Direct C-H alkylation and indole formation of anilines with diazo compounds under rhodium catalysis

Article abstract of DOI:10.1039/c5cc07767b

The rhodium(iii)-catalyzed direct functionalization of aniline C-H bonds with α-diazo compounds is described. These transformations provide a facile construction of ortho-alkylated anilines with diazo malonates or highly substituted indoles with diazo acetoacetates.

Full text of DOI:10.1039/c5cc07767b

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ARTICLE TYPE
Direct C−H alkylation and indole formation of anilines with diazo
compounds under rhodium catalysis
Neeraj Kumar Mishra, Miji Choi, Hyeim Jo, Yongguk Oh, Satyasheel Sharma, Sang Hoon Han, Taejoo
Jeong, Sangil Han, Seok-Yong Lee and In Su Kim*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The rhodium(III)-catalyzed direct functionalization of aniline
C−H bonds with α-diazo compounds is described. These
transformations provide the facile construction of ortho-
sp2 C−H functionalizations of a variety of (hetero)arenes using α-
diazo esters were also described.12
The indole nucleus is a privileged structural motif in natural
bioactive products, drugs, and other functional molecules.13 The
50 prevalence of indoles in bioactive molecules has led to lots of
efforts for the development of many useful methods for their
preparation. In particular, the Fischer indole synthesis14 and the
Larock’s indole synthesis15 represent valuable synthetic protocols.
Recently, direct synthesis of indoles based on the catalytic C–H
55 bond activation has attracted much attention owing to its
remarkable potential for atom economy and environmental
sustainability.16
10 alkylated anilines with diazo malonates or highly substituted
indoles with diazo acetoacetates.
Transition-metal-catalyzed C−H bond functionalization has
become an attractive alternative to traditional cross-coupling
reactions, because such methods avoid a multistep preparation of
15 preactivated starting materials and the production of
stoichiometric amounts of metallic waste.1 In this area, recent
progress has been made on the catalytic carbenoid insertion
reaction as a new approach toward C−H bond functionalization.
For example, Yu et al. first described the Rh(III)-catalyzed
20 carbene insertion of arene C−H bonds containing oxime and
carboxylic acid directing groups using α-diazo esters to afford
various ortho-functionalized arenes.2 Also, this protocol has been
successfully applied for the synthesis of isoquinolones through
ortho-alkylation of benzylamines followed by intramolecular
25 cyclization. In the meantime, Miura and coworkers reported the
Co(II)-catalyzed C−H functionalization of 1,3-azoles with N-
tosylhydrazones as carbene precursors.3 Rovis, Glorius, and Cui
respectively demonstrated the facile strategy for the formation of
various heterocycles such as γ-lactams,4 isoquinolines/pyridine
30 N-oxides,5 and azepinones6 using electron-deficient diazo
compounds under Rh(III) catalysis. Furthermore, the Rh(III)-
catalyzed construction of 1-aminoindole derivatives using 2-
acetyl-1-arylhydrazines and diazo compounds has been reported
by Wang and coworkers.7 Glorius et al. reported the Co(III)-
35 catalyzed C−H functionalization of N-heteroarylarenes with diazo
compounds to afford a new class of polycyclic hydrocarbons with
tunable emission wavelengths.8 Recently, Lee9a and Kim9b
independently reported the efficient synthesis of cinnolines using
azobenzenes and diazo compounds via C−H bond activation.
40 Wang et al. disclosed the efficient formation of ortho-alkenyl
phenol derivatives via the Rh(III)-catalyzed coupling reaction
between N-phenoxyacetamides and N-tosylhydrazones and diazo
esters.10 Wang and coworkers also demonstrated the direct C−H
alkylation of polyfluoroarenes with N-tosylhydrazones and diazo
45 compounds under Cu(I) catalysis.11 In addition, chelation-assisted
In continuation of our recent studies on the rhodium-catalyzed
C−H bond functionalization of aromatic compounds,17 we herein
60 present the Rh(III)-catalyzed ortho-C−H alkylation of anilines
with α-diazo esters. Additionally, the synthesis of indoles derived
from anilines and alkyl α-diazo acetoacetates is also described
(Figure 1).18
65
Figure 1 Rh(III)-catalyzed heterocycle synthesis using α-diazo esters.
