Broadening the catalyst and reaction scope of regio- and chemoselective C-H oxygenation: A convenient and scalable approach to 2-acylphenols by intriguing Rh(ii) and Ru(ii) catalysis

Article abstract of DOI:10.1039/c3ob27457h

A unique Rh(ii) and Ru(ii) catalyzed C-H oxygenation of aryl ketones and other arenes has been developed for the facile synthesis of diverse functionalized phenols. The reaction demonstrates excellent reactivity, regio- and chemoselectivity, good functional group compatibility and high yields. The practicality of this method has been proved by gram-scale synthesis of a few different 2-acylphenols. Its utility has been well exemplified in further applications in heterocycle synthesis and direct modifications of drug Fenofibrate.

Full text of DOI:10.1039/c3ob27457h

Organic &
Biomolecular Chemistry
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Broadening the catalyst and reaction scope of regio-
and chemoselective C–H oxygenation: a convenient
and scalable approach to 2-acylphenols by intriguing
Rh(II) and Ru(II) catalysis†
Cite this: Org. Biomol. Chem., 2013, 11,
2318
Gang Shan,‡a Xuesong Han,‡a,b Yun Lin,a Shanyou Yua and Yu Rao*a
A unique Rh(II) and Ru(II) catalyzed C–H oxygenation of aryl ketones and other arenes has been deve-
loped for the facile synthesis of diverse functionalized phenols. The reaction demonstrates excellent reac-
tivity, regio- and chemoselectivity, good functional group compatibility and high yields. The practicality of
this method has been proved by gram-scale synthesis of a few different 2-acylphenols. Its utility has been
well exemplified in further applications in heterocycle synthesis and direct modifications of drug
Fenofibrate.
Received 19th December 2012,
Accepted 7th February 2013
DOI: 10.1039/c3ob27457h
www.rsc.org/obc
greatly boost and improve the practicality and generality of this
chemistry. Therefore, the following two queries arose for us:
Introduction
Hydroxylated arenes are highly valuable chemicals related to (1) Is the ruthenium-catalyzed C–H oxygenation reaction is
pharmaceuticals, agrochemicals and polymers in industry.1 In applicable for different types of aryl ketones?9 The answer to
the last decade, direct functionalization of SP2 C–H bonds by this question has a multi-fold meaning. Firstly, although the
transition-metal2 emerged as a powerful method for generat- use of a ketone group as the directing group by Ru catalysts is
ing new C–O3 bonds. Although significant progress has been well known, the reaction only afforded the C–C coupling pro-
made in this field, there are still many difficulties and chal- ducts.10 By contrast, ketone directed C–O bond formation by
lenges for the direct C–H oxygenation in terms of catalyst ruthenium is unknown. Secondly, the potential C–O coupling
scope, reaction scope, regio- and chemoselectivity and practi- reaction of aryl ketones will afford 2-acylphenols which are an
cality of prevalent protocols. To date, the vast majority of important structural motif of a diversity of drugs and natural
research have been focused on the employment of transition products. Besides serving as useful synthetic intermediates for
metal Pd4 catalysts. In contrast, other metals, like Ru5 and preparation of a number of heterocycles, such as benzofuran-
Rh,6 are barely documented in this type of transformation. one, benzoxazole and dibenzoxazepine, etc., 2-acylphenols
Compared with palladium, ruthenium complexes are signifi- were also utilized as key building blocks for therapeutic agents
cantly less expensive and rhodium catalysts often show its dis- like Propafenone and Celiprolol. Thirdly, as aromatic ketones
tinct selectivity and efficiency in different types of are frequent scaffolds in drugs and synthetic intermediates for
transformations.
pharmaceuticals and natural products, this method potentially
Recently, our group have reported the first example of Ru(II) can provide a rapid access to hydroxylated analogues of a
catalyzed ortho-hydroxylation of benzoates.7,8 In our continu- broad range of substrates for early drug discovery. (2) Besides
ous studies of developing new general tools for functionalized ruthenium, we were curious if rhodium could efficiently
phenol synthesis, we try to further expand both substrate and promote this reaction as well. As rhodium has never been
catalyst scopes for this C–H oxygenation reaction, which would reported in this type of C–H oxygenation reaction, we are par-
ticularly interested in the possible new catalytic activity of
rhodium catalysts and its substrate scope. Moreover, expand-
ing the catalyst and reaction scope will provide more choice
and better control for its synthetic applications.
