Palladium-catalyzed chelation-assisted c-h bond halogenation: Selective chlorination of 2-arylpyridines with acid chlorides

Article abstract of DOI:10.1055/s-0033-1340869

Palladium-catalyzed chelation-assisted C-H bond halogenation of arenes using acid chlorides as chlorinating agents is reported. This method provides a monoselective, straightforward, and clean route for the construction of aryl chlorides as privileged mot

Full text of DOI:10.1055/s-0033-1340869

1224▌
PAPER
paper
Palladium-Catalyzed Chelation-Assisted C–H Bond Halogenation: Selective
Chlorination of 2-Arylpyridines with Acid Chlorides
Selective Chlorination of 2-Arylpyridines with Acid Chlorides
Farnaz Jafarpour,* Hamideh Hazrati, Samaneh Zarei, Sara Izadidana
School of Chemistry, College of Science, University of Tehran, 14155-6455 Tehran, Iran
Fax +98(21)66495291; E-mail: jafarpur@khayam.ut.ac.ir
Received: 15.01.2014; Accepted after revisxion: 05.02.2014
2-arylpyridines could be ortho-chlorinated using palladi-
Abstract: Palladium-catalyzed chelation-assisted C–H bond halo-
um(II) acetate as the catalyst and N-chlorosuccinimide as
genation of arenes using acid chlorides as chlorinating agents is re-
the halide source and terminal oxidant.8 However, the
ported. This method provides a monoselective, straightforward, and
yields were moderate and di-ortho-halogenated products
were typically observed unless a steric bias was intro-
duced into the substrates. Elegant work from the groups of
Yu9 and Kakiuchi12 on metal-catalyzed ortho C–H chlori-
nation of 2-arylpyridines using 1,1,2,2-tetrachloroethane
and aqueous hydrogen chloride as chlorinating agents,
was also accompanied by regioselectivity issues. Dong18
also highlighted the use of arylsulfonyl chlorides as chlo-
rinating agents for the chlorination of sterically encum-
bered 2-arylpyridines. Furthermore, recently Chen and
Cheng communicated a copper-catalyzed ortho-benzox-
ylation of 2-arylpyridine C–H bonds and extend it to an
ortho-chlorination reaction with benzoyl chloride.19 De-
spite the importance of this communication, the reactions
required the use of overstoichiometric amounts of lithium
carbonate as the base and the use of 2-arylpyridines with
a blocked ortho-position or increased steric hindrance, ei-
ther on the aryl or pyridinyl ring, to circumvent overhalo-
genation.
clean route for the construction of aryl chlorides as privileged mo-
tifs in organic transformations.
Key words: aroyl chlorides, C–H activation, chlorination, palladi-
um, 2-phenylpyridine
Halogenated compounds are a crucial class of materials
because of their vital applications in many synthetic
routes. They also serve as key intermediates for the con-
struction of several biologically active molecules, natural
products, pharmaceuticals, and agrochemicals.1 Further-
more, aryl chlorides are employed as precursors of
organolithium2 and Grignard reagents,3 and they are also
among the most versatile sources of aryl motifs in transi-
tion-metal-catalyzed cross-coupling reactions.4,5 In this
context, metal-catalyzed site selective direct halogenation
of C–H bonds of arenes bearing directing groups has at-
tracted much attention in the last few years.6
The pioneering work of Fahey in 1970 demonstrated the
viability of the direct conversion of C–H bonds into C–
halogen bonds by palladium-catalyzed chlorination of
azobenzene using chlorine as an oxidant.7 However, it re-
sulted in a mixture of mono-, di-, tri-, and tetrachlorinated
adducts and the use of chlorine as a toxic halogen source
limited its wide application. Related elegant studies on
chelation-assisted C–H bond halogenations were further
developed by the groups of Sanford,8 Yu,9 Bedford,10
Shi,11 Kakiuchi,12 Fabis,13 and Xu.14 Among the various
halogenating agents investigated, N-halosuccinimides
have received widespread research attention, and it use
has been extended to a wide variety of directing groups,
including oxime ethers,8e,f,13 isoquinolines,8e pyridine
derivatives,8b,e,f,10b,15a ketones15a, amides,8e,f,10a,b,15a carba-
mates,8e,15a,b isoxazolines,8b,e,f,10a,b,13,15 esters,16 and ni-
triles.17
Thus, the need for new methodology for the selective
monohalogenation of certain C–H bonds remains an im-
portant challenge in organic synthesis. Herein, we report
the monoselective ortho-chlorination of 2-arylpyridines
under mild base and oxygen-free reaction conditions us-
ing a palladium catalytic system and readily available, in-
expensive, and versatile acid chloride agents.
