Copper-Catalyzed Three-Component Carboamination of Arynes: Expeditious Synthesis of o-Alkynyl Anilines and o-Benzoxazolyl Anilines

Article abstract of DOI:10.1021/acs.orglett.9b01427

A copper-catalyzed three-component reaction of in situ formed arynes, terminal alkynes, and O-benzoylhydroxylamines has been developed. By adjusting reaction conditions, the nucleophiles in this transformation can be extended from terminal alkynes to benzoxazoles. These procedures provide a modular and facile approach to o-alkynyl anilines and o-benzoxazolyl anilines from easily available substrates in only one step.

Full text of DOI:10.1021/acs.orglett.9b01427

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Three-Component Carboamination of Arynes:
Expeditious Synthesis of o‑Alkynyl Anilines and o‑Benzoxazolyl
Anilines
Sheng-Li Niu, Jiangtao Hu, Kuicheng He, Ying-Chun Chen,* and Qing Xiao*
College of Pharmacy, Third Military Medical University, Chongqing 400038, China
S
* Supporting Information
ABSTRACT: A copper-catalyzed three-component reaction of in situ formed arynes, terminal alkynes, and O-
benzoylhydroxylamines has been developed. By adjusting reaction conditions, the nucleophiles in this transformation can be
extended from terminal alkynes to benzoxazoles. These procedures provide a modular and facile approach to o-alkynyl anilines
and o-benzoxazolyl anilines from easily available substrates in only one step.
nilines exist widely in biologically active natural products,
Avaluable pharmaceuticals, and significant synthetic
building blocks.1,2 Although numerous powerful method-
ologies toward formation of C(sp2)−N bonds have been
developed recently,3−5 the general and practical routes to
access ortho-substituted anilines have still been limited.
Traditional methods usually make use of coupling reactions
of 1,2-dihaloarenes or o-iodoanilines to achieve this goal.
However, it is not easy to regulate the stepwise coupling
reactions and the approaches are restricted to the availability of
the starting materials. As an umpolung strategy of traditional
cross-couplings, electrophilic amination has been a significant
tool for constructing C−N bonds.6 By merging the electro-
philic amination and Catellani reaction, o-alkynyl anilines can
be easily prepared via the Pd-catalyzed multicomponent
reaction of ortho-substituted aryl iodides, electrophilic
aminating agents, and alkyne precursors (Figure 1A).7
Nevertheless, when aryl iodides bearing no ortho substituent
Figure 1. Strategies to synthesize ortho-substituted anilines using
electrophilic aminating reagents.
were used, two amino groups were introduced at the adjacent
positions to iodine through C−H bond activation. And
terminal alkynes cannot be directly used in this reaction.
Simultaneously, transition-metal-catalyzed aryne chemistry
transformations can be realized with non-noble copper
has become a powerful platform for 1,2-difunctionalization of
arenes.8−10 In this field, Greaney et al. developed a Cu-
catalyzed one-pot reaction of arynes, heteronucleophiles, and
O-benzoylhydroxylamines affording 1,2-dihetero-functional-
ized arenes in high yields (Figure 1B).10t Xu et al. reported a
Cu-catalyzed multicomponent reaction of arynes, terminal
alkynes, and electrophilic sulfenylating reagents to synthesize
o-alkynyl arylsulfides (Figure 1C).10v From the perspective of
mechanism, the fundamental mode of these reactions can be
described as nucleophilic addition to arynes and successive
electrophilic trap tandem reactions. In contrast with the
strategy of precious-metal-catalyzed C−H activation, these
catalysts.
In this context, we conceived that the copper-catalyzed
three-component reaction of in situ formed benzynes, carbon
nucleophiles, and electrophilic aminating reagents would
provide a modular and facile synthetic approach to ortho-
substituted anilines (Figure 2). Although the design of this
reaction is relatively mature, there are still several challenges to
overcome: (1) the direct amination of int-I to generate
byproduct 5 through path A;11 (2) the direct protonation of
Received: April 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01427
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the fluorine resource and base in our reaction (entries 2−6).
The solvent, temperature, and concentration of substrates also
have significant impact to this transformation. When the THF
was replaced with toluene or 1,2-dichloroethane, the yield of
4a decreased greatly (entries 7, 8). The reaction was also
enabled at room temperature, but in a low yield (entry 9). A
specific substrate concentration is necessary for the reaction
(entries 10, 11). We believe that all these factors affect the
formation rate of benzyne and the concentration of various
active intermediates, thus affecting the efficiency and selectivity
of the reaction. The unexpected byproduct 4a′′ might be
produced from the attack of terminal alkynes on int-III. In this
side reaction, electrophilic amination reagents only act as
oxidants.
