A Novel Catalyst-Free Synthesis of 2,2-Diaryl Enamides from Stilbenes via a Nitrene Transfer Reaction

Article abstract of DOI:10.1002/ejoc.201900885

A novel catalyst-free nitrene transfer reaction between stilbenes and iminoiodinanes was achieved for the first time, which provides an efficient and environmentally friendly way to access variously substituted 2,2-diaryl enamides under mild conditions. Mechanistic investigations suggested the reaction proceed via nitrene transfer and aromatic rearrangement with iminoiodinane also acting as a Lewis acid, benefitting the ring-opening of the aziridine intermediate.

Full text of DOI:10.1002/ejoc.201900885

A Journal of
Accepted Article
Title: A novel catalyst-free synthesis of 2, 2-diarly enamides from
stilbenes via nitrene transfer reaction
Authors: Yuanyuan Zhang, Wenjing Ye, Hui Zhang, and Xiao Xiao
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900885
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900885
Supported by

10.1002/ejoc.201900885
European Journal of Organic Chemistry
COMMUNICATION
A novel catalyst-free synthesis of 2, 2-diaryl enamides from
stilbenes via nitrene transfer reaction
Yuanyuan Zhang[a], Wenjing Ye[a], Hui Zhang[a], Xiao Xiao* [a,b]
Rh-catalyzed arylation of azavinyl carbene ref. 14-16
Abstract: A novel catalyst-free nitrene transfer reaction between
X
stilbenes and iminoiodinanes was realized for the first time, which
provides an efficient and environmentally friendly way to access
variously substituted 2, 2-diaryl enamides under mild conditions.
Mechanistic investigations suggested the reaction proceed via nitrene
transfer and aromatic rearrangement. The iminoiodinane also acted
as a Lewis acid, benefitting the ring-opening of intermediate aziridine.
N
NSO2R1
[Rh]
N
R2
Ar
X
B(OH)2
NHSO2R1
Ar
R2
R2
R2
(X = OR3, NR3
)
2
R2
Rh
Ar
NHSO2R1
or
NHSO2R1
R2
Ar
or
Introduction
NHSO2R1
Ar
Nitrenes have been recognized as the important active
intermediates in the synthesis of nitrogen-containing
compounds that are ubiquitous in organisms and natural
products as well as pharmaceuticals.1 The four non-bonding
valence electrons of a mono-valent nitrene are arranged
such that one pair of electrons occupies one orbital, while the
other two electrons may either be spin-antiparallel or spin-
parallel, corresponding to an excited singlet or ground-state
triplet, respectively (Scheme 1). In both states, nitrene is a
highly reactive electrophilic reagent and can easily insert into
C-H bond to form C-N bond.2 Sulfonyliminoiodinanes
(RSO2N=IPh) are a class of nitrene precursors that were first
synthesized and utilized by Abramovitch3 and Yamada4 in
the 1970s, and further investigated by Breslow,5 Mansuy,6
Evans and Müller 7. These species have been widely applied
in nitrogen transfer reactions involving C-H amination and
aziridination of alkenes. 8,9 The key role of organonitrogen
compounds within many fields such as chemistry, medicine,
biology, and materials, drives the development of new
nitrene methodologies to achieve highly efficient synthesis of
C-N bonds.
Novel metal-free nitrene transfer reaction
R1
R2
O
S
R3 this work
PhI=N
R1
NHSO2Ar
O
R2
Scheme 2 Synthesis of enamides via carbene/nitrene reagent
Enamides are among the most crucial synthetic tools for
introducing nitrogen-containing functional groups.10 Various
useful N-containing molecular backbones in organic synthesis
and medicinal chemistry can be afforded with enamides as
substrates or intermediates.11,12 The increasing interest in
nitrogen-atom chemistry necessitates the development of
synthetic approaches to enamides.13 However, the example of 2,
2-diaryl enamide preparation from nitrene is rare. Recently,
Fokin14 reported an aryl boronic acid mediated Rh-catalyzed
arylation of N-sulfonyl triazoles that generated azavinyl carbene
by eliminating nitrogen gas, giving 2, 2-diaryl enamides as the
products of C-C coupling. Similar processes have also been
described by Anbarasan15 and Lee16 (Scheme 2). Herein, we
would like to report an effective method to synthesize 2, 2-diaryl
enamides directly from alkenes via nitrene transformation in the
absence of metal catalyst.
N
N
R
R
singlet
triplet
Results and Discussion
Scheme 1 Singlet and triplet states of nitrenes.
