Sulfoxonium-Ylide-Directed C-H Activation and Tandem (4 + 1) Annulation

Article abstract of DOI:10.1021/acs.orglett.0c00451

Rh(III)-catalyzed C-H activation and cyclization of sulfoxonium ylide with acrylates leads to an efficient synthesis of indanone derivatives. The reaction proceeds under mild and external metal-oxidant-free conditions. The sulfoxonium ylide acts as a trac

Full text of DOI:10.1021/acs.orglett.0c00451

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Letter
Sulfoxonium-Ylide-Directed C−H Activation and Tandem (4 + 1)
Annulation
Vinayak Hanchate,† Ravi Devarajappa,† and Kandikere Ramaiah Prabhu*
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ABSTRACT: Rh(III)-catalyzed C−H activation and cyclization of
sulfoxonium ylide with acrylates leads to an efficient synthesis of
indanone derivatives. The reaction proceeds under mild and
external metal-oxidant-free conditions. The sulfoxonium ylide acts
as a traceless directing group as well as an internal oxidant. (4 + 1)
Annulation after C−H activation leads to the formation of a
carbocyclic ring, and the byproduct obtained is DMSO, which can
be easily separated.
ransition-metal-catalyzed C−H activation has emerged as
Ta powerful alternative to the traditional metal-catalyzed
cross-coupling reactions1 because these reactions offer the
advantages of using unfunctionalized starting material,
reducing the halogen waste, being step-economical.2 In recent
advancements in the transition-metal-catalyzed C−H activa-
tion strategy, the construction of aryl-fused heterocyclic and
carbocyclic rings by tandem C−H activation and cyclization
has been achieved using noninnocent directing groups.3,4
There are several reports on the construction of heterocyclic
rings using heteroatom-based directing groups.3 Reports on the
construction of carbocyclic rings are scanty.4 This is due to the
difficulties associated with the in situ generation of carbon-
based directing groups. Nevertheless, enolate- and carbenoid-
based directing groups are reported.4
core. Paucifloral F is an antiosteoporosis agent,6a donepezil is
an approved drug molecule for treating Alzheimer’s disease,6b
indanocine is an anticancer agent,6c piperazine-derived
indanone derivative has shown serotonin receptor antagonist
activity,6d aquilide A is a natural product isolated from Pteris
ensiformis Burm.,6e and dilemmaone A is natural product
isolated from Ectyonanchora flabellata.6f
There is a plethora of methods reported for synthesizing
indanone derivatives.7 Most of these methods require either
prefunctionalized starting materials or a multistep synthesis. A
transition-metal-catalyzed C−H activation and tandem (4 + 1)
cyclization strategy can lead to a straightforward and step-
economical method for synthesizing indanone derivatives.
Greaney’s group reported the synthesis of 2,3-disubstituted
indanone derivatives employing the Ru(II)-catalyzed C−H
activation of phenyl acetophenone and coupling to olefins
(Scheme 1a).8 In this study, they disclosed cascade C−H
activation, oxidative olefination, and radical cyclization in the
presence of a stoichiometric amount of Cu(II) salt. Li’s group
reported the synthesis of indanone derivatives using phenacyl
phosphonium salt and acrylates (Scheme 1b),9 employing a
carbenoid-based directing group for the construction of a
carbocyclic ring under oxidative conditions. These oxidative
annulation reactions require a stoichiometric amount of
metaloxidants. In the latter report, triphenylphosphine oxide
is the byproduct, which is difficult to separate from the reaction
mixture. Recently, there have been many reports on
sulfoxonium ylides, which are used as carbenoid-based
Indanones are an integral part of many natural products and
pharmaceutically active molecules and are essential synthons
for a few target molecules.5,6 Figure 1 depicts a few natural
products and biologically active molecules having an indanone
Received: February 4, 2020
Figure 1. Selected examples of natural products and medicinally
important indanone derivatives
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A

