Metal-free arylation of pyrimidines through a photochemical process

Article abstract of DOI:10.1039/c5cc08927a

Pyrimidinyl and pyrazinyl radicals were generated under moderate energetic irradiation conditions (UVA), and proved to be prompt to undergo C-C bond formation processes. Hetero-biaryl derivatives were obtained in good to high yields with highly interesting functional group selectivities. Bis hetero-biaryls were also easily accessible leading to original compounds, ready for further transformations. Experiments supporting radical processes have been reported.

Full text of DOI:10.1039/c5cc08927a

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DOI: 10.1039/C5CC08927A
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ARTICLE
Metal-Free Arylation of Pyrimidines Through Photochemical
Process
Jonas Ruch,a Ariane Aubin,a Guillaume Erbland,a Audrey Fortunato,a Jean-Philippe Goddard a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Pyrimidinyl and pyrazinyl radicals were generated under moderate energetic irradiation conditions (UVA), and proved to
be prompt to undergo C-C bond formation processes. Hetero-biaryl derivatives were obtained in good to high yields with
highly interesting functional group selectivities. Bis hetero-biaryls were also easily accessible leading to original
compounds, ready for further transformations. Experiments supporting radical processes have been reported.
www.rsc.org/
derivatives. UV irradiation has been used for the formation of aryl
radicals.14,15 Surprisingly, only one publication reports the
photolysis of 5-iodopyrimidine with a low-pressure mercury arc at
Introduction
Diazines are ubiquitous scaffolds found in life and material science
254 nm and the trapping of the resulting radical with
heteroaromatic compounds.16 This transformation is limited to the
photolysis of C-I bond under high energy irradiation. Such process
would have strong synthetic impact if it could proceed with lower
energy light and a broader scope of substrates.
and also extensively represented among natural products and
1,2,3
pharmaceuticals.
Based on their large field of application, the
development of new processes that allow versatile chemical
alteration of diazines under mild conditions is of primary
importance. Moreover, economical and environmental issues have
to be considered. The construction of diazine skeletals are generally
dedicated to early steps synthesis of complex molecules.4 To
improve the flexibility of synthetic pathways, late derivatization of
such scaffolds might be more suitable.
Results and Discussion
We decided to investigate versatile and mild photochemical
transformations of pyrimidines dedicated to C-C bond
formation and focused on heterobiaryl scaffolds. In a first task,
Diazines are electron-deficient heteroarene having a low energy
LUMO. For this reason, nucleophilic aromatic substitution (SNAr)
has been intensively used as a functionalization pathway. However,
a stoichiometric amount of strong base and the need of directing
and protective groups are important limitations.5,6 C-C Bond
formation involving C-X partners using a catalytic organometallic
process is a valuable alternative but the C-H activation of electron-
deficient (hetero)arenes still requires improvements.7,8 Radical
functionalizations of diazines, and particularly pyrimidines, have
been reported with heteroarenes serving as acceptors for carbon-
centered radicals according to Minisci or Gomberg–Bachmann type
reactions, known to proceed with low regioselectivities.9,10 Then,
the selectivity can be enhanced by generating a radical center at the
suitable position of the diazine moiety. While pyridyl radicals have
been studied widely,11 only a few publications deal with pyrimidinyl
and pyrazinyl radicals, and most of them involve the use of tin or
silanes derivatives as the radical chain mediator.12,13 To develop
mild and sustainable C-C bond formation on pyrimidines and
pyrazines, photochemistry is certainly a valuable alternative to the
overstoichiometric use of radical chain mediators or toxic tin
5-bromopyrimidine
1 was used for optimization of the
photochemical arylation. Irradiation was conducted in
a
Rayonet photoreactor at 254, 300 and 350 nm or with a
medium-pressure mercury-vapors lamp (100 W) equipped
with an anticaloric filter (338-500 nm, UV-A/visible, see SI)
(Table 1). The latter proved to be the most efficient and
versatile irradiation system. By using acetonitrile as solvent in
the presence of 20 equivalents of benzene, 5-phenylpyrimidine
2a was obtained in 27% yield (Table 1, entry 1). Use of THF,
DMF, DMSO, MeOH, AcOEt and tBuOH did not improve the
arylation outcome. Slight improvements were observed in
DCM/ACN mixture (95/5 v/v) furnishing 2a and 2b in 41 and
31% yield (Table 1, entries 2-3).
