Selective Reductive Removal of Ester and Amide Groups from Arenes and Heteroarenes through Nickel-Catalyzed C?O and C?N Bond Activation

Article abstract of DOI:10.1002/anie.201612624

An inexpensive nickel(II) catalyst and a hydrosilane were used for the efficient reductive defunctionalization of aryl and heteroaryl esters through a decarbonylative pathway. This versatile method could be used for the removal of ester and amide functional groups from various organic molecules. Moreover, a scale-up experiment and a synthetic application based on the use of a removable carboxylic acid directing group highlight the usefulness of this reaction.

Full text of DOI:10.1002/anie.201612624

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201612624
German Edition: DOI: 10.1002/ange.201612624
Reductive Defunctionalization
Selective Reductive Removal of Ester and Amide Groups from Arenes
À
À
and Heteroarenes through Nickel-Catalyzed C O and C N Bond
Activation
Huifeng Yue, Lin Guo, Shao-Chi Lee, Xiangqian Liu, and Magnus Rueping*
Dedicated to Professor Dr. Hans-Joachim Gais on the occasion of his 75th birthday
Abstract: An inexpensive nickel(II) catalyst and a hydrosilane
were used for the efficient reductive defunctionalization of aryl
and heteroaryl esters through a decarbonylative pathway. This
versatile method could be used for the removal of ester and
amide functional groups from various organic molecules.
Moreover, a scale-up experiment and a synthetic application
based on the use of a removable carboxylic acid directing
group highlight the usefulness of this reaction.
T
he reduction of readily available carboxylic acid deriva-
Scheme 1. Reduction of esters and amides with hydrosilanes.
tives, such as esters and amides, is one of the most
fundamental and important transformations in synthetic
[
1]
organic chemistry.
sodium borohydride and lithium aluminum hydride, are
Organometallic hydrides, such as
ylative borylation, amination, and silylation reactions of
[
14,15]
esters and amides.
[
2]
used for such transformations. However, the use of these
reactive reducing agents may have the disadvantages of
cumbersome workup procedures and poor functional-group
tolerance. Highly chemoselective catalytic hydrosilylation
reactions of carboxylic acid derivatives with easy-to-handle
As part of our continuing efforts to develop viable
protocols for the activation of inert bonds by nickel cataly-
[
16]
sis, we herein describe the first nickel-catalyzed reductive
defunctionalization of aryl and heteroaryl esters through
a decarbonylative pathway. This transformation provides
a versatile method for the late-stage removal of ester as
well as amide groups from organic molecules.
[
3]
and practical hydrosilanes have also been developed. In
general, alcohols, ethers, or aldehydes are obtained from
[
4]
[5]
[6]
[7]
[8]
the hydrosilylation of esters, whereas amines or nitriles are
formed upon the hydrosilylation of amides (Scheme 1).
However, to date, despite the great importance of replacing
functional groups in organic synthesis, very little is known
about the removal of ester and amides from aromatic and
We initiated our studies by evaluating the reductive
defunctionalization of phenyl naphthalene-2-carboxylate
(1a) with inexpensive, nontoxic, and air-stable polymethyl-
hydrosiloxane (PMHS) as the reducing agent and [Ni(cod)2]
as the catalyst. Since ligands typically play an important role
in nickel-catalyzed transformations, we first tested several
phosphine ligands (Table 1, entries 1–3). The bidentate phos-
phine ligand 1,2-bis(dicyclohexylphosphanyl)ethane (dcype),
which provided the desired product in 69% yield, was
identified as the optimal ligand. The addition of potassium
fluoride as a base had no beneficial effect on the reaction
yield (entry 4). However, the yield was significantly improved
to 88% just by extending the reaction time (entry 5). A series
of nickel catalysts were subsequently examined, and inex-
pensive and air-stable Ni(OAc) ·4H O was found to give
[
9]
heteroaromatic rings through a catalytic reductive pathway.
Regarding the activation/functionalization of CÀO and
[
10,11]
CÀN bonds of esters and amides,
progress has been made
[
10–16]
in recent years.
