Facile synthesis of polycyclic fluorene derivatives via a palladium-catalyzed coupling, propargyl-allenyl isomerization and schmittel cyclization sequence

Article abstract of DOI:10.1002/adsc.200900600

A stepwise process involving Sonogashira coupling, propargyl-allenyl isomerization and Schmittel cyclization has been realized, leading to an efficient synthesis of polycyclic fluorene derivatives from readily available starting materials. The reaction features the formation of three new carbon-carbon bonds to construct the benzene unit together with an efficient assembly of three or four rings in a single operative step.

Full text of DOI:10.1002/adsc.200900600

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DOI: 10.1002/adsc.200900600
Facile Synthesis of Polycyclic Fluorene Derivatives via a
Palladium-Catalyzed Coupling, Propargyl-Allenyl Isomerization
and Schmittel Cyclization Sequence
Ruwei Shen,a Linzhu Chen,a and Xian Huanga,b,*
a
Department of Chemistry, Zhejiang University, Hangzhou 310028, Peopleꢀs Republic of China
Fax : (+86)-571-8880-7077; e-mail: huangx@mail.hz.zj.cnhuangx@zju.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, Shanghai 200032, Peopleꢀs Republic of China
Received: August 31, 2009; Revised: October 16, 2009; Published online: November 18, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900600.
Abstract: A stepwise process involving Sonogashira
coupling, propargyl-allenyl isomerization and
Schmittel cyclization has been realized, leading to
an efficient synthesis of polycyclic fluorene deriva-
tives from readily available starting materials. The
reaction features the formation of three new
carbon-carbon bonds to construct the benzene unit
together with an efficient assembly of three or four
rings in a single operative step.
erated vinylallene intermediate would undergo a
Diels–Alder reaction under mild conditions, providing
an efficient synthesis of structurally complex poly-
cycles with 2,3-dihydrofuran units[5a] and structurally
diverse fused dihydroisobenzofuran derivatives.[5b]
Furthermore, our continuing study led us to an unex-
pected observation that the involved vinylallene inter-
mediates can also be trapped by an intramolecular
Alder-ene reaction to produce some structurally inter-
esting 2,3-dihydrofurans, making these reactions an
attractive diversity-oriented approach to access a vari-
ety of useful skeletons.[5c]
Keywords: allenes; fluorenes; Schmittel cyclization;
sequential reactions
As part of our continuing efforts in this chemistry,
herein we envisioned a novel sequence of palladium-
catalyzed coupling, propargyl-allenyl isomerization
and Schmittel cyclization.[6] The enyne-allene inter-
The synthesis of various carbocycles via efficient for- mediate B, which is expected to be generated in situ
mation of carbon-carbon bonds is a highly desired ac- from the palladium-catalyzed coupling reaction of
tivity in organic synthesis.[1] Sequential reactions that electron-deficient vinyl iodide 1 with 1-(2-alkynylphe-
often feature formation of several bonds and stereo- nyl)propargyl ether 2 followed by propargyl-allenyl
centers in a single step, have received great attention isomerization, may preferentially cyclize to produce a
because they address fundamental principles of syn- benzofulvene diradical C via Schmittel cyclization;[6,7]
thetic efficiency and reaction processing.[1,2] With deli- then an intramolecular 1,6-diradical coupling reaction
cate design, these reactions often allow an efficient and 1,5-H shift may proceed to furnish the fused
and straightforward access to novel and complex car- cyclic compound 3 (Scheme 1). Appealingly, with this
bocycles and heterocycles from simple or readily ac- strategy three new carbon-carbon bonds can be
cessible starting materials, and have been widely em- formed to construct the benzene ring while three or
ployed for the synthesis of the core skeleton of many four rings could be efficiently assembled in a single
important natural products.[3]
stroke, resulting in an attractive synthesis of fluorene
Mꢁller et al. pioneered the Sonogashira coupling- derivatives, which are of considerable interest[8] since
isomerization reactions for the synthesis of a variety fluorenes are a useful class of compounds with high
of useful compounds including chalcones, pyrazolines, utility as building blocks for advanced materials with
pyrroles, fluorescent spirocycles, and some other phar- unique electrical and optical properties[9] and the fluo-
maceutically interesting heterocycles.[4] Starting from rene unit frequently shows up in bioactive mole-
electron-deficient vinyl iodides and propargyl ethers, cules.[10] In this paper we wish to report our results on
we previously established an interesting palladium- this sequential reaction.
catalyzed sequential reaction wherein the in situ gen-
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Ruwei Shen et al.
