Cobalt(III)-catalyzed annulation of esters and alkynes: A facile route to indenones

Article abstract of DOI:10.1039/c6cc02468h

An efficient protocol for the synthesis of indenones has been developed via the annulation of benzoic esters and internal alkynes by exploiting the cobalt catalyst.

Full text of DOI:10.1039/c6cc02468h

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Cobalt(III)-Catalyzed Annulation of Esters and Alkynes: A Facile
Route to Indenones
Wenlong Yu,a Wei Zhang,a Zhanxiang Liua and Yuhong Zhang*,a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient protocol for the synthesis of indenones has been
developed from the annulation of benzoic esters and internal
alkynes by exploiting cobalt catalyst.
Transition metal-catalyzed reactions involving C-H activation
have attracted considerable interest because these
transformations do not require the prefunctionalization of C-H
bonds and thus change the way that chemists approach the
complex molecules by allowing nontraditional retrosynthetic
disconnections.1 In particular, the C-M organometallic species
generated from the direct metalation of C-H bonds is capable
to undergo the nucleophilic addition reactions with polar
multiple bonds to construct C-C and C-X bonds.2 In the past
decades, significant advances have been achieved for the
additions of C-TM (TM = transition metal) upon aldehydes,3
ketones,4 imines,5 amides,6 and nitrile3e,7 by using Pd,5h,7 Rh,3f-
j,4c-d,5a-e,6a Ru,4e,6gRe,3c Ir,3a-b,4a-b Mn,3d-e and Co catalysts.3k,5f-g,6b-f
Ester is one of the most fundamental functional groups and
has long been the subject of investigation in preparative
organic chemistry.8 However, the addition of C-TM species via
C-H cleavage upon esters9 remains the challenge due to the
lower electrophilicity of esters.
Scheme1. Transition metal-catalyzed annulation of alkynes and
amides/esters via C-H activation.
pyrroloindolones by the use of low cost cobalt catalyst
(Scheme 1b).6b For the first time, they demonstrated the
unique nucleophilic reactivity of organocobalt species toward
the carbamoyl group. However, the addition of C-TM
nucleophilic species to esters remains the challenge. Li and co-
workers reported the rhodium-catalyzed annulations of N-tert-
butyl-α-arylnitrones with internal alkynes(Scheme 1c).5c In this
reaction, nitrone functions as an electrophile to capture the C-
TM nucleophilic species. Herein, we report the cobalt-
catalyzed annulation of benzoic esters with internal alkynes
(Scheme 1d). It is found that the alkenyl-Co species, which is
generated by insertion of alkyne into site-selective ortho C-H
activation intermediate of benzoic esters, processes high
nucleophilic activity to undergo the nucleophilic addition into
the esters. This transformation provides a complementary
route to important indenone motifs occurring prevalently in
bioactive molecules, ligand scaffolds, and material science.
We initially set out to optimize the reaction with methyl 4-
methoxybenzoate (1a) and diphenylacetylene (2a) as model
substrates and the results are summarized in Table 1.
[Cp*RhCl2]2 was firstly examined by using Cu(OAc)2 or KOAc as
additives in the presence of AgSbF6 (20 mol%) in DCE (1 mL) at
Indenones are useful skeletons in pharmaceutical and
material sciences, and much interest has focused on the
development of approaches to indenones.10 In 2012, Shi and
coworkers demonstrated the rhodium-catalyzed annulation of
benzimides and alkynes in the presence of 20 mol % Cu(OAc)2
to give indenones (Scheme 1a).6a In this transformation,
benzoic ester was sluggish to give the indenones in 42% GC
yield. More recently, Kanai, Matsunaga, and coworkers
reported the C2 site-selective annulation of N-
carbamoylindoles with alkynes for the synthesis of the
o
130 C for 24 h (entries 1-2). However, the expected indenone
3aa was only obtained in trace. To our delight, the desired
annulation product 3aa was obtained in 32% (entry 3) by
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Table 1. Optimization of the reaction conditionsa,b
O
O
Cp*Co(CO)I2 (10 mol%)
AgSbF6 (20 mol%), KOAc (20 mol%)
OMe
Ph
Ph
Ph
R1
+
R1
DCE (0.1 M), 130 oC, 24 h, N2
H
Ph
3aa-3sa
2a
1a
O
O
O
O
Ph
Ph
Ph
Ph
Yield(%)b
MeO
Entry
Catalyst (mol%)
Ag salt
Additive (mol%)
