DDQ-promoted direct C5-alkylation of oxazoles with alkylboronic acids: Via palladium-catalysed C-H bond activation

Article abstract of DOI:10.1039/c7ob01083d

A novel and concise C5-alkylation of oxazoles using alkylboronic acids as alkyl donors via Pd(ii)-catalysed C-H bond activation has been achieved in moderate to good yields with satisfactory functional group tolerance. Instead of commonly used BQ as a key promoter, DDQ was discovered to be a better additive that significantly promoted this alkylation. This efficient and advanced method represents the first C(sp²)-C(sp³) cross-coupling reaction at the C5-position of oxazoles, which is particularly attractive due to its potential applications in the late-stage functionalization of oxazole-containing bioactive molecules.

Full text of DOI:10.1039/c7ob01083d

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DDQ-promoted direct C5-alkylation of oxazoles
with alkylboronic acids via palladium-catalysed
C–H bond activation†
Cite this: DOI: 10.1039/c7ob01083d
Received 4th May 2017,
Accepted 1st July 2017
Bowen Lei, Xiaojiao Wang, Lifang Ma,* Huixuan Jiao, Lisi Zhu and Ziyuan Li
*
DOI: 10.1039/c7ob01083d
rsc.li/obc
A novel and concise C5-alkylation of oxazoles using alkylboronic several groups.5 Besides these reports, we have reported a
acids as alkyl donors via Pd(II)-catalysed C–H bond activation has series of C(sp2)–C(sp2) coupling reactions at this position,
been achieved in moderate to good yields with satisfactory func- including the first C5-homocoupling, alkenylation and aryl-
tional group tolerance. Instead of commonly used BQ as a key pro- ation of oxazoles through double C–H bond activation,6 as well
moter, DDQ was discovered to be a better additive that signifi- as the first aryl–aryl coupling via palladium-catalysed C–H/C–P
cantly promoted this alkylation. This efficient and advanced cleavage.7 Exploring novel methodologies for the C5-
method represents the first C(sp2)–C(sp3) cross-coupling reaction functionalization of oxazoles via C–H bond activation is of
at the C5-position of oxazoles, which is particularly attractive due great importance and consistently attractive.
to its potential applications in the late-stage functionalization of
Since 2006, a series of pioneering studies by the Yu group8
on the Pd-catalysed alkylation of pyridine-, oxazoline- or car-
boxylate-directed simple benzenes through C(sp2)–H/C(sp3)–B
cleavage have been reported (Scheme 1a). Other groups9 have
also made much effort on developing these new types of
C(sp2)–C(sp3) cross-coupling reactions of benzenes with mis-
cellaneous directing groups, such as isoxazole,9a pyridylsulfi-
nyl,9b pyrimidylsilyl,9c and pyridylsilyl9d directing groups, or
without a directing group.9e Alternatively, this alkylation of
simple benzenes could be achieved via C(sp3)–halogen bond
cleavage.10 Besides simple benzenes, the Pd-catalyzed alkyl-
oxazole-containing bioactive molecules.
Oxazoles are among the most significant heterocyclic moieties
in a variety of marine natural products with promising bio-
activities.1 Owing to the fact that the biosynthesis of this cat-
egory of natural products relies on serine or threonine as the
starting material,2 most of them bear methyl or no substi-
tution at the C5-position of the oxazole moieties. However,
miscellaneous C5-alkylated oxazole-based artificial com-
pounds have been reported to possess diverse and potent
bioactivities,3 and these compounds were generally prepared
via annulations of complicated pre-alkylated building blocks
which were required to be synthesized in advance. Such cir-
cumstance is not only detrimental to the efficiency of prepa-
ration, but also severely limits the diversity of these bioactive
molecules, which makes developing novel methodologies for
the preparation of C5-alkylated oxazoles very desirable.
