Electronic effects of ligands on the cobalt(II)-porphyrin-catalyzed direct C-H arylation of benzene

Article abstract of DOI:10.1002/ejic.201100780

The electronic effects of the porphyrin ligand on the cobalt(II)-porphyrin- catalyzed direct C-H arylation of benzene with 4-iodotoluene were explored. The investigation was facilitated by the easy preparation of various substituted porphyrin ligands. The reaction rates were found to be dependent on the electron richness of the cobalt(II)-porphyrins. Such effects are consistent with the electronic influences on the rate-determining aryl iodide bond cleavage. Copyright

Full text of DOI:10.1002/ejic.201100780

FULL PAPER
DOI: 10.1002/ejic.201100780
Electronic Effects of Ligands on the Cobalt(II)–Porphyrin-Catalyzed Direct
C–H Arylation of Benzene
Tek Long Chan,[a] Ching Tat To,[a] Bei-Sih Liao,[b] Shiuh-Tzung Liu,*[b] and
Kin Shing Chan*[a]
Keywords: Electronic effects / Cobalt / C–H arylation / Rate-determining step / Ligand effects / Steric hindrance
The electronic effects of the porphyrin ligand on the co-
balt(II)-porphyrin-catalyzed direct C–H arylation of benzene
with 4-iodotoluene were explored. The investigation was fa-
cilitated by the easy preparation of various substituted por-
phyrin ligands. The reaction rates were found to be depend-
ent on the electron richness of the cobalt(II)–porphyrins. Such
effects are consistent with the electronic influences on the
rate-determining aryl iodide bond cleavage.
tained, the commercially available ligands for preliminary
studies may not be a final choice for maximum optimiza-
Introduction
Utilization of ligands in homogeneous transition-metal- tion when the scope of the reaction is broadened, such as
catalyzed reactions is essential. The ligands form transition- the cross coupling of aryl chloride with C–Cl bond cleav-
metal complexes with the inorganic metal salts, providing age.[1] Moreover, a systematic evaluation of ligand elec-
solubility and stability. However, the manner in which the tronic effects on the reaction will be a useful tool for mech-
ligands modify the reactivity of ligated metal centres anistic investigations.
through electronic effects is far more important to a reac-
tion system. A demonstrative example is the successful oxi-
dative addition of Pd complexes to strong C–Cl bonds of
aryl chlorides with the use of electron-rich PtBu3 or PCy3
ligands,[1] but it fails for the more commonly available PPh3
ligand. The underlying principle involves the enhanced nu-
cleophilicity of the Pd centre in the presence of very strong
σ-donor ligands. Therefore, the Pd centre more readily at-
tacks the σ* orbital of a C–Cl bond, facilitating the oxidat-
ive addition.[2] This shows that a suitable choice of ligand
with varying electronic effects greatly facilitates the rate of
the fundamental reaction step and makes reactions more
facile.[3] The issue is particularly true when reaction path-
ways are ionic in nature or sensitive to electronic environ-
ments.
The realization of ligand control can be accomplished by
installing different substituents to tune the electronic fac-
tors, while keeping the steric environment minimally al-
tered. Indeed, Busacca et al. discovered the ligand elec-
tronic dependence of stereoselectivity in asymmetric Heck
reactions.[4] Very recently, Sanford et al. achieved the re-
gioselective α-arylation of naphthalene through the system-
atic modulation of ligand substituent electronics.[5]
The macrocyclic porphyrin ligand is attractive for its
thermal stability and easily tunable substituents at both the
meso and β positions. Many porphyrin derivatives can be
prepared from readily available aldehydes and pyrroles to
meet the electronic requirements.[6] Therefore, the use of
first-row transition-metal metalloporphyrins as catalysts,
such as cobalt(II)–porphyrins, is attractive for the ease of
ligand modulability and as a cheap metal source. Zhang
et al. have extensively studied the chemistry of cobalt(II)–
porphyrins on C–H amination,[7] cyclopropanation[8] and
aziridination.[9] The studies revealed that the electronic fac-
tor of the porphyrin ligands is critical to the reaction out-
come. Similarly, Sharghi et al. discovered that the elec-
tronics of cobalt(II)–porphyrins affect the reaction yields as
well as the reaction rates in the thiocyanation of epoxides,
because of the different formation constants for thiocyanate
complexation.[10]
Therefore, finding the best ligand for a reaction system
is crucial to optimization. Initial ligand screening is gen-
erally a random approach. Several readily available ligands
are selected on the basis of their denticities, binding atoms,
cone angles, hardness/softness or donor/acceptor proper-
ties. Although distinctive experimental results can be ob-
[a] Department of Chemistry,
The Chinese University of Hong Kong, Shatin,
New Territories, Hong Kong, People’s Republic of China
Fax: +852-26035057
Aryl–aryl bond formations from two aromatics are of
fundamental importance, and the search is always on for
more user-friendly approaches.[11] Obtaining biaryls
through direct C–H arylation is desirable.[12] Recently, we
E-mail: ksc@cuhk.edu.hk
[b] Department of Chemistry, National Taiwan University,
Taipei, Taiwan 106, Republic of China
Fax: +886-2-23636359
E-mail: stliu@ntu.edu.tw
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FULL PAPER
have reported the cobalt(II)–porphyrin-catalyzed direct C– CoII(t4-CF3pp), decreased the reaction rates significantly
H arylation of unactivated arenes with aryl halides.[13] In compared with other porphyrin ligands (Table 1, Entries 4
view of the easily tunable porphyrin ligands and thus its and 5). The poor solubility of CoII(t4-CF3pp) in the reaction
electronic effects on reaction systems, we report on the in- mixture also accounts for its lower performance. Interest-
fluences of electron-rich and -poor cobalt(II)–porphyrins ingly, the reaction-rate enhancement was not observed
on the direct C–H arylation of benzene. The axial ligand when electron-rich and sterically hindered CoII(tmp) was
effects will also be discussed.
