PhI(OCOCF3)2-mediated ruthenium catalyzed highly site-selective direct ortho-C-H monoarylation of 2-phenylpyridine and 1-phenyl-1H-pyrazole and their derivatives by arylboronic acids

Article abstract of DOI:10.1039/c5ra18402a

We report a concise, versatile, and practical method for PhI(OCOCF₃)₂ mediated direct ortho C-H monoarylation of 2-phenylpyridine and its derivatives and 1-phenyl-1H-pyrazoles via Ru catalyzed reaction. The significant advantage of t

Full text of DOI:10.1039/c5ra18402a

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PhI(OCOCF3)2-mediated ruthenium catalyzed
highly site-selective direct ortho-C–H
monoarylation of 2-phenylpyridine and 1-phenyl-
1H-pyrazole and their derivatives by arylboronic
acids†
Cite this: RSC Adv., 2015, 5, 105347
Ganapam Manohar Reddy, Naidu Samba Siva Rao, Ponnam Satyanarayana
*
and H. Maheswaran
Received 9th September 2015
Accepted 30th November 2015
We report a concise, versatile, and practical method for PhI(OCOCF3)2 mediated direct ortho C–H
monoarylation of 2-phenylpyridine and its derivatives and 1-phenyl-1H-pyrazoles via Ru catalyzed
reaction. The significant advantage of this transformation is the creation of highly site selective C–C
bond by using arylboronic acids as arylating agent under mild reaction conditions.
DOI: 10.1039/c5ra18402a
www.rsc.org/advances
Aromatic C–H bonds are ubiquitous, an attribute that makes derivatives.10,11 In the former case, one of the two ortho-C–H
them attractive for the construction of complex molecules via bonds in the phenyl ring of 2-phenylpyridine is blocked by alkyl
direct C–H activation, and functionalization under transition- substitution which prevents the formation of non-selective
metal catalyzed conditions. However, this same characteristic biarylated products. In the latter, site-selective monoarylations
is also a great obstacle to developing practical methods for bearing meta-alkyl-substituents are controlled by steric inter-
highly site-selective C–H bond functionalization reactions. This actions. Unfortunately, similar reactions still remain chal-
is because to achieve useful yields of a single product, the lenging for 2-phenylpyridine and its derivatives that contain two
aromatic C–H functionalization must occur with a high degree ortho aromatic C–H bonds that can be directly monoarylated
of site-selectivity for one C–H bond over the others within with preference of one over another with high degree of site-
a molecule. In recent years, however, the advances in the selectivity.12,13 However, serendipitously, we have discovered
development of homogeneous catalysis using various Pd, Ru, that the [Ru(p-cymene)Cl2]2 catalyst system in the presence of
and Rh complexes as catalysts has set the stage for such chal- PhI(OCOCF3)2 (a hyper-valent iodine oxidant)12 in catalytic
lenging C–H bond functionalization.1–7 The key to this devel- amounts, without requiring any external base or inorganic
opment is a good understanding of C–H bond cyclometalation additives, directly enables highly site-selective direct mono-
steps that occur with transition-metals via chelation effect of the arylation of 2-phenylpyridine with arylboronic acids.9f This new
substrate that contains at least one chelating atom in proximity nding and the scope of this protocol for a range of substrate
to the C–H bond that needs to be functionalized selectively.1–7,8a combinations for site-selective direct ortho C–H monoarylation
Substituted pyridines such as 2-phenylpyridine and its in 2-phenylpyridine and its derivatives forms the focus of this
derivatives not only constitute important bioactive compounds paper.
but also form excellent substrates for cyclometalation with
We began our preliminary investigation by studying the
transition-metal catalysts.8 Therefore, they are ideal substrates direct aromatic ortho C–H monoarylation of 2-phenylpyridine
for facile site-selective ortho aromatic C–H bonds functionali- using three different Ru catalysts with phenyl-boronic acid as
zation for important C–C cross-coupling reactions such as arylating agent with various additives in different solvents at
direct C–H monoarylation. Such site-selective reactions have 100 ꢀC in a sealed tube. The screening of the reaction conditions
been reported for substituted pyridines using Rh, Ru, and Pd are summarized in Table 1. As can be seen, a variety of
catalysis.7,9 However, the available reports are oen pertinent to parameters such as nature of Ru catalyst, additives, and solvent
either ortho-alkyl or meta-alkyl substituted 2-phenylpyridine play crucial role not only on the efficacy but also on the effi-
ciency of the reaction. Notably, in the absence of Ru catalyst, as
well as when additives alone were evaluated as catalyst, the
direct C–H arylation reaction did not proceed at all for the
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology,
Hyderabad, India. E-mail: maheswaran_dr@yahoo.com; Fax: +914027193510; Tel:
model reaction. Among three Ru catalysts, namely, RuCl3,
+914027191532
Ru(PPh)3Cl, and [Ru(p-cymene)Cl2]2 evaluated for the model
† Electronic supplementary information (ESI) available: Experimental procedure,
and NMR data available.
