Rh-Catalyzed Annulation of Benzoic Acids, Formaldehyde, and Malonates via ortho-Hydroarylation to Indanones

Article abstract of DOI:10.1021/acs.orglett.0c02986

A three-component reaction from readily available low-cost materials of benzoic acids, formaldehyde, and malonates for the preparation of indanones by rhodium catalysis is reported. The annulation is initiated by an ortho-hydroarylation of benzoic acids, and a Lewis acid is not required. The solvent has a significant influence to the reaction, and 2-substituted or nonsubstituted indanones are obtained by the change of solvent.

Full text of DOI:10.1021/acs.orglett.0c02986

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Letter
Rh-Catalyzed Annulation of Benzoic Acids, Formaldehyde, and
Malonates via ortho-Hydroarylation to Indanones
Shuling Yu, Ningning Lv, Chao Hong, Zhanxiang Liu,* and Yuhong Zhang*
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ABSTRACT: A three-component reaction from readily available low-cost materials of benzoic acids, formaldehyde, and malonates
for the preparation of indanones by rhodium catalysis is reported. The annulation is initiated by an ortho-hydroarylation of benzoic
acids, and a Lewis acid is not required. The solvent has a significant influence to the reaction, and 2-substituted or nonsubstituted
indanones are obtained by the change of solvent.
elective functionalization of C−H bonds assisted by
carboxylate group through transition-metal-catalyzed C−
involving the reaction of benzoic acids with α,β-unsaturated
keones by Shi and Gooßen (Scheme 1).11a,17 In Gooßen’s
work, a carboxylate directed hydroarylation carried out by Rh/
In catalysis, and a Claisen-type condensation, occurs directly
from the free carboxylic acid rather than its corresponding
ester (Scheme 1, eq b).17b They further improved the yields of
simple alkenyl methyl ketones by a bimetallic Ir/In system, and
the substrate scope was successfully extended to 3-aryl
alkenones (Scheme 1, eq c).11a Acrylates are important
coupling partners and have been extensively used in organic
synthesis and transition-metal-catalyzed reactions. However,
the effort to apply acrylates in both Rh/In and Ir/In system
failed. Herein, we report a new rhodium catalytic system for
the preparation of indanones from benzoic acids and
methylene malonates without the use of Lewis acids. The
phthalide derivatives, which are produced in the reactions of
benzoic acids with acrylates by Rh18 and Ru19 catalysis, are not
detected. Significant solvent impact on the reaction is
observed. 2-Substituted indanones are obtained by the use of
dichloroethane (DCE), and nonsubstitued indanones are
produced in hexafluoroisopropanol (HFIP). Importantly, the
catalytic system can be exploited in a multicomponent reaction
from readily available benzoic acids, formaldehyde, and
malonates, which significantly simplifies the synthetic route
and affords this annulation a practical advantage.
S
H activation continues to attract broad interest.1 The
utilization of carboxylate as directing group possesses several
synthetically attractive features: (1) a wide range of substrates,
(2) easy removal by decarboxylation as traceless groups after
installation of the desired functionality, and (3) further
construction of new carbon-heteroatom or carbon−carbon
bonds as nucleophile or nucleophilic center. In this context, a
variety of coupling partners, including alkenes,2 alkynes,3 α,β-
unsaturated ketones,4 acrylates,5 arylhalide,6 epoxide,7 organo-
boron,8 sulfonyl azide,9 and so on,10 have been identified to be
powerful for building a huge useful molecules. Despite the
encouraging development and progress, the application of
multicomponent reaction, which is an attractive strategy for
rapidly building complexity, is extremely underdeveloped for
the transformation of C−H activation by the use of carboxylate
as directing group.11
Indanones are valuable organic molecules serving as versatile
building blocks in organic synthesis and occur in pharmaceut-