employed (Table 1, Entry 6). This is likely attributed to the
offset of the electronic effect by bulky mesityl substituents.
Meanwhile, reduction of 4-iodotoluene occurred in all cases
as toluene was detected from the GC–MS analysis of the
reaction mixtures. Therefore, cobalt(II)–porphyrins bearing
electron-donating meso-aryl substituents increased the rate
of direct C–H arylation of benzene.
To obtain a quantitative comparison of the cobalt(II)–
porphyrin electronic effect, the reactions of 4-iodotoluene
with benzene catalyzed by various CoII(por) complexes at
200 °C were monitored by withdrawing aliquots from the
reaction mixtures for GC–MS analyses. The consumptions
of 4-iodotoluene were plotted against reaction times. The
plots fit well with a first-order exponential decay function
(Figure 2) and confirmed the first-order dependence on
4-iodotoluene. The rate law can be expressed as rate ϰ
kobs[4-iodotoluene]. The initial concentrations of benzene,
CoII(por) and 4-iodotoluene were 10.1 m, 5.07 mm and
101 mm, respectively.
Results and Discussion
With the successful discovery of the direct C–H
arylation of benzene with 4-iodotoluene catalyzed by
CoII(t4-OMepp),[13] we further examined the porphyrin elec-
tronic effect by employing CoII(tpp), CoII(ttp),
CoII(t4-Clpp), CoII(t4-CF3pp) and CoII(tmp) as the catalysts
(Figure 1). These cobalt(II)–porphyrins were easily pre-
pared by condensation of the corresponding substituted al-
dehydes with pyrrole, followed by metallation with
Co(OAc)2·4H2O.[14c] Electron-donating and -withdrawing
meso-aryl substituents were installed with reference to
CoII(tpp).
Figure 1. Structures of cobalt(II)–porphyrin complexes.
To our delight, we observed that the rate of direct C–H
arylation increased with the electron richness of the
CoII(por) catalysts (Table 1). Electron-donating porphyrin
CoII(t4–OMepp) resulted in the fastest C–H arylation in 2 h
to give 79% of 4-methylbiphenyl (Table 1, Entry 1). A
slightly longer reaction time was needed when less electron-
rich CoII(ttp) was used (Table 1, Entry 2). However, elec-
tron-poor cobalt(II)–porphyrins, such as CoII(t4-Clpp) and
Figure 2. Reaction time profile of 4-iodotoluene.
Table 1. Electronic effects of porphyrins on the direct C–H aryl-
ation of benzene with 4-iodotoluene.
The kobs values for the CoII(t4-OMepp)- CoII(ttp)- and
CoII(t4-Clpp)-catalyzed direct C–H arylation were measured
to be 2.1ϫ10–4 s–1, 2.0ϫ10–4 s–1 and 9.2ϫ10–5 s–1 at
200 °C, respectively (Table 2). Hence, the rate of consump-
tion of 4-iodotoluene in the reaction with benzene catalyzed
by CoII(por) follows the order: CoII(t4-OMepp) Ͼ CoII(ttp)
Ͼ CoII(t4-Clpp).
Entry
CoII(por)
Time [h]
Yield of 1 [%][a]
Table 2. Observed rate constants of the CoII(por)-catalyzed direct
C–H arylation at 200 °C.
