Combining rhodium and photoredox catalysis for C-H functionalizations of arenes: Oxidative heck reactions with visible light

Article abstract of DOI:10.1002/anie.201400560

Direct, oxidative metal-catalyzed C-H functionalizations of arenes are important in synthetic organic chemistry. Often, (over-)stoichoimetric amounts of organic or inorganic oxidants have to be used in these reactions. The combination of rhodium and photo

Full text of DOI:10.1002/anie.201400560

Angewandte
Chemie
DOI: 10.1002/anie.201400560
À
C H Functionalization
À
Combining Rhodium and Photoredox Catalysis for C H Functionali-
zations of Arenes: Oxidative Heck Reactions with Visible Light**
David C. Fabry, Jochen Zoller, Sadiya Raja, and Magnus Rueping*
À
Abstract: Direct, oxidative metal-catalyzed C H functionali-
zations of arenes are important in synthetic organic chemistry.
Often, (over-)stoichoimetric amounts of organic or inorganic
oxidants have to be used in these reactions. The combination of
rhodium and photoredox catalysis with visible light allows the
though improved procedures using internal oxidants have
been reported in the last few years, they are still dependent
upon specific functional-directing moieties which limit the
general applicability to the synthesis of complex substrate
classes.[10] On the basis of our studies on photoredox
catalysis[11] we questioned to what extent the recyclization
of the metal catalyst in the oxidative olefination reaction can
be successfully accomplished by a photoredox-based process
(Scheme 1).[12,13]
À
direct C H olefination of arenes. Small amounts (1 mol%) of
a photoredox catalyst resulted in the efficient C H function-
alization of a broad range of substrates under mild conditions.
À
À
T
he concept of C H activation and functionalization has
proven to be a powerful tool in organic chemistry in recent
years.[1] Metal-catalyzed C H functionalization has also been
À
established as a reliable method for the construction of
complex natural products.[2] In this important field of C H
À
functionalization, it is the oxidative Heck reaction that has
gained prominent attention, since a variety of metals, such as
palladium, rhodium, and ruthenium, allow the introduction of
synthetically useful building blocks.[3] In this context, Miura
and co-workers reported the first Rh-catalyzed ortho-mono-
and bisolefination of phenylpyrazoles with alkenes.[1a,o,p,4,5]
The use of Cu(OAc)2 led to regeneration of the rhodium
catalyst. Glorius and co-workers described a rhodium-cata-
lyzed olefination and vinylation of arenes by using a directing
acetamide motif and Cu(OAc)2 as the oxidant.[6,7] Liu and co-
workers,[7a] as well as Feng and Loh,[7b] showed that this
catalyst system can also be transferred to phenyl carbamates
by using overstoichiometric amounts of Cu(OAc)2. As
independently shown by several research groups, the oxida-
tive Heck reaction can also be conducted with other substrate
classes by using Rh[6a,7c,d] or Ru catalysts[8,9] in combination
with different amounts of Cu(OAc)2.
Scheme 1. Working hypothesis for the combined rhodium- and photo-
À
redox-catalyzed C H functionalization. Cp*=C5Me5.
Herein, we report the first combination of photoredox and
À
rhodium catalysis for the direct, oxidative C H olefination of
aryl amides using visible light.
Nevertheless, the necessity of directing groups and
relatively harsh reaction conditions still limit the sustainabil-
ity of this method, and the dependency on stoichiometric
amounts of an external oxidant, in particular, poses a major
drawback of these olefination reactions. In these procedures,
the intermediary metal–hydride complex that is formed
through b-H elimination is reoxidized by an oxidant. Even
Initial studies using the photoredox catalyst [Ru(bpy)3]-
À
(PF6)2 in the rhodium-catalyzed C H functionalization of
amide 1a with acceptor 2a showed a significant yield (65%)
of the olefinated product 3a after 16 h (Table 1, entry 7).[14,15]
It must be highlighted that not only is 1 mol% photoredox
catalyst sufficient for the described reaction, but the usual
high reaction temperatures could also be lowered from 1208C
to a moderate 808C. A subsequent solvent screening revealed
that chlorinated solvents led to conversion, and that chloro-
benzene was the best solvent, affording the product in 89%
yield (Table 1, entry 8).
