Palladium/copper-catalyzed aerobic intermolecular cyclization of enediyne compounds and alkynes: Interrupting cycloaromatization for (4 + 2) cross-benzannulation

Article abstract of DOI:10.1021/ja506795u

A tandem coupling-ketooxygenation reaction of readily accessible enediyne-carboxylic compounds with inner alkynes has been developed that utilizes the PdCl₂/CuBr₂ catalytic system under an O ₂ atmosphere and assembles a cl

Full text of DOI:10.1021/ja506795u

Communication
pubs.acs.org/JACS
Palladium/Copper-Catalyzed Aerobic Intermolecular Cyclization of
Enediyne Compounds and Alkynes: Interrupting Cycloaromatization
for (4 + 2) Cross-Benzannulation
Fei Ling,† Zexiang Li,† Chenguang Zheng, Xiang Liu, and Cheng Ma*
Department of Chemistry, Zhejiang University, 20 Yugu Road, Hangzhou 310027, China
S
* Supporting Information
Scheme 1. Moiety-Directed Annulations of Acyclic Enediyne
Skeletons
ABSTRACT: A tandem coupling−ketooxygenation reac-
tion of readily accessible enediyne−carboxylic compounds
with inner alkynes has been developed that utilizes the
PdCl2/CuBr2 catalytic system under an O2 atmosphere
and assembles a class of isoindolinones and o-acylbenzoic
acids. The two oxygen atoms are regioselectively
incorporated into enediyne units at the 1- and 6-positions
from atmospheric molecular oxygen and H2O, respectively,
during the present process. This study uncovered a formal
[4C + 2C] benzannulation−diketonization of enediynes
and alkynes via a coupling and decoupling strategy.
ince the discovery of natural enediyne antibiotics, the
compounds (1 and 2) with inner alkynes, leading to
isoindolinone and o-acylbenzoic acid scaffolds.6 As a comple-
ment to the Pd(0)-catalyzed highly regioselective enyne−alkyne
[4 + 2] cross-benzannulation7 established by Gevorgyan and
Yamamoto,7e−h this research discloses a formal [4C + 2C]
benzannulation−diketonization of enediyne units and alkynes by
a coupling and decoupling strategy8 to override the innate
cyclization reactivity of enediynes.
S
cycloaromatization of (Z)-hexa-1,5-diyn-3-enes has emerged
as an attractive approach for the preparation of versatile aromatic
compounds.1 In this area, pioneered by the studies of Wang and
Finn2a and Ohe, Uemura, and co-workers,2b recent advances
have remarkably uncovered the unique cyclization reactivity of
acyclic enediyne scaffolds in the presence of various transition-
metal catalysts, such as Ru, Pt, Au, Pd, and Cu salts.2 O’Connor et
al.2c disclosed that the cycloaromatization of enediynes could be
achieved at ambient temperature by the generation of metal
complexes. Liu and co-workers notably found that Ru-catalyzed
aromatization of enediynes can involve nucleophilic addition of
O-, N-, and C-centered nucleophiles to enediynes to form
functionalized aromatics.2d Meanwhile, variations in substitution
at the terminal alkyne carbons have revealed new reactivity
trends in consecutive intramolecular cyclization.2e−i In addition,
fascinating cascade cyclizations of benzenoid enediynes upon
dual gold catalysis1g were independently reported by the groups
of Hashmi2j and Zhang2k in 2012. Despite these advances, the
self-cyclization tendency of enediyne units in turn causes
problems in developing transition-metal-catalyzed intermolecu-
lar carbocyclization reactions,3 which have been rarely addressed
despite their apparent merits in greatly expanding the scope of
potential applications.4
The initial assays were carried out with enediyne−imide 1a
and hex-3-yne (3a) in DMF at 35 °C (Table 1). While Pd(PPh3)4
was ineffective, we found that PdCl2 (10 mol %) enabled a cross-
coupling−oxygenation reaction between 1a and 3a, producing 3-
hydroxyisoindolinone 4aa in 45% isolated yield after 12 h
(entries 1 and 2). Similar yields of 4aa indeed were obtained with
either high-purity PdCl2 or Pd(OAc)2 instead under an air
atmosphere (entries 3 and 4).9 A stoichiometric amount of PdCl2
resulted in rapid consumption of 1a (0.5 h), but no increase in
product yield was observed (entry 5). We envisioned that a
radical process was possibly involved and that Cu salts might be
used as cocatalysts to facilitate this Pd(II)-catalyzed aerobic
transformation.10 Gratifyingly, under air or O2 (1 atm), addition
of 20 mol % CuBr2 accelerated this reaction, affording 4aa in 81%
and 93% yield, respectively (entries 6 and 7). Switching CuBr2 to
other copper salts did not improve the yield (entries 8 and 9).
