Nickel-catalyzed reductive defunctionalization of esters in the absence of an external reductant: Activation of C-O bonds

Article abstract of DOI:10.1039/c9cc07710c

The nickel-catalyzed reductive cleavage of esters in the absence of an external reductant, which involves the cleavage of an inert acyl C-O bond in O-Alkyl esters is reported. Various groups, such as N-containing heterocycles, esters, amides, and even arene rings can function as a directing group.

Full text of DOI:10.1039/c9cc07710c

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DOI: 10.1039/C9CC07710C
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Nickel-Catalyzed Reductive Defunctionalization of Esters in the
absence of an External Reductant: Activation of C-O Bonds
Received 00th January 20xx,
Accepted 00th January 20xx
Yasuaki Iyori, Kenjiro Takahashi, Ken Yamazaki, Yusuke Ano and Naoto Chatani*
DOI: 10.1039/x0xx00000x
The nickel-catalyzed reductive cleavage of esters in the absence of an aryloxy group is a good leaving group, compared with an
an external reductant, which involves the cleavage of an inert acyl alkoxy group in O-alkyl esters (-C(O)-OR). Nevertheless, there
C-O bond in O-alkyl esters is reported. Various groups, such as N- are only two examples of the reductive removal of ester
containing heterocycles, esters, amides, and even arene rings can groups (Figure 1). The first report, which was reported by our
function as a directing group.
group, involved the ruthenium-catalyzed reductive cleavage of
O-pyridylmethyl esters using ammonium formate as a hydride
source.15 Rueping recently reported on the removal of ester
groups from O-phenyl esters with hydrosilanes in the presence
of a nickel catalyst.12b However, in these studies, the addition
of an external reducing agent was required for achieving a
successful reaction. Herein, we wish to report on the nickel-
catalyzed reductive cleavage of esters in the absence of an
external reductant. The reaction involves the cleavage of an
inert acyl C-O bond in an O-alkyl ester.16 The presence of a
N(sp2)- or O(sp2)-directing group accelerates the reaction. It is
noteworthy that an arene ring also can be employed as a
Organic synthesis involves the breaking of an existing bond
and the formation of a new one to convert simple, small
compounds into complex and large targeted molecules by C-C
bond and C-heteroatom bond formation.1 Thus, C-C bond and
C-heteroatom bond formation has long been a subject of
interest in the field of organic synthesis. In sharp contrast,
much less attention has been given to the defunctionalization
of organic molecules, except for the decarbonylation of
aldehydes, a reaction that is widely used in the synthesis of
natural products.2 In recent years, defunctionalization
reactions have attracted more attention since they are related
to the preparation of useful compounds from readily available
and sustainable organic resources, such as biomass and
naturally occurring carboxylic acids.3 A key challenge in
defunctionalization reactions involves the activation of
relatively strong and unreactive chemical bonds, such as C-C,4
C-O,5 and C-N bonds.6 Reductive defunctionalization is an
example of such a reaction. The majority of reductive
defunctionalization reactions reported thus far have involved
the use of an external hydrogen source, such as H2 or a
hydrosilane. For example, the defunctionalization of anisole
derivatives, which is recognized as a model reaction in the
degradation of lignin, leading to the formation of arenes was
achieved using H2 and hydrosilane as reducing agents.7 The
reductive defunctionalization of aniline derivative,8 ketones,9
acid fluorides,10 thioethers,11 amides,12 CN,13 and nitro14
derivatives have also been studied. In most cases, a hydrogen
source is required for the success of such reactions.
directing group.17
Previous works (in the presence of reductants)
Our group (2001, 2004)
Rueping (2017)
O
O
PMHS
good leaving group
Ni/
HCO NH
directing group
Ru/
2
4
O
OPh
N
This work (in the absence of reductants)
DG
DG
none
Ni/
O
R'
H
O
H
activation of C(O)-OR (R = alkyl) bond
no an external reductant
DG = N(sp2) and O(sp2) atoms and arene ring!!
