Cp*Co(III)-Catalyzed C?H Alkenylation of Aromatic Ketones with Alkenes

Article abstract of DOI:10.1002/adsc.201801385

Cost-effective and air-stable high-valent cobalt(III)-catalyzed weakly coordinating, ketone-directed regioselective mono-alkenylation of arenes and heteroarenes with alkenes is demonstrated. Various electron-rich and electron-deficient arenes are tolerate

Full text of DOI:10.1002/adsc.201801385

Title: Cp*Co(III)-Catalyzed C–H Alkenylation of Aromatic Ketones with
Alkenes
Authors: Md Raja Sk, Sourav Sekhar Bera, and Modhu Maji
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cp*Co(III)-Catalyzed C–H Alkenylation of Aromatic Ketones
with Alkenes
Md Raja Sk, Sourav Sekhar Bera, and Modhu Sudan Maji*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, W. B., India.
*E-mail: msm@chem.iitkgp.ac.in
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Although the expensive noble metals, mainly
cobalt(III)-catalyzed weakly coordinating, ketone-directed Pd,[8b] Rh,[9a,j] Ru,[10c] and Ir,[11e] have been used for
Abstract. Cost-effective and air-stable high-valent
regioselective
mono-alkenylation
of
arenes
and
the ketone-directed oxidative C–H alkenylations, the
use of the earth-abundant, cheap, first-row transition
metals (3d-metals) is scarce.[16] Among the 3d-metals,
recently cobalt have been a subject of investigation as
an alternative to the noble metals. Particularly, after
the pioneering work by Kanai and Matsunaga, air
stable Cp*Co(III)-catalysis has gained much attention
for various C–H functionalizations.[17] However, the
low reactivity of base-metals (such as cobalt),
challenges with ketones (acetophenones), and the
requirement of harsh reaction conditions for the
oxidative alkenylation, are main obstacles for ketone-
directed C–H alkenylation with alkenes under cobalt
catalysis.[18] To overcome such challenges, we
heteroarenes with alkenes is demonstrated. Various
electron-rich and electron-deficient arenes are tolerated
under the reaction conditions, providing E-alkenylated
products exclusively. The tert-butyl-acrylates serve as an
acrylic acid surrogate to provide cinnamic acid derivatives
via direct C–H alkenylation. A two-step synthesis of a γ-
PPAR antagonist, the synthesis of indanone, and the
modification of divinylsulfone are reported as applications.
The mechanistic details suggest
intermolecular electrophilic substitution reaction pathway.
a
base-assisted
Keywords: cobalt(III)-catalyzed; ketone-directed; C–H
alkenylation; cinnamic acids; indanone and indenone
developed
a
method of ketone-directed C–H
alkenylation of aromatic ketones with alkenes under
Co(III)-catalysis (Scheme 1). The method offers
stereo- and regio-selective mono-alkenylation of
arenes and heteroarenes with moderate to good yields.
Alkenyl-arenes are a very important structural motif
present in several biologically active compounds,[1]
natural products,[2] organic materials,[3] and most
significantly, they are utilized as synthetic
intermediates to achieve different target molecules.[4]
Hence, different methods have been developed to
introduce the alkene functional unit into arenes or
heteroarenes. In this regard, due to the step- and
atom-economy[5] and regioselectivity, chelating
group-assisted, oxidative Heck-type[6,7] coupling has
become an attractive strategy to synthesize
alkenylated arenes using transition metals such as
Pd,[8] Rh,[9] Ru[10] and others.[11–12]
Scheme 1. Ketone-directed C–H alkenylation with alkene.
