Facile synthesis of indene and fluorene derivatives through AlCl3-catalyzed cyclization of in situ formed iminium ions

Article abstract of DOI:10.1002/aoc.3863

A simple AlCl₃-catalyzed condensation/cyclization cascade process between aldehydes and sulfonamide is reported, in which two new bonds and one five-membered ring are simultaneously formed with water as the byproduct. This method provides a rapid access to indenamine and 9-aminofluorene derivatives. Additionally, these products can be transformed into corresponding indanones and fluorenones under the developed conditions.

Full text of DOI:10.1002/aoc.3863

Received: 20 January 2017
DOI: 10.1002/aoc.3863
Revised: 12 March 2017
Accepted: 25 March 2017
C O M M U N I C A T I O N
Facile synthesis of indene and fluorene derivatives through
AlCl3‐catalyzed cyclization of in situ formed iminium ions
Lei Chen1 | Wei Teng1 | Xin‐Le Geng1 | Yi‐Fan Zhu1 | Yong‐Hong Guan1 | Xiaohui Fan1,2,3
1 School of Chemical and Biological
Engineering, Lanzhou Jiaotong University,
Lanzhou 730070, China
2 Beijing National Laboratory for Molecular
Sciences (BNLMS), College of Chemistry,
Peking University, Beijing 100871, China
A simple AlCl3‐catalyzed condensation/cyclization cascade process between alde-
hydes and sulfonamide is reported, in which two new bonds and one five‐membered
ring are simultaneously formed with water as the byproduct. This method provides a
rapid access to indenamine and 9‐aminofluorene derivatives. Additionally, these
3 YMU‐HKBU Joint Laboratory of
products can be transformed into corresponding indanones and fluorenones under
Traditional Natural Medicine, Yunnan Minzu
University, Kunming 650500, China
the developed conditions.
KEYWORDS
Correspondence
AlCl3, exo‐model cyclization, fluorene, iminium ion, indene
Xiaohui Fan, School of Chemical and
Biological Engineering, Lanzhou Jiaotong
University, Lanzhou 730070, China.
Email: fanxh@mail.lzjtu.cn
Funding information
Beijing National Laboratory for Molecular
Sciences, Grant/Award Number: 20140133;
National Natural Science Foundation of
China, Grant/Award Number: 21162013 and
21562030
1 | INTRODUCTION
Indene and fluorene are two important classes of
structural motifs present in a number of naturally occurring
The development of simple and efficient methods for the
synthesis of carbo‐ and heterocyclic molecular scaffolds
continues to be a principal issue in synthetic chemistry. In
this context, the iminium ion cyclization reaction has proven
to be an extremely useful approach for the rapid and
convergent construction of complex structures.[1] One such
example is the Pictet‐Spengler reaction, an invaluable
method being widely used in both laboratory and industry
for the preparation of a diverse array of isoquinoline and
indole alkaloids.[1,2] Despite the cyclization reactions
involving iminium ions have led to profound developments
in synthetic chemistry, where, very often, they focused on
the construction of nitrogen‐containing heterocycles through
an endo‐model.[2,3] In sharp contrast, the exo‐model
cyclizations leading to the formation of all‐carbon ring
skeletons have received considerably less attention
(Scheme 1), the reports pertaining to this class of reaction
remain scarce.[4]
molecules and pharmaceutically useful compounds.[5]
Moreover they also serve as valuable building blocks in
material science and ligands in organometallic chemistry.[6]
For example, the fluorenyl‐based Pd complexes and the
indenyl‐based Ni complexes have been used as efficient
catalysts in Suzuki coupling and olefin polymerization
reactions, respectively.[6] Consequently, these unique
carbocycles have and continue to receive significant synthetic
interest. To date, a number of effective strategies for the
synthesis of various indene or fluorene derivatives have been
developed.[7,8] Despite these advances, catalytic protocol to
construct both of indene and fluorene skeletons is absent in
the literature. In addition, many existing methods more or
less have suffered from some drawbacks, including the
necessity of expensive transition metal catalysts, special
starting materials and lengthy synthetic sequences. Therefore,
development of simple and atom‐economic methodologies to
furnish these structural motifs is still in high demand.
Appl Organometal Chem. 2017;e3863.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3863

