Pd-Catalyzed asymmetric hydrogenation of 3-(toluenesulfonamidoalkyl)indoles

Article abstract of DOI:10.1039/c1ob06777j

A series of 2-substituted 3-(toluenesulfonamidoalkyl)indoles was synthesized by application of (EtO)₂POH or iodine as the catalyst, and was hydrogenated using chiral Pd catalyst, giving the 2,3-disubstituted indolines with up to 97% ee. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1ob06777j

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PAPER
Pd-Catalyzed asymmetric hydrogenation of 3-(toluenesulfonamidoalkyl)-
indoles†
Ying Duan, Mu-Wang Chen, Qing-An Chen, Chang-Bin Yu and Yong-Gui Zhou*
Received 23rd September 2011, Accepted 3rd November 2011
DOI: 10.1039/c1ob06777j
A series of 2-substituted 3-(toluenesulfonamidoalkyl)indoles was synthesized by application of
(EtO)2POH or iodine as the catalyst, and was hydrogenated using chiral Pd catalyst, giving the
2,3-disubstituted indolines with up to 97% ee.
Introduction
Homogeneous catalytic asymmetric hydrogenation has emerged
to be a powerful tool for constructing enantiomerically pure
architectures, such as chiral alcohols and amines.1 Due to the
simple operation and convenient after-treatment, great progress
has been made in asymmetric hydrogenation. However, the asym-
metric hydrogenation of aromatic and heteroaromatic compounds
is still a challenge because of their aromaticity as well as the
severe conditions needed to overcome this stability.2,3 To overcome
these difficulties, some groups have devoted to searching for
effective approaches to activate aromatic rings.4,5 Recently, our
group developed a novel strategy for highly enantioselective Pd-
catalyzed hydrogenation of unprotected indoles with Brønsted
acid as the activator.6 Based on this strategy, we also documented
the dehydration triggered asymmetric hydrogenation of 3-(a-
hydroxyalkyl)indoles7 and consecutive Brønsted acid/Pd-complex
promoted tandem reactions of simple indoles and aldehydes for
the rapid synthesis of chiral 2,3-disubstituted indoline derivatives.8
The 3-(toluenesulfonamidoalkyl)indole structural motif, 1, is
embedded in synthetic indole derivatives and numerous alkaloids.9
Reaction of indole derivatives, 1, with a Brønsted acid or a
Lewis acid would produce an elimination of toluenesulfonamide
(TsNH2),10 leading to vinylogous iminium intermediate, I (Scheme
1),11 which was also involved in many other transformations.12
Considering palladium complexes have been successfully applied
to asymmetric hydrogenation of indoles,6–8 we envisioned that
3-(toluenesulfonamidoalkyl)indoles 1 would be hydrogenated in
the presence of Brønsted acid using chiral Pd catalyst, affording
the chiral 2,3-disubstituted indolines. Herein, we report a series
of 2-substituted 3-(toluenesulfonamidoalkyl)indoles that was syn-
thesized and hydrogenated, giving the 2,3-disubstituted indolines
with up to 97% ee.
Scheme 1 The process for Pd-catalyzed asymmetric hydrogenation of
3-(toluenesulfonamidoalkyl)indoles 1.
Results and discussions
Although indoles are known to undergo Friedel–Crafts reactions
with N-tosyl imines under both Lewis acidic and Brønsted acidic
conditions, affording 3-(toluenesulfonamidoalkyl)indoles,10b,10g,13
the Friedel–Crafts reactions between 2-substituted indoles 3 and
imines 4 are rarely reported.14 When we followed the standard
procedure of You’s work,13b only the bisindole product was isolated
with racemic phosphoric acid (rac-1,1¢-binaphthalene-2,2¢-diyl
phosphoric acid) as the catalyst. Considering the relatively strong
acidity of phosphoric acid, thereafter, we tried weaker acidity
diethyl phosphonate (EtO)2POH as the catalyst. Gratifyingly,
several substrates were synthesized with low to excellent yields.15
In this process, at least 3.0 equivalents of 2-substituted indoles
were needed, along with bisindole by-products, and the yields
varied with the steric effect of N-tosyl imines (Table 1, entries
1, 3–6). In addition, for the aliphatic aldehyde derived N-tosyl
imines, no desired product was obtained (Table 1, entries 2,
10 and 11). After screening several other catalysts, fortunately,
molecular iodine (I2) was found to be a good catalyst to promote
this reaction.16 Although the mechanism of the reaction catalyzed
by I2 was not clear, I2 showed its robust action in this reaction.
