Rh(III)-catalyzed coupling of nitrones with alkynes for the synthesis of indolines

Article abstract of DOI:10.1016/S1872-2067(15)60866-X

Rh-catalyzed redox-neutral coupling between N-aryl nitrones and alkynes has been achieved under relatively mild conditions. The reaction proceeded via C-H activation at the N-aryl ring with subsequent O-atom transfer, affording trisubstituted indolines in good chemoselectivity and moderate to good diasteroselectivity.

Full text of DOI:10.1016/S1872-2067(15)60866-X

Chinese Journal of Catalysis 36 (2015) 925–932
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c h n j c
Article
Rh(III)-catalyzed coupling of nitrones with alkynes for the synthesis
of indolines
Lingheng Kong, Fang Xie, Songjie Yu, Zisong Qi, Xingwei Li *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Rh-catalyzed redox-neutral coupling between N-aryl nitrones and alkynes has been achieved under
relatively mild conditions. The reaction proceeded via C–H activation at the N-aryl ring with subse-
quent O-atom transfer, affording trisubstituted indolines in good chemoselectivity and moderate to
good diasteroselectivity.
Received 22 April 2015
Accepted 30 April 2015
Published 20 July 2015
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Rhodium(III)
Carbon-hydrogen activation
Oxygen-atom transfer
Nitrogen-aryl nitrones
1.
Introduction
of multi-task DGs.
In this context, stable Cp*Rh(III) complexes have stood out
as efficient catalysts for the selective C–H functionalization of a
large scope of arenes [16–19], and this area has experienced
tremendous progress. In particular, Fagnou and Glorius pio-
neered in C–H activation of arenes assisted by an oxidizing N–O
bond as a DG under redox-neutral conditions [20–24]. These
systems overcame limited roles of DGs in general because they
participated in subsequent reactions as internal oxidants, espe-
cially leading to efficient synthesis of heterocycles under mild
conditions when alkenes and alkynes were used as the coupling
partner [25–36]. In addition, these reactions are re-
dox-economic since no external oxidant is necessary. Inspired
by this strategy, the oxidizing DGs have been extended to other
oxidizing bonds such as N–N [37–44], N–S [45], and C–N [46].
On the other hand, the cleavage of these oxidizing groups al-
most always generates a small molecule such as water, alcohol,
acid, or amine as a co-product [25–46], which limited the at-
Metal-catalyzed C–H activation of arenes has received in-
creasing attention in the past several decades and this strategy
has allowed the development of a plethora of new synthetic
methods to access complex structures [1–8]. This strategy is
appealing in that no prior functionalization of arenes is neces-
sary. In most cases, the C–H activation of less intrinsically active
arenes requires the installation of a directing group typically at
the ortho position [9–14]. The C–H bond is cleaved with con-
current formation of a metalacycle, which provides a driving
force to enhance both reactivity and selectivity. Despite the
progress, the presence of a pendent directing group in the cou-
pled product is often undesirable. To address this limitation, a
number of coupling systems with functionalizable or transfera-
ble DGs in arene substrates have been developed [15]. Howev-
er, challenges remain and it is necessary to develop more effi-
cient and selective C–H activation systems by taking advantage
*
Corresponding author. Tel: +86-411-84379089; E-mail: xwli@dicp.ac.cn
This work was supported by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and the National Natural Science Foundation of
China (21272231, 21472186, 21372266).
DOI: 10.1016/S1872-2067(15)60866-X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 7, July 2015

9
26
Lingheng Kong et al. / Chinese Journal of Catalysis 36 (2015) 925–932
2.3. Spectral data for products
om-economy of the coupling system.
We recently reported the Rh(III)-catalyzed redox-neutral
coupling between quinolone N-oxides and alkynes, leading to
the synthesis of diaryl-substituted acetophenones [47,48]. In
this reaction, the N–O bond acted as an oxidizing DG to realize
C–H activation at the 8-position. Significantly, the oxygen was
retained via an intramolecular OAT process with 100% at-
om-economy. So far this C–H activation-OAT system has been
achieved for only one arene system. It is necessary to extend
this strategy to other related arenes for the preparation of syn-
thetically useful complex products. Arenes such as benzalde-
hyde-derived nitrones are structurally related to quinolone
N-oxides, and our previous studies revealed that
N-tert-butyl-α-arylnitrones coupled with alkynes under Rh
catalysis to afford indenones [49]. However, no OAT to alkyne
was involved although the polar N–O bond proved to be a via-
ble DG to effect endo C–H activation. On the other hand, Wan
and coworkers reported in one example the exo-type C–H acti-
vation of nitrone in the coupling with diphenylacetylene, af-
fording an indole [50–53]. We reasoned that N-aryl nitrone
may still undergo C–H activation-OTA coupling with alkynes
under appropriate Rh(III)-catalyzed conditions. We now report
synthesis of indolines via this type of coupling, and the indoline
motif has been widely found in natural products and pharma-
ceuticals [54–56].
