C?O Activation by a Rhodium Bis(N-Heterocyclic Carbene) Catalyst: Aryl Carbamates as Arylating Reagents in Directed C?H Arylation

Article abstract of DOI:10.1002/anie.201610409

Despite recent progress in the catalytic transformation of inert phenol derivatives as alternatives to aryl halides and triflates, attempts at the cross-coupling of inert phenol derivatives with the C?H bonds of arenes have met with limited success. Herein, we report the rhodium-catalyzed cross-coupling of aryl carbamates with arenes bearing a convertible directing group. The key to success is the use of an in situ generated rhodium bis(N-heterocyclic carbene) species as the catalyst, which can promote activation of the inert C(sp²)?O bond in aryl carbamates.

Full text of DOI:10.1002/anie.201610409

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610409
German Edition: DOI: 10.1002/ange.201610409
Homogeneous Catalysis
Hot Paper
À
C O Activation by a Rhodium Bis(N-Heterocyclic Carbene) Catalyst:
Aryl Carbamates as Arylating Reagents in Directed C H Arylation
À
Mamoru Tobisu,* Kosuke Yasui, Yoshinori Aihara, and Naoto Chatani*
Abstract: Despite recent progress in the catalytic transforma-
tion of inert phenol derivatives as alternatives to aryl halides
and triflates, attempts at the cross-coupling of inert phenol
À
derivatives with the C H bonds of arenes have met with limited
success. Herein, we report the rhodium-catalyzed cross-cou-
pling of aryl carbamates with arenes bearing a convertible
directing group. The key to success is the use of an in situ
generated rhodium bis(N-heterocyclic carbene) species as the
2
À
catalyst, which can promote activation of the inert C(sp ) O
bond in aryl carbamates.
M
etal-catalyzed cross-coupling of organometallic nucleo-
philes with aryl halides has been established as the predom-
inant method for the functionalization of aromatic com-
pounds.[1] Recently, phenol and its unactivated derivatives,
such as ethers and esters, have emerged as less expensive and
more environmentally benign alternatives to aryl halides and
triflates.[2] An even more important advantage of using these
phenol derivatives is that their metal-coordinating ability and
robustness allows new synthetic strategies, including late-
stage functionalization and directing-group manipulation.
À
À
Scheme 1. C H/C O cross-coupling.
À
Nickel is the best catalyst to activate C(aryl) O bonds, and
it mediates a range of cross-coupling reactions with inert
Ackermann and Song achieved cross-coupling of non-acidic
phenol derivatives.[2] Considering that C H cross-coupling
unactivated C H bonds with aryl carbamates using a cobalt
À
À
reactions have become increasingly popular methods,[3] it is
catalyst.[7] Although this reaction represents an important
advance, the requirement for the use of excess Grignard
reagent leaves several issues to be addressed: 1) electrophilic
functional groups, such as ketones and nitriles, are not
compatible, and 2) the applicable directing group is limited
to a robust but synthetically less attractive pyridine ring. With
these considerations in mind, we herein report a rhodium-
catalyzed cross-coupling of arenes bearing a convertible
directing group with aryl carbamates in the absence of
a strong base.
À
natural to expect that C H arylation with inert phenol
derivatives should enable a dramatic increase in the scope and
application of C O cross-coupling reactions of inert phenol
derivatives (Scheme 1a). However, attempts at C H cross-
coupling with inert phenol derivatives have found limited
success. Itami, Yamaguchi and co-workers first reported this
type of reaction in the nickel-catalyzed cross-coupling of aryl
pivalates with azoles.[4] Shi and co-workers recently reported
À
À
À
that a C H bond in perfluorinated arenes can be arylated by
nickel/copper dual catalysis with aryl carbamates.[5] Although
these two reactions provide valuable products related to
To realize the cross-coupling of unactivated C H bonds
À
with inert phenol derivatives, the catalyst needs to efficiently
À
À
pharmaceuticals and organic materials, the substrates are
mediate the activation of both C H and C O bonds. Based
[6]
limited to those bearing a relatively acidic C H bond.