Then, we proceeded to study the scope of the substrates
(Figure 3). Substituent phenyl alkynes with either an electron-
donating or electron-withdrawing group on the ortho-, meta-,
and para-position of the benzene ring were able to undergo the
three-component reaction to generate the corresponding
products in good to excellent yields (4b−4i). Vinyl and alkyl
alkynes have also proven to be useful starting materials for the
efficient construction of o-alkynyl anilines (4j−4o). Besides,
Figure 2. Proposed mechanism of the copper-catalyzed carboamina-
tion of benzynes to ortho-substituted anilines.
int- II to generate byproduct 6 through path B;10e (3) the
insertion of another benzyne into the C−Cu bond of int-II and
sequential protonation to generate byproduct 7 through Path
C.10f,g To avoid these undesirable side reactions in our
transformation, it is essential to select reactivity-matched
carbon nucleophiles, seek appropriate catalysts and ligands,
and control the benzyne formation rate by using suitable
fluorides and bases. In continuation of our interest in
transition-metal-catalyzed benzyne chemistry,12 we herein
report our preliminary results in this issue: a copper(I)-
catalyzed multi component reaction of Kobayashi reagents,
terminal alkynes/benzoxazoles, and O-benzoylhydroxylamines
to afford ortho-substituted anilines (Figure 1D).
First, we optimized the conditions of the model reaction of
Kobayashi reagent 1a, alkyne 2a, and O-benzoylhydroxylamine
3a (Table 1). The 5 mol % CuI catalyzed process at 60 °C in
tetrahydrofuran (THF) in the presence of KF, Cs2CO3, and
18-crown-6 furnished o-alkynyl aniline 4a in 82% isolated yield
(entry 1). Control experiments established the importance of
Table 1. Reaction Condition Optimization I
b
isolated yield (%)
entry variation of the standard conditions I
4a/4a′/4a′′
a
1
2
3
none
82/<5/<5
60/11/16
44/9/31
CsF instead of KF
without 18-crown-6, CsF instead of
KF
4
5
6
7
8
9
K2CO3 instead of Cs2CO3
DBU instead of Cs2CO3
DIPEA instead of Cs2CO3
toluene instead of THF
Cl2CHCHCl2 instead of THF
room temperature instead of 60 °C
66/11/21
trace/47/19
67/9/19
c
24/8/17
18/10/11
d
e
58/9/11
10
volume of THF was changed to
4.0 mL
volume of THF was changed to
2.0 mL
65/11/16
61/5/5
11
a
0.26 mmol of 1a, 0.26 mmol of 2a and 0.20 mmol of 3a in 2.5 mL of
Figure 3. Scope of the three-component reaction of benzyne
precursors, terminal alkynes, and O-benzoylhydroxylamines. Unless
otherwise noted, all the reactions were carried out under the standard
conditions I in Table 1. a 5 mol % CuI/dppb was used as catalyst. b 10
mol % CuI/dppb was used as catalyst. 0.52 mmol of 1a, 0.52 mmol
of 2a, and 0.20 mmol of 3l in 8 mL of THF.
THF in the presence of 0.40 mmol of Cs2CO3, 0.40 mmol of KF, 0.40
mmol of 18-crown-6, and 0.01 mmol of CuI. The yield of 4a was
b
calculated on the basis of 3a. The yields of 4a′ and 4a′′ were
c
d
c
calculated on the basis of 2a. 37% 3a was recovered. 51% 3a was
e
recovered. 19% 3a was recovered.
B
DOI: 10.1021/acs.orglett.9b01427
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Organic Letters
Letter
the reaction conditions were compatible with alkyl, bromide,
chloride, fluoride, methoxy, ester, and trifluoromethyl groups.
Further exploration demonstrated that the reaction proceeded
successfully with a variety of N,N-disubstituted O-benzoyl-
hydroxylamines (4p−4z). Remarkably, the allyl or benzyl
substituted amine products (4w−4y) can be easily converted
to corresponding primary or secondary amines. Gratifyingly,
the electrophilic aminating reagent with two active sites
prepared from piperazine can also participate in this reaction
smoothly, producing highly symmetrical o-alkynyl aniline (4z).
Various aryne precursors can also be effectively used in this
reaction (4aa−4ae). Only one regioisomer (4aa) could be
efficiently generated from 3-methoxyl substituted silylphenyl
triflate.13 Additionally, o-alkynyl naphthylamine 4ab or 4ac
could be obtained from the corresponding silylnaphthyl triflate
in a moderate yield. The 4-methyl substituted silylphenyl
triflate gave a 1.3:1 mixture of inseparable 4ad and 4ad′ in a
68% overall isolated yield. When 3-methyl substituted
silylphenyl triflate was used, 4ae and 4ae′ were isolated in
50% and 19% yields, respectively.