1
3aa. H NMR (400 MHz, CDCl ) δ 7.63 – 7.51 (m, 2H), 7.44 –
3
7.35 (m, 1H), 7.30 – 7.20 (m, 2H), 7.17 (td, J = 7.7, 1.2 Hz, 1H),
7.12 – 6.95 (m, 9H), 6.82 (d, J = 7.6 Hz, 1H), 6.73 – 6.60 (m, 3H),
6.17 (s, 1H), 4.28 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 197.5,
152.2, 140.0, 138.4, 137.2, 132.2, 131.0, 129.4, 128.6, 128.4,
128.1, 128.0, 127.99, 127.7, 127.3, 127.1, 126.3, 119.0, 109.8,
3
72.3, 70.3. HRMS: m/z: [M+H]+ calculated for C27
H21NO:
376.1696, Found: 376.1696.
1
3aa’. H NMR (400 MHz, CDCl
3
) δ 7.68 – 7.60 (m, 2H), 7.57 –
7.50 (m, 1H), 7.32 (t, J = 7.6 Hz, 2H), 7.27 – 7.21 (m, 4H), 7.07
(m, 3H), 7.00 (m, 1H), 6.88 (t, J = 7.7 Hz, 2H), 6.85 – 6.78 (m,
3H), 6.67 (d, J = 7.7 Hz, 1H), 5.50 (s, 1H), 4.26 (s, 1H). 13C NMR
(100 MHz, CDCl ) δ 197.6, 148.7, 145.1, 141.2, 138.1, 131.9,
131.8, 129.7, 129.6, 128.4, 128.3, 128.2, 128.0, 127.9, 127.5,
3
127.45, 124.8, 119.74, 109.8, 71.4, 70.3. HRMS: m/z: [M+H]+
calculated for C27
H21NO: 376.1696, Found: 376.1697.
1
3ba. H NMR (400 MHz, CDCl
3
) δ 7.48 (d, J = 7.4 Hz, 2H),
7.29 (t, J = 7.4 Hz, 1H), 7.13 (t, J = 7.9 Hz, 2H), 7.00 – 6.86 (m,
9H), 6.73 (s, 1H), 6.63 (d, J = 7.9 Hz, 1H), 6.57 (d, J = 7.3 Hz, 2H),
6.05 (s, 1H), 4.03 (s, 1H), 2.02 (s, 3H). 13C NMR (100 MHz,
CDCl ) δ 197.6, 149.8, 140.0, 138.3, 137.1, 132.0, 130.8, 129.7,
128.5, 128.3, 128.27, 128.1, 127.9, 127.8 127.5, 127.1, 126.9,
3
+
126.6, 109.7, 72.3, 70.5, 21.1. HRMS: m/z: [M+H] calculated for
C
28
H
23NO: 390.1852, Found: 390.1855.
1
2.
Experimental
3ca. H NMR (400 MHz, CDCl
3
) δ 7.53 (d, J = 7.9 Hz, 2H), 7.40
(
t, J = 7.4 Hz, 1H), 7.23 (m, 2H), 7.11 – 6.97 (m, 8H), 6.87 (td, J =
8.7, 2.5 Hz, 1H), 6.72 (m, 2H), 6.62 (d, J = 7.7 Hz, 2H), 6.16 (s,
H), 4.14 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 196.9, 156.6 (d,
JC-F = 233.9 Hz), 148.2, 148.2, 139.4, 137.7, 136.7, 132.4, 130.7,
2.1. General
1
3
All chemicals were from commercial sources and were used
as received unless otherwise noted. All reactions were carried
out using Schlenk techniques or in an N
-filled glovebox. Col-
umn chromatography was carried out on silica gel (300–400
mesh) using triethylamine/petroleum ether or ethyl acetate
128.3, 128.25, 128.1 (d, JC-F = 1.4 Hz), 127.7 (d, JC-F = 7.8 Hz),
127.5, 127.3, 127.2, 115.7 (d, JC-F = 23.3 Hz), 115.0 (d, JC-F = 25
Hz), 110.0 (d, JC-F = 8.1 Hz), 72.1, 70.8. HRMS: m/z: [M+H] cal-
2
+
culated for C27
H
20FNO: 394.1602, Found: 394.1604.
1
(EA)/petroleum ether (PE). NMR Spectra were recorded on a
3da. H NMR (400 MHz, CDCl
3
) δ 7.57 – 7.48 (m, 2H), 7.42 –
Bruker 400 MHz NMR spectrometer in the solvents indicated.
The chemical shift is given in dimensionless δ values and is
frequency referenced relative to TMS in H and 13C NMR spec-
troscopy. The ratio of trans and cis was determined on the basis
of H NMR analysis. High resolution mass spectra were ob-
tained on an Agilent Q-TOF 6540.
Diphenylacetylene and 4-octyne were obtained from com-
mercial sources. Other nitrones [50] and diarylacetylenes [57]
were prepared according to literature reports and the NMR
data agree with those in the literature reports.
7.37 (m, 1H), 7.23 (m, 2H), 7.08 (m, 2H), 7.03 – 6.93 (m, 8H),
6.71 (d, J = 8.4 Hz, 1H), 6.67 – 6.61 (m, 2H), 6.16 (s, 1H), 4.26 (s,
1
1H). 13C NMR (100 MHz, CDCl
) δ 196.9, 150.7, 139.3, 137.6,
3
136.5, 132.4, 130.7, 129.2, 128.3, 128.2, 128.1, 128.02, 128.0,
127.9, 127.6, 127.3, 127.2, 123.2, 110.4, 72.2, 70.5. HRMS: m/z:
1
+
[M+H] calculated for C27
H
20ClNO: 410.1306, Found: 410.1302.
