Direct arylation of 6-phenylpurine and 6-arylpurine nucleosides by ruthenium-catalyzed Ci-H bond activation

Article abstract of DOI:10.1002/anie.201104035

One aryl or two? The title reaction predominantly gives the monoarylation products with various amounts of diarylation product being observed in almost all cases (see scheme). Aryl iodides as well as aryl bromides were reactive under the optimized reactio

Full text of DOI:10.1002/anie.201104035

Communications
DOI: 10.1002/anie.201104035
À
C H Bond Activation
Direct Arylation of 6-Phenylpurine and 6-Arylpurine Nucleosides by
À
Ruthenium-Catalyzed C H Bond Activation**
Mahesh K. Lakshman,* Ashoke C. Deb, Raghu Ram Chamala, Padmanava Pradhan, and
Ramendra Pratap
À
C H bond activation represents an efficient approach to
idine as well as 9-benzyl-6-phenyl-9H-purine. The distances
between the metal-directing nitrogen atom and the ortho-
hydrogen atom on the phenyl ring were calculated from the
energy-minimized structures, and are shown in Figure 1.
molecular functionalization.[1,2] In directed C H bond acti-
À
vation, Lewis basic sites are exploited to draw the catalyst
proximal to the reactive center. We have been interested in
À
the C H bond activation and arylation of nucleobases and
nucleosides using the nitrogen atoms of the purine itself as the
Lewis basic sites. In this context, 6-arylpurine has embedded
2-phenylpyridine and 4-phenylpyrimidine motifs. 2-Arylpyr-
idines, and benzo[h]quinoline, which can be considered as
containing a rigidified 2-phenylpyridine structure, have been
À
the subject of C H bond activation/arylation strategies using
Pd, Ru, Rh, and Fe.[3–6] However, any metal-catalyzed
conversion of purines and purine nucleosides is a challenging
proposition owing to the presence of four nitrogen atoms in
the nucleobases, plus additional oxygen atoms in the sugar
unit; all of these heteroatoms could participate in metal
sequestration and deactivation of catalytic processes.
As shown in Scheme 1, the N1 nitrogen atom of purine is
À
well positioned to direct C H bond activation. Alternatively,
N7 can also function in a similar capacity. To gain preliminary
insight, energy minimization was performed, using DFTat the
B3LYP/6-311 ++ G(2d,2p) level of theory, on 2-phenylpyr-
Figure 1. Energy-minimized structures of 2-phenylpyridine (left) and
9-benzyl-6-phenyl-9H-purine (right).
Consistent with known X-ray structures of 2-phenylpyr-
idine derivatives,[7] the aryl rings in 2-phenylpyridine are not
coplanar. The distance between the pyridyl nitrogen atom and
the ortho-hydrogen atom of the aryl ring in this case is 2.49 ꢀ.
By comparison, in the energy-minimized structure of 9-
benzyl-6-phenyl-9H-purine, the C6-aryl group is coplanar
with the purine ring. We next compared the computed
structure of 9-benzyl-6-phenyl-9H-purine to the crystallo-
graphic structures of 9-benzyl-6-phenyl-8-(p-tolyl)-9H-purine
and 8,8’-bis(9-benzyl-6-phenyl)-9H-purine.[8] In both com-
pounds, the C6-phenyl group is coplanar with the purinyl
system. We also evaluated the distance between N1 and the
ortho-hydrogen atom of the C6-aryl group for these cases. In
9-benzyl-6-phenyl-8-(p-tolyl)-9H-purine, this distance is
2.43 ꢀ whereas the distance to N7 is 2.34 ꢀ. Similar distances
of 2.43 ꢀ and 2.36 ꢀ, respectively, were obtained for 8,8’-
bis(9-benzyl-6-phenyl)-9H-purine. These data closely match
the DFT-derived distances for 9-benzyl-6-phenyl-9H-purine
shown in Figure 1. Given the similar distances between the
nitrogen atom and the aryl hydrogen atom in 9-benzyl-6-
À
Scheme 1. N-directed C H bond activation in 2-phenylpyridine, and
À
two plausible modes of N-directed C H bond activation in C6-aryl
purines and nucleosides.