We checked the influence of basic additives to trap the
released HBr and facilitate the aromatization step (Table 1,
entries 4-5 and SI). With K2CO3 (3 equiv.), 2b was obtained in
47% yield. Comparable results were observed in acetonitrile
(Table 1, entry 6) and with only 1.1 equivalent of K2CO3 (Table
1, entry 7). Moreover, we were able to decrease the amount
of the acceptor from 20 to 10 equivalents with even better
yields of 2b (54%) and 2a (67%) (Table 1, entries 8-9). Light is
crucial while no reaction was observed in the dark (Table 1,
entry 10). At this point, a direct nucleophilic substitution of 5-
bromopyrimidine can be excluded.
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Table 1. Optimization of the photoarylation method.
tolerated as substituent of the acceptor. Interestingly, aryl
bromides were advantageously engaged in the photochemical
arylation reaction. The selective cleavage of the 5-
bromopyrimidine C-Br bond has been done in the presence of
other aryl C-Br bond to form exclusively the 5-pyrimidinyl
radical which added onto the aryl bromide acceptor (2q-u).
Br
Ar
UVA
ArH:
Cl
ArH (equiv.)
N
N
N
N
solvent, rt
additive
A
B
1
2a-b
This selectivity allowed
a one-step synthesis of biaryl
pyrimidine derivative having an aryl bromide function readily
available for further transformations like cross coupling
reaction. The yields were moderate to good as 59% of 2u was
isolated.
Entry
ArH (equiv.)
A (20)
solvent
ACN
Additive
Yield (%)[c-d]
1
2
-
27
41
A (20)
CH2Cl2/ACN[a]
CH2Cl2/ACN [a]
CH2Cl2/ACN [a]
CH2Cl2/ACN [a]
ACN
-
3
4
B (20)
B (20)
B (20)
B (20)
B (20)
B (10)
A (10)
A (10)
-
31
47
42
50
51
54
67
0[b]
Br
Ar
UVA
K2CO3 (3)
Cs2CO3 (3)
K2CO3 (3)
K2CO3 (1.1)
K2CO3 (1.1)
K2CO3 (1.1)
K2CO3 (1.1)
K2CO3 (1.1 equiv.)
5
Arene acceptor (10 equiv.)
N
N
N
N
6
1
ACN, rt, 24 h
2a-u
OMe
OMe
7
ACN
Arene acceptors:
MeO
8
ACN
2
4
2
9
ACN
3
OMe
2d, 63%
OMe
4
10
ACN
2a, 67%
2c, 63%
2e, 79%
C2:C3:C4 68:18:14
C2:C4 41:59
[a] CH2Cl2/MeCN 95/5 (v/v). [b] Reaction was conducted in the dark. [c]
Irradiation was done with a medium-pressure mercury lamp (100 W) with an
anticaloric filter (338-500 nm). [d] See Scheme 2 for regioselectivity with B.
OMe
OMe
OMe
OMe
OMe
OMe
2
3
5
3
OMe
MeO
4
4
4
In our hands, this photoarylation process proved to be
superior to classical radical chain reaction using TTMSS/AIBN
system (Scheme 1). 2a and 2b were formed in low yields and
isolated from complex mixtures.
2f, 56%
C3:C4 30:70
2g, 83%
2h, 67%
2i, 48%
C4:C5 80:20 C2:C3:C4 57:25:18
2
3
5
Cl
4
4
Br
Ar
Ar-H:
2j, 57%
C2:C4:C5 40:53:7
2k, 62%
C3:C4 48:52
2l, 50%
2m, 63%
AIBN (2 equiv.)
TTMSS (2 equiv.)
