Palladium-catalyzed direct arylations of 1,2,3-triazoles with aryl chlorides using conventional heating

Article abstract of DOI:10.1002/adsc.200800016

Generally applicable, palladium-catalyzed direct arylations of 1,2,3-triazoles with aryl chlorides were accomplished through conventional heating at reaction temperatures of 105-120°C. Thereby, intra- and intermolecular C-H bond functionalizations were achieved with a variety of differently substituted chlorides as electrophiles, bearing numerous valuable functional groups.

Full text of DOI:10.1002/adsc.200800016

FULL PAPERS
DOI: 10.1002/adsc.200800016
Palladium-Catalyzed Direct Arylations of 1,2,3-Triazoles with
Aryl Chlorides using Conventional Heating
Lutz Ackermann,a,* RubØn Vicente,a and Robert Borna
a
Institut für Organische und Biomolekulare Chemie, Georg-August-Universitaet, Tammannstrasse 2, 37077 Goettingen,
Germany
Fax : (+49)-551-39-6777; e-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Received: January 10, 2008; Published online: March 12, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Generally applicable, palladium-catalyzed ed chlorides as electrophiles, bearing numerous val-
direct arylations of 1,2,3-triazoles with aryl chlorides uable functional groups.
were accomplished through conventional heating at
reaction temperatures of 105–1208C. Thereby, intra-
and intermolecular C H bond functionalizations Keywords: arylation; C C coupling; C H activation;
were achieved with a variety of differently substitut- palladium; triazoles
À
À
À
Introduction
benign nature. Among the known direct arylations of
electron-rich heteroarenes, the majority has been ac-
1,2,3-Triazoles are important substructures of various complished under rhodium or palladium catalyses, ap-
compounds with biological activities of relevance to plying aryl iodides, bromides, or triflates as electro-
medicinal chemistry. Thus, various strategies for their philes.[15–17] Thus, Gevorgyan and co-workers commu-
synthesis have been devised, with Huisgenꢀs 1,3-dipo- nicated recently on direct arylations of 1,2,3-triazoles
lar [3+2]cycloadditions of azides and alkynes[1] being with aryl bromides.[18] However, aryl chlorides are ar-
the most commonly used approach.[2] The efficient guably the most useful single class of halides due to
rate enhancement and excellent regioselectivity ach- their lower cost and the wider diversity of (commer-
ieved with either copper[3,4] or ruthenium catalysts,[5] cially) available compounds.[19,20] As part of our pro-
allowed more recently for their widespread applica- gram on the use of more easily accessible and less ex-
tions in diverse research areas, ranging from bioor- pensive electrophiles, such as chlorides[21–23] and tosy-
ganic chemistry to material sciences.[6] While these lates,[24] in direct arylation reactions,[25,26] we became
catalytic [3+2]cycloadditions proceeded efficiently interested in probing palladium-catalyzed direct aryla-
and highly regioselectively with terminal alkynes, the tions of 1,2,3-triazoles with aryl chlorides.[27] A very
corresponding transformations of internal alkynes to recent report by Oshima and co-workers on a non-
fully substituted 1,2,3-triazoles were found to be thermal microwave effect on palladium-catalyzed
either not generally applicable or gave rise to mix- direct arylations of 1,2,3-triazoles with aryl chlor-
tures of regioisomers.[5,7–9] Alternative methodologies ides,[28] prompted us to disclose our own results on the
for their syntheses, such as metalations of disubstitut- direct arylations of 1,2,3-triazoles with aryl chlorides
ed triazoles, or cycloadditions with either deprotonat- through conventional heating.[27] Importantly, these
À
ed C H acidic compounds or magnesiated terminal studies highlight that microwave heating and a reac-
alkynes,[2] often rely on stoichiometric amounts of tion temperature of 2508C are not mandatory for effi-
À
highly reactive organometallic reagents, and therefore cient catalysis to proceed. Instead, C H bond func-
show significant limitations in scope.
tionalizations of 1,2,3-triazoles with aryl chlorides can
in be conveniently achieved through conventional heat-
[10–14]
À
The direct functionalization of C H bonds
1,2,3-triazoles could constitute an attractive alterna- ing at significantly milder reaction temperatures of
tive to traditional cross-couplings with organometallic 105–1208C.