1
3ea. H NMR (400 MHz, CDCl
3
) δ 7.43 (d, J = 7.4 Hz, 2H),
7.32 (t, J = 7.4 Hz, 1H), 7.18 – 7.12 (m, 3H), 6.99 (t, J = 4.9 Hz,
2H), 6.96 – 6.86 (m, 7H), 6.58 (t, J = 8.3 Hz, 3H), 6.08 (s, 1H),
4.20 (s, 1H). 13C NMR (100 MHz, CDCl
37.6, 136.5, 132.4, 132.0, 130.7, 130.65, 128.4, 128.3, 128.13,
128.1, 128.0, 127.6, 127.3, 127.2, 110.9, 110.1, 72.2, 70.4.
3
) δ 196.9, 151.1, 139.2,
1
2.2. General procedure for the synthesis of compounds 3
HRMS: m/z: [M+H]+ calculated for C27
H20BrNO: 454.0801,
Nitrones (0.2 mmol), diarylacetylenes (0.3 mmol),
Found: 454.0801.
1
[
Cp*RhCl ]
2 2
(4 mol%), AgSbF
6
(16 mol%), PivOH (2.0 equiv)
3fa. H NMR (400 MHz, CDCl
3
) δ 7.79 (dd, J = 8.3, 1.4 Hz,
and ethyl acetate (2 mL) were charged into the sealed tube. The
reaction mixture was stirred at 80 °C for 15 h. After cooled to
room temperature, the solvent was removed under reduced
pressure and the residue was purified by silica gel chroma-
tography using triethylamine/PE or PE/EA to afford com-
pounds 3.
1H), 7.64 (s, 1H), 7.47 (d, J = 7.5 Hz, 2H), 7.30 (t, J = 7.4 Hz, 1H),
7.15 (m, 2H), 7.01 – 6.83 (m, 8H), 6.63 (t, J = 8.0 Hz, 3H), 6.14 (s,
1H), 4.67 (s, 1H), 4.09 (m, 2H), 1.14 (t, J = 7.1 Hz, 3H). 13C NMR
(100 MHz, CDCl ) δ 197.1, 166.4, 155.9, 139.1, 137.7, 136.4,
3
132.3, 132.1, 130.7, 130.2, 128.3, 128.2, 128.0, 127.9, 127.6,
127.4, 127.2, 125.4, 120.5, 108.1, 71.8, 70.2, 60.2, 14.3. HRMS:

Lingheng Kong et al. / Chinese Journal of Catalysis 36 (2015) 925–932
927
m/z: [M+H]+ calculated for C30
H
25NO
: 448.1907, Found:
7.8 Hz, 1H), 6.67 (t, J = 7.5 Hz, 1H), 6.61 (d, J = 7.4 Hz, 2H), 6.13
(s, 1H), 4.24 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 197.4, 151.6,
138.8, 138.0, 136.9, 132.3, 130.9, 130.5, 130.1, 129.3, 128.4,
128.1, 127.9, 127.5, 127.3, 126.3, 121.1, 119.2, 109.9, 71.9, 69.6.
3
4
48.1906.
3
1
3ga. H NMR (400 MHz, CDCl
3
) δ 7.55 (d, J = 7.4 Hz, 2H),
7.37 (t, J = 7.4 Hz, 1H), 7.22 (m 2H), 7.08 – 6.94 (m, 8H), 6.87 (d,
J = 7.8 Hz, 1H), 6.72 – 6.61 (m, 3H), 6.46 (d, J = 7.8 Hz, 1H), 6.13
s, 1H), 4.19 (s, 1H), 2.29 (s, 3H). 13C NMR (100 MHz, CDCl
) δ
97.3, 152.3, 140.0, 139.4, 138.4, 136.9, 132.0, 130.8, 128.4,
28.2, 127.9, 127.8, 127.6, 127.5, 127.1, 126.9, 123.1, 119.8,
10.5, 71.9, 70.2, 21.6. HRMS: m/z: [M+H]+ calculated for
HRMS: m/z: [M+H]+ calculated for C27
H20BrNO: 454.0801,
Found: 454.0802.
(
3
1
1
1
1
C
3na. H NMR (400 MHz, CDCl
3
) δ 7.61 – 7.56 (m, 2H), 7.40 (t,
J = 7.4 Hz, 1H), 7.24 (m, 2H), 7.19 – 7.11 (m, 2H), 7.07 (t, J = 7.4
Hz, 2H), 7.02 (d, J = 7.6 Hz, 1H), 6.94 (dd, J = 5.0, 1.0 Hz, 1H),
6.84 (m, 2H), 6.74 (m, 3H), 6.68 (td, J = 7.6, 0.9 Hz, 1H), 6.37 (s,
28
H
23NO: 390.1852, Found: 390.1852.
1
1H), 4.36 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 197.2, 151.2,
3ha. H NMR (400 MHz, CDCl
3
) δ 7.59 (d, J = 7.6 Hz, 2H),
3
7
.37 (t, J = 7.4 Hz, 1H), 7.21 (t, J = 7.8 Hz, 2H), 7.00 (m, 8H), 6.90
143.6, 137.9, 136.9, 132.3, 130.9, 129.2, 128.1, 128.0, 127.9,
127.6, 127.3, 126.4, 126.1, 125.9, 124.7, 119.4, 110.3, 71.6, 66.7.