Initially we examined the feasibility of the monoselective
ortho-chlorination of 2-phenylpyridine (1a) with benzoyl
chloride (2a) using reaction conditions similar to those of
Dong and co-workers:18 PdCl2(PhCN)2 (10 mol%), CuCl2
(10 mol%), and 4 Å molecular sieves in N,N-dimethylfor-
mamide at 140 °C for 24 hours (Table 1, entry 1), howev-
er, the desired adduct 3a was obtained in <5% yield.
Varying the palladium source did not improve the yield
further (entries 2–4). Next we examined the use of various
solvents and found that the use of 1,4-dioxane led to the
formation of the monochlorinated 2-phenylpyridine 3a in
71% yield (entries 4–7). Varying the oxidant showed pal-
ladium(II) chloride/copper(II) chloride to be the most ef-
fective catalytic system (entries 7–11). A control reaction
also showed the crucial role of copper(II) chloride in the
formation of chloroarenes in the palladium(II)-catalyzed
reaction of 2-phenylpyridine derivatives with aroyl chlo-
rides (entries 12). Further optimization showed that low-
ering the amount of the aroyl chloride to two equivalents
Although selective ortho-halogenation of 2-arylpyridines
is broadly useful in organic synthesis and medicinal
chemistry, there are few examples of the monoselective
chlorination of these scaffolds. A significant advance in
this area was reported by Sandford who demonstrated that
SYNTHESIS 2014, 46, 1224–1228
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DOI: 10.1055/s-0033-1340869; Art ID: SS-2014-N0545-OP
© Georg Thieme Verlag Stuttgart · New York

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Selective Chlorination of 2-Arylpyridines with Acid Chlorides
1225
substantially increased the yield of 3a (entry 13); however employed (Scheme 1). Electron-rich aroyl chlorides 2b–
lowering the reaction temperature or running the reaction d,i reacted smoothly with 1a to afford 3a in high to excel-
without palladium decreased the yield remarkably (entries lent yields. The use of 4-methoxybenzoyl chloride (2b)
15 and 16). The optimized conditions, 2-phenylpyridine was the most effective, resulting in the formation of 3a in
(1a), benzoyl (2 equiv), palladium(II) acetate and cop- almost quantitative yield. Electron-deficient aroyl chlo-
per(II) chloride in 1,4-dioxane at 140 °C for 24 hours, rides with nitro and chloro substituents 2e–h, were also
generate the chlorinated 2-phenylpyridine 3a in 92% yield tolerated under the reaction conditions and successfully
with excellent regioselectivity. To the best of our knowl- afforded 3a, albeit in lower yields. Next we examined the
edge, this transformation represents the first successful use of aliphatic acid chlorides and intriguingly found that
palladium-catalyzed monoselective ortho-C–H chlorina- highly monoselective chlorination of 2-phenylpyridine
tion of 2-arylpyridines. Furthermore, aroyl chlorides are (1a) was achieved in 80% yield using 2-chloroacetyl chlo-
less toxic and they are readily available chlorinating ride (2j) as the chlorinating agent. 2-Phenylacetyl chloride
agents that may provide a valuable starting point in the sp2 (2k) also afforded the desired adduct 3a in 38% yield, but
C–H chlorination of arenes. Although the use of benzoyl benzyl chloride (2l) gave 3a in only 22% yield.
chloride detracts from the atom economy of this process,
their cost, stability, and availability makes this an interest-
Py
Py
ing source of electrophilic chloride.
