Visible-light-triggered direct benzoyloxylation of electron-rich arenes at room temperature without chelation assistance

Article abstract of DOI:10.1002/ejoc.201201093

A Ru^II photocatalytic method was developed for the direct mono-benzoyloxylation of electron-rich aromatic and heteroaromatic systems even with an excess amount of benzoyl peroxide. The reaction was conducted at room temperature under visible light. The direct Ar_C-H benzoyloxylations occur without the assistance of any directing groups and can also tolerate various functional groups that have already shown diverse reactivities in transition-metal catalysis.

Full text of DOI:10.1002/ejoc.201201093

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201093
Visible-Light-Triggered Direct Benzoyloxylation of Electron-Rich Arenes at
Room Temperature without Chelation Assistance
Honghua Rao,[a][‡] Ping Wang,[a,b] and Chao-Jun Li*[a]
Keywords: Redox chemistry / Directing groups / Photochemistry / Non-chelation assistance / Electron transfer
A RuII photocatalytic method was developed for the direct
mono-benzoyloxylation of electron-rich aromatic and het-
eroaromatic systems even with an excess amount of benzoyl
peroxide. The reaction was conducted at room temperature
under visible light. The direct ArC–H benzoyloxylations occur
without the assistance of any directing groups and can also
tolerate various functional groups that have already shown
diverse reactivities in transition-metal catalysis.
Introduction
Aromatic organic compounds abundant in C–H bonds
are ubiquitous in natural products, pharmaceuticals, fine
chemicals, and functional materials.[1] Functionalization of
these traditionally inert C–H bonds can provide straightfor-
ward strategies and tools for the facile manufacture of com-
pounds with useful molecular architectures and building
blocks in both academia and industry.[2] Over the past dec-
ade, transition-metal-catalyzed direct C–H functionaliza-
tion has emerged as a powerful method in organic synthesis.
Tremendous achievements have culminated in these pion-
eering discoveries, most of which involved substrates con-
taining one or more pre-existing directing groups at rela-
tively elevated temperatures (most of the time above
100 °C).[3] To facilitate potential applications in complex
natural product synthesis, deliberate attempts have been
made to develop mild and operationally simple methods for
direct ArC–H functionalization in the presence of certain
functional groups, such as anilides, arylketones, and α-aryl-
acetamides.[4] However, in contrast to direct ArC–H func-
tionalization assisted by directing groups,[2–5] there are few
examples on non-chelation-assisted ArC–H functionalization
under mild conditions, and therefore, it still remains a very
challenging subject (Scheme 1a).[6]
Scheme 1. Direct ArC–H functionalization with/without directing
groups.
As is well recognized, photoredox catalysis represents a
powerful tool for the activation of molecules in both inor-
ganic and organic chemistry.[7] In particular, RuII–polypyri-
dine complexes are noteworthy as a result of their impres-
sive photoredox properties under visible light at room tem-
perature,[8] and many researchers, including Stephen-
son,[8i,8j,8l,8m,8q] and Yoon,[8h,8s] have already made great
progress in this area. Some achievements on photoredox
functionalization of sp2 C–H bonds have also been dis-
closed in recent years. For example, Deronzier[9] and San-
ford[10] demonstrated the Pschorr reaction with Ru(bpy)3-
Cl2 as the photocatalyst through intra- and intermolecular
ArC–H activation, respectively. Recently, Li’s group ac-
complished an aerobic visible-light-assisted photocatalytic
intramolecular ArC–H activation/cyclization of thioanil-
ides.[11] Most inspiringly, in late 2011, MacMillan disclosed
a photoredox catalyst for the direct trifluoromethylation of
electron-rich aromatic/heteroaromatic C–H bonds at room
temperature to provide products in good to excellent yields
[a] Department of Chemistry, McGill University
Montreal, QC, H3A 0B8, Canada
Fax: +1-514-398-3797
E-mail: cj.li@mcgill.ca
Homepage: http://cjli.mcgill.ca/
[b] Department of Chemistry, Tsinghua University,
100084 Beijing, P. R. China
[‡] Current address: Department of Chemistry, Capital Normal
University,
100048, Beijing, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201093.