icals and biologically active analogues widely.12 Therefore, new
approaches to prepare indanones in an easier manner would be
of considerable significance. Traditionally, indanones are
synthesized by intramolecular Friedel−Crafts acylation13 and
Nazarov cyclization,14 which often suffer from substrate
limitations and harsh reaction conditions. Transition metal
catalyst has expanded the preparation of indanones, leading to
the establishment of a range of useful cross-coupling
methods,15 but with preactivated substrates. In recent years,
direct C−H activation has emerged as an attractive alternative
in the synthesis of indanones, and several chelation groups
such as amides and carbenoids have been employed in these
methods.16 A recent breakthrough has been developed
Received: September 7, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02986
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Indanones from Carboxylic Acids
Table 1. Optimization of Reaction Conditions
and α,β-Unsaturated Compounds
b
b
entry
catalyst
base
solvent
3aa (%)
4aa (%)
1
2
3
4
5
6
7
8
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[Ru(cy)Cl2]2
[Rh(COD)Cl2]2
Pd(OAc)2
Cp*Co(CO)I2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
Na2CO3
CsOAc
KOAc
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
THF
dioxane
toluene
TFE
trace
42
58
78
49
42
nd
nd
nd
37
56
69
nd
nd
nd
nd
<5
10
<5
8
nd
nd
nd
nd
nd
<5
<5
49
70
81
NaOAc
NaOPiv
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
9
10
11
12
13
14
HFIP
HFIP
c
15
a
Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol), catalyst (5 mol
b
%), base (1.0 equiv), solvent (0.4 mL), 140 °C, 12 h, N2. Isolated
yield. 150 °C, 18 h. nd = not detected. cy = p-cymene.
c
We initiated to develop a catalytic system for extending the
coupling partner from α,β-unsaturated ketones to acrylates for
the preparation of indanones because acrylates are also typical
electron-deficient alkene and even more widely used in the
coupling reactions than α,β-unsaturated ketones. We tested the
reaction of benzoic acid 1a and ethyl acrylate by the use of Rh/
In catalyst,17b but it failed. When we removed Lewis acid
In(OTf)3, which plays key role in the reaction of benzoic acids
with α,β-unsaturated ketones in Claisen and retro-Claisen
condensation steps,17b exclusively phthalide derivative was
formed.18 We reasoned that an electron-withdrawing group
would facilitate the formation of Claisen product through the
stabilization of Rh enolate. Therefore, methylene malonate 2a
was studied as the substrate. In this case, trace desired
indanone product 3aa was observed (Table 1, entry 1). To our
delight, indanone 3aa was isolated in 42% yield by replacing
Na2CO3 with CsOAc (Table 1, entry 2). Further screening of
additives, such as KOAc, NaOAc, and NaOPiv, indicated that
NaOAc was the best choice to give indanone 3aa in 78% yield
(Table 1, entries 3−5). The ruthenium catalyst showed poor
reactivity (Table 1, entry 6). Other tested catalysts were almost
ineffective (Table 1, entries 7−9). The solvent has a significant
impact on the reactivity and selectivity. THF, dioxane, and
toluene afforded 3aa, but in lower yields (Table 1, entries 10−
12). Dichloroethane (DCE) provided superior reaction
outcome (Table 1, entry 4). Notably, the nonsubstituted
indanone 4aa was isolated in 49% yield and 3aa was not
observed when trifluoroethanol (TFE) was used as solvent
(Table 1, entry 13). The improved yield was obtained by the
use of hexafluoroisopropanol (HFIP) (Table 1, entry 14) and
the elevation of temperature gave optimal result to produce
4aa in 81% yield (Table 1, entry 15). Thus, two optimal
reaction conditions are obtained: condition A in DCE for 2-
substitued indanones (3aa) and condition B in HFIP for
nonsubstitued indanones (4aa).
Scheme 2. Coupling of Carboxylic Acids with Methylene
Malonates
a b
,
a
Reaction conditions; A: 1 (0.3 mmol), 2 (0.6 mmol), [RhCp*Cl2]2
(5 mol %), NaOAc (0.3 mmol), DCE (0.4 mL), 140 °C, 12 h, N2.
