Catalytic system for inhibition of amination-type reaction and palladium-catalysed direct arylation using non-protected pyrazole derivatives

Article abstract of DOI:10.1002/adsc.201100726

The palladium-catalysed direct arylation at C-4 of non-protected 5-aminopyrazoles was found to proceed in high yields using a variety of aryl bromides. The choice of potassium acetate as the base was found to be crucial to inhibit the amination reaction a

Full text of DOI:10.1002/adsc.201100726

UPDATES
DOI: 10.1002/adsc.201100726
Catalytic System for Inhibition of Amination-Type Reaction
and Palladium-Catalysed Direct Arylation using Non-Protected
Pyrazole Derivatives
Fazia Derridj,a Julien Roger,b Safia Djebbar,a and Henri Doucetb,*
a
Laboratoire d’hydromꢀtallurgie et chimie inorganique molꢀculaire, Facultꢀ de Chimie, U.S.T.H.B. Bab-Ezzouar, Alger
Institut Sciences Chimiques de Rennes, UMR 6226 CNRS – Universitꢀ de Rennes. “Catalyse et Organometalliques”,
b
Campus de Beaulieu, 35042 Rennes, France
Fax : (+33)-2-2323-6939: phone: (+33)-2-2323-6384; e-mail: henri.doucet@univ-rennes1.fr
Received: September 19, 2011; Revised: November 9, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100726.
Abstract: The palladium-catalysed direct arylation
at C-4 of non-protected 5-aminopyrazoles was
found to proceed in high yields using a variety of
aryl bromides. The choice of potassium acetate as
the base was found to be crucial to inhibit the ami-
nation reaction and to promote the direct arylation.
the direct coupling of such heteroaromatics with aryl
halides, would be a considerable advantage for indus-
trial applications and for sustainable development.
We have recently reported results on the direct ary-
lation of some free NH2 substituted thiophenes.[16] In
this update, we wish to report on the reaction of pyra-
zoles derivatives bearing unprotected amino functions
with a set of electronically and sterically diverse aryl
bromides.
The direct arylation of heteroaromatics bearing a
free NH2 is not limited to the use of thiophenes. Pyra-
zoles were also found to be suitable reactants
(Scheme 1, Scheme 2, and Table 1). We observed that
À
Keywords: atom economy; catalysis; C H activa-
tion; heteroarenes; palladium
Mixed bi
ACHTUNGTRENNUNG
active or physical properties and their preparation 2 mol% PdCl
N
ACHTUNGTRENNUNG
constitutes an active field of research in organic bromoacetophenone led to the C.4 arylated pyrazole
chemistry.[1] In recent years, the palladium-catalysed 1 in a high yield of 85% (Table 1, entry 1). It should
direct arylation of heteroaromatics has emerged as a be noted that, under these reaction conditions, no ary-
very powerful method for the preparation of arylated lation at the NH2 group was observed. Quite similar
heteroaromatics.[2–7] However, there are still limita- results were obtained in the presence of 4-bromopro-
tions for these reactions in terms of functional group piophenone, 4-bromobenzonitrile or 4-(trifluorome-
tolerance of the heteroaromatic compound. While the thyl)bromobenzene, and products 2–4 were obtained
presence of acetyl, formyl, nitrile, and methyl alcohol in 64–83% yields (Table 1, entries 2–4). A lower yield
as the functional groups on the thiophenes has been of 48% in 5 was obtained with 4-bromonitrobenzene
described,[8] on the other hand, the use of free NH2 due to the formation of side-products (Table 1,
substituents has attracted much less attention. To the entry 5). 4-Fluorobromobenzene, bromobenzene, 4-
best of our knowledge, only purines bearing a free bromotoluene or 2- bromobenzonitrile also led to the
NH2 on the six-membered ring have been em- direct arylation products 6–9 in moderate yields due
ployed.[9,10] In a few cases, protected amines have to partial conversions of these aryl bromides (Table 1,