HRMS: m/z: [M+H]+ calculated for C25
H20NOS: 382.1260,
(
d, J = 8.1 Hz, 1H), 6.86 (d, J = 1.4 Hz, 1H) 6.68 (dd, J = 8.1, 1.5
Hz, 1H), 6.60 (d, J = 7.4 Hz, 2H), 6.17 (s, 1H), 4.18 (s, 1H), 1.30
s, 9H). 13C NMR (100 MHz, CDCl
) δ 197.3, 152.8, 151.9, 139.9,
38.7, 137.0, 132.1, 131.1, 128.5, 127.9, 127.8, 127.4, 127.1,
26.9, 126.88, 123.6, 116.3, 107.2, 71.7, 70.4, 34.8, 31.5. HRMS:
23NO: 432.2322, Found:
(
3
Found: 382.1258.
1
1
1
3ab. H NMR (400 MHz, CDCl
3
) δ 7.50 (d, J = 8.3 Hz, 2H),
7.13 (td, J = 7.8, 1.0 Hz, 1H), 7.06 – 6.94 (m, 8H), 6.78 (dd, J =
8.0, 1.8 Hz, 3H), 6.64 (td, J = 7.6, 0.8 Hz, 1H), 6.49 (d, J = 8.1 Hz,
2H), 6.13 (s, 1H), 4.33 (s, 1H), 2.29 (s, 3H), 2.18 (s, 3H). 13C NMR
m/z: [M+H]+ calculated for C28
H
432.2327.
1
3
ia. H NMR (400 MHz, CDCl
3
) δ 7.58 (m, 4H), 7.40 (q, J = 7.1
(100 MHz, CDCl ) δ 197.0, 152.0, 142.7, 140.0, 136.5, 135.4,
3
Hz, 3H), 7.32 (m, 1H), 7.23 (m, 2H), 7.10 – 6.96 (m, 10H), 6.88
dd, J = 7.9, 1.6 Hz, 1H), 6.77 – 6.67 (m, 2H), 6.20 (s, 1H), 4.32 (s,
134.2, 131.2, 129.1, 128.6, 128.55, 128.4, 128.3, 127.7, 127.4,
127.1, 126.8, 118.8, 109.7, 71.7, 70.2, 21.5, 21.0. HRMS: m/z:
(
H). 13C NMR (100 MHz, CDCl
39.8, 138.3, 136.9, 132.1, 130.9, 128.7, 128.4, 128.3, 128.1,
28.0, 127.9, 127.6, 127.4, 127.2, 127.1, 127.06, 125.3, 118.1,
08.3, 72.0, 70.4. HRMS: m/z: [M+H] calculated for C28
) δ 197.2, 152.6, 142.5, 141.1,
[M+H] calculated for C29
+
H
25NO: 404.2009, Found: 404.2005.
1
1
1
1
4
3
1
3ac. H NMR (400 MHz, CDCl
3
) δ 7.64 (m, 2H), 7.17 (m, 1H),
7.10 – 6.98 (m, 6H), 6.81 (d, J = 7.8 Hz, 1H), 6.73 (d, J = 9.0 Hz,
2H), 6.67 (t, J = 7.5 Hz, 1H), 6.50 (q, J = 8.9 Hz, 4H), 6.17 (s, 1H),
4.23 (s, 1H), 3.79 (s, 3H), 3.69 (s, 3H). 13C NMR (100 MHz,
+
H24NO:
52.2009, Found: 452.2006.
1
3
3ja. H NMR (400 MHz, CDCl ) δ 7.54 (d, J = 8.2 Hz, 2H), 7.37
CDCl ) δ 196.0, 162.6, 158.4, 151.8, 139.9, 133.5, 131.0, 129.5,
3
(
t, J = 7.4 Hz, 1H), 7.21 (t, J = 7.8 Hz, 2H), 7.14 (t, J = 7.5 Hz, 1H),
129.4, 129.0, 128.4, 127.5, 127.4, 127.3, 127.1, 118.8, 113.2,
113.1, 109.7, 71.0, 70.2, 55.3, 55.2. HRMS: m/z: [M+H] calcu-
+
7
2
1
.07 (t, J = 7.2 Hz, 1H), 7.00 (t, J = 7.2 Hz, 3H), 6.87 (d, J = 7.8 Hz,
H), 6.79 (d, J = 7.3 Hz, 3H), 6.66 (d, J = 7.5, 2H), 6.62 (d, J = 7.5,
H), 6.11 (s, 1H), 4.19 (s, 1H), 2.17 (s, 3H). 13C NMR (100 MHz,
lated for C29
H
25NO
3
: 436.1907, Found: 436.1907.
1
3ad. H NMR (400 MHz, CDCl
3
) δ 7.62 (m, 2H), 7.18 (td, J =
CDCl
3
) δ 197.5, 152.1, 138.3, 137.0, 136.8, 136.6, 132.0, 130.8,
7.8, 1.1 Hz, 1H), 7.08 – 6.89 (m, 8H), 6.82 (d, J = 7.7 Hz, 1H), 6.74
– 6.64 (m, 3H), 6.59 – 6.51 (m, 2H), 6.15 (s, 1H), 4.25 (s, 1H). 13
NMR (100 MHz, CDCl ) δ 195.5, 164.9 (d, JC-F = 253.4 Hz), 161.8
(d, JC-F = 245.7 Hz), 151.9, 139.3, 134.1 (d, JC-F = 3.3 Hz), 133.6
(d, JC-F = 9.0 Hz), 132.8 (d, JC-F = 3.2 Hz), 130.0 (d, JC-F = 8.0 Hz),
1
1
C
29.2, 128.5, 128.2, 128.16, 127.9, 127.8, 127.78, 127.0, 126.3,
18.8, 109.7, 72.0, 70.1, 21.1. HRMS: m/z: [M+H] calculated for
C
+
3
28
H23NO: 390.1852, Found: 390.1853.