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CHEN ET AL.
TABLE 1 Optimization of reaction conditionsa
SCHEME 1 Cyclization models of iminium ion
Aluminum chloride (AlCl3) has attracted the attention of
synthetic organic chemists for a long time, since it is a cheap,
easy available and less toxic Lewis acid and has been found
to show promising catalytic abilities in a number of organic
transformations, such as Friedel‐Crafts reaction, Diels‐Alder
reaction, Michael addition, Prins reaction, [3 + 2] cycloaddi-
tion, ring expansion, and cascade arylation/cyclization, etc.[9]
In recent years, our group has been interested in the develop-
ment of eco‐friendly methods for the synthesis of C‐C, C‐N
bonds and carbocycles through functionalization of the
C‐O/C=O bonds.[10] As part of our ongoing program on this
area, herein, we report an AlCl3‐catalyzed two‐component
cascade reaction between aldehydes and sulfonamide that
provides a facile access to indenamine and 9‐aminofluorene
derivatives. Additionally, derivation of these compounds
was also investigated, we found that indenamine and 9‐
aminofluorene can be transformed into corresponding
indanone and fluorenone under the developed conditions.
Yield (%)b / t (h)
Catalyst
(mol%)
Entry
Solvent T (°C)
3a
3o
1
FeCl3 (20)
AlCl3 (10)
AlCl3 (15)
AlCl3 (20)
AlCl3 (20)
AlCl3 (30)
AlCl3 (20)
AlCl3 (20)
AlCl3 (20)
AlCl3 (20)
AlCl3 (20)
FeCl3 (20)
TiCl4 (20)
CuI (20)
Toluene
CH3NO2
CH3NO2
CH3NO2
40
80
80
80
98/4h Mess/0.5h
2
45/4h
60/4h
90/4h
89/4h
85/4h
30/16h
51/16h
80/14h
81/8h
3
4
5
CH3NO2 100
6
CH3NO2
Toluene
EtOH
80
80
80
40
60
80
77/14h
7
75/4h Trace/12h
8
0/8h
42/8h
0/12h
37/8h
0/12h
16/12h
0/12h
9
DCM
10
11
12
13
14
15
16
17
18
19c
20
THF
DCE
10/24h
CH3NO2 100
64/4h Mess/2h
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
80 Mess/8h Trace/12h
2 | RESULTS AND DISCUSSION
80
80
80
80
80
80
80
0/4h
30/8h Trace/12h
0/8h 0/12h
58/4h Mess/2h
0/12h
BiCl3 (20)
ZnCl2 (20)
BF3·Et2O (20)
Cu(OTf)2 (20)
AlCl3 (20)
HCl (20)
Recently, our group disclosed a novel FeCl3‐catalyzed cas-
cade process to construction of indene frameworks via con-
densation of the readily accessible cinnamylaldehydes and
sulfonamides.[4d] In continuation of this research, we began
to wonder about the possibility of using this protocol to
synthesize both indene and fluorene skeletons. Initially, we
subjected 2‐phenylbenzaldehyde (1o) to the optimized
reaction conditions identified in our previous work
(Table 1, entry 1).[4d] However, reaction of 1o with TsNH2
gave an intractable mixture rather than the corresponding
cyclization product of 9‐aminofluorene (3o) even at room
temperature, presumably due to the instability of 3o under
the catalysis of FeCl3.[11] Recently, the iron‐catalyzed cleav-
age of the C‐N bond of benzylic sulfonamides has emerged
as a useful strategy in C‐C bond formation reactions.[12]
Inspired by the previous works,[9] AlCl3 was then selected
as the catalyst. To our delight, both 1a ((E)‐2‐methyl‐3‐
phenylacryl‐aldehyde) and 1o can react smoothly with TsNH2
in nitromethane to give the desired cyclization products
(Table 1, entry 2). Encouraged by this result, the reaction con-
ditions were then systematically investigated. These examina-
tions revealed that treating 1a or 1o (1.0 equiv.) with 1.2
equiv. of TsNH2 in CH3NO2 in the presence of 20 mol% AlCl3
gave the best results (Table 1, entries 4 and 5). Under the opti-
mized reaction conditions, the desired cyclization products 3a
38/4h
88/4h
0/4h
26/16h
82/14h
0/12h
aReaction conditions: 1a (0.3 mmol), TsNH2 (0.36 mmol), solvent (3 mL).
bIsolated yield.
cUnder argon.
and 3o could be isolated in 90% and 81% yields, respectively.
Replacing AlCl3 with FeCl3, the reaction of 1o with TsNH2 in
CH3NO2 also resulted in an inseparable mixture (Table 1,
entry 12). Other Lewis acids, such as TiCl4, CuI, BiCl3,
ZnCl2, BF3·OEt2 and Cu(OTf)2, were found to be less effec-
tive or ineffective in promoting this transformation (Table 1,
entries 13–18). Further investigation of this reaction under
an argon atmosphere had no obvious influences on the yields
(Table 1, entry 19). In addition, hydrochloric acid showed no
catalytic activity in this transformation, which suggests that
the present reactions is catalyzed by AlCl3 rather than its
hydrolyzed species (Table 1, entry 20).
With the optimized reaction conditions in hand (Table 1,
entry 3), the generality of this AlCl3‐catalyzed reaction was

