Rhodium-catalyzed C-H functionalization with N-acylsaccharins

Article abstract of DOI:10.1039/c6ob02526a

A rhodium-catalyzed C-H functionalization with activated amides by decarbonylation has been developed. Notably, this is the first C-H arylation employing N-acylsaccharins as coupling partners to give biaryls in good to excellent yields. The highlight of the work is the high tolerance of functional groups such as formyl, ester, and vinyl and the use of a removable directing group.

Full text of DOI:10.1039/c6ob02526a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C6OB02526A
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Rhodium-Catalyzed C–H Functionalization with N-Acylsaccharins
Hongxiang Wu,ac Tingting, Liu,ac Ming Cui,a Yue Li,a Junsheng Jian,a Hui Wanga and Zhuo Zeng*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
While the progress in the C–H functionalization area was
achieved, the difficulty of utilizing amides as coupling partners
still exist due to the high activation energy required for an acyl-
nitrogen bond cleavage in amides. Recently, Szostak and Shi
utilized twisted amides as the coupling partners in the transition-
metal-catalyzed system, which demonstrated a novel route in
decarbonylation.8 Inspired by the challenge of employing amides
as the arylation precursors, we turned our attention to designing
an alternative methodology.
A rhodium-catalyzed C–H functionalization with activated amides
by decarbonylation has been developed. Notably, this is the first
C-H arylation by employing N-acylsaccharins as coupling partners
to give biaryls in good to excellent yield. The highlight of the work
is high functional group tolerance such as formyl, ester, vinyl, and
the use of a removable directing group.
Carbon–hydrogen bonds are the most common chemical bonds
in organic compounds and a ubiquitous structural motif in
organic chemistry. In the past decades, great advances have been
achieved in transition-metal-catalyzed direct C–H bond
functionalization reactions.1 Site-selective C–H functionalization
afforded a direct conversion of carbon– hydrogen (C–H) bonds
into new carbon–carbon (C–C) bonds or carbon–heteroatom (C–
X) bonds.2 These C–H activation technologies have been widely
utilized in the synthesis of biologically important natural
products and advanced functional materials.3
Herein, we report a strategy for C–H functionalization by
utilizing N-acylsaccharins as coupling partners, which proceeds
via C–H/C–N cleavage (Scheme 1b).
N-acylsaccharins provide several advantages: (a) N-
acylsaccharins can be efficiently synthesized from readily
available and low-cost saccharin. (b) In comparison with acyl
chlorides and anhydrides, N-acylsaccharinsare shelf-stable
andeasy to handle white powder.Also, no decomposition was
observed when N-acylsaccharins suspendedin the water for two
weeks. (c) The twisted structure of N-acylsaccharins resulted in a
chemoselective cleavage of acyl-nitrogen bond (Figure 1).
In this context, a variety of electrophiles such as aryl halides,
tosylates and even aryl sulfonyl chlorides were selected as cross-
5
coupling partners in C–H functionalization.4, However, the
majority of these methods have relatively complicated catalytic
systems, and also have limited substrate scope. To extend the
substrate scope, organometallic reagents, aryl boronic acids, and
arenes were employed as arylation sources in oxidative C–H
functionalization.6 These oxidative approaches have advantages,
but there are some drawbacks such as the requirement of large
amounts of oxidants (Cu or Ag salt) in these reactions and
normally air sensitive.
Also, this protocol tolerated functional groups such as halides,
aldehydes, ketones, esters and heterocycles. And notably, this
study demonstrates a new model in acyl-nitrogen bond cleavage.
Initially, N-benzoylsaccharin 1a was selected as an
electrophile to couple with benzo[h]quinoline 2a to form 10-
phenyl benzo[h]quinoline 3a in the presence of a rhodium
catalyst. For the optimization studies (Table 1). [Rh(COD)Cl]2
was used as the catalyst and toluene as the solvent to investigate
the hypothesis. No reaction occurred when this approach was
tested at 110 oC (Table 1, entry 1). Inspired by the previous study
in the decarbonylation and the preliminary experiment, the
A significant progress has been made by using acyl chlorides and
anhydrides as arylation reagents in C–H functionalization via
extrusion of carbon monoxide (Scheme 1a).7 Both of these
protocols can be efficiently worked in the absence of oxidants,
and demonstrated high functional group tolerance.
Figure 1. Slective Cleavage of C-N.