1
3ka. H NMR (400 MHz, CDCl
3
) δ 7.47 (d, J = 7.8 Hz, 2H),
7
–
7
1
1
1
1
C
.29 (t, J = 7.3 Hz, 1H), 7.13 (m, 2H), 7.06 (t, J = 7.6 Hz, 1H), 7.02
6.96 (m, 1H), 6.92 (m, 3H), 6.83 (d, J = 8.3 Hz, 2H), 6.71 (d, J =
.8 Hz, 1H), 6.62 – 6.49 (m, 3H), 6.45 (d, J= 8.5 Hz, 2H), 6.02 (s,
129.5, 128.2, 127.7, 127.4, 127.2, 126.2, 119.1, 115.1 (d, JC-F =
21.5 Hz), 114.9 (d, JC-F = 21.3 Hz), 109.9, 71.2, 70.2. HRMS: m/z:
+
[M+H] calculated for C27
H
19
F
2
NO: 412.1507, Found: 412.1506.
H), 4.07 (s, 1H), 3.58 (s, 3H). 13C NMR (100 MHz, CDCl
3ae. H NMR (400 MHz, CDCl
1
3
) δ
3
) δ 7.43 (m, 2H), 7.14 (t, J = 6.0
97.6, 158.8, 152.0, 138.4, 137.1, 132.0, 131.7, 130.8, 129.4,
29.2, 128.5, 128.0, 127.8, 127.7, 127.0, 126.4, 118.8, 112.9,
09.7, 71.9, 69.8, 55.2. HRMS: m/z: [M+H]+ calculated for
Hz, 2H), 7.09 (t, J = 7.7 Hz, 1H), 6.99 – 6.84 (m, 8H), 6.72 (d, J =
7.8 Hz, 1H), 6.59 (t, J = 7.6 Hz, 1H), 6.42 (dd, J = 8.5, 1.4 Hz, 2H),
6.05 (s, 1H), 4.15 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 195.6,
3
28
H
23NO
2
: 406.1802, Found: 406.1805.
151.9, 139.1, 138.8, 136.8, 134.9, 133.2, 132.3, 129.7, 129.6,
129.1, 128.3, 128.2, 127.7, 127.6, 127.2, 125.8, 119.1, 110.0,
1
3la. H NMR (400 MHz, CDCl
3
) δ 7.56 (d, J = 8.0 Hz, 2H), 7.39
(t, J = 7.4 Hz, 1H), 7.26 – 7.19 (m, 2H), 7.16 (t, J = 7.3 Hz, 1H),
71.4, 70.2. HRMS: m/z: [M+H]+ calculated for C27
H
19Cl NO:
2
7
1
4
1
1
.09 (t, J = 7.3 Hz, 1H), 7.05 – 6.93 (m, 7H), 6.80 (d, J = 7.8 Hz,
H), 6.65 (t, J = 7.5 Hz, 1H), 6.59 (d, J = 7.5 Hz, 2H), 6.13 (s, 1H),
444.0916, Found: 444.0912.
1
3af. H NMR (400 MHz, CDCl
3
) δ 7.41 (q, J = 8.7 Hz, 4H), 7.18
.22 (s, 1H). 13C NMR (100 MHz, CDCl
3
) δ 197.4, 151.7, 138.2,
(t, J = 7.6 Hz, 1H), 7.11 (d, J = 8.5 Hz, 1H), 7.08 – 6.97 (m, 5H),
6.94 (d, J = 7.6 Hz, 1H), 6.81 (d, J = 7.8 Hz, 1H), 6.67 (t, J = 7.5 Hz,
1H), 6.44 (d, J = 8.5 Hz, 2H), 6.13 (s, 1H), 4.23 (s, 1H). 13C NMR
38.0, 136.9, 132.9, 132.3, 130.9, 129.7, 129.3, 128.4, 128.1,
27.9, 127.6, 127.57, 127.2, 126.3, 119.1, 109.9, 71.9, 69.5.
HRMS: m/z: [M+H]+ calculated for C27
H20ClNO: 410.1306,
(100 MHz, CDCl ) δ 195.8, 151.9, 139.0, 137.3, 135.3, 132.4,
3
Found: 410.1309.
131.3, 131.1, 130.1, 129.6, 128.1, 127.8, 127.6, 127.6, 127.2,
125.6, 121.4, 119.1, 110.0, 71.4, 70.1. HRMS: m/z: [M+H] cal-
1
+
3ma. H NMR (400 MHz, CDCl
3
) δ 7.57 (d, J = 7.6 Hz, 2H),
7
.41 (t, J = 7.4 Hz, 1H), 7.28 – 7.21 (m, 2H), 7.18 (m, 1H), 7.11 (t,
culated for C27
H
19Br
2
NO: 531.9906, Found: 531.9906.