Figure 1. Transition-metal-catalyzed reductive removal of an ester group
Our study began by optimizing the conditions for the reaction
of ethyl benzoate with an oxazolyl group at the ortho position,
as in 1a (Table S1). Among NHC ligands examined, free IMes
were found to be the most effective. Finally, the following
conditions were determined as a standard reaction conditions:
1a (0.2 mmol), Ni(cod)2 (0.02 mmol), IMes (0.02 mmol) in
toluene (0.5 mL) at 180 ℃ for 14 h.
Cross-coupling reactions of esters involving the cleavage of C-
O bonds have been extensively studied.5i-l However, aryl esters
(-C(O)-OAr) were generally used as the starting ester because
We next examined the effect of an alkoxy group in O-alkyl
esters (Table 1). All esters with primary alkoxy groups and
secondary alkoxy groups gave the desired product 2 in good
yields. However, the reaction of an ester with tertiary alkoxy
group 1e gave the desired product but in low yield.
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita,
Osaka 565-0871 (Japan) E-mail: chatani@chem.eng.osaka-u.ac.jp
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1: Screening of substituents on an alkoxy groupa
as in 9 and 10. In the case of a substrate with two VeieswteArrtigclreoOunplinse,
as in 10c, reductive cleavage of an ester group occurred only at
O
DOI: 10.1039/C9CC07710C
O
H
Ni(cod)2
IMes
10 mol%
10 mol%
N
the ortho-position of an oxazolyl group and the other ester
group at the meta-position remained intact. Various functional
groups, such as CF3, ester, amide, NMe2, and OMe were
tolerated. However, the reaction of a substrate containing a 4-
pyridyl group 11 gave no desired product and 91% of the
starting material was recovered. An amido group also
promoted the reductive defunctionalization, as in 12, 13 and
14. The reaction was also applicable to π-extended aromatic
esters, 15 and 16 and the indole derivative 17.
N
OR
toluene 0.5 mL
180 °C, 14 h
O
2
1
0.2 mmol
GC yields
entry
1
R
2
1
1b
79%
87%
97% trace
77% 5%
3%
3%
Me (
)
2
3
4
5
Et (
1a
)
1c
Bu (
)
i
1d
Pr (
)
)
t
25% 43%
1e
Bu (
Ni(cod)2
10 mol%
Ar
ICy·HCl 20 mol%
Ar
H
N
O
NaOtBu 25 mol%
N
OEt
N
N
N
N
toluene 0.5 mL
180 °C, 14 h
OEt
OEt
OEt
OEt
O
Me
O
O
O
O
R
a
b
1a
3
4
5
87%
87%
O
95%
90%
52%a
27%
21%
19a
(
R = H (
NMe2
)
O
O
OEt
OEt
19b
)
OEt
NEt2
OiPr
OiPr
OiPr
19c
Ph (
CF3
)
NHPh
OiPr
O
O
50%a
O
19d
(
)
a
a
20
18
38%
13%
O
O
O
O
c
a
6
7
8
38%
74%
57%
Scheme 1. Screening of directing groups. Reaction conditions: ester (0.2 mmol),
Ni(cod)2 (0.02 mmol), IMes (0.02 mmol) in toluene (0.5 mL) at 180 ℃ for 14 h.
Yields shown are isolated yields, unless otherwise noted. aGC yield. bNMR yield.
cICy⋅HCl (0.04 mmol) and NaOtBu (0.04 mmol) were used instead of IMes.
OEt
OEt
OEt
OEt
O
O
O
O
21
24
34%
71%
23
50%
22
36%
O
R
Me
R = H (10a)
10b
)
91%
75%
O
O
Me
N
N
CF3
(
N
10c
93%
CO2Et (
)
OEt
OEt
OEt
OEt
10d
CONMe2
(
) 98%
87%
OEt
O
N
Me
26
10e
NMe2
(
)
a
O
O
9
58%
O
O
O
O
O
49%
77%
59%b
27
N
O
25
NEt2
OiPr
NEt2
OiPr
NEt2
OiPr
N
Scheme 3. Substrate scope. Reaction conditions: ester (0.2 mmol), Ni(cod)2 (0.02
mmol), ICy⋅HCl (0.04 mmol), NaOtBu (0.05 mmol) in toluene (0.5 mL) for 14 h at
180 °C. Yields shown are isolated yields. aGC yield bNaOtBu (0.06 mmol) was used.
