Fe(OTf)3-catalyzed reaction of benzylic acetates with organosilicon compounds

Article abstract of DOI:10.1055/s-0030-1259313

Fe(OTf)₃-catalyzed reaction of benzylic acetates with allyltrimethylsilane, azidotrimethylsilane, and cyanotrimethylsilane afforded the corresponding allylated, azido, and cyano products in high yields. 2-Trimethylsilyl-substituted benzofuran and indole worked well to furnish the benzyl-substituted benzofuran and indoles. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1259313

LETTER
415
Fe(OTf)3-Catalyzed Reaction of Benzylic Acetates with Organosilicon
Compounds
R
eaction of Ben
i
zylic Acetate
Y
s
w
ith
O
rganosilic
a
on Compou
n
nds Chan, Sundae Kim, Wan Ting Chung, Chong Long, Sunggak Kim*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
Fax +6567911961; E-mail: sgkim@ntu.edu.sg
Received 20 October 2010
ylsilane (entry 3) were used under the same conditions,
ether 3 was isolated exclusively, whereas the desired azi-
dation product 2 was obtained when employing azidotri-
methylsilane (entry 4).
Abstract: Fe(OTf)3-catalyzed reaction of benzylic acetates with al-
lyltrimethylsilane, azidotrimethylsilane, and cyanotrimethylsilane
afforded the corresponding allylated, azido, and cyano products in
high yields. 2-Trimethylsilyl-substituted benzofuran and indole
worked well to furnish the benzyl-substituted benzofuran and in-
doles.
Table 1 Reaction of Diphenylmethanol with Organosilanesa
Ph
Ph
Key words: iron-catalyzed reaction, allylation, azidation, cyana-
tion, benzylic acetates, organosilanes
OH
Ph
Fe(OTf)3
DCE
+
+
TMS-X
Ph
O
Ph
Ph
Ph
Ph
X
1
2
3
Entry
TMSX
Time
(h)
Isolated yield Isolated yield
Lewis acids play a very important role in promoting car-
bon–carbon bond formation in organic synthesis.1 In re-
cent years, much attention was focused in such bond
formation with the aid of catalysts such as BF3,2 Tf2NH,3
InX3,4 BiX3,5 and ZrX4.6 In some cases, high reaction tem-
perature and even microwave conditions7 are necessary to
make the reaction possible. Although these transition met-
als are efficient, they are generally costly. As such, it is es-
sential to come out with more effective methods in
carrying out the bond formation.
of 2 (%)
of 3 (%)
1
2
3
4
2
31
–
59
95
93
–
TMS
TMS
TMS
2
2
–
TMSN3
2
96
a Conditions: 1 (0.30 mmol), TMSX (0.36 mmol), FeCl3 (5 mol%),
and AgOTf (15 mol%) in 1,2-dichloroethane (DCE) at r.t.
Iron exists in abundant and can be easily obtained from
nature, thus it is regarded as one of the lowest cost and en-
vironmentally friendly metals that can efficiently catalyze
many organic reactions as compared to the other conven-
tional Lewis acids.8 Iron-mediated organic reactions have
attracted a great deal of recent attention which include the
Friedel–Crafts type reactions,9 cross-coupling reactions,10
allylations,11 and other transformations.12 We have been
interested in the catalytic activity of iron salts as the Lewis
acid in organic reactions and report a new efficient
iron(III)-catalyzed reaction of benzylic acetates with var-
ious organosilanes.
OAc
Ph
Ph
Fe(OTf)3
TMS
+
DCE, r.t., 0.5 h
Ph
Ph
4a (96%)
4
Scheme 1 Allylation of 4 with allyltrimethylsilane
Table 2 Optimizing Reaction Conditions with Various Catalysts
OAc
[Fe]
TMS
+
DCE
Ph
Ph
5
5a
Entry Catalyst (mol%)
Temp (°C) Time (h) Yield (%)a
We initially explore the efficiency of iron salts in allyla-
tion of benzylic alcohols as shown in Table 1. When
diphenylmethanol (1) was treated with allyltrimethylsi-
lane in the presence of 5 mol% iron triflate in 1,2-dichlo-
roethane at room temperature for two hours, a 31:59
mixture of allylated product 2 and ether 3 was obtained,
whereby similar observations were previously noted.13
The formation of 3 depended very much on the nature of
organosilanes and this was exemplified in the results.
When vinyltrimethylsilane (entry 2) and ethynyltrimeth-
1
2
3
4
5
6
Fe(acac)3 (5)
FeF3 (5)
80
80
r.t.
r.t.
r.t.
r.t.
15
15
15
3
0
0
FeCl2 (5)
48 (40)b
89
FeBr3 (5)
FeCl3 (5)
9
89
FeCl3 (5), AgOTf (15)
0.5
96
a Isolated yields.
b Recovery yield of starting material.
SYNLETT 2011, No. 3, pp 0415–0419
x
x.
x
x.
2
0
1
1
Advanced online publication: 13.01.2011
DOI: 10.1055/s-0030-1259313; Art ID: U09610ST
© Georg Thieme Verlag Stuttgart · New York