Initially, (E)-1,2-di-o-tolylethene (1a) was used in the Pd/Fe
catalyzed
amidation17
with
p-bromobenzenesulfonyl
[a] Y. Zhang, Dr. W. Ye, Dr. H. Zhang, Dr. X. Xiao
iminoiodinane (PhI=NBs) as a nitrogen donor, forming 2, 2-di-o-
tolyl enamide as an unexpected by-product in low yield (12%
yield) at room temperature (Scheme 3). To start our investigation,
the stilbene 1a and PhI=NBs (2a) were chosen as standard
substrates with PdCl2 as a catalyst, for optimization studies. As
shown in Table 1, the enamide 3a was obtained in 45% yield with
o-tolylimines (o-tolylC=NBs) and o-tolualdehyde as the main
Key Laboratory of Structure-Based Drug Design and Discovery, Ministry. of
Education, Shenyang Pharmaceutical University,
Shenyang 110016, PR China.
[b] Department of Chemistry, Temple University
1901 N. 13th St., Philadelphia, PA 19122, USA
E-Mail: xiao.xiao0002@temple.edu
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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10.1002/ejoc.201900885
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Table 1 Optimization of reaction conditionsa
competitive products, when the reaction performed in 1,2-
dichloroethane (DCE) at 80 °C (entry 1, Table 1). Encouraged by
the obtained result, other metals were further investigated in this
nitrene transformation reaction. Increasing reactivity (51% yield)
was observed when Pd2(dba)3 was used (entry 2, Table 1). Ru
Entry
1
Catalyst
PdCl2
Solvent
DCE
Yield (%)b
45
2
3
Pd2(dba)3
RuCl2(PPh3)2
DCE
DCE
51
44
4
[Ru(p-cymene)Cl2]2 DCE
72
5
AuCl3
DCE
61
6
7
CuBr2
--
DCE
DCE
31
75
8
9
--
--
--
--
--
--
--
--
Toluene
EtOH
CH3CN
1,4-
Dioxane
DMF
DCE
53
25
66
26
--
67
71
38(36)f
10
11
12
13c
14d
15e
Scheme 3 Pd/Fe catalyzed formal amidation of stilbene
s
complexes such as RuCl2(PPh3)2, and [Ru(p-cymene)Cl2]2 were
also involved in the reaction (entries 3–4, Table 1). It was found
that the yield was increased to 72%, when using the [Ru(p-
cymene)Cl]2 as a catalyst. Enamide 3a could be obtained in
moderate yield when AuCl3 (entry 5, Table 1) was used as a
catalyst. CuBr2 was also tested, but the lower yield (31%) was
observed (entry 6, Table 1). To our delight, the best results were
obtained by using the catalyst-free condition and the designed
product 3a could be obtained in 75% yield, which indicated the
iminoiodinane might work more efficiently than metal-nitrene
intermediates in this process (entry 7, Table 1). Different solvents
including toluene, ethanol, acetonitrile, 1,4-dioxane, and N,N-
dimethyl formamide, were subsequently screened for the reaction
(entries 8–12, Table 1). However, lower reactivity was observed
in those solvents, indicating the 1,2-dichloroethane would be the
best choice. A higher reaction temperature (150 °C) and a longer
reaction time (48 hours) decreased the yields to 67% and 71%,
respectively (entries 13-14, Table 1). The nitrene transfer reaction
with PhI(OAc)2 and NH2Bs, which generated PhI=NBs in situ,
afforded the enamide product in 38% yield (12 hours) and 36%
yield (48 hours).
With optimal conditions in hand, the substrate scope of this
catalyst-free nitrene transfer reaction was examined with
various substituted trans-stilbenes 1 and iminoiodinanes 2,
affording a broad range of 2, 2-diaryl enamides 3 in 28–75%
yields (Table 2). Firstly, stilbenes 1a-n were subjected to the
nitrene transformation using PhI=NBs (2a) as a nitrene reagent.
Symmetrically substituted stilbenes containing 3-methyl, 2-chloro,
2-methoxyl, and 2, 5-dimethyl substituted aromatic groups
afforded products 3b-e in 32–69% yields. Besides the imine,
complicated side-products were formed in the reaction mixture
when highly activated stilbene 1d was used, leading to 3d in low
yield. 2, 2-Diphenylethene underwent the optimized process to
furnish the product 3f in 49% yield. The 1,2-di(naphthalen-2-yl)
ethene 1g underwent the nitrene transfer reaction smoothly to
yield the corresponding 2, 2-dinaphthyl enamides (3g) in 63%
yield. To our delight, the nitrene transfer reaction of
unsymmetrically substituted stilbenes 1h-k was also tolerated,
providing more sterically hindered 2-o-tol, 2-aryl cis-enamides (3’-
DCE
a
All reactions were carried out with stilbene 1a (0.25 mmol),
PhI=NBs (2a) (0.55 mmol), and catalyst (0.025 mmol), in solvent
(0.5 mL) at 80 °C for 12 hours. Isolated yield.c The reaction
b
d
e
temperature was 150 °C. The reaction time was 48 hours.
