Synthesis of fluorene derivatives through rhodium-catalyzed dehydrogenative cyclization

Article abstract of DOI:10.1002/anie.201201526

Doubling up: Two C-H bond activations took place efficiently upon treatment of 1 with a rhodium catalyst to form dehydrogenative cyclization products 2. Furthermore, 3 undergoes similar cyclization and subsequent decarboxylation through the cleavage of two C-H bonds and one C-C bond. Both reactions provide straightforward routes to the fluorene framework. Copyright

Full text of DOI:10.1002/anie.201201526

Angewandte
Chemie
DOI: 10.1002/anie.201201526
CÀH Activation
Synthesis of Fluorene Derivatives through Rhodium-Catalyzed
Dehydrogenative Cyclization**
Keisuke Morimoto, Masaki Itoh, Koji Hirano, Tetsuya Satoh,* Yu Shibata, Ken Tanaka, and
Masahiro Miura*
Polycyclic aromatic hydrocarbons (PAHs) have attracted
considerable attention because of their optical and electronic
properties, and their application as p-conjugated functional
[1]
materials. Fluorene is one of the simplest motifs in PAHs
and its derivatives have been employed as important building
blocks in a broad range of fields, including light-emitting
devices, organic field-effect transistors (OFET), organic
[
2]
photovoltaic cells (OPV), biosensors, etc. Fluorenyl func-
tions have also played important roles in the field of organic
synthesis, mainly as unique protecting groups in peptide
[
3]
synthesis. In spite of the simple structures, their construction
has been conducted under harsh reaction conditions or
through complicated multistep procedures.
Scheme 1. Synthesis of fluorenes through CÀH bond cleavage. Prece-
[4]
dents for a, b, and c. No precedent for d.
Meanwhile, transition-metal-catalyzed organic reactions
involving CÀH bond cleavage have been significantly devel-
oped in recent years as atom- and step-economical tools in
In an initial attempt, triphenylmethylamine (1a) was
treated under the standard reaction conditions for our
rhodium-catalyzed oxidative coupling. The reaction of 1a in
the presence of [{Cp*RhCl } ] (2 mol% Rh) and Cu-
[
5]
precise organic synthesis. Such procedures can provide
simple, functional-group-tolerable methods for fluorene syn-
thesis. As shown in Scheme 1, several palladium-catalyzed
cyclization reactions of halogenated aromatic substrates were
2
2
(OAc)·H O (2 equiv) as a catalyst and an oxidant, respec-
2
[6]
recently reported (routes a–c).
tively, in o-xylene at 1308C for 10 h gave the dehydrogenative
cyclization product 9-phenyl-9H-fluoren-9-amine (2a) in
65% yield (Table 1, entry 1). Upon elevating the reaction
temperature to 1508C, the yield was improved up to 96%
(entry 2). The reaction proceeded quantitatively in the
Although valuable, the development of a straightforward
method involving the cleavage of two CÀH bonds (route d,
[
7–9]
Scheme 1)
is strongly desired. Herein, we report such
a dehydrogenative cyclization. Thus, in the course of our
[
10,11]
E
E
study of rhodium-catalyzed direct oxidative coupling,
we
presence of [{Cp RhCl } ] (Cp = 1,3-bis(ethoxycarbonyl)-
2 2
have found that 1-amino-1,1-diarylalkanes efficiently
undergo cyclization to furnish 9H-fluoren-9-amine deriva-
tives without any protection of the amine. Furthermore,
related substrates such as 2,2-diphenylalkanoic acids can also
be transformed into the corresponding fluorenes through
dehydrogenative cyclization and subsequent decarboxylation.
2,4,5-trimethylcyclopentadienyl),
which
was
recently
employed for the oxidative coupling of acetanilides with
[12]
alkynes as a catalyst in place of [{Cp*RhCl } ], even at
2
2
1308C (entry 3). To our surprise, the reaction was effectively
[
a]
Table 1: Reaction of triphenylmethylamine (1a).
