Highly efficient synthesis of polysubstituted fluorene via iron-catalyzed intramolecular Friedel-Crafts alkylation of biaryl alcohols

Article abstract of DOI:10.1016/j.tetlet.2012.08.005

An efficient and mild Fe(III)-catalyzed intramolecular Friedel-Crafts alkylation of biaryl methanol derivatives has been developed to achieve the substituted fluorene derivatives. The present reaction provides an excellent alternative to published methods

Full text of DOI:10.1016/j.tetlet.2012.08.005

Tetrahedron Letters 53 (2012) 5544–5547
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Tetrahedron Letters
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Highly efficient synthesis of polysubstituted fluorene via iron-catalyzed
intramolecular Friedel–Crafts alkylation of biaryl alcohols
⇑
Soumen Sarkar, Sukhendu Maiti, Krishnendu Bera, Swapnadeep Jalal, Umasish Jana
Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and mild Fe(III)-catalyzed intramolecular Friedel–Crafts alkylation of biaryl methanol deriv-
atives has been developed to achieve the substituted fluorene derivatives. The present reaction provides
an excellent alternative to published methods because of its high yields, operational simplicity, mild con-
ditions, and use of inexpensive and sustainable catalyst.
Received 10 May 2012
Revised 3 August 2012
Accepted 5 August 2012
Available online 10 August 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Fluorenes
Friedel–Crafts alkylation
Substitution of alcohols
Iron
The chemistry of polycyclic aromatic hydrocarbons (PAHs) has
become a field of increasing interest during the past decades due
to their unique properties in material science.1 For example, they
are widely used in organic electronics, such as light-emitting
diodes (OLEDs), field-effect transistors, and solar cells.2 For this
reason, development of an efficient synthetic method for the func-
tionalized PAHs is highly desirable in modern science and technol-
ogy. In particular, functionalized fluorenes are very important
structural motifs found in many advanced materials, and have
been extensively utilized as dyes and optical brightening agents.3
In addition, a few fluorene bearing molecules exhibit interesting
bioactivities.4 Moreover, they are also frequently utilized as effec-
tive ligands in organometallic chemistry.5 Despite this widespread
utility of fluorenes, a limited number of preparative methods have
been developed. The classical method for the synthesis of fluorenes
includes intramolecular FriedelꢀCrafts alkylations promoted by
Brønsted or Lewis acids.6 Recently, several groups have indepen-
dently reported palladium-catalyzed coupling reactions to synthe-
size substituted fluorenes.7 Rh- and Cu-catalyzed intramolecular
aromatic carbenoid insertion of biaryldiazoacetates also offered a
convenient route to the synthesis of fluorene.8 Very recently, a
Bi(III)-catalyzed domino reaction of (Z)-pent-2-en-4-yl acetates
and ethynylarenes 2,20-bipyridine at 80 °C has also been reported.9
Although, few of the above-said methods are useful for the selec-
tive synthesis of fluorenes, most of these methods are associated
with significant practical drawbacks including nonavailability of
the substrates, drastic reaction conditions, use of large amount of
a strong Brønsted acid or Lewis acids, or the use of toxic and expen-
sive chemicals, and production of large amounts of waste, which
limit their applications for industrial purposes. Therefore, develop-
ment of simple, straightforward, high yielding, practical, and envi-
ronmentally benign method for the synthesis of substituted
fluorene derivatives represents a highly desirable goal in organic
synthesis.
Friedel–Crafts alkylations are among the most powerful C–C
bond-forming processes for the synthesis of functionalized aro-
matic compounds.10 Generally, drastic reaction conditions with re-
gard to temperature, use of pre-activated alcohols as electrophilic
partner, stoichiometric quantities of electrophile activators, and
the production of significant amount of byproducts limit their
applications due to environmental concerns. Therefore, a great deal
of attention has been given toward the design and development of
new catalytic protocols with the aim of minimizing hazard waste
production. Although the use of alcohols in Friedel–Crafts alkyl-
ation reactions has long been known,10 but due to irreversible
coordination of catalysts with the generating hydroxyl group or
water, stoichiometric amount or large excess of Lewis acids isre-
quired. Therefore, catalytic Friedel–Crafts alkylation with the di-
rect use of alcohols has been successfully developed only
recently.11a,b Various Lewis acids and Brønsted acids have been re-
ported to catalyze this process. Iron got special attention in this
context, as iron is an abundant, economical, sustainable, and envi-
ronmentally friendly metal. A few iron-salt catalyzed intermolecu-
lar coupling reactions of alcohols with various nucleophiles have
been demonstrated by our group and others.12 However, intramo-
lecular Friedel–Crafts alkylation with alcohols is very rare.13 Dur-
ing our continuing interest in the development of iron catalyzed
⇑
Corresponding author. Tel.: +91 9051230927; fax: +91 33 2414 6584.