been used.[11–15]
However, the direct use of heteroaromatics bearing
unprotected functions, such as NH2 would be more
useful in organic synthesis since it would allow one to
avoid the protection/deprotection sequence, and would
provide a more environmentally and economically at-
tractive access to such arylated heteroaromatics.
Therefore, the discovery of effective conditions, for Scheme 1.
Adv. Synth. Catal. 2012, 354, 747 – 750
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Fazia Derridj et al.
UPDATES
Table 1. Direct arylations of 1,3-dimethylpyrazol-5-amine
(Scheme 1).[a]
Entry Aryl bromide
1
Product
Yield [%]
85
1
2
3
4
5
6
7
8
2
3
4
5
6
7
8
64
83
70
48
40
20
43
Scheme 2.
base inhibits the amination reaction and promotes the
direct arylation. This result is consistent with a con-
certed metallation-deprotonation mechanism.[17]
A
wide variety of substituents such as acetyl, propionyl,
nitrile, nitro, trifluoromethyl, or fluoro on the aryl
bromide is tolerated. To the best of our knowledge,
this is the first method for direct arylation of free
NH2 subsituted pyrazoles. This procedure is attractive
as it allows one to prepare arylated pyrazoles bearing
free NH2 substituents without a protection/deprotec-
tion sequence and therefore should provide a “green-
er” and more economic access to such compounds.
Experimental Section
General Remarks
All reactions were performed in Schlenck tubes under
argon. DMAc analytical grade was not distilled before use.
Potassium acetate 99+ was used. Commercial aryl bromides
and heteroaromatic derivatives were used without purifica-
tion. 1H (300 MHz), 13C NMR (75 MHz) spectra were re-
corded in CDCl3 solutions. Chemical shifts are reported in
ppm relative to CDCl3 (1H: 7.26 and 13C: 77.0). Flash chro-
matography was performed on silica gel (230–400 mesh).
9
9
21
10
10 52
[a]
Reaction conditions: PdCl
N
ACHTUNGERTN(NUNG dppb) (0.02 equiv.), aryl
General Procedure for the Synthesis of Products 1–13
bromide (1 equiv.),
(2 equiv.), KOAc (2 equiv.), DMAc, 20 h, 1508C.
As a typical experiment, the reaction of the aryl bromide
(1 mmol), the pyrazole derivative (2 mmol) and KOAc
(0.196 g, 2 mmol) at 120–1508C during 20 h in DMAc
entries 6–9). A yield of 52% in 10 was obtained in the
presence of 3-bromopyridine (Table 1, entry 10).
(3 mL) in the presence of PdCl
ACHUTGTNRENN(UG C3H5)ACHUTNGTREN(NUNG dppb) (12.2 mg,
0.02 mmol) under argon afforded the coupling product after
The reactivity of a pyrazole substituted at C-3 by a extraction with dichloromethane, evaporation and filtration
on silica gel (pentane/ether).
phenyl and at C-5 by a free NH2 was also examined
(Scheme 2). This substrate was found to be slightly
less reactive than 1,3-dimethylpyrazol-5-amine. How-
ever, the desired C-4 arylated products 11–13 were
obtained in 42–62% yield.
In summary, we have demonstrated that when ap-
propriate reaction conditions are employed, the palla-
dium-catalysed direct arylations at C-4 of some free
NH2 substituted pyrazole derivatives with aryl bro-
1-[4-(5-Amino-1,3-dimethylpyrazol-4-yl)phenyl]ethanone
(1): 4-Bromoacetophenone (0.199 g, 1 mmol), 1,3-dimethyl-
pyrazol-5-amine (0.222 g, 2 mmol) and KOAc (0.196 g,
2 mmol) at 1508C during 20 h in DMAc (3 mL) in the pres-
ence of PdClACTHNUTGRENN(UG C3H5)ACHTUTGNREN(NUNG dppb) (12.2 mg, 0.02 mmol) under
argon afforded 1; yield: 0.195 g (85%).
1-[4-(5-Amino-1,3-dimethylpyrazol-4-yl)phenyl]propan-1-
one (2): 4-Bromopropiophenone (0.213 g, 1 mmol), 1,3-di-
methylpyrazol-5-amine (0.222 g, 2 mmol) and KOAc
mides proceed nicely. The choice of KOAc as the (0.196 g, 2 mmol) at 1508C during 20 h in DMAc (3 mL) in
748
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 747 – 750