on its remarkable activity in C H activation,[8] we decided to
À
À
use a rhodium catalyst, even though rhodium complexes are
rarely used for C O bond activation processes. Reported
[9]
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À
[*] K. Yasui, Y. Aihara, N. Chatani
rhodium complexes that can activate the C(aryl) O bond
require the use of a pincer-type ligand[9a,b] or boron-based
Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: chatani@chem.eng.osaka-u.ac.jp
reagents,[9c–e] both of which cannot be directly applied to our
À
À
target C H/C O cross-coupling reactions. We chose ortho
arylation of 1a[10] with aryl carbamate 2a as our model
reaction for catalyst development. It should be noted that the
oxazoline substrates are readily accessible from the corre-
sponding carboxylic acids through condensation with 2-
aminopropan-1-ol. Initial ligand screening led us to identify
the NHC-based ligand L6 as a potential lead for further
M. Tobisu
Center for Atomic and Molecular Technologies
Graduate School of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: tobisu@chem.eng.osaka-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610409.
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optimization.[11,12] Interestingly, the use of [RhCl(C2H4)2]2
instead of [RhCl(cod)]2 as the catalyst precursor along with
L6 considerably improved the yield of 3aa, although the
yields were variable between experiments (Scheme 2). It was
Scheme 2. Optimization of rhodium-catalyzed cross-coupling of arene
1a with aryl carbamate 2a. [a] Results of six independent experiments.
[b] RhCl(C2H4)]2, L6·HCl, KOtBu, and Na2CO3 were stirred at 608C for
1 h prior to being used for the catalytic reaction. [c] Yield of isolated
product. OCar=N,N-diisopropylcarbamate, Ox=4-methyl-4,5-dihy-
droxooxazol-2-yl.
Scheme 3. Reaction scope. [a] RhCl(L6)2 was prepared by stirring
a mixture of [RhCl(C2H4)]2 (0.015 mmol), L6·HCl (0.060 mmol), KOtBu
(0.066 mmol), and Na2CO3 (0.060 mmol) at 608C for 1 h. Reaction
conditions: 1 (0.30 mmol), 2 (0.45 mmol) and catalyst in toluene
(1.0 mL) for 24 h. The yield refers to the isolated yield. [b] [RhCl(C2H4)]2
(0.023 mmol), L6·HCl (0.090 mmol) and KOtBu (0.099 mmol) were
used.
found that a consistently high yield of 3aa can be obtained
using a catalyst generated by preheating (608C, 1 h) a mixture
of [RhCl(C2H4)2]2 and L6. FAB-MS and 13C NMR analysis of
the preheated solution of [RhCl(C2H4)2]2 and L6 suggested
exclusive generation of the bis-NHC species[13] [(L6)2RhCl+:
802.3420; 13C NMR of C2: d = 190.2 ppm (d, JRh-C = 40 Hz)],
whereas only the mono-NHC species [(L6)RhCl(cod)+:
578.1935; 13C NMR of C2: d = 185.1 ppm (d, JRh-C = 52 Hz)]
was generated from a preheated solution of [RhCl(cod)]2 and
L6.
With the method to generate an active catalyst in hand, we
À
À
next examined the scope of the C H/C O cross-coupling
with respect to the aryl carbamate component. As shown in
Scheme 3, this transformation is applicable to aryl carbamates
bearing a range of functional groups, including ketones (3ab),
nitriles (3ad), fluorides (3ac and 3ae), and amides (3ak).
Moreover, heteroaromatic carbamates (3al and 3am) can
also be successfully coupled with 1a. The scope with respect
to the arene substrate was next evaluated. When meta-
substituted arenes were reacted with 2a, arylation exclusively
À
occurred at the less hindered C H bond, as in 3ca and 3da.
À
However, a sterically congested C H bond can be arylated
Scheme 4. Synthetic applications.
when it is the only reactive site, as evidenced by the formation
of 3ea. This arylation is also applicable to fused arenes (3ia)
and heteroarenes (3ja and 3ka).
completely stable under the conditions used for standard
palladium-catalyzed cross-coupling of aryl halides, such as the
Mizoroki–Heck and Sonogashira reactions, sequential func-
The 4-methyl-4,5-dihydrooxazol-2-yl group used as the
directing group in the present study can be readily converted
into the corresponding carboxylic acid derivative, which
À
À
tionalization of C X and C O bonds is possible (Scheme 4b).
