII(ttp) and PhBr with KOH.
chemical reactions. The reaction mixtures in Teflon-screw- CoII(ttp) (1a, 10.0 mg, 0.014 mmol), PhBr (2a, 145.0 μL,
capped tubes were heated in heat blocks on heaters. The reac- 1.400 mmol) and KOH (7.8 mg, 0.140 mmol) were added to
tions in sealed NMR tubes were heated in GC-ovens. The crude benzene (1.0 mL) and the mixture was degassed for three
mixture was dried under high vacuum. The products were freeze–pump–thaw cycles in a Teflon screw capped tube and
further purified by aluminum oxide (Merck, 70–230 mesh, filled with N2. The reaction was then heated at 150 °C for 2
neutral, pre-treated with 10 wt% of water) column chromato- days. The solvent was then removed under vacuum and the
graphy eluting with a solvent mixture of hexane–CH2Cl2. crude product was purified by column chromatography over
Single crystal of 3e for X-ray crystallography was grown from alumina eluted with CH2Cl2–hexane (1 : 2) to give CoIII(ttp)Ph
CH2Cl2–MeOH via slow evaporation. Unless otherwise specified, (3a, 8.2 mg, 0.010 mmol, 73%). Rf = 0.84 (CH2Cl2–hexane =
the residual benzene proton signal of benzene-d6 solvent was 1 : 2). 1H NMR (400 MHz, CDCl3): δ 0.27(d, 2H, o-Ph), 2.66
used as an internal standard to estimate the yield of cobalt (s, 12H, methyl-H), 4.67 (t, 2H, m-Ph), 5.26 (t, 1H, p-Ph),
porphyrin species by 1H NMR spectroscopy. CoII(ttp) (1a), 7.49 (d, 8H, tolyl-H), 7.93 (s, 8H, tolyl-H), 8.83 (s, 8H, pyrrole-
CoII(tap) (1b), CoII(tpp) (1c), and CoII(t4-Clpp) (1d) were pre- H). 13C NMR (CDCl3, 100 MHz) δ 21.7, 121.4, 122.0, 123.0,
pared according to literature procedures.31
127.7, 132.6, 133.0, 133.7, 137.5, 138.8, 145.5; HRMS (FABMS):
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calcd for [C54H41CoN4]+([M]+): m/z 804.2658. Found: m/z. heated at 200 °C for 1.5 hours to give CoIII(ttp)Ph (3a, 7.5 mg,
804.2682.
0.009 mmol, 67%).
(c) Reaction between CoII(ttp) and PhBr with KOH and
(k) Reaction between CoII(ttp) and PhBr with 10 equiv.
10 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg, 0.014 mmol), KOH and 10 equiv. tBuOH at 150 °C in THF. CoII(ttp)
PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) (10.0 mg, 0.014 mmol), PhBr (145.0 μL, 1.400 mmol), KOH
and tBuOH (13.0 μL, 0.140 mmol) were added to benzene (7.8 mg, 0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were
t
(1.0 mL) and the mixture was degassed and then heated at added to THF (1.0 mL) and the mixture was degassed and then
150 °C for 10 hours to give CoIII(ttp)Ph (3a, 8.2 mg, heated at 150 °C for 4 days to give CoIII(ttp)Ph (0.6 mg,
0.010 mmol, 73%).
0.001 mmol, 5%).
(d) Reaction between CoII(ttp) and PhBr with 10 equiv.
KOH and 30 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg,
4.4. Porphyrin ligand electronic effect
(a) Reaction with CoII(tap). CoII(tap) (1b, 10.0 mg,
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg,
t
0.140 mmol) and BuOH (40.0 μL, 0.421 mmol) were added to 0.013 mmol), PhBr (2a, 133.0 μL, 1.263 mmol), KOH (7.1 mg,
t
benzene (1.0 mL) and the mixture was degassed and then 0.126 mmol) and BuOH (60.0 μL, 0.632 mmol) were added to
heated at 150 °C for 6 hours to give CoIII(ttp)Ph (3a, 8.7 mg, benzene (1.0 mL) and the mixture was degassed for three
0.011 mmol, 77%).
freeze–pump–thaw cycles in a Teflon screw capped tube and
(e) Reaction between CoII(ttp) and PhBr with 10 equiv. filled with N2. The reaction was then heated at 150 °C for
KOH and 50 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg, 1.5 hours. The solvent was then removed under vacuum and
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg, the crude product was purified by column chromatography
t
0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were added to over alumina eluted with CH2Cl2–hexane (1 : 2) to give CoIII-
benzene (1.0 mL) and the mixture was degassed and then (tap)Ph32 (3b, 7.6 mg, 0.009 mmol, 69%). 1H NMR (CDCl3,
heated at 150 °C for 3 hours to give CoIII(ttp)Ph (3a, 8.9 mg, 400 MHz) δ 0.32 (d, 2 H, o-Ph), 4.04 (s, 12 H, methyl-H), 4.71
0.011 mmol, 79%).
