Synthesis of indolines: Via a palladium/norbornene-catalyzed reaction of aziridines with aryl iodides

Article abstract of DOI:10.1039/c8cc01062e

A Pd- and norbornene-catalyzed domino procedure has been developed to synthesize indoline compounds. This reaction provides efficient access to indolines by employing aryl iodides with aziridines as new electrophiles. The transformation is scalable and tolerates a range of functional groups.

Full text of DOI:10.1039/c8cc01062e

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Synthesis of Indolines via Palladium/Norbornene-Catalyzed
Reaction of Aziridines with Aryl Iodides
Ce Liu,a Yujie Liang,b Nian Zheng,a Bo-Sheng Zhang, a Yuan Feng,a Siwei Bi*b and Yong-Min Liang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Pd- and norbornene-catalyzed domino procedure has been
developed to synthesize indoline compounds. This reaction
provides efficient access to indolines by employing aryl iodides
with aziridines as new electrophiles. The transformation is
scalable and tolerates a range of functional groups.
(a) Classical Catellani reaction
R
R
I
E
alkyl
=
Nu
E
Pd(0), NBE
(since 1997)
E
+
LG
+
Nu
Y
aryl
H
(or olefin)
(since 2001)
amino
(since 2013)
acyl
(since 2015)
carboxyl
(since 2016)
LG + Y
R
R
E+
PdX
E
Heterocyclic compounds are ubiquitous and are present in
pharmaceuticals, agrochemicals, and biologically active
molecules.1 Transition-metal catalysts for the preparation of
heterocyclic compounds, especially palladium-catalyzed
reactions have been reported by many research groups in
recent years.2 Palladium/norbornene (NBE) chemistry plays an
important role here. Compared with many other strategies,
this protocol does not need directing groups (DGs) and
performs both iso and ortho-functionalization of the aryl
halides in one step under mild conditions. Significantly, as a
key step, the Catellani reaction has been applied to
heterocyclic natural products synthesis, such as (±)-
goniomitine, aspidospermidine,3 (+)-linoxepin4 and rhazinal.5
Pd/NBE chemistry was pioneered by Catellani in 19976 and
- NBE
Pd
ANP
I
(b) This work
R
R
Ts
N
Ts
N
Pd(0), NBE
H2C
+
C
A
G
H
H2
Pd(0)
R
R
Ts
Pd
N
R
F
Pd
B
C
Pd
ANP
H2
N
Ts
NTs
NBE
pathway b
SN2
R
R
R
Pd
OA
pathway a
Pd(IV)
RE
Pd
NTs
Pd
has become
a powerful tool for performing bi-/tri-
N
H2C
Ts
N
Ts
C
functionalization to construct polysubstituted arenes. Over the
D
E
Ligands on Pd are omitted for clarity.
OA denotes oxidative addition, RE denotes reductive elimination.
past two decades, varied termination reagents (Nu–Y) have
been well studied by the Catellani, Lautens, and other groups.7 Scheme 1. Palladium/norbornene catalysis.
However, electrophiles (E–LG) that react with the key five-
membered aryl-norbornene-palladacycle (ANP) intermediate
to install functional groups at the ortho-position were mainly
confined to alkyl and aryl halides (Scheme 1a).8 Until 2013,
amino,9 acyl,10 and carboxyl11 electrophiles have been
developed to achieve ortho-functionalization successively.
Very recently, epoxides as electrophilic reagents were
reported by Dong’s and Zhou’s group.12
In pursuit of our interest in exploring the Catellani
reaction,9d,
10a, 13
aziridines, a new and special type of
electrophilic partner, are studied in this paper. The three-
membered strained N-containing heterocycles are capable of
reacting with diversified nucleophiles to produce nitrogen-
containing organic frameworks.14 Given the chemical
properties of aziridines, we reasoned that amino groups could
act as nucleophiles to terminate the reaction for the synthesis
of the desired indolines after C-N bond cleavage by the ANP
intermediate. One main obstacle is the selective oxidation
between the ANP intermediate and the initial Pd(0) catalyst by
aziridines.15 Fortunately, the experimental results achieved the
desired targets. No leaving group (LG) or other small molecules
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Table 1. Scope of indoline formationa
Table 2. New optimization of the reaction conditions
Entry
Change from the Condition A
none
Yield (%)a
29b
24c
1
2
3
4
5
6
7
8
9
DME instead of toluene
toluene/DME (v/v 1:1) instead of toluene
H2O (2 equiv) + toluene/DME (v/v 1:1)
H2O (2 equiv) + toluene/DME (v/v 1:1)
EtOH (2 equiv) + toluene/DME (v/v 1:1)
NH4Cl (2 equiv) + toluene/DME (v/v 1:1)
MXne (2 equiv) + toluene/DME (v/v 1:1)
BF3·Et2O (2 equiv) + toluene/DME (v/v 1:1)
36c
64
71d
61d
63d
traced
traced
a Isolated yields. b Recovery of 4a = 74%. c Recovery of 4a < 5%. d 4a (2.0 equiv),
105°C, and 30 h. e MXn = FeCl3, FeCl2, CuCl2, CuCl. DME denotes ethylene glycol
dimethyl ether.