Due to the advantage of easy installation and facile
functional group interconversion, simple and
ubiquitous ketones have been used as directing
groups (DGs) in the recent years. Since the path-
breaking work by Murai et al. on Ru-catalyzed
ketone-directed C–H alkylation, ketones have been
When the 1,3-dioxole group is introduced at the 3
and 4 position of acetophenones or other DGs, the
substrates show the activation of the more hindered
C–H bond due to a very weak coordinating nature of
the 1,3-dioxole oxygen. Influenced by this
regioselectivity, we initiated our optimization taking
benzo[d][1,3]dioxol-5-yl)ethan-1-one 1a as a model
substrate (Table 1). When n-butyl acrylate 2a is used
as an electrophile, 1a provided the desired product 3a
in 27% yield (entry 1). The introduction of the
fluorinated 2,2,2-trifluoro ethanol solvent was
employed
as
DGs
for
several
the
C–H
functionalizations.[13,14]
However,
weak-
coordinating nature of ketones (weak Lewis basicity)
makes the cyclometallated species vulnerable in the
transition state, and the enolizable α-C–H bonds
make ketones, more specifically acetophenones,
challenging DGs.[15]
1
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
effective to improve the yield (entry 2). Other
solvents did not respond well (entries 3–4). This
observation and our earlier report[18] corroborate the
recent report on the assistance of fluorinated solvents
on
cobalt-catalyzed
C–H
bond
functionalizations.[17m,p,q] Varying the oxidant to
anhydrous copper acetate provided 3a in 43% yield
(entry 5). No significant improvement was observed
in the yield when other oxidants (entries 6–7) or
silver salts (entries 8–10) were incorporated.
Gratifyingly, we obtained the best yield by using 20
mol% of Cp*Co(CO)I2, 50 mol% of AgSbF6 and 90
mol% of Cu(OAc)2 (about 100% conversion of 1a,
entry 11). Yields decrease rapidly on reducing the
amount of Cu(OAc)2 oxidant (entries 12–14).
Table 1. Optimization of the reaction conditions.[a]
Scheme 2. Scope of aromatic ketones. 1a-1s (0.3 mmol),
2a or 2b (0.45 mmol) and 2 mL TFE were used. All yields
are isolated yields.
Entry Solvent Ag-salt Oxidant (mol%)
Yield
[3a%][b]
1
2
DCE
TFE
AgSbF6 Cu(OAc)2·H2O (100) 27
AgSbF6 Cu(OAc)2·H2O (100) 34
The readily oxidizable sulfur-containing SMe
group was also survived under this oxidative reaction
conditions (3j, 47%). Unlike 1a, the acetophenone
with 3,4-dimethoxy substitution yielded 3k in 46%
yield where the alkenylation occurs at the sterically
less hindered position indicates that here steric-effect
overcomes electronic-effect. Ketone bearing electron-
withdrawing CO2Me also responded well to give 3l in
41% yield. To avoid peri-interaction 1m produced
3m. The alkyl aryl ketone 1n, diaryl ketone 1o–1p,
chalcone 1q, heteroaryl ketone 1r, and biologically
relevant chromone 1s also provided mono-
alkenylated products 3n–3s in 44–61% yields.
3
4
5
6
7
8
9
10
11[c]
12[c]
13[c]
14[c]
C6H5Cl AgSbF6 Cu(OAc)2·H2O (100) 16
C6H5CF3 AgSbF6 Cu(OAc)2·H2O (100) 19
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
AgSbF6 Cu(OAc)2 (100)
AgSbF6 Co(OAc)2·4H2O (100) 17
AgSbF6 Mn(OAc)3·2H2O (100) 0
AgBF4 Cu(OAc)2 (100)
AgOTf Cu(OAc)2 (100)
AgOAc Cu(OAc)2 (100)
43
29
36
22
72
59
27
0
Cu(OAc)2 (90)
AgSbF6
AgSbF6 Cu(OAc)2 (60)
AgSbF6 Cu(OAc)2 (30)
AgSbF6 Cu(OAc)2 (0)
[a]1a (0.3 mmol), 2a (0.45 mmol) and 2.0 mL of TFE were
used. [b]Isolated yield. [c]20 mol% of Cp*Co(CO)I2 and 50
mol% of the Ag-salt were used.
With the optimized conditions in hand (Table 1,
entry 11), we first studied the scope of the reaction by
varying different aromatic ketones (Scheme 2). The
reactions were effective for both electron-rich and -
deficient substrates. Halogen groups, for examples,
fluoro, bromo, and iodo at the para position of
acetophenone provided 3b–3d in 47–51% yields.
Simple acetophenone furnished 3e in 59% yield. The
methyl, phenyl or methoxy groups at the para
position of the acetophenone also provided products
3f–3h in good yields (53–64%). The free hydroxyl
group of piceol 1i does not require any protection for
this alkenylation reaction (3i, 58%).