R
OEt
O
O
O
14a
R = OMe (
)
50%b,c
76%b
b
b
11
12
13
0%
57%
64%
14b
)
CF3
(
It is noteworthy that an arene ring also functions as a directing
group. While ethyl [1,1'-biphenyl]-4-carboxylate (18) gave
biphenyl only in 13% yield, the reaction of ethyl [1,1'-
biphenyl]-2-carboxylate (19a) gave the biphenyl in 52% yield,
indicating that a phenyl group at the ortho-position would
coordinate to Ni, which would then accelerates the reaction.17
It was found that an electron-withdrawing group on the phenyl
ring facilitates the reaction, as in 19d. A naphthalene ring, as in
21 and 22 and heteroaromaric rings, as in 24 and 25 were also
found to function as a directing group. An alkene also serves as
a directing group, as in 27.
Mechanistic studies were conducted in an attempt to gain
insights into the reaction mechanism (Scheme 4). The reaction
of 1f gave 2 in 92% yield along with 5-nonanone in quantitative
yield, suggesting that the Hα atom is transferred to the ortho-
position (Scheme 4a). In fact, in the reaction of 1a-d5, a
deuterium atom was transferred to the ortho-position,
although some deuterium loss was observed (Scheme 4b). A
crossover experiment using 1a-d5 and 10e was conducted
(Scheme 4c). Curiously, deuterium atoms were scrambled,
O
O
O
O
OiPr
OiPr
NEt2
OiPr
N
OEt
N
O
b
b,d
15
16
17
64%
85%
45%
Scheme 2. Substrate scope. Reaction conditions: ester (0.2 mmol), Ni(cod)2 (0.02
mmol), IMes (0.02 mmol) in toluene (0.5 mL) for 14 h at 180 °C. Yields shown are
isolated yields, unless otherwise noted. aGC yield bICy⋅HCl (0.04 mmol) and
NaOtBu (0.04 mmol) were used instead of IMes. cN,N-Diethylbenzamide was
obtained in 12% NMR yield. dNapthalene was also obtained in 17% GC yield.
We next turned our attention to evaluating the effect of
directing groups (Scheme 1). The reaction was applied to
aromatic esters with an N-heteroaromatic ring, as in 3, 4, and
5. Curiously, we found that amide and ester groups also
function as directing groups. Secondary and tertiary amide
groups, as in 6 and 7 can be used as directing groups. When
using diisopropyl phthalate (8), isopropyl benzoate was
obtained in 38% yield.
We next examined the scope of the reaction with respect to
substrates. An oxazoyl group served as a good directing group,
2 | J. Name., 2012, 00, 1-3
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although no H/D exchange was observed in the recovered with the elimination of acetaldehyde.22 Finally, reductive
View Article Online
DOI: 10.1039/C9CC07710C
starting materials. To gain additional insights into the reaction elimination gives 2 with the regeneration of the catalytic active
mechanism, the reaction of 10e with a deuterium labeled Ni(0) species. In a crossover experiment (Scheme 4c), no H/D
aldehyde was examined. The reaction gave the desired exchange between the starting materials was detected,
product albeit in low yield and 26% of the atoms introduced in suggesting that the cleavage of
a C-O bond and/or
the ortho position of the product were deuterium atoms decarbonylation is irreversible. The results obtained from a
(Scheme 4d). The reaction of 1a with a boronic ester gave the crossover experiment (Scheme 4c), in which deuterium atom
coupling product in 29% yield (Scheme 4e).18 This result scrambling was observed in the products and the result
indicates that the present reaction proceeds via a similar obtained from Scheme 4d suggest that the β-hydride
pathway to that of previously reported coupling reactions of elimination step is reversible. The results obtained from
esters.5i
Scheme 4e indicate that an alkoxy intermediate C is a key
(a)
intermediate.