[*] K. Morimoto, M. Itoh, Dr. K. Hirano, Prof. Dr. T. Satoh,
Prof. Dr. M. Miura
Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: satoh@chem.eng.osaka-u.ac.jp
miura@chem.eng.osaka-u.ac.jp
Homepage: http://www.chem.eng.osaka-u.ac.jp/~miura-lab/
index-Eng.htm
[
b]
Entry
[Rh]
T [8C]
t [h]
Yield [%]
1
2
3
4
5
6
7
[{Cp*RhCl } ]
[{Cp*RhCl } ]
[{Cp RhCl } ]
[{RhCl(cod)} ]
RhCl ·3H O
–
[{RhCl(cod)}2]
130
150
130
130
130
130
100
10
6
6
65
96
>99
2
2
2
2
E
2
2
Y. Shibata, Prof. Dr. K. Tanaka
2
c]
2
>99 (98)
[
Department of Applied Chemistry, Graduate School of Engineering,
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
3
2
10
10
10
27
0
88
[
**] This work was partly supported by Grants-in-Aid from the MEXTand
[
(
a] Reaction conditions: 1a (0.5 mmol), [Rh] (0.005 mmol), Cu-
JSPS (Japan).
OAc) ·H O (1 mmol), o-xylene (3 mL) under N . [b] GC yield based on
2
2
2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201526.
the amount of 1a used. Value within parentheses indicates yield after
purification. [c] RhCl ·3H O (0.01 mmol) was used.
3
2
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1
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.
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Communications
promoted by the use of [{RhCl(cod)} ] as a catalyst precursor.
95% yield (entry 7). Expectedly, the reaction of an unsym-
metrically substituted substrate, (4-chlorophenyl)bis(4-
methoxyphenyl)methylamine, gave a mixture of two cycliza-
tion products. It was found that the present procedure is
applicable to constructing a pentacyclic system. Thus, 12-
methyl-12H-dibenzo[b,h]fluoren-12-amine (2i) was obtained
predominantly upon treatment of 1-amino-1,1-di(2-naph-
thyl)ethane (1i), along with a minor amount of an unidenti-
fied isomer (entry 8).
2
Thus, in the absence of any Cp ligand, 2a was obtained in
[
13]
a quantitative yield within 2 hours (entry 4). Under similar
reaction conditions, treatment of N-acetylated triphenyl-
methylamine gave no cyclization product. RhCl ·3H O
3
2
showed poor activity (entry 5). It was confirmed that the
reaction did not proceed at all without any rhodium catalyst
(
entry 6). Under the reaction conditions using [{RhCl(cod)} ],
2
2
a was formed in 88% yield even at 1008C (entry 7).
Under the optimized reaction conditions (Table 1,
A plausible mechanism for the reaction of 1 is illustrated
in Scheme 2, in which neutral ligands are omitted. In either
entry 4), various 1-amino-1,1-diphenylalkanes (1b–e) also
underwent the cyclization to produce the corresponding 9-
alkyl-9H-fluoren-9-amines 2b–e in good to excellent yields
I
III
the case of using Rh or Rh precursors, an RhX active
3
(
Table 2, entries 1–4). Note that a series of fluorenes bearing
branched- and long-chain alkyl groups at their 9-position can
be prepared through this reaction. 3,6-The disubstituted 9-
methyl-9H-fluoren-9-amines 2 f and 2g were smoothly syn-
thesized from the 1-amino-1,1-diarylethanes 1 f and 1g,
respectively (entries 5 and 6). The reaction of tris(4-methyl-
phenyl)methylamine (1h) also gave a cyclized product 2h in
[
a]
Table 2: Reaction of 1-amino-1,1-diarylalkanes 1.
Scheme 2. Plausible mechanism for the reaction of 1.
Entry
1
T
t
Product
Yield
[%]
species seems to be formed under oxidative conditions.