Eur. J. Org. Chem. 2012, 6503–6507
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6503

H. Rao, P. Wang, C.-J. Li
SHORT COMMUNICATION
with high regioselectivities. This study can be regarded as Table 1. Optimization of the reaction conditions by using Ru(bpy)3-
Cl2·6H2O as the photocatalyst at room temperature.[a]
the seminal work addressing the challenge of direct ArC–H
functionalization without the assistance of directing groups
under mild conditions (Scheme 1b).[12] Herein, we demon-
strate a ruthenium photoredox catalysis for the direct ben-
zoyloxylation of non-chelation assisted ArC–H bonds at
room temperature (Scheme 1c).[3a,13]
Entry
Solvent
Additive Yield [%][b]
o/p[b]
1
2
3
4
5
6
7
8
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
propionitrile
chlorobenzene
benzene
KOtBu
Cs2CO3
K3PO4
K2HPO4
K2CO3
KHCO3
Na2CO3
NaHCO3
NEt3
12
15
39
42
44
47
44
65
0
0
50
47
34
17
41
trace
21
26
1.9:1
1.3:1
1.6:1
1.5:1
1.6:1
1.8:1
1.9:1
2.9:1
–
Results and Discussion
At the outset of our study on direct ArC–H functionaliza-
tion under mild conditions, we were inspired by the discov-
ery that organic peroxides (ROOR) could undergo peroxide
bond cleavage through a single-electron transfer (SET) pro-
cess to generate the RO· radical and the RO– anion.[14] The
RO· species may be well-suited to undergo addition to the
most electron-rich position of arenes through a photoredox
catalytic process, and the RO– anion may act as a suitable
base for the abstraction of a proton.
9
10
11
12
13
14
15
16
17
18
19
20
21[c]
22[d]
23[e]
24[f]
DBU
–
–
2.5:1
2.2:1
1.7:1
1.2:1
1.3:1
–
1.1:1
2.7:1
2.3:1
0.9:1
–
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
1,4-dioxane
1,2-dimethoxyethane
ether
tetrahydrofuran
ethyl acetate
1,2-dichloroethane
acetonitrile
acetonitrile
acetonitrile
acetonitrile
To test this hypothesis, we treated p-anisole with
(BzO)2 (benzoyl peroxide) in the presence of 2 mol-%
Ru(bpy)3Cl2·6H2O (commercially available) by using acet-
onitrile as the solvent. A 26-W compact fluorescent light
bulb (daylight GE Energy Smart™ with 1600 lumens) was
chosen as the visible-light source other than the 9-W (with
550 lumens) and 13-W (with 860 lumens) bulbs. As screened
in Table 1, initially, KOtBu (sometimes worked well in radi-
cal ArC–H arylations)[15] was first evaluated in the optimiza-
tion process. The benzoyloxylation product was obtained in
12% yield with a 1.9:1 ratio at the ortho and para positions
(Table 1, entry 1) and no decarboxylative byproduct was de-
11
58
trace
trace
56
–
1.8:1
2.4:1
61
[a] Reactions were carried out at room temperature with a 26-W
household bulb, anisole (0.25 mmol), (BzO)2 (0.50 mmol), Ru(bpy)3-
Cl2·6H2O (2 mol-%), solvent (2 mL), additive (0.75 mmol), under
argon, 60 h. [b] Yield determined by 1H NMR spectroscopy with
mesitylene as the internal standard. [c] Without Ru(bpy)3Cl2·6H2O.