1
2
3
4
5
6
CoII(t4–OMepp)
2
3.5
4
8
20
4
79
88
70
78
66
84
CoII(ttp)
CoII(tpp)
Entry
CoII(por)
kobs [s–1]
Relative rate
CoII(t4–Clpp)
CoII(t4–CF3pp)
CoII(tmp)
1
2
3
CoII(t4–OMepp)
2.1ϫ10–4
2.0ϫ10–4
9.2ϫ10–5
2.3
2.2
1.0
CoII(ttp)
CoII(t4–Clpp)
[a] GC yield.
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Cobalt(II)–Porphyrin-Catalyzed Direct C–H Arylation of Benzene
We then examined the catalyst loading effects where 2.5,
5, 10 and 20 mol-% of CoII(t4-OMepp) catalyst was em-
ployed. Reducing the catalyst loading from 5 to 2.5 mol-%
increased the reaction time without affecting the yield of 1
(Table 3, Entry 1). This is in line with the rate-determining
formal iodine atom abstraction step with CoII(t4-OMepp),
since the decreased concentration of CoII(t4-OMepp) reduces
the rate of aryl radical generation.[13] However, when the
catalyst loading was increased to 10 mol-%, the reaction
was slower compared with that of 5 mol-% (Table 3, En-
tries 2 and 3), but there was a small increase in the reaction
yield. This observation is interesting as the reaction rate did
not increase with catalyst loading or attained saturation.
Therefore, we then studied the catalytic arylation with
20 mol-% of CoII(t4-OMepp) loading. The catalysis required
an unexpectedly long reaction time of 17 h, with only 41%
of 1 formed. The low yield of 1 was accounted for by the
possible formation of CoIII(t4-OMepp)(4-tolyl) from the
trapping of the 4-tolyl radical with CoII(t4-OMepp)
(Scheme 1).
Scheme 2. Combination rate constants of some CoII complexes
with alkyl radicals. B12r = cobalamin; CoII(DMG)2 = bis(dimethyl-
glyoximato)cobalt(II).
In addition to the enhanced effect of the electron-donat-
ing porphyrin ligand on the rate of direct C–H arylation
of benzene with 4-iodotoluene, the axial ligand effect was
investigated. It has been reported that in the presence of σ-
donor ligands, the energy level of the singly occupied dz2
orbital of CoII(por) rises, enriching the metalloradical char-
acter.[16] Pyridine and PPh3 were thus added to form the
more reactive LCoII(por) (L = pyridine or PPh3) in situ,
hoping to facilitate the iodine atom abstraction step. How-
ever, both pyridine and PPh3 were inefficient as longer reac-
tion times were required (Table 4, Entries 2 and 5). The for-
mation constant of the 1:1 adduct (py)CoII(t4-OMepp) at
200 °C was estimated to be 49.1 m–1,[17] indicating that
about 87% of the added CoII(t4-OMepp) exists as (py)-
CoII(t4-OMepp) in solution. The reduction of vacant sites on
CoII(t4-OMepp) from two to one could account for the lower
reactivity but is less reasonable. A rate enhancement should
still have occurred. We favour the existence of a dispropor-
tionation equilibrium of axially ligated CoII(t4-OMepp) in
the presence of L (L = OH–, pyridine or PPh3) to give
CoI(t4-OMepp)– and LnCoIII(t4-OMepp)+ (n = 1,2) at high
temperatures (Scheme 3).[2b,18] This side reaction decreases
the effective concentration of CoII(t4-OMepp) and, hence, the
rate of iodine atom abstraction. Thus, the electronic effect
of the porphyrin on the rate is unique.
Table 3. Catalyst loading effects on the direct C–H arylation of
benzene with 4-iodotoluene.
Entry Catalyst loading [mol-%]
Time [h]
Yield of 1 [%][a]
1
2
3
4
2.5
5
10
20
4
2
4
77
79
81
41
17
[a] GC yield.
Table 4. σ-Donor ligand effects on direct C–H arylation of benzene
with 4-iodotoluene.
Scheme 1.
Formation
of
CoIII(t4-OMepp)(4-tolyl)
from
CoII(t4-OMepp) and the 4-tolyl radical.
