Palladium-Catalyzed Regioselective Coupling Cyclohexenone into Indoles: Atom-Economic Synthesis of β-Indolyl Cyclohexenones and Derivatization Applications

Article abstract of DOI:10.1021/acs.orglett.0c01763

Herein, we report a palladium-catalyzed dehydrogenative cross-coupling of indoles with cyclic enones to give β-indolyl cyclic enones under mild and neutral reaction conditions. The key to the success is to explore a mild condition, which ensures the indole C-H activation and subsequent syn β-hydride elimination through rapid enolization isomerization of Pd(II)-enolate while suppressing other undesired side reactions. Synthetic utility has also been demonstrated in the flexible transformation of the coupling products to meta-phenols and benzo[a]carbazoles.

Full text of DOI:10.1021/acs.orglett.0c01763

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Letter
Palladium-Catalyzed Regioselective Coupling Cyclohexenone into
Indoles: Atom-Economic Synthesis of β‑Indolyl Cyclohexenones and
Derivatization Applications
Zhen-Kang Wen,* Xiao-Xue Wu, Wen-Kai Bao, Jing-Jing Xiao, and Jian-Bin Chao
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ABSTRACT: Herein, we report a palladium-catalyzed dehydro-
genative cross-coupling of indoles with cyclic enones to give β-
indolyl cyclic enones under mild and neutral reaction conditions.
The key to the success is to explore a mild condition, which
ensures the indole C−H activation and subsequent syn β-hydride
elimination through rapid enolization isomerization of Pd(II)-
enolate while suppressing other undesired side reactions. Synthetic
utility has also been demonstrated in the flexible transformation of
the coupling products to meta-phenols and benzo[a]carbazoles.
he numerous applications of functionalized indole in
pharmaceuticals, agrochemicals, and functional materials
through the C−H activation strategy is highly desirable. In this
context, we conceived that direct cross-coupling of indole with
cyclohexenone to produce β-indolyl-substituted cyclohexenone
would undoubtedly expand diversified derivatization of indoles
(Scheme 1c), such as unique accessibility to meta-indolyl
phenols and benzo[a]carbazoles9. However, to the best of our
knowledge, direct coupling of cyclohexenone to the indole
moiety through C−H bond activation has not been reported
owing to the substantial challenges by employing both sensitive
substrates.
The challenge posed by our envisioned dehydrogenative
couplings between cyclohexenone and indole is to identify a
mild reaction condition which must be compatible and orderly
in good delay with indole C−H activation and syn β-hydride
elimination (Scheme 1d). First, selective palladation of the
indole C−H bond and migratory insertion to cyclohexenone
must be fast enough because cyclohexenone is sensitive to
strong acidic/basic conditions (self-aldol condensation) as well
as oxidative dehydrogenation (phenol) in palladium cata-
lysis.6e,10 In addition, both the substrate and product
containing indole units are susceptible to decomposition
under oxidative reaction conditions.11 Thus, developing a mild
and near neutral reaction conditions is highly desirable.
T
have triggered long-standing and continuous efforts for the
exploration of efficient and applicable approaches for the
installation of these valuable compounds.1 Transition-metal-
catalyzed inert C−H bond functionalization of indole
represents one of the most straightforward and efficient
strategies to access structurally diversified indoles owing to its
high efficiency in atom and step economy as well as
sustainability (Scheme 1a).2 Among them, palladium-catalyzed
direct cross-coupling of indole with acyclic acrylate3 has been
well studied and holds a unique position in this thriving
research area because of the synthetic flexibility of olefin
functionality in these products, which provide opportunities for
further useful transformations,4 such as reductive additions,
pericyclic additions, and so on. Thus, exploration of new
coupling partners to expand this reactivity regime would
constitute significant interest in direct modification of indole
pharmacophores.
As a commercially available and relatively cheap reagent,
cyclohexenone has been demonstrated as an excellent
precursor to produce phenol.5 In particular, dehydrogenative
oxidation of β-functionalized cyclohexenones opens a new
entry to access meta-substituted phenols,6 which are more
difficult to prepare in traditional electrophilic substitution
methods owing to the intrinsic ortho/para-directing propensity
of the hydroxyl group.7 As a consequence, in order to
overcome the limitations of currently used coupling reactions
from prefunctionalized substrates8 such as aryl halides and aryl
boronic acids (Scheme 1b), exploring efficient synthetic
approaches for β-functionalized cyclohexenones preparation
Received: May 25, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01763
© XXXX American Chemical Society
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A

