Domino synthesis of fluorine-substituted polycyclic aromatic hydrocarbons: 1,1-difluoroallenes as synthetic platforms

Article abstract of DOI:10.1002/anie.201302740

Rather crafty: 1,1-Difluoroallenes bearing an aryl group and a cyclopentene moiety undergo indium(III)-catalyzed Friedel-Crafts-type cyclization with subsequent ring expansion and dehydrogenation to afford fluorinated polycyclic aromatic hydrocarbons in high yields. The introduction of an Ar group was effected by in situ halogenation of the intermediary indium species and a subsequent Suzuki-Miyaura reaction. Copyright

Full text of DOI:10.1002/anie.201302740

Angewandte
Chemie
Communications
Arenes
K. Fuchibe, Y. Mayumi, N. Zhao,
S. Watanabe, M. Yokota,
J. Ichikawa*
&&&& — &&&&
Domino Synthesis of Fluorine-
Substituted Polycyclic Aromatic
Hydrocarbons: 1,1-Difluoroallenes as
Synthetic Platforms
Rather crafty: 1,1-Difluoroallenes bearing
an aryl group and a cyclopentene moiety
undergo indium(III)-catalyzed Friedel–
Crafts-type cyclization with subsequent
ring expansion and dehydrogenation to
afford fluorinated polycyclic aromatic
hydrocarbons in high yields. The intro-
duction of an Ar group was effected by
in situ halogenation of the intermediary
indium species and a subsequent
Suzuki–Miyaura reaction.
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DOI: 10.1002/anie.201302740
Arenes
Domino Synthesis of Fluorine-Substituted Polycyclic Aromatic
Hydrocarbons: 1,1-Difluoroallenes as Synthetic Platforms**
Kohei Fuchibe, Yuka Mayumi, Nan Zhao, Shumpei Watanabe, Misaki Yokota, and
Junji Ichikawa*
Fluorinated unsaturated compounds such as 1,1-difluoro-1-
alkenes are useful synthetic intermediates because of the
unique properties of the fluorine substituents, which stabilize
a carbocations by donating their lone pairs into the vacant
p orbitals of the cationic centers
(+ M effect, Figure 1).[1] A flu-
orine substituent, in addition,
can also act as a leaving group
(FÀ). By using these unique
properties, we have been study-
Figure 1. a-Cation-stabilizing
ing the chemistry of fluorine-
effect of fluorine substituent
stabilized carbocations (CF2
(+M Effect).
cations) generated from 1,1-
difluoro-1-alkenes
(Scheme 1).[2]
Difluoroalkenes bearing two aryl groups were treated
with a stoichiometric amount of Magic Acid (FSO3H·SbF5)
which has a Hammett acidity function Ho = À22.9 (Sche-
Scheme 1. Domino cyclizations of 1,1-difluoro-1-alkenes and 1,1-
difluoro-1,3-alkadienes via CF2 cations (a) and allylic CF2 cations (b).
me 1a).[3] Selective protonation took place at the position b to
the fluorine substituents to form CF2 cations, which readily
underwent Friedel–Crafts-type cyclization in a domino fash-
ion to give helicenes and related polycyclic aromatic hydro-
carbons (PAHs) in good to high yields.[4]
Tf =trifluoromethanesulfonyl.