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Scheme 1. Reaction design for a tandem palladium-catalyzed coupling, propargyl-allenyl isomerization, Schmittel cyclization
sequence.
We began our investigations into the feasibility of ceeded as well at room temperature although a pro-
this cascade process using 3-iodo-5,5-dimethylcyclo- longed reaction time was needed, affording 3a in 60%
hex-2-enone (1a) and 1-[2-(2-phenylethynyl)phenyl]- yield (entry 4). Further experiments demonstrated
prop-2-ynyl methyl ether (2a), treatment of which that the reaction conducted in solvents such as THF,
with a catalytic amount of PdCl2ACTHNUTRGEN(UNG Ph3P)2 and CuI in MeCN and DMF, furnished 3a in inferior yields (en-
toluene and Et3N at 1008C for 5 h successfully led to tries 5–7). Therefore, we established the proposed se-
the desired product 3a in 73% yield (Table 1, entry 1). quential reaction conducted in toluene and Et3N at
When the reaction was performed at 608C, the yield 608C under the catalysis of 5 mol% PdCl2ACTHNUTRGEN(NUG Ph3P)2 and
was improved to 84% (entry 2). The reaction pro- 5 mol% CuI.
Table 1. Solvent and temperature effects on the sequential reaction.[a]
Entry
Solvent
Temperature [oC]
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
Toluene
Toluene
Toluene
Toluene
THF
100
80
60
5
7
7
24
6
73
76
84
60
61
67
53[c]
room tempertaure
60
60
60
MeCN
DMF
5
3
[a]
Reactions were carried out using 1a (0.2 mmol), 2a (0.24 mmol), PdCl
solvent and 0.6 mL of Et3N.
Isolated yields.
2ACHTUNGRTNE(NUNG PPh3)2 (5 mol%), and CuI (5 mol%) in 1.8 mL of
[b]
[c]
Contaminated with small amount of unknown by-products.
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Facile Synthesis of Polycyclic Fluorene Derivatives
Table 2. Sequential reaction of electron-deficient vinyl iodides 1 with 1-(2-alkynylphenyl)propargyl ethers 2.[a]
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Entry
1
1
2
Time [h]
Product 3
Yield [%][b]
R3
Ph
R4
Me (2a)
7
3a
84
2
3
4
5
6
1a
1a
1a
1a
1a
p-EtC6H4
p-ClC6H4
Ph
p-ClC6H4
Ph
Me (2b)
Me(2c)
Bn (2d)
Bn (2e)
Allyl (2f)
7
6
7
6.5
6.5
3b
3c
3d
3e
3f
87
75
84
80
85
7
Ph
Me (2a)
7
3g
3h
83
8
9
1b
1b
Ph
n-C6H13
Hept-2-ynyl (2g)
Me (2h)
5
9
74
–
[c]
–
10
Ph
Me (2a)
6
3i
73
11
12
1c
1c
p-EtC6H4
p-ClC6H4
Me (2b)
Me (2c)
7
6
3j
3k
70
71
13
14
p-EtC6H4
Me (2b)
4.5
3l
88
Ph
Me (2a)
Me (2b)
7
6
3m
3n
41
47
15
1e
p-EtC6H4
[a]
Reactions were carried out using 1 (0.2 mmol), 2 (0.24 mmol), PdCl
toluene and 0.6 mL of Et3N at 608C.
Isolated yields.
2ACHTUNGRTNE(NUNG PPh3)2 (5 mol%), and CuI (5 mol%) in 1.8 mL of
[b]
[c]
Small amount of unidentified mixture was obtained.
With the optimal conditions in hand, we next exam- no such significant effect on this reaction although it
ined the reaction scope. Typical results are summar- was observed that the yields differed to some extent
ized in Table 2. As for 1-(2-alkynylphenyl) propargyl with this factor. R4 can be Me or Bn, and interestingly
ethers 2 wherein R3 is an aromatic group such as allyl or propargyl groups were also well tolerated. For
phenyl, p-ethylphenyl and p-chlorophenyl group, the examples, when 2f and 2g were employed to react
reactions with 3-iodocyclohex-2-enone 1a or 1b pro- with 3-iodocyclohex-2-enone, the corresponding prod-
ceeded smoothly under the established conditions, de- ucts 3f and 3h were obtained in 85% and 74% yields,
livering the fluorenes 3 in good yields (Table 2, en- respectively. Here it is worth noting that in either
tries 1–8). However, when R3 is an alkyl group, for ex- case we did not observe the formation of any Diels–
ample, 2h, the reaction did not give the expected Alder cycloadduct of the vinylallene intermediate
product but a small amount of an unidentified mix- with the alkene or alkyne moiety of R4,[5a,b] indicating
ture (entry 9), probably due to the dominating occur- that the Schmittel cyclization of the presumed enyne-
rence of the Myers–Saito cyclization which complicat- allene intermediate such as B (Scheme 1) proceeds
ed the reaction.[11] The substituents R4 seem to have preferentially rather than the Diels–Alder reaction. In
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Ruwei Shen et al.