Both aliphatic and aromatic aldehyde derived imines reacted well
with only 1.0 equivalent of 2-substituted indoles in 5 min to
give 3-(toluenesulfonamidoalkyl)indoles 1 in moderate to good
yields (Table 1, entries 2, 5, 6, 8, and 10–14). By comparison with
diethyl phosphonate, I2 displayed its generality in the synthesis of
3-(toluenesulfonamidoalkyl)indoles.
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R.
China. E-mail: ygzhou@dicp.ac.cn
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c1ob06777j
After accomplishing the synthesis of 3-(toluenesulfonami-
doalkyl)indoles 1, their asymmetric hydrogenation was also ex-
plored. Initially, 3-(toluenesulfonamidoalkyl)indole 1a was chosen
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Table 1 The synthesis of 3-(toluenesulfonamidoalkyl)indoles 1a
Table 2 Optimization of the asymmetric hydrogenation of 1aa
Entry
Solvent
Acid
Conv. (%)b
ee (%)c
d
1
2
3
4
5
6
7
8
Toluene
THF
TFE
DCM
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
CF3SO3H
TFA
PhCO2H
L-CSA
D-CSA
TsOH·H2O
TsOH·H2O
TsOH·H2O
—
—
—
—
71
66
83
85
80
74
41
—
81
76
81
87
84
d
Entry
R1/R2/R3
Method
Yield (%)b
79
92
>95
>95
>95
76
1
H/Me/Ph
H/Me/Cy
A
90 (1a)
2c
3
B
50 (1b)
DCM/TFE (1/2)
DCM/TFE (1/1)
DCM/TFE (2/1)
DCM/TFE (1/1)
DCM/TFE (1/1)
DCM/TFE(1/1)
DCM/TFE (1/1)
DCM/TFE (1/1)
DCM/TFE (1/1)
DCM/TFE (1/1)
DCM/TFE (1/1)
H/Me/4-FC6H4
H/Me/4-MeC6H4
H/Me/3-MeC6H4
H/Me/2-MeC6H4
H/n-Bu/Ph
H/phenethyl/Ph
Me/Me/Ph
Me/Me/Cy
Me/Me/i-Pr
Me/Me/4-MeC6H4
Me/Me/3-MeC6H4
Me/Me/2-MeC6H4
A
A
64 (1c)
79 (1d)
4
5
6
7
8
9
B (A)
B (A)
A
79 (15) (1e)
82 (30) (1f)
85 (1g)
9
78
—
d
10
11
12
13e
14f
15g
>95
>95
>95
>95
>95
B
74 (1h)
A
99 (1i)
10c
11c
12
13
14
B
54 (1j)
B
47 (1k)
B (A)
B (A)
B
70 (25) (1l)
71 (29) (1m)
71 (1n)
a Reaction conditions: Method A: 3 (3 equiv.), 4 (0.25 mol L-1) in toluene,
(EtO)2POH (10 mol%), 12 h, 0 ◦C–rt; Method B: 3 (1 equiv.), 4 (0.25 mol
L-1) in CH2Cl2, I2 (10 mol%), 5 min, 0 ◦C. b Isolated yields. c No desired
product was isolated by method A.
a Reaction conditions: Pd(OCOCF3)2 (2 mol%), L1 (2.4 mol%), 1a (0.25
mmol), H2 (600 psi), acid (0.25 mmol), solvent (3 mL), 50 ◦C, 16–20 h.
b Determined by 1H NMR. c Determined by HPLC analysis. d 2-Methyl-3-
benzylindole was obtained. e Ligand L2 was used. f Ligand L3 was used.
g Ligand L4 was used.
