Experimental and computational studies on H2O-promoted, Rh-catalyzed transient-ligand-free ortho-C(sp2)-H amidation of benzaldehydes with dioxazolones

Article abstract of DOI:10.1039/c8cc04904a

An efficient and convenient ligand-free, rhodium-catalyzed ortho-C(sp²)-H amidation of benzaldehydes with dioxazolones using H₂O as the key promoter is described. Using this protocol, a wide range of benzaldehyde substrates were selectively amidated in good to excellent yields with broad functional group compatibility. KIE experiments revealed that the C-H bond activation was likely the rate-limiting step. In addition, computational studies indicated that the catalyst precursor interacted with water and dioxazolones to generate the active catalytic species. Notably, the practicality and efficacy of this method were illustrated by a late-stage amidation of an estrone-derived molecule and further transformations of the amidated product.

Full text of DOI:10.1039/c8cc04904a

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Experimental and Computational Studies on H2O-Promoted, Rh-
Catalyzed Transient-Ligand-Free Ortho-C(sp2)–H Amidation of
Benzaldehydes with Dioxazolones
Received 00th January 20xx,
Accepted 00th January 20xx
Jun Ding,a Wei Jiang,a He-Yuan Bai,a Tong-Mei Ding,a Dafang Gao,b Xiaoguang Bao*b and Shu-Yu
Zhang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
regenerate the aldehyde group is required, and this method
requires stoichiometric ligands (Scheme 1B). More recently,
Shi, Yu, Chen and He11 independently developed elegant Ir-
catalyzed ortho-amidations of benzaldehydes with sulfonyl
azides as aminating reagents using substituted anilines as
transient directing groups (TDG) to reversibly form imine
directing groups (Scheme 1C). Nevertheless, no promising or
ideal C–H amination protocol that can avoid the waste
generation associated with using stoichiometric directing
groups based on the aldehyde group of benzaldehyde has
been reported to date.
An efficient and convenient ligand-free, rhodium-catalyzed ortho-
C(sp2)−H amidation of benzaldehydes with dioxazolones using H2O
as the key promoter is described. Using this protocol, a wide range
of benzaldehyde substrates were selectively amidated in good to
excellent yields with broad functional group compatibility. The KIE
experiments revealed that the C−H bond activation was likely the
rate-limiting step. In addition, computational studies indicated
that the catalyst precursor interacted with water and
dioxazolones to generate the active catalytic species. Notably, the
practicality and efficacy of this method were illustrated by a late-
stage amidation of an estrone-derived molecule and further
transformations of the amidated product.
Previous work:
A) Transition-metal-catalyzed C-H amination via directing group (DG)
DG
DG
Cat. [M]
The catalytic construction of C–N bonds has been of great
interest due to the ubiquity of N-containing molecules in
organic and medicinal chemistry1. Among the extensively
explored C–N bond-forming methods2, direct amination of
unactivated C–H bonds under a range of transition-metal
catalysis has drawn significant attention for its atom- and step-
economy and because it does not require prefunctionalization
of the arene precursors3. To control the regioselectivity of the
C(sp2)–H amination of arenes, most of these reactions rely on
directing groups (DG)4, such as amides5, ketones6 and esters7
DG=Amide, Ketone, Ester, etc.
M=Rh, Ru, Ir, Co, Cu, Ni, Pd, etc.
aminating reagents
H
NR1R2
B) Rh- or Ir-catalyzed C-H amidation of benzaldehydes via preinstalled directing group
R
N
O
[Rh] or [Ir]/Ag
H+
H
H
amidating reagents
NHR3
H
C) Ir-catalyzed C-H amidation of benzaldehydes
transient directing group (TDG)
via
O
O
[Ir]/Ag
ArNH2 (TDG):
H
H
ArNH2 (TDG)
a) Shi (2016): 3-CF3PhNH2
b) Yu (2017): 2-F-5-CF3PhNH2
c) Chen & He (2017): 3,5-diCF3PhNH2
TsN3
H
NHTs
Ar
via
Ir
N
(Scheme 1A). In particular, given the ubiquity and importance
of 2-aminobenzaldehyde and its derivatives as building blocks
for numerous bioactive molecules8, directed C(sp2)–H
amination reactions provide direct access to ortho-amidated
benzaldehydes from relatively abundant starting materials.
