Weakly Coordinating, Ketone-Directed Cp?Co(III)-Catalyzed C-H Allylation on Arenes and Indoles

Article abstract of DOI:10.1021/acs.orglett.7b03440

Weakly coordinating, ketone-directed, regioselective monoallylation of arenes and indoles is reported using a stable and cost-effective high-valent cobalt(III)-catalyst to access several important molecular building blocks. The allylation proceeds smoothl

Full text of DOI:10.1021/acs.orglett.7b03440

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Weakly Coordinating, Ketone-Directed Cp*Co(III)-Catalyzed C−H
Allylation on Arenes and Indoles
Md Raja Sk, Sourav Sekhar Bera, and Modhu Sudan Maji*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, W. B., India
S
* Supporting Information
ABSTRACT: Weakly coordinating, ketone-directed, regioselective
monoallylation of arenes and indoles is reported using a stable and
cost-effective high-valent cobalt(III)-catalyst to access several
important molecular building blocks. The allylation proceeds
smoothly with a variety of substrates in the presence of various
electron-rich and -deficient substituents. The method was applied to
the formal synthesis of an ancisheynine alkaloid, a highly conjugated
azatetracene, and isochroman. The mechanistic study reveals that the
allylation reaction follows a base-assisted intermolecular electrophilic
substitution pathway.
ortho-Allyl aromatic ketones are very useful synthons for many
biologically active natural products,1 and they are also used as
important intermediates for different structural conversions.2
They can also serve as key building blocks to access several π-
extensions directly.3 Method development to construct these
core moieties is an important synthetic task. In this context,
ketone-directed ortho C−H allylation of an sp2 bond would be the
most atom-4 and step-economical pathway.5 Previously, the
stitching of allyl groups was mostly dependent on Lewis acid
assisted Friedel−Crafts allylations6 and metal-catalyzed cross-
coupling reactions.7 But low regioselectivities, overallylation,
limited substrate scopes, and multistep reactions restrict their
applications, hence rendering direct C−H allylation a superior
method. To our knowledge, although weakly coordinating,
ketone-directed C−H functionalizations with different electro-
philes are reported,8,9 ketone-directed allylation is still not
encountered.
Cp*Rh(III)-catalytic systems. Later, from various reports,15,16
the unique reactivity and selectivity of Cp*Co(III) was
established over its Rh congener. Here, we have expanded this
newreactivityandexplored Cp*Co(III)asaneffectivecatalystfor
weakly coordinating, ketone-directed allylation reactions. To the
best of our knowledge, we report the first example of weakly
coordinating, ketone-directed stable, high valent cobalt catalyzed
C−H allylation of acetophenones, diaryl ketones, chalcones, and
indoles (Scheme 1). Our method offers complete regioselective
monoallylated products exclusively without any alkene isomer-
ization.
Scheme 1. Ketone-Directed C−H Allylation
However, oftenpreinstalled coordinating groups areobligatory
forthereactionconditions, butdetachingthesedirectinggroupsis
not operationally simple, which impedes the feasibility of
synthetic applications. In this context, weakly coordinating,
simple, and ubiquitous ketones can add value as a preferred
alternative.8,9 But, challenges with ketones include their weak
Lewis basicity, which decreases the coordination power to a metal
and, hence, diminishes the stability of the transition state.8a In
addition, its enolizable α proton is another issue for directed ortho
C−H activation.9e,10 Hence, we looked to develop an elegant
method of C−H allylation directed by ketones.
Over the past few years, several directed C−H allylations were
reported on arene and indole systems using different transition
metals such as Rh, Ru, Pd, Mn, Ni, Fe, etc.11−13 In particular,
Cp*Rh(III) catalysts had established their superiority due totheir
stability, high catalytic efficiency, and decent functional group
compatibility. But their low abundance and high price demand a
pragmatic substitute. In 2013, Kanai and Matsunaga,14 in their
pioneering work, reported Cp*Co(III) as an alternative to
We started our optimization taking 4-methoxy acetophenone
1a as a model substrate (Table 1). Though simple allyl alcohol
provided 3a in 44% yield,17 more facile β-oxygen elimination with
allyl carbonate 2 makes it a better allylating agent for our reaction.