1
J = 8.5 Hz, 3H), 7.02 (m, 3H), 6.92 (d, J = 8.2 Hz, 2H), 6.82 (d, J =
3ag. H NMR (400 MHz, CDCl
3
) δ 7.50 (s, 1H), 7.25 – 7.11 (m,

928
Lingheng Kong et al. / Chinese Journal of Catalysis 36 (2015) 925–932
3
2
7
2
1
1
1
H), 7.05 (t, J = 7.7 Hz, 1H), 7.03 – 6.95 (m, 6H), 6.93 – 6.83 (m,
H), 6.79 (d, J = 7.8 Hz, 1H), 6.64 (t, J = 7.5 Hz, 1H), 6.49 (d, J =
.4 Hz, 1H), 6.40 (s, 1H), 6.12 (s, 1H), 4.25 (s, 1H), 2.24 (s, 3H),
the coupling between N-phenyl nitrone 1a and diphenylacety-
lene (2a) in the presence of [RhCp*Cl catalyst (4 mol%, Table
]
2 2
1). The reaction proceeded in poor yield and low selectivity in
DCE in the absence of any acid additive (entry 1). Addition of
PivOH or AcOH increased the conversion and the selectivity
(entries 2 and 3), and the major product was identified as a
2,3,3-trisubstituted indoline (3aa). Product 3aa was fully
characterized, including by X-ray crystallography (not shown).
However, diastereomer 3aa’ and indole 4 were also detected as
the minor products, but no indenone product has been ob-
served. These products were generated via exo C–H activation
assisted by the O-coordination. It should be noted that the con-
ditions that were optimal for related C–H activation-O-atom
transfer reaction between quinoline N-oxide and an alkyne
only gave poor results (entries 2,3) [47]. Switching to other Ag
.01 (s, 3H). 13C NMR (100 MHz, CDCl
) δ 197.7, 152.1, 140.0,
3
38.1, 137.7, 137.5, 137.1, 132.8, 131.3, 129.2, 129.1, 128.3,
28.1, 128.0, 127.8, 127.6, 127.5, 127.4, 127.1, 126.2, 125.4,
+
18.8, 109.6, 72.2, 70.1, 21.4, 21.2. HRMS: m/z: [M+H] calcu-
lated for C29
H
26NO: 404.2009, Found: 404.2009.
1
3
ah. H NMR (400 MHz, CDCl
3
) δ 7.32 – 7.22 (m, 2H), 7.21 –
7
6
1
1
1
1
1
2
1
.15 (m, 2H), 7.14 – 7.08 (m, 1H), 7.05 (m, 1H), 6.97 (m, 6H),
.83 – 6.75 (m, 2H), 6.67 (t, J = 7.5 Hz, 1H), 6.41 (m, 2H), 6.12 (s,
H), 4.30 (s, 1H). 13C NMR (100 MHz, CDCl
) δ 195.4 (d, JC-F =
.9 Hz), 162.7 (d, JC-F = 247.5 Hz), 162.1 (d, JC-F = 248.2 Hz),
52.0, 140.6 (d, JC-F = 7.2 Hz), 139.2, 138.8 (d, JC-F = 6.3 Hz),
29.7, 129.5 (d, JC-F = 2.5Hz), 129.4 (d, JC-F = 3.2 Hz), 128.0,
27.7, 127.6, 127.6, 126.5 (d, JC-F = 3.1 Hz), 125.0, 124.3 (d, JC-F
.7 Hz), 119.3 (d, JC-F = 21.4 Hz), 119.1, 117.4 (d, JC-F = 23.3 Hz),
3
=
salts or to Zn(OTf)
2
only gave negative results (entry 4). Signif-
icant solvent effects have been observed. Using DCM as the
solvent, both the selectivity and the conversion of the reaction
were improved, and product 3aa was isolated in 63% yield.
Extensive screening of solvents revealed that the yield was
optimal in ethyl acetate (68%, entry 8). Control experiments
also proved that the Rh catalyst was necessary; omission of the
Rh catalyst or switching to the Ir congener only resulted in
decomposition of the starting nitrone.
15.5 (d, JC-F = 22.7 Hz), 114.2 (d, JC-F = 21.2 Hz), 109.9, 71.8 (d,
J
C-F = 1.3 Hz), 70.14. HRMS: m/z: [M+H]+ calculated for
C
27
H
19 2
F
NO: 412.1507, Found: 412.1509.
1
3ai. H NMR (400 MHz, CDCl
3
) δ 7.67 (t, J = 1.8 Hz, 1H), 7.38
(m, 1H), 7.33 – 7.28 (m, 1H), 7.21 – 7.12 (m, 2H), 7.08 – 6.95 (m,
7
7
H), 6.92 (t, J = 7.9 Hz, 1H), 6.82 (d, J = 7.8 Hz, 1H), 6.68 (td, J =
.6, 0.9 Hz, 1H), 6.63 (t, J = 1.8 Hz, 1H), 6.53 – 6.47 (m, 1H), 6.11
(
s, 1H), 4.32 (s, 1H). 13C NMR (100 MHz, CDCl
40.0, 139.1, 138.2, 134.4, 134.3, 132.3, 130.7, 129.8, 129.2,
29.1, 128.8, 128.4, 128.0, 127.8, 127.6, 127.5, 127.4, 126.6,
24.9, 119.2, 110.0, 71.8, 70.2. HRMS: m/z: [M+H] calculated
3
) δ 195.4, 152.0,
With the optimized conditions established, we next ex-
plored the scope and generality of this coupling (Table 2). The
scope of the nitrone was first examined in the coupling with
1
1
1
+
diphenylacetylene.