O
Ni(cod)2
IMes
10 mol%
10 mol%
O
H
Ni(cod)2
+ NHC
O
O
O
Bu
N
O
N
+
toluene 0.5 mL
180 °C, 14 h
Bu
H
N
N
Bu
- 2 cod
+ CO
OEt
O
Bu
H
Ni(NHC)
O
1
2
1f 0.2 mmol
2
92% (GC)
>99% (GC)
reductive
elimination
(b)
O
N
O
N
Ni(cod)2
IMes
10 mol%
10 mol%
O
O
N
toluene 0.5 mL
180 °C, 14 h
OC2D5
N
CO
D
Ni NHC
OEt
Ni
O
89% D
>99% D
H
NHC
D
O
1a-d5
0.2 mmol
2-d 72% (GC)
A
(c)
-H elimination
Me2N
O
C-O cleavage
O
O
Ni(cod)2
IMes
0.04 mmol
0.04 mmol
CH3CHO
N
N
O
N
+
OC2D5
toluene 1.0 mL
180 °C, 3 h
O
N
OC2H5
O
>99% D
0.2 mmol
CO
Ni
Ni NHC
OEt
1a-d5
10e 0.2 mmol
EtO
NHC
O
B
C
decarbonylation
Scheme 5. Proposed mechanism.
Me2N
O
O
N
N
1a-d5
10e
+
+
+
H/D
H/D
34% D
In conclusion, we report the nickel-catalyzed reductive
removal of an ester group in the absence of an external
reductant. This new reaction proceeds via a mechanism
involving the oxidative addition of the inert C–O bond of an O-
alkyl ester, which was achieved by the assistance of a directing
group. In the course of our investigations of the substrate
scope, we found that an aryl group also functioned as a
directing group and new mechanistic aspects of the reaction
were revealed. Studies aimed at expanding this new reactivity
of nickel complexes to reactions without a directing group are
ongoing in our laboratories.
54% D
46%
17%
30%
2-d
60% (GC)
(d)
Me2N
O
O
O
N
+
OC2H5
C11H23
D
95% D
10e
0.5 mmol
Me2N
0.5 mmol
Ni(cod)2
IMes
10 mol%
10 mol%
O
N
toluene 1 mL
180 °C, 14 h
H/D
4% (isolated by GPC)
26% D
Conflicts of interest
There are no conflicts to declare.
(e)
O
PhBnep
Ni(cod)2
IMes
1.5 equiv.
10 mol%
10 mol%
2.0 equiv.
O
O
Na2CO3
N
N
N
+
toluene 0.5 mL
180 °C, 14 h
OC2H5
Ph
H
O
Notes and references
29% (GC)
1a
0.2 mmol
29%
1
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Scheme 4. Mechanistic Studies.
A proposed reaction mechanism is shown in Scheme 5. The
coordination of a nitrogen atom to Ni(0) follow by the
oxidative addition of a C-O bond gives the complex B19 through
A.20 Decarbonylation in complex B affords the complex C,21
which undergoes β-hydride elimination to form complex D
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CC07710C
Journal Name
COMMUNICATION
Nickel-Catalyzed Reductive Defunctionalization of Esters in the
absence of an External Reductant: Activation C-O Bonds
Yasuaki Iyori, Kenjiro Takahashi, Ken Yamazaki, Yusuke Ano and Naoto Chatani*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
DG
DG
none
Ni/
O
R'
H
O
H
activation of C(O)-OR (R = alkyl) bond
no an external reductant
DG = N(sp2) and O(sp2) atoms and arene ring!!
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan. E-mail: chatani@chem.eng.osaka-u.ac.jp
‡ The authors contributed equally
† Electronic Supplementary Information (ESI) available: Experimental details and
Spectroscopic data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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