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Table 1 Optimization of ruthenium catalysed arylation of 2-phenyl Table 2 Direct ortho C–H monoarylation reactions of diverse aryl-
pyridine with phenyl-boronic acid and various additivesab
boronic acids with 2-phenylpyridine and its derivatives
Entry
Substrate
Product
Code
Yield (%)
1
1a
85
2
2a
89
87
92
90
65
72
78
83
85
75
89
Entry Catalyst
Additive
Solvent
Yield (%)
1
2
3
[Ru(p-cymene)Cl2]2 Cu(OAC)2
[Ru(p-cymene)Cl2]2 AgSbF6
[Ru(p-cymene)Cl2]2 Cu2O
Toluene Trace
Toluene Trace
Toluene Trace
Toluene 25
Toluene 20
Toluene 10
Toluene 85
Toluene Trace
CH3CN
Dioxane 20
THF 40
3
3a
4
5
6
[Ru(p-cymene)Cl2]2 PhCOOH
[Ru(p-cymene)Cl2]2 PhI(OCOCH3)2
[Ru(p-cymene)Cl2]2 Ag2O
7
[Ru(p-cymene)Cl2]2 PhI(OCOCF3)2
8
RuCl3
PhI(OCOCF3)2
4
4a
9
[Ru(p-cymene)Cl2]2 PhI(OCOCF3)2
[Ru(p-cymene)Cl2]2 PhI(OCOCF3)2
[Ru(p-cymene)Cl2]2 PhI(OCOCF3)2
10
10
11
12
13
14
15
16
[Ru(p-cymene)Cl2]2 KOtBu + PhI(OTf)2 Toluene Tracec
[Ru(p-cymene)Cl2]2 PhI(OCOCF3)2
Ru(PPh3)2Cl2
[Ru(p-cymene)Cl2]2
Toluene 20d
Toluene Trace
Toluene Trace
Toluene <10%
5
5a
PhI(OCOCF3)2
—
PhI(OCOCF3)2
—
6
6a
a
Optimized reaction conditions: 1.0 mmol 2-phenylpyridine, 1.2 mmol
phenyl-boronic acid, 2.5 mol% Ru-catalyst, 20.0 mol% additive, and 2
b
c
mL of solvent for 3 h. Isolated yields. Reaction with 20 mol% each
of KOtBu and PhI(OTf)2. Biarylated product formed when reaction
d
7
7a
was performed with 2.4 mmol of phenyl-boronic acid.
8
8a
reaction, only the latter catalyst gave promising yields for
desired C–H monoarylated product with additives such as
benzoic acid (25%), bis(acetoxy)iodobenzene (20%) and silver
oxide (10%) in toluene. A dramatic increase in the yield of
desired monoarylated product up to 85% was obtained when
bis(tri-uoroacetoxy)iodobenzene oxidant was used as additive
with Ru(p-cymene)Cl2 catalyst in toluene. This result is indeed
very surprising because the use of structurally and functionally
similar bis(acetoxy)iodobenzene oxidant gave only 20% yield for
desired product. It should be noted that 1 : 1.2 loading of
reactants are desirable because at higher loadings (1 : 2.4) the
reaction gave non-selective biarylated product in low yields
(20%) (Table 1; entry 13). Finally, it should be noted that in all
the reactions screened using stoichiometric amounts of
reagents, we have obtained only site-selective monoarylated
products.
Using optimized reaction conditions in hand, the scope of
the reaction was evaluated for various substrates, and the
results from this study are presented in Table 2. As was the case
with phenyl-boronic acid (1a), we have obtained excellent yields
as high as high as 89% for monoarylated product (2a) in 2-
phenylpyridine when 2-naphthylboronic acid was used as ary-
lating agent. Similarly, arylboronic acids containing electron-
donating groups in the phenyl ring such as p-methyl (3a), p-
9
9a
10
11
12
10a
11a
12a
13
13a
91
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Table 2 (Contd.)
with 2-phenylpyridine (1a–6a), we did not observe formation of
non-selective biarylated products.
