Synthesis of multisubstituted indenes via iron-catalyzed domino reaction of benzylic compounds and alkynes

Article abstract of DOI:10.1055/s-0032-1317705

A novel approach to synthesizing multisubstituted indenes by iron-catalyzed domino reaction of benzylic compounds and alkynes under mild conditions was developed. This system could be applied to various available substrates in a one-step synthetic procedure in moderate to good yields. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317705

130▌
LETTER
lSetyternthesis of Multisubstituted Indenes via Iron-Catalyzed Domino Reaction of
Benzylic Compounds and Alkynes
Synthesis of Multisubstituted Indenes
Yongxin Chen,a,b Kangning Li,a,b Xiang Liu,a,b Jiaoyan Zhu,a,b Baohua Chen*a,b
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
b
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, P. R. of China
Fax +86(931)8912582; E-mail: chbh@lzu.edu.cn
Received: 18.10.2012; Accepted after revision: 07.11.2012
cient way of synthesizing multisubstituted indenes by in
Abstract: A novel approach to synthesizing multisubstituted in-
situ generated benzyl bromides as benzyl cation precur-
denes by iron-catalyzed domino reaction of benzylic compounds
sors in an iron-catalyzed, one-step synthetic procedure
(Scheme 1, B).
and alkynes under mild conditions was developed. This system
could be applied to various available substrates in a one-step syn-
thetic procedure in moderate to good yields.
A) previous work
LG
Key words: indenes, iron catalysis, domino reaction, one-step pro-
cedure, annulation
R3
R
R2
R3
R
R2
+
Ar
R1
O
O
R1
The indene core is one of the most important cyclic motifs
that are very common in natural products and biologically
active compounds.1 In addition, it has tremendous appli-
cations in various research fields including material
chemistry, analytical chemistry, and synthetic organic
chemistry.2 Great progress has been made to successfully
construct this motif, but traditional methods required ex-
pensive and difficult-to-prepare catalysts/reagents, te-
dious reaction procedures, moreover, it is difficult to
directly introduce multifunctional groups into the indene
core.3 Recently, several new approaches4 to generating in-
denes from the reaction between carbon–carbon triple
bonds and benzyl cation intermediates that are obtained
by the use of leaving groups have been developed
(Scheme 1, A).
LG = OH, NHTs,
R
R3
B) this work
H
R
R2
R3
R2
+
Ar
R1
Ar
R1
(Ar = RC6H4)
R1
Scheme 1 Synthesis of indene derivatives
At the outset of our study, diphenylacetylene (1a) and di-
phenylmethane (2a) were chosen as model substrates to
optimize the reaction conditions (Table 1). Compounds
3aa and 4aa were obtained in 10% and 56% yields, re-
spectively, with FeCl2 (10 mol%) as the catalyst and DDQ
(2.0 equiv) as the oxidant in DCE at 80 °C (Table 1,
entry1). The yield of 3aa increased to 66% when NBS
was used as the oxidant, while other oxidants such as
NCS, TBHP, and Cu(OAc)2 showed no effect on the pro-
motion of this domino reaction (Table 1, entries 2–5).
Subsequently, a series of solvents (toluene, acetonitrile,
nitromethane, and tetrahydrofuran) were also evaluated
but they showed disappointing results (Table 1, entries 6–
9). Furthermore, we investigated the reaction using other
iron, copper, and zinc salts as catalyst, but no better results
were obtained (Table 1, entries 10–14). It is noteworthy
that the amount of NBS had a significant effect on this re-
action. Both 1.5 equivalents and 3.0 equivalents of NBS
offered the desired product 3aa in a low yield; 2.2 equiv-
alents of NBS were the most effective and resulted in 75%
yield (Table 1, entries 15–17). In addition, the yield was
found to decrease with the temperature changed to 60 °C
or 120 °C (Table 1, entries 18 and 19), and only a trace
amount of product was observed in the absence of NBS or
FeCl2.