CHEN ET AL.
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then investigated on a series of aldehydes. As shown in
Scheme 2, the optimized reaction conditions show good sub-
strate compatibility with a wide variety of cinnamylaldehydes
and arylated benzaldehydes, providing the corresponding
indene and fluorene derivatives in moderate to good yields
(3a–k; 3m–t). However, the starting cinnamylaldehyde
has an α‐substituent is crucial to this reaction, for example,
(Z)‐cinnamaldehyde and (E)‐3‐phenylbut‐2‐enal can not take
place this transformation even under harsh reaction condi-
tions. The only exception to this structural requisite is the
aldehyde (1h) that can carry out this reaction successfully
without an α‐substituent, presumably due to its
stereoconvergent structure. These results are consistent with
our previous observations.[4d] In addition, the electronic
natures of the substrates also have obvious influences on
the reaction outcomes. When aldehydes 1l and 1u bearing
an electron‐withdrawing group on the phenyl ring were sub-
jected to this transformation, the cyclization reactions were
totally inhibited, only corresponding imines along with the
unreacted aldehydes were observed; in contrast, when elec-
tron‐donating substituents such as alkyl or methoxy groups
were placed on the phenyl ring, the reaction could carry out
at 40 °C or room temperature within a short reaction time
(3b–f). However, the methoxy group substituted substrates
gave relatively low yields (3b and 3e; 3c and 3f), whereas
increasing the catalyst loading led to inferior yields. The rea-
son for this phenomenon is unclear, we think that the elec-
tronic properties and the coordination abilities of these
substrates should be responsible for the observed results.[13]
In addition, 1‐naphthyl and 2‐naphthyl substituted acroleins
1m and 1n can also be applied to this reaction, providing a
facile method to access benzene‐fused indene type of tricy-
cles (3m–n), which are also useful carbocyclic skeletons.[14]
To our disappointment, aromatic and cinnamic ketones such
as (E)‐3‐methyl‐4‐phenylbut‐3‐en‐2‐one and 2‐acetyl‐
bipheny can not undergo this transformation even under
harsh reaction conditions, which can probably be attributed
to the lower electrophilicity of the ketone.
Subsequently, derivation of the cyclization products was
carried out using 3a and 3o as the substrates. However,
removal of the tosyl‐moiety from 3a and 3o proved much
more challenging, continued efforts to achieve this goal
under a variety of conditions[15] (SmI2, sodium naphthalide,
Mg/CH3OH, LAH/THF, NaOH/CH3OH) failed to give the
deprotected products. Surprisingly, upon treatment of these
compounds with SmI2/pyrrolidine/water[16] in THF, 3a was
smoothly transformed into indanone 4a in 91% yield without
the formation of detosylation product (Scheme 3), whereas
SCHEME 2 Reaction of TsNH2 with various aldehydes
SCHEME 3 Formation of indanone 4a and fluorenone 4o