The results suggested that introducing an electron-rich
alkene moiety to 1,1-difluoroalkenes provides easy access to
CF2 cations under mild reaction conditions. We next tried to
generate allylic CF2 cations starting from other substrates, 1,1-
difluoroallenes, which have an electron-rich alkene moiety
next to the electron-deficient difluoroalkene moiety
(Scheme 2).[6] A metal catalyst would readily interact with
difluoroallenes through p coordination with the non-fluori-
nated electron-rich alkene moiety,[7,6f] thus leading to the
formation of allyllic CF2 cations followed by the cyclizations
described above. Herein we describe the efficient synthesis of
fluorine-substituted PAHs (F-PAHs) on the basis of a catalytic
domino ring-forming process of 1,1-difluoroallenes. F-PAHs
are promising basic molecules for the synthesis of advanced
materials such as semiconductors and pharmaceuticals.[8]
The required 1,1-difluoroallenes 1a–h (Figure 2) were
readily prepared from aldehydes using our difluorovinylide-
nation method (Scheme 3).[9] Benzylic nitriles 2 were sub-
jected to double allylation under basic conditions to introduce
a cyclopentene moiety, which was reserved for the formation
of an extra benzene ring (see below). Nitriles 3, thus obtained,
Although the above-mentioned reactions are useful, they
have one drawback: they use a highly acidic reagent, Magic
Acid, to protonate the electron-deficient difluoroalkene
moiety. This drawback was overcome by introducing a fluo-
rine-free alkene moiety to the 1,1-difluoroalkene, that is,
using 1,1-difluoro-1,3-alkadienes as substrates (Scheme 1b).[5]
Protonation at the electron-rich termini enabled the gener-
ation of allylic CF2 cations by using a relatively mild acid,
trifluoromethanesulfonic acid (TfOH, pKa = À14), to pro-
mote the Friedel–Crafts-type cyclization.
[*] Dr. K. Fuchibe, Y. Mayumi, N. Zhao, S. Watanabe, Dr. M. Yokota,
Prof. Dr. J. Ichikawa
Division of Chemistry, Faculty of Pure and Applied Sciences
University of Tsukuba
Tsukuba, Ibaraki 305-8571 (Japan)
E-mail: junji@chem.tsukuba.ac.jp
Homepage: http://www.chem.tsukuba.ac.jp/junji/index.html
[**] This research is partly supported by the MEXT KAKENHI Grant
Number 24106705 (Grants-in-Aid for Scientific Research on
Innovative Areas, No.2105), JSPS KAKENHI Grant Numbers
25288016 and 25620158, and the Asahi Glass Foundation. We
acknowledge the generous gifts of (CF3)2CHOH (HFIP) from the
Central Glass Co., Ltd. and CF3CH2I from Tosoh F-Tech, Inc.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302740.
Scheme 2. Concept for the catalytic generation of allylic CF2 cations
from 1,1-difluoroallenes.
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Scheme 4. Synthesis of fluorophenanthrene. DDQ=2,3-Dichloro-5,6-
dicyano-1,4-benzoquinone.
Figure 2. List of substrates.
Scheme 3. Preparation of 1,1-difluoroallenes. DMF=N,N-dimethyl-
formamide, LDA=lithium diisopropylamide, NMP=N-methylpyrroli-
done, THF=tetrahydrofuran.
Scheme 5. Plausible catalytic cycle and reaction mechanism.
were subjected to half reduction with diisobutylaluminum
hydride (DIBAL) to afford aldehydes 4 in high yield.
Aldehydes 4 were converted into 3,3-difluoroallylic acetates
(not shown), which in turn underwent b elimination upon
treatment with zinc metal to give 1a–h in good yields.
Difluoroallene 1i was prepared from the corresponding 1-
phenylindan-1-carbonitrile, which was obtained from 1-inda-
nol in three steps.[10]
After several attempts, we found that a catalytic amount
of indium(III) bromide specifically promoted the generation
of the desired allylic CF2 cations, and the domino cyclization/
ring expansion took place readily (Scheme 4).[11] The difluoro-
allene 1a was treated with 2 mol% of indium(III) bromide in
1,2-dichloroethane (DCE) at room temperature. After stir-
ring for 30 minutes, 19F NMR and mass analysis revealed that
the fluorodihydrophenanthrene 5a was formed. Subsequent
addition of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
smoothly promoted the dehydrogenation of 5a to afford
a 90% yield of 9-fluorophenanthrene (6a) in a one-pot
operation.
