Construction of N-Alkyl- and N-Arylaziridines from Unprotected Amines via C-H Oxidative Amination Strategy

Article abstract of DOI:10.1021/acs.orglett.8b03799

A copper-promoted intramolecular C-H oxidative amination reaction between secondary amine (N-H) and C(sp³)-H at the benzylic position of azaarenes or α-position of ketones for the synthesis of aziridine derivatives has been developed. Moreover,

Full text of DOI:10.1021/acs.orglett.8b03799

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Construction of N‑Alkyl- and N‑Arylaziridines from Unprotected
Amines via C−H Oxidative Amination Strategy
Yang Yu,† Meijuan Li,† Yong Zhang,† Yonghai Liu,† Lei Shi,§ Wei Wang,*,†,‡ and Hao Li*,†
†State Key Laboratory of Bioengineering Reactor, Shanghai Key Laboratory of New Drug Design, and School of Pharmacy, East
China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
‡Department of Pharmacology and Toxicology and BIO5 Institute, University of Arizona, 1703 East Mabel Street, P.O. Box 210207,
Tucson, Arizona 85721-0207, United States
§Corporate R&D Division, Firmenich Aromatics (China) Co., Ltd., Shanghai 201108, China
S
* Supporting Information
ABSTRACT: A copper-promoted intramolecular C−H oxidative amination reaction between secondary amine (N−H) and
C(sp3)−H at the benzylic position of azaarenes or α-position of ketones for the synthesis of aziridine derivatives has been
developed. Moreover, a practical annulation of electron-deficient vinylarenes with an unprotected primary alkyl amine by a
Yb(OTf)3−CuI relay system has also been reported. The reactions were carried out with oxygen as the sole oxidant to give the
N-alkyl- and N-arylaziridines in good yields.
Scheme 1. Transition-Metal-Catalyzed or Promoted
Synthesis of N-Unprotected Aziridines
ziridines and their derivatives are important classes of N-
Acontaining heterocycles occurring in numerous natural
products,1 biologically active compounds,2 and intermediates in
organic synthesis.3 Thus, aziridines, the smallest ring motifs,
have been of great interest to chemists for years.4 The main
approaches for the construction of aziridines5 involve nitrogen−
olefin additions,6 carbon−imine cyclizations,7 intramolecular
substitutions,8 and other reactions.9 The nitrogen−olefin
addition strategy requires stable N-protecting groups such as
p-toluenesulfonyl (Ts) or p-nitrophenylsulfonyl (Ns), which are
often troublesome to remove and lead to a reduced overall
yield.10 Furthermore, the other methods require prefunctional-
ization of the reaction substrates with a leaving group such as
OTs, OAc, halogen, ylide, and azide, which is not atom-
economic. Therefore, the development of practical methods for
the straightforward synthesis of aziridines by using non-
functionalized reagents remains a challenge.
Recently, Falck and co-workers developed a new synthetic
method for the formation of unprotected N−H and N-Me
aziridines from simple olefins via homogeneous rhodium
catalysis (Scheme 1).11 In addition, Gaunt’s group reported
palladium-catalyzed direct intramolecular C−H amination for
the synthesis of unprotected aziridine derivatives starting from
cyclic aliphatic amines (Scheme 1).12 However, the aziridine
scope in this method is very limited. It is highly desirable to
Received: November 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03799
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Organic Letters
Letter
develop a facile and efficient procedure for constructing
unprotected N-aryl- or N-alkylaziridines. Recently, an oxidative
cross-coupling reaction between two nucleophiles has been
developed as a powerful synthetic methodology to construct
functional molecules.13 As far as we know, the synthesis of
aziridines from simple amines by using oxidative cross-coupling
has not been reported. Based on our early investigation of
functionalization of azaarenes,14 we want to develop a new
strategy that nucleophilic amine reacts with nucleophilic
benzylic C−H of azaarenes15 to produce structure diversified
aziridines through an oxidative cross-coupling pathway. Herein,
we present an account of our recent work on copper-mediated16
annulation from secondary amine and C(sp3) at the benzylic
position of azaarenes or α-position of ketones for the synthesis of
N-alkyl- and N-arylaziridines.
In the initial attempt on the formation of aziridines, secondary
amine 1a was synthesized by Michael addition between aniline
and 5-nitro-2-vinylpyridine catalyzed by Yb(OTf)3 (see the
Supporting Information). The solution of 1a (0.02 mmol) in the
presence of CuBr (0.02 mmol) and 2,3,4,6,7,8,9,10-
octahydropyrimido[1,2-a]azepine (DBU) (0.04 mmol) in
toluene was stirred for 2 h at 65 °C. The desired product
aziridine 2a was obtained in 59% yield (Table 1, entry 1).
that more polar solvents, such as dioxane, dichloroethane
(DCE), and DMF, dioxane, and DCE gave slightly lower yields
(entries 7 and 8), and DMF gave very poor yield (entry 9).
