Substituent-dependent, iron-mediated tandem cyclization of diynes with benzaldehyde acetals to form highly functionalized indene derivatives

Article abstract of DOI:10.1002/chem.201101667

A three-ring circus: The FeCl₃-and FeBr₃-mediated tandem cyclization of diynes with diethyl benzaldehyde acetals to form fused tricyclic indenes has been accomplished (see scheme). The consecutive formation of three new C-C bonds all

Full text of DOI:10.1002/chem.201101667

COMMUNICATION
DOI: 10.1002/chem.201101667
Substituent-Dependent, Iron-Mediated Tandem Cyclization of Diynes with
Benzaldehyde Acetals to Form Highly Functionalized Indene Derivatives
Tongyu Xu,[a] Qin Yang,[a] Wenjing Ye,[a] Quanbin Jiang,[a] Zhaoqing Xu,[a]
Jiping Chen,[a] and Zhengkun Yu*[a, b]
Dedicated to Professor Christian Bruneau on the occasion of his 60th birthday
The construction of functionalized carbo- and heterocy-
cles from relatively simple and readily available starting ma-
terials in a convergent manner is always desired in organic
synthesis.[1] Diynes have been used as versatile building
blocks for the preparation of carbo- and heterocycles
through transition-metal catalyzed carbocyclization, cycload-
dition, and cycloisomerization reactions.[2] The indene back-
bone is frequently found in natural products,[3] pharmaceuti-
cals,[4] functional materials,[5] and metallocene complexes,[6]
and a variety of synthetic methods have been developed for
their construction.[7] Although enediynes have been docu-
mented for the synthesis of indenes,[8] diynes have seldom
been applied for this purpose.[9] Recently, iron catalysis has
emerged as a promising, environmentally benign alternative
to traditional transition-metal catalysis due to the many ad-
vantages of utilizing iron-catalyst precursors, such as low
cost, nontoxicity, good stability, and straightforward methods
of handling.[10–12] Very recently, we discovered that FeCl3
and FeCl3·6H2O can efficiently promote the intramolecular
cyclization of monoalkynyl aldehyde acetals to generate sol-
vent-dependent Prins-type products[13a] and a,b-unsaturated
cyclic ketones,[13b] respectively (Scheme 1). Herein, we
report the FeCl3- and FeBr3-mediated tandem cyclization of
simple diynes with benzaldehyde acetals for the synthesis of
highly functionalized indenes.
indene 3a in 55% isolated yield (Table 1, entry 1). Although
several other products could be seen in the TLC analysis of
the reaction mixture, no isomers of 3a or other potential
products were successfully isolated. At À108C, 3a was
formed in a better yield (63%), although formation of 3a
(66%) was further improved by performing the reaction at
08C (Table 1, entries 2 and 3). The reaction was less efficient
Scheme 1. The iron-mediated intramolecular cyclization of monoalkynyl
aldehyde acetals (Ts=tosyl).[13]
Table 1. The screening of reaction conditions for the cyclization of diyne
1a with diethyl benzaldehyde acetal (2a).[a]
In our initial studies, the reaction of diyne 1a with diethyl
benzaldehyde acetal 2a was investigated to screen the reac-
tion conditions (Table 1). Under a nitrogen atmosphere,
compound 1a was reacted with an equimolar amount of 2a
in the presence of FeCl3 (1.0 equiv) in CH2Cl2 at ambient
temperature, to give 3-chlorovinyl-2,3-trisubstituted 1H-
Lewis acid
([equiv])
Solvent
T
[8C]
t
Yield[b]
[%]
G
[h]
1
2
3
4
5
6
7
8
9
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.2)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3/CH3COCl (0.1:2.0)
FeCl3/Me3SiCl (0.1:2.0)
FeCl3·6H2O (1.0)
SnCl4 (1.0)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
RT
À10
0
0
0
RT
RT
RT
RT
0
1
1
0.5
0.5
1
1
1
10
4
4
55
63
66
61
36
34
40
24
37
n.r.
70
48
[a] T. Y. Xu, Q. Yang, W. J. Ye, Q. B. Jiang, Dr. Z. Q. Xu,
Prof. Dr. J. P. Chen, Prof. Dr. Z. K. Yu
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian, Liaoning 116023 (P.R. China)
Fax : (+86)411-8437-9227
DCE
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
E-mail: zkyu@dicp.ac.cn
[b] Prof. Dr. Z. K. Yu
10
11
12
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry (CAS)
354 Fenglin Road, Shanghai 200032 (P. R. China)
0
0
0.5
0.5
TiCl4 (1.0)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101667.
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), solvent (5 mL),
N2 atmosphere. [b] Isolated yields of 3a. DCE=1,2-dichloroethane.
Chem. Eur. J. 2011, 17, 10547 – 10551
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10547