416
L. Y. Chan et al.
LETTER
Table 3 Reactions of Benzylic Acetates with Various Organosilanes
OAc
Nu
FeCl3 (5 mol%), AgOTf (15 mol%)
DCE
+
Nu
Ar
R
Ar
R
Entry
1
Acetate
Nu
Temp (°C) Time (h)
Product
Yield (%)a
97
N3
OAc
TMSN3
r.t.
50
1
1
Ph
Ph
Ph
Ph
4b
4
CN
Ph
2
3
4
4
TMSCN
88
92
Ph
Ph
4c
O
OTMS
OMe
r.t.
6
Ph
OMe
4d
Ph
4
5
4
4
80
12
1
0
TMS
Ph
4e
Ph
r.t.
88
TMS
O
Ph
O
4f
Ph
Ph
6
4
r.t.
1
90
TMS
N
N
Me
Me
4g
OAc
7
8
9
r.t.
r.t.
80
0.5
96
86
0
TMS
Ph
Ph
5a
5
N3
5
TMSN3
1
Ph
5b
CN
5
5
5
TMSCN
14
Ph
5c
O
OTMS
OMe
10
11
12
13
14
80
80
r.t.
r.t.
80
3
16
1
0
69
67
85
78
Ph
OMe
5d
TMS
O
Ph
O
5e
OAc
TMS
MeO
MeO
6
6a
N3
6
6
TMSN3
2
MeO
6b
CN
TMSCN
2
MeO
6c
Synlett 2011, No. 3, 415–419 © Thieme Stuttgart · New York

LETTER
Reaction of Benzylic Acetates with Organosilicon Compounds
417
Table 3 Reactions of Benzylic Acetates with Various Organosilanes (continued)
OAc
Nu
FeCl3 (5 mol%), AgOTf (15 mol%)
DCE
+
Nu
Ar
R
Ar
R
Entry
Acetate
Nu
Temp (°C) Time (h)
Product
Yield (%)a
Me
N
15
6
r.t.
2
66
TMS
N
MeO
Me
6d
OAc
16
17
18
19
80
80
80
r.t.
18
18
24
2
0
0
TMS
TMS
TMS
7
7a
OAc
Cl
Cl
8
8a
Ph
Ph
Ph
71
82
Ph
OAc
Ph
9a
9
9
TMSN3
Ph
N3
9b
Ph
20
21
9
80
80
24
59
56
TMS
TMS
O
O
Ph
Ph
9c
9
24
2
N
N
Me
Ph
Me
9d
Ph
OAc
Ph
22
23
r.t.
r.t.
87
TMS
Ph
Ph
10
10a
N3
Ph
10
TMSN3
0.5
60:37
Ph
Ph
Ph
10b
10c
Conditions: 0.30 mmol of benzylic acetate, 0.36 mmol of Nu, 5 mol% FeCl3 and 15 mol% AgOTf in DCE.
a Isolated yields.
To obviate the problem of the unwanted ether formation, FeBr3 and FeCl3 dramatically increased the reactivity,
benzhydryl acetate 4 was employed in the allylation reac- thus were discovered to be very effective catalysts under
tion. When 4 was subjected under the same conditions, the mild reaction conditions (entries 4 and 5). The addition of
desired product was isolated in 96% yield without any for- AgOTf further improved the efficiency of iron catalyst
mation of ether 3 as shown in Scheme 1. Thus, the re- (entry 6), which reduced the reaction time significantly
maining reactions were carried out with the benzylic and gave high yield (96%). Although FeCl3 still produced
acetates.14 To determine the optimum conditions, the cat- excellent and comparable yields, it still proceeded at a
alytic efficiency of various iron salts was also examined much slower rate without AgOTf. Thus, remaining reac-
using 1-phenylethyl acetate (5). As shown in Table 2, tions were carried out using 5 mol% Fe(OTf)3 (generated
Fe(acac)3 and FeF3 proved no catalytic activity in allyla- in situ),15 whereas FeCl3 and FeBr3 are expected to be
tions (Table 2, entries 1 and 2), whereas FeCl2 was effec- used in the present study. With the optimized conditions
tive to some extent in the catalysis (entry 3). The use of on hand, we studied reactions of benzylic acetates with
Synlett 2011, No. 3, 415–419 © Thieme Stuttgart · New York