(m, 2 H, m-Ph), 5.28 (d, 1 H, p-Ph), 7.22 (d, 2 H, tolyl-H), 7.98
(f) Reaction between CoII(ttp) and PhBr with 10 equiv. (s, 8 H, tolyl-H), 8.88 (s, 8 H, pyrrole-H).
KOH and 100 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg,
(b) Reaction with CoII(tpp). CoII(tpp) (1c, 10.0 mg,
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg, 0.015 mmol), PhBr (2a, 157.0 μL, 1.491 mmol), KOH (8.3 mg,
t
0.140 mmol) and tBuOH (133.0 μL, 1.400 mmol) were added to 0.148 mmol) and BuOH (70.0 μL, 0.737 mmol) were added to
benzene (1.0 mL) and the mixture was degassed and then benzene (1.0 mL) and the mixture was degassed then heated at
heated at 150 °C for 6 hours to give CoIII(ttp)Ph (3a, 7.5 mg, 150 °C for 2 hours to give CoIII(tpp)Ph33 (3c) (6.1 mg,
1
0.011 mmol, 67%).
0.008 mmol, 55%). Rf = 0.68 (CH2Cl2–hexane = 2 : 1) H NMR
(g) Reaction between CoII(ttp) and PhBr with 20 equiv. (CDCl3, 400 MHz) δ 0.28 (d, 2 H, o-Ph), 4.70 (t, 2 H, m-Ph), 5.28
KOH and 30 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg, (d, 1 H, p-Ph), 7.71 (m, 12 H, por-Ph), 8.03 (s, 8 H, por-Ph),
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (15.6 mg, 8.83 (s, 8 H, pyrrole-H).
t
0.280 mmol) and BuOH (40.0 μL, 0.421 mmol) were added to
(c) Reaction with CoII(t4-Clpp). CoII(t4-Clpp) (1d, 10.0 mg,
benzene (1.0 mL) and the mixture was degassed and then 0.012 mmol), PhBr (2a, 130.0 μL, 1.234 mmol), KOH (7.0 mg,
t
heated at 150 °C for 8 hours to give CoIII(ttp)Ph (3a, 8.4 mg, 0.125 mmol) and BuOH (58.0 μL, 0.611 mmol) were added to
0.010 mmol, 75%).
benzene (1.0 mL) and the mixture was degassed for three
(h) Reaction between CoII(ttp) and PhBr with 50 equiv. freeze–pump–thaw cycles in a Teflon screw capped tube and
KOH and 30 equiv. tBuOH. CoII(ttp) (1a, 10.0 mg, filled with N2. The reaction was then heated at 150 °C for
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (39.0 mg, 5 hours. The solvent was then removed under vacuum and the
t
0.700 mmol) and BuOH (40.0 μL, 0.421 mmol) were added to crude product was purified by column chromatography over
benzene (1.0 mL) and the mixture was degassed and then alumina eluted with CH2Cl2–hexane (1 : 2) to give CoIII(t4-Clpp)-
heated at 150 °C for 18 hours to give CoIII(ttp)Ph (3a, 7.8 mg, Ph (3d) (7.8 mg, 0.009 mmol, 73%). 1H NMR (CDCl3, 400 MHz)
0.010 mmol, 69%).
δ 0.18 (d, 2 H, o-Ph), 4.68 (t, 2 H, m-Ph), 5.28 (t, 1 H, p-Ph),
(i) Reaction between CoII(ttp) and PhBr with 10 equiv. 7.68 (d, 8 H, por-Ph), 7.96 (s, 8 H, por-Ph), 8.82 (s, 8 H, pyrrole-
KOH and 50 equiv. BuOH at 120 °C. CoII(ttp) (1a, 10.0 mg, H). 13C NMR (CDCl3, 100 MHz) δ 120.8, 121.7, 123.3, 127.3,
t
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg, 132.4, 133.2, 134.5, 134.8, 139.8, 145.3; HRMS (FABMS): calcd
t
0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were added to for [C50H29Cl4CoN4]+([M]+): m/z 884.0478. Found: m/z
benzene (1.0 mL) and the mixture was degassed and then 884.0480.
heated at 120 °C for 2 days to give CoIII(ttp)Ph (3a, 8.2 mg,
4.5. Scope of base-promoted Ar–Br cleavage with CoII(ttp)
(a) Reaction with 4-bromoanisole. CoII(ttp) (1a, 10.0 mg,
0.010 mmol, 73%).