Catalytic System for Inhibition of Amination-Type Reaction and Palladium-Catalysed Direct Arylation
the presence of PdCl
G
G
the presence of PdClACTHNGUTREN(NUG C3H5)CAHTUNGTRNE(NUNG dppb) (12.2 mg, 0.02 mmol)
under argon afforded 12; yield: 0.165 g (60%).
under argon afforded 2; yield: 0.155 g (64%).
2-Methyl-4-(4-nitrophenyl)-5-phenylpyrazol-3-ylamine
(13): 4-Bromonitrobenzene (0.202 g, 1 mmol), 1-methyl-3-
phenylpyrazol-5-amine (0.207 g, 1.2 mmol) and KOAc
(0.196 g, 2 mmol) at 1208C during 20 h in DMAc (3 mL) in
the presence of PdClACTHNUTRGENN(UG C3H5)CAHTUNGTRNE(NUGN dppb) (12.2 mg, 0.02 mmol)
under argon afforded 13; yield: 0.123 g (42%).
4-(5-Amino-1,3-dimethylpyrazol-4-yl)benzonitrile (3): 4-
Bromobenzonitrile (0.182 g, 1 mmol), 1,3-dimethylpyrazol-5-
amine (0.222 g, 2 mmol) and KOAc (0.196 g, 2 mmol) at
1508C during 20 h in DMAc (3 mL) in the presence of
PdClACHTUNGTRENNUNG(C3H5)ACHTUNGTRENNUNG(dppb) (12.2 mg, 0.02 mmol) under argon afford-
ed 3; yield: 0.176 g (83%).
1,3-Dimethyl-4-[4-(trifluoromethyl)phenyl]pyrazol-5-yl-
amine (4): 4-(Trifluoromethyl)bromobenzene (0.225 g,
1 mmol), 1,3-dimethylpyrazol-5-amine (0.222 g, 2 mmol) and
KOAc (0.196 g, 2 mmol) at 1508C during 20 h in DMAc
Acknowledgements
(3 mL) in the presence of PdClACTHNUTRGENNU(G C3H5)ACHTUNGTRENN(UGN dppb) (12.2 mg,
J.R. is grateful to “Ministꢀre de la Recherche” for a grant. We
thank the CNRS and “Rennes Metropole” for providing fi-
nancial support, and PCAS SA for providing some amino-
thiophenes.
0.02 mmol) under argon afforded 4; yield: 0.178 g (70%).
1,3-Dimethyl-4-(4-nitrophenyl)pyrazol-4-ylamine (5): 4-
Bromonitrobenzene (0.202 g, 1 mmol), 1,3-dimethylpyrazol-
5-amine (0.222 g, 2 mmol) and KOAc (0.196 g, 2 mmol) at
1508C during 20 h in DMAc (3 mL) in the presence of
PdClACHTUNGTRENNUNG(C3H5)ACHTUNGTRENNUNG(dppb) (12.2 mg, 0.02 mmol) under argon afford-
ed 5; yield: 0.111 g (48%).
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ACHTUGNTERN(NUGN C3H5)
ACHTUNGTRENNUNG
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ACHTUGNTERN(NUGN C3H5)
ACHTUNGTRENNUNG
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Adv. Synth. Catal. 2012, 354, 747 – 750
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Adv. Synth. Catal. 2012, 354, 747 – 750