Coordination of the amino nitrogen of 1 to the rhodium
center and amino-directed CÀH rhodation takes place to give
[
b]
[
8C] [h]
the five-membered rhodacycle intermediate A. Subsequently,
the second cyclorhodation to form the six-membered species
B and reductive elimination may occur to produce 2.
1
2
3
4
1b: R=Me
1c: R=nPr
1d: R=iPr
130
130
130
130
4
2
4
2
2b: R=Me
2c: R=nPr
2d: R=iPr
98
96
78
87
Notably, in the early stages of the reaction of an isotope-
labeled substrate, 1-amino-1,1-bis(d -phenyl)ethane ([D ]-
5
10
1e: R=n-C H
2e: R=n-C H
9
19
9
19
1
b), considerable contamination by protons at the ortho po-
sitions of recovered [D ]-1b and at the 1- and/or 8-positions of
n
produced fluorene [D ]-2b took place (see the Supporting
n
Information). This result indicates that at least the first
cyclorhodation step to form A seems to be reversible.
Besides the amino group, a carboxylic function was also
found to act as a good directing group to trigger the
5
6
1 f: X=OMe
1g: X=Cl
130
150
4
2
2 f: X=OMe
2g: X=Cl
76
87
[
c]
[14]
cyclization. Although treatment of 2,2-diphenylpropionic
acid (3a) under the optimized reaction conditions for the
reaction of amine 1 did not give any cyclized product (Table 3,
entry 1), the corresponding fluorene product could be
obtained under modified reaction conditions. Thus, in the
presence of [{Cp*RhCl } ] (4 mol% Rh) and K CO (1 equiv)
2
2
2
3
7
8
1h
1i
130
130
2
2
2h
2i
95
in diglyme, 3a underwent dehydrogenative cyclization/
decarboxylation to afford 9-methyl-9H-fluorene (4a) in
2
7% yield (entry 3). Addition of AgSbF (8 mol%) improved
6
the 3a yield, in spite of retarding the reaction (entry 4). In this
E
[
c]
[d]
reaction, [{Cp RhCl } ] was found to be the catalyst precursor
2
2
73
of choice. Especially, with further addition of 2,6-
[a] Reaction conditions: 1 (0.5 mmol), [{RhCl(cod)} ] (0.005 mmol),
2
Me C H CO H (0.5 equiv) 4a was obtained in 73% yield
2
6
3
2
Cu(OAc) ·H O (1 mmol), o-xylene (3 mL) under N . [b] Yield of isolated
product based on the amount of 1 used. [c] The reaction was conducted
on half the scale: 1 (0.25 mmol), [{RhCl(cod)} ] (0.0025 mmol), Cu-
2
2
2
(entry 6).
Even under milder reaction conditions (1008C), 2,2-
2
diphenylacetic acid (3b) underwent the reaction smoothly
to produce 9-unsubstituted 9H-fluorene (4b) in 56% yield
(
OAc) ·H O (0.5 mmol) in o-xylene (2 mL). [d] Contaminated with an
2 2
isomer (2i/isomer=81:19).
2
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
[
a]
Table 3: Reaction of 2,2-diphenylalkanoic acids 3.
[
1] Selected reviews: a) J. E. Anthony, Angew. Chem. 2008, 120,
60; Angew. Chem. Int. Ed. 2008, 47, 452; b) A. R. Murphy,
Entry
Catalyst
Additive
T
t
[h]
Yield
[%]
4
[
b]
[
8C]
J. M. J. Frechet, Chem. Rev. 2007, 107, 1066; c) J. E. Anthony,
Chem. Rev. 2006, 106, 5028; d) M. Bendikov, F. Wudl, D. F.
Perepichka, Chem. Rev. 2004, 104, 4891; e) M. D. Watson, A.
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Mitschke, P. Bꢁuerle, J. Mater. Chem. 2000, 10, 1471; g) R. G.
Harvey, Polycyclic aromatic hydrocarbons, Wiley-VCH, New
York, 1996.