With regard to decarbonylative trans-
formations, the nickel-catalyzed CÀC bond formation of aryl
[
12]
esters was achieved. Additionally, Heck- and Suzuki-type
cross-coupling reactions of twisted amides through a decar-
[
13]
bonylative pathway were reported, as well as decarbon-
2
2
[
*] H. Yue, L. Guo, S.-C. Lee, Dr. X. Liu, Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
a slightly better result (90%; Table 1, entry 6) as compared to
0
II
the Ni species, whereas other Ni catalysts, such as NiCl2,
NiBr , and NiI , all gave none of the desired product
2
2
E-mail: magnus.rueping@rwth-aachen.de
(entries 7–9). The yield of the product decreased to 68%
Prof. Dr. M. Rueping
King Abdullah University of Science and Technology (KAUST)
KAUST Catalysis Center (KCC)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: magnus.rueping@Kaust.edu.sa
when 10 mol% of the dcype ligand was used (entry 10). The
use of other solvents, such as 1,4-dioxane, and a decrease in
the temperature proved to be less productive (entries 11 and
12). Control experiments demonstrated that the desired
product is not generated in the absence of the nickel catalyst
(entry 13).
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201612624.
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[
a]
[a,b]
Table 1: Optimization of the reaction conditions.
Table 2: Scope of the reaction of aryl esters.
[b]
Entry
[Ni]
Ligand (x mol%)
t [h]
Yield [%]
1
2
3
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
dcype (20)
12
12
12
12
24
24
24
24
24
24
24
24
24
69
trace
43
57
88
90
0
0
0
68
51
30
0
PCy (40)
3
PnBu (40)
3
[
c]
4
5
6
7
8
9
0
1
2
3
dcype (20)
dcype (20)
dcype (20)
dcype (20)
dcype (20)
dcype (20)
dcype (10)
dcype (20)
dcype (20)
dcype (20)
Ni(OAc)
NiCl2
NiBr2
NiI2
2 2
·4H O
1
1
1
1
Ni(OAc) ·4H O
2
2
[
[
d]
e]
Ni(OAc) ·4H O
2
2
Ni(OAc) ·4H O
2
2
–
[a] Reaction conditions: 1a (0.2 mmol), PMHS (0.4 mmol), [Ni]
(
0.02 mmol), ligand (0.04 mmol), toluene (1.0 mL), 1708C. [b] Yield after
purification. [c] KF (2 equiv) was added. [d] Dioxane was used as the
solvent. [e] The reaction was carried out at 1508C. Cy=cyclohexyl.
Having optimized the reaction conditions, we evaluated
the scope of the reaction with respect to the ester substrate
(
Table 2). A wide variety of aryl esters bearing electron-rich
and electron-poor functional groups were converted into the
corresponding products in good to excellent yields. For
example, naphthalen-2-yl esters 1a–c underwent the reaction
smoothly to give the products in high yields (71–99%).
The decarbonylative reduction of the polycyclic substrate
1
e and p-extended ester 1 f also proceeded in good to
excellent yields. Although the para and meta biphenyl esters
g and 1h underwent this transformation in high yields, the
1
ortho biphenyl ester 1i gave the product in only moderate
yield, thus indicating that the reaction is slightly hampered by
the increased steric hindrance. Other functional groups,
including double bonds (substrates 1j and 1k) and fluoro
(
substrates 1l, 1p, and 1q), trifluoromethyl (substrates 1m
and 1r), tert-butyl (substrate 1n), methylthio (substrate 1o),
ketone (substrate 1s), amide (substrate 1t), and acetal groups
(
substrate 1v), were tolerated under our reaction conditions,
which highlights the advantages of our newly developed
reductive protocol. Moreover, by this protocol, selective
reduction of the phenyl ester group was possible in the
presence of a methyl ester group (substrate 1u). Notably, the
estrone-derived phenyl ester 1w also underwent the defunc-
tionalization.
In contrast to previously reported defunctionalization
reactions, substrates with a methoxy (1c), thiomethyl (1o), or
fluoro group (1l, 1p, and 1q) were well tolerated, as well as
methyl ester or amide groups, thus highlighting the chemo-
selectivity of this new catalytic reductive decarbonylation.
