Iron-Catalyzed Nitrene Transfer Reaction of 4-Hydroxystilbenes with Aryl Azides: Synthesis of Imines via C=C Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.9b03160

C=C bond breaking to access the C=N bond remains an underdeveloped area. A new protocol for C=C bond cleavage of alkenes under nonoxidative conditions to produce imines via an iron-catalyzed nitrene transfer reaction of 4-hydroxystilbenes with aryl azides is reported. The success of various sequential one-pot reactions reveals that the good compatibility of this method makes it very attractive for synthetic applications. On the basis of experimental observations, a plausible reaction mechanism is also proposed.

Full text of DOI:10.1021/acs.orglett.9b03160

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed Nitrene Transfer Reaction of 4‑Hydroxystilbenes with
Aryl Azides: Synthesis of Imines via CC Bond Cleavage
Yi Peng, Yan-Hui Fan, Si-Yuan Li, Bin Li, Jing Xue, and Qing-Hai Deng*
The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai
Normal University, Shanghai 200234, China
S
* Supporting Information
ABSTRACT: CC bond breaking to access the CN bond remains an underdeveloped area. A new protocol for CC bond
cleavage of alkenes under nonoxidative conditions to produce imines via an iron-catalyzed nitrene transfer reaction of 4-
hydroxystilbenes with aryl azides is reported. The success of various sequential one-pot reactions reveals that the good
compatibility of this method makes it very attractive for synthetic applications. On the basis of experimental observations, a
plausible reaction mechanism is also proposed.
mines are not only versatile intermediates for the synthesis
oxidative cleavage of CC bonds to carbonyl derivatives7 and
Iof nitrogen-containing bioactive compounds, pharmaceut- the olefin metathesis, which could convert CC bonds to new
CC bonds in the presence of metal−carbene catalysts.8
icals, and fine chemicals but also useful starting materials for
various organic transformations.1 Traditionally, imines are
Recently, in addition to the examples of CC bond cleavage
to access nitriles,9 a few related works on oxidative cleavage of
synthesized by dehydrative condensation of amines with
carbonyl compounds, which usually are derived from the
CC bond to form the CN bond have been reported.10 In
corresponding alcohols.2 Apart from this, several alternative
2017, Zhu and co-workers reported a visible-light-mediated
oxidative cleavage of CC bonds to form CN bonds.10d
strategies for the synthesis of imines have been established.
This process converted alkenes to hydrazones under aerobic
oxidative conditions by using hydrazines as the nitrogen
sources. Recently, three methods for the preparation of
quinazolinones10b,c or benzothiadiazine-1,1-dioxides10a via an
oxidative cleavage of CC bonds were developed independ-
ently, in which the alkenes were first converted to the
corresponding aldehydes under oxidative10a,c or electronically
oxidative10b reaction conditions. As part of our ongoing
program on developing base-metal-catalyzed nitrene-transfer
reactions,11 we herein report the first example of an iron-
catalyzed nitrene transfer reaction of 4-hydroxystilbenes with
aryl azides via CC bond cleavage for the preparation of
imines (Scheme 1b). Importantly, owing to the simple
condition only with 10 mol % of FeCl2 as the catalyst, the
method can combine with many kinds of further reactions to
achieve various sequential one-pot transformations.
There were included self-condensation of primary amines upon
oxidation, oxidation of secondary amines, and oxidative
dehydrogenative coupling of alcohols with amines (Scheme
1a).3 To create environmentally benign processes, the direct
dehydrogenative versions, which eliminate the formation of
waste from oxidants and allow the further utilization of the
valuable hydrogen byproduct, have also achieved great
success.4 Recently, the methods for the synthesis of imines
through metal-catalyzed hydrogenation of nitriles5 and hydro-
amination of alkynes with amines6 have also been developed.
Although such efficient methodologies for the synthesis of
imines from several types of starting materials were successfully
developed, the exploration of alternative strategies to prepare
imines from other kinds of starting materials remains desirable.
