Rh(III)-Catalyzed Acceptorless Dehydrogenative Coupling of (Hetero)arenes with 2-Carboxyl Allylic Alcohols

Article abstract of DOI:10.1021/acs.orglett.7b03881

Rhodium(III)-catalyzed C-H activation of (hetero)arenes and redox-neutral coupling with 2-carboxyl allylic alcohols has been realized for the construction of β-aryl ketones. This reaction occurred efficiently at a relatively low catalyst loading via initial dehydrogenative alkylation to give a β-keto carboxylic acid, followed by decarboxylation.

Full text of DOI:10.1021/acs.orglett.7b03881

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rh(III)-Catalyzed Acceptorless Dehydrogenative Coupling of
(
Hetero)arenes with 2‑Carboxyl Allylic Alcohols
Jintao Xia, Zhipeng Huang, Xukai Zhou, Xifa Yang, Feng Wang, and Xingwei Li*,†
†
,‡
†,‡
†,‡
†,‡
†
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
*
S Supporting Information
ABSTRACT: Rhodium(III)-catalyzed C−H activation of
hetero)arenes and redox-neutral coupling with 2-carboxyl
(
allylic alcohols has been realized for the construction of β-aryl
ketones. This reaction occurred efficiently at a relatively low
catalyst loading via initial dehydrogenative alkylation to give a β-keto carboxylic acid, followed by decarboxylation.
1
1
n the past decades, metal-catalyzed C(aryl)−H activation
ketones with a quaternary stereogenic center. Although those
methods were generally efficient, oxidants were required for the
catalyst turnover. Alternatively, hydroarylation of olefins
represents a useful method to access β-aryl ketones/aldehydes
1
I
and functionalization has evolved as an attractive alternative
to classical organic synthesis which relies heavily on
prefunctionalized starting materials for construction of new
C−C bonds. By following this strategy, a great number of
highly efficient protocols have been developed to deliver
12
under redox-neutral conditions. However, the olefins are
limited to highly reactive enones/maleimides with stoichio-
metric amounts of base additives.
2
various organic functional materials and bioactive molecules.
We envisioned that as a special allylic alcohol, 2-carboxyl
allylic alcohol may serve as a suitable olefin substrate in C−H
activation. Significantly, the migratory insertion of a M−Ar
bond into the C = C bond of 2-carboxyl allylic alcohol and
subsequent β-H elimination would give a β-keto acid that can
readily undergo decarboxylation (Scheme 1c). We speculated
The coupling between an arene and a nucleophilic reagent
generally requires a stoichiometric amount of oxidant,
together with generation of waste, which may limit the atom-
economy and pose environmental issues. Ideally, the coupling
system includes hydrogen-releasing conditions.
1i,j,3
Olefins are commonly used substrates in C−H activation
1
e,4
systems. In oxidative olefination of arenes, the employment
of either external-oxidation or hydrogen-releasing conditions is
dictated by the fate of the metal hydride species M(H)(X) in
the catalytic cycle. Elimination of an HX from this metal
hydride species leads to a lower valent metal species that is
prone to re-oxidation to regenerate the active catalyst.
Alternatively, protonolysis of the M−H bond by an acid may
give rise to hydrogen evolution. However, this type of
dehydrogenative coupling system is rather limited. Despite
Scheme 1. Catalytic Synthesis of β-Aryl Aldehydes/Ketones
5a
5b
5c
5d,h
5e
the challenges, Zhang, Wang, Yang, Jeganmohan,
Li,
5
f
6
Dong, and other groups have independently developed
dehydrogenative coupling reactions via C−H activation. On the
7
other hand, β-aryl ketones are versatile synthetic building
blocks in organic transformations, and they have been accessed
via cross-coupling of aryl (pseudo)halides, arylboronic acids,
aryl carboxylic acids, or aryldiazonium salts with allylic alcohol8s
or aldehydes in the presence or absence of external oxidants.
Recently, Rh(III)-catalyzed C−H activation has provided new
avenues for synthesis of β-aryl ketones. In 2013, the Glorius
group and the Jiang group independently reported synthesis of
these products via C−H activation of arenes and coupling with
that decarboxylation and dehydrogenation might be integrated
13
into a symbiotic system, especially in the presence of a
carboxylic or Lewis acid which may facilitate the protonolysis of
the M−H bond for hydrogen evolution. We now report
efficient hydrogen-releasing and decarboxylative coupling of
arenes and 2-carboxyl allylic alcohols.
