Convergent Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Alkene Isomerization Sequence

Article abstract of DOI:10.1002/chem.201901519

A convergent synthetic method for the synthesis of fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/alkene isomerization sequence. The reactions proceed smoothly under mild conditions for a variety of substrate combinations, and extended π-conjugation systems are also readily accessible by utilizing this synthetic method. Optical properties of the obtained fluorene derivatives have also been examined.

Full text of DOI:10.1002/chem.201901519

A Journal of
Accepted Article
Title: Convergent Synthesis of Fluorene Derivatives by Rhodium-
Catalyzed Stitching Reaction/Alkene Isomerization Sequence
Authors: Masaki Nishida and Ryo Shintani
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To be cited as: Chem. Eur. J. 10.1002/chem.201901519
Link to VoR: http://dx.doi.org/10.1002/chem.201901519
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10.1002/chem.201901519
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Convergent Synthesis of Fluorene Derivatives by Rhodium-
Catalyzed Stitching Reaction/Alkene Isomerization Sequence
Masaki Nishida[a] and Ryo Shintani*[a]
Abstract: A new and convergent synthetic method of fluorene
convergent
fluorene synthesis
XH
derivatives has been developed by devising a rhodium-catalyzed
stitching reaction/alkene isomerization sequence. The reactions
proceed smoothly under mild conditions for a variety of substrate
combinations, and extended π-conjugation systems are also readily
accessible by utilizing this synthetic method. Optical properties of the
obtained fluorene derivatives have also been examined.
X
B(OR)2
I
IV
II
Rh-catalyst
stitching
reaction
alkene
isomerization
XH
Fluorenes constitute a structural motif often found in various
functional π-conjugated materials such as light-emitting diodes,
III
semiconductors, and
chemosensors based
on
their
optoelectronic properties.[1] They are also used as ligands for
transition metals in their anionic forms to provide effective
catalysts for several reactions such as olefin polymerization.[2] It
is therefore highly important to develop synthetic methods that
allow for a rapid access to diverse arrays of fluorenes and related
compounds with high efficiency.
Scheme 1. Synthetic strategy of (benzo)fluorenes by rhodium-catalyzed
stitching reaction followed by alkene isomerization (X = O, NR, etc.).
alkynes[13] and an aldehyde (or its derivative),[14] and subsequent
isomerization of the diene moiety toward a thermodynamically
more stable aromatic ring system accomplishes the formation of
(benzo)fluorene IV.
In addition to conventional synthetic approaches such as
Friedel–Crafts cyclization[3] and radical cyclization,[4] transition-
metal-catalyzed cyclization involving C–H bond activation and
subsequent C–C bond formation has recently been well
investigated[5] for both aromatic C–H bond activation[6] and
benzylic C–H bond activation.[7] However, most of them are
intramolecular processes based on the use of substituted biaryls
or diarylmethanes,[8] and it is more desirable to develop
intermolecular reactions in view of convergence of the process
and diversity of the accessible structures. In this regard, several
intermolecular synthetic methods of fluorenes have been reported,
including palladium-catalyzed tandem cross-coupling/cyclization
reactions,[6d,g,7a,b] metal-catalyzed [2+2+2] cycloaddition of diynes
and alkynes,[9] and gold-catalyzed annulation of enynes and
arylalkynes.[10,11] Although some of them seem effective, they
often suffer from limitations such as narrow substrate scope, high
reaction temperature, and low product selectivity. As a new and
complementary intermolecular approach, herein we describe the
development of a rhodium-catalyzed stitching reaction/alkene
isomerization sequence for the synthesis of fluorene derivatives
from easily accessible substrate combinations.