A
B
N
N
N
N
ArH (solvent)
80°C, 24 h
Br
OAc
Cl
2a, 25%
2b, 15%
1
2a-b
4
4
2
2
4
2
3
3
2p,354%
2q, 40%
Scheme 1. TTMSS/AIBN conditions for arylation of 1.
2n, 74%
2o, 51%
C2:C3:C4 36:37:27 C2:C3:C4 60:21:19 C2:C3+C4 50:50
The optimized conditions were applied to the photoarylation
of pyrimidine with various aromatic acceptors (Scheme 2).
Methoxy-substituted arenes were engaged in this process to
afford the corresponding 5-aryl-pyrimidines in good to high
yields. With anisole, 2c was obtained in 63% yield as a mixture
of regioisomers (C2:C3:C4 68:18:14). This regioselectivity is
consistent with the addition of the 5-pyrimidinyl electrophilic
radical onto an electron-rich arene. 1,2- and 1,4-
Dimethoxybenzene reacted in a similar manner to afford 2f
and 2d in good yields. When electron-rich trimethoxybenzene
acceptors were used, the arylation process proved to be very
efficient since 2g was obtained in 83% yield. Similar behaviour
was observed within the series of methyl-substituted arenes.
Regioisomeric mixtures were isolated with toluene, ortho- and
meta-xylene but still in good yields. 5-Pyrimidinyl radical
reacted more efficiently with nucleophilic durene and
mesitylene to afford 2m and 2n in 63% and 74% yields
respectively. Under our conditions, polymethyl-substituted
arenes, which are good hydrogen atom donors, were valuable
radical acceptors. Ester group (2o) and chloride (2p) were also
Br
2
2
MeO
Br
MeO
MeO
3
4
6
Br
5
Br
2u, 59%
2r, 41%
2s, 45%
mixture
2t, 43%
C2:C4+C6:C5 39:51:10
C2:C3 73:27
Scheme 2. Photoarylation of 5-bromopyrimidine.
This synthetically usefulness of our method was illustrated by
the selective functionalization of possessing two reactive
Csp2-Br bonds (Scheme 3). Under the standard conditions,
was irradiated in the presence of 1,3,5-trimethoxybenzene.
After 24 h, a conversion of 60% was observed and was
3
3
4
isolated in 44% yield without any traces of arylation product at
the phenyl ring.
Then, we extended the scope of the photoarylation to
substituted diazines (Scheme 4). 2-Fluoro-5-bromopyrimidine
1b was engaged under standard conditions in the presence of
arene acceptors. High yields of 2-fluoro-5-aryl-pyrimidine
2 | J. Name., 2012, 00, 1-3
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Br
HetAr
derivatives were obtained with benzene (5a, 74%), 1,3,5-
trimethoxybenzene (5b, 89%) and mesitylene (5c, 80%). With
2,4,6-trimethybromobenzene, 5d was isolated in 45% yield
through a selective photochemical activation of the C-Br bond
from pyrimidine 1b. 2-Chloro-5-bromopyrimidine 1c was also
an interesting partner since the corresponding phenyl (6a,
UVA
K2CO3 (1.1 equiv.)
Heteroarene
(10 equiv.)
ACN, rt, 24 h
N
N
N
N
X = H, F, Cl
X
X
HN
HN
N
79%), 2,4,6-trimethoxyphenyl
(6b, 77%) and 2,4,6-
trimethylphenyl (6b, 66%) derivatives were isolated in good to
N
N
N
high yields.
8c, 34%
(o:m:p 60:9:31)
N
N
N
8a, 60%
8b, 62%
OMe
Br
N
MeO
HN
HN
HN
3
UVA
N
K2CO3 (1.1 equiv.)
N
N
N
N
OMe
OMe
Br
ACN, rt, 24 h
conv. 60%
F
N
Cl
N
Cl
N
9b, 68%
9a, 62%
10, 64%
N
4, 44%
(10 equiv.)
OMe
Scheme 5. Photoarylation with heteroaromatic partners.