The generality of the present nickel-catalyzed reductive
defunctionalization protocol was further demonstrated with
a series of heteroaryl esters, which play an important role in
medicinal chemistry (Table 3). The reduction of quinoline-
derived ester 3a under the optimal conditions afforded the
[a] Reaction conditions: 1 (0.2 mmol), PMHS (0.4 mmol, 2 equiv),
Ni(OAc) ·4H O (0.02 mmol, 10 mol%), dcype (0.04 mmol, 20 mol%),
2
2
toluene (1 mL), 1708C, 24 h. [b] Yields after purification are reported.
c] The reaction was carried out with [Ni(cod) ] (0.02 mmol, 10 mol%)
[
2
for 48 h. [d] The yield was determined by GC owing to the volatility of the
product. [e] Reaction time: 72 h. pin=pinacolate.
corresponding product in good yield. Similar success was
observed for the pyridine-derived esters 3b–d. Besides
indole- and benzofuran-derived esters 3e–h, furan- and
2
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[
a,b]
[a,b]
Table 3: Scope of the reaction of heteroaryl esters.
Table 4: Scope of the reaction of aryl and heteroaryl amides.
[a] Reaction conditions: 5 (0.2 mmol), PMHS (0.4 mmol, 2 equiv),
[a] Reaction conditions: 3 (0.2 mmol), PMHS (0.4 mmol, 2 equiv),
Ni(OAc) ·4H O (0.02 mmol, 10 mol%), dcype (0.04 mmol, 20 mol%),
2 2
Ni(OAc) ·4H O (0.02 mmol, 10 mol%), dcype (0.04 mmol, 20 mol%),
toluene (1 mL), 1708C, 48 h. [b] Yields after purification are reported.
2
2
toluene (1 mL), 1708C, 24 h. [b] Yields after purification are reported.
c] The yield was determined by GC owing to the volatility of the product.
[c] The yield was determined by GC owing to the volatility of the product.
[
thiophene-derived esters 3i–l were also suitable substrates for
this transformation, with the desired products formed in
moderate to excellent yields.
Our reductive defunctionalization protocol could also be
readily extended to aryl and heteroaryl amides. As shown in
Table 4), the reaction of naphthyl amide 5a proceeded in
good yield. Whereas the para biphenyl amide 5b was reduced
in high yield, the ortho biphenyl amide 5c gave the desired
product in only moderate yield. A wide range of phenyl
amides possessing fluoro (substrate 5d), trifluoromethyl
(
substrate 5e), methyl ester (substrate 5 f), tert-butyl (sub-
strate 5g), methoxy (substrate 5h), and acetal functionalities
substrate 5i) were all suitable substrates, and the corre-
(
sponding products were obtained in good to high yields.
Furthermore, quinoline- and pyridine-derived amides 5j and
5
k both underwent this reaction in excellent yield.
To demonstrate the practicability of this newly developed
decarbonylative reduction method, we conducted a gram-
scale reaction of phenyl naphthalene-2-carboxylate (1a) in
the presence of only 5 mol% of the nickel(II) catalyst, and
isolated the desired product 2a in 72% yield (Scheme 2a).
Furthermore, on the basis of the ability of carboxylic acid
groups to act as directing groups in aromatic CÀH function-
Scheme 2. Gram-scale reaction and synthetic application.
tive defunctionalization sequence (Scheme 2b). Thus, ester
and amide groups often used in organic synthesis as directing
groups in aromatic substitution reactions or as ortho directing
groups in CÀH functionalization reactions can be readily
alization and the need for their subsequent removal by
reductive defunctionalization, we also examined a carboxylic-
[
17]
acid-directed CÀH functionalization /esterification/reduc-
replaced.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: amides · cleavage reactions · esters · nickel catalysis ·
reduction
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Angew. Chem. 2015, 127, 14726; b) S. Shi, G. Meng, M. Szostak,
Angew. Chem. Int. Ed. 2016, 55, 6959; Angew. Chem. 2016, 128,
Manuscript received: December 29, 2016
7073; c) G. Meng, M. Szostak, Org. Lett. 2016, 18, 796.
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Reductive Defunctionalization
H. Yue, L. Guo, S.-C. Lee, X. Liu,
M. Rueping*
&&&& — &&&&
Selective Reductive Removal of Ester and
Amide Groups from Arenes and
Heteroarenes through Nickel-Catalyzed
CÀO and CÀN Bond Activation
Now you see it… now you don’t: An
inexpensive nickel(II) catalyst and
a hydrosilane were used for the efficient
reductive defunctionalization of aryl and
heteroaryl esters by a decarbonylative
pathway (see scheme). This versatile
method is suitable for the removal of
ester and amide functional groups,
including carbonyl directing groups, from
a variety of organic molecules.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!