afforded their respective indoline products (3k-r) in excellent
yields (up to 93%). Attempting to further expand the scope,
aryl iodides without a substituent group at the 2-position
(R1=H) were carried out. The 3-nitroiodobenzene and methyl
3-iodobenzoate showed good compatibility in this
transformation and produced a single isomer in moderate
yields (3s, 58% yield and 3t, 61% yield). The configuration of
product 3s was unambiguously determined by X-ray analysis.
Methyl 4-iodobenzoate resulted in a lower yield (3u, 16%
yield). Nevertheless, iodobenzene without any substituent
groups was unable to give the corresponding product 3v. It is
likely attributable to “the ortho effect” that ortho-substituted
aryl iodides lead to better yields than those of unsubstituted
ones.21
a Condition A: 1 (0.15 mol, 1.0 equiv), 2 (2.5 equiv), Pd(OAc)2 (10 mol%), P(m-
ClC6H4)3 (20 mol%), NBE (50 mol%), and K2CO3 (2.0 equiv) in toluene (1 mL) at
b
100°C for 24 h. Isolated yields. Determined by gas chromatography-mass
spectrometry (GC-MS).
(Y) were produced during the catalytic cycle process (Scheme
1b). 1) In view of Yu’s16 and Dong’s17 studies; 2) because
almost all aziridine ring openings proceed via an SN2
mechanism;14e 3) and the desired indoline product with a
configuration inversion was obtained when chiral 2-aryl-
aziridine was used, we speculate that the transformation of
aryl iodides and aziridines occurs via either a Pd(IV)
intermediate (Scheme 1b, pathway a) or an SN2 nucleophilic
ring-opening reaction pathway (Scheme 1b, pathway b).
When 2-substituted aziridine 4a was combined with aryl
iodide 1g under previously optimized conditions (Condition A),
the desired indoline product 5a was obtained in 29% yield with
74% recovery of starting aziridine 4a (Table2, entry 1).
Meanwhile, the use of DME as solvent gave 24% yield with low
recovery of 4a (entry 2). In a mixed solvent of toluene and
DME (v/v 1:1), the yield of isolated 5a slightly increased (36%,
entry 3). The addition of 2 equiv of H2O could remarkably
enhance the yield (entries 4-5). Adding EtOH or NH4Cl, which
can provide proton hydrogen, gave moderate yields (entries 6-
7). Nevertheless, the use of Lewis acid as an additive
dramatically decreased the yields (entries 8-9).
Inspired by Lautens’ reports,18 2-iodotoluene 1a and 1-
tosylaziridine 2a were used as model coupling partners. With
the optimized conditions determined (see the ESI, Tables S1-
5), the scope was to further study the assay (Table 1). In
particular, it was found that P(m-ClC6H4)3 had the best
combination of electronic and steric properties to give the best
18b
yield.
First, different aryl iodides were examined using 1-
tosylaziridine 2a as the aziridine partner. Compared with 3a
(81% yield),19 which was characterized by single crystal X-ray
analysis,20 moderate yields of 3b (63% yield) and 3c (48% yield)
were obtained with the more sterically hindered Et and i-Pr
moieties at the 2-position (R1) of the benzene ring. Therefore,
with the 2-position being methyl, a list of substrates with both
electron-deficient and electron-rich groups at the 3-, 4- or 5-
positions of the phenyl ring were isolated in good to high
yields (3d-j), among which the aromatic iodine-bearing 4-nitro
group gave the desired indoline product 3g at a high yield of
87%. For a gram-scale reaction, the indoline 3g was obtained
in comparable yield (see the ESI). Next, when coupled with 1-
iodo-2-methyl-4-nitrobenzene 1g, the aziridines with different
arylsulfonyl groups on the nitrogen atom all worked well and
After the new optimized conditions were established (Table
2, entry 5), the scope of the reaction was studied using various
substituted aziridines. They tolerated aryl iodide 1g and
afforded the desired products in good yields (Table 3). When
R6 comprised monoalkyl substituents 4a-c, almost only single
isomers 5a-c derived from ring opening at the unsubstituted
carbon atom were observed (>20:1, 5:6). While 2-aryl-
aziridines 4d-j produced two isomers
(5d-j and 6d-j,
respectively), products 6d-j obtained via the regio-selective
cleavage of a benzylic C-N bond accounted for the highest
2 | J. Name., 2012, 00, 1-3
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Table 3. New scope of indoline formationa
Fig. 1 X-ray structures of products (R)-5n and (S)-6n
.