Ph
Ph
Ph
Ph
1
2c
[Cp*RhCl2]2 (5)
[Cp*RhCl2]2 (5)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
Cp*Co(CO)I2 (10)
none
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgNTf2
AgBF4
KOAc (20)
Cu(OAc)2 (20)
Cu(OAc)2 (20)
AgOAc(20)
NaOAc (20)
KOAc (20)
KOPiv (20)
AdCOOK (20)
MesCOOK(20)
PhCOOK(20)
KOAc (20)
KOAc (20)
KOAc (20)
KOAc (20)
KOAc (20)
KOAc (20)
none
NR
6
3ba
3aa
3ca
3da
, 60%
, 70%
O
, 71%
O
O
PhCOOR
R = Me, 74%
R = Et, 60%
R = i-Pr, 54%
3
32
Ph
Ph
Ph
4
25
t-Bu
Et2N
5
54
Ph
Ph
Ph
3ea, 64%
3fa, 68%
3ga, 25%
6
70
O
O
O
O
7
65
8
Trace
40
Ph
Ph
Ph
Ph
9
Ph
F
Cl
I
Ph
Ph
Ph
Ph
3ka
, 40%
10
11
12
13
14
15d
16
17
18
19
20
Trace
50
3ha
3ia
3ja
, 75%
, 50%
, 46%
MeO
O
NR
NR
NR
trace
NR
NR
NR
NR
NR
O
O
O
O
AgPF6
Ph
Ph
Ph
Ph
Ph
AgOTf
AgSbF6
none
F3C
Ph
Ph
, 48%
Ph
, 56%
OMe
Ph
, 21%
3la
3ma
3na
3oa
, 33%
O
OMe
AgSbF6
AgSbF6
AgSbF6
AgSbF6
O
O
O
KOAc (20)
KOAc (20)
KOAc (20)
Ph
Ph
Ph
Co(OAc)2 (10)
Cl
MeO
Ph
Ph
CoCl2 (10)
Ph
3ra, 54%
Ph
3pa, 60%
3qa, 62%
3sa, 65%
aReaction conditions: substrate 1a (0.1 mmol), diphenylacetylene2a (0.15
Scheme
indenonesa,baReaction
2
.
Scope of esters in the synthesis of substituted
conditions: substrate (0.1 mmol),
mmol), Cp*Co(CO)I2 (10 mol%), AgSbF6 (20 mol%), KOAc (20 mol%), DCE (1
c
mL), 130 °C, N2, 24 h. bIsolated yields. determined by GC-MS. dSolvent was
1
diphenylacetylene2a (0.15 mmol), Cp*Co(CO)I2 (10 mol%), AgSbF6 (20
mol%), KOAc (20 mol%), DCE (1 mL), 130 °C, N2, 24 h. bIsolated yields.
withdrawing groups such as halide (3ia-3ka) afforded the
corresponding indenones in relatively lower yields. The strong
electron-withdrawing substituents such as trifluoromethyl (3la
and ester (3ma) gave the desired indenones, but in low yields.
Both α- and β-naphthylesters participated in the reaction
TFE (1 mL).
employing 10 mol % Cp*Co(CO)I2, showing the unique catalytic
properties of cobalt catalyst over the rhodium catalysts in this
reaction. Since the carboxylate anion accelerates the C-H
metalation via a base-assisted concerted deprotonation,11 we
examined a series of carboxylate anions with different cation
partners, and KOAc provided the optimal results (entries 3-10).
It was found that Ag salts had a significant impact on the
reaction.12 No product was detected when AgOTf, AgBF4, and
AgPF6 were employed (entries 12-14). The use of AgNTf2 gave
the yield to 50% (entry 11), and AgSbF6 provided the highest
yield (entry 6). The addition of Lewis acids such as Cu(OTf)2,
Sc(OTf)3 and Zn(OTf)2 was not capable to improve the reaction
(See the ESI†).13 Among the different solvents tested, DCE
showed the best efficiency (See the ESI†). CF3CH2OH, a widely-
used solvent in Co(III)-catalyzed C-H activation reactions, gave
the desired product in trace (entry 15). No reaction was
observed in the absence of cobalt catalyst or silver/carboxylate
salts (entries 16-18). Simple cobalt salts were ineffective
(entries 19-20).
)
smoothly to furnish the annulation products
(3na-3oa)
efficiently. The esters bearing the methyl groups at the ortho
With the establishment of the optimized conditions, we first
evaluated the scope of esters with diphenylacetylene 2a as a
counterpart (Scheme 2). It was found that the alkoxy part of
esters had an obvious influence on the reaction efficiency. For
example, the methyl benzoate gave the indenone 3ba in 74%
yield, but ethyl/isopropyl benzoates afforded the product 3ba
in the diminished yields. A significant electronic effect of
arenes was observed in this annulations reaction. The esters
with various electron-donating groups such as methoxyl (3aa),
Scheme3. Scope of alkynes in the synthesis of multisubstituetedindenonesa,
baReaction conditions: substrate 1a (0.1 mmol), alkynes
(0.15 mmol),
2
Cp*Co(CO)I2 (10 mol%), AgSbF6 (20 mol%), KOAc (20 mol%), DCE (1 mL),
130 °C, N2, 24 h. bIsolated yields. C140°C. ddetermined by 1H-NMR.
and meta positions gave the desired products in the slightly
diminished yields, which might be attributed to the steric
hindrance (3da, and 3ea respectively). To our delight, this
alkyl (3ca, 3fa), N,N-diethylamino (3ga) and phenyl (3ha
)
showed the good reactivity to give the desired products in
moderate to good yields. However, the esters with electron-
annulation reaction was not limited to monosubstituted esters.
