Copper-Catalyzed Annulation Cascades of Alkyne-Tethered α-Bromocarbonyls with Alkynes: An Access to Heteropolycycles

Article abstract of DOI:10.1021/acs.orglett.8b00236

We here describe a new Cu-catalyzed annulation cascade of alkyne-tethered α-bromocarbonyls with common alkynes for the synthesis of various heteropolycycle frameworks, including 1H-benzo[de]quinolin-2(3H)-ones, 4H-dibenzo[de,g]quinolin-5(6H)-one, and benzo[de]chromen-2(3H)-one, which were constructed with high selectivity. This was achieved by two-component annulation cascades, rather than atom-transfer radical cyclization (ATRC), with alkyne-tethered α-bromocarbonyls for one-step accessing heteropolycycles via C-Br bond split, intramolecular cyclization, intermolecular [4 + 2] annulation, and aryl C(sp²)-H functionalization cascades.

Full text of DOI:10.1021/acs.orglett.8b00236

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Annulation Cascades of Alkyne-Tethered
α‑Bromocarbonyls with Alkynes: An Access to Heteropolycycles
Bang Liu,†,‡ Jiang-Xi Yu,†,‡ Yang Li,†,‡ Jin-Heng Li,*,†,‡,§ and De-Liang He*,†
†State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China
‡Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University,
Nanchang 330063, China
§State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: We here describe a new Cu-catalyzed annulation cascade of alkyne-tethered α-bromocarbonyls with common
alkynes for the synthesis of various heteropolycycle frameworks, including 1H-benzo[de]quinolin-2(3H)-ones, 4H-
dibenzo[de,g]quinolin-5(6H)-one, and benzo[de]chromen-2(3H)-one, which were constructed with high selectivity. This was
achieved by two-component annulation cascades, rather than atom-transfer radical cyclization (ATRC), with alkyne-tethered α-
bromocarbonyls for one-step accessing heteropolycycles via C−Br bond split, intramolecular cyclization, intermolecular [4 + 2]
annulation, and aryl C(sp2)−H functionalization cascades.
Scheme 1. Annulation of α-Bromoacetamides with Alkynes
he tandem annulation strategy has emerged as an ideal
Tone-step platform to build various complex cyclic systems,
particularly heteropolycycles, in chemical synthesis.1−3 In this
field, attractive methods include the annulation cascades
reaction of haloalkanes, particularly α-halocarbonyls, with
unsaturated hydrocarbons, which has proven to be an
importantly straightforward alternative to access diverse cyclic
compounds by means of either radical initiators2 or transition
metal redox catalysis.2j,3 However, this technology for the
synthesis of structurally diversified polycycles remains a
formidable challenge, probably because such events mainly
rely on the atom transfer radical cyclization (ATRC) process
which includes termination by halogen atoms to inhibit the
incorporation of other functional groups, thereby often leading
to the construction of monocyclic compounds.2−5 Further-
more, reports using alkynes reacted with haloalkanes and
functional groups that tender polycycles via the annulation
cascades rather than ATRC (Scheme 1a) remain scarce.4a−c
These successful protocols strongly relied on the use of special
functionalized haloalkanes and/or alkynes to achieve the
desired reactivity, thus resulting in a great limitation in product
structure variety. To date, such versions to build heteropoly-
cycle skeletons are limited to only one report by Stephenson
and Tucker, which uses terminal alkyne-tethered β-aryl-
substituted α-bromoamides allowing the formation of tricyclic
pyrrolidinones via visible-light-promoted Ir-catalyzed tandem
radical cyclization and sigmatropic rearrangement.4a However,
this method was achieved by employing the inherent aryl
C(sp2)−H bonds as the terminated functional groups, thereby
allowing the resulting product variety to only be dependent on
the nature of the starting materials. In order to address this
deficiency, a multicomponent technique that allows annulation
cascades of alkynes with α-bromocarbonyls and other func-
tional reagents would be highly desirable to offer the
opportunity for diversified construction of N-heteropolycyclic
skeletons, while use of the external functional reagents
terminated this event.
Recently, intermolecular [4 + 2] cycloaddition of alkynes
involving aryl C(sp2)−H bond functionalization has attracted
increased attention for building diverse cyclic backbones due to
their atom and step economy without prefunctionalizing
starting materials.6 On this basis, we reasoned that by
Received: January 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00236
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
simultaneously combining alkyne-tethered α-bromoamide
radical annulation cascades and an intermolecular [4 + 2]
cycloaddition of an external alkyne, a one-pot, two-component
annulation cascade technique that makes N-heteropolycyclic
skeleton diversified construction would be established. Herein,
we report a new Cu-catalyzed annulation cascade of 2-bromo-
N-(2-ethynylaryl)acetamides with alkynes toward diverse
heteropolycycles, namely, 1H-benzo[de]quinolin-2(3H)-ones,
4H-dibenzo[de,g]quinolin-5(6H)-one, and benzo[de]chromen-
2(3H)-one, which are an important class of structural skeletons
commonly found in numerous natural products, pharmaceut-
icals, functional materials, and organocatalysts (Scheme 1b).7,8
Using the CuSO4 catalyst and tris(pyridin-2-yl-methyl)amine
(TPMA) ligand enables the formation of three new C−C
bonds in a single reaction via a sequence of C−Br bond
splitting, intramolecular cyclization by radical addition across
the CC bond, and [4 + 2] annulation with the external
alkynes and internal aryl C(sp2)−H bonds.
yield (entry 12). Further survey of the base effect found K2CO3
to be optimal (entries 2, 13, and 14).
After optimizing the reaction conditions, the scope of this
annulation cascade protocol with respect to 2-bromo-N-(2-
ethynylaryl)acetamides 1 was first investigated (Scheme 2). In
Scheme 2. Variations of the 2-Bromo-N-(2-
ethynylaryl)acetamides (1) and Alkynes (2)
a
We initially investigated the annulation cascades of 2-bromo-
N,2-dimethyl-N-(2-(phenylethynyl)phenyl)propanamide (1a)
with 4-ethynylanisole (2a) (Table 1 and Table S1 in
a
Table 1. Screening of Optimal Reaction Conditions
entry
[Cu] (mol %)
CuSO4 (10)
ligand (L)
base
yield (%)
61
b
1
L1
L1
L1
L1
L1
L1
L1
−
L2
L3
L4
L1
L1
L1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3