Based on this synthetic strategy, we initially chose 2-((tert-
butyldimethysilyl)ethynyl)phenylboronate
1a
and
2-(1-
pentynyl)benzaldehyde (2a) as a model substrate combination for
the rhodium-catalyzed stitching reaction [Eq. (1)]. After some
investigation, we found that, by using [{Rh(OH)(cod)}2] (5 mol%
Rh) as the catalyst with additional 1,5-cyclooctadiene (25 mol%)
and H2O (1.3 equiv) in THF, the reaction proceeded smoothly
under mild conditions to give corresponding stitched product 3aa
in 94% NMR yield.[15] Treatment of crude 3aa with nBu4NF led to
clean formation of 11H-benzo[a]fluoren-5-ol 4aa through
desilylation and alkene isomerization, and crude 4aa was further
converted to corresponding acetate 5aa for the ease of isolation
(84% overall yield). It is worth noting that the first step of this
reaction sequence represents a new entry of the rhodium-
catalyzed stitching reaction, which enables the formation of a 5,6-
fused carbocyclic system by
a 6-membered ring-forming
termination process using an aldehyde as the electrophilic
component, instead of the previously reported 5-membered ring
formation with an aryl bromide as the termination step.[12]
Our synthetic strategy is illustrated in Scheme 1. Thus, we
imagined that (benzo)fluorenes would be readily synthesized from
2-alkynylarylboronates I and 2-alkynylbezaldehydes (or b-alkynyl-
a,b-enals) or their derivatives II as follows: a reaction of I with II in
the presence of a rhodium(I) catalyst in a stitching manner[12]
would lead to initial product III through the formation of three
consecutive carbon–carbon bonds via successive insertion of
SiMe2tBu
[{Rh(OH)(cod)}2]
(5 mol% Rh)
cod (25 mol%)
tBuMe2Si
B(neop)
1a (1.3 equiv)
OH
+
H2O (1.3 equiv)
THF, 20 °C, 20 h
O
nPr
3aa: 94% NMR yield
nPr
(neop = OCH2CMe2CH2O)
2a (0.10 M)
Ac2O (3.0 equiv)
Et3N (3.0 equiv)
DMAP (5 mol%)
OH
OAc
[a]
M. Nishida, Prof. Dr. R. Shintani
Division of Chemistry, Department of Materials Engineering Science
Graduate School of Engineering Science
nBu4NF•3H2O
(1.0 equiv)
(1)
CH2Cl2, RT, 3 h
THF, RT, 1 h
Osaka University, Toyonaka, Osaka 560-8531 (Japan)
E-mail: shintani@chem.es.osaka-u.ac.jp
nPr
4aa
nPr
5aa: 84% overall yield
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/chem.201901519
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X
limited to silyl groups, which will be removed by the treatment with
nBu4NF. Thus, phenyl and 2-naphthyl groups can be installed at
6-position by employing the corresponding arylboronates as
shown in Eq. (2) (4ga–4ha, 72–78% yield). It is also possible to
install substituents at this position by utilizing ortho-
functionalization of the phenol moiety of obtained
(benzo)fluorenes. For example, selective bromination of
compound 4ak proceeds smoothly by using 1.0 equiv of N-
bromosuccinimide (NBS) to give 4ik in 81% yield [Eq. (3)].
SiMe2tBu
[{Rh(OH)(cod)}2]
1. nBu4NF•3H2O
THF
RT or 40 °C
OR’
B(neop)
(5 mol% Rh)
cod (25 mol%)
1 (1.3 equiv)
X
+
H2O (1.3 equiv)
THF, 20 °C, 20 h
2. Ac2O, Et3N
DMAP, CH2Cl2
RT or 40 °C
O
Y
R
4 (R’ = H) or 5 (R’ = Ac)
R
Y
2 (0.10 M)
OAc
OAc
OAc
R
[{Rh(OH)(cod)}2]
(5 mol% Rh)
cod (25 mol%)
R
6
nPr
B(neop)
1 (1.3 equiv)
OH
R
H
Y
nBu4NF•3H2O
5aa (R = nPr): 84% yield 5ad (R = SiMe3): 71% yield 5ae (Y = OMe): 87% yield
+
(2)
5ab (R = Me): 86% yield
5ac (R = Ph): 42% yield
(10 mol% Rh, 40 °C)
5af (Y = Cl): 83% yield
THF, RT
H2O (1.3 equiv)
THF, 20 °C, 20 h
O
nPr
4ga (R = Ph): 78% yield
4ha (R = 2-naphthyl): 72% yield
OH
OAc
OAc
nPr
2a (0.10 M)
Y
N
Br
N
OH
nPr
OH
nPr
nPr
5aj: 83% yield
Me
5ag (Y = NMe2): 85% yield
5ah (Y = NO2): 81% yield
5ai (Y = Br): 80% yield
4ak: 72% yield
NBS (1.0 equiv)
MeCN, RT, 3 h
(3)
N
N
nPr
OH
OH
OAc
nPr
Me
4ak
Me
4ik: 81% yield
Me2N
S
It is worth noting that the present stitching reaction conditions
can also accommodate imines as the 6-membered ring-forming
termination moiety instead of aldehydes described above.[17] The
stitching reaction of arylboronate 1a with N-tosyl imine 6
proceeded smoothly under the same conditions, and subsequent
treatment with nBu4NF led to benzofluorene 7 possessing a
sulfonamido group at 5-position in 82% overall yield. Unlike the
reactions using aldehydes, tert-butyldimethysilyl group at 6-
position remained intact by the reaction with nBu4NF in this
particular case. Furthermore, an acylmethyl group can be
installed at 5-position by employing alkyne 8 having an a,b-
unsaturated ketone as the termination moiety [Eq. (5)].[18]
nPr
4al: 77% yield
nPr
nPr
5ba: 83% yield
4am: 74% yield (15 h)
Me
OAc
F
OAc
OAc
F
O
O
nPr
5ca: 87% yield
nPr
5da: 90% yield
nPr
5ea: 85% yield (40 °C)
OAc
OAc
Me2N
NO2
nPr
5fa: 88% yield
nPr
5bh: 76% yield
Scheme 2. Scope of rhodium-catalyzed stitching reaction followed by alkene
isomerization.
