Base-Controlled Divergent Synthesis of 5-Cyanobenzoxepines and Benzofuro[2,3- b]pyridines from 2-Bromophenylacetonitriles and Ynones

Article abstract of DOI:10.1021/acs.orglett.9b01700

An effective base-controlled divergent annulation reaction of 2-bromophenylacetonitriles and ynones has been developed. Various functionalized 5-cyanobenzoxepines and benzofuro[2,3-b]pyridines were obtained with a broad substrate scope and high regioselec

Full text of DOI:10.1021/acs.orglett.9b01700

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Base-Controlled Divergent Synthesis of 5‑Cyanobenzoxepines and
Benzofuro[2,3‑b]pyridines from 2‑Bromophenylacetonitriles and
Ynones
Lu-Lu Chen, Jing-Wen Zhang, Pei Chen, Shuai Zhang, Wan-Wan Yang, Ji-Ya Fu, Jun-Yan Zhu,
and Yan-Bo Wang*
Institute of Functional Organic Molecular Engineering, Henan Engineering Laboratory of Flame-Retardant and Functional
Materials, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China
S
* Supporting Information
ABSTRACT: An effective base-controlled divergent annula-
tion reaction of 2-bromophenylacetonitriles and ynones has
been developed. Various functionalized 5-cyanobenzoxepines
and benzofuro[2,3-b]pyridines were obtained with a broad
substrate scope and high regioselectivity in moderate to
excellent yield. Of importance, an unexpected O-rearrange-
ment reaction to access benzofuro[2,3-b]pyridines was
observed using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as
the base, and the possible mechanism was supported by 18O-
labeled experiments. In addition, the gram-scale synthesis and
further transformation of the product were studied.
Scheme 1. Previous Work and This Work for the
Cyclization Reaction of Ynones Used as the Four-Atom (1C,
2C, 3C, 1O) Synthon
enzoxepines and benzofurans, as important seven- and
five-membered oxygen-containing heterocyclic com-
B
pounds, are vital structural units due to their broad application
in biomedicinal natural molecules and pharmaceuticals.1,2
Consequently, various methods for preparing benzoxepines
or benzofurans from easily available materials by intermolec-
ular or intramolecular cyclization have been developed,
respectively.3,4 Although the above methodologies are
impressive, the development of an efficient and transition-
metal-free approach for the diversity-oriented synthesis of both
benzoxepines and benzofurans is still a challenging but
attractive task.
On the contrary, ynones, as electron-deficient and polarized
alkynes, have been widely investigated because of their
convenient preparation by the Sonogashira coupling of
terminal alkynes with acyl chlorides5 and easy functionalized
transformation to valuable compounds.6 It is noteworthy that
few cyclization reactions including the C−C triple bond and
carbonyl groups (1C, 2C, 3C, 1O) as the four-atom synthon
have been explored.7 For example, the synthesis of 1,4-
oxazepines from ynones and 2-bromoaniline or propargyl-
amine was described (Scheme 1a,b).7a−d A direct approach to
isoxazoles from ynones and trimethylsilyl azide under metal-/
catalyst-free conditions was also reported (Scheme 1c).7e
Inspired by the above excellent studies and our continuing
interest in the catalytic transitions of alkyne,8 we initially
envisioned that ynones as a four-atom synthon may react with
2-bromophenylacetonitriles via a formal [4 + 3] cyclization
reaction to construct benzoxepines in the presence of a suitable
base. To our surprise, the expected benzoxepines and
unexpected benzofuro[2,3-b]pyridines were both obtained
using different bases. Herein we report for the first time a
novel and efficient base-controlled divergent synthesis of 5-
Received: May 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01700
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Scope of the Cyclization Reactions Using LiOtBu
as Base
cyanobenzoxepines and benzofuro[2,3-b]pyridines from
ynones and 2-bromophenylacetonitriles with high regioselec-
tivity and a broad substrate scope under transition-metal-free
conditions (Scheme 1d).