( j) Reaction between CoII(ttp) and PhBr with 10 equiv.
t
KOH and 50 equiv. BuOH at 200 °C. CoII(ttp) (1a, 10.0 mg, 0.014 mmol), 4-bromoanisole (2e, 257.0 mg, 1.400 mmol),
t
0.014 mmol), PhBr (2a, 145.0 μL, 1.400 mmol), KOH (7.8 mg, KOH (7.8 mg, 0.140 mmol) and BuOH (65.0 μL, 0.685 mmol)
t
0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were added to were added to benzene (1.0 mL) and the mixture was degassed
benzene (1.0 mL) and the mixture was degassed and then and then heated at 150 °C for 7 hours to give CoIII(ttp)C6H4-
7776 | Dalton Trans., 2014, 43, 7771–7779
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(p-OMe) (3e, 8.6 mg, 0.010 mmol, 75%). Rf = 0.20 (CH2Cl2– 132.1, 132.2, 133.1, 133.6, 137.6, 138.7, 145.5; HRMS (FABMS):
hexane = 1 : 2) 1H NMR (CDCl3, 400 MHz) δ 0.15 (d, 2 H, o-Ph), calcd for [C54H40CoFN4]+([M]+): m/z 822.2569. Found: m/z.
2.65 (s, 12 H, methyl-H), 2.83 (s, 3 H, OMe-H), 4.43 (d, 2 H, m- 838.2564.
Ph), 7.49 (d, 8 H, por-Ph), 7.93 (s, 8 H, por-Ph), 8.83 (s, 8 H,
(f) Reaction with 4-bromo-nitrobenzene. CoII(ttp) (1a,
pyrrole-H). 13C NMR (CDCl3, 100 MHz) δ 21.7, 54.3, 109.2, 10.0 mg, 0.014 mmol), 4-bromo-nitrobenene (2j, 278.0 mg,
t
122.0, 127.7, 131.8, 133.0, 133.7, 137.5, 138.8, 145.6, 155.3; 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and BuOH (65.0 μL,
HRMS (FABMS): calcd for [C55H43CoN4O]+([M]+): m/z 834.2769. 0.685 mmol) were added to benzene (1.0 mL) and the mixture
Found: m/z. 834.2780.
was degassed and then heated at 150 °C for 18 hours to give
(b) Reaction with 4-bromotoluene. CoII(ttp) (1a, 10.0 mg, CoIII(ttp)C6H4(p-NO2) (3j, 9.2 mg, 0.011 mmol, 79%). Rf = 0.19
1
0.014 mmol), 4-bromotoluene (2f, 169.0 mg, 1.400 mmol), (CH2Cl2–hexane = 1 : 2). H NMR (CDCl3, 400 MHz) δ 0.53 (d,
t
KOH (7.8 mg, 0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) 2 H, o-Ph), 2.66 (s, 12 H, por-Me), 5.55 (d, 2 H, m-Ph), 7.51 (d,
were added to benzene (1.0 mL) and the mixture was degassed 8 H, por-Ph), 7.91 (s, 8 H, por-Ph), 8.88 (s, 8 H, pyrrole-H). 13C
and then heated at 150 °C for 2.5 hours to give CoIII(ttp)- NMR (CDCl3, 100 MHz) δ 21.7, 115.9, 122.3, 127.9, 132.7,
C6H4(p-Me) (3f, 10.0 mg, 0.012 mmol, 89%). Rf = 0.52 (CH2Cl2– 133.3, 133.6, 137.6, 138.3, 144.0, 145.4. HRMS (FABMS): calcd
hexane = 1 : 2) 1H NMR (CDCl3, 400 MHz) δ 0.20 (d, 2 H, o-Ph), for [C54H40CoN5O2]+([M]+): m/z 849.2514. Found: m/z 849.2503.