generates the second cationic intermediate C,[13–15] which
undergoes 1,2-migration, ring expansion to give 5a.[16,17]
The indium(III)-catalyzed domino reaction of 1,1-
difluoroallenes was applied to the synthesis of F-PAHs
(Table 1). The difluoroallenes 1b and 1c with a para substitu-
ent gave the corresponding fluorophenanthrenes 6b and 6c in
93% and 66% yields, respectively (entries 2 and 3). Cycliza-
tion of the meta-substituted 1,1-difluoroallene 1d proceeded
and fluorophenanthrenes 6d/6d’ were obtained in 96% yield
(entry 4). The domino reaction of the 2-naphthyl-substituted
difluoroallenes 1e and 1 f proceeded exclusively at the
a position to give 5-fluorochrysenes 6e and 6 f, respectively
(entries 5 and 6), and shows that the regioselectivity follows
the same trend as the conventional Friedel–Crafts reaction of
naphthalene. The reaction of the 1-naphthyl-substituted
difluoroallene 1g afforded the corresponding 6-fluoro[4]-
helicene (6g) along with 5-fluorochrysene (6e), which was
formed by sequential cyclization at the ipso position and
a double ring expansion (entry 7). The substrate 1h, which has
a benzocyclopentene moiety, reacted well, thus giving benzo-
chrysene 6h in 84% yield (entry 8). Ring expansion in the
domino reaction of substrate 1i, which has an indane moiety,
proceeded exclusively with the migration of the sp2-carbon
atom to give 6-fluorochrysene (6i) in 88% yield (entry 9). The
selective formation of each of the regioisomers 6e and 6i
We believe that 5a was formed by the mechanism shown
in Scheme 5. The difluoroallene 1a is transformed into the
allylic CF2 cation A, which undergoes Friedel–Crafts-type
cyclization to generate the organoindium intermediate B.[12]
À
À
The protonolysis of the C In bond and elimination of F
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Table 1: Synthesis of F-PAHs by the domino cyclization/ring expansion
bromine- and iodine-substituted fluorophenanthrenes (6j,k)
were obtained in quantitative and 94% yields, respectively.
The bromophenanthrene 6j readily underwent the Suzuki–
Miyaura cross-coupling reaction with aryl boronic acids to
give the corresponding arylated fluorophenanthrenes in high
yield (not shown).[10]
Finally, an aryl group was introduced without isolating 6j
by applying the Suzuki–Miyaura cross-coupling reaction to
the intermediate 5b (Table 2). The 1,1-difluoroallene 1a was
treated with 2 mol% of indium(III) bromide in the presence
of NBS (1.2 equiv), followed by 1.0 equivalents of TFA. After
confirming the formation of 5b by TLC analysis, the crude
reaction mixture was subjected to coupling conditions with
phenyl boronic acid. Extraction and DDQ dehydrogenation
of the intermediate gave the corresponding fluorinated
phenylphenanthrene 6l in 64% yield from 1a through the
of 1,1-difluoroallenes.[a]
Entry Difluoroallene 1 (R)
F-PAH
6
Yield
[%][b]
1
2
3
1a (H)
1b (Me)
1c (OMe)
6a 90
6b 93
6c 66[c]
6d
+
4
1d (Me)
96 (52:48)[d]
6d’
5
6
1e (H)
1 f (Me)
6e 85[e]
6 f 98
À
formation of three C C bonds (entry 1). Electron-donating,
electron-withdrawing, and sterically demanding aryl boronic
acids also afforded the corresponding phenanthrenes 6m–o in
good yields (entries 2–4). 1- and 2-Naphthyl boronic acids
successfully afforded the fluorinated naphthylphenanthrenes
6p and 6q in 62% and 63% yield, respectively (entries 5 and
6).
In summary, we have developed a novel method for
generating allylic CF2 cations from 1,1-difluoroallenes with an
indium(III) catalyst. The generated allylic CF2 cations readily
underwent a sequential process of Friedel–Crafts-type cycli-
zation/ring expansion and subsequent one-pot dehydrogen-
6g
+
7
8
1g
1h
80 (75:25)[d]
6e
6h 84
À
ation with DDQ to afford F-PAHs. The C In bond involved
in the intermediates was successfully used to functionalize the
F-PAH frameworks through halogenation and cross-coupling
reaction.