In our initial study, N-phenylpyrimidin-2-amine (1a) and
dimethyl 2-diazomalonate (2a) were chosen as model substrates
for optimizing the reaction conditions, and the selected results are
summarized in Table 1. To our delight, the rhodium complex,
70 derived from [Cp*RhCl2]2 and AgOAc, was found to promote the
coupling of 1a and 2a in dichloroethane (DCE) at 60 °C for 24 h
to give the ortho-alkylated product 3a in 16% yield (entry 1).
Screening of other solvents showed that MeOH was found to be
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DOI: 10.1039/C5CC07767B
the most effective in this transformation (entries 2−6). Exchange
of silver additive to AgSbF6 and AgNTf2 to generate cationic
sterically accessible position as well as the tolerance of the
reaction conditions to the chloro moiety, which provides a
rhodium catalysts afforded the decreased reactivity (entries 7 and 40 versatile synthetic handle for further reactions. Moreover, we
8). Other additives such as Ag2O and NaOAc were found to be
less effective in this coupling reaction (entries 9 and 10). In
addition, the use of both AgOAc and AgSbF6 provided our
desired product 3a in low yield (entry 11). In addition, treatment
of cobalt and iridium catalysts was found to display lower
reactivity under otherwise identical conditions (entries 12 and 13).
were pleased to observe C–H alkylation of ortho-substituted
aniline 1n, which provided the corresponding product 3u in 94%
yield.
5
Table 2 Scope of N-phenylpyrimidin-2-amines and diazo compounds a
10 Furthermore, this reaction furnished almost comparable yield of
the desired product 3a under 2 equiv. loading of 2a (entry 14).
Table 1 Selected optimization of the reaction conditionsa
MeO2C
CO2Me
H
N
H
[RhCp*Cl2]2
H
N
CO2Me
N
additive, solvent
+
N
N2
CO2Me
60 oC, 24 h
N
N
1a
2a
3a
Entry
1
Additive (mol %)
AgOAc (15)
AgOAc (15)
AgOAc (15)
AgOAc (15)
AgOAc (15)
AgOAc (15)
AgSbF6 (15)
AgNTf2 (15)
Ag2O (50)
Solvent
Yield (%)b
DCE
THF
16
trace
48
2
3
toluene
MeCN
MeOH
t-AmOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
4
10
5
62
6
5
7
30
8
26
9
15
10
11
12c
13d
14e
NaOAc (15)
31
a
45 Reaction conditions: 1a–1m and 4a (0.2 mmol), 2a–2g (0.24 mmol),
[RhCp*Cl2]2 (2.5 mol %), AgOAc (15 mol %), MeOH (1 mL) under air 60
oC for 24 h in sealed tubes. b Yield isolated by column chromatography.
AgOAc (15) + AgSbF6 (10)
AgOAc (15)
41
N.R.
25
AgOAc (15)
During the screening of substrate scope of diazo compounds,
we found that ethyl α-diazo acetoacetate (2h) was coupled with
50 1a to give C2- and C3-substituted indole 5a in 51% yield, which
might be formed via ortho-alkylation of 1a and subsequent
intramolecular condensation (Table 3). Thus, we investigated the
further optimization for the synthesis of indole from 1a and 2h.
Interestingly, we found that a pyridinyl directing group is unique
55 in its ability to facilitate high levels of indole formation under the
standard reaction conditions. Thus disubstituted indole 5b at C2-
and C3-positions was obtained in 94% yield. Further study
revealed that alkyl α-diazo acetoacetates 2i–2l were found to be
favored in indole formation reaction, affording the corresponding
60 products 5c–5f in good to high yields, whereas sterically
congested methyl 2-diazo-4,4-dimethyl-3-oxopentanoate (2m)
was unreactive under the current reaction conditions. In addition,
para-substituted aniline derivatives 4b–4e participated in the
alkylation and tandem cyclization to provide indole adducts 5h–
65 5k with good reactivity. Notably, the reaction of meta-substituted
N-phenylpyridin-2-amines 4f–4i preferentially occurred at the
less sterically hindered position to afford the corresponding
product 5l–5o as single regioisomers. To our pleasure, this
transformation could be applied to ortho-substituted aniline 4j
70 under the present conditions to furnish C7-substituted indole 5p
in 95% yield. To demonstrate the practicable synthesis of
substituted indoles, we scaled up the reactions to 4 mmol of N-
AgOAc (15)
62
a
Reaction conditions: 1a (0.3 mmol), 2a (0.36 mmol), [RhCp*Cl2]2 (2.5
15 mol %), additive (quantity noted), solvent (1 mL) under air at 60 oC for 24
b
h in reaction tubes. Isolated yield by flash column chromatography.
c
d
[CoCp*(CO)I2] was used as a catalyst. [IrCp*Cl2]2 was used as a
catalyst. e 2a (0.6 mmol, 2 equiv.) was used.
