The copper-catalyzed C-3-formylation of indole C-H bonds using tertiary amines and molecular oxygen

Article abstract of DOI:10.1002/adsc.201200345

A copper-catalyzed formylation reaction has been developed by employing oxygen (O₂) as the clean oxidant. The C-H bonds of indoles were C-3-formylated by tetramethylethylenediamine (TMEDA) and water (H₂O; in situ formed and external added water) as the carbonyl source in moderate to good yields with good functional group tolerance. Thus, it represents a facile procedure leading to 3-formylindoles. Copyright

Full text of DOI:10.1002/adsc.201200345

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DOI: 10.1002/adsc.201200345
À
The Copper-Catalyzed C-3-Formylation of Indole C H Bonds
using Tertiary Amines and Molecular Oxygen
b
b
b
b
a,
Jianbin Chen, Bin Liu, Dongfang Liu, Shan Liu, and Jiang Cheng *
a
School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, Peopleꢀs Republic of China
Fax : (+86)-577-5699-8939; e-mail: jiangcheng@cczu.edu.cn
College of Chemistry & Materials Science, Wenzhou University, Wenzhou 325000, Peopleꢀs Republic of China
b
Received: April 22, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200345.
Abstract: A copper-catalyzed formylation reaction
has been developed by employing oxygen (O ) as
2
the clean oxidant. The CÀH bonds of indoles were
C-3-formylated by tetramethylethylenediamine
(
TMEDA) and water (H O; in situ formed and ex-
2
ternal added water) as the carbonyl source in mod-
erate to good yields with good functional group tol-
erance. Thus, it represents a facile procedure lead-
ing to 3-formylindoles.
Keywords: aldehydes; CÀH activation; copper; for-
Scheme 1. Design plan.
mylation; tertiary amines
a new iminium intermediate b (step 3, Scheme 1), fol-
lowed by the trapping of H O as a new nucleophile
2
(
step 4, Scheme 1). Thus, it would provide a potential-
Iminium ions are versatile intermediates in organic ly synthetically useful pathway leading to the carbonyl
transformations because they can be trapped by nu- compounds.
cleophiles in situ to form a series of a-functionalized
In 2011, Li described the palladium-catalyzed oxi-
amines. And they can be formed via the oxidation of dative coupling of trialkylamines with aryl iodide pro-
a tertiary or secondary amine with a transition metal viding alkyl aryl ketones, where the carbon atom of
[1]
catalyst. Using this strategy, Murahashi reported the the amine adjacent to the nitrogen atom was installed
[
7]
ruthenium-catalyzed oxidative cyanation of tertiary into the carbonyl of the ketones. Subsequently, Su
[
2]
amines with sodium cyanide.
copper-catalyzed cross-dehydrogenative coupling tion of indoles using anilines as the carbonyl source.
CDC) of tertiary amines with alkynes, active methyl- However, excesses of tert-butyl hydroperoxide
ene compounds, nitromethane, and dialkyl H-phos- (TBHP) and pivalic acid (PivOH) were required as
Li pioneered the reported a state-of-the-art method foor the formyla-
[8]
(
[
3]
phonates, respectively. Afterwards, significant prog- oxidant and additive in the protocol. Employing mo-
ress has been made in the CDC reaction using imini- lecular oxygen as the terminal oxidant in such a trans-
um ions as the key intermediates. In addition, the formation would greatly improve its practicality.
[
4]
[9]
organometallic nucleophiles derived from CÀH bond Herein, we report a simple and practical procedure
cleavage, were good partners in such a transformation. for the copper-catalyzed C-3-formylation of indoles
For example, Li sequentially demonstrated the CDC using molecular oxygen as terminal oxidant and pres-
[
5]
reactions of tetrahydroisoquinolines with indoles. In ent a preliminary mechanistic study.
011, Zhang and co-workers reported the copper-cat- Initially, we selected 1-methyl-1H-indole (1a) and
alyzed oxidative coupling of N,N-dimethylanilines TMEDA as model substrates, and screened various
2
[
6]
with heteroarenes, such as indolizines and indoles.
reaction parameters such as Cu source, additive, sol-
Meanwhile, in light of the great power of the CDC vent, and temperature. Fortunately, the first glimpse
reaction (steps 1 and 2, Scheme 1), we envisaged that of success was obtained by using 20 mol% of CuI
a-functionalized amines could be further oxidized to under O in DMSO with 50 mL of H O at 1208C, de-
2
2
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Jianbin Chen et al.
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Table 1. Selected results of screening the optimal conditions. were also tested as the potential carbonyl sources (see
the Supporting Information, Scheme S1). The result
indicated that the choice of N-methyl-containing sub-
strates was crucial for this transformation, and
TMEDA was found to be the best in this transforma-
tion. The N-methyl-containing amides were totally in-
effective for this transformation. It is noteworthy that
in contrast with Suꢀs protocol, N-methylaniline, which
reacted efficiently in the Ru-catalyzed formylation of
indoles, resulted in no reaction in our procedure;
while N-methylbenzylamine and N,N-dimethylbenzyl-
amine furnished the formylation product in moderate
yields.