We initiated our exploration with the optimization studies of
the coupling of N-(2-pyrimidinyl)indole (1a) and carboxylic
9
10
allylic alcohols using Cu(OAc) as an oxidant. Our group
2
achieved synthesis of β-aryl ketones by taking advantage of the
ring strain in the oxidative coupling of cyclopropanols with
arenes. The Lu and Liu groups applied allenol or propargyl
alcohols as a coupling reagent for oxidative synthesis of β-aryl
Received: December 13, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03881
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
acid 2a (Table 1). Our initial studies revealed ketone 3aa was
isolated in 71% yield using [RhCp*Cl2]2 catalyst and
Scheme 2. Scope of Coupling N-(2-Pyrimidinyl)indoles and
2-Carboxyl Allylic Alcohols
a,b
a
Table 1. Screening of the Reaction Conditions
b
entry
additive (30 mol %)
solvent
yield (%)
c
1
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Cu(OAc)2
AgOAc
DCM
71
70
c
2
DME
c
3
acetone
DME
74
4
5
6
7
8
91
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
99
24
20
CsOAc
trace
99
d
9
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
e
1
1
1
1
0
1
2
3
99
,
ef
99
<5
g
Zn(OAc)2
a
Reaction conditions: 1a (0.2 mmol), 2a (0.5 mmol), Rh(III) catalyst,
b
additive in a solvent (3 mL) under nitrogen at 80 °C. Yield of the
isolated product. 2a (0.3 mmol) was used. [RhCp*Cl ] (2.5 mol
%
a
c
d
Reaction conditions: 1a (0.2 mmol), 2 (0.50 mmol), [RhCp*Cl2]
2
2
2
(
1.5 mol %), Zn(OAc) (0.3 equiv), acetone (3 mL) at 80 °C under
2
b
e
f
g
). [RhCp*Cl ] (1.5 mol %). Under air. No [RhCp*Cl ] .
2
2
2 2
air for 12 h. Isolated yield.
The arene substrates are not limited to N-pyrimidylindoles
(Scheme 3). N-Pyridylindole also coupled smoothly with 2a in
Zn(OAc)2 additive in DCM under rigorously oxidant-free
conditions (entry 1). A similar yield was obtained in DME or
acetone solvent (entries 2 and 3). The yield was drastically
improved to 91% in DME when the amount of 2a was
increased to 2.5 equiv (entry 4), and a nearly quantitative yield
was delivered in acetone (entry 5). These results might indicate
hydrogen evolution or hydrogen acceptance by the olefin or
a,b
Scheme 3. Scope of Other Arene Substrates
1
acetone. In fact, H NMR analysis of a reaction in acetone-d6
indicted that neither 2-propanol-d nor hydrogenation product
6
of the olefin was formed, and absence of the latter was further
confirmed by GC−MS and HRMS. The H and CO2
2
coproducts were directly observed by online GC-MS analysis.
Zn(OAc) play an important role in this system since inferior
2
results were obtained when Cu(OAc) , AgOAc, or CsOAc was
2
used (entries 6−8). Lowering the catalyst loading to 1.5 mol %
did not affect the reaction outcome (entries 9 and 10), and the
reaction proceeded equally well under air (entry 11). Control
experiments confirmed that both [RhCp*Cl ] and Zn(OAc)
2
2
2
were necessary (entries 12 and 13).
With the establishment of the optimal conditions in hand,
the scope and limitations of this system (Scheme 2) were next
examined. Introduction of electron-donating (OMe, OBn),
a
Reaction conditions: 4 (0.2 mmol), 2a (0.5 mmol), [RhCp*Cl2]2
(
2.5 mol %), Zn(OAc) (0.3 equiv), acetone (3 mL), 80 °C under air
2
-
withdrawing (CO Me, NO , CHO), and halogen groups (F,
b
c
2
2
for 12 h. Isolated yields. 2a (1.0 mmol) was used.
Cl, Br) into the C4−C6 positions of the indole were all
tolerated, and the products were isolated in 61−99% yields
(
3aa, 3ca−ta). A relatively lower yield was isolated when the
C3 (3ba) or C7 (3ua−wa) position was substituted, indicative
of steric effects. Of note, useful functional groups such as NO2
high efficiency (5aa). Pyrrole 4b coupled with 2a only to give
the monoalkylated product (5ba) in 83% yield, while both
mono- (5ba) and dialkylated (5ba′) products were obtained
when the amount of 2a was increased to 5 equiv. In addition,
N-phenylpyrazole was amenable to the reaction conditions
(5ca, 55%). Coupling of pyridone derivatives with 2a provided
products 5da and 5ea in moderate yields. Purine and pyridine
were also applicable directing groups for C−H activation of
(
2
3sa) and CHO (3ta) were also compatible. The scope of the
-carboxyl allylic alcohol was next explored in the coupling with
indole 1a. 2-(Hydroxymethyl)acrylic acid reacted to afford β-
aryl aldehyde 3ab in 59% yield, and moderate yields were
delivered when the C3 position of 2-carboxyl allylic alcohol was
substituted by a (cyclo)alkyl or a benzyl group (3ac−ag).