During the past few decades, academic community has wit-
nessed tremendous progress in palladium-catalysed cross-
coupling reactions via C–H bond activation,4 which has been
becoming an exceedingly valuable process of great significance
in modern organic synthesis. In light of the advances in this
realm, the direct C5-functionalization of oxazoles via palla-
dium-catalysed C–H bond activation has been reported by
Department of Pharmaceutical and Biological Engineering,
School of Chemical Engineering, Sichuan University, Chengdu 610065, China.
E-mail: liziyuan@scu.edu.cn, mlfang11@scu.edu.cn
†Electronic supplementary information (ESI) available: Detailed experimental
procedures, and 1H and 13C NMR data and spectra. See DOI: 10.1039/
c7ob01083d
Scheme 1 Pd-Catalysed alkylation of arenes via C(sp2)–H/C(sp3)–B
cleavage.
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ation of several heteroarenes including indole,11 thiophene12 toluene, affording 3a in 56% or 59% yield, respectively.
and quinoline13 with alkylboronic acids via C(sp2)–H/C(sp3)–B Subsequently, combinations of PhEt and AcOH with different
cleavage has been systematically studied (Scheme 1b). ratios were explored (entries 11–13). To our delight, in a com-
Likewise, an alkyl halide has also been reported to be used as bined solvent of PhEt and AcOH with a ratio of 1 : 1 (v/v), the
an alternative alkyl donor for the alkylation of heteroarenes.14 yield of 3a was considerably elevated to 80% (entry 13), and
Despite much progress in the Pd-catalysed C–H alkylation of increasing the loading of DDQ could not further improve this
arenes has been witnessed, the direct C5-alkylation of oxazoles yield (entry 14). The conversion of substrate 1a was remarkably
via C(sp2)–H bond activation has not been reported until now.
impeded when replacing the additive DDQ by traditionally
Herein, we would like to disclose a novel C(sp2)–H alkyl- used BQ or adding no quinone additive (entries 15 and 16),
ation at the C5-position of oxazoles with alkylboronic acids as giving the product 3a in dramatically declined yields, which
alkyl donors through palladium catalysis (Scheme 1c). It is demonstrated that, compared to BQ, DDQ indeed serves as a
noteworthy that DDQ (2,3-dichloro-5,6-dicyano-1,4-benzo- better promoter to facilitate this alkylation of oxazoles.
quinone), rather than the traditional additive BQ (1,4-benzo- Likewise, decreasing the loading of the oxidant AgOAc or the
quinone) that has been commonly used as a key promoter in boronic acid 2a was detrimental to the yield of 3a (entries 17
this reaction, was employed as a more effective additive which and 18). Therefore, the reaction conditions of entry 13 were
remarkably promoted this alkylation. To the best of our knowl- selected for the following investigations on the scope of sub-
edge, this is the first protocol on the direct C5-alkylation of strates and boronic acids.
oxazoles through transition-metal-catalysed C(sp2)–H bond
activation.
With the optimized reaction conditions established, various
oxazole-4-carboxylates with diverse substitutions on the C2-
Our study commenced with optimizing the suitable reac- phenyl group were first tested as shown in Table 2 (entries
tion conditions by using methyl-2-phenyloxazole-4-carboxylate 1–11). Generally, electron-rich substrates afforded the corres-
(1a) as the model substrate which was coupled with n-butyl- ponding products 3a–3e in good yields, while the electron-
boronic acid (2a) (Table 1). Initial investigation on three com- deficient trifluoromethyl substrate 1e gave product 3e in a rela-
monly used palladium catalysts in toluene suggested that tively lower yield. Unlike our previous reports on the C5-coup-
Pd(OAc)2 was the best catalyst (entries 1–3), and adding ling reactions of oxazoles,6,7 in which a steric hindrance
0.5 equivalents of DDQ which was rarely employed in C(sp2)–H showed some adverse influences on the yields, in this alkyl-
alkylation via C(sp3)–B cleavage increased the yield of the ation the yields were not severely encumbered with the steric
desired C5-butylated product 3a to 39% (entry 4). Then the hindrance, since the yields of more sterically hindered meta-
screening of solvents (entries 5–10) revealed that either ethyl- or ortho-substituted products 3c and 3e were comparable to
benzene (PhEt) or acetic acid (AcOH) was a better solvent than the yields of less sterically hindered para-substituted products
Table 1 Optimization of the reaction conditions for the Pd-catalysed C5-alkylation of oxazolesa
Entry
Catalyst (10 mol%)
Additive (equiv.)