Cleavage Path B
O
O
Ph
N
S
O
O
Cleavage Path A
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Scheme 1. Previous Works in C–H Functionalization via temperature was increased 140 oC, which led to the desired
View Article Online
Decarbonylation of Carboxylic Acid Derivatives.
product in 18% yield (Table 1, entry 2). To accelerate the
DOI: 10.1039/C6OB02526A
deprotonation process, a variety of bases were selected to
examine the protocol. K3PO4 display a remarkable result among
a series of bases, which led to a 61% yield (Table 1, entries 3-5).
aPrevious works
The solvent also plays an important role in determining the
yield. o-Xylene seems to be the best solvent in this approach
since a lower yield was observed when employing toluene,
xylene and chlorobenzene (Table 1, entries 5-8) under the same
conditions. Also, the loading of rhodium catalyst influenced the
yield of the target compound. For example, changing the loading
from 5.0 mol % to 1.0 mol % led to a lower yield (Table 1,
entries 9-10).
N
N
O
H
Ar
X
catalyst, base, solvent
145 o
C
O
O
O
O
N
O
X = Cl,
O
O
Ar,
,
tBu
O
O
Ar,
O
bThis works
O
With this condition in hand, the reaction temperature was further
N
N
o
H
Ar
N
S
optimized. The result of several experiments proved that 140 C
catalyst, base, solvent
140 o
is the best temperature of this protocol (Table 1, entries 11-12).
Finally, the optimal conditions of this C–H functionalization with
N-acylsaccharins were set as 5 mol % [Rh(COD)Cl]2 in the
presence of K3PO4 (1 equiv) in o-xylene at 140 oC for 12 h.
O
O
C
oxidant free
under air
Table 1 Effect of Catalyst, Solvent and Base
With the optimized reaction conditions in hand, a variety of N-
acylsaccharins were employed to probe the scope and limitations
of this approach (Scheme 2). To investigate the influence of
electronic and steric factors, a range of N-acylsaccharins with
neutral and electron-donating groups were coupled to
benzo[h]quinoline 2a. This provided corresponding C–H
functionalization compounds (3a-3f) in good to excellent yields.
Amides bearing strong electron-withdrawing groups generated
3g and 3h in 79% and 86% yields, respectively. p-Fluoro, p-
chloro and p-bromosubstituents all gave excellentyields (3i-3k).
When N-acylsaccharins contained formyl and acetylgroups in the
para-position, the arylation was accomplished in good yields.
The heterocycles furan and thiophene also provided good yields
in the typical procedure. Overall, these experiments indicated
O
O
N
2a
N
H
Ph
N
S
catalyst, base, solvent
O
O
1a
3a
entry
1
catalyst
baseb
none
temperature yield (%)
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(CO)2Cl]2
RhCl(PPh3)3
110 oC
140 oC
140 oC
140 oC
140 oC
140 oC
140 oC
140 oC
140 oC
140 oC
130 oC
150 oC
140 oC
140 oC
0
2
none
18
47
43
61
83
53
90
51
38
68
90
35
11
3
K2CO3
Na2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
4
Scheme 2. Amide scope in C–H Functionalization.
5
6c
7d
8e
9f
O
O
N
2a
R
N
S
H
[Rh(COD)Cl]2, o-xylene
K3PO4, 140 oC, 12 h
N
O
O
R
3
1
R=
OMe
OMe
10g
11
12
13
14
OMe
85%
3b
91%
3c
90%
3a
87%
3e
89%
3d
OMe
OMe
Cl
CN
CF3
F
OMe
79%
3g
86%
3h
56%
3f
92%
3i
90%
3j
OMe
CHO
Br
77%
3o
O
83%
3n
75%
91%
3m
88%
3k
O
aConditions: 1a (0.3 mmol), 2a (0.2 mmol), catalyst (5 mol %),
toluene (0.8 mL). bBase (0.3 mmol). cxylene. dchlorobenzene. eo-
xylene. f2.5 mol %. g1.0 mol %
3l
S
80%
3q
87%
3p
2 | J. Name., 2012, 00, 1-3
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Scheme 3. Amide scope in C2 arylation of N-(2-pyridyl)
Indoles.
Scheme 5. Indoles scope in C2 arylation of N-V(i2ew-PAyrtricilme Oidnylinle)
Indoles.