a b
,
Initially, we started to screen reaction conditions by the
model reaction of 2-bromophenylacetonitrile 1a and ynone 2a
(Table 1). To our delight, product 3aa was isolated in 58%
a
Table 1. Optimization of the Reaction Conditions
b
entry
base
1a/2a/base
solvent
t (h) yield (%) 3aa/4aa
1
2
3
4
5
6
7
8
K2CO3
Cs2CO3
KOtBu
LiOtBu
KOH
1/1/1.5
1/1/1.5
1/1/1.5
1/1/1.5
1/1/1.5
1/1/1.5
1/1/1.5
1/1/0.5
1/1/1
1/1/2
1/1/1
1/1/1
1/1/1
1/1/1.5
1/1/1.5
1/1/1.5
1/1.5/1.5
1/2/1.5
1/1.5/2
1/1.5/2
1/1.5/2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
58/trace
68/trace
70/10
89/trace
50/15
trace/47
trace/−
44/−
94/−
91/−
96/−
82/trace
trace/−
trace/56
−/45
10/41
trace/68
21/65
DBU
DABCO
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
DBU
DBU
DBU
DBU
DBU
9
DMF
DMF
NMP
10
11
12
13
14
15
16
17
18
19
DMSO
CH3CN
DMSO
NMP
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
12
12
12
12
12
DBU
DBU
DBU
trace/76
trace/65
trace/71
c
20
21
d
a
a
Reaction conditions: 0.3 mmol 1, 0.3 mmol 2, 0.3 mmol LiOtBu, 1.5
Reaction conditions: 1a (0.30 mmol, 1.0 equiv), 2a, base, and 1.5
b
b
c
d
mL of NMP, 100 °C, 5 h. Isolated yield.
mL of solvent. Isolated yields. 110 °C. 90 °C.
yield using K2CO3 as an inorganic base (Table 1, entry 1). On
the basis of the above experimental result, various inorganic
and organic bases were then investigated. The higher yield of
product 3aa was obtained in the presence of LiOtBu (Table 1,
entry 4). When the base was changed to DBU, an unexpected
major product 4aa was given in 47% yield (Table 1, entry 6).
Subsequently, the effects of different reaction parameters, such
as solvent, reaction time, reaction temperature, material ratio
of reactants, and usage of base, were further explored. The
experimental results show that the base LiOtBu is suitable for
product 3aa in 96% yield (Table 1, entry 11), and the base
DBU exhibits superior activity to prepare product 4aa in 76%
yield (Table 1, entry 19).
With the optimal reaction conditions in hand, we set out to
explore the formal [4 + 3] cyclization reaction to construct
benzoxepines in the presence of LiOtBu. A variety of ynones 2
with 2-bromophenylacetonitriles 1 were studied, and the
results are listed in Scheme 2. The R2 and R3 substituents
anchoring the ynones were first studied. Substrates 2b−i and
2m−v with common substituents containing alkyl, ether, halo,
CF3, or an ester group at the ortho, meta, or para positions of
the aryl groups could smoothly deliver the corresponding
products 3ab−ai and 3am−av in moderate to excellent yield.
Further investigation showed that heteroaromatic substrates
2j,k and 2x,y and naphthyl substrate 2w were also tolerated
under the standard conditions. It is noteworthy that product
3al was obtained in 86% yield, the structure of which was
unambiguously characterized by X-ray crystallography. Then,
we turned to investigate the scope of various 2-bromopheny-
lacetonitriles with ynone 2a. The electronic effect on the four-
position of 2-bromophenylacetonitriles has little influence on
the reactivity, and the 4-Me (1b), 4-OMe (1c), and 4-F (1d)
substitutive substrates were also competent in this cyclization
reaction. Additionally, substrate 1e with an electron-donating
substituent (R1 = 4,5-OMe) or naphthyl substrate 1f was
smoothly transformed into the corresponding product in
satisfactory yield.
Subsequently, the cyclization reactions of 2-bromophenyla-
cetonitriles 1 and ynones 2 to prepare benzofuro[2,3-
b]pyridines were investigated, and the results are shown in
Scheme 3. To our delight, substrates 2b−i and 2m−t at meta
or para positions of the aryl groups (R2, R3) with electron-
donating groups or electron-withdrawing groups were well
tolerated, and the corresponding products 4ab−ai and 4am−at
were obtained in moderate to good yield. Unfortunately,
substrates 2u,v at the ortho position of the aryl groups (R3)
failed to give products 4au,av, and benzoxepine products
3au,av were given in 51 and 85% yield, respectively. Similar
B
DOI: 10.1021/acs.orglett.9b01700
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Organic Letters
Letter
a b
,
Products 3 with a cyano group have an opportunity to
prepare new functionalized compounds, and product 5 was
obtained by the reduction of 3aa with excellent yield in the
presence of HAl(iBu)2 (Scheme 4a). Moreover, when 2-
Scheme 3. Scope of the Reactions Using DBU as Base
Scheme 4. Further Chemical Transformations of Product
3aa, Application Using Other 2-Halophenylacetonitriles as
Substrates, and Gram-Scale Reaction for the Synthesis of
Products 3aa and 4aa
a
Reaction conditions: 0.3 mmol 1, 0.45 mmol 2, 0.6 mmol DBU, 1.5
b
c
mL of DMSO, 100 °C, 12 h. Isolated yield. 3 equiv DBU.