reagents because of its ecologically and economically
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Results and Discussion
At the outset of our studies, we optimized reaction
conditions for the challenging palladium-catalyzed
direct arylation of triazole 1a with the electron-rich,
thus for an oxidative addition electronically-deactivat-
ed, aryl chloride 2a employing conventional thermal
heating. Interestingly, our studies showed that polar
aprotic solvents, such as NMP, DMF, or DMA, were
Figure 1. General structure of monophosphine biaryl ligands
11.
found to provide less satisfactory results, while 1,4-di- (entry 1). Aryl-substituted tertiary monophosphine li-
oxane and toluene as solvents along with K2CO3 as gands 4–6 gave also no conversion of the starting ma-
base allowed for efficient catalysis. Subsequently, we terials using conventional heating (entries 2–4). Simi-
probed the effect of various phosphine and carbene li- larly, commonly used bidentate phosphine ligands 7
gands in the palladium-catalyzed direct arylation, em- and 8 (entries 5, and 6) or N-heterocyclic carbene pre-
ploying economical Pd
A
and the challenging chloride 2a as electrophile ineffective.
(Table 1). Notably, a thermal reaction did not occur
However, more electron-rich, alkyl-substituted ter-
under phosphine ligand-free reaction conditions tiary phosphines enabled palladium-catalyzed direct
arylations with the electron-rich chloride 2a (en-
tries 8–12). While phosphonium salt 10 (entry 8) and
monophosphine biaryl ligands 11a–c (entries 9–11,
direct arylation of triazole 1a with electron-rich chloride
Table 1. Optimization studies for the palladium-catalyzed
2a.[a]
and Figure 1) gave rather unsatisfactory yields, com-
mercially available phosphine 12 proved superior in
the thermal palladium-catalyzed direct arylation of
triazole 1a (entries 12, and 13). This highly active cat-
alytic system enabled even an efficient direct aryla-
tion to occur at a reaction temperature of 1058C (en-
tries 14 and 15).
With this optimized catalytic system in hand, we
probed its scope in the direct arylation of N-alkyl-sub-
stituted 1,2,3-triazoles 1 with various aryl chlorides 2
(Table 2). 1,2,3-Triazoles with substituents on either
aromatic group of 1a were efficiently converted, and
the corresponding products 3ab and 3ac could be iso-
lated in high yields (entries 1, and 2). Simple N-(n-
alkyl)-substituted triazoles were efficiently arylated as
well with conventional heating (entry 3). However,
1,2,3-triazoles with two n-alkyl-substituents reacted
rather sluggishly under our reaction conditions. Here,
a significant rate-acceleration was accomplished with
catalytic amounts of pivalic acid[29–31] as additive (en-
tries 4 and 5). With respect to the electrophile, elec-
tron-rich aryl chlorides could generally be employed
(entries 1–5), even when being more sterically hin-
dered through ortho-substitution (entry 6). Important-
ly, a variety of valuable functional groups was tolerat-
ed by the catalytic system. Thus, aryl chlorides dis-
playing an ester substituent were efficiently converted
(entries 7 and 8). Remarkable, a chloride with an eno-
lizable ketone gave rise chemoselectively to the de-
sired product 3aj in high yields of isolated product
(entries 9–11). Here, as in the palladium-catalyzed
direct arylation with chloride 2f bearing a cyano
group (entries 12 and 13), substoichiometric amounts
of pivalic acid proved to generate a more effective
catalyst. A comparable isolated yield was obtained in
1,4-dioxane as solvent at 1058C (entry 10). Finally, we
Entry
Ligand
Yield
1
2
3
4
5
6
-
-
-
-
-
-
-
PPh3 (4)
P
ACHTREUNG
P
AHCTREUNG
BINAP (7)
dppf (8)
7
-
8
9
A
10%
10%
47%
48%
11a
11b
11c
PCy3 (12)
12
12
10
11
12
13
14
15
53%
96%[b]
92%[c]
84%[d]
12
[a]
Reaction conditions: 1a (1.0 mmol), 2a (1.5 mmol),
Pd(OAc)2 (4.0 mol%), ligand (8.0 mol%), K2CO3
A
ACHTREUNG
(2.0 mmol), PhMe (2 mL), 1208C, 7 h. GC yields using n-
heptadecane as internal standard.
20 h at 1208C.
1,4-Dioxane (2 mL), 24 h at 1058C; isolated yield.