After studying other allylating agents, allyl isobutyl carbonate 2a
proved to be the best one.17 After extensive screening of the
additives, 30 mol % of Cu(OAc)2·H2O and 30 mol % of AgSbF6
were found to be optimal (entry 2).17 Changing either solvents or
Received: November 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
in similar yield. Next, we introduced electron-donating functional
groups at the different positions of acetophenones. Methoxy-
substitition at the meta position offered allylated product 3g in
64% yield. Interestingly, here allylation at the more hindered
positionindicatesthatanelectroniceffectispredominantoverthe
steric effect. ortho-Methoxy acetophenone provided 3h in
excellent yield (92%). Other electron-rich acetophenones also
reacted well to provide 3i−3k in 48−93% yields. Considering the
applicability of this method, a gram scale synthesis of 3i was also
executed. An acetophenone derivative bearing a strongly
coordinating hydroxy functional group was also a suitable
substrate (3l, 63%). Other electron-donating functional groups
suchasSMe, NMe2, andphenylwerealsotoleratedinourreaction
conditions (3m−3o, 59−68% yields). Gratifyingly, substrates
having electron-withdrawing CO2Me and NO2 functional groups
at the para position also responded well (3p−3q, 40−52%).
Allylations on naphthalenes and anthracene were also achieved to
produce products 3r, 3s, and 3t. A 3-butene-2-ol derived isobutyl
carbonate also reacted well, and 3u was isolated in 58% yield as a
1.6:1 isomeric mixture. Similar results were recorded with other
substituted allylating agents (3v−3x, 44−66% yields).
Table 1. Optimization of Reaction Conditions
b
entry
1
solvent
catalyst
additive
none
yield [3a%]
DCE
DCE
Cp*Co(CO)I2
Cp*Co(CO)I2
trace
42
0
c
2
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2
3
4
5
tAmOH Cp*Co(CO)I2
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
[Cp*CoCl2]2
[Cp*RhCl2]2
64
78
75
28
35
42
40
18
0
c
6
Cu(OAc)2
7
8
9
NaOAc
Mn(OAc)2·4H2O
Cu(OAc)2
d
10
Cu(OAc)2
d
11
[Ru(p-cymene)Cl2]2 Cu(OAc)2
[Cp*IrCl2]2 Cu(OAc)2
d
12
a
1a (0.25 mmol), 2a (0.50 mmol), and 1.5 mL TFE were used.
Isolated yield. 1.0 equiv of additive. 5.0 mol % catalyst loading.
b
c
d
The scope of this reaction was further explored by varying the
directing group (Scheme 3). n-Butyl and tert-butyl ketones
silver additives did not produce any better result.17 Introduction
of the fluoro solvent TFE (2,2,2-trifluoro ethanol) significantly
improved the yield (entry 4). Gratifyingly, on changing the
additive to anhydrous copper acetate, we obtained our best yield
(78%, entry 5). Further optimization with other additives did not
provide any further improvement (entries 7−8). Incorporation of
other catalysts (entries 9−12) under our optimized conditions
(entry 5) did not produce encouraging results.
a b
,
Scheme 3. Scope of Diaryl Ketones and Chalcones
After having the optimized conditions, we first studied the
scope of this allylation and found a wide range of acetophenones
took part in the reaction (Scheme 2). Under our reaction
conditions, simple acetophenone provided 3b in 63% yield.
Halogens such as chloro, bromo, and iodo at the para position of
acetophenone also furnished 3c−3e in 55−60% yields. Likewise,
meta bromo-substituted acetophenone also delivered product 3f
a
1aa−1ah (0.25 mmol), 2a (0.50 mmol), and 1.5 mL TFE were used.
Isolated yield.
b
ab
,
Scheme 2. Scope of Allylation on Arenes
provided decent yields (4a−4b, 60−81% yields). Dinaphthyl
ketone and benzophenone having electron-withdrawing fluoro
and electron-donating methoxy groups reacted well (4c−4e, 56−
62% yields). Out of four possible C−H allylations, only
monoallylated products 4c−4e were obtained exclusively.
Chalcone is a common structural motif present in many natural
products and bioactive molecules.18a This attracted us to employ
the method to execute late-stage diversification of relatively
unexplored chalcones. Starting from respective chalcones,
products 4f and 4g were obtained in 64−68% yields. We were
also successful in achieving a late-stage allylation of a choleretic
drug, metochalcone,18b in excellent yield (4h, 85%).