E-mail address: jumasish2004@yahoo.co.in (U. Jana).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.005

S. Sarkar et al. / Tetrahedron Letters 53 (2012) 5544–5547
5545
atom-efficient organic transformations,12d,e,g–j,14 herein, we
R1
decided to study Fe(III)-catalyzed intramolecular Friedel–Crafts
alkylation reaction with direct use of biaryl alcohols 1 toward
the synthesis of structurally varied fluorene molecules 2 (Scheme
1). This methodology represents an ideal strategy for the synthesis
of fluorene compounds since they could avoid the need of pre-acti-
vation of alcohols. The reaction is operationally simple, high yield-
ing, selective, efficient, and environmentally friendly, and works
under extremely mild and neutral conditions.
The biaryl-methanol derivatives can easily be prepared by two-
step transformations in good yields, as depicted in Scheme 2. In the
first step, an aryl boronic acid or 2-formyl aryl boronic acid is cou-
pled with o-bromobenzaldehyde or aryl halides under normal Su-
zuki-coupling conditions to achieve 2-formyl biphenyl derivatives.
In the second step, a suitable Grignard reagent is treated to the
resulting 2-formyl biaryl derivatives to obtain the required biaryl
alcohols 1.
R1
R2
R1
R2
CHO
Br/B(OH)2
R3
OH
ii
i
CHO
+
B(OH)2/Br
R2
1
Scheme 2. Preparation of the biaryl-methanol derivatives. Reagents and condition:
(i) Na2CO3 (2.5 M), Pd(PPh3)4, toluene, reflux; (ii) R3MgBr, THF, NH4Cl.
Table 1
Optimization of reaction conditionsa
Catalyst
OH
Solvent
rt
Our investigation began with the optimization of the reaction
parameters including the catalyst, solvent, and temperature for
the model substrate 1a. The results are summarized in Table 1.
At first we screened various iron salts such as anhydrous FeCl3,
FeCl3ꢁ6H2O, FeBr3, Fe(acac)3, and Fe(OTf)3 for their catalytic effi-
ciency toward the intramolecular Friedel–Crafts alkylation of alco-
hols 1a. To our delight, when the reaction was performed in
MeNO2 solvent at room temperature in the presence of catalytic
anhydrous FeCl3 (5 mol %), the reaction was completed within
5 min and gave the desired fluorene 2a in 96% yield (Table 1, entry
5). In comparison to the other tested solvents such as 1,2-dichloro-
ethane, dichloromethane, and toluene (Table 1, entries 1–4),
MeNO2 has proven to be the best reaction medium in terms of
reaction efficiency, and it works under extremely mild conditions
within a very short period of time. Although this ring forming reac-
tion also works in toluene with comparable yield, it takes longer
time and high temperature. Halogenated solvents gave the lower
yield of the desired products. Increasing the amount of FeCl3 from
5 mol % to 10 mol % resulted in the same reaction time and gave 2a
almost in similar yield. However, decreasing the amount of FeCl3 to
2.5 mol % afforded slightly lower yield of 80% (Table 1, Entry 6) and
also took a longer time for complete conversion. Reduction of
yields was observed for other available iron salts such as
FeCl3ꢁ6H2O, FeBr3, and Fe(OTf)3. In case of Fe(acac)3 the substrate
1a has remained unreacted. Therefore, the Lewis acidity of various
iron-salts may play a crucial role in this intramolecular coupling
process. In addition, other commonly used Lewis acids for Fri-
edel–Crafts alkylations with alcohols such as Yb(OTf)3, AgOTf,
AlCl3, and InCl3 were also tested to check the efficiency of this par-
ticular intramolecular Friedel–Crafts alkylation reaction. In all
cases lower yields of the desired product were obtained compared
to the anhydrous FeCl3 under the similar reaction conditions. The
results show the superiority of FeCl3 as the catalyst in polar MeNO2
media. No conversion was observed at room temperature in the
absence of metal species. The reaction procedure was very simple
and straightforward.15
2a
1a
Entry
Solvent
Toluene
Catalyst (mol %)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
FeCl3 (5)
FeCl3 (5)
FeCl3 (5)
FeCl3 (5)
FeCl3 (10)
FeCl3 (2.5)
FeCl3ꢁ6H2O (5)
Fe(OTf)3 (5)
Fe(acac)3 (5)
FeBr3 (5)
60
30
30
5
92c
68
70
96
96
82
85
56
0
68
71d
75
69
81
1,2-Dichloroethane
Dichloromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
Nitromethane
5
15
10
15
90
5
9
10
11
12
13
14
InCl3 (5)
AlCl3 (5)
Yb(OTf)3 (5)
AgOTf (5)
5
15
15
15
The significance of bold values is the best reaction conditions to afford maximum
yield.