as the model substrate, and Pd(OCOCF3)2/(R)-BINAP/
TsOH·H2O were used to begin conditions screening. No desired
product was observed when the reaction was performed in toluene
and THF (Table 2, entries 1 and 2). To our delight, moderate
to good enantioselectivities and conversions were obtained in
TFE and DCM (Table 2, entries 3 and 4). Full conversions and
good to excellent ee values were achieved when the mixed solvent
was applied and DCM/TFE (1/1) delivered the best result with
85% ee (Table 2, entries 5–7). However, other Brønsted acids did
not improve the enantioselectivity (Table 2, entries 8–12). Next,
the effect of some bisphosphine ligands on the reactivity and
enantioselectivity was explored, and (R)-H8-BINAP L3 was found
to be the best one (Table 2, entries 13–15). Therefore, the optimal
conditions were established as the following: Pd(OCOCF3)2/(R)-
H8-BINAP/TsOH·H2O/DCM-TFE(1 : 1)/H2
Table 3 Pd-Catalyzed asymmetric hydrogenation of 3-(toluenesulfona-
midoalkyl)indoles 1a
Entry
1 (R1/R2/R3)
Yieldb
ee (%)c
1
2
3
4
5
6
7
8
1a (H/Me/Ph)
89 (2a)
97 (2b)
81 (2c)
84 (2d)
97 (2e)
93 (2f)
97 (2g)
95 (2h)
94 (2i)
90 (2j)
88 (2k)
87 (2l)
97 (2m)
97 (2n)
87
92
86
84
87
89
92
91
95
97
94
94
94
94
1b (H/Me/Cy)
1c (H/Me/4-FC6H4)
1d (H/Me/4-MeC6H4)
1e (H/Me/3-MeC6H4)
1f (H/Me/2-MeC6H4)
1g (H/n-Bu/Ph)
1h (H/phenethyl/Ph)
1i (Me/Me/Ph)
1j (Me/Me/Cy)
1k (Me/Me/i-Pr)
1l (Me/Me/4-MeC6H4)
1m (Me/Me/3-MeC6H4)
1n (Me/Me/2-MeC6H4)
(600 psi)/50 ◦C.
With optimized conditions in hand, the reaction scope was
examined, and the results are summarized in Table 3. Ex-
cellent yields and enantioselectivities were obtained with 3-
(toluenesulfonamidoalkyl)indoles 1 bearing various substituted
patterns. The electronic character of R3 had little impact on
reactivities and enantioselectivities (Table 3, entries 3 and 4).
In general, when R3 were alkyl groups, slightly higher enan-
tioselectivities were obtained than R3 as aromatic groups (Table
3, entries 2, 10 and 11). For 2-butyl and 2-phenethyl substi-
tuted 3-(toluenesulfonamidomethyl)indoles, 92% and 91% ee were
achieved, respectively (Table 3, entries 7 and 8). Notably, excellent
ee values were obtained with substrates bearing a methyl group at
the 7-position of the indole motif (Table 3, entries 9–14), which
might be ascribed to steric hindrance.
9
10
11
12
13
14
a Reaction conditions: Pd(OCOCF3)2 (2 mol%), (R)-H8-BINAP (2.4
mol%), 1 (0.25 mmol), H2 (600 psi), DCM/TFE = 1 : 1 (3 mL), TsOH·H2O
(0.25 mmol), 50 ◦C, 16–20 h. b Isolated yields. c Determined by HPLC.
and N-tosyl imines 4. First, Friedel–Crafts reaction products 1
formed in the presence of Brønsted acid. Then, TsNH2 dropped
out to give the intermediate vinylogous iminium I. Finally, an
asymmetric hydrogenation process was used to give the products.