H
Cl
H
H
PdCl2 (10 mol%)
+
R
Cl
CuCl2 (10 mol%)
1,4-dioxane
4 Å MS
Table 1 Screening Reaction Conditions for the C–H Chlorination of
2-Phenylpyridine (1a)a
1a
2a–l
3a
140 °C, 24 h
Entry Catalyst
Oxidant
Solvent
Yieldb (%)
O
O
O
O
1
2
3
4
5
6
7
8
PdCl2(PhCN)2 CuCl2
DMF
DMF
DMF
DMF
DMSO
<5
<5
<5
10
10
Pd2(dba)3
Pd(acac)2
PdCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
MeO
Me
Ph
2a, 92%
2b, 96%
2c, 82%
2d, 90%
O
O
O
O
PdCl2
PdCl2
1,4-dioxane 71
MeCN 20
Cl
O2N
Cl
Cl
2h, 75%
NO2
PdCl2
2e, 55%
2f, 60%
2g, 71%
PdCl2
Cu(OAc)2·2 H2O 1,4-dioxane 30
O
9c PdCl2
ZnCl2
O2
1,4-dioxane <5
1,4-dioxane <5
1,4-dioxane 45
O
O
Cl
10
PdCl2
11c PdCl2
NCS
–
Me
2i, 80%
2j, 80%
2k, 38%
2l, 22%
12
PdCl2
1,4-dioxane
0
Scheme 1 Scope of the chlorinating agents. Reagents and condi-
tions: 2-Arylpyridine (0.1 mmol, 1.0 equiv), acid chloride 2 (2.0
equiv), PdCl2 (10 mol%), CuCl2 (10 mol%), 4 Å MS, 1,4-dioxane (0.1
M), 140 °C, 24 h.
13d PdCl2
14e PdCl2
15f PdCl2
CuCl2
CuCl2
CuCl2
CuCl2
1,4-dioxane 92
1,4-dioxane 41
1,4-dioxane 33
1,4-dioxane 18
As summarized in Table 2, ortho-C–H chlorination of 2-
phenylpyridine using aroyl chlorides was extended to a
range of 2-arylpyridines. Chlorination took place smooth-
ly with ortho-blocked 2-phenylpyridine, or substrates
with increased hindrance on either the phenyl or pyridinyl
rings (entries 2–5 and 8). 2-Aryl-5-methylpyridines also
afforded monochlorinated arenes in good yields (entries 6
and 7). With 4-methylphenyl-substituted pyridine howev-
er, a competitive di-ortho-halogenation reaction resulted
in a mixture of mono- and dichloroarylpyridines (entry 4).
The ortho-chlorination reaction also worked well with a
benzoquinoline substrate and afforded the desired product
in 65% yield (entry 9). We were delighted to see that 1-
phenylpyrazole also reacted under the optimized reaction
16
–
a Reaction conditions: 2-phenylpyridine (1a, 0.1 mmol, 1.0 equiv),
benzoyl chloride (2a, 3.0 equiv), palladium catalyst (10 mol%), oxi-
dant (10 mol%), 4 Å MS, solvent (0.1 M), 140 °C, 24 h.
b Isolated yields.
c 20 mol% of oxidant was added.
d Benzoyl chloride (2.0 equiv).
e 12 h.
f 120 °C.
We next used the optimal reaction condition to examine
the scope of the ortho-C–H chlorination using a range of
aroyl chlorides. 2-(2-Chlorophenyl)pyridine (3a) was ob-
tained regioselectively when various aroyl chlorides were
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1224–1228

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F. Jafarpour et al.
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Table 2 Scope of the Chlorination Reaction of 2-Arylpyridinesa
Entry
1
Substrate
Aroyl chloride Product
Yield (%)
2a
3a
92
N
N
N
N
Cl
2
3
4
5
6
7
2b
2a
2f
3b: R1 = 6-Me, R2 = H
3c: R1 = 5-Me, R2 = H
3d: R1 = 4-Me, R2 = H
3e: R1 = H, R2 = 3-Me
3f: R1 = H, R2 = 5-Me
3g: R1 = 4-Me, R2 = 5-Me
68
55
82b
48
68
61
R2
R2
R1
R1
2a
2a
2a
Cl
N
N
8
2b
3h
61
Cl
N
N
9
2a
2a
3i
3j
65
63
Cl
N
Cl
N
N
N
10
a Reaction conditions: 2-arylpyridine (0.1 mmol, 1.0 equiv), aroyl chloride (2.0 equiv), PdCl2 (10 mol%), CuCl2 (10 mol%), 4 Å MS, 1,4-diox-
ane (0.1 M), 140 °C, 24 h.
b A mixture of mono and dichloroarylpyridines (1:1) was obtained.
All reagents and metal catalysts were commercially available and
used as received. All reactions were carried out in an oil bath using
Microwave Vials (2–5 mL). NMR spectra were recorded at r.t. on
500, 400, and 300 MHz spectrometers using CDCl3 as the NMR sol-
vent.
conditions to give the desired monochlorinated adduct 3j
in 63% isolated yield (entry 10).