With the optimized reaction conditions in hand, the substrate
20 scope of N-phenylpyrimidin-2-amines and diazo compounds was
examined, as shown in Table 2. The coupling of symmetrical α-
diazo esters 2b−2e and N-phenylpyrimidin-2-amine (1a) was
found to be favoured in the alkylation reaction to afford our
desired products 3b–3e in good to high yields. However, in case
25 of α-diazo esters 2f and 2g containing sulfonate and phosphonate
groups, decreased yields of ortho-alkylation adducts 3f and 3g
were obtained. This reaction was also found to be reactive with
aniline compound 4a containing a pyridinyl directing group to
furnish 3h in 72% yield. It should be noted that all reactions
30 exclusively afforded the monoalkylated products, and a trace
amount of dialkylated products was observed by 1H NMR or GC-
MS analysis. Furthermore, the reactions between para- and meta-
substituted anilines 1b–1m and 2c were screened under standard
reaction conditions. All reactions proceeded smoothly to afford
35 the desired products 3i–3t in satisfactory yields irrespective of the
electronic property of substrates. Particularly noteworthy were
the mono-selectivity and site-selectivity found at the more
2
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15 alkylation reaction using diisopropyl 2-diazomalonate (2c) can be
performed under slightly modified reaction conditions to provide
C7-alkylated product 7a in 90% yield (Scheme 1, eq. 1).12b
Furthermore, cyanation19 and amidation20 reactions were
achieved at indolic C7-position to give the corresponding
20 products 7b and 7c, respectively (Scheme 1, eqs. 2 and 3). While
performing the intramolecular cyclization between ester and
amine groups of 3a to afford indolin-2-one under basic conditions,
a tricyclic 6H-pyrimido[2,1-b]quinazoline 8a was unexpectedly
obtained in 42% yield (Scheme 2).
phenylpyridin-2-amine (4a), and obtained 1.04 g of 5b in 93%
isolated yield.
Table 3 Scope of indole synthesisa
X
H
N
H
COR2
[RhCp*Cl2]2 (2.5 mol %)
AgOAc (15 mol %)
N
X
N
+
N2
R1
R1
R2
N
CO2R3
MeOH, 60 oC, 20 h
CO2R3
5a-5p, %b
2h-2m
1a (X = N)
4a-4j (X = C)
N
N
N
N
N
N
N
N
N
Me
Me
Me
Me
CO2Et
5a, 51% (50%)c
CO2Et
5b, 94% (93%)d
CO2Me
CO2tBu
5d, 94%
25
5c, 90%
Scheme 2 Transformation of alkylated aniline.
N
N
N
N
Finally, we successfully removed the pyridinyl directing group
of indole 5b by treating with MeOTf and subsequent base-
mediated hydrolysis to result in the free (NH)-indole 9a in 63%
30 yield (Scheme 3).21
Me
Me
Me
Me
N
N
N
N
Me
Me
MeO
MeO
CO2Bn
CO2Et
CO2Me
CO2Et
5e, 90%
5f, 66%
5g, N.R.
5h, 94%
N
N
N
N
N
H
N
N
N
N
N
(i) MeOTf, DCM, rt, 24 h
Me
N
Me
Me
Me
Me
Me
(ii) 2 N NaOH, EtOH, 60 oC, 12 h
Me
Me
F3CO
Cl
Cl
CO2Et
CO2Et
CO2Et
CO2Et
5k, 93%
CO2Et
CO2Et
5b
9a
, 63% yield
5i, 78%
5j, 60%
5l, 84%
Scheme 3 Removal of a pyridinyl directing group.
Me
N
N
N
N
F
N
N
N
N
To gain a mechanistic insight, the kinetic isotope effect
experiments of 4a and deuterio-4a were carried out (Scheme 4).