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a
Scheme 1. Previous Reports
Table 1. Optimization of Reaction Conditions
b
entry
solvent (2 mL)
additive (1 equiv)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
DCE
EtOAc
DME
TFE
THF
CH3CN
toluene
MeOH
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
none
AcOH
AcOH
AcOH
AcOH
ClCH2COOH
AdCOOH
PivOH
NaOAc
40
20
22
68
18
18
12
18
55
76
68
47
27
c
directing groups under redox neutral conditions for the
construction of carbocyclic rings.10 We reported the synthesis
of 1-naphthols using sulfoxonium ylide as a directing group
with 4-hydroxy-2-alkynoate as a coupling partner.11 In light of
the literature precedence10 and our work,11 we envisioned a
C−H activation of sulfoxonium ylide followed by the insertion
of olefin, which may lead to (4 + 1) annulation to form C3-
substituted indanone derivatives (Scheme 1c). Because
sulfoxonium ylide acts as a traceless directing group as well
as internal oxidant, the reaction does not need an external
metal oxidant, and the byproduct obtained is DMSO, which
can be easily separated. With this hypothesis, we carried out a
reaction of sulfoxonium ylide 1a (0.2 mmol) with ethyl
acrylate 2a (0.3 mmol) in the presence of [Cp*RhCl2]2 (5 mol
%), AgSbF6 (20 mol %), and AcOH (1 equiv) in DCE (2 mL)
at 80 °C for 16 h in an argon atmosphere. To our delight, we
observed the formation of indanone derivative 3aa in 40%
yield (Table 1, entry 1).
With this initial lead, we began the optimization of the
reaction conditions by solvent screening. TFE proved to be a
better solvent, furnishing the product 3aa in 68% yield (entries
2−8). Temperature screening studies (entries 9−11) revealed
that 100 °C (entry 10) is an optimal temperature for the
reaction to obtain the product 3aa in 76% yield. The yield of
the product decreased in the absence of additive AcOH and
activator AgSbF6 (entries 12 and 13). Among the activators
screened (entries 14 and 15), AgBF4 is better suited for the
reaction, furnishing the product 3aa in 83% yield(entry 15).
The reaction carried out in an air atmosphere resulted in
decreasing the yield of 3aa to 70% (entry 16). Screening of
other acid and base additives did not increase the yield of 3aa
(entries 17−20). Further attempts to improve the yield
resulted in the decomposition of the starting material 1a.
Further investigation for exploring the scope of the reaction
was carried out using the optimal conditions, as in entry 15.
Next, we examined the scope of the reaction with different
sulfoxonium ylide derivatives (Scheme 2). Unsubstituted aryl
sulfoxonium ylides and alkyl substitution at various positions
underwent a smooth reaction under optimal conditions,
furnishing the corresponding indanone derivatives 3aa−da in
77−84% yield. An electron-donating methoxy group at various
positions did not influence the reaction, and the reaction
provided the corresponding annulated products 3ea−ga in
68−92% yield. Sulfoxonium ylide with a weak electron-
withdrawing halogen substitution on the aryl ring afforded
the products 3ha−ia in 67−69% yield. However, the dihalide-
substituted aryl sulfoxonium ylide afforded the corresponding
annulated product 3ja in 30% yield. The substitution of strong
d
e
f
g
72
83 (80)
70
h
63
67
57
60
a
Reaction conditions: 1a (0.2 mmol) and ethyl acrylate 2a (0.3
mmol), [Cp*RhCl2]2 (5 mol %), AgSbF6, entries 1−12; AgBF4,
entries 15−20 (20 mol %), additive (1 equiv), in solvent (2 mL) at 80
b
°C for 16 h under an argon atmosphere. 1H NMR yield calculated
using trimethoxybenzene as an internal standard; yield in parentheses
c
d
is the isolated yield. Reaction at 60 °C. Reaction at 100 °C.
e
f
g
Reaction at 120 °C. Reaction in the absence of AgSbF6. AgNTf2
h
used as activator. Reaction in an air atmosphere.
electron-withdrawing groups such as nitro and cyano groups
on the para position of aryl sulfoxonium ylide also afforded low
yields of the products 3ka and 3la (39 and 34%, respectively).
The reaction condition was tolerant to furan and thiophene-
derived sulfoxonium ylides, furnishing products 3ma, 3na, and
3oa in 87, 40, and 54% yield, respectively.
Next, we extended the scope of the reaction with a variety of
acrylate, acrylamide, and styrene derivatives (Scheme 3).
Methyl and n-butyl acrylates reacted well with sulfoxonium
ylide 1a to furnish products 4aa and 4ab in 75 and 76% yield,
respectively. Trichloroethyl and ethoxyethyl acrylates under-
went a smooth reaction, resulting in the formation of products
4ac and 4ad in 58 and 72% yield. The reactions with phenyl,
naphthyl, and benzyl acrylates also proceeded well, providing
the products 4ae, 4af, and 4ag in 59, 55, and 69% yield,
respectively. N,N-Dimethyl- and piperidine-derived acryla-
mides are also compatible under reaction conditions, furnish-
ing the products 4ah and 4ai in 43 and 45% yield, respectively.
Unactivated olefins such as styrene derivatives were also
subjected to the optimal reaction conditions. The electron-
withdrawing acetoxy-group-substituted styrene derivative fur-
nished the corresponding product 4aj in 66% yield. The
electron-donating methyl group offered product 4ak in 29%
yield.
The synthetic utility of the reaction has been demonstrated
by performing a gram-scale reaction between 1a and 2a to
obtain 3aa in 80% yield (Scheme 4a). Furthermore, the base-
B
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a b
,
Scheme 2. Scope of Sulfoxonium Ylide Derivatives
Scheme 4. Scale-up and Synthetic Utility of the Reaction
indanone derivative 3aa was also subjected to the aldol
condensation reaction to obtain 6aa in 85% yield (Scheme 4b).
To understand the reaction mechanism, a few control