Scheme 3. Scope of alkenylating agents. 1a or 1h (0.3
mmol), 2b-2h (0.45 mmol) and 2 mL of TFE were used.
All yields are isolated yields.
Next, the scope of the alkenylation was explored
with various alkenes (Scheme 3). The methyl and
2
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
ethyl acrylates were equally effective to furnish the
corresponding alkenylated products (4a–4b, 71–76%
yields). When the menthyl ester of the acrylic acid
was used, the yield of the alkenylated product was
reduced significantly (4c, 42%). In case of the phenyl
acrylate, along with the desired product 4d (44%), a
by-product 4e was also isolated due to in situ trans-
esterification with the solvent alcohol. Vinyl
phosphonate was also a suitable alkenylating partner
to provide 4f in 40% yield. The desired alkenylation
with vinyl sulfones underwent smoothly to provide
the corresponding products 4g–4h in 53–75% yields.
Interestingly, by employing 2.0 equivalent of 1a with
respect to divinylsulfone, two-fold C–H alkenylation
was also achieved to obtain 4i in 56% yield. We
observed, except for 3a, the unreacted ketones 1 were
detected in the reaction mixture and not consumed
even after prolonged reaction time.
The importance of 2-alkenylated aromatic ketones
was realized through the synthesis of various
important structural motifs.[19] A base mediated
intramolecular 1,4-addition of acetophenone to
acrylates produced indanone 6 (Scheme 5a), a very
demanding motif of biological importance.[19a]
Divinyl sulfone can be modified by incorporating two
different arenes using our alkenylation protocol. The
best yield was obtained when electron-rich aromatic
ketone was incorporated in the second step (Scheme
5b). Most importantly, a two-step, gram-scale
synthesis of
a
γ-PPAR antagonist 11 was
accomplished starting from the ketone 9, whereas the
reported synthesis required six-steps from the simple
starting materials.[19b]
Scheme 4. One-pot synthesis of cinnamic acids. 1 (0.3
mmol), 2i (0.45 mmol) and 2 mL of TFE were used. All
yields are isolated yields. [a]50 mol% of AgSbF6 used.
The direct C–H alkenylation by employing acrylic
acid electrophile is scarce in the literature.[10a] Our
attempts to employ acrylic acid as alkene partner
provided only a trace amount of 5a. However, during
our optimization study by using 2i we observed that
tert-butyl group of the product ester undergoes in situ
hydrolysis to provide cinnamic acid derivative 5a,
opening a route to the direct synthesis of several
cinnamic acids derivatives 5a–5e in 41–58% yields
(Scheme 4). Our method eliminated the use of strong
bases for the hydrolysis of cinnamates in the presence
of base-sensitive functional groups like enolizable
ketones.
Scheme 6. Intermolecular competition experiment,
deuterium labelling study, Kinetic Isotopic Effect study,
and detection of cyclocobalted intermediates 13-14 by LC-
MS analysis.
To understand the nature and mechanism of the
present alkenylation reaction, we first performed a
competition experiment between electron-rich 1h and
-deficient 1l acetophenones (Scheme 6a). The more
efficiency of the electron-rich acetophenone (3h/3l =
11.4:1 ratio) suggest
a base (acetate)-assisted
intermolecular electrophilic substitution (BIES)-type
mechanism.[20a] H/D scrambling was observed at the
ortho position of [D5]-1e and 1e in the absence of
alkene, supporting a reversible C–H activation step
(Scheme 6b-6c). Furthermore, the low value of
Scheme 5. Applications of the alkenylated ketones.
Standard conditions: 20 mol% of Cp*Co(CO)I2, 50 mol%
of AgSbF6 and 90 mol% of Cu(OAc)2 were used in TFE
solvent.