Although increasing the ratio of 1a to 3a would lead to some
undetermined byproducts, the use of 2 equiv of alkyne 3a still
gave 4aa in good yield (entry 10). Further exploration showed
that performing the reaction in an aqueous solvent system (10:1
DMF/H2O) or decreasing the loading of PdCl2 to 5 mol % had
We recently developed an efficient single-step procedure to
synthesize enediyne−imides 1, which can undergo Pd(II)-
catalyzed branching cycloisomerization−allylation to give
allylfuro[2,3]pyridines via vinylpalladium intermediates I
(Scheme 1).5 Encouraged by this result, we envisaged that an
alternative insertion of external alkynes into species I followed by
subsequent annulation might be achieved through a judicious
choice of catalytic system. Herein we report a palladium/copper-
catalyzed aerobic cross-coupling−ketooxygenation of enediyne
Received: July 6, 2014
Published: July 25, 2014
© 2014 American Chemical Society
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a
give 4ha in 76% yield. It was found that employing an aryl group
as R2 is critical for this reaction. For example, while 1i (R1 = Ph,
R2 = n-C5H11) resulted in a complex transformation without any
detectable 4ia, enediyne 1n (R1 = n-Bu, R2 = Ph) was a suitable
substrate to afford 4na, albeit in relatively low yield.
Table 1. Optimization of the Reaction Conditions
Next, the scope of this reaction with respect to alkynes 3 was
studied (Scheme 3). Symmetrical alkynes bearing alkyl
b
entry
Pd cat. (mol %)
additive
t (h)
air/O2
yield (%)
1
2
Pd(PPh3)4
PdCl2
−
−
−
−
12
air
air
air
air
air
air
O2
O2
O2
O2
O2
O2
O2
−
−
a
12
45
42
41
44
81
93
69
82
75
91
88
−
Scheme 3. Reactions of 1a and Alkynes 3
c
3
PdCl2
12
4
5
6
7
8
9
Pd(OAc)2
PdCl2 (100)
PdCl2
12
−
0.5
1.5
1.5
1.5
1.5
2.0
1.5
2.0
12
CuBr2
CuBr2
CuI
PdCl2
PdCl2
PdCl2
CuCl2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
d
10
e
PdCl2
11
PdCl2
12
13
PdCl2 (5)
−
PdCl2
f
14
12
<5
a
Unless otherwise noted, reactions were conducted at 35 °C on a 0.2
mmol scale of 1a with 3a (1.0 mmol), Pd catalyst (10 mol %), and
additive (20 mol %) in DMF (1.6 mL) in air or under 1 atm O2.
b
c
d
Isolated yields. PdCl2 (99.999% purity) was used. 0.4 mmol of 3a
e
f
was used. In 10:1 DMF/H2O. Under N2.
little influence on the reaction outcome (entries 11 and 12). In
contrast, no desired product was detected in the absence of Pd
catalyst, and only a trace of 4aa was observed under N2 (entries
13 and 14). These blank tests clearly indicated that both Pd(II)
catalyst and O2 are required for this reaction.
The scope of this reaction was then explored with a range of
substituted enediyne−imides 1 (Scheme 2). Variation of R1 and
a
Scheme 2. Reactions of 1 and Alkyne 3a
a
Enediyne 1a (0.2 mmol) and 3 (1.0 mmol) in DMF (1.6 mL) for 3 h.
b
c
Determined from the isolated yields. Determined by 1H NMR
analysis. The yields are of the isolated products.
substituents, including α-heteroatom-substituted compounds
3c and 3d, could participate in the process, although the yields
of the corresponding products 4ac and 4ad were lower that that
of 4ab. Diaryl-substituted alkynes, some with electron-with-
drawing and -donating groups, reacted smoothly to give 4ae−
4ah in decent yields. When 1,2-bis(4-methoxyphenyl)ethyne
(3f) was used as the coupling partner, an intramolecular
hydroarylation occurred, affording alcohol 4af′ as a pair of
diastereoisomers in addition to the targeted product 4af.
Nonsymmetrical dialkyl and aryl alkyl alkynes provided 4ai
and 4aj, respectively, with nearly 1:1 regioselectivity in good
yield. Pleasingly, captodative diarylethyne 3k having strong
electron-biased groups (−OMe and −NO2) at the para positions
of the phenyl rings was converted to 4ak with >20:1
regioselectivity in 79% yield. The challenging (bromoethynyl)-
benzene (3l) still successfully underwent this coupling−
oxygenation reaction, affording 4al in 56% yield with 2.5:1
regioselectivity.
a
Enediyne 1 (0.2 mmol) and 3a (1.0 mmol) in DMF (1.6 mL) for 3 h.
nd = not detected. The yields are of the isolated products.