1.21 (s, 3 H, tolyl-Me), 2.67 (s, 12 H, por-Me), 4.57 (d, 2 H,
(g) Reaction with 4-bromoacetophenone. CoII(ttp) (1a,
m-Ph), 7.50 (d, 8 H, por-Ph), 7.95 (s, 8 H, por-Ph), 8.85 (s, 8 H, 10.0 mg, 0.014 mmol), 4-bromoacetophenone (2k, 274.0 mg,
t
pyrrole-H). 13C NMR (CDCl3, 100 MHz) δ 19.0, 21.7, 122.0, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and BuOH (65.0 μL,
124.1, 127.7, 130.5, 131.9, 132.9, 133.7, 137.4, 138.9, 145.6; 0.685 mmol) were added to benzene (1.0 mL) and the mixture
HRMS (FABMS): calcd for [C55H43CoN4]+([M]+): m/z 818.2820. was degassed and then heated at 150 °C for 3 hours to give
Found: m/z. 818.2814.
CoIII(ttp)C6H4(p-C(O)Me) (3k, 6.6 mg, 0.008 mmol, 57%). Rf =
(c) Reaction with 1-bromo-4-tert-butylbenzene. CoII(ttp) 0.10 (CH2Cl2–hexane = 1 : 2). H NMR (CDCl3, 400 MHz) δ 0.47
(1a, 10.0 mg, 0.014 mmol), 1-bromo-4-tert-butylbenzene (2g, (d, 2 H, o-Ph), 1.66 (s, 3 H, C(O)Me-H), 2.66 (s, 12 H, por-Me),
242.5 μL, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and tBuOH 5.26 (d, 2 H, m-Ph), 7.50 (d, 8 H, por-Ph), 7.94 (s, 8 H, por-Ph),
(65.0 μL, 0.685 mmol) were added to benzene (1.0 mL) and the 8.86 (s, 8 H, pyrrole-H). 13C NMR (CDCl3, 100 Hz) δ 21.7, 25.6,
mixture was degassed and then heated at 150 °C for 1 day to 26.8, 121.7, 122.2, 127.4, 127.8, 128.4, 129.1, 132.7, 133.1,
give CoIII(ttp)C6H4(p-tert-butyl) (3g, 8.7 mg, 0.010 mmol, 72%). 133.6, 137.7, 138.6, 145.5; HRMS (FABMS): calcd for
Rf = 0.53 (CH2Cl2–hexane = 1 : 2). 1H NMR (CDCl3, 400 MHz) [C56H43CoN4O]+([M]+): m/z 846.2769. Found: m/z. 846.2763.
1
t
δ 0.21 (d, 2 H, o-Ph), 0.39 (s, 9 H, Bu), 2.66 (s, 12 H, por-Me),
4.71 (m, 2 H, m-Ph), 7.49 (d, 8 H, por-Ph), 7.93 (s, 8 H, por-Ph),
4.6. Stability of CoIII(ttp)C6H4(p-OMe) in benzene at 150 °C
8.85 (s, 8 H, pyrrole-H). 13C NMR (CDCl3, 100 MHz) δ 21.7,
30.8, 120.4, 121.9, 127.7, 131.7, 132.9, 133.7, 137.4, 138.9, CoIII(ttp)C6H4(p-OMe) (3e, 11.0 mg, 0.014 mmol), KOH
t
145.5; HRMS (FABMS): calcd for [C58H49CoN4]+([M]+): m/z (7.8 mg, 0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were
860.3289. Found: m/z 860.3284.
added to benzene (1.0 mL) and the mixture was degassed for
(d) Reaction with 1-bromo-4-chlorobenzene. CoII(ttp) (1a, three freeze–pump–thaw cycles in a Teflon screw capped tube
10.0 mg, 0.014 mmol), 1-bromo-4-chlorobenzene (2h, and filled with N2. The reaction was then heated at 150 °C for
263.0 mg, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and tBuOH 5 hours. The solvent was then removed under vacuum and the
(65.0 μL, 0.685 mmol) were added to benzene (1.0 mL) and the crude product was purified by column chromatography over
mixture was degassed and then heated at 150 °C for 2.5 hours alumina eluted with CH2Cl2–hexane (1 : 2) to give CoII(ttp) (1a,
to give CoIII(ttp)C6H4(p-Cl) (3h, 7.0 mg, 0.008 mmol, 61%). Rf = 1.6 mg, 0.002 mmol, 16%) and CoIII(ttp)C6H4(p-OMe) (3e,
1
0.45 (CH2Cl2–hexane = 1 : 2). H NMR (CDCl3, 400 MHz) δ 0.22 8.3 mg, 0.010 mmol, 71%).