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a b
,
Scheme 1. Overview and Context of This Work
Table 1. Identification of Reaction Conditions
a
Conditions: The reaction was conducted with 1a (0.25 mmol), 2a (1
mmol, 4 equiv), Pd(TFA)2 (0.025 mmol, 10 mol %), ligand (0.05
mmol, 20 mol %), oxidant (0.375 mmol, 1.5 equiv) in solvent (1.2
mL) at 50 °C stirred for 24 h. Isolated yield.
Second, syn-β-hydride elimination is inaccessible owing to the
restricted bond rotation of the six-membered ring conforma-
tion for β-indolyl Pd(II)-enolate, which request its rapid
enolization isomerization to align the metal for syn
elimination.6d Within our ongoing efforts in the development
of coupling diversifications by using challenging cyclic enones
as coupling partners,12 we have achieved the first palladium-
catalyzed dehydrogenative cross-coupling of cyclic enones with
indoles, which we report herein. Notable features such as
excellent regioselectivity, broad functional group tolerance, as
well as clean oxidant made our method more practical and
appealing in synthetic chemistry. In addition, the successful
transformation of coupling products to valuable meta-phenols
and benzo[a]carbazoles is also demonstrated.
b
essential to this reaction (entry 9). Thus, the efficient method
for the facile synthesis of β-indolyl cyclohexenone has been
established, starting from commercially available indole and
cyclohexenone.
With the optimized reaction conditions, we then examined
the generality of this dehydrogenative cross-coupling reaction.
As illustrated in Scheme 2, indoles bearing various functional
groups reacted efficiently with cyclic enones to give desired
coupling products in good yields. Remarkably, reactions
proceeded exclusively at the C3 position of indole moiety in
all cases. Regardless of the substitutes, the positions of indole
substrates except for C3 are all tolerated in this reaction
protocol and afforded the desired products in synthetic useful
yields. In addition, a wide range of functional groups, including
alkyl (3b, 3c), methoxyl (3d, 3e, 3f), hydroxyl (3g), ester
(3h), fluoro (3i, 3j, 3k), and chloro (3l, 3m), and even
susceptible bromo (3n, 3o, 3p, 3q), cyano (3r), aldehyde (3s),
and nitro (3t), are all compatible under reaction conditions.
Notably, substitutes at the C2 position of indoles, which could
potentially increase steric hindrance of the coupling reaction,
also afforded the corresponding products in good yields (3u,
3w). Furthermore, substitutions on the nitrogen atom of
indole also furnished the reaction to provide the corresponding
products in good yields (3x, 3z). Besides monosubstituted
indoles, disubstituted indoles (3v, 3y) also proved to be
suitable substrates in this reaction. Other cyclic enones such as
cyclopentenone and cycloheptenone can also couple with
indole to give the desired coupling products (3ab, 3ac),
respectively. Such high chemoselectivity and broad functional
We commenced our study by investigating reaction
conditions (Table 1) for dehydrogenative coupling of
commercially available indole (1a) with cyclohexenone (2a).
The reaction was initially performed in DMSO by using
Pd(OAc)2 as catalyst and tert-butylhydroperoxide as oxidant at
50 °C for 24 h. Preliminary catalyst examination (entry 1−3)
proved that more electrophilic Pd(TFA)213 could promote the
desired product formation in 20% yield. Extensive screening of
t
the oxidants revealed that employment of BuOOH is the
optimal oxidant to furnish the reaction at mild conditions
(entries 4−7); other commonly used oxidants such as Ag2CO3,
Cu(OAc)2, and BQ proved to be ineffective (for more details,
please see the Supporting Information). The yield of 3a was
enhanced slightly when mixed solvents DMSO/THF (2:1)
were used (entry 8). To further improve the yield, we tested
the ligand effect by using nitrogen ligands to tune the
electronic property and stability of the Pd center (Table 1,
evaluation of ligands).14 To our delight, use of L7 as ligand
dramatically improved the yield of 3a up to 68%. Finally,
control experiment indicated that the palladium catalyst was
B
https://dx.doi.org/10.1021/acs.orglett.0c01763
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a b
,
a b
,
Scheme 2. Scope of Substrate
Scheme 3. Synthetic Applications
a
Conditions: (A) The reaction was conducted with 3 (0.25 mmol), I2
(0.075 mmol, 30 mmol %), DMSO (1 mL) at 60 °C stirred for 31 h.
(B) The reaction was conducted under N2 balloon with 3 (0.25
mmol), CsF (1.5 mmol, 6 equiv), 5 (0.75 mmol, 3 equiv), CH3CN (2
b
mL) at room temperature stirred for 46 h. Isolated yield.