3
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
kinetic isotope effect (KIE) from both the
intermolecular competition experiment (kH/kD = 1.9)
and parallel experiment (kH/kD = 1.3) indicate that the
C–H bond cleavage is probably not involved in the
rate-limiting step (Scheme 6d).[20b–c] To eliminate the
possibility of alkylation followed by subsequent
oxidation to the corresponding alkene,[17l] first we
prepared the alkylated product 12 from 3h by
hydrogenation, and then subjected to the standard
reaction conditions. The absence of any alkenylated
product 3h in the reaction mixture further support the
β-hydride elimination pathway (Scheme 6e).
method will lead to the access of diverse
functionalized aromatic and hetero-aromatic ketones,
and this work could be an important finding to
develop such oxidative reactions with basemetals.
Experimental Section
General procedure for the alkenylation: The
aryl/heteroaryl ketone 1 (0.3 mmol, 1.0 equiv) was taken in
a 15.0 mL screw capped sealed tube and 2.0 mL of 2,2,2-
trifluro ethanol was added. Then catalyst Cp*Co(CO)I2
(28.5 mg, 0.06 mmol, 20.0 mol%), AgSbF6 (51.5 mg,
0.150 mmol, 50.0 mol%) and Cu(OAc)2 (49 mg, 0.27
mmol, 90.0 mol%) were added successively to the reaction
mixture and was stirred for 5 min at the room temperature.
After that, the alkenylating agent 2 (0.45 mmol, 1.5 equiv)
was added to the reaction mixture and the resultant
reaction mixture was allowed to stir at 90 °C for 24 h.
After completion of the reaction as indicated by TLC, the
crude product was directly purified by silica gel column
chromatography using petroleum ether/ ethylacetate eluent.
Based on previous reports[9a,10c,12] and kinetic
studies, we propose a plausible mechanism for the
alkenylation in Scheme 7. The catalyst Cp*Co(CO)I2
with AgSbF6 generates an active catalyst A that
forms a five-membered cyclometallated species B
reversibly with 1e. The analogous to the intermediate
B, a cyclometallated species 13 formed from 1a, was
observed by LC-MS analysis of the crude reaction
mixture (Scheme 6f).[21] Then, the coordination of 2a
to the cobalt center of B followed by a migratory
insertion of alkene delivers the intermediate D. The
acetate decordinated cationic species 14 was also
detected by LC-MS analysis of the crude reaction
mixture showing the incorporation of acrylate 2a to
the cobaltacycle 13 (Scheme 6g).[21] Finally, product
3e is formed by β-hydride elimination from
intermediate E and a reduced cobalt species
Cp*Co(I)Ln F is also generated which in the presence
of Cu(OAc)2 oxidizes to the active cobalt-catalyst A
for the next catalytic cycle.
General procedure for the one-pot cinnamic acid
derivative synthesis: The aryl/heteroaryl ketone 1 (0.3
mmol, 1.0 equiv) was taken in a 15.0 mL screw capped
sealed tube and 2.0 mL of 2,2,2-trifluro ethanol was added.
Then catalyst Cp*Co(CO)I2 (28.5 mg, 0.06 mmol, 20.0
mol%), AgSbF6 (41.2 mg, 0.150 mmol, 40.0 mol%) and
Cu(OAc)2 (49 mg, 0.27 mmol, 90.0 mol%) were added
successively to the reaction mixture and was stirred for 5
min at the room temperature. After that, the alkenylating
agent 2i (0.45 mmol, 1.5 equiv) was added to the reaction
mixture and the resultant reaction mixture was allowed to
stir at 90 °C for 24 h. After completion of the reaction as
indicated by TLC, the crude product was directly purified
by silica gel column chromatography by using petroleum
ether/ ethylacetate eluent.
Acknowledgements
M.S.M. gratefully acknowledges SERB, Department of Science
and Technology, New Delhi, India (Sanction No. EMR/2015/
000994) and IIT Kharagpur (ISIRD grant) for funding. M.R.S.
thanks IIT Kharagpur and S.S.B. thanks CSIR India for
fellowship.
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
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[21] See Supporting Information for LC-MS spectra.
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10.1002/adsc.201801385
Advanced Synthesis & Catalysis
UPDATE
Cp*Co(III)-Catalyzed C–H Alkenylation of
Aromatic Ketones with Alkenes
Adv. Synth. Catal. Year, Volume, Page – Page
Md Raja Sk, Sourav Sekhar Bera, and Modhu
Sudan Maji*
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