[*] M. Sc. D. C. Fabry, Dipl.-Chem. J. Zoller, Dr. S. Raja,
Prof. Dr. M. Rueping
Institut fꢀr Organische Chemie, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
E-mail: magnus.rueping@rwth-aachen.de
To gain further insight into the role of the photoredox
catalyst, the standard reaction was performed in the absence
of either the photoredox catalyst or light (Table 1, entries 11
and 12). In the absence of light, only traces of product could
be detected, whereas no conversion could be observed if no
photoredox catalyst was present during the reaction. The
photocatalytic step, is therefore, indispensable for the whole
process to occur.
[**] The research leading to these results has received funding from the
European Research Council under the European Union’s Seventh
Framework Programme (FP/2007-2013) / ERC Grant Agreement no.
617044 (SunCatChem).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400560.
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Table 1: Optimization of reaction conditions and control experiments[a]
Table 2: Optimization of the catalyst system.[a]
Entry Solvent
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
PhMe
MeCN
DMF
DMSO
EtOH
tBuOH
dichloroethane
PhCl
PhCl, 8 h
PhCl, 4 h
0
0
0
0
0
<5
65
89
79
54
11
12
13
PhCl, no photocatalyst, 16 h
PhCl, no light, 16 h
PhCl, inert atmosphere, 16 h, photocatalyst
(1 mol%)
0
<3
<3
14
15
16
17
PhCl, inert atmosphere, 16 h, photocatalyst (1 equiv) 48
[D5]PhCl, 16 h
[D5]PhCl, 8 h
[D5]PhCl, 4 h
81
72
58
[a] Reaction conditions: 0.1 mmol aryl amide 1a, 2 equiv acrylate 2a,
1 mL solvent, irradiation at a distance of 3 cm; reaction vessels were
wrapped with aluminum for reactions in the dark. [b] Yield after
purification by column chromatography.
To shed light on the role of the photoredox catalyst,
various catalysts with different properties were examined.
First, the coordination ability of the counterion was varied.
Changing from weakly coordinating PF6 to the more strongly
coordinating Cl anions did not have a significant impact on
the yield, whereas the electronic character of the ligands had
a substantial influence on the reaction (Table 2, row 1).
Increasing the electron density through sequential introduc-
tion of tBu groups on the bipyridine backbone led to a drop in
the yields from 89 to only 14%. Changing rhodium to iridium
also led to complete suppression of the conversion,[16]
irrespective of whether electron-rich or -poor ligands were
used (Table 2, rows 2 and 3 (left)). This finding indicated that
no radical oxo species[17] are directly involved in the rhodium–
hydride oxidation, since both [Ru(bpy)3]2+ and the corre-
sponding iridium complex are known to generate reactive
oxygen intermediates.
This observation suggested that the reduction potential of
the photoredox catalyst may be the crucial parameter in
guaranteeing a successful recycling. Furthermore, no con-
version was observed when stoichiometric amounts of either
eosin Y or titanium dioxide were applied as the photoredox
catalysts (Table 2, row 3 (right)). However, the following
experiments were of greater importance: When the reaction
was carried out using 1 mol% photoredox catalyst in an inert
atmosphere, a negligible yield of 3% was obtained, which
indicates no catalyst turnover occurs. In contrast, conducting
the same reaction with stoichiometric amounts of photoredox
catalyst led to a 48% yield of the product 3a, thus
[a] Reaction conditions: 0.1 mmol aryl amide 1a, 2 equiv acrylate 2a,
1 mL chlorobenzene, irradiation at a distance of 3 cm. Yield after
purification by column chromatography.
demonstrating that the photoredox catalyst is able to recycle
the Rh catalyst (Table 1, entries 13 and 14).
These findings reveal that the reduction potential of the
photoredox catalyst is the crucial parameter in the recycling
of the active rhodium catalyst. However, this also means that
oxygen participates in the recycling of the rhodium catalyst if
the reaction is performed with 1 mol% photoredox catalyst,
with hydrogen peroxide generated as a by-product.