A higher catalyst loading should have resulted in faster
4-tolyl radical generation, a more rapid homolytic substitu-
tion of benzene and a faster catalytic direct C–H aryl-
ation.[13] However, instead of only being a catalyst for io-
dine atom abstraction, CoII(t4-OMepp) can also act as an
aryl radical trap in the reaction mixture. Relatively high
concentrations of 4-tolyl radical and CoII(t4-OMepp) com-
bine to give CoIII(t4-OMepp)(4-tolyl) (Scheme 1) since the
combination rate constants of various CoII species with
alkyl radicals are fast and some are close to diffusion-con-
trolled.[15] Some examples of combination rate constants of
CoII complexes with alkyl radicals are illustrated in
Scheme 2.[15] This process competes with the homolytic
substitution for direct C–H arylation. Therefore, the rate of
the 4-tolyl radical attack on benzene is reduced, leading to
even slower reaction rates.
Entry
Additive Loading [mol-%][a] Time [h] Yield of 1 [%][b]
1
2
3
4
5
none
pyridine
PPh3
–
5
5
2
6
8
8
8
79
62
78
58
60
PPh3
PPh3
0.5
0.25
[a] With reference to 4-iodotoluene. [b] GC yield.
Scheme 3. Disproportionation of CoII(t4-OMepp) promoted by axial
ligands.
Eur. J. Inorg. Chem. 2012, 485–489
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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487

T. L. Chan, C. T. To, B.-S. Liao, S.-T. Liu, K. S. Chan
FULL PAPER
Scheme 4. Formal iodine atom abstraction as the rate-determining step.
2 H) ppm. M.p. 42.7–44.2 °C.[13] The same procedures were em-
ployed for the other CoII(por) complexes.
General Procedure for the CoII(t4-OMepp)-Catalyzed Direct C–H
Arylation of Benzene with 4-Iodotoluene with Various Catalyst
Loadings: CoII(t4-OMepp) (0.0055 mmol, 0.022 mmol and
0.044 mmol) was used in the same procedure as described above.
From the above findings and the reaction mechanism
proposed previously,[13] we conclude that the formal iodine
atom abstraction is rate-determining (Scheme 4). The sec-
ond-order rate constant for the formal iodine atom abstrac-
3–
tion of 2-iodopyridine with Co(CN)5 at 25 °C is ca.
10–3 –1 s–1.[19] The nucleophilic radical nature of CoII(por)
m
General Procedure for the CoII(t4-OMepp)-Catalyzed Direct C–H
Arylation of Benzene with 4-Iodotoluene with Various Additives: Pyr-
idine (0.011 mmol) and PPh3 were added. For 0.5 and 0.25 mol-%
PPh3, 0.56 mm and 0.28 mm standard solutions of PPh3 (2 mL) in
benzene were used as solvent, respectively.
is enhanced by electron-donating porphyrin ligands to facil-
itate the Ar–I bond cleavage.
Conclusions
The electronic effects of porphyrin ligands on the Co-
II(por)-catalyzed direct C–H arylation are presented. The
systematic modulation of ligand substituent electronics of-
fers improved reactivity of economical first-row transition-
metal catalysts. This provides another approach for reaction
optimizations and mechanistic studies. Application of this
concept to expand the scope of the ligand and substrate is
ongoing.
Acknowledgments
We thank the Research Grants Council of Hong Kong (No.
400308), a Special Equipment Grant (SEG/ CUHK09) of the
People’s Republic of China, and the National Science Council
(NSC-100-2113-M002-001-MY3) for financial support.
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Experimental Section
General: All reagents were purchased from commercial suppliers
and used without further purification. Hexane for chromatography
was distilled from anhydrous calcium chloride. Thin layer
chromatography was performed on precoated silica gel 60 F254
plates. Silica gel (Merck, 230–400 mesh) was used for column
chromatography in air. Melting points were measured with a Reich-
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General Procedure for the Various CoII(por)-Catalyzed Direct C–H
Arylations of Benzene with 4-Iodotoluene: CoII(t4-OMepp) (8.9 mg,
0.011 mmol), 4-iodotoluene (48.9 mg, 0.224 mmol), KOH (126 mg,
2.24 mmol), tBuOH (213 μL, 2.24 mmol) were added in benzene
(2.0 mL, 22.4 mmol). The mixture was degassed during three
freeze-pump-thaw cycles, flushed with N2 and heated at 200 °C.
After a GC–MS analysis of the reaction mixture to confirm com-
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rotary evaporation. The crude residue was purified by column
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afford 4-methylbiphenyl (1). 1H NMR (400 MHz, CDCl3): δ = 2.39
(s, 3 H), 7.24 (d, J = 7.9 Hz, 2 H), 7.31 (t, J = 7.3 Hz, 1 H), 7.42
(t, J = 7.6 Hz, 2 H), 7.49 (d, J = 8.0 Hz, 2 H), 7.57 (d, J = 7.4 Hz,
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Published Online: November 10, 2011
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