R2 showed that benzene rings with an electron-donating group
(−OMe) or an electron-withdrawing group (−F or −NO2) as
well as o-methyl were tolerated, leading to the corresponding
products in 79−95% yield. The reaction conditions are
compatible with Br and Cl, which are convenient handles for
further functionalization (4ea, 4fa, and 4ma). Moreover, a
heteroaromatic ring (2-thienyl) could be used in this reaction to
Intriguingly, a set of enediyne−carboxylic acids 2 derived from
1 or their ester precursors11 smoothly underwent the cross-
coupling−oxygenation with 3a without any modification to the
standard conditions for 1, giving diketones 5aa and 5ha instead
of the corresponding 3-hydroxyphthalide products becauses of
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Communication
ring−chain tautomerism (Scheme 4a).12,6 1,2-Diphenylethyne
(3e) was also compatible with this reaction, delivering 5ae in
Furthermore, in the presence of stoichiometric 2,2,6,6-
tetramethyl-1-piperidine-1-oxyl (TEMPO), the reaction of 1a
and 3a upon treatment with PdCl2 (100 mol %) and CuBr2 (20
mol %) in DMF-d7 under N2 cleanly yielded TEMPO adduct 7aa
after 0.5 h,15 while the standard conditions provided a mixture of
4aa and 7aa (eq 3). These results hinted that a benzyl radical is
generated in this process and would be responsible for the
subsequent O2 activation.
Scheme 4. Cyclization of Enediyne−Carboxylic Acids 2 and
Alkynes 3 and Protodecarboxylation of 5
On the basis of the above observations, a proposed reaction
pathway is depicted in Scheme 5. Initial 5-endo cyclization of 1a
Scheme 5. Proposed Reaction Pathway
66% yield. As expected, subjection of 2a to the pull−push alkyne
3k afforded 5ak with excellent regioselectivity (>20:1) in 85%
yield. Subsequent removal of the CO2H group of 5 by Cu-
catalyzed protodecarboxylation13 provided an efficient approach
for the regioselective preparation of 1,4-diacylbenzenes 6aa and
6ae, which would be difficult to prepare by conventional
methods (Scheme 4b).
To gain insight into the reaction mechanism, control
experiments with 18O2 and/or H218O isotopic labeling were
performed. Exposure of 4aa to 10 equiv of H218O under the
standard conditions did not form any obvious 18O-labeled
products after 12 h, as detected by HRMS. The reaction in the
presence of H218O under 16O2 afforded [18O]4aa and 4aa (5:2)
in 90% combined yield (eq 1). In contrast, [18O]4aa′ and [18O2]
through an intramolecular anti aminopalladation would provide
vinylpalladium species I′,16 which may carry out a syn
carbopalladation onto external alkyne 3 to afford intermediate
II. Presumably because of electronic effects of the remote
conjugated carbonyl group, the regioselective nucleophilic attack
of H2O on the triple bond of II followed by reductive elimination
would release enol IV and a Pd(0) species, which could be
oxidized to the Pd(II) catalyst using O2 as the terminal oxidant.17
Subsequent one-electron oxidation18 of the enol moiety of IV
facilitated by Cu(II) species would afford benzyl radical VI,
probably via putative radical V in favor of aromatization.
Thereafter, VI would react with O2 to afford product 4,
presumably through Fenton-type fragmentation of intermediate
superoxo radicals.19 On the other hand, trapping of radical VI
with TEMPO would give adduct 7.
In conclusion, we have presented a palladium/copper-
catalyzed aerobic cross-coupling−ketooxygenation of ene-
diyne−carboxylic compounds and inner alkynes involving O2
activation at room temperature. Upon combination of the [4 + 2]
cross-cyclization with aromatization, this process simultaneously
enables the formation of benzene rings and the regioselective
incorporation of two oxygen atoms from atmospheric molecular
oxygen and H2O, respectively. This study discloses the feasibility
of using a coupling and decoupling strategy to override the self-
cyclization reactivity of enediyne skeletons for intermolecular
cyclization.
4aa (5:2) were obtained when 18O2 (1 atm) and H216O were
used (eq 2). These isotopic labeling results clearly indicated that
the oxygen atom in the hydroxyl group of 4aa is wholly from O2
while the ketonic oxygen comes from H2O, presumably via a
Wacker-type oxidation reaction of the alkyne.14 Incomplete
incorporation of the 18O label into the ketone group was likely
due to the competing attack of both adventitious water and in
situ-generated water arising from catalyst regeneration with O2 as
the terminal oxidant.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data for 4ea, 4ak, and 5aa (CIF),
experimental procedures, and characterization data for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Journal of the American Chemical Society
Communication
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AUTHOR INFORMATION
Corresponding Author
mcorg@zju.edu.cn
■
Author Contributions
†F.L. and Z.L. contributed equally.
Notes
The authors declare no competing financial interest.
(9) When PdCl2 (99.999% purity) was used, deoxy-4aa was isolated in
7% yield, which could not be transformed into 4aa after 12 h.
ACKNOWLEDGMENTS
■
Financial support was provided by the Zhejiang Provincial
Natural Science Foundation of China (R4110055), the National
Natural Science Foundation of China (21372196), and the
Program for New Century Excellent Talents in University.
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Chem. Soc. Rev. 2012, 41, 3381. (e) Stahl, S. S. Angew. Chem., Int. Ed.
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