Compared to DBU, three other common organic bases,
2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine (TBD),
N,N-dimethylpyridin-4-amine (DMAP), and 1,4-
diazabicyclo[2.2.2]octane (DABCO), were proven to be less
efficient (entries 10−12). Moreover, the results showed that
temperature has an influence on the yields, the yield was
significantly lower at room temperature or 90 °C (entries 13 and
14). Thus, the optimal reaction condition was determined as
follows: 1 equiv of CuI and 2 equiv of DBU in toluene for 2 h at
65 °C.
With the optimal reaction conditions in hand, we then
examined the substrate scope of CuI-promoted C(sp3)−N
coupling for the formation of aziridines 2 from amines 1 with
azaarenes. As shown in Scheme 2, almost all of the tested
Scheme 2. Scope of N-Arylamines for the Synthesis of N-
a
Arylaziridines
a
Table 1. Optimization of Reaction Conditions.
b
entry
copper
CuBr
CuBr2
CuI
CuI/PIDA
CuI/TBHP
CuI
CuI
CuI
CuI
CuI
base
DBU
solvent
yield (%)
1
2
3
4
5
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
DCE
59
54
72
33
trace
51
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
TBD
DMAP
DABCO
DBU
DBU
c
d
e
6
7
8
9
63
48
a
Reaction conditions: a solution of 1a (0.2 mmol) and CuI (0.2
DMF
7
mmol) with DBU (0.4 mmol) in toluene (1.5 mL) was stirred for 2 h
10
11
12
13
14
toluene
toluene
toluene
toluene
toluene
11
b
at 65 °C. 0.24 mmol of CuI was used for 2m−w. Isolated yield. The
CuI
CuI
CuI
CuI
trace
trace
53
reaction was run for 2 h at 45 °C.
f
combinations produced the corresponding products in moder-
ate to high yields (2a−l). Electron-withdrawing groups (EWGs)
on the benzene rings such as halogen, CF3, and CO2Et in the
para-position on the phenyl ring had a slight impact on the yields
(2a−e, 70−75%). Even for the amine-bearing electron-donating
group (EDG) on the para-position of phenyl ring, the reaction
proceeded smoothly in 55% yield (2f). Furthermore, the
application of substrates bearing a meta-EWG substituent on
the phenyl ring provided the desired products in good yields as
well (2g−i, 67−84%). m-Me- and o-F-substituted amines also
transformed to the corresponding products 2j and 2k in good
yields. Moreover, aniline 1l with benzylic C−H at the para-
position of pyridine can also be suitable for this protocol to give
the corresponding product 2l in 71% yield. To further expand
the scope of the reaction, β-secondary amino ketones were
investigated (1m−w). In general, good yields were achieved
across a range of substituted α-aryl ketones. The simple 1-
phenyl-3-(phenylamino)propan-1-one 1m, promoted by 1.2
equiv of CuI, generated the aziridine product 2m in 81% yield.
The results showed that there was no significant influence
g
35
a
Unless specified, amine 1a (0.2 mmol), base (0.4 mmol), and copper
salt (0.2 mmol) in 1.5 mL of toluene was stirred for 2 h at 65 °C.
b
c
Isolated yield. 0.3 equiv of CuI and 2.0 equiv of PIDA were used
d
under nitrogen. 0.3 equiv of CuI and 2.0 equiv of TBHP were used
under nitrogen. 1.0 equiv of DBU was used. The reaction was
carried out at rt. The reaction was carried out at 90 °C.
e
f
g
Inspired by this result, the reaction conditions were optimized to
improve the reaction yield. CuBr2 gave a slightly lower reaction
yield than CuBr (entry 2). The yield was improved to 72% when
CuI was used (entry 3).
The effect of oxidants was also examined in this reaction. The
product 2a was obtained in 33% yield in the presence of CuI (0.3
equiv) and oxidant (diacetoxyiodo)benzene (PIDA). In
comparison, no product 2a was observed when PIDA was
replaced with 2-hydroperoxy-2-methylpropane (TBHP) (entry
5). Decreasing the amount of DBU to 1 equiv reduced the
reaction yield to 51% (entry 6). The solvent screening revealed
B
DOI: 10.1021/acs.orglett.8b03799
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Organic Letters
Letter
whether R1 was para-EWG (2n and 2o) or para-EDG (2p).
Similar results were also observed in the aromatic anilines with
EWGs and EDG at the meta-position of the phenyl ring of
ketones (2q−t). Furthermore, the ketones with p-Cl, p-F, and
bulky p-t-Bu on the phenyl ring also gave high yields (2u−w).