Table 2. The FeCl3-mediated cyclization of diynes 1a–d with acetals 2.[a]
if it was carried out in DCE or toluene, whereas di-
chloromethane seems to be the most suitable sol-
vent for the reaction (Table 1, entries 1–7). The use
of an excess of FeCl3 or catalytic amounts of FeCl3
with
Me3SiCl, led to a lower reaction efficiency (Table 1,
entries 4, 8, and 9). Unexpectedly, iron(III) trichlor-
a chloride source, such as CH3COCl or
AHCTUNGTRENNUNG
diyne (1)
Ar’ (2)
Product
(Y; yield[b] [%])
ide hydrate (FeCl3·6H2O) exhibited no activity in
this reaction (Table 1, entry 10). However, Lewis
acids SnCl4 and TiCl4 showed moderate to good ac-
tivity (Table 1, entries 11 and 12). Due to the easier
manipulation of FeCl3 than SnCl4, the former was
chosen as the mediator/chloride source for the
tandem cyclization/halogenation of 1a with benzal-
dehyde acetals. The reactions of 1a with PhCH-
AHCTUNGTRENNUNG
1
1a
1a
1a
1a
1a
1a
1a
Ph (2a)
3a (Cl; 66)
3b (Cl; 52)
3c (Cl; 51)
3d (Cl; 49)
3e (Cl; 46)
3 f (Br; 70)
3g (Br; 50)
2[c]
3[c]
4
4-MeOC6H4 (2b)
3-MeOC6H4 (2c)
3-CF3C6H4 (2d)
3-BrC6H4 (2e)
Ph (2a)
5
6[d]
7[d]
4-ClC6H4 (2 f)
ACHTUNGTRENNUNG(OMe)2 (2ab), PhCHAHCTUNGTREN(NUGN OCH2CH2O) (2ac), and their
8
9
1b
1c
4-MeOC6H4 (2b)
Ph (2a)
3h (Cl; 41)
parent aldehyde, PhCHO (2ad), were also carried
out under the same conditions, producing 3a in 48,
41 and 0% yields, respectively, and revealing that
diethyl benzaldehyde acetal 2a exhibits much
higher reactivity than its analogues (Scheme 2).
Under the optimized conditions, a variety of re-
actions of compounds 1 with compounds 2 were ex-
plored (Table 2). TsN-linked diyne 1a reacted with
diethyl benzaldehyde acetals 2a–e to form the de-
sired products 3a–e in 46–66% yields (Table 2, en-
tries 1–5). By using FeBr3 instead of FeCl3 as the
Lewis acid mediator/halogen source, the reactions
of 1a with 2a and 2 f gave the same type of prod-
ucts, that is, 3 f (70%) and 3g (50%), respectively,
3i (Cl; 25) + 4 (60)
3j (Cl; 62)
3j (Cl; 67)[e]
10
1d
Ph (2a)
[a] Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), FeCl3 (0.5 mmol), CH2Cl2 (5 mL),
N2 atmosphere, 08C, 0.5 h. [b] Isolated yields. [c] 258C. [d] FeBr3 (0.75 mmol) was
used instead of FeCl3. [e] SnCl4 (0.5 mmol) was used instead of FeCl3.
The formation of the desired product was clearly affected
by the substituents on the aryl moieties and by the tether
atom in diyne 1 (Table 2 and the Supporting Information).
Moreover, increasing the electron density of the aryl func-
tionalities in diynes 1e–g by introducing two electron-donat-
ing methyl groups led to the formation of 1-methylene-2,3-
disubstituted 1H-indenes 5 (51–72%) as the major product
(Table 3). The presence of electron-withdrawing substituents
on the aryl moiety of acetals 2 improved the yield of com-
pounds 5 from the reaction of 1e–g with compounds 2
(Table 3). The generation of indenes 3 and 5 was further
confirmed by X-ray crystallographic structural analysis of
Scheme 2. The reactions of diyne 1a with acetals 2ab–ad.
which have incorporated a bromovinyl moiety (Table 2, en-
tries 6 and 7). O-Tethered diyne 1b reacted with acetal 2b
and FeCl3 to generate 3h in 41% yield (Table 2, entry 8).
Unexpectedly, the reaction of C-tethered diyne 1c with
acetal 2a in the presence of FeCl3 formed the desired prod-
uct 3i (25%) as only the minor product, while alkynyl
enone 4 (60%) was the major product (Table 2, entry 9),
which suggests that one of the alkynyl moieties underwent
intramolecular cyclization with 2a.[13b] These results suggest
that the tethering heteroatoms may stabilize a cationic inter-
mediate during the reaction.[14] It is noteable that the reac-
tion of functionalized diyne 1d with 2a also formed the de-
sired product 3j in the presence of either FeCl3 or SnCl4
(Table 2, entry 10).
products 3a and 5i (Figure 1 and Figure 2, respectively), re-
vealing their fused tricyclic structures and the construction
of three new carbon–carbon bonds in the newly formed
backbones.
Possible mechanisms for the reaction are proposed in
Scheme 3. In pathway a, the oxocarbonium cation Ar’CH=
OEt+ (A) is generated in situ from the interaction of FeCl3
with diethyl benzaldehyde acetal 2, Ar’CH
ACHTUNGRTEN(NUNG OEt)2, by loss of
the FeCl3A
CTHUNGTRENNUNG
a [2+2] cycloaddition reaction to form oxete B, which is
shown to be preferentially formed by our theoretical study
of the reaction.[13b] Ring-opening of B gives 4p-cation C,
which undergoes a Nazarov-type cyclization reaction,[15]
yielding oxocarbonium cation F via species D and E. Intra-
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Chem. Eur. J. 2011, 17, 10547 – 10551