Encouraged by these results, we were devoted to searching
for other carbon nucleophiles to further expand the range of
substrates for the transformation. As already known, the acidity
of the active hydrogen of benzoxazole is slightly weaker than
that of the terminal alkyne. Therefore, electron-deficient
heterocyclic arenes such as benzoxazoles are carbon
nucleophiles similar to terminal alkynes under alkaline
conditions in the precence of a transition-metal catalyst.
Based on this inference, we designed a copper-catalyzed model
reaction of benzyne precursor 1a, O-benzoylhydroxylamine 3a,
and benzoxazole 5a and optimized the reaction conditions
(Table 2). We found the ligand used in this reaction was
critical. When other ligands were utilized instead of Brettphos,
the yield of 9a was greatly reduced (entries 3−12). Different
from the common practice, this reaction did not need a
fluoride source14 and the phase transfer regent was tetrabutyl
ammonium tetrafluoroborate (entries 13−15). Next, the
substrate range for the three-component reactions involving
benzoxazoles was investigated (Figure 4). A series of o-
benzoxazolyl anilines which are difficult to prepare by other
means were efficiently produced in satisfactory yields from the
combinations of various benzoxazoles, electrophilic aminating
reagents, and benzyne precursors. It is worth noting that the
9m and 9n are both unique isomers in their reactions. Besides,
the structure of 9p (CCDC 1906945) was confirmed by X-ray
crystallography. Although the yield was low, oxazole can also
participate in this reaction to afford product 9i. We also tried
benzothiazoles, but no corresponding product was obtained
under the reaction conditions. It may be due to the relatively
weak nucleophilicity of benzothiazoles compared with
benzoxazoles.
Table 2. Reaction Condition Optimization II
b
isolated yield (%)
entry variation of the standard conditions II
9a/9a′/9a′′
a
1
none
62/−/9
21/17/−
40/−/21
48/−/15
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Cu(MeCN)4BF4 instead of CuI
tBuBrettphos instead of Brettphos
Xphos instead of Brettphos
tBuXphos instead of Brettphos
Sphos instead of Brettphos
iPrSphos instead of Brettphos
Johnphos instead of Brettphos
Xantphos instead of Brettphos
10 mol % dppb instead of Brettphos
PPh3 instead of Brettphos
Without Brettphos
c
trace/−/−
34/−/15
32/−/11
23/8/11
trace/17/12
37/11/−
17/12/−
0/0/0
29/10/−
trace/<5/−
12/25/11
d
18-crown-6 instead of TBABF4
TBAF instead of TBABF4
CsF instead of Cs2CO3
c
a
0. 60 mmol of 1a, 0.20 mmol of 3a, and 0.26 mmol of 8a in 3.0 mL
of THF in the presence of 0.80 mmol of Cs2CO3, 0.80 mmol of
TBABF4, 0.04 mmol of Brettphos, and 0.02 mmol of CuI. The yield
b
of 9a was calculated on the basis of 3a. The yields of 9a′ and 9a′′
c
d
were based on 8a. Complex mixtures. 95% 8a was recovered.
Finally, further transformations of the generated o-alkynyl
aniline 4a were investigated (Figure 5). 3-Aroyl oxazino[4,3-
a]indole 10 could be achieved from 4a through an efficient
Cu(I)-catalyzed oxidative reaction.15a And in the presence of
iodine, 4a could be converted into 3-iodo indole 11 in an
excellent yield.15b
In summary, based on the understanding of the benzyne
difunctionalization mechanism, we have developed a copper-
catalyzed three-component reaction of aryne precursors,
terminal alkynes/benzoxazoles, and electrophilic aminating
reagents. It provides an efficient and modular approach to o-
alkynyl anilines and o-benzoxazolyl anilines which are
Figure 4. Scope of the three-component reaction of benzyne
precursors, benzoxazoles, and O-benzoylhydroxylamines. Unless
otherwise noted, all the reactions were carried out under the standard
conditions II in Table 2.
otherwise difficult to prepare. These reactions show excellent
functional group compatibility and can be realized from readily
available starting materials in only one step. We believed that
C
DOI: 10.1021/acs.orglett.9b01427
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Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01427.
Detailed experimental procedures, characterization data,
1
and copies of the H and 13C NMR spectra for all
previously unknown products (PDF)
Accession Codes
CCDC 1906945 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xiaoqing@tmmu.edu.cn (Q.X.).
*E-mail: ycchen@scu.edu.cn (Y.-C.C.).
ORCID
Ying-Chun Chen: 0000-0003-1902-0979
Qing Xiao: 0000-0002-9019-937X
Notes
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Greaney, M. F. J. Am. Chem. Soc. 2006, 128, 7426−7427. (b) Liu, Z.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported financially by the NSFC (Grant No.
21602251) and the Foundation for Transformation of Sci-tech
Achievements (Grant No. 2016XZH02) of Third Military
Medical University.
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