9
1i
6i 88
[a] Reaction conditions: a) 2 mol% InBr3, 1, DCE, RT, 30 min. b) DDQ
(1.6 equiv), DCE, reflux, 1 h. [b] Yield of isolated product. [c] 10 mol%
InBr3. [d] The regioisomeric ratio was determined by 19F NMR spectros-
copy. [e] Single regioisomer.
Table 2: Facile Synthesis of fluorine-substituted arylphenanthrenes by
the sequential cyclization/ring expansion/bromination/coupling of 1,1-
difluoroallenes.
(entries 5 and 9) was accomplished, thus clearly showing the
potential of this method for the synthesis of F-PAHs.[18]
The functionalization of F-PAH skeletons was success-
À
fully accomplished using the C In bond involved in the
intermediate B (Scheme 6). The difluoroallene 1a was treated
with 2 mol% of indium(III) bromide in the presence of N-
bromosuccinimide (NBS) or N-iodosuccinimide (NIS). After
sequential addition of trifluoroacetic acid (TFA)[19] and DDQ,
Entry
Ar
Yield [%][a]
1
2
3
4
5
6
Ph
6l: 64
p-MeC6H4
p-ClC6H4
o-PhC6H4
1-Naphtyl
2-Naphtyl
6m: 48
6n: 49
6o: 83
6p: 62
6q: 63
Scheme 6. Halogenation of the domino intermediate B.
[a] Yield of isolated product. TFA=trifluoroacetic acid.
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Chemie
[10] See the Supporting Information.
Experimental Section
[11] For reviews on reactions catalyzed or promoted by indium, see:
a) V. Nair, S. Ros, C. N. Jayan, B. S. Pillai, Tetrahedron 2004, 60,
1959 – 1982; b) F. Fringuelli, O. Piermatti, F. Pizzo, L. Vaccaro,
Curr. Org. Chem. 2003, 7, 1661 – 1689.
[12] For reactions involving indium(III) alkene/p complexes, see:
a) Y. Nishimoto, H. Ueda, Y. Inamoto, M. Yasuda, A. Baba, Org.
Lett. 2010, 12, 3390 – 3393; b) Y. Yuan, Z. Shi, Synlett 2007,
3219 – 3223; c) M. Weꢁwer, L. Coulombel, E. DuÇach, Chem.
Commun. 2006, 332 – 334; d) T. Tsuchimoto, S. Kamiyama, R.
Negoro, E. Shirakawa, Y. Kawakami, Chem. Commun. 2003,
852 – 853.
[13] We suppose that the elimination of a fluoride ion is also
promoted by indium(III) and that the dual role of indium(III), as
both a p and a s acid, is a key feature in the domino cyclization
of 1,1-difluoroallenes. See: T. Tsuchimoto, H. Matsubayashi, M.
Kaneko, Y. Nagase, T. Miyamura, E. Shirakawa, J. Am. Chem.
Soc. 2008, 130, 15823 – 15835.
[14] For elimination of bromide ion by indium(III) catalyst, see:
a) G. R. Cook, R. Hayashi, Org. Lett. 2006, 8, 1045 – 1048. See
also: b) Y. Onishi, T. Ito, M. Yasuda, A. Baba, Eur. J. Org. Chem.
2002, 1578 – 1581; c) T. Mukaiyama, T. Ohno, T. Nishimura, J. S.
Han, S. Kobayashi, Bull. Chem. Soc. Jpn. 1991, 64, 2524 – 2527.
[15] Alternatively, a transient six-membered cyclic monofluoroallene
could be formed by cyclization followed by elimination of
a fluoride ion. Its subsequent protonation might lead to the
formation of the carbocation C.
[16] For recent publications on reactions catalyzed or mediated by
indium(III) species, see: [reactions of alkynes] a) K. Takahashi,
M. Midori, K. Kawano, J. Ishihara, S. Hatakeyama, Angew.
Chem. 2008, 120, 6340 – 6342; Angew. Chem. Int. Ed. 2008, 47,
6244 – 6246; b) Y. Itoh, H. Tsuji, K. Yamagata, K. Endo, I.
Tanaka, M. Nakamura, E. Nakamura, J. Am. Chem. Soc. 2008,
130, 17161 – 17167; [reactions of alcohols] c) G. Chen, Z. Wang, J.