In recent years, the direct functionalization of benzylic
C(sp3)–H bonds has attracted extensive attention in organ-
ic synthesis.5–8 For example, Shi and others5 have reported
the cross-dehydrogenative coupling (CDC) between ben-
zylic C–H bonds and sp3, sp2, or sp C–H bonds. Direct
benzylic C–H amination through various catalytic sys-
tems have also been developed.6 Additionally, the Jiao
group7 demonstrated an iron-catalyzed transformation of
benzylic compounds into corresponding amide through
C–H and C–C bond cleavage. Among these reactions, iron
is a particularly attractive catalyst because of its sustain-
ability, easy availability, low price, and environmentally
friendly characteristics.9 Despite the significant progress
that has been achieved in this area, the cyclization reaction
is still a relatively unexplored field. Owing to the impor-
tance of indene derivatives and the lack of successful di-
rect annulations from benzylic compounds without
leaving groups, we initially envisioned a novel and effi-
Under the optimized reaction conditions, a series of sub-
stituted substrates was investigated to establish the scope
and limitations of this process (Table 2). The results
SYNLETT 2013, 24, 0130–0134
Advanced online publication: 06.12.2012
DOI: 10.1055/s-0032-1317705; Art ID: ST-2012-W0900-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

LETTER
Synthesis of Multisubstituted Indenes
131
Table 1 Optimization of the Reaction Conditionsa
Ph
Ph
Ph
Ph
+
Ph
catalyst, oxidant
Ph
Ph
Ph
+
Ph
Ph
solvent, T(°C)
Ph
Ph
1a
2a
3aa
4aa
Entry
Oxidant (equiv)
Catalyst
Solvent
Temp (°C)
Yield of 3aa (%)
Yield of 4aa (%)
1
2
DDQ (2.0)
NBS (2.0)
NCS (2.0)
TBHP (2.0)
Cu(OAc)2 (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (1.5)
NBS (2.2)
NBS (3.0)
NBS (2.2)
NBS (2.2)
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeBr2
FeCl3
DCE
DCE
DCE
DCE
DCE
PhMe
MeCN
MeNO2
THF
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
60
120
10
56
12
0
66
3
trace
trace
trace
15
4
0
5
0
6
42
0
7
0
8
0
0
9
0
0
10
11
12
13
14
15
16
17
18
19
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
43
0
44
15
20
0
Fe(acac)3
CuCl2
ZnCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
trace
trace
trace
56
0
20
0
75
47
0
67
0
60
0
a Reaction conditions: 1a (0.25 mmol), 2a (1.25 mmol), catalyst (0.025 mmol), solvent (2 mL), under N2, 12 h.
showed that substrates with various weakly electron-do- cept for 3ab and 3ad (Table 2, entries 11, 13–19; see also
nating and electron-withdrawing functional groups on ei- the Supporting Information). Confirmed by 1H NMR and
ther alkynes or benzylic compounds gave the 13C NMR analyses of intermediate product 4ae, we as-
corresponding indenes in moderate to good yields, and sumed the favorable isomer would be 3ae.4b A low yield
other strongly electron-donating and electron-withdraw- was obtained when the bis[4-(tert-butyl)phenyl]methane
ing substituents gave no desired products probably be- was used, probably because of the steric effects of the tert-
cause of strong electronic effect (Table 2, entries 7, 8, and butyl substituent (Table 2, entry 12). Besides diarylmeth-
17). The corresponding product 3da was obtained in good anes, simple benzylic substrates such as ethyl benzene
yield when the substrate with an ortho-substituted aryl also worked smoothly and gave a mixture of diastereo-
group was employed (Table 2, entry 4). Moreover, sub- mers (1:1 ratio) in 56% yield (Table 2, entry 16).