To overcome the weak coordinating ability and instability of
aldehydes9, preinstalled N-Ts imine or other prepared
removable directing groups (DG) have been developed by Cui,
Kim and Wang10. However, an additional hydrolysis step to
This work:
D) Rh-catalyzed transient-ligand free C-H amidation of benzaldehydes
CHO
CHO
O
R
[Rh]/Ag
H2
"DG or TDG Free"
O
R
Ar
NH
+
O
O
H
N
Ar
O
Scheme 1 Metal-catalyzed ortho-C(sp2)–H amination of arenes
Development of more efficient and direct methods for the
amination of arenes continues to be extensively investigated in
synthetic chemistry. Based on our ongoing interest in
developing effective C–H amination methods12, herein, we
report the first Rh-catalyzed direct ortho-C(sp2)–H amidation
of benzaldehydes using dioxazolones as the aminating reagent
with H2O as the key promoter under preinstalled DG- or TDG-
free conditions (Scheme 1D). This method provides a direct
and practical strategy for the rapid synthesis of a broad range
a.Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs & School of
Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai,
200240, China
b.College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123,
China
E-mail: zhangsy16@sjtu.edu.cn, xgbao@suda.edu.cn
Electronic Supplementary Information (ESI) available: Experimental details,
compound characterization, NMR spectra. CCDC 1819612 for 16, CCDC 1819622
for 40. For ESI and crystallographic data in CIF see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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of diversely functionalized 2-aminobenzaldehyde derivatives Table 2 Substrate scope of benzaldehydes and dioxazolonesa
with excellent regioselectivity and functional group
compatibility. Furthermore, the practicality and efficacy of this
method were illustrated by the late-stage amidation of an
estrone-derived molecule and further transformation of the
amidated product.
Table 1 Optimization of the Rh-catalyzed ortho-amidation of
benzaldehydesa
Entry
[Rh] Cat.
Ag salt
additive
Yieldb (%)
1
2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*(MeCN)3](SbF6)2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
AgSbF6
None
None
None
None
None
NaOAc
ZnCl2
17
trace
11
NR
23
39
trace
58
73
81
82
76
20
82
3c
4
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
5
6
7
8
CuCl2
PhCO2Na
1-AdCO2H
PhCO2H
H2O (1.0)
H2O (2.0)
H2O (0.5)
9
10
11d
12e
13f
[RhCp*Cl2]2
[RhCp*Cl2]2
AgSbF6
AgSbF6
H2O (2.0)
H2O (2.0)
14g
a Reaction conditions:
1 (0.2 mmol), 2 (0.4 mmol), [Rh] catalyst
(3 mol%), silver salt (12 mol%), and additive (1.0 equiv) in TFE
b
(1 mL) at 100 °C under Ar for 12 h. Isolated yield. c 6 mol%
catalyst. d 2.0 equiv H2O. e 0.5 equiv H2O. f At 80 °C. g At 120 °C.
Our investigation commenced with the intermolecular
amidation of benzaldehyde
1 with readily available
dioxazolones as they are highly efficient amidating reagents
2
developed by Chang and other groups. This amidating reagent
has been proven to be a robust amino source due to its high
reactivity and redox-neutrality13. After an initial screening of
Rh(III) complexes, the desired amidated product was isolated
in 17% yield using 3 mol% of [RhCp*Cl2]2and 12 mol% of
AgSbF6 in TFE (entry 1). In addition, control experiments
confirmed the pivotal role of AgSbF6 in the catalytic cycle
a
Reaction conditions: benzaldehyde (0.2 mmol), dioxazolone
(0.4 mmol), [RhCp*Cl2]2 (3 mol%), AgSbF6 (12 mol%), and H2O
(2.0 equiv) in TFE (1 mL) at 100 °C under Ar for 12 h. Isolated
yields are given. 5 mol% [RhCp*Cl2]2 and 20 mol% AgSbF6
b
were used.
(
entries 2 and 3).To promote the turnover of the amination
With the optimized conditions in hand, we first explored the
scope of benzaldehydes in the reaction with dioxazolone 2. In
reaction, a series of additives were tested in the amination
system (entries 4-9). Interestingly, a significant improvement
was achieved with the addition of 1-AdCO2H and PhCO2H as
additives, and they provided yields of 58% and 73%,
general, a broad range of benzaldehydes were well-tolerated
in the reaction and smoothly provided the corresponding
products in good to excellent yields. A variety of ortho-
substituents (Table 2A), such as alkyl, halogens, OMe, OCF3,
and CF3 groups, were well-tolerated in this reaction system
and afforded ortho-amidated benzaldehyde products 4-12 in
good to excellent yields, which will allow further
transformations for organic synthesis. Notably, di-amidated
product 13 was not observed, indicating that this amidation
process has high mono selectivity. The meta-substituted
benzaldehyde substrates (Table 2B) demonstrated that this
approach is highly regioselective, and only the less-hindered
ortho-C–H bond was activated to give desired C–H amidation
products 14-19 as the sole regioisomer. The regioisomer of the
amidation product was confirmed by X-ray crystallography of
respectively (entries 8 and
9). Based on the results of the
screening, we speculated that additional protons might be
beneficial for this amination reaction, and the complete
inhibition of the reactivity in the presence of PhCO2Na verified
this hypothesis to a certain extent (entry 7). Encouraged by
the impressive results, we further screened a variety of proton
sources (see supporting information). Surprisingly, the yield of
the reaction was further improved to 81% upon addition of 1.0
equiv of H2O7, 14, which is an ideal additive because it is clean,
green, and inexpensive (entry 10). A slightly increase to 82%
yield was obtained when the amount of H2O was increased to
2.0 equiv (entry 11). Moreover, raising or lowering the
reaction temperature did not give better reaction performance
compound
16
.