Inspired by the previous work,8 we also attempted the tandem
reaction (Friedel–Crafts/asymmetric hydrogenation) of indoles 3
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N-(Cyclohexyl(2-methyl-1H-indol-3-yl)methyl)-4-methyl-ben-
zenesulfonamide (1b). White solid, m.p. 94–95 ◦C; 1H NMR (400
MHz, d6-acetone) d 9.61 (s, 1H), 7.58 (d, J = 7.9 Hz, 1H), 7.22 (d, J
= 8.0 Hz, 2H), 7.08 (d, J = 8.0 Hz, 1H), 6.92 (t, J = 7.5 Hz, 1H), 6.82
(dd, J = 18.3, 7.6 Hz, 4H), 6.53 (d, J = 8.3 Hz, 1H), 4.29 (t, J = 9.2
Hz, 1H), 2.34–2.25 (m, 1H), 2.24 (s, 3H), 2.16 (s, 3H), 2.08–1.97 (m,
1H), 1.76 (dd, J = 9.2, 4.8 Hz, 1H), 1.56 (dd, J = 19.4, 11.5 Hz, 2H),
1.42–0.95 (m, 6H), 0.87–0.77 (m, 1H). 13C NMR (100 MHz, d6-
acetone) d 142.03, 136.75, 133.63, 128.83, 127.24, 126.80, 120.95,
119.64, 119.17, 111.04, 110.71, 57.46, 42.54, 31.85, 30.93, 27.11,
26.81, 26.66, 21.24, 11.79. HRMS calculated for C24H30N2O2NaS
[M+Na]+ 419.1769, found 419.1769; IR (KBr) n 3386, 2924, 2857,
1307, 1156, 670 cm-1.
Scheme 2 Tandem reactions of indoles 3 and N-tosyl imines 4.
Gratifyingly, the reaction proceeded smoothly without losing the
enantioselectivity (Scheme 2).
Conclusions
General procedure for Pd-catalyzed asymmetric hydrogenation of
3-(toluenesulfonamidoalkyl)indoles
In summary, we have synthesized a series of 2-substituted 3-
(toluenesulfonamidoalkyl)indoles 1 by using (EtO)2POH or iodine
as the catalyst, and their asymmetric hydrogenation reactions were
also explored using the chiral palladium catalyst in the presence
of Brønsted acid, giving the chiral 2,3-disubstituted indolines with
up to 97% ee. In addition, the tandem reactions of 2-substituted
indoles and N-tosyl imines under the standard reaction conditions
can also perform well with similar ee values. The readily accessible
starting materials, high enantioselectivity and high yields make
this method very useful for the synthesis of chiral 2,3-disubstituted
indolines.
(R)-H8-BINAP (3.8 mg, 0.006 mmol) and Pd(OCOCF3)2 (1.7
mg, 0.005 mmol) were placed in a dried Schlenk tube under
nitrogen atmosphere, and degassed anhydrous acetone 1 mL
was added. The mixture was stirred at room temperature for
1 h, and then solvent was removed under vacuum to give the
catalyst. In a glovebox, TsOH·H2O (0.25 mmol) and substrate
1 (0.25 mmol) were stirred in 1 mL solvent (DCM and TFE
were mixed in ratio of 1 : 1 prior to use) at room temperature
for 5 min. Subsequently, the above catalyst together with 2 mL
solvent was added to the reaction mixture. The hydrogenation
was performed at 50 ◦C under H2 (600 psi) in a stainless steel
autoclave for 16–20 h. After carefully releasing the hydrogen, the
resulting mixture was concentrated under vacuum and dissolved
in saturated aqueous NaHCO3 (5 mL). After stirring for 10 min,
the mixture was extracted with CH2Cl2 (3 ¥ 5 mL) and dried over
Na2SO4. After purification by silica gel chromatography using
petroleum ether/EtOAc (10/1) as eluent, the enantiomeric excess
of the products were determined by HPLC with chiral columns
(OJ-H, OD-H or AD-H).
(2R,3R)-(-)-2-Methyl-3-benzylindoline (2a). 89% yield, 87% ee,
[a]2D7 -68.0 (c 0.83, CHCl3); 1H NMR (400 MHz, CDCl3): d 1.23
(d, J = 6.5 Hz, 3H), 2.87 (dd, J = 13.8, 8.9 Hz, 1H), 2.97 (dd, J
= 13.9, 7.2 Hz, 1H), 3.53 (dd, J = 15.9, 7.8 Hz, 1H), 3.71 (br s,
1H), 3.96–4.03 (m, 1H), 6.54–6.65 (m, 3H), 7.00 (t, J = 7.4, 1H),
7.17–7.31 (m, 5H); HPLC (OJ-H, elute: hexanes/i-PrOH = 80/20,
detector: 254 nm, flow rate: 1.0 mL min-1), t1 = 10.4 min, t2 = 11.6
min (maj.).