SiMe2tBu
[{Rh(OH)(cod)}2]
(5 mol% Rh)
cod (25 mol%)
tBuMe2Si
6
B(neop)
1a (1.3 equiv)
NHTs
5
nBu4NF•3H2O
(4)
We subsequently found that the present reaction sequence
could be readily applied to the synthesis of various substituted
fluorene derivatives as summarized in Scheme 2. For example,
substituents on the alkyne of aldehydes 2 can be not only an alkyl
or aryl group but also a silyl group, which is regarded as a terminal
alkyne surrogate by treatment with nBu4NF, to give corresponding
benzofluorenes 5aa–5ad (42–86% yield). In addition, 2-(1-
pentynyl)benzaldehydes having various substituents as well as
heteroaromatic aldehydes and an alkenyl aldehyde are applicable
to give substituted benzofluorenes 5ae–5ai, heteroaryl-fused
fluorenes 5aj and 4ak–4al, and alkyl-substituted fluorene 4am,
respectively, in uniformly high yields (72–87% yield).[16] With
regard to the variation of arylboronates 1, several electronically
different arylboronates including a naphthylboronate are well
tolerated, giving corresponding products 5ba–5fa in 83–90% yield.
By employing arylboronate 1b having a dimethylamino group and
+
THF, RT, 2 h
H2O (1.3 equiv)
THF, 20 °C, 20 h
Ts
N
nPr
7: 82% yield
nPr
6 (0.10 M)
Ph
B(neop)
1g (1.3 equiv)
O
Me
Ph
[{Rh(OH)(cod)}2]
(10 mol% Rh)
cod (50 mol%)
+
nBu4NF•3H2O
O
5
(5)
Me
THF, 0 °C, 1 h;
RT, 4 h
H2O (1.3 equiv)
THF, 20 °C, 20 h
nPr
9: 63% yield
nPr
8 (0.10 M)
More extended π-conjugation systems are readily accessible
as well by using the present synthetic method. For example, the
two-fold stitching reaction of arylboronate 1g with dialkynyl
benzaldehyde 2h having
benzofluorene 5bh can also be synthesized (76% yield).
Furthermore, substituents on the alkyne of arylboronates are not
a
nitro group, donor-acceptor
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2
1
0
dialdehyde 10 under slightly modified conditions, and subsequent
treatment with nBu4NF followed by acetylation, quickly provides
dihydrodiindenoanthracene 11 in a relatively high yield [60%
yield; Eq. (6)]. As a different approach, benzofluorenyl triflate 5ad’,
which is obtained by triflation instead of acetylation in the final
step of the present synthetic sequence,[18] can be homo-coupled
under reducing conditions to give bibenzo[a]fluorenyl 12 in 68%
yield [Eq. (7)].[19]
5aa
5ba
5ah
5bh
11
ε
Ph
[{Rh(OH)(cod)}2]
B(neop)
(15 mol% Rh)
cod (75 mol%)
1. nBu4NF•3H2O
THF, RT, 10 h
1g (2.6 equiv)
+
250
300
350
400
450
500
550
600
wavelength (nm)
H2O (2.6 equiv)
THF, 10 °C, 15 h
2. Ac2O, Et3N
DMAP, CH2Cl2
RT, 5 h
O
Figure 1. UV-vis spectra of compounds 5aa (blue line), 5ba (green line), 5ah
(dark yellow line), 5bh (orange line), and 11 (red line) in CH2Cl2 (1.0 x 10–5 M)
at 25 °C.
nPr
nPr
Ph
OAc
nPr
O
10 (0.05 M)
(6)
nPr
5aa
AcO
5ba
Ph
1
5ah
11: 60% yield
5bh
11
OTf
Pd(OAc)2 (10 mol%)
Binap (10 mol%)
(7)
0.5
Zn (20 equiv)
DMF, 75 °C, 3 h
5ad’
12: 68% yield
0
With a series of new fluorene derivatives in hand, we began
to examine their optical properties. As summarized in Table 1, we
chose some of these compounds for comparison (5aa: parent,
5ba: 9-NMe2, 5ah: 3-NO2, 5bh: 9-NMe2-3-NO2, 11: doubly fused).