c
2
CuSO4 (10)
81 (72)
3
−
0
a
Reaction conditions: 1 (0.2 mmol), 2 (1 mmol), CuSO4·5H2O (10
4
CuSO4 (5)
76
74
76
72
trace
73
13
24
7
mol %), L1 (10 mol %), KI (25 mol %), K2CO3 (0.22 mmol), PhCF3
5
CuSO4 (20)
Cu(OTf)2 (10)
Cu(MeCN)4PF6 (10)
CuSO4 (10)
CuSO4 (10)
CuSO4 (10)
CuSO4 (10)
CuSO4 (10)
CuSO4 (10)
CuSO4 (10)
b
c
(3 mL), argon, 110 °C, and 16 h. 1 (1 mmol) and 48 h. 130 °C and
6
36 h.
7
8
9
the presence of 4-ethynylanisole 2a, CuSO4, L1, and K2CO3,
substrates 1b−d possessing a N-Bn group, a N-Ts group, or a
free N−H group were all viable to produce 3ba−da in 49%−
75% yields. Both electron-rich and -withdrawing aryl groups at
the terminal alkyne were well accommodated and gave 3ea−fa
in good yields. Aliphatic alkyne 1g was also compatible with the
optimal conditions (3ga), in which the ester group was
tolerated. Secondary alkyl bromide 1h was smoothly converted
into 3ha, albeit diminishing the yield. Unfortunately, primary
alkyl bromide 1i exhibited no reactivity (3ia). Substrates 1j−k
bearing an electron-donating group (Me or MeO) in the 5-
position of the N-aryl moiety was highly reactive and afforded
3ja−ka in good yields. Substrate 1l bearing an electron-
withdrawing CF3 group also showed high reactivity (3la; 64%
yield). Notably, substrate 1m could be efficiently annulated,
accessing 3ma, a cepharadione derivative.7,8 Gratifyingly, this
reaction was applicable to construct benzo[de]chromen-2(3H)-
one 3na, a valuable oxygen-containing heteropolycycle.
10
11
12
13
14
K3PO4
i-Pr2NH
74
77
a
Reaction conditions: 1a (0.2 mmol), 2a (1 mmol), [Cu] (10 mol %),
L (10 mol %), KI (25 mol %), base (0.22 mmol), PhCF3 (3 mL),
b
c
argon, 110 °C, and 16 h. Without KI. 1a (1 mmol).
Supporting Information (SI)). We found that a catalytic
amount of KI could improve the reaction. While in the absence
of KI the reaction of substrate 1a with alkyne 2a, CuSO4, L1
and K2CO3 afforded the desired 1H-benzo[de]quinolin-2(3H)-
one 3aa in a 61% yield (entry 1), the addition of 25 mol % KI
enhanced the yield to 81% (entry 2). Notably, both Cu
catalysts and ligands were necessary for the reaction to proceed
(entries 3 and 8). The amount of CuSO4 affected the reaction,
and 10 mol % CuSO4 proved to be optimal (entries 2, 4, and
5). Other alternative Cu catalysts, including Cu(OTf)2 and
Cu(MeCN)4PF6, showed lower catalytic activity than CuSO4
(entries 6 and 7). Although three other ligands L2−L4
exhibited activity, they were all inferior to L1 (entries 9−11).
The reaction could occur without a base, albeit with a lower
Next, we tested the feasibility of this Cu-catalyzed annulation
cascade protocol with a wide range of alkynes 2b−m. This
technology had the ability to engage various arylalkynes 2b−g
and high substituent compatibility (e.g., Me, Br, CN, and MeO)
(3ab−ag) (CCDC 1584061 (3ag)). While electron-rich alkyne
2b delivered 3ab in 83% yield, electron-deficient alkyne 2d
tendered 3ad with a decreased yield (69%). Alkyne 2e bearing a
B
DOI: 10.1021/acs.orglett.8b00236
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
meta-MeO group was more reactive than alkyne 2f having an
ortho-MeO group (3ae−af). 2-Ethynylpyridine 2h was
competent to access 3ah. Using ethynylcyclopropane 2i and
ethynyltrimethylsilane 2j succeeded in the formation 3ai−aj,
and cyclopropyl and trimethylsilane groups remain intact. Prop-
1-yn-1-ylbenzene 2k, an internal alkyne, was a suitable
substrate, albeit giving 3ak in a lower yield.
Scheme 4. Possible Mechanism
To highlight the applicability of this protocol, we attempted
to clarify annulation cascades of functional and/or bioactive
alkynes (Scheme 3). Ethynylferrocene (2l), in which the
Scheme 3. Other Alkynes and Control Experiments