A plausible mechanism for this transformation may in-
volve the initial formation of palladium(II) species 4 that
undergoes an oxidative addition to the aroyl chloride to
form the palladium(IV) species 5. Carbon–halogen bond-
forming reductive elimination from this species affords
the chlorinated product. It is interesting that C–Cl reduc-
tive elimination occurs in preference to C–C reductive
elimination (the acylated product was not observed in the
reaction mixture) (Scheme 2).
L
(II)
Pd
L
(II)
Pd
4
In summary, the palladium-catalyzed regioselective C–H
monochlorination of 2-phenylpyridine with acid chlorides
as a safe and available halogen source under mild reaction
conditions was developed. Importantly, both aroyl chlo-
rides and aliphatic acid chlorides could be utilized in this
halogenation reaction. It is noteworthy that under the op-
timized reaction conditions, overhalogenation was pre-
cluded, which is not feasible in similar copper-catalyzed
chlorination approaches. Furthermore, the protocol could
be readily extended to the monohalogenation of 2-phenyl-
pyrazole. Broadening the scope of the reaction to the ha-
logenation of simple arenes as a crucial class of materials
is currently under investigation.
L
ArCOCl
O
L
Cl
Cl
Pd
ArCHO
ArH
CO
Ar
+
5
L
O
Ar
Scheme 2 Possible mechanistic route to chlorinated pyridines
Synthesis 2014, 46, 1224–1228
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Selective Chlorination of 2-Arylpyridines with Acid Chlorides
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2-(2-Chlorophenyl)pyridine (3a);9d Typical Procedure
13C NMR (62 MHz, CDCl3): δ = 154.1, 149.8, 139.7, 136.5, 136.2,
131.8, 131.3, 130.5, 129.6, 127.9, 125.8, 124.4.
MS (EI, 70 eV): m/z = 217 [M+].
A vial equipped with a stirrer bar was charged with 2-phenylpyri-
dine (0.2 mmol, 1.0 equiv), benzoyl chloride (2.0 equiv), PdCl2 (10
mol%), CuCl2 (10 mol%), and 4 Å MS. Then anhyd 1,4-dioxane
(0.1 M) was added, and the vial was capped. The resulting mixture
was heated in an oil bath at 140 °C for 24 h, cooled, and then filtered
through a short plug of silica. Removal of the solvent gave a crude
mixture that was purified by flash column chromatography (hex-
anes–EtOAc gradient) to give a colorless oil; yield: 17 mg (92%).
Anal. Calcd for C13H12ClN: C, 71.72; H, 5.56; N, 6.43. Found: C,
72.03; H, 5.70; N, 6.65.
2-(2-Chloronaphthalen-1-yl)pyridine (3h)
Yellow solid; mp 86–87 °C (Lit.8e 85.7–86.4 °C); yield: 15 mg
(61%).
1H NMR (400 MHz, CDCl3): δ = 8.79–8.80 (m, 1 H), 7.95–8.03 (m,
3 H), 7.77 (d, J = 8.0 Hz, 1 H), 7.48–7.58 (m, 3 H), 7.43 (d, J = 7.6
Hz, 1 H), 7.25 (d, J = 7.9 Hz, 1 H).
1H NMR (500 MHz, CDCl3): δ = 8.73 (br s, 1 H), 7.77 (t, J = 7.2
Hz, 1 H), 7.65 (d, J = 7.5 Hz, 1 H), 7.60 (d, J = 6.7 Hz, 1 H), 7.48
(d, J = 7.1 Hz, 1 H), 7.38–7.34 (m, 2 H), 7.30 (d, J = 7.6 Hz, 1 H).
Anal. Calcd for C11H8ClN: C, 69.67; H, 4.25; N, 7.39. Found: C,
69.92; H, 4.37; N, 7.53.
Anal. Calcd for C15H10ClN: C, 75.16; H, 4.21; N, 5.84. Found: C,
74.92; H, 4.09; N, 5.67.
2-(2-Chloro-6-methylphenyl)pyridine (3b)9d
Colorless oil; yield: 14 mg (68%).
1H NMR (500 MHz, CDCl3): δ = 8.68 (d, J = 4.5 Hz, 1 H), 7.70 (t,
J = 7.2 Hz, 1 H), 7.33–7.29 (m, 3 H), 7.26–7.17 (m, 2 H), 2.10 (s, 3
H, Me).