strates with different substituents on R1 and R2 gave a
mixture of regioisomers; due to electronic effects, the ra-
Interestingly, it was found that substrates with a para-me-
thoxy group on the aryl ring could react smoothly by using
tio of isomers varied from 3.7:1 to 1.3:1 (Table 2, entries
DDQ as oxidant (Table 3, entry 1). Good yield was still
2, 4, 5; see also the Supporting Information).4a,b The ali-
achieved when the bis[4-(tert-butyl)phenyl]methane was
used (Table 3, entry 2).
phatic alkynes gave lower but still acceptable yields with
perfect regioselectivity (Table 2, entries 9 and 10).4a,b
To gain an insight into the mechanism of the above-men-
When unsymmetric diphenylmethanes were employed,
tioned process, the following control experiments were
exclusive regioselectivity of indenes were observed ex-
performed. As shown in Scheme 2, (bromometh-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 130–134

132
Y. Chen et al.
LETTER
ylene)dibenzene A was obtained via reaction of diphenyl- In summary, we have developed a novel iron-catalyzed
methane (2a) with NBS in DCE under optimized direct cyclization for the synthesis of multisubstituted in-
conditions (Scheme 2, A), and then diphenylacetylene denes with benzylic compounds and alkynes in moderate
(1a) and FeCl2 were added to the reaction mixture to good yields. This system could be applied to various
(Scheme 2, B), and 3aa was obtained in 74% yield.
available substrates in a one-step synthetic procedure. We
believe that this is one of the simplest and most straight-
forward methods available for the synthesis of indenes to
date.
On the basis of the above results, a tentative reaction
mechanism is illustrated in Scheme 2 (C). Treatment of
diphenylmethane (2a) with NBS produces (bromometh-
ylene)dibenzene A. The reaction of iron salt on A can lead
to benzylic cation, which regioselectively attacks 1a re-
sulting in the formation of vinyl cation C. Then C under-
goes cyclization and subsequent aromatization to provide
4aa.4a,b,10 Finally, 4aa and a second A react to afford the
desired product 3aa.11
Acknowledgment
We are grateful to the project sponsored by the Project of National
Science Foundation of P. R. of China (No. J11003307).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
Table 2 Synthesis of Indenes Derivatives from Alkynes and Benzylic Compoundsa
R1
R2
R1(R2)
R2(R1)
R
1
FeCl2 (10 mol%)
+
Ar
NBS (equiv)
DCE, 80 °C
Ar
R3
R3
R3
2 (Ar = RC6H4)
3
Entry
1 R1, R2
2 Ar, R3
NBS (equiv)
Yield of 3 (%)
1
2
1a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2a Ph, Ph
2b Ph, 4-MeC6H4
2.2
1.9
1.6
1.8
2.2
2.5
2.2
2.2
3.0
2.2
2.2
3aa 75
1b Ph, 4-MeC6H4
3ba/3ba′ (1:3.7) 72
3ca 53
3
1c 4-MeC6H4, 4-MeC6H4
1d Ph, 2-MeC6H4
1e Ph, 4-ClC6H4
1f 4-ClC6H4, 4-ClC6H4
1g Ph, 4-O2NC6H4
1h Ph, 4-MeOC6H4
1i Ph, Bu
4
3da/3da′ (1:1.3) 69
3ea/3ea′ (1:1.3) 83
3fa 74
5
6
7
3ga 0
8
3ha 0
9
3ia 55
10
11
12
13
14
15
16
17
18
19
1j n-Pr, n-Pr
3ja 42
1a Ph, Ph
3ab (Ar = Ph)/3ab′ (Ar = 4-MeC6H4) (1:1) 62
1a Ph, Ph
2c 4-t-BuC6H4, 4-t-BuC6H4 2.6
3ac 48c
1a Ph, Ph
2d Ph, 4-FC6H4
2e Ph, 4-ClC6H4
2f Ph, 4-F3CC6H4
2g Ph, Me
2.6
2.4
3.0
2.2
2.2
2.8
3.0
3ad (Ar = Ph)/3ad′ (Ar = 4-FC6H4) (2:1) 58
1a Ph, Ph
3ae 57
3af 63c
3ag 56d
3ah 0
1a Ph, Ph
1a Ph, Ph
1a Ph, Ph
2h Ph, 4-MeOC6H4
2e Ph, 4-ClC6H4
2f Ph, 4-F3CC6H4
1f 4-ClC6H4, 4-ClC6H4
1f 4-ClC6H4, 4-ClC6H4
3fe 72
3ff 45b
a Reaction conditions: 1 (0.25 mmol), 2 (1.25 mmol), FeCl2 (0.025 mmol), DCE (2 mL), 80 °C, 10 h, under N2.