[
[
c]
c]
1
2
3
4
5
6
[{RhCl(cod)}2]
[{Cp*RhCl } ]
[{Cp*RhCl } ]
–
–
120
120
120
120
120
120
8
8
8
20
20
20
0
0
27
45
65
2
2
K2CO3
K2CO3
K2CO3
K CO /acid
[2] Selected recent examples: a) J. L. Zafra, J. Casado, I. I. Per-
epichka, M. R. Bryce, F. J. Ramirez, J. T. L. Navarrete, J. Chem.
Phys. 2011, 134, 044520; b) M. Zhu, T. Ye, C.-G. Li, X. Cao, C.
Zhong, D. Ma, J. Qin, C. Yang, J. Phys. Chem. C 2011, 115,
2
2
[{Cp*RhCl } ]/AgSbF
6
2
2
E
[{Cp RhCl } ]/AgSbF
6
2
2
E
[d]
[{Cp RhCl } ]/AgSbF
73 (63)
2
2
6
2
3
17965; c) K.-Y. Pu, R. Zhan, B. Liu, Chem. Commun. 2010, 46,
1470; d) S. M. Aly, C.-L. Ho, W.-Y. Wong, D. Fortin, P. D.
Harvey, Macromolecules 2009, 42, 6902; e) H.-C. Yeh, C.-H.
Chien, P.-I. Shih, M.-C. Yuan, C.-F. Shu, Macromolecules 2008,
4
1, 3801; f) Y. Mo, X. Jiang, D. Cao, Org. Lett. 2007, 9, 4371.
E
[d]
[d]
7
8
9
[{Cp RhCl } ]/AgSbF
K CO /acid
120
100
100
20
48
48
51
56
57 (57)
[3] Selected examples: a) O. Bassas, J. Huuskonen, K. Rissanen,
A. M. P. Koskinen, Eur. J. Org. Chem. 2009, 1340; b) M. R.
Paleo, N. Aurrecoechea, K.-Y. Jung, H. Rapoport, J. Org. Chem.
2
2
6
6
6
2
3
E
[{Cp RhCl } ]/AgSbF
K CO /acid
2
2
2
3
E
[{Cp RhCl } ]/AgSbF
K2CO3
2
2
2003, 68, 130; c) B. W. Lee, J. H. Lee, K. C. Jang, J. E. Kang, J. H.
[a] Reaction conditions: 3 (0.5 mmol), Rh-cat (0.01 mmol), (AgSbF6
Kim, K.-M. Park, K. H. Park, Tetrahedron Lett. 2003, 44, 5905;
d) Y. Ding, J. Wang, K. A. Abboud, Y. Xu, W. R. Dolbier, Jr.,
N. G. J. Richards, J. Org. Chem. 2001, 66, 6381; e) H. Nishida, T.
Eguchi, K. Kakinuma, Tetrahedron 2001, 57, 8237; f) K. S.
Reddy, L. Sola, A. Moyano, M. A. Pericas, A. Riera, Synthesis
(
(
0.04 mmol)), (K CO (0.5 mmol)), Cu(OAc) ·H O (1 mmol), diglyme
2 3 2 2
3 mL) under N . [b] GC yield based on the amount of 3 used. Value
2
within parentheses indicates yield after purification. [c] In o-xylene
(
3 mL). [d] acid=2,6-Me C H CO H (0.25 mmol).
2 6 3 2
2000, 165; g) J. Savrda, J.-P. Mazaleyrat, M. Wakselman, F.
Formaggio, M. Crisma, C. Toniolo, J. Pept. Sci. 1999, 5, 61.
4] Selected examples: a) D. L. J. Clive, R. Sunasee, Org. Lett. 2007,
9, 2677; b) K. Wong, L. Chi, S. Huang, Y. Liao, L. Y. Wang, Org.
Lett. 2006, 8, 5029; c) S. Merlet, M. Birau, Z. Y. Wang, Org. Lett.
2002, 4, 2157.