B
DOI: 10.1021/acs.orglett.7b03881
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
benzene rings, as in formation of 5fa−ka in good to excellent
yields.
process, insignificant kinetic isotope effect (k /k = 1.1) of the
H
D
reaction of indole 1a was then obtained on the basis of two rate
constants, suggesting that the C−H cleavage was not involved
in the turnover-limiting step (Scheme 5c). A control experi-
ment using the analogous ester suggested the reaction likely
involve β-hydride elimination (Scheme 5d). In another control
experiment, coupling of simple allyl alcohol under the standard
conditions only afforded product 3ab in 9% yield, indicating
Several derivatization reactions were carried out to
demonstrate the synthetic applications. Scaling up the reaction
to 2 mmol gave 3aa in 94% yield (Scheme 2). Further C(7)−H
functionalization of 3aa with methyl acrylate and dimethyl 2-
diazomalonate afforded the olefination product 6 and alkylation
product 7, respectively (Scheme 4a). Attempted removal of the
olefination activation by the carboxylic group (Scheme 5d)1.
5
Scheme 4. Diversification of the Products
On the basis of mechanistic studies and literature reports,
a
plausible mechanism is proposed in Scheme 6. Starting from
Scheme 6. Proposed Mechanism
pyrimidyl group in 3aa afforded an NH indole 8 together with
2
-(2-(pyrimidin-2-yl)ethyl)-1H-indole (9), where 8 was gen-
10a,14
erated via Smiles rearrangement (Scheme 4b).
Further-
more, only 9 was isolated when NaOMe was used in 3 equiv
amounts.
A series of experiments have been performed to elucidate the
reaction mechanism (Scheme 5). H/D exchange between 1a
[
RhCp*(OAc)X] (X = OAc or Cl), cyclometalation of indole
produces a rhodacycle B. Subsequent coordination of 2a gives a
cationic olefin intermediate C, which undergoes selective
migratory insertion to generate a tertiary alkyl species D, and
Scheme 5. Preliminary Mechanistic Studies
this process is likely promoted by the Zn(OAc) additive which
2
15a,16
activates the carbonyl.
Subsequent β-hydride elimination
provides a rhodium hydride species E together with the enol
form of a β-keto acid. The β-keto acid G undergoes facile
decarboxylation to furnish the final product. The rhodium
hydride E is then protonolyzed by HX to regenerate the active
rhodium species with hydrogen liberation. It is possible that the
protonolysis is also facilitated by Zn(OAc)2.
In conclusion, we have developed an efficient process to
access β-aryl ketones via Rh(III)-catalyzed C−H activation of
arenes and coupling with 2-carboxyl allylic alcohols, and the
reactions tolerated a wide range of substrates with good
functional group compatibility with H2 and CO2 as the
coproducts. Efforts to explore other dehydrogenative coupling
systems are underway in our laboratories.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03881.
Detailed experimental procedures, characterization of
new compounds, and NMR spectra (PDF)
and CD CO D has been carried out in the presence of 2a
3
2
(
Scheme 5a). Deuterium incorporation at the C2 and C7
positions was observed for the recovered indole substrate,
suggestive of reversible C−H activation. In addition, H/D
exchange at the α-position of the carbonyl group of the product
was also detected. Rhodacyclic complex 10 was then
synthesized, and it showed good activity in both stoichiometric
and catalytic reactions (Scheme 5b), indicating relevancy of a
C−H activation pathway. To further probe the C−H activation
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: xwli@dicp.ac.cn.
ORCID
Feng Wang: 0000-0002-9167-8743
Xingwei Li: 0000-0002-1153-1558
C
DOI: 10.1021/acs.orglett.7b03881
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
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(
The authors declare no competing financial interest.
Soc. Rev. 2010, 39, 2093. (d) Tian, P.; Dong, H.-Q.; Lin, G.-Q. ACS
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Huang, K.; Jiang, H. Org. Lett. 2013, 15, 2330. (g) Oliveira, C. C.;
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ACKNOWLEDGMENTS
Financial support from the NSFC (Nos. 21525208 and
1472186) and the new technology of methanol conversion
of Dalian Institute of Chemical Physics (Chinese Academy of
Sciences) are gratefully acknowledged.
■
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DOI: 10.1021/acs.orglett.7b03881
Org. Lett. XXXX, XXX, XXX−XXX