a
Reaction condition: substrate
1 (0.5 mmol), nitromethane (1 mL), FeCl3
(0.025 mmol).
b
c
The yield refers to isolated product characterized from spectral data.
Performed at 60 °C.
Performed at 45 °C
d
the results are summarized in Table 2. We are pleased to find that
a range of functional groups were tolerated under the reaction con-
ditions and all of the reactants were smoothly converted into the
desired fluorene products in nitromethane solvent in the presence
of catalytic FeCl3 (5 mol %) within a very short period of time. Sub-
strates containing electron-donating groups such as methyl and
methoxy substituents on any of the aromatic rings at para-,
meta-, or ortho- gave the desired products in high yields (Table
2). Notably, substrates bearing electron-withdrawing substituents
such as, -F and -Cl at benzylic alcohols moiety (Table 2, entry 4
and 5) were also smoothly transformed into the desired fluorene
products 2d and 2e in excellent yields, 85% and 90%, respectively.
Moreover, the reactions also work efficiently in case of the sub-
strates bearing electron-withdrawing substituents such, as –Cl
and –Br (Table 2, entries 12 and 13) on the aromatic rings to be
alkylated, and gave the desired products 2l and 2m in 90 and
92% yields, respectively. The tolerance of chloro and bromo substit-
uents at either of the ring systems might provide an additional
opportunity for further synthetic transformation via transition me-
tal-catalyzed cross-coupling reactions. Furthermore, the bulky aro-
matic biaryl substrates such as naphthalene-1-yl and
phenanthrene-9-yl were also effective substrates and annulation
took place efficiently to afford the corresponding polyaromatic flu-
orene derivatives in excellent yields (Table 2, entries 6 and 7),
which indicated that the steric and electronic effects of biaryl-2-
yl methanol did not affect the reactivity of the substrates.
In order to explore the scope of the present catalytic protocols,
varieties of substituted biaryl-2-yl-methanols were synthesized
and were treated under the optimized reaction conditions, and
R1
R1
R3
Catalyst
H2O
R3
OH
R2
R2
1
2
Scheme 1. Lewis acid mediated intramolecular Friedel–Crafts alkylation for the
synthesis of fluorene.

5546
S. Sarkar et al. / Tetrahedron Letters 53 (2012) 5544–5547
Table 2
Iron(III) chloride-catalyzed intramolecular Friedel–Crafts alkylation with biaryl alcohols for the synthesis of various fluorenesa
R2
R2
R3
FeCl3 (5 mol%)
MeNO2, rt
R3
OH
R1
R1
2
1
Entry
1
R1
R2
R3
2
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
2a
2b
2c
2d
2e
2f
2g
2h
2i
5
5
5
15
5
15
15
5
5
5
96
95
90
85
90
92
92
89
96
96
92
90
92
p-Me-C6H4
p-MeO-C6H4
p-F-C6H4
p-Cl-C6H4
1-Naphthyl
9-Phenanthryl
p-Me-C6H4
C6H5
m,p-di-OMe
9
p-OMe
p-OMe
p-Me
p-Cl
p-Br
H
H
H
H
H
10
11
12
13
1j
1k
1l
p-Me-C6H4
C6H5
C6H5
2j
2k
2l
15
5
5
1m
C6H5
2m
a
Reaction condition: substrate 1 (0.5 mmol), nitromethane (1 mL), FeCl3 (0.025 mmol).