of the intermediate B occurs to deliver 4a. Radical addition to
the aryl ring in the intermediate C affords the aryl radical
intermediate D, which sequentially undergoes single-electron
oxidation by the CuIILn species to access the aryl cation
intermediate E. Finally, deprotonation of the intermediate E
offers 3.
In summary, we have developed a novel Cu-catalyzed
annulation cascade of 2-bromo-N-(2-ethynylaryl)acetamides
with alkynes for building heteropolycyclic skeletons, including
1H-benzo[de]quinolin-2(3H)-ones, 4H-dibenzo[de,g]quinolin-
5(6H)-one, and benzo[de]chromen-2(3H)-one. This protocol
employs readily available reagents, including alkyne-tethered α-
bromocarbonyls and common alkynes, to construct diverse
heteropolycyclic frameworks in a single reaction with a broad
substrate scope and high functional group tolerance.
Importantly, this reaction provides a straightforward access to
the tricyclic ABC and tetracyclic ABCD rings found in the
cepharadione family and other useful molecules. The synthetic
utility of this protocol is further demonstrated in the
incorporation of some functional and/or bioactive units into
the resulting 1H-benzo[de]quinolin-2(3H)-one architectures.
ferrocene moiety is a ligand, pharmaceutical, and material
scaffold, was annulated, giving 3al in 80% yield (eq 1).
Substrate 2m, an estrone analogue, was successfully converted
into complex product 3am that comprises both estrone and N-
heteropolycycle groups (eq 2). Control experiments in eq 3
showed that KI could improve the formation of 3aa, but
disfavored ATRC. Substrate 1a underwent the ATRC to afford
4a in the absence of alkynes 2. However, using 25 mol % of KI
suppressed the ATRC, as the yield of 4a decreased to 55%. It is
noteworthy that 4a can react with alkyne 2a to access 3aa, and
KI also affects the reaction (eq 3). Without KI, only a 45% yield
of 3aa was obtained together with recovery of 43% 4a; using 25
mol % of KI enhanced the yield to 82%. These results
suggested that product 4a is not the key intermediate in the
current protocol, and KI may act as a bromo replacement
reagent for the in situ generation of alkyl iodides and as an
electron-transfer reagent to improve this annulation cascade.9
The reaction of substrate 1a with alkyne 2a was completely
inhibited by TEMPO, a radical inhibitor (eq 4). Using 2,6-di-
tert-butyl-4-methylphenol (BHT) led to a sharply diminished
yield. These results support a radical process.
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.8b00236.
Descriptions of experimental procedures for compounds;
analytical characterization (PDF)
Accession Codes
CCDC 1584061 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.
Consequently, the possible mechanism for the current
annulation cascades protocol is proposed (Scheme 4).2−5,9
Splitting of the C−Br bond in 1 by the active CuILn species and
a base occurs via single-electron transfer (SET) to form the
alkyl radical intermediate A and the CuIILn species,4,5,9 followed
by intramolecular addition across the CC bond in the
intermediate A affording the vinyl radical intermediate B. In the
presence of alkyne 2 intermolecular radical addition of the
intermediate B across the CC bond in 2 gives the other vinyl
intermediate C, whereas in the absence of alkyne 2 the ATRC
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jhli@hnu.edu.cn.
*E-mail: delianghe@hnu.edu.cn.
ORCID
Jin-Heng Li: 0000-0001-7215-7152
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.8b00236
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of China (Nos.
21625203 and 21472039) and the Jiangxi Province Science and
Technology Project (Nos. 20171ACB20015 and
20165BCB18007) for financial support.
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