Introduction
of
electron-donating,
for C27
H
19Cl
2
NO: 444.0916, Found: 444.0915.
-withdrawing, and halogen groups at the para position of the
N-phenyl ring is fully tolerated. In all cases, both diastereomer-
ic products were generated, and diastereomeric ratio of the
1
3aj. H NMR (400 MHz, CDCl ) δ 7.85 (t, J = 1.7 Hz, 1H), 7.53
3
(
(
m, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.24 – 7.16 (m, 2H), 7.11 – 7.00
m, 4H), 7.00 – 6.93 (m, 3H), 6.89 – 6.80 (m, 2H), 6.76 (t, J = 1.7
1
crude products has been determined by H NMR spectroscopy.
Hz, 1H), 6.69 (m, 1H), 6.55 (d, J = 7.9 Hz, 1H), 6.11 (s, 1H), 4.27
s, 1H). 13C NMR (100 MHz, CDCl
) δ 195.2, 152.0, 140.3, 139.0,
38.3, 135.2, 133.7, 131.3, 130.4, 129.8, 129.5, 129.3, 129.2,
28.0, 127.8, 127.7, 127.5, 127.0, 124.8, 122.5, 122.5, 119.2,
10.0, 71.8, 70.2. HRMS: m/z: [M+H]+ calculated for
The major product corresponds to that with the two phenyl
groups in a cis orientation, and in most cases the major product
(
3
1
1
1
C
Table 1
Optimization studies.
27
H
20Br NO: 531.9906, Found: 531.9902.
2
1
3ak. H NMR (400 MHz, CDCl
3
) δ 7.33 – 7.19 (m, 5H), 7.15 (t,
J = 7.6 Hz, 1H), 7.03 (d, J = 7.4 Hz, 1H), 6.78 (q, J = 6.6 Hz, 2H),
4
2
1
.90 (s, 1H), 4.13 (s, 1H), 2.17 – 2.03 (m, 2H), 1.98 (t, J = 7.4 Hz,
H), 1.53 – 1.38 (m, 1H), 1.20 – 1.08 (m, 1H), 1.07 – 0.97 (m,
H), 0.94 (t, J = 7.2 Hz, 3H), 0.89 – 0.78 (m, 1H), 0.50 (t, J = 7.4
Isolated yield (%)
Entry
Additive
Acid Solvent
3aa 3aa’
4
<5
<5
<5
<5
trace
<5
<5
<5
<5
<5
nd
40
Hz, 3H). 13C NMR (100 MHz, CDCl
3
) δ 210.5, 151.4, 140.3, 129.6,
1
2
3
4
AgSbF
AgSbF
AgSbF
6
6
6
—
DCE
DCE
DCE
DCE
DCE
DCM
<5
36
24
12
<5
<5
1
3
C
28.5, 128.46, 127.9, 126.9, 125.8, 118.9, 109.4, 70.8, 67.5, 43.7,
8.6, 18.2, 16.7, 14.6, 13.5. HRMS: m/z: [M+H] calculated for
PivOH
AcOH
PivOH
PivOH
PivOH
+
Zn(OTf)
AgNTf
2
trace trace
21
H26NO: 308.2009, Found: 308.2009.
a
5
2
nd
63
49
68
40
nd
9
8
9
7
4
was prepared according to the general procedure but ad-
6
7
8
9
AgSbF
AgSbF
AgSbF
AgSbF
AgSbF
AgSbF
6
6
6
6
6
6
1
ditional Zn(OTf)
etone) δ 10.70 (s, 1H), 7.64 (d, J = 8.0 Hz, 1H), 7.56 (t, J = 8.2 Hz,
2
(20 mol%) was used. H NMR (400 MHz, Ac-
PivOH dioxane
PivOH EtOAc
PivOH Acetone
3
1
H), 7.50 – 7.44 (m, 4H), 7.43 – 7.33 (m, 4H), 7.25 (t, J = 7.6 Hz,
H), 7.14 (t, J = 7.5 Hz, 1H). The NMR data agree with those in a
10
11
12
PivOH t-AmOH trace trace
b
c
PivOH
PivOH
DCE
DCE
nd
nd
literature report [50].
6
AgSbF /Zn(OTf)
2
trace trace
Reaction conditions: nitrone 1a (0.2 mmol), diphenylacetylene (0.3
mmol), [Cp*RhCl (4 mol%), additive (16 mol%), acid (2 equiv) in
3.
Results and discussion
]
2 2
a
b
solvent (2 mL), 80 °C for 15 h. [IrCp*Cl
]
2 2
was used as a catalyst. No
c
catalyst was used. Additional Zn(OTf)
2
(20 mol%) was used.
We initiated our investigation with optimization studies on

Lingheng Kong et al. / Chinese Journal of Catalysis 36 (2015) 925–932
929
Table 2
Scope of substrates.