[*] Prof. M. K. Lakshman, Dr. A. C. Deb, Dr. R. R. Chamala,
Dr. P. Pradhan, Dr. R. Pratap
Department of Chemistry
The City College and The City University of New York
160 Convent Avenue, New York, NY 10031-9198 (USA)
E-mail: lakshman@sci.ccny.cuny.edu
[**] This work was partially supported by a PSC CUNY award to M.K.L.
Infrastructural support at CCNY was provided by the NIH RCMI
Grant G12 RR03060. We thank Ona Liu (undergraduate research
participant) for her contribution to this work, Frontier Scientific for
generous samples of arylboronic acids, and Dr. Bill Boggess and
Nonka Sevova (University of Notre Dame) for HRMS analyses (NSF
Grant CHE-0741793).
À
phenyl-9H-purine and 2-phenylpyridine, we reasoned that C
H bond activation in purines and nucleosides should be
feasible. In support of this hypothesis, serendipitously,
undesired arylation of the C6-phenyl group has been
observed in the Pd/Cu-mediated C8 arylation of 9-benzyl-6-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104035.
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À
Table 2: C H bond activation/arylation of 9-benzyl-6-phenyl-9H-purine
phenyl-9H-purine with iodobenzene or p-iodotoluene (2–
10 equiv).[8]
(1) with aryl iodides and aryl bromides.[a]
In initial experiments, [{RuCl2(benzene)}2] (A) and
[{RuCl2(p-cymene)}2] (B) were selected as catalysts for
À
assessing the C H bond activation of 9-benzyl-6-phenyl-9H-
purine (1), and 2 equivalents of iodobenzene were used to
ensure complete consumption of 1. Results from a prelimi-
nary screen of reaction conditions are shown in Table 1.
Notably, 5 mol% of catalyst A in combination with 40 mol%
of PPh3 resulted in full conversion (Table 1, entries 1–4), and
catalyst A appeared marginally superior to catalyst B
(Table 1, entries 4 and 5). Replacement of K2CO3 with
Cs2CO3 led to a slower reaction (Table 1, entries 6 and 7),
and the mono/diarylation ratio was substantially altered. This
Entry
1
Aryl halide
Yield [%][b]
2/3
2a: 76
3a: 17
4.5:1
2b: 81
3b: 18
2
4.5:1
5.6:1
À
2c: 79
3c: 14
indicates an unknown but crucial role of the base in these C
H bond activation processes.
3[c]
2d: 82
3d: trace
4
NA[d]
À
Table 1: Preliminary evaluation of the C H bond activation process for
9-benzyl-6-phenyl-9H-purine (1).[a]
2a: 72
3a: 12
5
6
6:1
2b: 73
3b: 17
4.3:1
2d: 75
3d: trace
7
NA[d]
Entry
1
Catalytic system[b]
Result[c]
A (2.5 mol%), PPh3 (20 mol%), K2CO3 (3 equiv)
1/2a/3a=
83:15:2[d]
1/2a/3a=
83:15:2[d]
1/2a/3a=
63:34:3[d]
2a:76%
[a] Reaction conditions: 1 (0.2m) in anhydrous NMP, aryl halide
(2 equiv), [{RuCl2(benzene)}2] (5 mol%), PPh3 (40 mol%), K2CO3
(3 equiv), 1208C. [b] Yields are of isolated and purified products. [c] The
reaction was conducted using 10 mol% of [{RuCl2(benzene)}2] and 80
mol% of PPh3. [d] Not applicable since only a trace of the diaryl product
was detected.