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CHEN ET AL.
3o led to an inseparable mixture. Other indenamine deriva-
tives such as 3k can also efficiently undergo this transforma-
tion. The mechanism for this transformation is not clear at
this stage,[17] further experimental and mechanistic studies
are underway in our group. In the case of 3o which can be
readily converted into corresponding fluorenone 4o upon
treatment with tetrabutylammonium fluoride (TBAF).[18]
With the aim of understanding the mechanism, control
experiments were conducted (Scheme 4). First, prior‐
prepared aldimines 5a and 5o were subjected to this reaction
under the optimal conditions. Strikingly, both 5a and 5o
failed to give the cyclization products 3a and 3o even under
improved reaction temperature and prolonged reaction time,
which led to partially decomposition of 5a and 5o into corre-
sponding aldehydes. Second, treating 5a and 5o with
20 mol% of hydrochloric acid, most of them were hydrolyzed
into 1a and 1o without the formation of 3a and 3o. Third,
leaving aldehyde 1a or 1o in typical reaction conditions with-
out addition of TsNH2, no reaction was observed.
Based on the above results and previous works,[13,4]
a
tentative reaction pathway is described in Scheme 5. Initially,
AlCl3‐catalyzed condensation of the aldehyde 1a (or 1o) and
TsNH2 leads to the formation of resonance‐stabilized
iminium ion A (or C), A can isomerize to its Z isomer B.
Subsequently, intermediate B (or C) undergoing an intramo-
lecular aza‐Friedel‐Crafts reaction or a 4π electrocyclization
process would give rise to the corresponding cyclization
product. In the case of aldehyde 1a, isomerization of interme-
diate A through bond rotation to give its Z isomer B is neces-
sary before cyclization can occur. We speculate that
SCHEME 5 Proposed mechanism
this process is facilitated by the cinnamylaldehyde bearing
an α‐substituent, which has an A1,2 strain between the α‐alkyl
and the aromatic group. Additionally, formation of
the delocalized cationic species would also benefit from the
α‐alkyl due to its electron‐donating nature.
3 | CONCLUSIONS
In summary, an AlCl3‐catalyzed two‐component cyclization
reaction between aldehydes (i.e. cinnamylaldehydes and
arylated benzaldehydes) and sulfonamide for the convenient
synthesis of fluorene and indene derivatives has been devel-
oped. The advantages of this method include readily available
starting materials, cheap catalyst, simple reaction conditions,
and atom‐economic way. Additionally, this method can also
be used as an alternative for the synthesis of indanones and
fluorenones. Further refining this method in organic synthe-
sis is in progress in our laboratory and will be reported in
due course.
3.1 | Experimental
To a stirred solution of aldehydes 1a‐1u (0.3 mmol) in
CH3NO2 (3 mL) was added 4‐methylbenzenesulfonamide
(0.36 mmol, 61.6 mg) and AlCl3 (20 mol%, 8.0 mg). The
resulting mixture was stirred at rt‐100 °C. After the aldehyde
was completely consumed (monitored by TLC), the reaction
was quenched by addition of H2O (3 mL) and then extracted
SCHEME 4 Control experiments

CHEN ET AL.
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ether/EtOAc =10/1) to afford the corresponding products.
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We thank the National Natural Science Foundation of China
(21162013 and 21562030) and Beijing National Laboratory
for Molecular Sciences (20140133) for financial support,
and Prof. Zhi‐Xiang Yu for helpful discussions.
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How to cite this article: Chen L, Teng W, Geng X‐L,
Zhu Y‐F, Guan Y‐H, Fan X. Facile synthesis of indene
and fluorene derivatives through AlCl3‐catalyzed
cyclization of in situ formed iminium ions. Appl
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