Entry
1
Product
Isolated yield (%)
68
dra
Entry
13
Product
Isolated yield (%)
81
dra
3aa
3ba
7.1:1
3ma
3na
5.5:1
5.2:1
2
3
54
61
5.9:1
4.2:1
14
15
50
76
3ca
3da
3ea
3fa
3ab
3ac
3ad
3ae
7.0:1
8.3:1
7.9:1
6.6:1
4
69
73
79
4.0:1
3.7:1
6.0:1
16
17
18
50
75
66
5
6
7
Ph
O
EtO2C
Ph
Ph
N
H
3ga
3ha
45
64
7.9:1
9.9:1
19
20
3af
72
61
6.0:1
7.2:1
8
9
3ag
3ia
3ja
58
61
71
7.1:1
6.2:1
6.2:1
21
22
23
3ah
3ai
3aj
53
48
56
5.9:1
5.9:1
5.5:1
1
0
1
1
3ka
Ph
O
Ph
12
3la
76
8.2:1
24
3ak b
56
5.5:1
Cl
N
H
Reaction conditions: nitrone (0.2 mmol), alkyne (0.3 mmol), [Cp*RhCl
2
]
2
(4 mol%), AgSbF
. trans:cis ratio. DCM solvent was used in a sealed tube.
6
(16 mol%), PivOH (2.0 equiv), ethyl acetate (2 mL), 80 °C,
a
b
1
5 h, sealed tube under N
2

930
Lingheng Kong et al. / Chinese Journal of Catalysis 36 (2015) 925–932
can be isolated by column chromatography, but the minor iso-
mer overlaps with the major one or other impurities and defies
further purification. Introduction of meta substituents such as
methyl, tert-Bu and Ph resulted in steric bias and the C–H bond
at the less hindered position underwent coupling with high
selectivity. In contrast, variation of the spectating C-phenyl ring
in the nitrone is also tolerated. Introduction of elec-
tron-donating and halogen group or using a heteroaryl group
caused no negative effect. In all cases, the reaction gave nearly
full conversion and dr ranges from 3.7–9.9:1, and the major
product was isolated in moderate to high yield.
The scope of the alkyne substrate was next explored.
Diphenylacetylenes bearing substituents such as m- and
p-halogen, -OMe, and -alkyl groups all reacted with 1a in
moderate to good yield and the dr ranges from 5.5–8.3:1. The
alkyne is not limited to diaryl substituents, and 4-octynes
reacted under the optimized conditions (in DCM) with
moderate yield. It should be noted that aryl- and alkyl-mixed
alkynes such as PhC≡CMe are also viable; however, the
complication of regio- and stereoselectivity rendered it difficult
to isolate the desired product.
To probe the intramolecularity of the O-atom transfer
process, the coupling of 1a and 4-octyne were performed in the
presence of H
O18 (2 equiv). MS analysis of the product
2
indicated that no O18 atom was incorporated, and this
observation is in line with that in our previously reported
coupling between quinoline N-oxide and alkynes, where the
O-atom transfer is intramolecular. To probe the C–H activation
process, kinetic isotope effect has been measured from the
Scheme 1. Proposed mechanism of Rh(III)-catalyzed coupling of
nitrones with alkynes.
competitive coupling between 1a and 1a-d in equimolar ratio
5
o
(
Eq. 1). The reaction was performed at 40 C in DCM and was
stopped after with low conversion (ca. 15%). The crude
product was purified by chromatography to afford a mixture of
Alternatively, intramolecular nucleophilic attack of the enol
form of the ketone yields the final indoline product as a mixture
of diastereomers.
3
ak and 3ak-d
. 1H NMR analysis of the mixture revealed a KIE
4
value of 5.3, indicating that C-H cleave is likely involved in the
rate-limiting step.
4
.
Conclusions
We have achieved the Rh(III)-catalyzed C–H activation of
N-aryl nitrones in the coupling with alkynes with rare oxygen
atom transfer under redox-neutral conditions. The reaction
proceeded under relatively mild conditions with high
chemoselectivity and moderate to good diastereoselectivity
and provided trisubstituted protic indolines as synthetically
useful products. In all cases, the C–H activation occurred at the
N-aryl ring (exo C–H activation), which differs from the endo
selectivity in our previously reported Rh(III)-catalyzed C–H
activation of nitrones. The chemoselectivity also stays contrast
to that previously reported in Rh(III) catalysis. A broad scope of
substrates has been established, and this method may find
applications in the synthesis of complex structures.
On the basis of previous reports, a plausible catalytic cycle is
proposed (Scheme 1) starting from an active RhCp*X
2
(X = SbF
6
or OPiv) catalyst. O-coordination-assisted C–H activation of the
nitrone affords a rhodacyle A. Coordination of the alkyne
followed by migratory insertion of the Rh–C bond generates a
seven-membered rhodacyclic intermediate B, which is
proposed to undergo C–O reductive elimination to give a
cationic six-membered heterocycle (C) [47] together with a
Cp*Rh(I) species. The N–O bond in heterocycle C then
oxidatively adds back to the Rh(I) to generated a Rh(III)
enolate species E, which accounts for the oxygen transfer.
Protonolysis of the Rh–O bond in E by an acid (HX) releases an
imine F. The fate of F can be found in two subsequent
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931
Graphical Abstract
Chin. J. Catal., 2015, 36: 925–932 doi: 10.1016/S1872-2067(15)60866-X
Rh(III)-catalyzed coupling of nitrones with alkynes for the synthesis of indolines
Lingheng Kong, Fang Xie, Songjie Yu, Zisong Qi, Xingwei Li*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
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