The structure of the products was determined by the single-
crystal X-ray diffraction analysis of 2g.17
Scheme 4. Highly Regioselective Ring-Opening Reactions of
2v
Besides N-arylaziridines, N-alkylaziridines were also success-
fully synthesized via this C(sp3)−N coupling reaction. A
Yb(OTf)3/CuI relay system has been developed via a
Michael-oxidative cycloaddition tandem pathway. A solution
of 2-vinylpyridine 3 (0.4 mmol), alkylamine 4 (0.8 mmol), and
Yb(OTf)3 (0.02 mmol) in toluene (2 mL) was stirred for 0.5 h at
65 °C. Then DBU (0.8 mmol) and CuI (0.32 mmol) were added
into the solution for another 2 h under the O2 atmosphere to
afford the aziridine products 5 (Scheme 3). Primary amines with
a
Scheme 3. Scope of N-Alkylamines
yield via CuSO4-catalyzed azidation. Interestingly, a tandem
reaction occurred rapidly (10 min) to provide dibrominated 9v
in 83% yield. Bromination of arene may take place first to give 1
equiv of HBr, which then opened the aziridine ring. Moreover,
the polybrominated product 10v was obtained by prolonging
the reaction time and adding K2CO3 as base.
Based on the above results, a possible reaction mechanism is
proposed in Scheme 5. Initially, CuI was oxidized by oxygen to
Scheme 5. Proposed Mechanism and Mechanistic
Experiments
a
Reaction conditions: a solution of 2-vinylpyridine (0.4 mmol),
alkylamine (0.8 mmol), and Yb(OTf)3 (0.02 mmol) in toluene (2
mL) was stirred for 0.5 h at 65 °C. Then DBU (120 μL, 0.8 mmol)
and CuI (60 mg, 0.32 mmol) were added into the solution for 2 h
b
under O2 atmosphere. The first step of the reaction was stirred for 2
h.
mono-, di-, and trisubstituents were all tolerable in this reaction.
For example, phenethylamine 4a formed 5a in 71% yield.
Benzylamine, hexylamine, 2-(benzyloxy)ethanamine, and 2-
(thiophene-2-yl)ethanamine rendered the corresponding prod-
ucts in moderate yields (5b−e). Additionally, reactions with
amines bearing five-, six-, or seven-membered rings resulted in
moderate yields (5f−h). It is noteworthy that the reaction of
more bulky bioactive amantadine proceeded smoothly as well
(5i). Disubstituted 2-vinylpyridine 2-chloro-6-vinylnicotinoni-
trile could afford product 5j in acceptable yield (55%). Besides
azaarenes, 2,4-dinitrobenzene, as an activating moiety, produced
the desired product 5k in moderate yield.
The ring opening of aziridines played an important role
toward the preparation of a large variety of N-containing
compounds. However, the ring-opening studies of aziridine-2-
ketones and their derivatives was very limited. To test the
applicability of our methodology, the initially obtained aziridine
2v was converted into a variety of highly functionalized ketones
with high yield and regioselectivity. As depicted in Scheme 4,
treating 2v with HCl or benzyl bromide gave products 6v and 7v
via C−N bond cleavage between nitrogen and carbon at the α-
position of ketone. Additionally, azide 8v was generated in good
CuII, which chelated with the substrate 1a to generate the
intermediate I. The CuII intermediate II was formed under basic
conditions and converted to the intermediate III18 and IV19 in
sequence. Finally, the radical intermediate IV underwent
cyclization reaction via a single-electron transfer process
affording the aziridine product 2a. To validate the reaction
mechanism, 2 equiv of 2,2,6,6-tetramethylpiperidine 1-oxyl was
added into the reaction solution. No desired aziridine 2a was
found, and the coupling product 11a was obtained in 11% yield.
This result indicated that the radical intermediate IV was
generated during the reaction.
In summary, an efficient copper-promoted C−H oxidative
amination approach providing N-alkyl- and N-arylaziridines has
been developed. The secondary amines containing azaarenes or
ketones tolerated the reaction to give the aziridine products in
moderate to good yields, and a practical Yb(OTf)3 combined
with CuI-promoted annulation of electron-deficient vinylarenes
C
DOI: 10.1021/acs.orglett.8b03799
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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developed as well. This procedure proceeds via Michael addition
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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.orglett.8b03799.
Experimental details and spectroscopic data (PDF)
Accession Codes
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CCDC 1867799 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 Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: wwang@pharmacy.arizona.edu.
*E-mail: hli77@ecust.edu.cn
ORCID
(11) Jat, J. L.; Paudyal, M. P.; Gao, H.; Xu, Q.-L.; Yousufuddin, M.;
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Wei Wang: 0000-0001-6043-0860
Hao Li: 0000-0002-8978-0247
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21572054, 21572055, and 21738002),
the program for Professor of Special Appointment (Eastern
Scholar) at Shanghai Institutions of Higher Learning, the
Fundamental Research Funds for the Central Universities, and
the China 111 Project (Grant B07023).
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