To gain further insight into the mechanism, the reaction
was conducted in chlorobenzene and in deuterated chloro-
benzene, since the solubility and reactivity of any potentially
1
involved singlet oxygen O2 should have an impact.[18] The
yields in both cases were almost identical (Table 1, entries 8–
10 and 15–17), which indicates that singlet oxygen does not
have a direct impact on the reaction outcome. Glorius and co-
workers reported the use of halogenated arenes as stoichio-
metric oxidants.[19] Thus, we employed deuterated chloroben-
zene in the reaction. However, no formation of pentadeute-
rium benzene was detected, which led to the conclusion that
chlorobenzene does not act as an oxidant. Moreover, no
incorporation of deuterium into product 3a could be
observed.
2
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To exclude the possibility that the N-methoxy group of the
Weinreb amide acts as internal oxidant,[10b] dialkylamide 1j
was applied in the dual catalysis (Scheme 2). Product 3j was
obtained in a yield of 91%, thus demonstrating that an
internal oxidant can be excluded and the rhodium catalyst
regeneration can be exclusively attributed to the photoredox
catalyst. This also shows that the carbonyl oxygen atom or the
amide nitrogen atom acts as a directing group to stabilize the
Scheme 2. Control reactions to exclude internal oxidants.
Table 3: Substrate scope of the aryl amide.[a]
À
Rh complex and to enable C H insertion.
With the optimized reaction conditions and a general
understanding of the mechanism in hand, the substrate scope
of the reaction was investigated. Electron-poor aryl amides
could be successfully converted in very good yields (Table 3,
3b 84%). Good yields could also be obtained with electron-
poor arenes (Table 3, 3c 63%, 3d 59%). To further prove the
synthetic utility of the dual catalysis, different terminal olefins
were tested, which revealed that the reaction is neither
limited to different acrylates (3e–g) nor to vinyl sulfones (3h)
and silanes (3i) as the electrophile.
As described above (Scheme 2), it was shown that alkyl
amides can be converted in this reaction. Thus, this substrate
class was also examined in terms of their electronic and steric
effects. Similar to the case with Weinreb amides, the reaction
showed a broad acceptance of electron-rich and -poor arenes
(3k–m). Additionally, cyclic amides (3q,r) and hydroxyalkyl
amides (3s) could be transformed in good yields. The latter
also reveals the tolerance of the reaction towards unprotected
hydroxy groups and thereby the attractiveness of the method
for the synthesis of complex compounds. As previously
mentioned for the Weinreb amides, variations of the acrylate
did not have a significant impact on the reaction. Further-
more, the substrate scope could also be extended to secondary
(3t,u) and primary alkyl amides (3n). The products were
obtained in moderate to good yield, which highlights the
general applicability of the method for the ortho olefination
of different amides.
In summary, a dual catalysis concept for the ortho olefi-
nation of aryl amides has been developed and its applicability
to a broad range of substrates was highlighted. The previously
often used (over)stoichiometric amounts of oxidants can be
readily substituted with a photoredox catalyst, which makes
this system both attractive and a more environmentally sound
alternative. Further attempts to apply this new type of
combined catalysis to other substrate classes and metal
catalysts are currently underway.
Received: January 18, 2014
Published online: && &&, &&&&
À
Keywords: C H activation · olefination ·
oxidative Heck reaction · photoredox catalysis ·
rhodium catalysis
.
À
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À
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À
C H Functionalization
D. C. Fabry, J. Zoller, S. Raja,
M. Rueping*
&&&& — &&&&
Combining Rhodium and Photoredox
À
Catalysis for C H Functionali-
zations of Arenes: Oxidative Heck
Reactions with Visible Light
Much milder and environmentally
plex to be catalytically regenerated. A
friendly reaction conditions can be used
for oxidative Heck reactions through the
combined use of rhodium and redox
catalysis. This allows the rhodium com-
broad range of substrates was tolerated
in the reaction and afforded different
amides in good to very good yields.
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