Condition B: [RhCp*Cl2]2 (5 mol %), NaOAc (0.3 mmol), HFIP
b
(0.4 mL), 150 °C, 18 h, N2. Isolated yield.
reactivity and gave slightly higher yields than its electron-
deficient analogs to give 2-substitued indanones under the
reaction condition A (3aa−3ad). Importantly, 2-cyanoacry-
lates exhibited good reactivity and delivered the corresponding
2-substitued indanones under the reaction condition A in good
yields (3ae−3ai). Furthermore, nonsubstitued indanones were
obtained under condition B successfully in HFIP (4aa−4ad).
Dimethyl ethylidenemalonate is inactive.
After we demonstrated the success of annulation of benzoic
acids with methylene malonates under condition A and
condition B, we began to investigate the reactivity of three
component reaction of benzoic acids with malonates as well as
Our catalytic system was found to be powerful enough to
produce indanones by the reaction of benzoic acids with
methylene malonates as shown in Scheme 2. It was found that
benzoic acids bearing electron-donating group showed better
B
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formaldehyde under the optimal reaction conditions since
methylene malonates are tedious to be prepared and
unstable.20 Obviously, malonates and formaldehyde are readily
available chemicals, and a three-component reaction in one pot
would enhance the practicability of the method significantly.
To our delight, the three-component strategy is successfully
performed in the optimal reaction conditions as shown in
Scheme 3, and a variety of 2-substituted indanones were
Scheme 4. Multicomponent Synthesis of Nonsubstituted
Indanones
a b c
, ,
Scheme 3. Multicomponent Synthesis of 2-Substituted
a b
,
Indanones
a
Condition B: 1 (0.3 mmol), 5a (0.6 mmol), 6 (0.6 mmol),
[RhCp*Cl2]2 (5 mol %), NaOAc (0.3 mmol), HFIP (0.4 mL), 150
b
c
°C, 18 h, N2. Isolated yield. HFIP (1.0 mL), 160 °C, 12 h.
phenyl, methoxy, phenoxy, amide, and fluoro groups, were
tolerated. With respect to the 2,3- and 2,4-disubstituted
benzoic acids, the reaction proceeded equally well to give the
nonsubstitued indanones in moderate to good yields (4ab, 4ac,
8ah−8at). The tolerance of chloro and bromo groups allows
the as-prepared indanones for further synthetic elaborations.
The substrates of 2,5-disubstituted benzoic acids gave reduced
yields (8au−8aw), which might be attributed to the steric
hindrance. The use of benzoic acid itself shows poor
regioselectivity to give three indanone isomers (8ax, 8ax′
and 8ax′′). Naphthoic acid and 6-methyl naphthioc acid
exhibited good reactivity to afford the nonsubstitued indanone
products (8ay−8az), which is the key structure motif of
agricultural fungicides.21 Tetrahydronaphthalic acid showed
good reactivity to deliver the product of cyclopenta-
naphthalenone 8za. The indanone 8zb was obtained in 87%
yield from benzodioxine carboxylic acid. Good yield was
achieved for this three-component reaction by 2,5-bistrifluoro-
ethoxy benzoic acid as well (8zc).
a
Condition A: 1 (0.3 mmol), 5 (0.6 mmol), 6 (0.6 mmol),
[RhCp*Cl2]2 (5 mol %), NaOAc (0.3 mmol), DCE (0.4 mL), 140
b
°C, 12 h, N2. Isolated yield.
prepared through the three-component reaction of benzoic
acids, malonates, and formaldehyde under condition A. The
transformation displayed good functional group tolerance.
Benzoic acids with methyl, ethyl, benzyl, acyloxyl, chloro,
bromo, and fluoro groups all gave the corresponding 2-
substitued indanones in moderate to good yields (7aa−7am).
Slightly lower yields were obtained for the benzoic acids
bearing electron-withdrawing groups (7ah−7am). Naphthoic
acid underwent the reaction smoothly to give 71% yield. A
range of dialkyl malonates showed good reactivity to give the
diverse indanones (3aa, 7ao−7ar). Notably, malonate
substrates could be successfully extended to various
cyanoacetates, affording the diverse 2-substitued indanones
under reaction condition A (3ae, 7as−7av).