The yield refers to isolated product characterized from spectral data.
b
Next we extended the present intramolecular alkylation reac-
tion with various other biaryl systems (Table 3). Using the similar
reaction conditions, naphthyl fused fluorene 2n was synthesized in
94% yield (Table 3, entry 1). Thiophene containing fluorene was
also investigated, as recent studies have shown that thiophene
incorporated fluorenes exhibit efficient photoluminescence prop-
erties.16 Thus the development of new efficient method is highly
desirable for the synthesis of thiophene incorporated fluorene.
Gratifyingly, thiophene containing biaryl derivative also produced
corresponding fluorene derivatives in 80% yield within 5 min in the
presence of 5 mol % FeCl3 (Table 3, entry 2). Moreover, the present
method has also worked efficiently for intramolecular Friedel–
Crafts alkylation reaction of various (2-phenylcyclohex-1-yl)meth-
anol derivatives under the optimized reaction conditions and in all
cases high yields of the desired products were obtained (Table 3,
entries 3–5). Furthermore, to check the regioselectivities of the
cyclization process a meta-substituted arene was tested, which
possesses two nonequivalent ortho-hydrogen atoms on the aryl
ring to be alkylated (Table 3, entry 6). We observed that the result-
ing regioselectivity was poor and only provided mixtures of two
nonseparable isomers in almost similar ratio (43:57), albeit in high
yields of the mixture.
Table 3
Iron(III) chloride-catalyzed intramolecular Friedel–Crafts alkylation with varieties of
biaryl systems for the synthesis of various fluorene analoguesa
Entry Substrate 1
Product 2
Time
(min)
Yieldb
(%)
Ph
OH
1n
Ph
2n
1
20
94
Ph
OH
2
5
80
78
Ph
S
1o
S
2o
Ph
Ph
3
4
15
OH
2p
1p
Finally, we attempted to synthesize 9,90-spirobifluorene via
iron-catalyzed intramolecular Friedel–Crafts alkylation, outlined
in Scheme 3. Due to their high morphological stability and higher
solubility of 9,90-spirobifluorene derivatives compared with the
corresponding non-spiro-linked parent compounds, spirobifluo-
rene derivatives have been extensively studied as potential candi-
dates in organic electronic devices.17 In particular, the most recent
study into blue light-emitting materials has centered on spiro-
based derivatives on account of their high solution and solid state
photoluminescence quantum yield.
Ph
OH
1q
Ph
20
15
88
88
2q
Ph
Ph
OH
5
6
2r
1r
OMe
OMe
Ph
OH
Ph
96c
Ph
2s'
10
+
i)
Mg, I2
Diethyl ether
2s
1s
OH
FeCl3, (5 mol %)
Br
Nitromethane
ii) Fluorenone
14h, Reflux
iii) NH4Cl
r.t, 5 min
a
Reaction condition: substrate
FeCl3(0.025 mmol).
1
(0.5 mmol), nitromethane (1 mL),
95%
4
3
85%
b
The yield refers to isolated product characterized from spectral data.
Isomeric yield.
c
Scheme 3. Iron-catalyzed synthesis of 9,90-spirobifluorene 4.

S. Sarkar et al. / Tetrahedron Letters 53 (2012) 5544–5547
5547
Pérez, D.; Guitián, E. Chem. Soc. Rev. 2004, 33, 274–283; (d) Buess, C. M.;
Lawson, D. D. Chem. Rev. 1960, 60, 313–330; (e) Sergeyev, S.; Pisula, W.; Geerts,
Y. H. Chem. Soc. Rev. 2007, 36, 1902–1929.
These compounds are generally prepared by the cyclization of
the corresponding tertiary alcohols in a boiling mixture of acetic
acid and hydrochloric acid. To our delight under the similar reac-
tion conditions tertiary carbinol 3 underwent smooth electrophilic
alkylation in the presence of FeCl3 (5 mol %) at room temperature
and gave the desired 9,90-spirobifluorene 4 in 95% yield within
5 min.
2. (a) Leclerc, M. J. Polym. Sci. Part A 2001, 39, 2867–2873; (b) Scherf, U.; List, E. J.