Accordingly, based on our experience and understanding of
this chemistry, we envisioned that ruthenium and rhodium
catalysts, under certain acidic conditions, could promote C–H
bond activation via an orthometalation process through weak
aDepartment of Pharmacology and Pharmaceutical Sciences, School of Medicine and
School of Life Sciences, Tsinghua University, Beijing, 100084, China.
E-mail: yrao@mail.tsinghua.edu.cn; Fax: (+86)10-62783404
bShenyang Pharmaceutical University, Shenyang, 110016, China
†Electronic supplementary information (ESI)
10.1039/c3ob27457h
available. See DOI:
‡These authors contributed equally.
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which showed similar catalytic efficiency as [RuCl2(p-
cymene)]2. In contrast, either none or only trace amount of
product were formed, if RuCl3 and RuO2 were employed under
the same conditions. In parallel, various rhodium metals were
also applied to probe their potential catalytic activities. Sur-
prisingly and intriguingly, when Rh(OAc)2 was applied to the
reaction, desired phenol 2 was obtained with a yield of 53%
(entry 14). However, no reaction occurred at all with Rh(III) and
Rh(I) catalysts (entries 16 and 17), even with elongated reaction
times. This initial result suggests that Ru(II) and Rh(II) cata-
lyzed C–H oxygenation with the carbonyl oxygen of aryl
ketones as effective coordinating group could occur under
proper acidic conditions. Encouraged by the preliminary
results, we set out to optimize the reaction conditions. After
extensive testing, we found that the ratio of TFA–TFAA around
7 : 3 is most suitable for the reaction. There are only slight
differences with the addition of ligands such as pyridine in
terms of reaction rates and conversion ratios. In general, the
reaction will proceed to completion with 2.5% equiv. of
[RuCl2(p-cymene)]2 or Rh(OAc)2 and 2.0 equiv. of oxidants
Scheme 1 Developing a new approach for C–H oxygenation of diverse arenes
with Rh and Ru catalysts.
coordination with carbonyl oxygen of aryl ketones. Sub-
sequently, a C–O bond formation is possible via the reductive
elimination to afford corresponding phenols with suitable
acids and oxidants11 (Scheme 1). Herein, we report a novel of
Rh(II) and Ru(II) catalyzed regio- and chemoselective C–H oxy-
genation of various arenes including aryl ketones, benzoates
and benzamides, and its convenient applications in hetero-
cycle synthesis and direct modification of drugs.
Results and discussion
To test our hypothesis, a model study was initiated with aro- (K2S2O8 or PhI(OAc)2) in the presence of TFA–TFAA (7 : 3/v : v)
matic ketone 1 in the presence of various ruthenium catalysts within 3–4 hours at 80 °C (Table 1).
in TFA/TFAA solvent system with commercially available
K2S2O8
With the optimized conditions in hand, we next began to
as the terminal oxidant. To our delight, the for- survey this new reaction scope with both Ru(II) and Rh(II) cata-
12
mation of phenol product 2 was observed in 57% yield with lysts. As showed in Table 2, a variety of aromatic ketone sub-
K2S2O8 in the presence of [RuCl2(p-cymene)]2 after stirring for strates were efficiently transformed into the desired phenol
2 h at 80 °C (entry 1). A variety of other oxidants were also products not only in moderate to excellent yields, but also with
examined in the reaction, such as PhI(OAc)2, KIO4, NaIO4, excellent regio- and chemoselectivity. Compared to Rh(OAc)2,
selectfluor13 and Na2S2O8, etc. Among them, selectfluor and the overall catalytic activity of [RuCl2(p-cymene)]2 is better in
PhI(OAc)2 was noted to have a similar capability as K2S2O8 terms of efficiency and both catalysts share a similar selecti-
(entries 2 and 9). Other Ru(II) catalysts were attempted in the vity. [Yields of substrates with Rh(OAc)2 are shown in brackets.]
reaction as well, such as RuCl2(PPh)3 and Ru(CO)HCl(PPh)3, We found that the scope of the substituents was very broad.