10-Chlorobenzo[h]quinoline (3i)
Yellow solid; yield: 14 mg (65%); mp 79–80 °C (Lit.9d 81–82 °C).
1H NMR (400 MHz, CDCl3): δ = 9.12 (d, J = 4.1, 1 H), 8.17 (d,
J = 7.8 Hz, 1 H), 7.83 (d, J = 8.0 Hz, 2 H), 7.77 (d, J = 8.2 Hz, 1 H),
7.69 (d, J = 8.2 Hz, 1 H), 7.54 (t, J = 7.0 Hz, 2 H).
Anal. Calcd for C12H10ClN: C, 70.77; H, 4.95; N, 6.88. Found: C,
71.01; H, 5.06; N, 7.02.
Anal. Calcd for C13H8ClN: C, 73.08; H, 3.77; N, 6.56. Found: C,
72.84; H, 3.65; N, 6.40.
2-(2-Chloro-5-methylphenyl)pyridine (3c)16
Colorless oil; yield: 11 mg (55%).
1H NMR (400 MHz, CDCl3): δ = 8.73 (d, J = 4.2 Hz, 1 H), 7.74 (t,
J = 7.3 Hz, 1 H), 7.65 (d, J = 7.5 Hz, 1 H), 7.42 (br s, 1 H), 7.35 (d,
J = 7.6 Hz, 1 H), 7.24–7.30 (m, 1 H), 7.1 (d, J = 7.8 Hz, 1 H), 2.37
(s, 3 H, Me).
1-(2-Chlorophenyl)-1H-pyrazole (3j)8e
Colorless oil; yield: 11 mg (63%).
1H NMR (400 MHz, CDCl3): δ = 7.88 (d, J = 1.8 Hz, 1 H), 7.76 (d,
J = 1.2 Hz, 1 H), 7.57–7.61 (m, 1 H), 7.51–7.54 (m, 1 H), 7.31–7.40
(m, 2 H), 6.47–6.48 (m, 1 H).
Anal. Calcd for C9H7ClN2: C, 60.52; H, 3.95; N, 15.68. Found: C,
60.78; H, 4.09; N, 15.85.
Anal. Calcd for C12H10ClN: C, 70.77; H, 4.95; N, 6.88. Found: C,
71.06; H, 5.09; N, 7.04.
2-(2-Chloro-4-methylphenyl)pyridine (3d)6c
Colorless oil; yield: 17 mg (82%).
Acknowledgment
1H NMR (400 MHz, CDCl3): δ = 8.71 (d, J = 4.3 Hz, 1 H); 7.74 (t,
J = 7.6 Hz, 1 H), 7.64 (d, J = 7.6 Hz, 1 H), 7.49 (d, J = 7.1 Hz, 1 H),
7.26–7.31 (m, 2 H), 7.16 (d, J = 7.6 Hz, 1 H), 2.38 (s, 3 H, Me).
We gratefully acknowledge the financial support of the University
of Tehran and Iran National Science Foundation (INSF).
Anal. Calcd for C12H10ClN: C, 70.77; H, 4.95; N, 6.88. Found: C,
70.98; H, 5.05; N, 7.00.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
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2-(2-Chlorophenyl)-3-methylpyridine (3e)9d
Colorless oil; yield: 10 mg (48%).
1H NMR (400 MHz, CDCl3): δ = 8.52 (d, J = 4.2 Hz, 1 H), 7.59 (d,
J = 6.4 Hz, 1 H), 7.48–7.46 (m, 1 H), 7.36–7.31 (m, 3 H), 7.24 (d,
J = 7.3 Hz, 1 H), 2.18 (s, 3 H, Me).
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2-(2-Chlorophenyl)-5-methylpyridine (3f)
Yellow oil; yield: 14 mg (68%).
IR (neat): 1693, 2853, 2923 cm–1.
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H), 7.48 (d, J = 6.3 Hz, 1 H), 7.28–7.39 (m, 2 H), 2.42 (s, 3 H).
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71.13; H, 5.11; N, 7.12.
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Yellow oil; yield: 13 mg (61%).
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(d, J = 8.0 Hz, 1 H), 7.29 (s, 1 H), 7.17 (d, J = 8.7 Hz, 1 H).
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