Moreover, various
para-substituted
(entries 13 and 14). Finally, the optimal reaction conditions
benzaldehydes were examined as shown in Table 2C. A range
of substrates with electron-donating (alkyl, OMe, etc.) and
electron-withdrawing groups (halogens, CF3, ester, aldehyde,
were established as follows: 3 mol% [RhCp*Cl2]2, 12 mol%
AgSbF6 and 2.0 equiv of H2O in TFE at 100 °C under Ar for 12 h.
2 | J. Name., 2012, 00, 1-3
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etc.) were well-tolerated, and high monoselectivity was
observed in all cases (20-34).The lower efficiency of para-CN-
(
33) and para-NO2- (34) substituted benzaldehydes may have
been due to chelation to the heteroatoms. Multi-substituted
benzaldehydes (Table 2D) also reacted efficiently to furnish
the sterically favored polysubstituted C–H amidation products
(
35-40) with high regioselectivity and excellent yields.
We next investigated the generality of dioxazolones as
shown in Table 2E. While the methyl- and benzyl-substituted
dioxazolones were not reactive under the current conditions
(
58 and 59), to our delight, the aryl dioxazolones with various
electron-withdrawing and electron-donating substituents on
the phenyl ring could be used as amidating reagents, and they
all reacted smoothly to afford the corresponding amidated
products in moderate to good yields (41-57).
To elucidate the reaction mechanism, a series of preliminary
deuteration experiments were carried out as shown in Scheme
2. An obvious D/H exchange of 35% in the product was
observed when [D5]-benzaldehyde was used as the starting
material, suggesting that the C–H activation was reversible
under these conditions. Moreover, the parallel and
intermolecular kinetic isotope effects (KIE) were determined to
be 2.5 and 2.2, respectively, which indicated that the C–H
cleavage might be involved in the rate-limiting step of the
catalytic cycle. In addition, the possibility of a free radical
mechanism was excluded by the radical capture experiment
(see supporting information).
Scheme 3 Proposed mechanistic pathway
Scheme 2 Preliminary mechanistic experiments
Though the imine-directed process seemed feasible
according to previous studies11d,15, However, our control
experiments were conducted to exclude the possible
Scheme 4 Synthetic utility of this C−H amidation method
To further demonstrate the synthetic utility of this C−H
amidation reaction, a gram-scale reaction was carried out
under reduced catalyst loading, and the reaction afforded 3 in
66% yield (Scheme 4A). We also tested the feasibility of this
amidation protocol in the late-stage modification of complex
molecules. For example, estrone derivative 60, bearing an
aldehyde group, was converted into desired C2-amidation
product 61 with high regioselectivity, and none of the C4-
regioisomer was formed (Scheme 4B).The rapid intermolecular
formation of imine which could act as the directing group.16
A
plausible mechanism using aldehydes directly as the directing
groups was proposed as shown in Scheme 3 based on above
experiments together with DFT calculations17. Initially, the
anion exchange between [RhCp*Cl2]2 and AgSbF6 followed by
the coordination of one molecule of water could form a
complex
Lewis base, the coordinated water molecule could dissociate
to generate the active intermediate . Subsequently, the
yielded hydroxide anion in could lead to the concerted
metalation-deprotonation (CMD) with the ortho C–H bond of
to afford the rhodacyclic intermediate . Next, coordination of
A. With the assistance of substrate 2, behaving as a
B
cyclization of
3 with α,γ-dimethylallenoates was conducted to
B
deliver the 2-quinolin-2-yl β-ketoester bearing a quaternary
carbon center in almost quantitative yield; diverse
transformations of the aldehyde group were performed, and
the newly introduced benzamide group survived in these
reactions (Scheme 4C)19.
In summary, we have developed a highly efficient and
convenient Rh-catalyzed method for the amidation of the
ortho C−H bonds of benzaldehydes in the absence of
preinstalled directing groups or transient directing groups. The
key features of this protocol include the addition of water to
1
C
2
to Rh center of
nitrenoid species
to form species
the final amidated product
species
. The roles of H2O18 in the catalytic cycle were
C
D
followed by extrusion of CO2 would afford
. Then, intramolecular migratory insertion
E
and subsequent protonolysis of
E could yield
3
and regenerate the active Rh
B
probably to coordinate to the metal center to form the active
Rh species and to facilitate the protonation step.
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facilitate the reaction cycle and the ability to provide a broad
range of ortho-amidated benzaldehydes with good functional
group tolerance and excellent regioselectivity. The late-stage
amidation of a natural product-derived molecule was also
achieved, showing the value of this transformation in organic
synthesis. Furthermore, applications of this C−H amidaꢀon
methodology in the synthesis of more complex natural
alkaloids and more detailed mechanistic investigations are
currently under investigation.
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This work was supported by the “Thousand Youth Talents
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17 See Figure S1 in supporting information for more details.
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