Solvent (2 mL)
Yieldb (%)
1
2
3
4
5
6
7
8
Pd(OAc)2
PdCl2
PdTFA2
—
—
—
Toluene
Toluene
Toluene
Toluene
Ethylbenzene (PhEt)
Xylene
4-Isopropyltoluene
Isopropylbenzene
DMA
32
27
26
39
56
40
34
54
23
59
64
65
80
72
45
43
63
68
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.5)
DDQ (0.8)
BQ (0.5)
—
9
10
11
12
13
14
15
16
17 d
18 e
AcOH
PhEt/AcOH (2 : 1c)
PhEt/AcOH (1 : 2c)
PhEt/AcOH (1 : 1c)
PhEt/AcOH (1 : 1c)
PhEt/AcOH (1 : 1c)
PhEt/AcOH (1 : 1c)
PhEt/AcOH (1 : 1c)
PhEt/AcOH (1 : 1c)
DDQ (0.5)
DDQ (0.5)
a The reaction was performed with 1a (0.3 mmol), 2a (1.2 mmol), palladium catalyst (0.03 mmol), AgOAc (1.2 mmol), quinone additive and the
solvent (2 mL) at 120 °C for 24 hours. b Isolated yields based on oxazole 1a after column chromatography on silica gel. c v/v. d The loading of
AgOAc was decreased to 0.9 mmol. e The loading of 2a was decreased to 0.9 mmol.
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Table 2 Scope of oxazoles with the optimized reaction conditionsa
Table 3 Scope of alkylboronic acids with the optimized reaction
conditionsa
Entry R1 =
R2 =
1/3
Yieldb (%)
Entry
1
Alkylboronic acids (2)
3
Yieldb (%)
70
1
2
3
4
Phenyl
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
1a/3a
1b/3b
1c/3c
1d/3d
1e/3e
1f/3f
1g/3g
1h/3h
1i/3i
1j/3j
1k/3k
1l/3l
1m/3m 70
1n/3n
80
81
75
71
74
60
75
68
72
69
62
65
3r
4-Methylphenyl
2-Methylphenyl
4-Methoxylphenyl
3-Methoxylphenyl
2
3
4
5
3s
3t
73
77
68
72
5
6
7
8
9
10
11
12
13
14
15
16
17
4-Trifluoromethylphenyl CO2Me
4-Fluorophenyl
3-Fluorophenyl
2-Fluorophenyl
4-Chlorophenyl
4-Bromophenyl
nPropyl
Phenyl
Phenyl
Phenyl
Phenyl
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CONHEt
CONHnBu
3u
3v
6
7
3w
3x
64
83
69
63
45
78
CONHcycHex 1o/3o
CONEt2
Phenyl
1p/3p
1q/3q
Phenyl
8
3y
40
a The reaction was performed with 1a–1q (0.3 mmol), 2a (1.2 mmol),
Pd(OAc)2 (0.03 mmol, 10 mol%), AgOAc (1.2 mmol, 4 equiv.), DDQ
(0.15 mmol, 0.5 equiv.) and PhEt/AcOH (1 mL/1 mL) at 120 °C for
24 hours. b Isolated yields based on oxazole 1 after column chromato-
graphy on silica gel.
a The reaction was performed with 1a (0.3 mmol), 2b–2i (1.2 mmol),
Pd(OAc)2 (0.03 mmol, 10 mol%), AgOAc (1.2 mmol, 4 equiv.), DDQ
(0.15 mmol, 0.5 equiv.) and PhEt/AcOH (1 mL/1 mL) at 120 °C for
24 hours. b Isolated yields based on oxazole 1a after column chromato-
graphy on silica gel.