Formation of Functionalized Indene Derivatives
COMMUNICATION
Table 3. The FeCl3-mediated cyclization of diynes 1e–g with acetals 2.[a]
Diyne (1)
Ar’ (2)
Product
(yield[b] [%])
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
1e
1e
1e
1e
1e
1e
1e
1 f
1 f
1 f
Ph (2a)
5a (67)
5b (51)
5c (72)
5d (61)
5e (53)
5 f (60)
5g (60)
5h (61)
5i (60)
5j (70)
4-MeOC6H4 (2b)
3-BrC6H4 (2e)
4-ClC6H4 (2 f)
4-MeC6H4 (2g)
2-MeOC6H4 (2h)
2-naphthyl (2i)
Ph (2a)
Figure 2. The molecular structure of 5i.
ing intermediate H and FeCl2ACTHNUGRTNEUNG(OEt). The Lewis acid pro-
motes elimination of EtOH from H then affords product 3.
On the other hand, pathway b more plausibly describes
the diverging route that allows the formation of products 3
and 5. Tandem intra- and intermolecular interactions of
diyne 1 with oxocarbonium cation A produce vinyl cation I,
which reacts with an external chloride anion to give inter-
mediate J if the aryl groups in the diyne substrate are insuf-
ficiently electron-rich for the alternative pathway. Lewis
acid FeCl3 or Cl2FeOEt then initiates a Nazarov-type cycli-
zation to form product 3 via cations K and L (pathway b’).
However, if the aryl groups in the diyne substrates are elec-
tron-rich, for example, 3,4-dimethylphenyl groups, one of
these two aryl groups behaves as the nucleophile to capture
the developing vinyl cation in I and form product 5 (path-
way b’’) via intermediates M and N.
3-BrC6H4 (2e)
4-NO2C6H4 (2j)
10
11
1g
Ph (2a)
5k (70)
[a] Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), FeCl3 (0.5 mmol),
CH2Cl2 (5 mL), N2 atmosphere, 08C, 0.5 h. [b] Isolated yields.
In summary, the FeCl3- and FeBr3-mediated cyclization of
diynes with diethyl benzaldehyde acetals has been realized,
producing highly functionalized indene derivatives in which
three new carbon–carbon bonds have been formed. The sub-
stituents on the aryl moieties of the diyne direct the forma-
tion of either 3-halovinyl-2,3-disubstituted or 1-methylene-
2,3-disubstituted 1H-indenes. This synthetic protocol pro-
vides a new route to fused functionalized tricyclic indenes.
Experimental Section
Typical procedure: The iron-mediated formation of 3a: Under a nitrogen
atmosphere, a suspension of FeCl3 (81 mg, 0.5 mmol) in CH2Cl2 (5 mL)
was cooled to 08C. While this mixture was being stirred, diyne 1a
(200 mg, 0.5 mmol) and then benzaldehyde diethyl acetal 2a (90 mg,
0.5 mmol) were added. The mixture was stirred at 08C for 30 min, and
then quenched with water (10 mL). The organic layer was separated off
and the aqueous phase was extracted with CH2Cl2 (3ꢁ10 mL). The com-
bined organic layers were dried over anhydrous Na2SO4, filtered, and
concentrated under reduced pressure. The resulting residue was purified
by silica gel flash column chromatography (eluent: petroleum ether (60–
908C)/diethyl ether, 20:1, v/v) to give 3a as a white solid (173 mg, 66%).
Figure 1. The molecular structure of 3a.
molecular cyclization occurs in F to generate vinyl cation G,
which is traped by a chloride anion from FeCl3ACTHNUTRGNEUNG
(OEt)À, form-
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10549

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Keywords: acetals · cyclization · diynes · indenes · iron ·
Lewis acids
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Formation of Functionalized Indene Derivatives
COMMUNICATION
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Received: June 1, 2011
Published online: August 17, 2011
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10551