PhI=NBs was replaced by PhI(OAc)2 (0.75 mmol) and NH2Bs (0.75
mmol). f The mixture were stirred for 48 hours in the condition of
entry 15
methyl, 4’- bromo, 4’-isobutyl, and 3’-methyl-4’-chloro) in 47–61%
yields with high stereoselectivity (3h-k). Aziridine, which was not
observed in other examples, was isolated as the main product of
the reaction of stilbene 1l with PhI=NBs (2a). A mixture of cis and
trans enamides were obtained in moderate to good activity when
unsymmetrically alkene 1m and 1n were used. In this case, the
existence of isomers might arise from the facial tautomerization to
more stable E-enamides. It is worth noting that the nitrene transfer
did not occur when stilbenes with para-CH3, MeO, Br, substituents
on both phenyl rings were used; these substrates underwent
complicated reactions to afford some unknown mixture of
products. Next, the substituted iminoiodinane 2 was studied with
substrates
1a
and
1f.
Ortho-
and
para-toluene
sulfonyliminoiodinanes (2b and 2c) gave the enamide products
3o-q in 60–68% yields. Iminoiodinanes containing activating
groups (fluorine and tert-butyl) at the C4 position of phenylsulfonyl
were more reactive than those bearing deactivating groups
(trifluoromethyl and nitryl) (3r-v). The structure of products 3a and
3i were confirmed by the single-crystal X-ray analysis.18
To identify the mechanism of the reaction, several control
experiments were performed. Under the standard reaction
condition, cis-stilbene 1f’ couldn’t undergo tautomerization to give
trans-stilbene 1f (Scheme 4, eq 1). When cis-stilbene 1f’ and
PhI=NTs (2b) were used as the substrates of nitrene transfer
reaction under the standard condition, affording enamide 3f in 42%
yield and recovering 28% alkene mixture (cis/trans = 2/1)
(Scheme 4, eq 2). That result suggested the existence of unstable
four-membered ring intermediate 4a19, which generated
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Table 2 Substrate scope of the catalyst-free synthesis of 2, 2-diaryl enamidesa
a
All reactions were carried out with stilbenes
1
(0.50 mmol), iminoiodinane
2
(1.1 mmol), in DCE (1.0 mL) at 80 °C for 12 hours.
b
The geometries of enamide products
3 were tentatively proposed by analogy with the X-ray structure of enamide 3a and 3i.
Corresponding aziridine 5l was obtained instead of an enamide (see supporting information). c The isomer ratio of stilbene 1n is
1.5 : 1 (E : Z).
Ts
thermodynamic stable trans-stilbene 1f during the elimination of
Ph
N
H
PhI=NTs from 4a. In most cases, the aziridine, which is the general
product of nitrogen transfer reaction between iminoiodinanes and
alkenes9, was not observed in this method except for product 5l.
As shown in Scheme 5, no enamide product was detected when
aziridine 5a was subjected to the standard conditions for 12 hours.
To our surprise, when a catalytic amount PhI=NTs was used, it
could promote the rearrangement of aziridine 5a, giving enamide
3f in 57% yield. These control experiments indicate that the
(1)
N
DCE, 80 oC, 12 h
Ph
Ts
Ph
Ph
5f
3f
Ts
Ph
PhI=NTs (10% mol)
DCE, 80 oC, 12 h
H
N
N
(2)
Ph
Ts
Ph
Ph
3f
57%
5f
Scheme 5 Control experiments
Ph
Ph
Ph
Ph
(1)
iminoiodinane may act not only as a nitrogen donor in the
formation of aziridine but also a Lewis acid which catalyzed the
ring-opening and rearrangement of aziridine.20 This result can also
explain: 1) the corresponding aziridine intermediates of those
para-substituted (4-methyl, 4-MeO, 4-Br) substrates might be too
reactive and would rapidly decompose to complex mixtures, so no
enamide products were obtained for these para-substituted
stilbenes; 2) when most electron-deficient subtrate 1l was used,
the corresponding aziridine had relatively low reactivity of ring-
opening, only giving aziridine 5l as a final product.