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The disubstituted esters (3pa-3sa) were smoothly transformed Furthermore, we measured the KIE in the intermolecular
into the corresponding products as well with acceptable yields competitive experiment and a large KIE value of 5 was
under the optimized condition. The acrylate esters and observed, suggesting that the initial C-H bond activation may
heteroarene esters failed to give the desired products on the be involved in the rate-limiting step. We also performed a
reaction conditions.
We next examined the scope of the alkynes in the coupling solution reacted for 1 h. A series of signals identified with
with methyl benzoate 1a (Scheme 3). A variety of symmetrical species
are detected as shown in Fig. 1 (See the ESI†),
MALDI-TOF analysis experiment using a mixture of the reaction
C
internal alkynes reacted smoothly with the ester and provided displaying the possible presence of alkenyl-Co(III) species in
the corresponding products in moderate to good yields (3bb- the catalytic cycle.
3bg). The halides such as bromide, chloride were well
Based on the our mechanistic studies and previous
tolerated in the reaction, which provided ample opportunity reports,6,9 a plausible catalytic cycle is proposed as shown in
for further derivatization of the obtained indenones (3be, 3bf). Scheme 5. Iodide abstraction from precatalyst Cp*Co(CO)I2 by
The unsymmetrical internal alkynes participated in the AgSbF6 and the subsequent ligand exchange generates the
reaction smoothly to give the two regioisomers 3bh and 3bh’ catalyst
in a 2:1 ratio (3bh/3bh’). Terminal alkynes and symmetrical of to yield the cobaltacycle
alkyl alkynes was inactive. alkynes into cobaltacycle forms the intermediate
was observed by MALDI-TOF as shown in Fig. 1 (See the ESI†).
The Co−C bond of intermediate undergoes migratory
insertion into the ester to form the intermediate , which
undergoes the elimination to give the product and produce the
Co(III) alkoxide species . The protonolysis of the metal
regenerates Co(III) catalyst to fulfil the catalytic
A
, which undergoes the ester-assisted C-H metalation
. The migratory insertion of
, which
B
B
C
O
Cp*Co(CO)I2 (10 mol%)
AgSbF6 (20 mol%), KOAc (20 mol%)
COOMe
C
Ph
(a)
(b)
DCE, 130 oC, 24 h
Ph
Ph
D
Ph
3ba not observed !
4a
E
O
Cp*Co(CO)I2 (10 mol%)
AgSbF6 (20 mol%), KOAc (20 mol%)
alkoxide
cycle.
E
+
H4/D4
Ph
1b
1b-
d
+
2a
5
DCE, 130 oC, N2, 30 min
Ph
5%; 3ba+3ba-d4
KH/KD = 5
Scheme 4.Mechanistic studies.
In order to gain insights into the mechanism, the preliminary
experiments were performed as shown in Scheme 4. The
GC−MS analysis was performed with the reaction solution and
the annulation product 3ba was observed, accompanied with
the formation of the hydroarylation product 4a 14
We
.
supposed that the olefin 4a might be the intermediate. To
probe this, the olefin 4a was synthesized and treated under
the standard reaction condition. To our surprise, no desired
product was observed. This result indicated that the olefin 4a
was not capable to cyclize under the reaction conditions.
4
x10
YWL6 0:G16 MS Raw
8
Scheme 5. Proposed mechanism.
In conclusion, we have developed a cobalt(III)-catalyzed
annulation of benzoate with internal alkynes for the facile
synthesis of indenones. The reaction shows good functional
group compatibility. Notably, cobalt catalyst displayed the
unique catalytic reactivity over the rhodium catalyst to give
the annulation products in high efficiency. Further studies to
explore the cobalt(III)-catalyzed C-H functionalizations are
ongoing in our lab.
6
4
2
This work was financially supported by the Natural Science
Foundation of China (No. 21272205, 21472165), National Basic
Research Program of China (No. 2011CB936003), and the
Program for Zhejiang Leading Team of S&T Innovation
(2011R50007).
0
562
564
566
568
570
572
574
576
m/z
Fig.
1
Matrix-assisted laser desorption ionization time-of-flight mass
(exact mass: 567.2304) obtained from
spectroscopy (MALDI-TOF-MS) of
C
Notes and references
the reaction of 1b and 2a under standard conditions for 1h.
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