With the optimized conditions in hand, we further
pursued the scope of the process, and a series of sub-
strates with different substituents on both nitrogen
atoms as well as aromatic and pyrrole rings of 1 was
examined, and the results are summarized in Figure 1.
As expected, the reaction proceeded smoothly with
yields ranging from moderate to excellent. This proce-
dure tolerated both electron-donating and electron-
[
a]
Reaction conditions: All reactions were run with 1- withdrawing groups such as methoxy, fluoro, chloro,
methyl-1H-indole 1a (0.5 mmol), TMEDA (149 mL, bromo, formyl groups and sulfonic esters on the aro-
1
.0 mmol), O2 (1 atm.), the indicated copper source
matic moiety. Particularly, the heteroaryl indoles, such
as N-methyl-1H-pyrrolo[2,3-b]pyridine 1q, provided
(20 mol%), and dry DMSO/H O (0.25 mL/50 mL), 1208C,
2
AHCTUNGTRENNUNG
isolated yields.
Air.
N2.
the desired product 2q in 74% yield. It was notewor-
thy that halogen functional groups remained intact
under the standard procedures, offering handles for
other valuable manipulations (2l–2n). The effects of
[
[
[
b]
c]
d]
1
008C.
2
substituents (R ) on the pyrrole ring of 1 were also
livering the C-3 formylation product 2a in 39% yield examined. Both electron-donating and electron-with-
within 4 h (entry 1, Table 1). Encouraged by this drawing groups, such as methyl, phenyl, N-methylami-
promising result, other copper sources, such as do, were well-tolerated and delivered the desired
CuBr·SMe , Cu ACHTUNGTRENNUNG( OAc) , and CuF were tested in the products in moderate to good yields (2r–2t). Further
2
2 2
procedures and gave the desired product in moderate investigation showed that substituent groups such as
yields (entries 2–4, Table 1). Gratifyingly, CuCl af- N-ethyl, N-allyl, and N-benzyl analogues did not
2
forded the desired product 2a in 83% yield (entry 5, cause any difficulties for this transformation (2b, 2c,
Table 1). Acceptable yields were also obtained using and 2e). Interestingly, other substituent groups such
either 10 or 5 mol% of CuCl , albeit a prolonged time as N-acetyl transformed to free NÀH in our proce-
2
was required (entries 6 and 7, Table 1). The blank ex- dure (2d). Notably, under this procedure, irrespective
periment confirmed that the reaction did not run in of whether the C-2 positions of indoles were blocked
the absence of a Cu source (entry 9, Table 1). Under or not, the formylation of indoles furnished the corre-
air, the yield dramatically decreased to 43%, and sponding products in moderate to good yields (2a, 2r
under N only a trace of formylation product was ob- and 2s). However, no reaction took place when the C-
2
served (entry 5, Table 1). These results implied that 3 of indole was blocked. Evidently, the formylation
the O may take part in the procedure. In addition, predominantly took place on the C-3 of indoles. No
2
the temperature played an important role in this regioisomeric product was observed by GC-MS and
1
transformation since the yield was sharply lowered at
008C (entry 5, Table 1).
H NMR spectroscopy. Among the free (NÀH) in-
1
doles, only the 2-phenyl-1H-indole 1u delivered the
With these considerations in mind, initial studies formylation product 2u in an acceptable 61% yield,
were performed by using 1-methyl-1H-indole (1a) while the others gave mixtures. For methyl-substituted
with TMEDA as the model reaction. After a series of substrates, the order of activity is 6-> 5->7->2-sub-
trials, we found that 1a could be successfully formyled stituted in terms of yields (2f–2h and 2r).
with 20 mol% of CuCl and 1 atm of O in DMSO at
When 1-methyl-1H-indole 1a was treated with
2
2
1
208C, affording the desired product 2a in 83% yield CuCl (1 equiv.) under O at room temperature for
2
2
(
see the Supporting Information, Table S1). More- 4 h followed by quenching with D O (0.5 mL), 1-
2
over, some other N-methyl -containing substrates methyl-1H-indole was recovered with 20% D on C-3
2
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The Copper-Catalyzed C-3-Formylation of Indole CÀH Bonds using Tertiary Amines
[
12]
philic metalation pathway.
Thus, a cuprate of the
indole CÀH bonds may be involved in this procedure.
However, the test of intermolecular kinetic isotopic
effect for the C-3 CÀH bond in indole failed because
of the rapid exchange between the C-3 CÀH bond
[
10]
with D O (or H O) in our system.