DCE, 80 oC, 12 h
1f
1f'
Ph
PhI=NTs (2.2 eq)
DCE, 80 oC, 12 h
H
Ph
Ph
Ph
1f+1f'
(2)
N
Ph
Ph
Ts
1f'
3f
42%
28% (E:Z = 2:1)
Ts
Ph
Ph
N
I
Ph
Ph
(minor)
Ph
PhI=NTs
Ph
1f (major)
Ph
4a
2b
1f'
On the basis of the experimental results and previous
reports19,20, a plausible mechanism was proposed in scheme 6.
We envisioned that the iminoiodinane might mediated the aryl
[1, 2] migration of aziridine to form the enamide product
Scheme 4 Nitrene transfer reaction between cis-stilbene 1f’ and
iminoiodinane 2b.
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acknowledge Prof. K. Zheng for X-ray crystallographic
assistance.
generates the four-membered ring
(Scheme 6). First, the [2+2] cyclization of stilbenes 1 with
iminoiodinane
2
intermediate 4, followed by elimination of iodobenzene to give
aziridine 5. Sequentially, the iminoiodinane 2, as a role of Lewis
acid, interacts with aziridine 5, forming the dinitrogen-ligands
hypervalent iodine 721. Similar to an aza pinacol
Keywords: catalyst-free • 2, 2-diaryl enamides • nitrene transfer
reaction • iminoiodinanes
rearrangement20a
,
the irreversibly facial-selective [1, 2]
1
2
(a) Nitrenes, ed. W. Lwowski, Interscience, New York, 1970;(b) Nitrenes
and Nitrenium Ions, ed. D. E. Falvey, and A. D.Gudmundsdottir, John
Wiley & Sons, Inc, Hoboken, 2013.
migration of the aromatic group might proceed through
intermediate 7, to give the 2, 2-diaromatic imine 8. Compound
8 undergoes automerization to afford the final product enamide
3 with high stereoselectivity.
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Sasakura, A. Eguchi, K. Okamoto and K. Ohe, Chem. Eur. J. 2019, 25,
3156.
PhI NSO2Ar'
2
PhI
6
Ar'O2S
PhI NSO2Ar'
Ph
Ar'O2S
N
I
(2)
PhI NSO2Ar'
Ar2
2
3
R. A.Abramovitch, T. D.Bailey, T. Takaya and V. Uma, J. Org. Chem.
1974, 39, 340.
N
Ar2
Ar1
Ar2
Ar1
(Ar2 > Ar1)
Ar1
4
5
6
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Chem. Commun.1984, 1161.
1
4
5
SO2Ar’
N-
SO2Ar'
N
7
8
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H
Ar2
Ar1
Ar2 SO2Ar'
NH
I
Ph
Ar'O2S
Ar1
[1,3] H shift
N
Ar1
Ar2
PhI=NSO2Ar'
2
7
8
3
Scheme 6 Proposed mechanism
Conclusions
9
We have developed an effective and convenient synthesis
strategy of 2, 2-diaryl enamide via catalyst-free nitrene transfer
from sulfonyliminoiodinane to substituted stilbene. This method
provides an unprecedented route to 2, 2-diaryl enamides, which
have important synthetic utility. A probable reaction pathway
was demonstrated involving nitrene insertion and aromatic
group rearrangement of aziridine.
10 The Chemistry of Enamines, ed. Z. Rappaport and J. Wiley, Chichester,
UK, 1994.
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Experimental Section
Representative procedure of nitrene transfer reaction
To a oven-dried sealed tube charged with stilbene 1a (0.104 g,
0.50 mmol) in DCE (1.0 mL) was added PhI=NBs (2a) (0.482 g, 1.1
mmol). Upon stirring at 80 oC for 12 h, the crude product was
concentrated and purified by flash chromatography (silical gel,
petroleum ether/EtOAc = 10: 1) to give enamide 3a as a white solid.
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Acknowledgements
14 N. Selander, B. T. Worrell, S. Chuprakov, S. Velaparthi and V. V. Fokin,
The authors gratefully acknowledge the National Natural
Science Foundation of China (No. 21402127) and Shenyang
Pharmaceutical University for financial support. We also
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Entry for the Table of Contents
COMMUNICATION
Yuanyuan Zhang, Dr. Wenjing Ye, Dr.
Hui Zhang, Dr. Xiao Xiao*
Page No. – Page No.
A novel catalyst-free synthesis of 2,
2-diaryl enamides from stilbenes via
nitrene transfer reaction
An unexpected nitrene transfer from imioiodinane to stilbene in absence of catalyst
has been described, directly giving a series of 2, 2-diaryl enamides in 28-75%
yields. This reaction involved aziridination, ring-opening of aziridine, and aromatic
rearrangement, in which iminoiodinane played a dual role.
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