(
Table 3, entry 8). In this case, the addition of 2,6-
[
Me C H CO H was not necessary (entry 9). In contrast, the
corresponding amine, diphenylmethylamine, could not be
used for the cyclization. Under standard oxidative conditions,
only a small amount of dehydrogenation/condensation
product was detected (8%).
In summary, we have demonstrated that a series of
fluorene derivatives can be constructed through dehydrogen-
ative cyclization of diphenylmethane moieties of substrates
under rhodium catalysis. Work is underway toward further
development of relevant reactions.
2
6
3
2
[15]
[
5] Selected reviews: a) D. A. Colby, A. S. Tsai, R. G. Bergman,
J. A. Ellman, Acc. Chem. Res. 2012, DOI: 10.1021/ar200190g;
b) K. M. Engle, T.-S. Mei, M. Wasa, J.-Q. Yu, Acc. Chem. Res.
2012, DOI: 10.1021/ar200185g; c) S. H. Cho, J. Y. Kim, J. Kwak,
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Ellman, Chem. Rev. 2010, 110, 624; k) C.-L. Sun, B.-J. Li, Z.-J.
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A. R. Kapdi, Angew. Chem. 2009, 121, 9976; Angew. Chem. Int.
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Yu, Angew. Chem. 2009, 121, 5196; Angew. Chem. Int. Ed. 2009,
Experimental Section
General procedure for cyclization of 1-amino-1,1-diarylalkenes: The
amine
1 (0.5 mmol), [{RhCl(cod)}2] (0.005 mmol, 2.5 mg), Cu-
(OAc) ·H O (1 mmol, 200 mg), 1-methylnaphthalene (ca. 50 mg) as
2
2
internal standard, and o-xylene (3 mL) were added to a 20 mL two-
necked flask with a reflux condenser, a balloon, and a rubber cup. The
resulting mixture was then stirred under nitrogen at 100–1508C (bath
temperature) for 2–10 h. After cooling, the reaction mixture was
extracted with ethyl acetate (100 mL) and ethylenediamine (2 mL).
The organic layer was washed by water (100 mL, three times), and
dried over Na SO . After evaporation of the solvents under vacuum,
the product 2 was isolated by column chromatography on silica gel
using n-hexane/ethyl acetate (3:1 or 1:1, v/v) as the eluant. Character-
ization data of products are summarized in the Supporting Informa-
tion.
4
8, 5094; n) O. Daugulis, H.-Q. Do, D. Shabashov, Acc. Chem.
Res. 2009, 42, 1074; o) P. Thansandote, M. Lautens, Chem. Eur. J.
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2
Rev. 2009, 38, 2447; q) C.-J. Li, Acc. Chem. Res. 2009, 42, 335;
r) A. V. Vasilꢂev, Russ. J. Org. Chem. 2009, 45, 1; s) F. Kakiuchi,
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4
5318; w) D. Alberico, M. E. Scott, M. Lautens, Chem. Rev. 2007,
1
07, 174; x) K. Godula, D. Sames, Science 2006, 312, 67; y) F.
Received: February 24, 2012
Revised: March 23, 2012
Published online: && &&, &&&&
Kakiuchi, N. Chatani, Adv. Synth. Catal. 2003, 345, 1077; z) G.
Dyker, Angew. Chem. 1999, 111, 1808; Angew. Chem. Int. Ed.
1
999, 38, 1698.
[
6] Route a: a) L.-C. Campeau, M. Parisien, A. Jean, K. Fagnou, J.
Am. Chem. Soc. 2006, 128, 581; Route b: b) C.-C. Hsiao, Y.-K.
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Keywords: CÀC coupling · CÀH activation · cyclizations ·
.
homogeneous catalysis · rhodium
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
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.
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Communications
3
267; Pd-Catalyzed annulation, in which cyclization through
[11] Selected examples from other groups: a) M. E. Tauchert, C. D.
Incarvito, A. L. Reingold, R. G. Bergman, J. A. Ellman, J. Am.