22 h at 1058C.
[b]
[c]
[d]
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Palladium-Catalyzed Direct Arylations of 1,2,3-Triazoles with Aryl Chlorides
FULL PAPERS
Table 2. Palladium-catalyzed direct arylations of N-alkyl-substituted triazoles 1 with aryl chlorides 2.[a]
Entry
1
Alk
Bn
R
Ar
Product
Isolated Yield
4-MeOC6H4
1b
1c
1d
1e
4-MeC6H4
2b
2b
2b
2b
3ab
3ac
3ad
3ae
74%
80%
70%
66%[b]
2
3
4
PMB
Oct
Bn
Ph
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
Pent
5
6
7
Oct
Bn
Bn
Hex
Ph
1f
1a
1a
4-MeC6H4
2b
2c
2d
3af
3ag
3ah
68%[b]
95%
2-MeOC6H4
4-EtO2CC6H4
Ph
70%
8
Oct
Bn
Ph
Ph
1d
1a
4-EtO2CC6H4
2d
2e
3ai
63%
9
10
11
(37%)[c]
63%[d]
67%[b]
4-Me(O)CC6H4
3aj
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Lutz Ackermann et al.
Isolated Yield
FULL PAPERS
Table 2. (Continued)
Entry
Alk
R
Ar
Product
12
13
C
Bn
Ph
1a
4-NCC6H4
2f
3ak
14
Bn
Bn
Ph
Ph
1a
1a
3-py
2-py
2g
2h
3al
58%
82%
15
3am
[a]
Reaction conditions: 1 (0.50–1.00 mmol), 2 (0.75–1.50 mmol), Pd(OAc)2 (4.0 mol%), 12 (8.0 mol%), K2CO3 (1.00–
A
2.00 mmol), PhMe (2 mL), 1208C, 18–24 h; PMB=4-MeOC6H4CH2.
t-BuCO2H (30 mol%).
GC conversion.
t-BuCO2H (30 mol%), 1,4-dioxane (2 mL), 1058C, 18 h.
[b]
[c]
[d]
found that heteroaromatic chlorides could be em-
However, the chloride was found to be a signifi-
ployed in these thermal direct arylation reactions as cantly better leaving group when compared with a to-
well. Thus, 3- and even 2-chloropyridine (2g, and 2h) sylate, as illustrated by the highly selective coupling
gave rise to triazoles 3al and 3am, respectively, in of electrophile 2p to afford triazole 3bc (Scheme 2).
good yields (entries 14 and 15).
Thus, these experiments suggest the following order
Regioselective palladium-catalyzed direct arylations in reactivity of leaving groups in the electrophile:
of arenes bearing Lewis-basic N-containing directing Br> Cl> OTs.
groups are well established.[14,15,32] With respect to N-
In agreement with this reactivity trend, the direct
aryl-substituted 1,2,3-triazoles 1 this might result in a arylation of 1,2,3-triazole 1a proceeded highly effi-
competitive arylation of the carbo- and heterocyclic ciently with bromide 13 and chloride 2b as electro-
moieties. Therefore, we tested the regioselectivity of
thermal palladium-catalyzed direct arylations with N-
aryl-substituted 1,2,3-triazoles 1 (Table 3). Notably,
1,2,3-triazoles 1 with N-aryl groups being either un-
substituted (entry 1), or meta- (entries 2–4) as well as
ortho-substituted (entries 5–14) were converted to
single regioisomers with excellent selectivities through
arylations of the electron-rich heterocycles. Thereby,
a variety of important functional groups, such as ester
Scheme 1. Chemoselective catalytic direct arylation with
(entries 1–3, and 5–8), ketones (entries 9–11), or a
electrophile 2o.
silyl substituent (entry 8), was tolerated. Also the
challenging electron-rich aryl chlorides were efficient-
ly transformed into the desired fully substituted 1,2,3-
triazoles in high yields (entries 4 and 12–14).
Subsequently, we tested the chemoselectivity of
thermal palladium-catalyzed direct arylations of 1,2,3-
triazoles within competition experiments. Thus, bro-
mochlorobenzene 2o reacted highly selectively, giving
rise to 1,2,3-triazole 3bb in good isolated yield
through coupling of the aryl bromide functionality
(Scheme 1).
Scheme 2. Chemoselective catalytic direct arylation with
electrophile 2p.