Table 1 Optimization of the reaction conditions
Entry
Catalyst
Oxidantc
Conditions
Yielda (%)
1
2
3
4
5
6
7
8
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
RuCl3
RuO2
Ru3(CO)12
Dichlorotris(1,10-phenanthroline)ruthenium(II) hydrate
Ru(CO)ClH(PPh3)3
RuCl2(PPh3)3
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
Rh(OAc)2
K2S2O8
TFA/TFAA (1.4 : 0.6), 80 °C, 2 h
TFA/TFAA (1.4 : 0.6), 80 °C, 2 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA (2.0 : 0), 80 °C, 2 h
TFA/TFAA (1.4 : 0.6), 80 °C, 3 h
TFA/TFAA(1.0 : 1.0), 80 °C, 2 h
TFA/TFAA (0.5 : 1.5), 80 °C, 2 h
TFA/TFAA (0 : 2.0), 80 °C, 2 h
TFA/TFAA (1.8 : 0.2), 80 °C, 17 h
TFA/TFAA (1.8 : 0.2), 80 °C, 17 h
TFA/TFAA (1.8 : 0.2), 80 °C, 17 h
TFA/TFAA (1.8 : 0.2), 80 °C, 17 h
57%
41%
3%
NR
5%
Selectfluor
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
K2S2O8
NR
38%
40%
51%
72%b
46%
35%
34%
53%b
26%
NR
9
10
11
12
13
14
15
16
17
Rh(OAc)2
RhCl(PPh3)3
[(RhCp*Cl2)2]
Selectfluord
Selectfluor
Selectfluor
NR
a Conversion ratio. b Isolated yield. c 2.0 equiv. d 1.5 equiv.
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Table 2 Ruthenium and rhodium catalyzed regio- and chemoselective
C–H oxygenation of aryl ketonesa,b
Table 3 Representative ketone, benzoate and benzamide as substrates
under rhodium(II) catalysisa
a Isolated yield. b Unoptimized.
Scheme 2 Gram-scale synthesis of 2-acylphenols under ruthenium(II) catalysis.
aIsolated yield. bStarting material.
a Isolated yield. b Ada: 1-adamantyl; t-Bu: tert-butyl.
1% [RuCl2(p-cymene)]2 catalyst loading (Scheme 2). Interest-
ingly, besides the mono-hydroxylation product, a considerable
amount of the di-hydroxylation product, 10b, 2b and 26b, was
formed as well, which could be readily transformed into
1-hydroxyxanthone or 1-chloroxanthone under different con-
ditions. In comparison with conventional approaches requir-
ing air-/moisture-proof conditions, the current method is
advantageous for rapid access to these types of molecules
because of its operational simplicity and broad availability of
starting materials.
As shown in Scheme 3, to demonstrate the synthetic utility
of this C–H oxygenation reaction, a variety of biologically
important O-containing heterocyclic compounds were pre-
pared from 2-acylphenols. Not only benzofuran 36 and cou-
marin 37,14 but also demanding xanthone derivative 39 can be
The strong electron withdrawing and electron donating func-
tional groups (nitro, halides, ester, amide, methoxyl and alkyl
etc.), as well as various ortho, meta and para-substituted aryl
groups were well tolerated. It was noticed that the more elec-
tron-rich parts of unsymmetrical diaryl ketones were hydroxy-
lated prior to less electron-rich parts (7, 9, 16). Quite
impressively, in a few cases, synthetically challenging di-
hydroxylated products (5, 9, 11, 25, 26) can be obtained as
major products. Additionally, to our delight, remarkable
chemo- and regioselectivity can be achieved with aryl ketone
substrates (23–33), which contain reactive alpha-protons. It is
notable because these phenols are difficult-to-prepare products
or usually need tedious steps to prepare using known
methods. It is noteworthy that this protocol was conducted
without the need for air- or moisture-proof conditions.