2
B (2.5 mol%), PPh3 (20 mol%), K2CO3 (3 equiv)
A (5 mol%), PPh3 (20 mol%), K2CO3 (3 equiv)
A (5 mol%), PPh3 (40 mol%), K2CO3 (3 equiv)
B (5 mol%), PPh3 (40 mol%), K2CO3 (3 equiv)
A (5 mol%), PPh3 (40 mol%), Cs2CO3 (3 equiv)
A (5 mol%), PPh3 (40 mol%), Cs2CO3 (3 equiv)
3
4
3a:17%
5
1/2a/3a=
3:84:13[d]
1/2a/3a=
7:52:41[d]
2a:56%
We next explored arylations of the more complex 2’-
deoxynucleoside substrates, which are quite labile and prone
to facile deglycosylation.[9] The requisite 6-arylpurine 2’-
deoxyribonucleosides (6-aryl 2’-deoxynebularines) are read-
ily available by our previously reported procedures.[10] Results
from the nucleoside arylation reactions are shown in Table 3.
The results in Table 3 indicate that the procedure is
readily applicable to the sensitive 2’-deoxyribonucleoside
substrates. However, 10 mol% of the Ru catalyst was needed
to obtain complete consumption of the precursor. In contrast
to the reactions of purine 1, for which a 1:8 ratio of Ru
catalyst/PPh3 was needed for complete reaction (Table 1
entry 4), use of a 1:4 and 1:8 ratio of the Ru catalyst/PPh3 led
to full conversion of 4a (Table 3 entries 1–4), but there were
some differences in the reaction results. With iodobenzene,
increasing the amount of PPh3 resulted in a better yield and
better mono/diarylation ratio (Table 3, entry 1 versus
entry 2). With p-iodotoluene, again a better ratio of mono/
diarylation was observed with more PPh3, although the yield
of the monoarylation remained almost the same (Table 3,
entry 3 versus entry 4). Arylation reactions with electron-rich
p-iodoanisole and electron-deficient p-iodoacetophenone
were both successful (Table 3, entries 5 and 6, respectively).
Interestingly, for reasons currently unknown, when R = F
(4b) or OPh (4c), more diarylation was observed in reactions
6
7[e]
3a:36%
[a] Reaction conditions: 1 (0.2m) in anhydrous NMP, iodobenzene
(2 equiv), 1208C, 24 h. [b] A: [{RuCl2(benzene)}2], B: [{RuCl2(p-
cymene)}2]. [c] Yields are of the isolated and purified products. [d] The
reaction was incomplete. The ratio was determined from 1H NMR
spectra by the integration of the benzyl CH2 resonances, which appear at
d=5.49 ppm for 1, d=5.41 ppm for 2a, and d=5.32 ppm for 3a. [e] The
reaction time was 32 h. Bn=benzyl, NMP=N-methylpyrrolidone.
We evaluated the use of aryl iodides and aryl bromides for
the direct arylation of 9-benzyl-6-phenyl-9H-purine (1) under
the optimal reaction conditions developed in Table 1. Results
from these experiments are shown in Table 2.
Results in Table 2 indicate that reactions can be accom-
plished with both aryl iodides and aryl bromides, in yields of
75–99% (combined for mono- and diarylated products).
Yields of reactions with aryl bromides were respectable
(Table 2, entries 5–7) but a little lower than those with aryl
iodides.
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Communications
with iodobenzene (Table 3, entries 7 and 9). The same trend
was also observed when using catalyst B for the arylation of
4c with iodobenzene (Table 3, entry 10). In contrast, for 4d
(R = OMe), greater monoarylation was observed (Table 3,
entries 11 and 12). As with purine 1, aryl bromides were also
reactive (Table 3, entries 13 and 14).
À
Using catalyst A, we then examined the C H bond
activation/arylation of m-nitrophenyl nucleoside derivative
7[10] and 2-amino-6-phenylpurine nucleoside 8[11] (Scheme 2)
with iodobenzene. In the case of 7, no product formation was
seen but some precursor decomposition occurred, thus
indicating very electron-deficient aromatic rings, not surpris-
ingly, are resistant to arylation (see the mechanism in
Scheme 3). Substrate 8, which is structurally similar to 4a,
but has a C2-amino group, remained largely intact and
showed no product formation. It appears therefore that the
presence of the amino group abolishes any reactivity.