To obtain insight into the mechanism, several experiments
were performed. When aryl carbamate 2a was subjected to
À
À
allows further synthetic elaboration of the C H/C O cou-
pling products (Scheme 4a). Because the carbamate group is
2
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the rhodium-catalyzed conditions in the absence of an arene
deprotonation pathway,[15] gives diarylrhodium D, which
substrate, naphthalene (5) was formed in 56% yield (Sche-
finally gives arylated product 3 with concurrent regeneration
of A.
me 5a). This observation indicates that RhI(L6)2 can activate
In summary, we have developed a rhodium-catalyzed
2
À
arylation of non-acidic C(sp ) H bonds using aryl carbamates
as the arylating reagent. The key to success is the use of
a bis(NHC) complex of rhodium(I) as the catalyst, which
2
À
facilitates activation of inert C(sp ) O bonds in aryl carba-
mates. This readily generated rhodium species enables the
activation of inert C(sp ) O bonds in the absence of a strong
base, thereby allowing the use of a synthetically useful
2
À
À
À
directing group in C H/C O coupling. Further studies
aiming at the catalytic transformation of inert bonds using
rhodium bis(NHC) complexes are underway.
Scheme 5. Mechanistic studies.
À
the C O bond of 2a, presumably through oxidative addi-
Acknowledgements
tion,[9] and the resulting arylrhodium(III) species undergoes
protonation to form 5. Labeling studies revealed that RhI-
(L6)2 can also activate a C H bond (Scheme 5b). When
deuterated arene 1a-d7 was reacted with 2a in the presence of
a RhI(L6)2 catalyst for 1 h (62% conversion), the deuterium
content of the recovered 1a was significantly reduced.
Interestingly, H/D exchange took place not only at the
ortho position of 1a but also at the meta, para, and even
This work was supported by ACT-C (J1210B2576) from JST,
Japan and Scientific Research on Innovative Area “Precisely
Designed Catalysts
(16H01022)from MEXT, Japan. We also thank the Instru-
mental Analysis Center, Faculty of Engineering, Osaka
University, for their assistance with HRMS.
À
with
Customized Scaffolding”
À
benzylic positions. Therefore, non-selective C H activation
by RhI(L6)2 can occur under these conditions.[9e,14] The
decrease in the deuterium content of 1a was less when the
rhodium-catalyzed reaction of 1a-d7 with 2a was performed
in deuterated solvent, thus indicating that the solvent toluene
is the main source of hydrogen incorporated into the
recovered 1a (see the Supporting Information).
Conflict of interest
The authors declare no conflict of interest.
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À
Keywords: arylation · C H activation · C O activation ·
N-heterocyclic carbenes · rhodium
A possible mechanism is shown in Scheme 6. Although
I
À
À
Rh (L6)2 species A can activate both C H and C O bonds,
À
the initial C H activation only leads to non-productive H/D
exchange via intermediate B. The catalytic cycle that gives
À
arylated product 3 begins with oxidative addition of the C O
bond in carbamate 2 to form arylrhodium(III) intermediate
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À
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À
[12] Our works on inert C O bond activation using NHC ligands:
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À
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[6] Intramolecular C H/C O coupling was reported: J. Wang,
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À
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Manuscript received: October 24, 2016
Final Article published: && &&, &&&&
4
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Communications
Homogeneous Catalysis
M. Tobisu,* K. Yasui, Y. Aihara,
N. Chatani*
&&& — &&&
À
C O Activation by a Rhodium Bis(N-
Heterocyclic Carbene) Catalyst: Aryl
Carbamates as Arylating Reagents in
One for the rhodium: The rhodium-
catalyzed cross-coupling of aryl carba-
mates with arenes bearing a convertible
directing group is described. The key to
success is the use of an in situ generated
rhodium bis(N-heterocyclic carbene)
À
Directed C H Arylation
species as the catalyst, which can pro-
2
À
mote activation of the inert C(sp ) O
bond in aryl carbamates.
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!