Entry
Substrate
Product
Code
Yield (%)
76
As discussed earlier, Pfeffer et al. reported C–H bond elec-
trophilic activation between [Ru(benzene)Cl2]2 and 2-phenyl-
pyridine.7a Recently, several groups reported C–H activation by
cooperative action of Ru(II) catalyst and base (concerted metal-
ation deprotonation pathway).14 Notably, only very poor yield
was observed in our transformation in the presence of base
(Table 1, entry 12). We have also obtained very low yield of the
desired product when the reaction was performed using
only PhI(OOCCF3)2 additive in the absence of Ru(II) (Table 1;
entry 16).
14
14a
15
15a
76
We envisioned expanding the scope of this reaction to
include other useful nitrogen-containing directing groups. As
shown in Table 3, 1-phenyl-1H-pyrazole (4a) and a variety of aryl
methoxy (4a), methylenedioxy (5a) also gave excellent yields for
desired monoarylated products (87–91%) with 2-phenyl-
pyridine. Interestingly, the aryl-boronic acids that contain
electron-withdrawing groups such as m-NO2, (6a) and p-OH (7a)
in the aromatic ring gave desired products only in moderate
yields, such as 65% and 72%, respectively.
Table 3 Direct ortho C–H monoarylation reactions of different aryl-
boronic-acids with 1-phenyl-1H-pyrazole and its derivatives
Entry
Substrate
Product
Code
Yield (%)
We also have evaluated the feasibility of similar site-selective
monoarylation reactions involving meta-alkyl substituted 2-
phenylpyridine derivatives using our optimized reaction
conditions. These studies are needed because we would like to
know whether steric effects would also work favourable under
our optimized reaction conditions. As a rst step, we have
reacted 2-([1,10-biphenyl]-2-yl)-4-methylpyridine with phenyl-
boronic acid, and this reaction gave desired monoarylated
product (8a) in 78% yield. When the same pyridine substrate
was reacted with 2-naphthylboronic acid, about 83% yields for
the desired product (9a) was obtained. When 1-naphthylboronic
acid was used, we have obtained site selectivity as high as 85%
for monoarylated product (10a). Interestingly, when 3,4-meth-
ylenedioxyboronic acid is used as arylating agent, the ortho
monoarylated product (11a) was obtained only in 75% yield. In
contrast, when p-methoxyphenyl-boronic acid was reacted with
2-([1,10-biphenyl]-2-yl)-4-methylpyridine, a much higher yield
(89%) for (12a) was obtained. This result is consistent with our
earlier observation that electron-donating p-methoxy functional
bearing phenyl-boronic acid gave highest site-selectivity (92%)
for mono-arylated product (4a) in reaction with 2-phenyl-
pyridine. To better understand the role of p-OMe functional
group, we synthesized 2-(-4-methoxyphenyl)-pyridine wherein p-
OMe group is located in the phenyl ring of the 2-phenylpyridine
instead of at the phenyl ring of boronic acid. This pyridyl
substrate when reacted with 3,4-methylenedioxyboronic acid
gave excellent yield (91%) for desired product (13a). Notably,
our optimized monoarylation protocol are also applicable to p-
methyl substituted 2-naphthylpyridine substrate. For example,
the reaction of 4-methyl-2-(naphthalene-2-yl)pyridine with
phenyl-boronic acid, we have obtained monoarylated product in
76% yield (14a). We have also performed the same reaction
using p-methoxyphenyl-boronic acid so as to obtain desired
product in 76% yield (15a). As was the case with direct arylation
1
1a
78
2
3
4
2a
3a
4a
73
85
64
5
6
5a
6a
71
75
7
7a
68
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boronic acids were subjected to the optimized reaction condi- substrate combinations. On the basis of these results, we
tions, leading to the corresponding monoarylated products in reasoned that both concerted metalation deprotonation
good yields (64–85%). As was the case with direct arylation with pathway as well as initial electrophilic C–H bond cleavage and
2-phenylpyridine (1a–6a) and its derivatives, we did not observe subsequent trans-metalation with boronic acid were not deter-
formation of non-selective biarylated products with 1-phenyl- minant factor in the rate determining step of catalytic cycle even
1H-pyrazole (4a) substrate. However, our optimized reaction though cyclo-metalated species could be involved in reaction
conditions when applied to 9-(pyridin-2-yl)-9H-carbazole mechanism.
substrate, as enumerated in Table 4, the monoarylation reac-
tion did not proceed at all. It is unclear at this time why the
negative result we have obtained with different substituted
boronic acid with 9-(pyridin-2-yl)-9H-carbazole under our opti-
mized reaction conditions (Table 4).