In both UV-vis absorption (Figure 1) and fluorescence spectra
(Figure 2), introduction of an electron-donating group at 9-position
(5ba) or an electron-withdrawing group at 3-position (5ah) led to
a red-shift in their UV-vis absorption and emission band maxima
compared to the parent compound (5aa), although the quantum
yields did not show much difference (FF = 0.21–0.32). Even more
significant red-shifts of UV-vis absorption and emission band
300
400
500
600 700
wavelength (nm)
800
900
1000
Figure 2. Fluorescence spectra of compounds 5aa (blue line; lex = 266 nm),
5ba (green line; lex = 345 nm), 5ah (dark yellow line; lex = 384 nm), 5bh (orange
line; at 2.0 x 10–4 M, lex = 458 nm), and 11 (red line; lex = 321 nm) in CH2Cl2
(1.0 x 10–4 M except for 5bh) at 25 °C.
maxima were observed for donor-acceptor compound 5bh,
reaching near-IR region in its emission maximum (lmax = 723 nm).
On the other hand, compared with benzofluorene 5aa, doubly
fused dihydrodiindenoanthracene 11 showed much larger
absorption coefficient with a significant red-shift while keeping the
overall absorption pattern, and the emission band maximum was
also red-shifted with a similar quantum yield. These results
demonstrate that the optical properties can be effectively tuned
by changing the substituents and/or extending the core structure
of these (benzo)fluorene derivatives.
In summary, we have developed a new and convergent
synthetic method of fluorene derivatives by devising a rhodium-
catalyzed stitching reaction/alkene isomerization sequence. The
reactions proceed smoothly under mild conditions for a variety of
substrate combinations, and extended π-conjugation systems are
also readily accessible by utilizing the present synthetic method.
Future studies will be directed toward further development of new
and efficient synthetic methods for various functional organic
compounds.
Table 1. Optical properties of compounds 5aa, 5ba, 5ah, 5bh, and 11 in
CH2Cl2 at 25 °C.
UV-vis absorption[a]
l
Fluorescence[b]
lmax/nm (lex/nm)
Compound
F
F (lex/nm)[b]
max/nm (e/104 M–1cm–1
)
5aa
257 (4.6), 266 (7.1), 286 (1.1), 359, 372 (266)
295 (1.3), 308 (1.3), 319 (1.1),
348 (0.2)
0.25 (266)
5ba
5ah
290 (3.2), 345 (3.5)
458 (345)
0.32 (345)
0.21 (384)
281 (2.3), 295 (2.0), 303 (1.8), 537 (384)
385 (1.3)
5bh
11
266 (2.9), 307 (1.9), 458 (1.9) 723 (458)[c]
— (458)[c,d]
0.30 (321)
257 (2.8), 309 (7.9), 321 (17.5), 456, 485 (321)
358 (0.5), 378 (0.8), 398 (1.1),
415 (1.0), 440 (0.7)
[a] At 1.0 x 10–5 M. [b] At 1.0 x 10–4 M. [c] At 2.0 x 10–4 M. [d] Not determined.
Acknowledgements
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COMMUNICATION
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Keywords: fluorene • isomerization • π-conjugated compound •
rhodium • stitching reaction
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[15] Notes: a) The reaction becomes less clean at higher temperatures; b)
The reaction also proceeds in the absence of H2O, but addition of H2O is
beneficial, presumably accelerating the transmetalation step; c)
Compound 3aa is relatively stable, but decomposes on silica gel
chromatography.
[5]
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10.1002/chem.201901519
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COMMUNICATION
A new and convergent synthetic
Masaki Nishida, Ryo Shintani*
method of fluorene derivatives has
been developed through a rhodium-
convergent
fluorene synthesis
Page No. – Page No.
XH
catalyzed stitching reaction/alkene
isomerization sequence. The
reactions proceed smoothly under
mild conditions for a variety of
substrate combinations, and extended
π-conjugation systems are also
readily accessible by utilizing this
method.
X
B(OR)2
Convergent Synthesis of Fluorene
Derivatives by Rhodium-Catalyzed
Stitching Reaction/Alkene
alkene
isomerization
Rh-cat.
Isomerization Sequence
stitching
reaction
XH
This article is protected by copyright. All rights reserved.