418
L. Y. Chan et al.
LETTER
various organosilanes such as vinyl-, cyano-, and azido- creased to 84% together with a small amount of 10c (6%,
trimethylsilane. Scheme 2).
The experimental results with primary and secondary ben- In conclusion, we have developed efficient Fe(OTf)3-
zylic acetates (4–6, 9) using 5 mol% Fe(OTf)3 in 1,2- catalyzed reactions of benzylic acetates with various orga-
dichloroethane were summarized in Table 3. The reaction nosilanes. The present method proved a wide scope of
of 4 with azidotrimethylsilane and TMS enol ether pro- interest, making it a useful and attractive process.1,7,18
ceeded smoothly at room temperature to give 4b and 4c in
97% and 92% yield (entries 1 and 3), respectively. Simi-
larly, 4 worked well with cyanotrimethylsilane (entry 2)
Acknowledgment
We gratefully acknowledge financial support from Nanyang Tech-
nological University. W.T.C. and C.L. thank the Division of Che-
mistry and Biological Chemistry (CBC) for supporting the
undergraduate summer research.
but the reaction was somewhat slow and required gentle
heating at 50 °C for one hour. In the case of vinyltrimeth-
ylsilane, the reaction did not occur, even under prolonged
heating at 80 °C (entry 4), although the similar conversion
was achieved with InCl3 and BiBr3.1,6 We next examined
the possibility of using electron-rich alkenylsilanes such References and Notes
as 2-trimethylsilyl-substituted benzofuran and indole.
(1) Acid Catalysis in Modern Organic Synthesis, Vol. 1 and 2;
When 4 was reacted with 2-trimethylsilyl benzofuran and
indole under the standard conditions, the reaction pro-
ceeded cleanly at room temperature, yielding the desired
product in 88% and 90% yield, respectively (entries 5 and
6). In the case of 5, allyltrimethylsilane and azidotrimeth-
ylsilane worked well, yielding the desired products in
high yields (entries 7 and 8). However, 5 did not react with
cyanotrimethylsilane (entry 9) and TMS enol ether (entry
10) in refluxing 1,2-dichloroethane, although it reacted
slowly with 2-trimethylsilylbenzofuran at reflux (entry
11). In the case of primary benzylic acetates, the reactivity
depended very much on the nature of the substituent of the
phenyl ring. For example, benzyl acetate (7) did not react
readily with allyltrimethylsilane even at prolonged heat-
ing conditions (entry 16), and the starting material was re-
covered unchanged. A similar result was observed with 4-
chlorobenzyl acetate (8, entry 17). However, 4-methoxy-
benzyl acetate (6) was much more reactive than benzyl ac-
etate and reacted with allyltrimethylsilane (entry 12) at
ambient temperature within one hour. In addition, 6
worked well with azidotrimethylsilane, cyanotrimethylsi-
lane, and 2-trimethylsilylindole (entries 13–15). This is
because 6 contained the electron-donating methoxy group
attached at the aromatic ring that gave stabilizing effects
to the carbocation intermediate via resonance. This accel-
erated the rate of reaction, hence 6 proceeded faster under
mild conditions as compared to 5 and 7. Secondary (E)-
1,3-diphenylallyl acetate (9) was also reacted with several
organosilanes. However, 9 was not very reactive, and thus
required prolonged heating for allylation (entry 18) and
alkenylation (entries 20 and 21) to occur. Finally, allyla-
tion of tertiary 1,1-diphenylethyl acetate (10) proceeded
smoothly at room temperature (entry 22) but the azidation
afforded a 60:37 mixture of the desired azide 10b and ole-
fin 10c (entry 23). When alcohol 11 was used, 10b was in-
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OH
Ph
N3
Ph
Fe(OTf)3
+
TMSN3
+
DCE
r.t., 1 h
Ph
Ph
Ph
Ph
11
10b
10c
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6%
Scheme 2 Azidation of 11
Synlett 2011, No. 3, 415–419 © Thieme Stuttgart · New York