(1)
As discussed earlier, Wan and Pfeffer et al. reported C–H
bond electrophilic activation between [Ru(benzene)Cl2]2 and 2-
phenylpyridine with kH/kD values of 1.5.7a,9f Recently, several
groups reported C–H activation by cooperative action of Ru(II)
catalyst and base (concerted metalation deprotonation
(2)
pathway).14 Notably, only very poor yield was observed in our
transformation in the presence of base (Table 1, entry 12). We
have also obtained very low yield of the desired product when
the reaction was performed using only PhI(OOCCF3)2 additive
As an attempt to rationalize high site-selectivity observed for
in the absence of Ru(II) (Table 1; entry16). On addition of 2
monoarylation, a plausible mechanism for the Ru(II)-catalyzed
equiv. of D2O to the reaction system containing 2-phenyl-
direct arylation of 2-phenylpyridine through aromatic C–H
activation is presented in Scheme 1. First, the reaction of 2-
phenylpyridine with bis(triuoroacetoxy)iodobenzene affords
formation of charge–transfer complexes. Such charge–transfer
complexes are well known in the literature between arenes and
PhI(OOCCF3)2 reagent.15 This species, then reacts with Ru(II)
pyridine in the absence of boronic acid, deuterium was not
incorporated into the starting materials (eqn (1)). In the pres-
ence of boronic acid, however, the reaction gave, a large amount
of non D-incorporated monoarylated product (eqn. 2), this
demonstrates that preferential formations of non deuterated
mono-substituted products were observed even when excess
precursor and in a single electron transfer (SET) pathway to
D2O is used for the reaction (eqn. 2). Notably, these results are
form cationic radical species labelled A. This species in turn
not surprising because, as described earlier, we have obtained
undergo cyclo-metalation reaction to form species labelled B
highly site-selective formation of monoarylated products for as
reagent.15 This species, then reacts with Ru(II) precursor and in
high as seven important substrates with arylboronic acids for 23
a single electron transfer (SET) pathway to form cationic radical
species labelled A. This species in turn undergo cyclo-
metalation reaction to form species labelled B. This species in
turn reacts with arylboronic acid to form corresponding mon-
oarylated Ru(II) intermediate labelled C. Then, the reductive
Table 4 Direct ortho C–H monoarylation reactions arylboronic-acids
with 9-(pyridin-2-yl)-9H-carbazole
Entry
Substrate
Product
Code
Yield (%) elimination of this species produce desired monoarylated
1
1a
<10
2
3
2a
3a
<10
<10
Scheme 1 Plausible mechanism for Ru catalyzed direct ortho C–H
monoarylation of 2-phenylpyridine with arylboronic acid.
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product and Ru(0) species. Then the catalytic cycle continues
with reactions with PhI(OOCCF3)2 and Clꢁ ion to regenerate
active Ru(II) species.
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In summary, we developed a novel direct C–H arylation
protocol using Ru-catalyzed oxidative coupling of arylboronic
acids with arenes bearing 2-pyridyl and 2-pyrazole functionality.
The outstanding site-selectivity of the ruthenium catalyst was
reected by the fact that lack of formation of undesired biary-
lated product originating from high nucleophilic reactivity of
arylboronic acids. This operationally simple C–H functionali-
zation was accomplished, in particular, through the use of
hyper-valent based iodine based PhI(OCOCF3)2 reagent as
oxidant in catalytic amounts under fairly mild conditions. The
proposed charge transfer complex formation step in the
proposed mechanism may partly explain observed high site-
selectivity for monoarylated products for as many as 23
substrate combinations.
Acknowledgements
GMR and NSR thank the council of scientic and industrial
research (CSIR), and UGC, New Delhi, India for fellowship and
nancial support.
9 For selective mono-arylation using ortho- and meta-
substituted
2-phenylpyridine
derivatives
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2-phenylpyridine with arylsiloxanes; see: (a) W. Li, Z. Yin,
X. Jiang and P. Sun, J. Org. Chem., 2011, 76, 8543; (b) also
see ref. no. 9k for Rh based catalytic system for the
monoarylation of 2-phenylpyridine with arylsiloxanes; (c)
For Rh catalysed both selective and non selective ortho
C–H arylation using arylstannanes with 2-phenylpyridine
105352 | RSC Adv., 2015, 5, 105347–105352
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