Wu, K. Ding, Org. Lett. 2008, 10, 4573 – 4576; d) M. Yasuda, T.
Saito, M. Ueba, A. Baba, Angew. Chem. 2004, 116, 1438 – 1440;
Angew. Chem. Int. Ed. 2004, 43, 1414 – 1416; [reactions of
aldehydes or imines] e) Y. Kuninobu, T. Tatsuzaki, T. Matsuki,
K. Takai, J. Org. Chem. 2011, 76, 7005 – 7009; f) K. Miura, K.
Yamamoto, A. Yamanobe, K. Ito, H. Kinoshita, J. Ichikawa, A.
Hosomi, Chem. Lett. 2010, 39, 766 – 767; g) S. Harada, R. Takita,
T. Ohshima, S. Matsunaga, M. Shibasaki, Chem. Commun. 2007,
948 – 950; h) R. Takita, Y. Fukuta, R. Tsuji, T. Ohshima, M.
Shibasaki, Org. Lett. 2005, 7, 1363 – 1366; i) N. Sakai, M.
Hirasawa, T. Konakahara, Tetrahedron Lett. 2003, 44, 4171 –
4174; [cross-coupling reactions] j) Y. Nishimoto, R. Moritoh,
M. Yasuda, A. Baba, Angew. Chem. 2009, 121, 4647 – 4650;
Angew. Chem. Int. Ed. 2009, 48, 4577 – 4580; k) Y.-H. Chen, M.
Sun, P. Knochel, Angew. Chem. 2009, 121, 2270 – 2273; Angew.
Chem. Int. Ed. 2009, 48, 2236 – 2239; l) K. Takami, H. Yorimitsu,
K. Oshima, Org. Lett. 2004, 6, 4555 – 4558; m) M. A. Pena, I.
Perez, J. P. Sestelo, L. A. Sarandeses, Chem. Commun. 2002,
2246 – 2247.
[17] Reactions of allenes catalyzed or mediated by indium(III) are
limited. See: a) N. Hayashi, Y. Hirokawa, I. Shibata, M. Yasuda,
A. Baba, Org. Biomol. Chem. 2008, 6, 1949 – 1954; b) E. Soriano,
J. Marco-Contelles, Organometallics 2006, 25, 4542 – 4553.
[18] X ray crystal structure analysis of single crystals of 6e and 6i
confirmed that they have the fluorine substituent at the
predicted 5- and 6-positions. CCDC 911785 (6e) and 911786
(6i) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Synthesis of 9-fluorophenanthrene (6a): InBr3 (2 mg, 0.006 mmol)
was added to a DCE solution (6 mL) of 1,1-difluoroallene 1a (53 mg,
0.24 mmol) at room temperature. After being stirred for 30 min at the
temperature, DDQ (65 mg, 0.29 mmol) was added in a solid form.
After being refluxed for 1.5 h, the reaction mixture was cooled to
room temperature. The solvent was removed under reduced pressure,
and the residue was purified by column chromatography (n-hexane)
on silica gel to give 6a (42 mg, 90%).
Received: April 3, 2013
Published online: && &&, &&&&
Keywords: allenes · arenes · cyclization · fluorine · indium
.
[1] For reviews on fluorinated carbocations, see: a) A. D. Allen,
T. T. Tidwell in Progress in Carbocation Chemistry, Vol. 1 (Ed.:
X. Creary), JAI, London, 1989, pp. 1 – 44; b) C. G. Krespan,
V. A. Petrov, Chem. Rev. 1996, 96, 3269 – 3302. See also: c) G. A.
Olah, Y. K. Mo, J. Org. Chem. 1972, 37, 1028 – 1034.
[2] a) J. Ichikawa, M. Yokota, T. Kudo, S. Umezaki, Angew. Chem.
2008, 120, 4948 – 4951; Angew. Chem. Int. Ed. 2008, 47, 4870 –
4873. See also: b) J. Ichikawa, H. Jyono, T. Kudo, M. Fujiwara,
M. Yokota, Synthesis 2005, 39 – 46.
[3] a) G. A. Olah, Angew. Chem. 1995, 107, 1519 – 1532; Angew.