LETTER
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Reaction of Benzylic Acetates with Organosilicon Compounds
419
(14) Reaction of(methoxymethylene)dibenzene (Ph2CHOMe)
with allyltrimethylsilane in the presence of 5 mol% Fe(OTf)3
in DCE at r.t. for 5 h afforded 4a (73%) but the allylation of
3 did not occur, even under prolonged heating at 80 °C.
(15) Fe(OTf)3 was prepared from 5 mol% FeCl3 and 15 mol%
AgOTf in DCE. Filtration was done to remove the AgCl
precipitate, and the catalyst was used to repeat the allylation.
The reaction time and yield observed were comparable to the
Fe(OTf)3 generated in situ without filtration. For instance,
the allylation of 4 under the similar conditions afforded 4a in
89% yield.
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(17) General Procedure for the Reaction of Benzylic Acetates
with Organosilanes: But-3-ene-1,1-diyldibenzene (4a)
Anhydrous FeCl3 (2.4 mg, 0.015 mmol) and AgOTf (11.5
mg, 0.045 mmol) was carefully weighed inside a glove box
and stirred in DCE (2 mL) for 5 min. Allyl trimethylsilane
(41.1 mg, 0.36 mmol) and benzhydryl acetate 4 (67.9 mg,
0.3 mmol) were then added to the prepared catalyst solution
and stirred for 0.5 h at r.t. The residual crude product was
concentrated in vacuo and purified by silica gel column
chromatography using n-hexane as eluent to afford the
desired product 4a (60.0 mg, 96% yield). 1H NMR (400
MHz, CDCl3): d = 7.29–7.23 (m, 8 H), 7.19–7.15 (m, 2 H),
5.75–5.68 (m, 1 H), 5.05–4.93 (m, 2 H), 4.01 (t, J = 7.8 Hz,
1 H), 2.84–2.80 (m, 2 H). 13C NMR (100 MHz, CDCl3): d =
144.5, 136.8, 128.4, 127.9, 126.2, 116.3, 51.2, 39.9.
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Compounds
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2-Benzhydryl-1-methyl-1H-indole (4g)
1H NMR (400 MHz, CDCl3): d = 7.29–7.17 (m, 13 H), 6.97
(t, J = 7.6 Hz, 1 H), 6.40 (s, 1 H), 5.66 (s, 1 H), 3.68 (s, 1 H).
13C NMR (100 MHz, CDCl3): d = 144.1, 137.4, 129.0, 128.7,
128.2, 127.3, 126.1, 121.6, 120.0, 118.8, 118.2, 109.1, 48.8,
32.6. HRMS (EI+): m/z calcd for C22H20N [M + 1]: 298.1596;
found: 298.1603.
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2-(4-Methoxybenzyl)-1-methyl-1H-indole (6d)
1H NMR (400 MHz, CDCl3): d = 7.51 (d, J = 7.6 Hz, 1 H),
7.29–7.18 (m, 4 H), 7.06 (t, J = 7.4 Hz, 1 H), 6.82 (d, J = 8.4
Hz, 2 H), 6.72 (s, 1 H), 4.04 (s, 2 H), 3.77 (s, 3 H), 3.71 (s, 3
H). 13C NMR (100 MHz, CDCl3): d = 157.7, 137.1, 133.5,
129.5, 127.8, 127.0, 121.5, 119.2, 118.7, 114.7, 113.7,
109.1, 55.2, 32.6, 30.6, 29.2. HRMS (EI+): m/z calcd for
C22H20N [M + 1]: 252.1388; found: 252.1393.
(E)-2-(1,3-Diphenylallyl)-1-methyl-1H-indole (9d)
1H NMR (400 MHz, CDCl3): d = 7.42 (d, J = 8.0 Hz, 1 H),
7.37–7.18 (m, 13 H), 7.01 (t, J = 7.4 Hz, 1 H), 6.74–6.69 (m,
2 H), 6.43 (d, J = 16.0 Hz, 1 H), 5.11 (d, J = 7.6 Hz, 1 H),
3.72 (s, 3 H). 13C NMR (100 MHz, CDCl3): d = 143.5, 137.5,
137.4, 132.7, 130.4, 128.4, 127.3, 127.2, 127.1, 126.3,
121.6, 120.0, 118.8, 117.0, 109.2, 46.1, 32.7. HRMS (EI+):
m/z calcd for C24H22N [M + 1]: 324.1752; found: 324.1746.
Synlett 2011, No. 3, 415–419 © Thieme Stuttgart · New York

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