[d] Without light. [e] A loading of 4 mol-% Ru(bpy)3Cl2·6H2O was
tected, which may be because of the low reaction tempera- used. [f] Reaction time: 72 h.
ture.[10,16] Encouraged by these results, other inorganic salts
such as cesium carbonate, potassium and sodium salts, as
well as organic bases such as NEt3 and DBU were tested
(Table 1, entries 2–10). Among the inorganic salts, the use
of NaHCO3 as the additive gave the highest yield of the
Hence, the substrate scope for the direct ArC–H benzoyl-
benzoyloxylation product along with the highest ortho/para oxylation of arenes without assistance of directing groups
ratio (65%, 2.9:1; Table 1, entry 8). No transformation was was explored under visible light at room temperature for
observed with organic bases (Table 1, entries 9, 10). Inter- 60 h by using 2 mol-% Ru(bpy)3Cl2·6H2O as the photocata-
estingly, when conducting the reaction in the absence of ad- lyst, NaHCO3 (3 equiv.) as the additive, and acetonitrile
ditives, the desired product was obtained in 50% yield (2 mL) as the solvent. As presented in Table 2, all of the
(Table 1, entry 11). Afterwards, solvents other than acetoni- arene substrates examined showed certain regioselectivities
trile, including propionitrile, aromatic hydrocarbons, ethers, and moderate to good reactivities toward this transforma-
esters, and chlorinated solvents, were examined (Table 1, en- tion. Notably, in the presence of an excess amount of benz-
tries 12–20); however none of these solvents gave better re- oyl peroxide (2 equiv.), we observed exclusive mono-benz-
sults than acetonitrile. Importantly, when the reactions were oyloxylation at the aromatic ring. Intriguingly, MOM
carried out in the absence of Ru(bpy)3Cl2·6H2O or light, (methoxymethyl)-protected 4-tert-butylphenol underwent
only a trace amount of the desired benzoyloxylation prod- direct benzoyloxylation smoothly (Table 2, entry 2). To dis-
uct was detected by GC–MS (Table 1, entries 21, 22), indi- play the regioselectivity of this strategy, simple arenes with
cating that the photocatalyst and visible light are crucial for substituents at various positions on the aromatic ring were
this direct ArC–H benzoyloxylation at room temperature.[17] tested. For 1,3-dimethoxybenzene, benzoyloxylation mainly
Furthermore, the use of a larger amount of the catalyst or occurred at the C4 position (C4/C2 = 2:1), which may be
a longer reaction time did not have a positive effect on this due to steric hindrance at the C2 position (Table 2, entry 3).
transformation (Table 1, entries 23, 24).
As for 2,3-dimethoxytoluene, benzoyloxylation at the C5
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Visible-Light-Triggered Benzoyloxylation of Electron-Rich Arenes
Table 2. Substrate scope for the Ru(bpy)3Cl2·6H2O induced direct position was the only detectable outcome (Table 2, entry 5).
ArC–H benzoyloxylations under visible light at room temperature.[a]
Substrates in which the aromatic ring was substituted with
an alkyl group, such as methyl, exhibited lower reactivities
than substrates substituted with methoxy groups, because
of much weaker electron-donating ability of the methyl
group (Table 2, entries 7, 8). Particularly noteworthy is the
tolerance of a halogen atom and the aldehyde and ketone
groups, which remained intact during this photoredox pro-
cess (Table 2, entries 9–12). Furthermore, methoxy-substi-
tuted naphthalene and quinoline also showed good reactiv-
ities (Table 2, entries 13, 14). When free thiophene was sub-
jected to the reaction conditions, direct benzoyloxylation
took place at C2 exclusively, although a lower yield of the
desired product was delivered (Table 2, entry 15). Lastly, di-
rect modification of O-methylated fluorescein[18] was dem-
onstrated under our optimal conditions, and it was found
to undergo direct benzoyloxylation in 51% yield with a
preference for the C4Ј position (C4Ј/C2Ј
= 2.19:1,
Scheme 2). This result further displays the capability of
drug-like structures to readily participate in our benzoylox-
ylation protocol.
Scheme 2. Direct photoredox benzoyloxylation of fluorescein-
OMe.