We further explored the substrate scope of this three-
component reaction for the nonsubstituted indanones under
reaction condition B in Scheme 4. In general, this reaction
exhibited a broad substrate scope, and electron-rich benzoic
acids showed better reactivity than electron-poor substrates.
The ortho-substituted benzoic acids always afforded the
corresponding indanones smoothly (4aa, 8aa−8ag), and a
range of functional groups, such as methyl, ethyl, benzyl,
The efficiency of this catalytic system was further
demonstrated by running the reaction on a gram scale and
the indanone (8ac) was obtained in 64% isolated yield
(Scheme 5).
Scheme 5. Gram-Scale Synthesis
C
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Control experiments were performed for the better under-
standing of this three component reaction as shown in Scheme
6. Methylene malonate 2aa was detected by GC−MS in the
Scheme 7. Proposed Reaction Mechanism
Scheme 6. Experimental Mechanistic Studies
In conclusion, a new protocol for the preparation of
indanones through the C−H activation of benzoic acids has
been developed by a simple rhodium complex catalyst. The
method described in this paper allows the formation of
indanones from readily available starting materials of benzoic
acids, formaldehyde, and malonates via ortho-selective hydro-
arylation of benzoic acids using the carboxylic moiety as a
directing group. The reaction tolerates a wide range of
functional groups, and selectively gives 2-substituted or
nonsubstituted indanones by using the different solvent. This
simple rhodium catalytic system enables free carboxylic acids
to undergo a cyclization and affords Claisen-type condensation
products.
reaction solution, which should be formed in situ by malonate
5a and formaldehyde under the reaction condition (Support-
ing Information, S7). The yield of 2-substituted indanone 7aa
gradually increased along with the prolonging of the reaction
time under the condition A, and only trace of the
nonsubstituted indanone 4aa was observed (Scheme 6, eq
a). In contrast, under the condition B, 2-substitued 7aa, which
was isolated in 20% yield for 1 h, disappeared after the reaction
carried out for 10 h, and nonsubstituted indanone 4aa was
delivered in 68% yield (Scheme 6, eq b). This result indicated
that nonsubstituted indanone 4aa should be generated from 2-
substituted indanone 7aa in condition B. To testify this, 7aa
was subjected to condition B and expected excellent yield of
4aa was indeed obtained (Scheme 6, eq c).
An experiment of hydrogen−deuterium exchange was
performed by the treatment of benzoic acid 1a under condition
A and condition B. The selective ortho-deuteration was 84%
for condition A and 80% for condition B, suggesting that the
carboxylate directed ortho C−H cleavage is reversible in this
rhodium catalyzed three-component reaction (Scheme 6, eq
d). This result is consistent with the findings of Gooßen’s
group.11a,17b We conducted the KIE experiments, and a KIE
value of 0.7 under condition A and 0.8 under condition B was
obtained. The results implicate that the cleavage of the C−H
bond might not be involved in the rate-determing step
(Scheme 6, eq e). On the basis of previous reports11a,17 and
our study, a plausible pathway for this three component
reaction is depicted in Scheme 7. Initially, the ligand exchange
of [Cp*RhCl2]2 with NaOAc leads to the formation of active
catalyst Cp*Rh(OAc)2 I, which gives aryl-rhodium intermedi-
ate II through a carboxylate directed ortho-C−H activation.
The subsequent hydroarylation of intermediate II with
methylene malonates that generated in situ from malonates
with formaldehyde affords intermediate III, which undergoes
cyclization to give indanone 7.11a,17b The decarboxylation of 7
generates nonsubstitued indanone 4aa.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02986.
Experimental procedures, characterization data, NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Yuhong Zhang − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China; State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000,
China; orcid.org/0000-0002-1033-3429;
Email: yhzhang@zju.edu.cn
Zhanxiang Liu − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China; Email: liuzhanx@zju.edu.cn
Authors
Shuling Yu − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China
Ningning Lv − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China
Chao Hong − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02986
Notes
The authors declare no competing financial interest.
D
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182−198.
ACKNOWLEDGMENTS
■
Funding from Natural Science Foundation of China (No.
21672186) and Important Project of Zhejiang province
(2017C03049) is highly acknowledged.
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