35 The KIE value of 1.02 was observed, thus indicating that C−H
cleavage might not be involved in the rate-determining step (see
Supplementary Information for details).22
Me
Me
Me
Me
CO2Et
5m, 80%
CO2Et
5n, 68%
CO2Et
5o, 93%
CO2Et
5p, 95%
a
Reaction conditions: 1a and 4a–4j (0.2 mmol), 2h–2m (0.24 mmol),
5 [RhCp*Cl2]2 (2.5 mol %), AgOAc (15 mol %), MeOH (1 mL) under air 60
oC for 24 h in sealed tubes. b Yield isolated by column chromatography.
c
d
2h (0.6 mmol, 2 equiv.) was used. Scale-up experiment of 4a (4 mmol
scale).
iPrO2C
CO2iPr
N
CO2iPr
CO2iPr
[RhCp*Cl2]2 (2.5 mol %)
AgSbF6 (10 mol %)
N
5b
+
N2
2c
(1)
Me
Scheme 4 KIE experiments.
EtOH, rt, 24 h
(150 mol %)
CO2Et
, 90% yield
1
H
N
/2 [Cp*RhCl2]2 + AgOAc
7a
N
N
H
4a
NH
COMe
CO2Et
[Cp*Rh(OAc)2]
CN
N
[RhCp*Cl2]2 (5 mol %)
AgSbF6 (20 mol %)
NC
Ph
N
N
5b
+
HOAc
(2)
Me
Ts
NaOAc (30 mol %)
DCE, 110 oC, 24 h
D
HOAc
6a
(200 mol %)
CO2Et
7b, 46% yield
N
H
H
N
N
NH
OH
Me
N
N
Rh
Ln
HN
O
Rh
Me
Ln
N
[RhCp*Cl2]2 (5 mol %)
AgSbF6 (20 mol %)
MeOC
Me
CO2Et
CHO2Et
N
5b
+
A
E
C
(3)
Me
OCN
DCE, 100 oC, 24 h
6b (300 mol %)
CO2Et
, 61% yield
H
N
COMe
CO2Et
N2
7c
10
N
N
Scheme 1 Catalytic functionalization of indole C-7 position.
N
Rh
2h
Me
Ln
MeOC
CO2Et
Next, we investigated the catalytic C7-functionalization of
CO2Et
5b
B
40
indole 5b, generated from our indole formation protocol, with
Scheme 5 Proposed reaction pathway.
various coupling partners. First, we were delighted to find that
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Based on the precedent literatures on C–H functionalization of
aromatic compounds using α-diazo esters,2,5,12c a plausible
reaction pathway for ortho-alkylation anilines and subsequent
indole formation is depicted in Scheme 5. First, coordination of a
pyridinyl directing group on 4a to a Rh(III) catalyst and
subsequent C–H cleavage generates a six-membered rhodacycle
A. Then coordination of α-diazo compound 2h to A and
subsequent release of N2 affords a metal-carbenoid intermediate
B. Migratory insertion delivers a 7-membered rhodacycle species
65
70
75
80
85
90
95
5
10 C, which undergoes protonation to yield the ortho-alkylated
product D and an active Rh(III) catalyst. Enol intermediate E,
formed through keto-enol tautomerization, can undergo
dehydration process to afford indole product 5b.
In conclusion, we disclosed the rhodium(III)-catalyzed C−H
15 alkylation reaction of N-phenylpyrimidin-2-amines with α-diazo
compounds. Notably, anilines containing a pyridine directing
group were easily transformed with α-diazo acetoacetates into
highly substituted indoles, which are known to be crucial
scaffolds of biologically active molecules. Furthermore, the
20 formed indole adducts were subsequently used in the sequential
C−H functionalization process to give C7-alkylated, cyanated,
and amidated indoles. Our ongoing studies seek to expand the
scope to the alkylation of sp3 C−H bonds and the synthesis of
complex heterocycles.
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25
This research was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
Government (MSIP) (Nos. 2013R1A2A2A01005249 and
2014R1A1A2056809).
Notes and references
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30 School of Pharmacy, Sungkyunkwan University, Suwon 440-746,
Republic of Korea. Fax: 82 31 292 8800; Tel: 82 31 290 7788; E-mail:
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procedures and spectroscopic data for all compounds. See
35 DOI: 10.1039/b000000x/
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