Chem. Int. Ed. Engl. 1995, 34, 1393 – 1405; b) V. Gold, K. K.
Laali, K. P. Morris, L. Z. Zdunek, J. Chem. Soc. Chem. Commun.
1981, 769 – 771; c) G. A. Olah, R. H. Schlosberg, J. Am. Chem.
Soc. 1968, 90, 2726 – 2727. See also: d) B. J. Dahl, N. S. Mills, J.
Am. Chem. Soc. 2008, 130, 10179 – 10186; e) K. K. Laali, D. S.
Nagvekart, J. Org. Chem. 1991, 56, 1867 – 1874.
[4] For related work on catalytic cyclization of 1,1-difluoroalkenes
with palladium complexes, see: a) H. Tanabe, J. Ichikawa, Chem.
Lett. 2010, 39, 248 – 249; b) M. Yokota, D. Fujita, J. Ichikawa,
Org. Lett. 2007, 9, 4639 – 4642.
[5] K. Fuchibe, H. Jyono, M. Fujiwara, T. Kudo, M. Yokota, J.
Ichikawa, Chem. Eur. J. 2011, 17, 12175 – 12185.
[6] For the reactions of 1,1-difluoroallenes in synthesis, see: a) M. F.
Kuehnel, T. Schlçder, S. Riedel, B. Nieto-Ortega, F. J. Ramꢀrez,
J. T. L. Navarrete, J. Casado, D. Lentz, Angew. Chem. 2012, 124,
2261 – 2263; Angew. Chem. Int. Ed. 2012, 51, 2218 – 2220; b) M.
Mae, J. A. Hong, B. Xu, G. B. Hammond, Org. Lett. 2006, 8, 479 –
482; c) B. Xu, G. B. Hammond, Angew. Chem. 2005, 117, 7570 –
7573; Angew. Chem. Int. Ed. 2005, 44, 7404 – 7407; d) Q. Shen,
G. B. Hammond, J. Am. Chem. Soc. 2002, 124, 6534 – 6535; e) Y.-
Y. Xu, F.-Q. Jin, W.-Y. Huang, J. Fluorine Chem. 1995, 70, 5 – 6;
f) W. R. Dolbier, Jr., Acc. Chem. Res. 1991, 24, 63 – 69; g) W. H.
Knoth, D. D. Coffman, J. Am. Chem. Soc. 1960, 82, 3873 – 3875.
[7] DFT calculation suggested that the b- and g-carbon atoms to the
fluorine substituents are negatively charged, which would foster
coordination of difluoroallenes to main group metal catalyst
with their fluorine-free alkene moiety: a) K. Fuchibe, M. Ueda,
M. Yokota, J. Ichikawa, Chem. Lett. 2012, 41, 1619 – 1621. See
also: b) L. N. Domelsmith, K. N. Houk, C. Piedrahita, W. J.
Dolbier, Jr., J. Am. Chem. Soc. 1978, 100, 6908 – 6911; c) D.
Lentz, N. Nickelt, S. Willemsen, Chem. Eur. J. 2002, 8, 1205 –
1217.
[8] a) T. Okazaki, K. K. Laali, Adv. Org. Synth. 2006, 2, 353 – 380.
See also: b) Y. Sakamoto, T. Suzuki, M. Kobayashi, Y. Gao, Y.
Fukai, Y. Inoue, F. Sato, S. Tokito, J. Am. Chem. Soc. 2004, 126,
8138 – 8140.
[9] a) K. Oh, K. Fuchibe, M. Yokota, J. Ichikawa, Synthesis 2012, 44,
857 – 861; b) K. Oh, K. Fuchibe, J. Ichikawa, Synthesis 2011, 881 –
886; c) M. Yokota, K. Fuchibe, M. Ueda, Y. Mayumi, J.
Ichikawa, Org. Lett. 2009, 11, 3994 – 3997.
[19] Treatment with TFA was necessary to promote the ring
expansion, especially when the reaction was performed in the
presence of NIS.
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