Copper-catalyzed tandemcis-carbometallation/cyclization of imine-ynamides with arylboronic acids

Article abstract of DOI:10.1039/d0cc01424a

An efficient copper-catalyzed tandem regioselectivecis-carbometallation/cyclization of imine-ynamides with arylboronic acids has been developed. This method leads to a facile and practical synthesis of valuable 2,3-disubstituted indolines in moderate to e

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Copper-catalyzed tandem cis-carbometallation/cyclization of
imine-ynamides with arylboronic acids
Received 00th January 20xx,
Accepted 00th January 20xx
Hao-Ran Wang,a En-He Huang,a Chen Luo,a Wen-Feng Luo,b Yin Xu,a Peng-Cheng Qian,*b Jin-Mei
Zhoua and Long-Wu Ye*ac
DOI: 10.1039/x0xx00000x
www.rsc.org/
substrates are restricted to imidazolidinone- and oxazolidinone-
derived ynamides as the carbonyls of these substrates provide
coordination with metal catalyst to achieve the good
regioselectivity (Scheme 1a). As a continuation of our work on
developing ynamide chemistry for heterocycle synthesis,10 we
herein report an efficient copper-catalyzed highly regioselective
arylative cyclization of imine-ynamides with arylboronic acids
(Scheme 1b).11 This protocol enables facile and practical synthesis
of valuable 2,3-disubstituted indolines12 in moderate to excellent
yields and features a broad substrate scope and wide functional
group tolerance.
An efficient copper-catalyzed tandem regioselective cis-
carbometallation/cyclization of imine-ynamides with arylboronic
acids has been developed. This method leads to facile and
practical synthesis of valuable 2,3-disubstituted indolines in
moderate to excellent yields and features a broad substrate scope
and wide functional group tolerance. Other significant features of
this protocol include the use of readily available starting materials,
high flexibility, simple procedure and mild reaction conditions.
Transition-metal catalyzed tandem carbometallation/cyclization of
alkynes by organoboronic acids with unsaturated moieties is a
powerful method for the rapid assembly of structurally complex
heterocycles and carbocycles.1-6 However, compared with the
carbonyl moieties,2 transition-metal catalyzed arylative cyclization
of alkynes with imine moieties has been far less exploited,6 and
these arylative cyclizations have been mostly limited to noble metal
catalysts such as rhodium and palladium or special substrates such
as 2-iminoarylboronic acids6a,c and dimethyl-tethered imine-alkynes
due to Thorpe-Ingold effect.6b A likely reason for this paucity is that
organoboronic acids may attack the highly electrophilic imine
moieties directly.
Ynamides have proven to be powerful and versatile reagents in
organic synthesis, and various efficient synthetic methods have
been established in the past decade.7 Despite these significant
achievements, the above mentioned arylative cyclization based on
ynamides has seldom been explored.8,9 The only example was
reported by Lam and co-workers for the synthesis of 2-
amidoindenols or 2-amidoindenes through rhodium-catalyzed
annulation of ynamides with arylboronic acids or esters containing
an electrophilic functional group.8a Notable is that the reaction
Scheme
arylboronic acids.
1
Tandem carbometallation/cyclization of ynamides with
At the outset, Bus-protected imine-ynamide 1a and
phenylboronic acid 2a were used as the model substrates, and
selected results are outlined in Table 1. We were delighted to find
that the desired 2,3-disubstituted indoline 3a could be formed in 50%
yield in the presence of 10 mol % of CuOTf as catalyst and MeOH as
o
solvent at 50 C (Table 1, entry 1). Other typical copper(I) catalysts
such as CuI and Cu(MeCN)4PF6 were not effective (Table 1, entries 2
and 3), and significant formation of hydrolyzed byproduct 3a was
observed in the former case (Table 1, entry 2). Gratifyingly, 90%
yield of product 3a was achieved by employing CuOAc as catalyst
(Table 1, entry 4). It is notable that the use of Cu(OAc)2 as catalyst
or the reaction at room temperature led to poor results (Table 1,
entries 5 and 6). When 1.5 equiv of 2a was employed, the yield of
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Table 2 Copper-catalyzed reaction of imine-ynamide 1aViwewitAhrtidclieffOenrelinnet
3a significantly decreased (Table 1, entry 7). Finally, it was found
that 3a was obtained as the main product in case of using EtOH as
solvent (Table 1, entry 8) or Ts-protected imine ynamide 1aas
substrate.13
arylboronic acids 2a
DOI: 10.1039/D0CC01424A
Table 1 Optimization of reaction conditionsa
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), catalyst (0.01 mmol),
solvent (0.25 mL), rt–50 oC, 0.5–120 h, in Schlenk tubes. b Measured by 1H
NMR using diethyl phthalate as the internal standard. c 22% of 1a remained
unreacted. d 1.5 equiv of 2a was used. Bus = tert-butylsulfonyl.
Under the optimal reaction conditions (Table 1, entry 4), the
scope of the copper-catalyzed carbometallation-cyclization was
examined. As shown in Table 2, a wide range of arylboronic acids
with electron-withdrawing substituents such as F, Cl, Br, I on para-
position and F, Cl, Br, CHO on meta-position were suitable
substrates, delivering the corresponding 2,3-disubstituted indolines
3b–3h and 3o–3r in 44–98% yields. In addition, arylboronic acids
bearing electron-donating substituents such as Me, OMe, SMe,
phenyl on para-position and Me, OMe, OBn, CH2OH on meta-
position were also tolerated to produce the desired 3i–3n and 3s–
3v in 53–90% yields. Moreover, the reaction was also extended to
the disubstituted arylboronic acids, leading to the desired 3w–3z in
58-76% yields. Finally, the reaction proceeded smoothly with 2-