Fig. 2 X-ray structures of products E and F.
configuration. The two absolute configurations were
determined by X-ray crystallography (Fig. 1). Based on the
occurrence of Walden inversion and almost all aziridine ring
openings proceed via an SN2 mechanism,14e it is likely that a
nucleophilic ring-opening process occurs in this reaction.
a Condition B: 1g (0.15 mmol, 1.0 equiv), 4 (2.0 equiv), Pd(OAc)2 (10 mol%), P(m-
ClC6H4)3 (20 mol%), NBE (50 mol%), H2O (2.0 equiv), and K2CO3 (2.0 equiv) in
toluene/DME (1 mL, v/v 1:1) at 105°C for 30 h. Isolated yields. Ratios were
determined by 1H NMR. ND denotes not determined.
Mechanistic experiments have been carried out to get a
deeper understanding of the reaction pathway. At first, we
spent a lot of time isolating possible intermediates with
phosphorous ligand, but it didn’t work. Several outstanding
studies, in the Pd/NBE chemistry field, have been reported by
Catellani et al. via adding phenanthroline as a ligand to
stabilize the reactive palladium intermediates.23 Inspired by
proportion. The differences between alkyl- and aryl-
substituted aziridines probably occurred because the ring
opening of aziridines is controlled by a balance between
electronic effects and steric hindrance.14 The results showed
good agreement with the SN2 reaction mechanism under
alkaline conditions. The 1-tosyl-1,1a,6,6a-tetrahydroindeno
[1,2-b] azirine 4k was also suitable for this transformation. A
single regio-isomer 6k was isolated in 66% yield. Its
configuration was determined by X-ray crystallography. Other
poly-substituted aziridines, like gem-disubstituted 4l and 1,2-
disubstituted 4m aziridines were incompatible with aryl iodide
1g. Nevertheless, no desired products were tested when
unsaturated aziridine and heterocyclic substrates were used
(see the ESI).
these studies, palladacycle complex
B (Scheme 1b) bearing
phenanthroline was chosen as the substrate. Much to our
delight, after nearly a year of effort, two key X-ray crystal
structures of intermediates
two crystal structures are the important evidence that
carbon elimination of norbornene (norbornene deinsertion)
happens to the eight-membered palladacycle to give the six-
membered palladacycle in this Catellani reaction. Further
E and F were obtained (Fig. 2). The
β-
E
F
mechanistic studies are under investigation.
Previous work by Lautens suggested that inversion of the
configuration likely proceeds during the oxidative addition of
the enantioenriched secondary alkyl iodides to Pd(II) species to
generate the Pd(IV) species.22 Inspired by this study, an
additional experiment was carried out using chiral aziridine (S)-
4n and 1-iodonaphthalene 1o (Scheme 2). Cleaving of the N-C2
bond generated (R)-5n with retention of configuration, while
cleaving of the N-C1 bond gave (S)-6n with inversion of
In summary, we have developed a novel strategy to
synthesize indoline compounds from commercial and readily
accessible aryl iodides and N-sulfonated aziridines. This
process involves aziridine ring-opening followed by palladium-
catalyzed coupling-cyclization containing Calkyl-Caryl and N-Caryl
bond formation in one step. In addition, aziridines were shown
to be new electrophilic reagents that further extend the scope
of palladium/norbornene chemistry. The occurrence of the
Walden inversion suggests that an SN2 nucleophilic ring-
opening process is likely to proceed in this transformation.
Further applications of this method to synthesize other
heterocycles are underway.
Financial support was received from the National Natural
Science Foundation of China (NSF21472073, NSF21532001,
and NSF21473100) and the “111” project. We thank Dr. Ping-
Xin Zhou, Dr. Lian-Hua Li, Dr. Hui-Liang Hua, and Dr. Dao-Qian
Chen for helpful discussions. We would like to thank LetPub
(www.letpub.com) for providing language editing during the
preparation of this manuscript.
Scheme 2. Synthesis of enantioenriched products (R)-5n and (S)-6n
.
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Conflicts of interest
There are no conflicts to declare.
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