For the proposed photoredox catalytic cycle, we assumed
that the SET process would be crucial for this transforma-
tion at room temperature under visible light. As illustrated
in Scheme 3, the initial visible-light excitation of
2+
Ru(bpy)3
3
could generate photoexcited state
[Ru(bpy)32+]* 4,[8,12] which could be oxidized to Ru(bpy)3
3+
5, concurrently with the first SET reduction of (PhCO2)2
–
·
to the PhCO2 anion and the PhCO2 radical.[14] Then, the
·
ensuing PhCO2 radical could add to the most electron-rich
position of the arenes and heteroarenes.[19] Afterwards, re-
sultant radical 6 could participate in a second SET process
with strongly oxidizing photocatalyst Ru(bpy)33+ 5 to afford
reactive cation 7 while regenerating ground-state photocata-
[a] Reactions were carried out with a 26-W household bulb for
60 h, arene (0.25 mmol), (BzO)2 (0.50 mmol), Ru(bpy)3Cl2·6H2O
(2 mol-%), solvent (2 mL), NaHCO3 (0.75 mmol), r.t., under argon.
[b] Isolated yield. [c] The minor regioisomeric position is labeled
with the respective carbon atom number.
–
lyst 3. Finally, the previously formed PhCO2 anion could
abstract a proton from 7 (to form benzoic acid, which could
be defined by GC–MS) to provide the desired benzoyloxyl-
ation product.
Eur. J. Org. Chem. 2012, 6503–6507
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6505

H. Rao, P. Wang, C.-J. Li
SHORT COMMUNICATION
tion (CFI), Natural Sciences and Engineering Research Council
(NSERC) of Canada, Fonds Québécois de la Recherche sur la Na-
ture et les Technologies (FQRNT) Centre for Green Chemistry and
Catalysis, and McGill University for support of our research.
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Scheme 3. Proposed photoredox catalytic cycle for the direct
ArC–H benzoyloxylation at room temperature.
Conclusions
In summary, we have developed a RuII photocatalyst-
based method that allows for the direct benzoyloxylation
of electron-rich aromatic and heteroaromatic systems. This
strategy is conducted at room temperature under visible
light. Remarkably, the direct ArC–H benzoyloxylations oc-
cur with a certain regioselectivity, notably without the as-
sistance of any directing groups to control site selectivity.
Furthermore, it shows good tolerance toward various func-
tional groups such as halogens, aldehydes, ketones, and
quinolines. The success of the direct benzoyloxylation of
fluorescein-OMe demonstrates that this protocol might be
of wide application in synthetic and medicinal chemistry.
Further investigations to broaden the utility of this photo-
catalytic system are ongoing in our laboratory.
Experimental Section
General Procedure: An oven-dried sample vial (15ϫ45 mm) was
equipped with a magnetic stirring bar, Ru(bpy)3Cl2·6H2O (2 mol-
%, 3.75 mg), NaHCO3 (0.75 mmol, 63.0 mg), and (BzO)2
(0.50 mmol, 121.1 mg). The vial was fitted with a septa cap and
degassed by quickly alternating vacuum evacuation and argon
backfill (ϫ3) before anhydrous MeCN (2.0 mL) and the arene/het-
eroarene (0.25 mmol) were added under an atmosphere of argon.
The reaction mixture was stirred under radiation of a 26-W house-
hold fluorescent bulb at room temperature. After 60 h, the resulting
mixture was filtered through a short silica gel pad and washed with
ethyl acetate. The above solution was evaporated under vacuum,
and the residue was purified by silica gel column (hexane/ethyl acet-
ate, 35:1 to 2:1) to give the analytically pure benzoyloxylation prod-
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We are grateful to the Canada Research Chair (Tier 1) and E. B.
Eddy Foundations (to C. J. L.), the Canada Foundation for Innova-
6506
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[19] For an example of the addition of the oxylation radical to an
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Received: August 12, 2012
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Published Online: October 18, 2012
Eur. J. Org. Chem. 2012, 6503–6507
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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