MeO
Br
2,4-Dimethyl-3-ethylpyrrole
dimethylpyrrole were engaged in the photoarylation process
with 5-bromopyrimidine . Indole and Benzofuran have aslo
(kryptopyrrole)
and
2,4-
Scheme 3. Selective C-Br bond cleavage in photoarylation conditions.
1
Ar
Br
UVA
K2CO3 (1.1 equiv.)
been investigated as radical acceptors but afforded mixture of
arylation products. Compounds 8a and 8b were isolated in
60% and 62% yield respectively. The selective formation of 8b
is in agreement with the generally observed radical addition at
the 2-position of pyrrole derivatives.17 The radical addition of
the 5-pyrimidinyl radical onto pyridine gave 8c in a low yield
(34%). This can be explained by the electro-deficient character
of the pyridine which does not fit in term of polar effect with
the electrophilic pyrimidinyl radical. In contrary, good yields
were obtained with 2-chloro- and 2-fluoro-5-bromopyrimidine
with similar selectivity. It is noteworthy that free pyrrole
derivatives were smoothly substituted under the conditions
while ionic or organometallic transformations generally
require a nitrogen protective group.
(N)
(N)
Ar-H (10 equiv.)
N
N
ACN, rt, 24 h
X
X
OMe
Br
MeO
OMe
N
N
N
N
N
N
N
N
5a, 74%
5b, 89%
5c, 80%
OMe
F
F
F
F
5d, 45%
MeO
N
OMe
MeO
OMe
R
Br
OTMP
UVA
TMP:
K2CO3 (1.1 equiv.)
OMe
N
N
N
N
N
6b, 77%
N
N
N
7a (R = Cl), 79%
7b (R = H), 34%
TEMPO (1 equiv.)
ACN, rt, 24 h
6a, 79%
6c, 66%
N
N
N
N
N
1
Cl
Cl
11, 34%
Cl
OTMP
Br
OTMP
N
Scheme 4. Photoarylation of substituted diazines.
UVA
K2CO3 (1.1 equiv.)
To our delight, the photoarylation method succeeded also with
pyrazine scaffold since 2-bromo-6-chloropyrazine was
smoothly arylated to afford 7a in 79% yield. Unfortunately, 2-
bromopyrazine gave the corresponding biaryl compounds in
lower yield. From a synthetic point of view, this selective
arylation process opens interesting opportunities for the
construction of valuable heterobiaryl scaffolds through ionic or
organometallic coupling reactions of the resting C-F, C-Cl or C-
Br bond.
1-octene (10 equiv.)
TEMPO (1 equiv.)
ACN, rt, 24 h
N
N
1
N
N
12, 50%
11, 16%
Scheme 6. Direct radical trapping experiment and tandem radical addition/trapping.
To gain insight into the mechanism, we carried out radical
trapping experiments (Scheme 6). 5-Bromopyrimidine
1 was
irradiated under typical conditions in the presence of TEMPO
(1 equiv.). Adduct 11 was isolated in 34% yield, what strongly
supports the formation of free 5-pyrimidinyl radical,
subsequently trapped by TEMPO. The lower efficiency of this
reaction could be explained by a low reaction rate of coupling
two electrophilic radicals. When 1-octene was added to the
reaction mixture, the 5-pyrimidinyl radical added first onto the
olefin and the resulting secondary alkyl radical was trapped by
One step further was to delineate the scope of arylation with
heteroaromatic acceptors (Scheme 5). We focused on
nitrogen-containing heteroarenes because they are key
building blocks for pharmaceutical, agrochemical and materials
chemistry.
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Conclusions
We developed an efficient process for the photoarylation of
pyrimidine derivatives under mild UVA irradiation with K2CO3
as the sole additive. We demonstrated that valuable
heteroaryls and bisheteroaryls can be obtained in good to high
yields under these conditions from commercially available
substrates without the need of modification. The high
selectivity of the reaction toward halogen-substituted
substrates or acceptors makes this transformation valuable for
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4 | J. Name., 2012, 00, 1-3
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