naphthyl and 5-indolyl boronic acids to afford the anticipated
indolines 3aa (68%), 3ab (85%), respectively. The molecular
structure of 3b was confirmed by X-ray diffraction (Figure 1).14
a Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), CuOAc (0.02 mmol),
b
MeOH (0.5 mL), 50 oC, 0.5 h, in Schlenk tubes; isolated yields are reported.
1 h. c 3 equiv of 2 was used.
The treatment of phenylboronic acid 2a with different imine-
ynamides 1 was then explored under the above copper catalysis. As
depicted in Table 3, different N-protection groups such as MBS,
SO2Ph, Bs and Ms were compatible with this cyclization to produce
the corresponding 2,3-disubstituted indolines 3ac–3af in 52–82%
yields. The cascade reaction also occurred smoothly with various
ynamides bearing electron-withdrawing groups on both meta- and
para-positions and electron-donating groups, leading to the desired
products 3ag–3an in good to excellent yields. In addition, different
substituents such as F, Cl, and Me on the phenyl ring were also
examined and the expected products 3ao–3as were formed in 61–
98% yields. Of note, attempts to extend the reaction to the alkyl-
substituted and terminal ynamides or the styrylboronic acid only
led to the formation of the corresponding products in low yields
(<30%).13 Thus, this method provides a highly efficient and practical
route for the preparation of synthetically useful 2,3-disubstituted
indolines.
Further synthetic transformations of the 2,3-disubstituted
indolines were then explored, as shown in Scheme 2. First, indoline
3a could be synthesized on a preparative scale in 81% yield under
standard conditions, Interestingly, attempts to deprotect the N-Ts
group in 3a by the treatment with Na/naphthalene led to the
formation of the 2-benzhydryl-1H-indole 4a in 76% yield. In addition,
Figure 1 Structure of compound 3b in its crystal. Thermal ellipsoids are
shown at 50% probability.
2 | J. Name., 2012, 00, 1-3
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the treatment of 3a with AlCl3 and anisole afforded the
carbometallation/cyclization of alkynes,1-6 a plausible mechanism
DOI: 10.1039/D0CC01424A
for the formation of 2,3-disubstituted indolines 3 is proposed
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Table 3 Copper-catalyzed reaction of phenylboronic acid 2a with different
imine-ynamides 1a
(Scheme 3). Initially, the transmetallation between arylboronic
acids 2 and copper(I) species A produces aryl copper intermediates
B, which react with imine-ynamides 1 via regioselective cis-
carbometallation, generating the vinyl copper intermediates C. Of
note, previous nucleophilic addition to ynamides generally occurs
on the α position of ynamides.7a,9,16 Subsequent intramolecular
cyclization leads to the N-copper intermediates D, which can be
eventually transformed into the target products 3 upon protonation
and regenerate the copper catalyst.
a Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), CuOAc (0.02 mmol),
MeOH (0.5 mL), 50 oC, 0.5 h, in Schlenk tubes; isolated yields are reported.
PG
= protecting group, MBS = 4-methoxybenzenesulfonyl, Bs = 4-
bromobenzenesulfonyl.
Scheme 3 Plausible reaction mechanism.
corresponding 2-indolylmethanol 5a in 57% yield, which may serve
as a versatile building block in catalytic cycloadditions.15 The
molecular structure of 5a was confirmed by X-ray diffraction (Figure
2).14
In summary, we have developed an efficient copper-catalyzed
tandem cis-carbometallation/cyclization of imine-ynamides with
arylboronic acids, where a unique regioselectivity is achieved. This
method leads to facile and practical synthesis of valuable 2,3-
disubstituted indolines in moderate to excellent yields and features
a broad substrate scope and wide functional group tolerance. Other
significant features of this protocol include the use of readily
available starting materials, high flexibility, simple procedure and
mild reaction conditions. Further investigations into the asymmetric
version of the current protocol are in progress in our laboratory.
We are grateful for financial support from the National Natural
Science Foundation of China (21772161 and 21622204), the Natural
Science Foundation of Fujian Province of China (2019J02001), the
President Research Funds from Xiamen University (20720180036),
the Foundation of Wenzhou Science & Technology Brueau (No.
W20170003), NFFTBS (No. J1310024), PCSIRT, and Science &
Technology Cooperation Program of Xiamen (3502Z20183015). We
thank Mr. Zanbin Wei from Xiamen University (College of Chemistry
and Chemical Engineering) for assistance with X-ray crystallographic
analysis.
Scheme 2 Preparative-scale reaction and synthetic transformations.
Conflicts of interest
There are no conflicts to declare.
Figure 2 Structure of compound 5a in its crystal. Thermal ellipsoids are
shown at 50% probability.
Notes and references
1
For selected reviews: (a) S. W. Youn, Eur. J. Org. Chem., 2009, 2597;
(b) T. Miura and M. Murakami, Chem. Commun., 2007, 217.
Based on the above experimental observations and previous
protocols
on
the
transition-metal-catalyzed
tandem
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2
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9
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,
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Bus
DOI: 10.1039/D0CC01424A
NHBus
Ar2
N
N
R
R
CuOAc
(10 mol %)
Ar1
45 examples
up to 98% yield
Ar2B(OH)2
Ar1
N
PG
PG
 regioselective cis-carbometallation
 valuable N-heterocycles
 non-noble metal catalysis
 wide substrate scope
An efficient copper-catalyzed regioselective cis-carbometallation/cyclization of
imine-ynamides with arylboronic acids enables the facile and practical synthesis of
valuable 2,3-disubstituted indolines.