À
Table 3: C H bond activation/arylation of 3’,5’-di-O-silyl 6-arylpurine
2’-deoxyribonucleosides (4) with aryl iodides and aryl bromides.
Entry
1[b]
R
Aryl halide
Yield [%][a]
5/6
5a: 46
6a: 23
H
2:1
5a: 65
6a: 14
2[c]
3[b]
4[c]
5[c]
H
H
H
H
4.6:1
3.2:1
7.7:1
7.6:1
Scheme 2. Nucleoside substrates that did not yield arylation products.
TBDMS=tert-butyldimethylsilyl.
5b: 52
6b: 16
5b: 54
6b: 7
Since in previous experiments 2 equivalents of aryl halide
were used to achieve complete consumption of the purine or
nucleoside precursor, we evaluated the arylation of 1 and 4a
with a variety of stoichiometries of iodobenzene; all other
reaction conditions remained the same. These results are
shown in Table 4. With purine 1, incomplete reaction was
observed with 1 equiv of iodobenzene, but substantially more
monoarylated product 2a was formed (Table 4, entry 1).
5c: 53[d]
6c: 7
5d: 62
6d: 15
6[c]
H
4.1:1
5e: 31
6e: 44
7[c]
F
1:1.5
5 f: 42
6 f: 20
8[c]
F
2.1:1
Table 4: Reactions of 1 and 4a using various stoichiometries of
5g: 29
6g: 53
iodobenzene.
9[c]
OPh
OPh
OMe
OMe
H
1:1.8
Entry
Substrate
Mol%
of PhI
t [h]
Yield [%][a]
Mono/diaryl
products
5g: 24
6g: 72
10[e]
11[c]
12[c]
13[c]
14[c]
1:3
2.4:1
2.7:1
2:1
1[b]
2[b]
3[b]
4[b]
5[d]
6[d]
7[d]
1
100
120
400
400
120
400
400
24
24
24
48
30
30
60
2a: 91
3a: 2
2a/3a=45:1[c]
2a/3a=6.7:1
2a/3a=2:1
5h: 39
6h: 16
1
2a: 80
3a: 12
2a: 60
3a: 30
2a: 59
3a: 32
5a: 62
6a: 10
5a: 62
6a: 12
5a: 53
6a: 13
5i: 46
6i: 17
1
5a: 36
6a: 18
1
2a/3a=1.8:1
5a/6a=6.2:1
5a/6a=5.2:1
5a/6a=4.1:1
4a
4a
4a
5b: 57
6b: 18
H
3.2:1
[a] Yields are of the isolated and purified products. [b] Reaction
conditions: 4a (0.2m) in anhydrous NMP, aryl halide (2 equiv),
[{RuCl2(benzene)}2] (10 mol%), PPh3 (40 mol%), K2CO3 (3 equiv),
1208C, 30 h. [c] Reaction conditions: 4a–d (0.2m) in anhydrous NMP,
aryl halide (2 equiv), [{RuCl2(benzene)}2] (10 mol%), PPh3 (80 mol%),
K2CO3 (3 equiv), 1208C. [d] Owing to the similar Rf values of the
products, a trace of the diarylated product was present. [e] Reaction
conditions were the same as [c] except that [{RuCl2(p-cymene)}2] was
used in place of [{RuCl2(benzene)}2].
[a] Yields are of isolated and purified products. [b] Reaction conditions:
1 (0.2m) in anhydrous NMP, PhI (see Table), [{RuCl2(benzene)}2]
(5 mol%), PPh3 (40 mol%), K2CO3 (3 equiv), 1208C. [c] Reaction was
incomplete, 4% of 1 was recovered. [d] Reaction conditions: 4a (0.2m)
in anhydrous NMP, PhI (see Table), [{RuCl2(benzene)}2] (10 mol%),
PPh3 (80 mol%), K2CO3 (3 equiv), 1208C.