Experimental section
General procedure for the synthesis of
3-(toluenesulfonamidoalkyl)indoles 1
3-(Toluenesulfonamidoalkyl)indoles 1a–n were synthesized from
the corresponding 2-substituted indoles and N-tosyl imines ac-
cording to either method A or B.
Method A: In a dry Schlenk tube, N-tosyl imines 4 (1 mmol)
and (EtO)2POH (0.1 mmol) were dissolved in toluene (4 mL) under
nitrogen. The solution was stirred◦for 10 min at room temperature
and then for another 5 min at 0 C. Subsequently, 2-substituted
indoles 3 (3 mmol) were added in one portion at 0 ◦C. The reaction
mixture was allowed to warm to room temperature naturally. After
the reaction was complete (monitored by TLC), 10% NaHCO3 (5
mL) was added to quench the reaction. The mixture was extracted
with ethyl acetate (10 mL). The organic layer was washed with
brine (10 mL), separated, and dried over anhydrous Na2SO4. The
solvents were removed under reduced pressure and the residue was
purified by flash chromatography (ethyl acetate/petroleum ether
= 1/5) to afford the product.
General procedure for Pd-catalyzed tandem reactions of
2-substituted indoles and N-tosyl imines
Method B: In a dry Schlenk tube, 2-substituted indoles 3 (1
mmol) and I2 (10 mol%) were dissolved in 4 mL dry CH2Cl2.
Then the resulting mixture was stirred at 0 ◦C for 2 min before N-
tosyl imines 4 (1 mmol) was added. Finally, a saturated solution
of sodium subsulfite was not added to quench the reaction until
the starting materials were consumed as indicated by TLC (about
5 min). The mixture was extracted with CH2Cl2 (10 mL). The
organic layer was washed with brine (10 mL), separated, and dried
over anhydrous Na2SO4. The solvents were removed under reduced
pressure and the residue was purified by flash chromatography
(ethyl acetate/petroleum ether = 1/5) to afford the product.
(R)-H8-BINAP (3.8 mg, 0.006 mmol) and Pd(OCOCF3)2 (1.7 mg,
0.005 mmol) were placed in a dried Schlenk tube under nitrogen
atmosphere, and degassed anhydrous acetone was added. The
mixture was stirred at rt for 1 h, then the solvent was removed
under vacuum to give the catalyst. In a glovebox, acid (0.25 mmol)
and indole (0.25 mmol) were stirred in 1 mL DCM/TFE at room
temperature for 1 min. Subsequently, N-tosyl imine (0.25 mmol)
was added to the solution. Finally, the above catalyst together
with 2 mL DCM/TFE was added to the reaction mixture. The
hydrogenation was performed at 50 ◦C under H2 (600 psi) in
a stainless steel autoclave for 16 h. After carefully releasing the
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hydrogen, the resulting mixture was concentrated under vacuum
and dissolved in saturated aqueous NaHCO3 (5 mL). After stirring
for 10 min, the mixture was extracted with CH2Cl2 (3 ¥ 5 mL) and
dried over Na2SO4. After purification by silica gel chromatography
using petroleum ether/EtOAc (10/1) as eluent, the enantiomeric
excess of the products were determined by HPLC with chiral
column.
7 D.-S. Wang, J. Tang, Y.-G. Zhou, M.-W. Chen, C.-B. Yu, Y. Duan and
G.-F. Jiang, Chem. Sci., 2011, 2, 803.
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Acknowledgements
This work was financially supported by Natural Science Founda-
tion of China (21032003 & 20921092) and National Basic Research
Program of China (2010CB833300).
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1238 | Org. Biomol. Chem., 2012, 10, 1235–1238
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