3b and 3d. It is notable that C–halogen bonds such as C–Cl
and C–Br were well-tolerated in this alkylation (3j, 3k), which
might provide opportunities for further functionalization
through oxidative addition of transition metals to the C–X
bond. Besides 2-phenyloxazole-4-carboxylates, a 2-alkyl sub-
strate 1l was explored under the optimized conditions, and the
alkylated product 3l was acquired in a slightly declined yield
(entry 12). Several 2-phenyloxazole-4-formamides were also
tested (entries 13–16). The results showed that
N-monosubstituted formamides gave the corresponding C5-
alkylated formamides 3m–3o in higher yields than the N,N-di-
substituted formamide 3p. In addition, 2,4-diphenyloxazole
(1q) could provide alkylated product 3q in a competent yield
compared to oxazole-4-carboxylic derivatives (entry 17), indicat-
ing that, unlike our previous studies on other C–H functional-
izations at the C5-position of azole,6 a strong electron-with-
drawing group at the C4-position might not be prerequisite to
proceed this reaction.
According to our previously proposed mechanisms for C–C
cross-coupling reactions via palladium-catalysed C(5)–H acti-
vation,6,7 as well as the proposed pathway for the alkylation of
benzenes8 and other heteroarenes,11,12 a plausible mechanism
is given in Scheme 2 for the C5-alkylation of oxazoles via Pd-
catalysed C(sp2)–H/C(sp2)–B cleavage. Pd(OAc)2 first attacks the
oxazole substrate 1 at the C5-position and the C(sp2)–H bond
activation process affords the palladated intermediate A, fol-
lowed by transmetallation of the intermediate A with alkylboro-
nic acid 2 which generates intermediate B.11,12 Then inter-
mediate B undergoes a reductive elimination step, providing
Then the scope of the optimized conditions was expanded
to a variety of alkylboronic acids and the results are summar-
ized in Table 3. Primary alkylboronic acids 2b–2g afforded the
corresponding products in good yields, although the yields of
the hexylated product 3r and the isobutylated product 3t with
a higher steric hindrance declined slightly. With regard to sec-
ondary alkylboronic acids, isopropylboronic acid 2h provided
the corresponding product 3u in a competent yield, while the
yield of 3v generated from 2-cyclohexylboronic acid 2i dramati-
cally dropped, only giving C5-cyclohexylated product 3v in a
moderate yield.
Scheme 2 Plausible reaction mechanism for C(5)–H alkylation of
oxazoles.
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catalyst to close the catalytic cycle. Compared to previously
used BQ, the additive DDQ serves as a better promoter in this
reaction to regenerate the Pd(II) catalyst.
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Conclusions
In conclusion, we have developed a concise and efficient Pd(II)-
catalysed C5-alkylation of oxazoles with alkylboronic acids via
C(sp2)–H/C(sp3)–B bond cleavage in satisfactory yields. This
protocol represents the first alkylation reaction at the C5-posi-
tion of oxazoles via C(sp2)–H bond activation, and serves as a
novel and alternative route for the preparation of C5-alkylated
oxazole-based bioactive molecules, especially for the late-stage
functionalization15 of oxazole-containing natural products.
Further understanding of the reaction mechanism including
the role of the key promoter DDQ, as well as the applications
of this protocol to other synthetically significant substrates,
are under way in our laboratory.
Acknowledgements
This work is supported by the Fundamental Research Funds
for the Central Universities (2016SCU11020). Prof. Hequan Yao
and Dr Yue Huang from China Pharmaceutical University are
highly acknowledged for the assistance with the HRMS
analysis.
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