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However, with 120 mol% of iodobenzene the result was
nearly the same as that obtained with 200 mol% (Table 4,
entry 2 versus Table 2, entry 1). Increasing the amount of
iodobenzene to 400 mol% yielded a greater proportion of the
diarylated product 3a, but complete conversion to the
diarylated 3a did not occur (Table 4, entry 3). Doubling the
reaction time did not increase diarylation either (Table 4,
entry 4). The stoichiometry of iodobenzene had very little
effect on the mono/diarylation ratio of the nucleoside
precursor 4a (Table 4, entries 5–7 and Table 2, entry 2),
although a prolonged reaction time (60 h) was detrimental
and led to a lower yield (Table 4, entry 7).
directed electrophilic attack by the aryl/RuIV complex onto
the C6-aryl ring, no reaction was observed with the m-
nitrophenyl nucleoside derivative 7. This is because reaction
would have to occur at the electron-deficient para or
ortho positions to the nitro group.
À
Owing to interest in oxidative C H cross-coupling
processes,[3c,13–17] we evaluated whether the purinyl nitrogen
À
À
directed C H bond activation could be used to promote C H
cross-coupling.[18] This reaction gave dimerized 8,8’-bis(9-
benzyl-6-phenyl)-9H-purine[8] in ca. 20% yield, thus indicat-
ing a facile dimerization of 1 under oxidative conditions (50%
of 1 was recovered).
À
While this work was in progress, a complementary Pd-
We have demonstrated that Ru-catalyzed C H bond
À
catalyzed C H bond activation of 6-arylpurines and three
activation/arylation can be accomplished with purines and
deoxyribonucleosides. Both aryl iodides and aryl bromides
are reactive, leading to densely functionalized products in
relatively few steps. Metal-catalyzed approaches, such as that
described here, provide facile routes to novel entities that are
otherwise not easily accessed. This is important in the context
of the high biological value placed on purine and nucleoside
derivatives. Further work is currently ongoing in our labo-
ratories on these and related chemical processes, including
understanding the role, if any, of the purinyl N7 atom.
acetate-protected
6-arylpurine
ribonucleosides
was
reported.[12] It is therefore reasonable to offer a comparison
of the two approaches. In contrast to the reactions reported
here, those catalyzed by Pd required 30 equiv of aryl iodide,
with reaction times from 48 hours (nucleosides) to 60 hours
(purines), at 1208C.[12] Thus, the present reaction conditions
use substantially less of the aryl halide and generally provide
faster reactions. Pd catalysis results in only monoarylation,[12]
plausibly because of the slow reactions, whereas under Ru
catalysis both mono and diaryl products are formed. Use of
AcOH as the solvent at 1208C and a N2 atmosphere have
been stated as crucial for the Pd-catalyzed reactions.[12]
Exposure to AcOH at elevated temperature may be unsuit-
able for acid-sensitive substrates such as the deoxyribonucleo-
sides described here. Also, an inert atmosphere is strictly
necessary for Pd-catalyzed reactions. Plausibly, this is to
suppress undesired dimerization of the purine, as dimeriza-
tion has been observed previously when air was introduced
into Pd-catalyzed arylation reactions.[8] Most notably, aryl
bromides can be used in the Ru-catalyzed chemistry, whereas
aryl bromides and chlorides were unreactive under Pd
catalysis.[12]
Received: June 13, 2011
Published online: September 28, 2011
À
Keywords: arylation · C H activation · homogeneous catalysis ·
.
nucleosides · ruthenium
À
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catalytic cycle resulting in diarylation. Consistent with an N-
À
Scheme 3. Possible mechanism for the C H bond activation/arylation
of 6-arylpurine and 6-arylpurine 2’-deoxyribonucleosides.
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Communications
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