2.45 KIE value suggested that C−H bond cleavage of indole is
possibly involved in the rate-limiting step. Furthermore, the
deuterium retaining 100% in the 2-position of the coupling
product revealed that H/D exchange of the 2-position of the
indole moiety was not involved in this dehydrogenative
coupling reaction (eq 3). Thus, a plausible mechanism for
the catalytic cycle was proposed in Scheme 4. Initially, with the
a
Conditions: The reaction was conducted with 1 (0.25 mmol), 2 (1
mmol, 4 equiv), Pd(TFA)2 (0.025 mmol, 10 mol %), ligand (0.05
mmol, 20 mol %), BuOOH (0.375 mmol, 1.5 equiv), in DMSO/
t
b
THF (0.8 mL/0.4 mL) at 50 °C stirred for 24 h. Isolated yield.
c
d
Stirred for 39 h. Stirred for 48 h, yields are based on recovery of
Scheme 4. Proposed Mechanism
starting materials.
group compatibility are likely attributed to the mild reaction
temperatures as well as avoiding use of strong acids and bases.
To illustrate the practicality and utility of this methodology,
a scale up reaction of 1a with 2a was conducted and afforded
the corresponding product 3a in 64% yield (eq 1). Addition-
ally, in the presence of iodine catalyst, the coupling products
(e.g., 3a, 3s, 3f, 3x) could be transformed to meta-indolyl
phenols (Scheme 3) in high yields, which affords an efficient
and straightforward route to access these value-added
products. Furthermore, treatment of 3 with benzyne precursor
at room temperature yielded valuable benzo[a]carbazoles
bearing the free NH unit in good yields (Scheme 3), thus
providing a more efficient and easily handled [4 + 2]
annulation approach but avoiding the use of preformed α-
diazo carbonyl compounds.15
C−H bond cleavage of indole with more electrophilic Pd(II),
an indolyl-Pd(II) intermediate (A) was formed; subsequent
migratory insertion to cyclohexenone would generate Pd(II)-
enolate (B), which underwent a rapid enolization isomer-
ization (B−B1−B2) to maintain the Pd and β hydrogen atom
at the cis position for facile syn-β-hydride elimination to
To acquire the mode of action of this dehydrogenative cross-
coupling reaction, an intermolecular KIE reaction study (eq 2)
was operated under standard reaction conditions for 2 h. The
C
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provide the desired product (C) with the concomitant
generation of Pd(0), wherein DMSO is expected as a suitable
base and polar solvent to facilitate enolization isomerization.16
Finally, in the presence of tert-butylhydroperoxide, the active
Pd(II) catalyst could be regenerated to accomplish the
catalytic cycle.
In conclusion, we have developed an efficient method for the
facile synthesis of β-indolyl cyclic enones, starting from the
commercially available indoles and cyclic enones. Various
substituted indoles with broad functional group compatibility
are tolerated in this protocol to give the desired coupling
products in moderate to high yields. Further transformation of
the β-indolyl cyclohexenones to valuable meta-phenols and
benzo[a]carbazoles demonstrated the utility of our method-
ology. We expect that this cost-efficient and easy-to-handle
strategy has great prospective applications in indole
modification chemistry.
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.0c01763.
Detailed experimental procedures and spectral data for
all products (PDF)
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AUTHOR INFORMATION
Corresponding Author
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Zhen-Kang Wen − School of Chemistry and Chemical
Engineering, Shanxi University, Taiyuan 030006, China;
orcid.org/0000-0003-1253-9583; Email: zkwen@
sxu.edu.cn
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Authors
Xiao-Xue Wu − School of Chemistry and Chemical Engineering,
Shanxi University, Taiyuan 030006, China
Wen-Kai Bao − School of Chemistry and Chemical Engineering,
Shanxi University, Taiyuan 030006, China
Jing-Jing Xiao − School of Chemistry and Chemical Engineering,
Shanxi University, Taiyuan 030006, China
Jian-Bin Chao − Scientific Instrument Center, Shanxi University,
Taiyuan 030006, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01763
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21502106), the National
Natural Science Foundation for Young Scientists of Shanxi
Province (201701D221028), and the Ministry of Human
Resources and Social Security Foundation of China for High-
level Returned Talents. We are also very grateful for the test
platform provided by Scientific Instrument Center of Shanxi
University.
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