b At 100 °C, 24 h.
c At 100 °C.
d At 60 °C.
Synlett 2013, 24, 130–134
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Multisubstituted Indenes
133
Table 3 Synthesis of Indene Derivatives Using DDQ as Oxidanta
R1
R1
R2
R
FeCl2 (10 mol%)
+
R2
DDQ (2.5 equiv)
DCE, 100 °C
Ar
R3
R3
3
2 (Ar = RC6H4)
Entry
1 R1, R2
2 Ar, R3
Yield of 4 (%)
1
2
1h Ph, 4-MeOC6H4
1a Ph, Ph
2a Ph, Ph
4ha 55
4ac 65
2c 4-t-BuC6H4, 4-t-BuC6H4
a Reaction conditions: 1 (0.25 mmol), 2 (1.25 mmol), FeCl2 (0.025 mmol), DDQ (0.625 mmol), DCE (2 mL), 100 °C, 10 h, under N2.
Br
NBS
A)
B)
Ph
Ph
Ph
Ph
DCE
2a
A
Ph
Ph
Br
FeCl2 (10%)
DCE
Ph
Ph
Ph
Ph
+
Ph
Ph
Ph
Ph
1a
3aa
A
Br
NBS
Ph
B
C)
–
Ph
Ph
Ph
FeCl2Br
2a
A
Br
FeCl2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
FeCl2
Ph
1a
Ph
Ph
Ph
B
Ph
Ph
Ph
Ph
3aa
4aa
Ph
Ph
Ph
+
H
D
Ph
C
Scheme 2 A tentative reaction mechanism
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Synlett 2013, 24, 130–134

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Y. Chen et al.
LETTER
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(11) General Procedure for the Iron-Catalyzed Domino
Reaction – Synthesis of 6-Benzhydryl-1,2,3-triphenyl-
1H-indene (3aa)
Diphenylacetylene 1a (44.5 mg, 0.25 mmol), FeCl2 (3.1 mg,
10 mmol%), and NBS (97.9 mg, 0.55 mmol) were added to
a flask with a magnetic stirring bar. The tube was evacuated
and refilled with N2, and then diphenylmethane (2a, 210 μL,
1.25 mmol) and DCE (2 mL) was added. The resulting
mixture was stirred at 80 °C for 10 h. After cooling to r.t., the
mixture was diluted with EtOAc and filtered. The filtrate
was removed under reduced pressure to get the crude
product, which was further purified by silica gel
chromatography (PE as eluent) to give product 3aa (75%
yield); white solid; mp 155–157 °C. 1H NMR (300 MHz,
CDCl3): δ = 7.40–7.28 (m, 5 H), 7.22–7.13 (m, 8 H), 7.10–
6.98 (m, 14 H), 6.98–6.94 (m, 1 H), 5.50 (s, 1 H), 5.03 (s, 1
H). 13C NMR (75 MHz, CDCl3): δ = 148.4, 145.5, 144.1,
144.0, 143.4, 141.6, 140.4, 139.7, 135.5, 129.4, 129.2,
129.1, 128.6, 128.5, 128.2, 128.1, 127.8, 127.4, 126.5,
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Synlett 2013, 24, 130–134
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