2
2
In the course of our investigation into the detailed
processes of this transformation, an iminium ion salt
was detected from the reaction of 1-methyl-1H-indole
(
1a) with TMEDA under standard conditions by HR-
MS (see the Supporting Information). Thus, the imini-
um ion intermediate might be a key intermediate in
the reaction system.
To gain more mechanistic insights into the newly
developed formylation of CÀH bonds, isotope label-
ling experiments were conducted, as shown in the
Supporting Information, Table S2. The isotope label-
ling experiments firmly gave some conclusions as fol-
lows: (i) the aldehydic hydrogen atom in the formylat-
ed product does not come from H O; (ii) the aldehy-
2
dic oxygen atom originates from the in situ formed
and externally added water. Thus, O acted as the oxi-
2
dant and released water in our procedures. Photo-ini-
tiated singlet oxygen was usually present in the aero-
[
13]
bic oxidation reaction system. To investigate wheth-
er a photosensitized singlet oxygenation was involved
in the present copper-catalyzed oxidation reaction,
the singlet oxygen inhibitor 1,4-diazabicyclo-
AHCTUNGTRENNGNU[ 2.2.2]octane (DABCO) was added to the model reac-
tion. However, the reaction could not be inhibited,
and 2a was obtained in 100% conversion (GC-MS).
Furthermore, the reaction also proceeded smoothly in
the dark. Based on the above results, we envisioned
that singlet molecular oxygen might not be involved
in the present reaction system.
Finally, a possible mechanism is depicted in
Scheme 2 based on the experiments and earlier re-
[
1–8,14]
ported literature.
In cycle A, in the presence of
Cu(II) and O , the oxidation of a tertiary amine pro-
2
vides the iminium ion intermediate 4. Meanwhile, the
cupration of C-3 CÀH bonds of indoles takes place to
form the copper species 5. Then, the addition of the
copper species 5 to the iminium 4 takes place, produc-
ing tertiary amine intermediate 6. The intermediate 6
Figure 1. Copper-catalyzed formylation of indoles CÀH enters catalytic cycle B and forms a new iminium ion
[a]
bonds.
7, which was supported by HR-MS. Finally, hydrolysis
of 7 delivers the formylation product. The released
Cu(I) in cycle A and B is oxidized by O to regener-
2
while under N only a trace of D (<5%) was ob- ate the Cu(II) catalyst.
2
served
on
C-3
(Supporting
Information,
In conclusion, we have developed a simple, copper-
[
10]
Scheme S2). However, in the absence of CuCl , no catalyzed, directly formylation of indole CÀH bonds
2
H/D exchange was detected at room temperature. using oxygen and TMEDA, which provided a practical
Moreover, addition of TEMPO (50 mol%) as a radical approach to access 3-formylindoles.
scavenger had a small effect on the reaction outcome,
which ran counter to the copper-mediated single-elec-
[
11]
tron-transfer (SET) mechanism proposed by Yu.
This phenomena was likely consistent with an electro-
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Jianbin Chen et al.
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Scheme 2. Proposed mechanism.
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Experimental Section
Typical Experimental Procedure for the C-3
Formylation of Indoles
2
007, 1675; k) J. Xie, H. Li, J. Zhou, Y. Cheng, C. Zhu,
Under air, a 20-mL Schlenk tube equipped with a stir bar
was charged with indole 1 (0.5 mmol), TMEDA (149 mL,
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10340.
1
mmol), CuCl2 (13. 5 mg, 20 mol%), freshly distilled
DMSO (0.25 mL) and H O (50 mL). The tube was sealed
2
with a Teflon-lined cap and charged with O . The reaction
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2
mixture was stirred at 1208C for 4 h in an oil bath. After
cooling to room temperature, the resultant mixture was
evaporated with EtOAc (5 mL) under reduced pressure and
the residue was purified by flash column chromatography
on a silica gel to give the products.
1
1-Methyl-1H-indole-3-carbaldehyde
(2a):
H NMR
(
CDCl , 300 MHz): d=9.96 (s, 1H), 8.31–8.30 (m, 1H), 7.63
3
13
(
s, 1H), 7.34–7.30
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(m, 3H), 3.83 (s, 3H); C NMR (CDCl3,
5 MHz): d=184.3, 139.2, 137.8, 125.2, 123.9, 122.8, 121.9,
18.0, 109.8, 33.6.
7
1
[
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Acknowledgements
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We thank the National Natural Science Foundation of China
7
(
No. 20972115), the Natural Science Foundation of Zhejiang
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6
The Copper-Catalyzed C-3-Formylation of Indole CÀH
Bonds using Tertiary Amines and Molecular Oxygen
Adv. Synth. Catal. 2012, 354, 1 – 6
Jianbin Chen, Bin Liu, Dongfang Liu, Shan Liu,
Jiang Cheng*
6
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