Considering that alkenes are generally ideal raw materials for
chemical synthesis due to their ready availability, low cost, as
well as diversity, the development of new environmentally
friendly strategies to synthesize imines via reactions of alkenes
with proper nitrogen sources is an extremely attractive yet
challenging subject.
We started the investigations by reacting 4-hydroxystilbene
(1a) with 1-azido-4-methoxybenzene (2a) as the model
reaction (Table 1). Following the previously reported
procedure,11 the reaction was carried out in acetonitrile
On the other hand, the cleavage of the carbon−carbon
double bond is one of the primary transformations in organic
chemistry. In this respect, prominent examples include the
Received: September 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03160
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Letter
(MeCN) at 80 °C in the presence of FeCl2 (10 mol %) to
provide the desired products 3aa in 73% yield and 4aa in 66%
(entry 1). Increasing the temperature to 100 °C improved the
yields dramatically (95% of 3aa and 93% of 4aa, entry 2).
FeBr2 afforded the product in moderate yields (entry 3), and
Fe(OAc)2 showed no catalytic activity (entry 4). A further
examination of the solvent revealed that MeCN was optimal
(entries 2 and 5−9). Reducing the catalyst loading to 5 mol %
and the usage of azide 2a to 2.5 equiv both resulted in lower
yields (entries 10 and 11). As expected, there was no product
formation without FeCl2 (entry 12), and a comparable result
was obtained in the presence of ultrapure FeCl2 (entry 13),
thus suggesting that the catalyst system is iron-based. Notably,
the comparable yields of corresponding products were
obtained when the reaction was conducted under an
atmosphere of air (entry 14). Finally, the reaction can be
readily scaled up to 0.40 mmol for the substrate scope
investigations (entry 15).
Scheme 1. Strategies for the Synthesis of Imines
We investigated the protocol using different 4-hydroxys-
tilbenes 1 under the optimized reaction conditions (Scheme
2). It should be noted that the reaction mixture was reduced to
a,b
Scheme 2. Scope of 4-Hydroxystilbenes 1
a
Table 1. Optimization of the Reaction Conditions
temp
(°C)
time
(h)
yield of
b
yield of
b
entry
catalyst
solvent
3aa (%)
4aa (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
FeCl2
FeCl2
FeBr2
Fe(OAc)2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
MeCN
MeCN
MeCN
MeCN
PhCl
80
100
100
100
100
100
100
100
100
100
100
100
100
100
100
16
8
24
48
8
73
95
78
66
93
62
c
NR
43
46
70
NR
NR
65
79
NR
94
93
95
36
42
55
DCE
8
toluene
EtOH
DME
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
12
36
40
8
8
36
8
c
c
a
d
62
76
Reactions were conducted with 1 (0.40 mmol), 2a (1.20 mmol),
FeCl2 (10 mol %), and MeCN (2 mL) at 100 °C under an
atmosphere of argon. After completion of the reaction and removal of
MeCN, the mixture was reduced by NaBH4 (1.60 mmol) and MeOH
(15 mL) at room temperature under open atmosphere. PMP = p-
e
c
f
FeCl2
FeCl2
FeCl2
92
92
93
g
14
15
8
8
b
c
methoxyphenyl. Yields of isolated products are reported. FeCl2 (20
h
d
mol %) was used. PhCl as solvent.
a
Reaction was conducted with 1a (0.20 mmol), 2a (0.60 mmol),
catalyst (10 mol %), and solvent (2 mL) under an atmosphere of
b
argon. Yields of crude products were determined by 1H NMR
amines by NaBH4 for confirming the isolated yields of imines.