Chem. Soc. 2012, 134, 1482; b) P. Wang, H. Rao, R. Hua, C.-J. Li,
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2
006, 118, 2347; Angew. Chem. Int. Ed. 2006, 45, 2289; Route c:
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b) J. A. Ashenhurst, Chem. Soc. Rev. 2010, 39, 540. For
intermolecular dehydrogenative aryl – aryl coupling, see
Refs. [5c,f,i], and [5p].
[
[
[
8] Rhodium-catalyzed oxidative annulation involving the cleavage
of two or four CÀH bonds: a) N. Umeda, K. Hirano, T. Satoh, N.
Shibata, H. Sato, M. Miura, J. Org. Chem. 2011, 76, 13; b) N.
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120, 4083; Angew. Chem. Int. Ed. 2008, 47, 4019; c) G. Song, X.
Gong, X. Li, J. Org. Chem. 2011, 76, 7583.
9] During the preparation of this manuscript, Glorius and co-
workers reported the rhodium-catalyzed intermolecular aryl–
aryl coupling involving two-fold CÀH functionalization: a) J.
[12] a) Y. Shibata, K. Tanaka, Angew. Chem. 2011, 123, 11109;
Angew. Chem. Int. Ed. 2011, 50, 10917. For original reaction
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130, 16474.
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Chem. 2012, 124, 2290; Angew. Chem. Int. Ed. 2012, 51, 2247. For
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Nimphius, F. W. Patureau, F. Glorius, Chem. Asian J. 2012, DOI:
[13] Under the reaction conditions using [{RhCl(cod)} ], AgOAc was
2
found to be less effective as oxidant. Thus, the yield of 2a
decreased to 21%.
10.1002/asia.201101018; c) F. W. Patureau, C. Nimphius, F.
Glorius, Org. Lett. 2011, 13, 6346.
[14] In contrast, treatment of triphenylmethanol, benzophenone
imine, and benzanilide gave no or trace amounts of cyclization
products.
[15] a) K. Morimoto, K. Hirano, T. Satoh, M. Miura, Chem. Lett.
2011, 40, 600; b) X. Lang, H. Ji, C. Chen, W. Ma, J. Zhao, Angew.
Chem. 2011, 123, 4020; Angew. Chem. Int. Ed. 2011, 50, 3934.
Similar dehydrogenation also took place upon treatment of N-
methylated triphenylmethylamine.
[
10] Selected recent reports from our group: a) K. Morimoto, K.
Hirano, T. Satoh, M. Miura, J. Org. Chem. 2011, 76, 9548; b) S.
Mochida, K. Hirano, T. Satoh, M. Miura, J. Org. Chem. 2011, 76,
3024; c) K. Morimoto, K. Hirano, T. Satoh, M. Miura, Org. Lett.
2010, 12, 2068; d) T. Fukutani, N. Umeda, K. Hirano, T. Satoh,
M. Miura, Chem. Commun. 2009, 5141; e) M. Shimizu, K.
Hirano, T. Satoh, M. Miura, J. Org. Chem. 2009, 74, 3478; f) K.
Ueura, T. Satoh, M. Miura, J. Org. Chem. 2007, 72, 5362; g) K.
Ueura, T. Satoh, M. Miura, Org. Lett. 2007, 9, 1407.
4
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
CÀH Activation
Doubling up: Two CÀH bond activations
took place efficiently upon treatment of
1 with a rhodium catalyst to form dehy-
drogenative cyclization products 2. Fur-
thermore, 3 undergoes similar cyclization
and subsequent decarboxylation through
the cleavage of two CÀH bonds and one
CÀC bond. Both reactions provide
K. Morimoto, M. Itoh, K. Hirano,
T. Satoh,* Y. Shibata, K. Tanaka,
M. Miura*
&&&& — &&&&
Synthesis of Fluorene Derivatives through
Rhodium-Catalyzed Dehydrogenative
Cyclization
straightforward routes to the fluorene
framework.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!