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Palladium-Catalyzed Direct Arylations of 1,2,3-Triazoles with Aryl Chlorides
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Table 3. Palladium-catalyzed direct arylations of N-aryl-substituted triazoles 1 with aryl chlorides 2.[a]
Entry
1
Ar
Ph
R1
R2
Product
Isolated Yield
Bu
1g
1h
4-CO2Et
2d
2d
3an
3ao
65%
65%
2
3-MeC6H4
Bu
4-CO2Et
3
4
3-MeC6H4
3-MeC6H4
Bu
Bu
1h
1h
3-CO2Me
3-OMe
2i
2j
3ap
3aq
69%
60%
5
2-MeC6H4
Bu
1i
4-CO2Et
2d
3ar
68%
6
7
8
2-MeOC6H4
2-MeC6H4
2-MeC6H4
Bu
1j
1i
4-CO2Et
3-CO2Me
4-CO2Et
2d
2i
3as
3at
62%
96%
47%
Bu
CH2TMS
1k
2d
3au
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Lutz Ackermann et al.
Isolated Yield
FULL PAPERS
Table 3. (Continued)
Entry
Ar
R1
R2
Product
9
2-MeC6H4
Bu
1i
1i
4-C(O)Ph
2k
2l
3av
95%
80%
10
11
2-MeC6H4
2-MeC6H4
Bu
Bu
3-C(O)Ph
3aw
1i
4-C(O)-t-Bu
2m
3ax
95%
12
13
2-MeC6H4
2-MeC6H4
2-MeC6H4
Bu
Bu
Bu
1i
1i
1i
4-Me
2b
2a
2n
3ay
3az
3ba
70%
87%
90%
4-OMe
14
3,5-(OMe)2
[a]
Reaction conditions: 1 (0.50–1.00 mmol), 2 (0.75–1.50 mmol), Pd
2.00 mmol), PhMe (2 mL), 1208C, 18–24 h.
A
philes (Scheme 3). On the contrary, the corresponding
aryl iodide gave under these reaction conditions no
conversion to the desired triazole 3bd.
1,5-Disubstituted 1,2,3-triazoles can be prepared
highly regioselectively through ruthenium-catalyzed
1,3-dipolar cycloadditions of azides and terminal al-
kynes.[5] Therefore, we probed our catalytic system in
the thermal catalytic direct arylation of triazole 1l
Scheme 3. Catalytic direct arlyation with different halides as
electrophiles.
À
(Scheme 4). This C H bond functionalization pro-
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Palladium-Catalyzed Direct Arylations of 1,2,3-Triazoles with Aryl Chlorides
FULL PAPERS
Scheme 6. Intramolecular palladium-catalyzed direct aryla-
tion.
Scheme 4. Palladium-catalyzed direct arylation of 1,5-disub-
stituted triazole 1l.
action temperature of 2508C, but can be achieved at
ceeded only sluggishly, which can be explained with a significantly milder reaction temperatures of 105–
electrophilic substitution-type mechanism.[18a]
1208C.
This reactivity pattern was reflected by the palladi-
um-catalyzed direct arylation of mono-substituted tri-
azole 1m (Scheme 5). Hence, 1,5-disubstituted triazole Experimental Section
3bf was isolated as the major product, along with only
small amounts of the fully substituted triazole 3bg. General Remarks
Furthermore, there was no indication for the forma-
Catalytic reactions were carried out under an N2 atmosphere
tion of the corresponding 1,4-disubstituted triazole by
GC/MS analysis of the crude reaction mixture.
Finally, we probed our protocol in a thermal intra-
molecular direct arylation of a 1,2,3-triazole with a
chloride[33] as leaving group (Scheme 6). The excellent
catalytic efficacy allowed for an efficient cyclization
of substrate 1n, providing annulated triazole 3bh in
83% isolated yield.
using pre-dried glassware. Triazoles 1 were prepared in anal-
ogy to previously described methodologies.[5,34–36] Additional
starting materials were obtained from commercial sources,
and were used without further purification. Yields refer to
isolated compounds, estimated to be >95% pure as deter-
mined by 1H NMR and GC. Flash chromatography: Macher-
ey–Nagel silica gel 60 (70–230 mesh). NMR: spectra were
recorded on Varian-NMR 300, 500 or 600 instruments in the
solvent indicated; chemical shifts (d) are given in ppm.