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matic halides. We found that the reaction is applicable
to those with moderate electron-withdrawing groups
such as 4-iodobenzonitrile (5b), 1-(4-iodophenyl)etha-
none (5c) and 4-bromobenzaldehyde (5d), affording
benzofluorenes 6b–e in moderate to good yields
(Table 3, entries 2–5). However, as for 1-iodo-4-nitro-
benzene (5e) in which a strong electron-withdrawing
group is incorporated, the reaction failed to give any
identifiable product (entry 6), indicating that the
Scheme 2.
addition to 3-iodocyclohex-2-enone 1a and 1b, the re- nature of the electron-withdrawing group on the aro-
action was also applicable to other electron-deficient matic ring may play a significant role in this reaction.
vinyl iodides such as 3-iodobutenolides 1c and 1d, In addition, as an example for comparison we also
producing the corresponding products in good yields conducted the reaction of 1-iodobenzene (5a) and 2a,
(entries 10–13). When (Z)-methyl 3-iodo-3-phenyl- which exclusively gave the coupling product 6a in a
ACHTUNGTRENNUNGacrylate (1e) was employed, the desired products 3m yield of up to 95% (entry 1). The benzofluorene com-
and 3n were also obtained, albeit in lower yields (en- pound was not formed in this case mainly due to the
tries 14 and 15). However, when we carried out the
reaction of (Z)-ethyl 3-iodoacrylate (1g) and 2a under
the established conditions, only a small amount of flu-
Table 3. Sequential reaction of aromatic halides 5 and 1-(2-
alkynylphenyl)propargyl ethers 2.[a]
orene compound 3o was isolated with the indene 4 as
the main product (Scheme 2). Compound 4 may
result from other transformations of the Schmittel-
cyclization diradical intermediate of the involved
enyne-allene intermediate.[6c,12] All of the products
were characterized by spectroscopic methods, and 3i
was further confirmed by X-ray crystallography
(Figure 1).[13]
Furthermore, to expand the scope of this reaction,
we also investigated a range of electron-deficient aro-
{a]
Reactions were carried out using
1 (0.2 mmol), 2
(0.24 mmol), PdCl2ACHTNUTRGNE(UGN PPh3)2 (5 mol%), and CuI (5 mol%)
in 1.8 mL of toluene and 0.6 mL of Et3N at 1008C.
Isolated yields.
[b]
Figure 1. ORTEP representation of 3i.
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Facile Synthesis of Polycyclic Fluorene Derivatives
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(2.0 mg, 5 mol%). The Schlenk tube was sealed and then
evacuated and backfilled with N2 (3 cycles). A solution of 1a
(50 mg, 0.2 mmol) and 2a (59 mg, 0.24 mmol) in 1.8 mL of
toluene and 0.6 mL of Et3N was subsequently injected to
the Schlenk tube. The reaction mixture was stirred at 608C
for 7 h. After removal of the solvent under vacuum, the resi-
dues were purified with flash chromatography (silica/petro-
leum ether:ethyl acetate 8:1) to afford 3a; yield 62 mg
(84%).
fact that the absence of an electron-withdrawing
group makes the propargyl-allenyl isomerization of 6a
reluctant to produce an allene species, which is crucial
for the subsequent transformation.
The control experiments also revealed that the in
situ generation of an allene intermediate was crucial
in the reaction. It was found that no reaction occurred
by direct heating of the coupling product 7 in toluene,
whereas the addition of amine base, which may facili-
tate a propargyl-allenyl isomerization process, suc-
cessfully led to the formation of 3a (Scheme 3).
Indeed, after extensive study on the reaction of 1a
with a variety of 1-arylpropargyl methyl ethers, quite
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (Project Nos. 20732005 and 20872127) and National
Basic Research Program of China (973 Program,
2009CB825300) for financial support.
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