DOI: 10.1039/C6OB02526A
O
O
H
N
Ph
N
S
R2
R5
N
R2
R1
R1
O
O
O
2b
R3
R4
O
R3
R
R
N
S
H
N
Ph
1a
[Rh(COD)Cl]2, o-xylene
K3PO4, 140 oC, 12 h
N
N
R4
N
[Rh(COD)Cl]2, o-xylene
K3PO4, 140 oC, 12 h
O
O
N
N
R5
N
N
3
1
R=
OMe
2d-n
3
OMe
OMe
Ph
Ph
Ph
Ph
92%
3s
90%
3t
93%
3r
51%
91%
3u
N
N
N
N
3v
N
N
N
N
N
N
N
N
O
Cl
Br
F
75%
92%
88%
89%
Br 3da
3fa
3ea
3ga
90%
3w
93%
3x
82%
3z
91%
3y
87%
3aa
F
Br
Ph
Cl
Ph
Ph
S
Ph
N
N
N
N
N
N
N
N
N
N
N
81%
3ab
N
that halides, aldehydes, ketones, esters and heterocycleswere
tolerated, even in the case where the N-acylsaccharin contained a
functional group in the ortho-position 3e.
94%
90%
92%
93%
3ja
3ha
3ia
3ka
NC
MeO
MeOOC
Ph
Ph
Ph
Indoles are among the most investigated heterocycles, widely
appearing in natural products and pharmaceuticals.9 Hence the
method was expanded to the direct C2 arylation of indoles via
using a removable N-pyridyl directing group (Scheme 3). The
result demonstrated that utilizing N-acylsaccharins containing
N
N
N
N
N
N
N
N
N
85%
3na
87%
3la
81%
3ma
electron-donating groups in the para, meta, and ortho-positions
in the benzene ring, generated the corresponding products (3r-3v)
in excellent yields. p-Fluoro, p-chloro and p-bromo groups also
gave the C2 arylated products in high yields (3w-3y).
Remarkably, N-styrylsaccharin could be utilized in this approach
to give the corresponding vinyl product in 82% yield (3z). Also,
2-furyl and 2-thienyl amides are compatible with the reaction
conditions (3aa, 3ab).
the functional groups tolerance of this protocol. Irrespective of
methyl occupied in the C4, C5 and C6 position of the indole,
even in C3, corresponding products could be obtained in good to
excellent yields (3ca-3fa). Also, indoles containing halides were
tolerated, particularly bromide (3ga-3ja). Notably, the reaction is
tolerant to electron-donating and electron-withdrawing
substituents on the benzene ring of indole moiety to give desired
products in good yields (3ka-3ma). The result highlight that,
direct access to C-2 arylated indoles via decarbonylation is a
viable method in synthetic chemistry (Scheme 5).
To investigate the influence of N-protecting group, N-(2-
Pyrimidyl) indole (2c) was probed under the same conditions to
provide (3ac-3af) in excellent yields (Scheme 4). Next, a variety
of N-(2-Pyrimidyl) indole derivatives were selected to examine
Scheme 6. Controlled studies.
Scheme 4. Amide scope in C2 arylation of N-(2-Pyrimidyl)
Indoles.
H
N
N
H
N
2b
O
Ph
N
N
O
Ph
O
X
O
N
[Rh(COD)Cl]2, o-xylene
2c
N
R
K3PO4, 140 oC, 12 h
R
N
S
N
[Rh(COD)Cl]2, o-xylene
K3PO4, 140 oC, 12 h
X=
N
O
N
O
O
O
3
1
N
Ts
N
N
N
OMe
R=
S
N
Ph
O
O
O
O
Cl
Br
F
1s
1t
0%
1r
1a
1u
0%
91%
3ac
93%
3ad
89%
3ae
87%
3af
93%
37%
0%
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Conclusions
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In conclusion, utilization of an acDtiOvIa:t1e0d.103a9m/Cid6eOB0a2s52a6nA
electrophilic participant in a decarbonylative catalytic C-H
functionalization procedure has been reported. The variety of
functional groups tolerated using this approach demonstrated the
utilitarian nature of the methodology, allowing efficient access to
biaryl compounds. It also provides an alternative route in C–H
functionalization and a new activation model of C-N amide
bonds
Figure 2. Crystal structure of 1b.
Acknowledgements
The authors gratefully acknowledge the support of National
Natural Science Foundation of China (21272080).The Scientific
Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry
.
Notes and references
Several amides (1r, 1s, 1t) were employed in this catalysis
systems and a similar amide 1u were utilized to compare with N-
benzoyl saccharin under the same conditions. N-benzoyl
saccharin demonstrated higher reactivity than other amides
(Scheme 6).
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To further investigate the selective cleavage of N-acyl saccharins,
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O
O
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N
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[Rh (COD)Cl]2 (I)
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R
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-CO
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