results were observed for the substrates bearing alkyl (2l) and
naphthyl (2w) groups, which were all converted into
benzoxepine products. Failure to access the desired products
may be affected by the steric and electronic effect using the
above ynones (2l, 2u−2w) as substrates. In addition, the
reaction of heteroaromatic substrates 2j,k and 2x,y with 2-
bromophenylacetonitrile 1a proceeded smoothly, leading to
the desired products (4aj,ak, 4ax,ay). The effect on the
catalytic activity of 2-bromophenylacetonitriles 1 with ynone
2a was further studied. Compared with the ynones 2, the
substituted 2-bromophenylacetonitriles 1b−f showed low
catalytic activity and underwent a cyclization reaction to
deliver the target products 4ba−fa in the presence of 3 equiv
DBU. When 2-bromophenylacetonitriles with 4-Me (1b) or 4-
F (1d) group was used as the substrate, the corresponding
product 4ba or 4da was obtained in moderate yield. In
particular, product 3da was obtained in 49% yield,
accompanied by product 4da. However, 2-bromophenylaceto-
nitriles with strong electron-donating group 4-OMe (1c) or
3,4-OMe (1e) gave product 4ca or 4ea in low yield. The
naphthyl substrate 1f was also suitable for this reaction in
satisfactory yield. The structures of 4aa and 4af were
determined unambiguously by single-crystal X-ray crystallog-
raphy (see Figure S1). Additionally, the photophysical
properties of 4aa were analyzed using ultraviolet−visible and
photoluminescence (PL) spectrometry (see Figure S2).
iodophenylacetonitrile or 2-chlorophenylacetonitrile was em-
ployed to react with 2a under standard conditions, the
identical product 3aa or 4aa was obtained in moderate to
excellent yield (Scheme 4b,c). To further demonstrate the
practicality of our current method, gram-scale reactions were
carried out. When 3.0 mmol of substrate 1a was used under
standard conditions, the desired products 3aa and 4aa could be
obtained in 90 and 56% yield, respectively (Scheme 4d,e).
On the basis of the present results and previous
literature,3l,7a−d a plausible mechanism for the base-controlled
divergent preparation of 5-cyanobenzoxepines and benzofuro-
[2,3-b]pyridines from 2-bromophenylacetonitriles 1 and
ynones 2 is proposed in Scheme 5. First, 2-bromophenylace-
tonitriles 1 could add to the triple bond of ynones 2 by
nucleophilic attack in the presence of a base, affording
intermediate A. For path I using LiOtBu as the base,
deprotonation and allyl and keto−enol tautomerism of A
successively occur to yield the new zwitterions B, the
intramolecular nucleophilic aromatic substitution (SNAr) of
which would give product 3 (path I). Both intermediates A
and B could be detected by MS and 1H NMR (see Figures S3
and S4). For path II using DBU as base, the ethenimines C
were obtained by the hydrogen transfer of A, the intra-
molecular cyclization of which between ethenimines with the
carbonyl carbon occurs to release the bridged intermediates D.
Subsequently, the base snatches the hydrogen of D; then, the
C
DOI: 10.1021/acs.orglett.9b01700
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
unexpected products 4 by O-rearrangement was proposed and
supported by the 18O-labeled experiments. Further research of
the catalytic transition involving ynones as four-atom synthons
is ongoing in our laboratory.
Scheme 5. Plausible Mechanism for the Synthesis of
Products 3 and 4
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.9b01700.
Experimental procedures, characterization of products,
1
copies of H and 13C NMR, X-ray crystal structures of
3al, 4aa, and 4af, and photophysical properties of 4aa
and mechanistic studies (PDF)
Accession Codes
CCDC 1913757−1913759 contain the supplementary crys-
tallographic 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.
O-rearrangement intermediates E were obtained by the C−O
bond cleavage promoted by electron transfer. The target
products 4 were formed by the intramolecular nucleophilic
aromatic substitution (SNAr). According to previous report-
s,3l,7a path I to access the expected products 3 was easily
understood by the formal [4 + 3] cyclization reaction. To
further understand the mechanisms of this novel cyclization for
the preparation of products 4 and to clarify path II undergoing
the O-rearrangement reaction, the 18O-labeled substrate 2a
was synthesized (see Figure S5), and 18O-labeled experiments
were carried out (Scheme 6). When 1a reacted with 18O-
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wangyanbokf@henu.edu.cn.
ORCID
Yan-Bo Wang: 0000-0002-5676-1437
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (no. U1504205), the Key
Research Project of Education Department of Henan Province
(no. 17A150002), and Henan University (yqpy20170009).
Scheme 6. Controlled Experiments Using 18O-2a or H218O
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