In general, two desired amine products 5 and 6 were obtained
with almost similar yields in each reaction. The substrates with
different substituents on the phenyl ring (R1) were first
screened. 4-Hydroxystilbenes with electron-withdrawing sub-
stituents (F, Cl, Br, NO2, and CN) at the 4′-position afforded
the desired products in moderate to high yields (72−91% for
5ba−fa and 67−83% for 6aa), and the substrates bearing
c
spectroscopy using 4-nitrotoluene as internal standard. No reaction.
d
e
f
5 mol % of FeCl2 was used. 2.5 equiv of 2a was used. Ultrapure
FeCl2 (99.998% based on trace metals, Sigma-Aldrich) was used.
g
h
The reaction was carried out under the atmosphere of air. Reaction
was conducted with 1a (0.40 mmol), 2a (1.20 mmol), FeCl2 (10 mol
%), and MeCN (2 mL) under the atmosphere of argon.
B
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t
electron-donating groups (Me, Bu, and OMe) provided the
corresponding products in moderate to good yields (65−86%
for 5ga−ia and 69−90% for 6aa). The reaction efficiency was
affected obviously by the substituent position on the phenyl
ring (R1). For example, 4′-chloro-4-hydroxystilbene (1c)
afforded the desired product 5ca in 91% yield, while the
counterparts at the meta- and ortho-position (1j, 1k) were
converted to the corresponding amines 5ja (80%) and 5ka
(74%), respectively. Multiple substituted substrates such as
pterostilbene (1l), 1′,4′,6′-trimethyl-4-hydroxystilbene (1m),
and 4′-vinyl-4-hydroxystilbene (1n) as well as the complex
substrates (1o, 1p) were all well tolerated to afford the
corresponding amines 5la−pa in 63−95% yields. The
substrates (1q−t) with different substituents (R2) at the 4-
hydroxyphenyl group were also suitable for the reaction to
afford the desired amine 5aa in 71−95% yields, while the
products 6qa−ta were obtained in lower yields.
It is noteworthy that 4-hydroxystilbenes such as 1a and
complex estrone derivative 9 can be prepared smoothly in high
yields via Pd-catalyzed Heck−Matsuda coupling of the
corresponding alkenes with 4-hydroxybenzenediazonium salts
(Scheme 4).12 The substrate 9 was readily converted to the
desired imine 10 in 62% yield without destruction of the
original carbonyl group (Scheme 4a, eq 1). This result
indicated that this strategy could easily achieve carbon−carbon
double-bond cleavage of complex styrene derivatives after
incorporation of the 4-hydroxyphenyl group via Heck−
Matsuda reaction.
Imines are versatile but relatively unstable N-containing
synthons. Therefore, the direct use of imines without isolation
and purification as reaction partners is more appealing from
the viewpoint of easy operation and step economy. To further
demonstrate the synthetic utility of this methodology, a series
of sequential one-pot transformations were investigated
(Scheme 4b). After the synthesis of imines without workup
and isolation, the secondary step of all these reactions
proceeded subsequently in one pot by adding the extra
reagents. Polycyclic compounds 12 and 13 were prepared in
good total yield (73%) with low diastereoselectivity (1.3:1)
through Povarov reaction of in situ generated imine 3aa with
2,3-dihydrofuran (11) (Scheme 4b, eq 2).13 It is unfortunate
that the imine 4aa was easily decomposed in the second
transformation, so the corresponding products were not
obtained. Curiously, in the selected examples such as the
Strecker reaction for the synthesis of α-aminonitriles 14 (85%)
and 15 (55%) (eq 3),14 aerobic oxidation reaction of imine
with o-phenylenediamine (16) to benzimidazoles 17 (84%)
and 18 (83%) (eq 4),15 as well as cyclization reaction to access
quinoline 19 (72%) (eq 5),16 the iron salt was still efficient for
the following step. Furthermore, aminophosphonates 20
(90%) and 21 (80%) were easily obtained via hydro-
phosphonylation with diethyl phosphonate (eq 6).17 The
success of these one-pot transformations demonstrated that
this methodology, due to its simple and nonoxidizing reaction
conditions, can provide in situ generated imines for diverse
further transformations.