Subsequently, the reaction conditions with Rh(OAc)2 as the
catalyst were applied to other substrates including benzoates,
benzamides and phenylacetates. As displayed in Table 3, we
are very pleased to find that these three types of molecules
were successfully transformed into the corresponding phenol
products in modest to good yields. To the best of our know-
ledge, this represents the first example of Rh(II) catalyzed regio-
and chemoselective C–H oxygenation of these substrates.
To further prove both practicality and effectiveness of this
method in the synthesis, we prepared 2-acylphenol 2, 10 and
26 on a gram scale under the optimized conditions with only
Scheme 3 Synthetic applicability of 2-acylphenols. Conditions: (a) 2-bromo
acetophenone, K2CO3, MeCN, reflux, 16 h, 95%; (b) K2CO3, acetone, reflux, 4 h.
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synthesis of a variety of functionalized phenols from easily
accessible arenes. The reaction demonstrates excellent reacti-
vity, ortho-selectivity, good functional group tolerance and high
yields. The practicality of this new method was tested by a few
gram-scale syntheses of 2-acylphenols. Its utility has been well
exemplified in further applications in heterocycle synthesis
and direct modification of drug Fenofibrate. By employing a
combination of Rh(II) and Ru(II) catalysts, oxidants and
TFA/TFAA, we have discovered a highly efficient catalyst system
for this unique regio- and chemoselective C–H oxygenation
reaction. Further studies into the scope and the mechanism of
this reaction are in progress in our laboratory.
Scheme 4 Direct C–H oxygenation of drug Fenofibrate. aIsolated yield.
Acknowledgements
Scheme 5 Kinetic isotope effect studies.
This work was supported by the national ‘973’ grant (grant #
2011CB965300), NSFC grant (grant # 21142008) and Tsinghua
University 985 Phase II funds and Tsinghua University Initia-
tive Scientific Research Program.
readily obtained from corresponding the 2-acylphenols using
known transformations.
Direct modification of drugs or drug candidates via C–H
activation is a particularly attractive approach for modern drug
discovery, which highly depends on the practicality of such
chemical processes. Therefore, in Scheme 4, we show the feasi-
bility of our new reaction by performing direct C–H oxygen-
ation of Fenofibrate, a prescription drug commonly used to
treat unhealthy triglyceride and cholesterol levels. Pleasingly,
both mono- and di-hydroxylated products (41a, 41b) were
found in a total yield of 54%. This method can provide a new
approach to prepare novel analogues of a broad range of drugs
which contain aromatic ketone scaffolds.
Further investigations were performed to gain some insight
of the reaction mechanism, which is illustrated in Scheme 5.
The consistent and significant KIE values were observed
from both Rh(II)-catalyzed (kH/kD = 2.1) and Ru(II)-catalyzed
(kH/kD = 2.7) isotope effect studies, which indicated that the
C–H bond cleavage step might be involved in the rate-limiting
step of this transformation.
Although details about the mechanism remain unclear, on
the basis of our studies, a plausible mechanism for this reac-
tion is demonstrated as followings. Step (i) involves chelation
of Rh(II)/Ru(II) to carbonyl oxygen atom from the ketone sub-
strates and the following chelate-directed C–H activation of the
substrate could afford a five-membered cyclometalate(II) inter-
mediate. In the second step (ii), Rh(II)/Ru(II) was oxidized into
possible Rh(IV)/Ru(IV) intermediate. The final step (iii) involves
carbon–oxygen bond-forming reductive elimination to afford
the trifluoroacetated product and turned Rh(IV)/Ru(IV) back
into Ru(II)/Rh(II). The trifluoroacetated product was converted
to hydroxylated ketones after the later aqueous workup.15
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