(d, 2 H, o-Ph), 2.67 (s, 12 H, por-Me), 4.74 (d, 2 H, m-Ph), 7.50
(d, 8 H, por-Ph), 7.92 (s, 8 H, por-Ph), 8.86 (s, 8 H, pyrrole-
4.7. Exchange reaction between CoIII(ttp)Ph and 4-methoxy-1-
bromobenzene at 150 °C
CoIII(ttp)Ph (3a, 11.0 mg, 0.014 mmol), 4-bromoanisole (2e,
H).13C NMR (CDCl3, 100 MHz) δ 21.7, 122.1, 122.5, 127.7,
133.1, 133.6, 137.6, 138.6, 145.5; HRMS (FABMS): calcd for
[C54H40ClCoN4]+([M]+): m/z 838.2274. Found: m/z. 838.2258.
(e) Reaction with 1-bromo-4-fluorobenzene. CoII(ttp) (1a, 257.0 mg, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and tBuOH
10.0 mg, 0.014 mmol), 1-bromo-4-fluorobenzene (2i, 150.0 μL, (65.0 μL, 0.685 mmol) were added to benzene (1.0 mL) and the
t
1.400 mmol), KOH (7.8 mg, 0.140 mmol) and BuOH (65.0 μL, mixture was degassed for three freeze–pump–thaw cycles in a
0.685 mmol) were added to benzene (1.0 mL) and the mixture Teflon screw capped tube and filled with N2. The reaction was
was degassed and then heated at 150 °C for 3 hours to give then heated at 150 °C for 6 hours. The solvent was then
CoIII(ttp)C6H4(p-F) (3i, 8.4 mg, 0.010 mmol, 74%). Rf = 0.42 removed under vacuum and the crude product was purified by
1
(CH2Cl2–hexane = 1 : 2). H NMR (CDCl3, 400 MHz) δ 0.19 (t, column chromatography over alumina eluted with CH2Cl2–
2 H, o-Ph), 2.66 (s, 12 H, por-Me), 4.57 (t, 2 H, m-Ph), 7.50 (d, hexane (1 : 2) to give CoIII(ttp)C6H4(p-OMe) (3e, 3.2 mg,
8 H, por-Ph), 7.92 (s, 8 H, por-Ph), 8.86 (s, 8 H, pyrrole-H). 13C 0.004 mmol, 27%), CoIII(ttp)Ph (3a, 6.9 mg, 0.009 mmol, 61%)
NMR (CDCl3, 100 MHz) δ 21.7, 109.4, 109.6, 122.0, 127.8, and trace of CoII(ttp) (1a).
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4.8. Detection of anisole and 4-methoxybiphenyl from
reaction between CoII(ttp) and 4-methoxylbromobenzene in
benzene
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257.0 mg, 1.400 mmol), KOH (7.8 mg, 0.140 mmol) and tBuOH
(65.0 μL, 0.685 mmol) were added to benzene (1.0 mL) and the
mixture was degassed for three freeze–pump–thaw cycles in a
Teflon screw capped tube and filled with N2. The reaction was
then heated at 150 °C for 7 hours. The solvent was then col-
lected in a liquid-N2 cooled trap under vacuum and subjected
to GC/MS analysis using Decalin as the internal standard.
Anisole (54%) was detected. The crude product was purified by
column chromatography over alumina eluted with CH2Cl2–
hexane (1 : 2) to give CoIII(ttp)C6H4(p-OMe) (3e, 8.5 mg,
0.010 mmol, 73%) and 4-methoxylbiphenyl (4.8 mg,
0.026 mmol, 187%).
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4.9. Detection of anisole and 4-methoxybiphenyl from the
hydrolysis of CoIII(ttp)C6H4(p-OMe) in benzene
CoIII(ttp)C6H4(p-OMe) (3e, 11.0 mg, 0.014 mmol), KOH
t
(7.8 mg, 0.140 mmol) and BuOH (65.0 μL, 0.685 mmol) were
12 L. M. Alcazar-Roman and J. F. Hatwig, Organometallics,
2002, 21, 491.
added to benzene (1.0 mL) and the mixture was degassed for
three freeze–pump–thaw cycles in a Teflon screw capped tube
and filled with N2. The reaction was then heated at 150 °C for
7 hours. The solvent was then collected in a liquid-N2 cooled
trap under vacuum and subjected to GC-MS analysis using
Decalin as the internal standard. Anisole (50%) was detected.
The crude product was purified by column chromatography
over alumina eluted with CH2Cl2–hexane (1 : 2) to give CoII(ttp)
(1a, 3.3 mg, 0.004 mmol, 36%).
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Acknowledgements
We thank the Research Grants Council (no. 400308) and a
Special Equipment Grant (no. SEG/CUHK09) from the Univer-
sity Grants Committee of Hong Kong SAR, People’s Republic
of China, for financial support.
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