The substrate scope of azides was then explored by the
reactions of 4-hydroxystilbene 1a with various aryl azides 2b−j
(Scheme 3). The reactions could be carried out smoothly
a,b
Scheme 3. Scope of Azides 2
In order to understand the reaction mechanism, several
control experiments were carried out as described in Scheme 5.
2-Hydroxystilbene (22) was also efficiently reacted with 4-
methoxyphenyl azide (2a) under the optimized conditions to
provide the desired imines 3aa and 23 in moderate yields and a
byproduct 24 in low yield (eq 7). However, the reaction did
not work for 3-hydroxystilbene (25) (eq 8) or 4-methox-
ystilbene (26) (eq 9). These results indicated that the 4- or 2-
hydroxylphenyl group is crucial to the reaction. The reaction in
the presence of a radical quencher 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) under the optimal reaction con-
ditions was completely suppressed, and the addition of a
radical inhibitor such as 1,1-diphenylethylene or (1-
cyclopropylvinyl)benzene dramatically inhibited the reaction
(eq 10). These results suggested that the process probably
involves a radical pathway. Because aryl azides can be
converted to the corresponding azobenzenes via dimerization
of metal nitrene species,18 azobenzenes 27 and 28 were
prepared to investigate the reactivities. Azobenzene 27 did not
participate in the reaction of 1a with 2a and failed to transfer
to the expected imines 3ab and 4ab (eq 11). Additionally,
azobenzene 28 did not react with substrate 1a under the
standard reaction conditions (eq 12). These results exposed
a
Reactions were conducted with 1a (0.40 mmol), 2 (1.20 mmol),
FeCl2 (10 mol %), and MeCN (2 mL) at 100 °C under an
atmosphere of argon. After completion of the reaction and removal of
MeCN, the mixture was reduced by NaBH4 (1.60 mmol) and MeOH
b
(15 mL) at room temperature under open atmosphere. Yields of
isolated products are reported.
regardless of the electronic nature of the substituents at the
para-position of the phenyl ring, providing the corresponding
amines 5ab−ah (40−82%) and 6ab−ah (46−81%) in
moderate to good yields. These results showed that 4-
alkoxyphenyl azides (2a and 2g) are the optimal reagents for
CC bond cleavage in this protocol. Complex azide 2i was
also tolerated under the reaction conditions to obtain the
desired amines bearing 9H-fluorene units 5ai (73%) and 6ai
(71%). Notably, sterically hindered 2,4,6-trimethylphenyl azide
(2j) could be converted to the isolated imines 3aj (66%) and
4aj (61%).
C
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Letter
Scheme 4. Further Utilities of the Methods
In general, the iron nitrene species has significant diradical
character,19 and 4-hydroxystilenes can be easily converted to
the corresponding persistent free radicals under proper
conditions.20 Based on these known reports and the above
experimental results, a possible mechanism is proposed
(Scheme 6). In the initial step, azide 2a reacts with FeCl2 to
generate iron nitrene complex A,21 which abstracts a hydrogen
atom from substrate 1a to form iron aminyl species B and
radical C. In addition, intermediate B could assist another 1a
to become intermediate C because a trace of 4-methoxyaniline
was detected in the reaction mixture. Radical C subsequently
Scheme 5. Control Experiments for Mechanistic
Investigations
Scheme 6. Plausible Mechanism
that neither azobenzenes nor the four-ring species 2910d is
probably the intermediate.
D
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03160.
Experimental procedures, characterization data, and
copies of NMR spectra for all new products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qinghaideng@shnu.edu.cn.
ORCID
Qing-Hai Deng: 0000-0001-7793-1434
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Program of the Professor of Special
Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning, Shanghai Engineering Research Center of
Green Energy Chemical Engineering (18DZ2254200), the
National Natural Science Foundation of China (21402119,
21772122), and Shanghai Rising-Star Program
(16QA1403100) is greatly appreciated.
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DOI: 10.1021/acs.orglett.9b03160
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