W. Adv. Mater. 2002, 14, 477–487; (c) Bendikov, M.; Wudl, F.; Perepichka, D. F.
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Thus, the present method is quite efficient, simple, and can be
performed in the presence of air and moisture, and only a catalytic
amount of FeCl3 is necessary. Most of the reactions were completed
within a few minutes and without the pre-activation of alcohols.
Compared to the other reported method for intramolecular Fri-
edel–Crafts alkylation reaction using excess of Brønsted or Lewis
acids such as, HCl/HOAc, PPA at refluxing temperatures or excess
amount of BF3ꢁEt2O, or using catalytic strong, hygroscopic, and
expensive Brønsted acid such as TfOH, our present method is much
more efficient in terms of amount of catalyst loading, time required
for the completion of the reaction, and the yields of the products.6
Moreover, TfOH mediated reaction often required pre-activated
alcohols such as acetate to make the reaction faster and to improve
the yields of the desired products. So, the direct use of alcohols
worked much more efficiently in the presence of FeCl3 (5 mol %)
in MeNO2 solvent compared to any other catalytic species which
are often used for direct substitution of alcohols. This proves the
superiority of the iron-catalysis for intramolecular Friedel–Crafts
alkylations with direct use of alcohols. In addition, this method
was also compatible with a broad range of functional groups such
as chloride, bromide, fluoride, methoxy, and methyl group.
The notable advantages of this method over the previous meth-
odologies are: (a) only a 5 mol % FeCl3 was sufficient to catalyze
this reaction under extremely mild and neutral conditions with a
wide ranges of substrates; (b) all of the reactions proceeded at
room temperature within a few minutes, and special precautions
for exclusion of moisture or air are not necessary to afford exclu-
sively the desired fluorene; (c) alcoholic functional group directly
has been used instead of using acetate or halides; (d) this method
permits the synthesis of a variety of unsymmetrical fluorenes and a
spirobifluorene in very good to excellent yields; (e) furthermore,
FeCl3 is very cheap, easy to handle, sustainable, and environmen-
tally friendly. We believe that many important novel materials
can be obtained by applying this synthetic route. Additional devel-
opments in this transformation are currently underway in our
laboratories.
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15. Representative experimental procedure for the synthesis of 9-phenyl-9H-fluorene
(2a): Compound 1a (130 mg, 0.5 mmol) was taken in a 5 mL round bottom
flask containing 1 mL of dry nitromethane solvent. Anhydrous FeCl3 (5 mg,
0.025 mmol) was added and the resulting solution was stirred at room
temperature for 5 min. After completion of the reaction, solvent was
evaporated in vacuo and the residue was purified by silica gel column
chromatography using petroleum ether (bp 60–80 °C) to afford the desired
product 2a (116 mg, 0.96 mmol, mmol, 96%) as a white solid,18 mp 132 °C. 1H
NMR (CDCl3, 300 MHz) d 5.08 (s, 1H), 7.11–7.12 (m, 2H), 7.25–7.36 (s, 7H), 7.42
(t, J = 7.29 Hz, 2H), 7.84 (d, J = 7.6 Hz, 2H) ppm.13C NMR (CDCl3,75 MHz) d 54.4,
119.8, 125.3, 126.8, 127.3, 128.3, 128.7, 141.0, 141.6, 147.8 ppm.
16. Wong, K.-T.; Hwu, T.-Y.; Balaiah, A.; Chao, T.-C.; Fang, F.-C.; Lee, C.-T.; Peng, Y.-
C. Org. Lett. 2006, 8, 1415–1418.
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(b) Pei, J.; Ni, J.; Zhou, X.-H.; Cao, X.-Y.; Lai, Y.-H. J. Org. Chem. 2002, 67, 4924–
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Acknowledgments
We acknowledge the financial and infrastructural support from
the UGC-CAS program of the Department of Chemistry, Jadavpur
University. The DST-PURSE program is also gratefully acknowl-
edged. S.S., K.B., and S.J. are thankful to the CSIR, New Delhi, India,
for their fellowships. S.M. is also thankful to the UGC, Jadavpur
University for his fellowship.
References and notes
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2001, 101, 1267–1300; (b) Chandrasekhar, S. Liq. Cryst. 1993, 14, 3–14; (c)
18. Schmid, M. A.; Alt, H. G.; Milius, W. J. Organomet. Chem. 1996, 15, 525.