Representative Procedure for Palladium-Catalyzed
Direct Arylations: Preparation of Triazole 3av (Table
3,entry 9).
Conclusions
We have developed a highly active catalyst for ther-
mal intermolecular palladium-catalyzed direct aryla-
tions of 1,2,3-triazoles with inexpensive aryl chlorides.
Both N-aryl- as well as N-alkyl-substituted 1,4-disub-
stituted 1,2,3-triazoles were converted with high cata-
lytic efficacy to the fully substituted triazoles. Impor-
tantly, the catalyst proved broadly applicable, thus,
tolerating a variety of important functional groups.
Also more challenging electron-rich aryl chlorides
were converted efficiently, and an intramolecular
direct arylation allowed for the preparation of an an-
nulated triazole. Hence, we have shown that generally
applicable direct arylations of 1,2,3-triazoles with aryl
chlorides do not require microwave heating and a re-
A suspension of Pd(OAc)2 (9.0 mg, 4.0 mol%), PCy3 (22 mg,
N
8.0 mol%), K2CO3 (276 mg, 2.00 mmol), 1i (215 mg,
1.00 mmol) and 2k (325 mg, 1.50 mmol) in PhMe (2 mL)
was stirred under N2 for 22 h at 1208C. Et2O (75 mL) and
H2O (75 mL) were added to the cold reaction mixture. The
separated aqueous phase was extracted with Et2O (2”
75 mL). The combined organic layers were washed with
aqueous NH4Cl (50 mL), H2O (50 mL) and brine (50 mL),
dried over Na2SO4 and concentrated in vacuum. The re-
maining residue was purified by column chromatography on
silica gel (n-hexane/EtOAc, 5:1) to afford 3av as a colourless
1
oil; yield: 382 mg (95%). H NMR (300 MHz, CDCl3): d=
7.76–7.69 (m, 4H), 7.60–7.53 (m, 1H), 7.48–7.42 (m, 2H),
7.36–7.29 (m, 1H), 7.24–7.17 (m, 5H), 2.82 (t, J=8.0 Hz,
Scheme 5. Palladium-catalyzed direct arylation of mono-substituted triazole 1m.
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Lutz Ackermann et al.
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2H), 1.95 (s, 3H), 1.77 (quint, J=7.9 Hz, 2H), 1.39 (tq, J=
7.5, 7.5 Hz, 2H), 0.91 (t, J=7.3 Hz, 3H); 13C NMR
(75 MHz, CDCl3): d=195.8 (Cq), 145.8 (Cq), 137.4 (Cq),
137.0 (Cq), 135.8 (Cq), 135.0 (Cq), 133.9 (Cq), 132.7 (CH),
131.5 (Cq), 131.2 (CH), 130.3 (CH), 130.0 (CH), 129.9 (CH),
128.8 (CH), 128.4 (CH), 127.6 (CH), 126.8 (CH), 31.7
(CH2), 25.1 (CH2), 22.5 (CH2), 17.6 (CH3), 13.8 (CH3); IR
(NaCl): n=2957, 2860, 1660, 1610, 1498, 1447, 1317, 1177,
996, 939, 924, 857, 766, 668 cmÀ1; MS (EI): m/z (relative in-
tensity)=395 (3) [M+], 367 (65), 324 (100), 207 (31), 179
(8), 158 (8), 118 (9), 105 (29), 91 (15), 77 (12), 65 (8); HR-
MS (ESI): m/z=396.2072, calcd. for C26H26N3O: 396.2070.
2006, 62, 11483–11498; d) O. Daugulis, V. G. Zaitsev,
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Chem. 2006, 4, 4041–4047, and references cited there-
in.
[14] L. Ackermann, Top. Organomet. Chem. 2007, 24, 35–
60.
[15] D. Alberico, M. E. Scott, M. Lautens, Chem. Rev. 2007,
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[18] a) S. Chuprakov, N. Chernyak, A. S. Dudnik, V. Ge-
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Acknowledgements
Support by the DFG, the Spanish Ministerio de Educación y
Ciencia (fellowship to RV), the Fonds der Chemische Indus-
trie, the Rçmer-foundation, and Sanofi-Aventis (Frankfurt) is
most gratefully acknowledged.
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Adv. Synth. Catal. 2008, 350, 741 – 748