C−H bond functionalization at the C2-position of indoles
primarily relies on the N-protected indoles.19 These protections
alwaysrestrict theirusebyimposing additionalstepsofprotection
and deprotection. In a few cases, deprotection requires very
rigorous reaction conditions. Interestingly, on applying our
method, C2-allylation was achieved for N-unprotected 3-
acetylindoles. The scope of the reaction was also excellent, as
several bromo, chloro, methoxy, and methyl substituted 3-
acylindoles 5a−5e underwent smooth reaction to provide 6a−6f
in moderate to high yields (Scheme 4).
a
b
1 (0.25 mmol), 2 (0.50 mmol), and 1.5 mL TFE were used. Isolated
yield.
B
DOI: 10.1021/acs.orglett.7b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 4. Scope of Allylation on Indoles
Scheme 6. Competition Experiment, Deuterium Labelling
Study, and Kinetic Isotopic Effect (KIE) Study
a
5 (0.25 mmol), 2 (0.50 mmol), and 1.5 mL of TFE were used.
Isolated yield.
b
The allylated products can easily be functionalized to various
important structural motifs. Dioxabicyclo core structure 7,
present in many biologically active molecules, has been
synthesized as an application (Scheme 5a).20a,b Because
Based on kinetic studies and previous reports, we propose that
the allylation reaction proceeds via a facile BIES-type C−H
cobaltationstep(Scheme7). InthepresenceofAgSbF6, theactive
Scheme 5. Applications of Allylated Products
Scheme 7. Proposed Mechanism
azaacenes are important materials for application in field effect
transistors (OFET), organic light-emitting diodes (OLED), solar
cells,andmemorydevices,20c azatetracene8wassynthesizedfrom
3t in a one-pot procedure (Scheme 5b). From the allylated
product 3i, biologically active isochroman 9, a precursor for anti-
HIV analogue, has been synthesized in 2 steps (Scheme 5c),
whereas Konnig et al. made this structural motif over 10 steps.20d
Furthermore, a formal synthesis of an ancisheynine alkaloid was
achieved by Wacker oxidation of 3i to furnish the advanced
intermediate 10 (Scheme 5d).20e
To understand the catalyst’s mode of action, we first performed
an intermolecular competition experiment and deuterium
labeling study. A competition experiment between electron-rich
and -deficient benzophenones revealed that electron-rich
acetophenone reacted more efficiently, with an almost 7.6:1
ratio (Scheme 6a). This result indicates that a base-assisted
intermolecular electrophilic substitution (BIES) mechanism is
operative.21a−c H/D Exchange with D2O and CD3OD did not
provide a significant amount of deuterium scrambling at the ortho
position of 1b (Scheme 6b), suggesting an irreversible C−H
activation step. A kinetic value of 3.0 from an intermolecular
competition experiment and kH/kD value of 3.8 from a parallel
experiment (Scheme 6c) provide information about the rate-
determining C−H activation step. This conclusion was further
corroborated by the kH/kD value (∼2.0) from an intramolecular
competition experiment (Scheme 6d).21d,e
catalyst A is formed. Then in situ generated A reacts with 1b to
assemble the cyclometalated species B by acetate assisted C−H
bond activation, and then in the presence of allylating agent, C is
generated from B. 1,2-Migratory insertion on the allyl double
bond of 2a and coordination with carbonate oxygen delivers
species D. Unlike the free alcohol, an additional coordination to
the cobalt center for the carbonates probably facilitates the
geometrically more favored β-oxygen elimination from D over β-
hydride elimination. Finally, D produces the desired allylated
product 3b along with the release of alcohol and CO2 as
byproducts.
In conclusion, weakly coordinating, simple, and ubiquitous
ketone-directed regioselective monoallylation of arenes, chal-
cones, and N-unprotected indoles has been achieved. The unique
reactivity of a cost-effective, high-valent cobalt catalyst over-
powered the other expensive transition metal catalysts in our
reaction conditions. The synthesized allylated products hold the
key in preparing various synthetic intermediates. We have
successfully prepared highly conjugated azatetracene as a unique
application. Finally, a mechanistic investigation suggests a BIES
mechanism is operative here. We believe this method will be
highly useful for the rapid generation of several functionalized
ketones.
C
DOI: 10.1021/acs.orglett.7b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b03440.
■
S
Experimental details, analytical data for all new com-
pounds, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: msm@chem.iitkgp.ernet.in.
ORCID
Modhu Sudan Maji: 0000-0001-9647-2683
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(14) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. Angew. Chem.,
Int. Ed. 2013, 52, 2207.
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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Org. Chem. 2017, 82, 420. (b) Wei, D.; Zhu, X.; Niu, J.-L.; Song, M.-P.
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DOI: 10.1021/acs.orglett.7b03440
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