CL-140690
Received: July 19, 2014 | Accepted: August 6, 2014 | Web Released: August 12, 2014
Rhodium-catalyzed Intramolecular Dehydrogenative Aryl-Aryl Coupling
Using Air as Terminal Oxidant
Hannah Baars,1,2 Yuto Unoh,1 Takeshi Okada,1 Koji Hirano,1 Tetsuya Satoh,*1,3
Ken Tanaka,3,4 Carsten Bolm,2 and Masahiro Miura*1
1Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871
2Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany
3ACT-C, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
4Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8550
(E-mail: satoh@chem.eng.osaka-u.ac.jp)
R
DG
DG
Intramolecular dehydrogenative cyclization involving twofold
Csp2-H bond cleavages directed by the amino- or carboxy group
proceeds smoothly when using a rhodium-copper catalyst system
under air as the terminal oxidant. A variety of fluorene derivatives
can be prepared by the environmentally benign procedure.
Rh(III)
2Cu(I)
1/2O2 + 2H+
H H
R
Rh(I)
2Cu(II)
H2O
Chelation-assisted catalytic C-H bond transformation reac-
tions utilizing directing groups (DGs) are now recognized as
powerful tools in the precise synthesis of functional organic
molecules.1 In particular, dehydrogenative C-H arylation and
alkenylation at specified positions of substituted arenes are highly
useful for constructing π-conjugated molecules in an atom- and
step-economical manner. In the course of our continuous study on
rhodium- and iridium-catalyzed dehydrogenative coupling reac-
tions,2,3 we found that di- and triphenylmethylamines undergo
intramolecular dehydrogenative aryl-aryl coupling through amino-
directed C-H bond cleavage (Scheme 1, DG = NH2).4 Similar
cyclizations directed by carboxy (DG = CO2H) and hydroxy
groups (DG = OH) also proceeded efficiently when using a
rhodium- or an iridium catalyst.4,5 In the former case (DG =
CO2H), the cyclization was accompanied by decarboxylation.
These reactions provide straightforward routes to fluorene frame-
works, which are important structural units in the field of organic
materials.6 However, a stoichiometric amount of copper salt is
usually needed as the oxidant to carry out these reactions
smoothly.
+ 2H+
Scheme 2.
As reported in our previous paper,4 treatment of tritylamine
(1a) (0.5 mmol) in the presence of [RhCl(cod)]2 (0.005 mmol, cod:
1,5-cyclooctadiene) and Cu(OAc)2•H2O (1 mmol) in o-xylene at
130 °C for 2 h under N2 gave a dehydrogenative cyclization
product, 9-phenylfluoren-9-amine (2a), in 98% yield (Entry 1 in
Table 1). When 1a (0.25 mmol) was treated with a reduced amount
of Cu(OAc)2•H2O (0.05 mmol) under air, the yield of 2a decreased
to 22% (Entry 2). In diglyme, a higher 2a yield of 45% was
obtained (Entry 7) as compared to those in other solvents such as
PhCl, DMF, DMAc, and NMP (Entries 3-6). Interestingly, addition
of a carboxylic acid (0.25 mmol) was found to improve the yield of
2a (Entries 8-12). In particular, pivalic acid (PivOH) and 2,6-
difluorobenzoic acid were the most effective among the acids
examined. Increasing the amounts of reagents relative to that of
the rhodium catalyst significantly reduced the yield (Entry 13).
However, even with a lower loading of rhodium catalyst, a high 2a
yield (90%) was obtained by increasing the reaction temperature to
150 °C (Entry 14). Note that the reaction with a catalytic amount of
Cu(OAc)2•H2O could be readily scaled up to a gram scale. Thus,
from 1a (5 mmol), 2a was obtained in 95% yield (1.23 g, Entry 15).
Under the conditions using [RhCl(cod)]2-Cu(OAc)2•H2O as
the catalyst in air, 1,1-diphenylethan-1-amine (1b) underwent the
cyclization to produce 9-methylfluoren-9-amine (2b) (Entry 1 in
Table 2). The cyclization of other 1-amino-1,1-diarylalkanes 1c-
1e also proceeded efficiently to afford the corresponding fluoren-
amine derivatives (Entries 2-4). In the reaction of 1,1-di(2-
naphthyl)ethan-1-amine (1e), a mixture of isomers 2e and 2e¤ was
obtained (Entry 4). In this case, cyclization at a sterically less
hindered position took place preferably. 1,1,1-Tris(4-substituted
phenyl)methylamines 1f-1h underwent the reaction smoothly to
give 2f-2h in 67-89% yields (Entries 5-7). Treatment of unsym-
metrically substituted triarylmethylamines 1i and 1j gave the
corresponding mixtures of fluorene isomers (Entries 8 and 9).
This cyclization seems to proceed through a similar pathway
to that proposed in our previous work,4 involving coordination of
To perform the above-mentioned reactions in a more environ-
mentally benign manner, it is highly desirable to use molecular
oxygen as the terminal oxidant (Scheme 2).3h,7 Consequently, we
have explored a new catalyst system and succeeded in conducting
the coupling under air in the presence of a rhodium complex, a
copper salt, and a carboxylic acid as the catalyst, cocatalyst, and
promoter, respectively. The results obtained with this second-
generation catalytic system are described herein.
R
DG
DG = NH2, OH
R
DG
Rh- or Ir-cat.
Cu-salt
R
DG = CO2H
–CO2
H H
Scheme 1.
© 2014 The Chemical Society of Japan
Baars, Hannah