experiments were performed (Scheme 5). A deuterium labeling
Scheme 5. Control Experiments
a
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), [Cp*RhCl2]2 (5
mol %), AgBF4 (20 mol %), and AcOH (1.0 equiv) in TFE (2 mL) at
100 °C for 16 h under an argon atmosphere. Isolated yields.
b
a b
,
Scheme 3. Scope of Olefin Derivatives
experiment of 1a in the presence of CD3OD (10 equiv) at 60
°C led to the formation of dueterio-1a in 75% yield (Scheme
5a). In this labeling experiment, 46% of deuterium has been
incorporated at the ortho positions of the aryl moiety, and
surprisingly, 98% of deuteration was observed on the α-
position of the carbonyl group. This experiment suggests that
the C−H activation is a reversible step, and the α-carbon to
the carbonyl group may be involved in the metallacycle during
C−H activation. The kinetic isotopic effect (KIE) is calculated
by parallel reactions of 1a and pentadeuterated sulfoxonium
ylide 1a-d5 with ethyl acrylate under optimal conditions for 5
min, and the KIE was found to be 1.38 (Scheme 5b). This
experiment suggests that the C−H activation step may not be
rate-determining in the catalytic cycle. A reaction carried out
between 1a and the trideuterated derivative of acrylate 2f-d3
(97% deuterated; see the SI for more details) under the
optimal reaction conditions furnished the product 4af-d3 (95%
deuterated) in 52% yield (Scheme 5c). The deuterium shift
observed in 4af-d3 from the β-position to the α-position of
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), [Cp*RhCl2]2 (5
mol %), AgBF4 (20 mol %), and AcOH (1.0 equiv) in TFE (2 mL) at
100 °C for 16 h under an argon atmosphere. Isolated yields.
b
catalyzed hydrolysis of the ester group of the product 3aa
furnished the corresponding acid 5aa in 97% yield, which can
be further functionalized to other functional groups. The
C
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ester carbonyl strongly suggests a reversible β-hydride
elimination followed by 1,3-insertion, leading to the coordina-
tion slip of the metal from the α-position to the β-position of
ester carbonyl. Parallel reactions of trideuterated derivative of
acrylate 2f-d3 and undeuterated acrylate 2f with sulfoxonium
ylide 1a were carried out under standard reaction conditions
for 5 min, and the KIE was found to be 2.12 (Scheme 5d).
This experiment reveals that the reversible β-hydride
elimination and 1,3-insertion may be the rate-determining
step in the catalytic cycle.
membered rhodacycle B. Hence, the reaction preferably
proceeds via path A compared with path B.
In conclusion, we have developed an efficient synthesis of
indanone derivatives by Rh(III)-catalyzed C−H activation and
cyclization of sulfoxonium ylide with acrylates and acrylamides.
Tandem (4 + 1) annulation after C−H activation led to the
formation of a carbocyclic ring. The reaction was carried out
under mild and external metal-oxidant free conditions using
sulfoxonium ylides as a traceless directing group as well as an
internal oxidant, and the byproduct obtained is DMSO can be
easily separated.
On the basis of literature precedence,10−12 and control
experiments, we propose a plausible mechanism (Scheme 6).
ASSOCIATED CONTENT
* Supporting Information
■
Scheme 6. Plausible Mechanism
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00451.
Detailed experimental procedures, characterization of
new compounds, and copies of NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Kandikere Ramaiah Prabhu − Department of Organic
Chemistry, Indian Institute of Science, Bangalore 560012,
Karnataka, India; orcid.org/0000-0002-8342-1534;
Email: prabhu@iisc.ac.in
Authors
Vinayak Hanchate − Department of Organic Chemistry, Indian
Institute of Science, Bangalore 560012, Karnataka, India;
orcid.org/0000-0003-0786-2060
Ravi Devarajappa − Department of Organic Chemistry, Indian
Institute of Science, Bangalore 560012, Karnataka, India;
orcid.org/0000-0002-9351-4849
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00451
Author Contributions
†V.H. and R.D. contributed equally.
Notes
In the presence of Rh(III)-catalyst, silver activator, and acetic
acid, 1a undergoes C−H activation by concerted metalation
and deprotonation to form a five-membered rhodacycle A.
Alkene insertion to A takes place to form a seven-membered
rhodacycle B. The β-hydride elimination of B leads to the
formation of alkenylated intermediate C. After alkenylation, (4
+ 1) annulation can take place in two possible ways. In path A,
the 1,3-insertion of hydride leads to the formation of a six-
membered rhodacycle D.12 The internal oxidation of Rh(III)
to Rh(V) by the elimination of DMSO13 from rhodacycle D
results in the formation of intermediate E, and a reductive
elimination leads to the formation of intermediate F. In path B,
the intermediate C undergoes internal oxidation by the
elimination of DMSO to form the intermediate G, which
undergoes 1,3-insertion, furnishing the intermediate H,
followed by intramolecular 1,4-addition to form the
intermediate F.10d Later, proto-demetalation of F in the
presence of AcOH furnishes the product 3aa. From the control
experiment (Scheme 5c), it is evident that a reversible β-
hydride elimination and 1,3-insertion leads to a coordination
slip of metal to form six-membered rhodacycle D from seven-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by SERB (EMR/2016/006358),
New-Delhi, Indian Institute of Science, R L Fine Chem,
Bangalore, and SYNMR Chemicals Pvt. Ltd., Bangalore. We
thank the NMR Research Centre, Indian Institute of Science
for recording NMR spectra. V.H. thanks CSIR, New-Delhi for
a fellowship.
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