TBHP mediated oxidation of N-2-alkynylphenyl α-amino carbonyl compounds to oxalic amides using visible light photoredox catalysis and their application in the synthesis of 2-aryl indoles

Article abstract of DOI:10.1039/c4ra17232a

A visible light promoted and TBHP mediated oxidative reaction of N-2-alkynylphenyl α-amino carbonyl compounds to N-2-alkynylphenyl oxalic amides was developed. In the presence of CuBr and photocatalyst Ru(bpy)₃Cl₂·6H₂O, the reaction proceeded smoothly to afford the corresponding oxalic amides under the irradiation of a 26 W compact fluorescence bulb at room temperature. Furthermore, N-2-alkynylphenyl oxalic amides could be subsequently transferred to 2-aryl indoles without an additional deacylation step through a favored 5-endo-dig N-cyclization process using AgNO₃ as catalyst.

Full text of DOI:10.1039/c4ra17232a

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TBHP mediated oxidation of N-2-alkynylphenyl a-
amino carbonyl compounds to oxalic amides using
visible light photoredox catalysis and their
application in the synthesis of 2-aryl indoles†
ab
Cite this: RSC Adv., 2015, 5, 17383
Received 29th December 2014
Accepted 3rd February 2015
Wei Liu,‡a Sheng Liu,‡a Hongqi Xie,ab Zhixing Qing,a Jianguo Zengab and Pi Cheng
*
DOI: 10.1039/c4ra17232a
www.rsc.org/advances
A visible light promoted and TBHP mediated oxidative reaction of N-2- methylquinoline derivatives and N-aryl glycine to provide
alkynylphenyl a-amino carbonyl compounds to N-2-alkynylphenyl b-quinolinyl a-amino acid esters.4m
oxalic amides was developed. In the presence of CuBr and photo-
However, the CDC a-functionalization is not the
catalyst Ru(bpy)3Cl2$6H2O, the reaction proceeded smoothly to afford predominant transformation of a-amino carbonyl during the
the corresponding oxalic amides under the irradiation of a 26 W radical-mediated protein damaging process in biological
compact fluorescence bulb at room temperature. Furthermore, N-2- systems. In fact, the formation of a-carbon radicals of
alkynylphenyl oxalic amides could be subsequently transferred to 2- a-amino carbonyl units generally leads to the peptide back-
aryl indoles without an additional deacylation step through a favored bone oxidations and fragmentations via oxalic acid deriva-
5-endo-dig N-cyclization process using AgNO3 as catalyst.
tives and imine intermediates.3 Recently, Huo reported an
aerobic auto-oxidative of glycine derivatives to obtained
oxalic acid derivatives using dioxygen.7 Li and coworkers also
demonstrated that visible light could promote the oxidation
of N-aryl glycine ester to oxalic amide under dioxygen atmo-
sphere with Ru(bpy)3Cl2 as photocatalyst.6g In the last half
decade, visible light photoredox catalysis with transition
metal complexes such as polypyridyl complexes of ruthenium
has received much attention.8 Although that molecular
oxygen has also been used as oxidant in the photoredox-
mediated oxidative organic reactions,9 these reactions were
oen catalyzed by photocatalysts in combination with stoi-
chiometric amounts of chemical oxidants such as BrCCl3,10
K2S2O8,11 Co(acac)3,12 m-dinitrobenzene (m-DNB)13 and aryl
diazonium salts.14 Following our exploring process for the
visible light promoted oxidative reaction,15 we found that
visible light could promote the chemoselective oxidation of
N-2-alkynylphenyl glycine ester 1a to the oxalic amide deriv-
atives 2a with stoichiometric oxidant tert-butyl hydroper-
oxide (TBHP) (Scheme 1).
The a-amino carbonyl is an important structural unit of many
natural and synthetic compounds with diverse biological
activities.1,2 In biological systems, the peptide backbone is also
constructed of a-amino carbonyl units.3 For these reasons, the
a-functionalization of a-amino carbonyl compounds represents
an important transformation in organic chemistry.4 Among
these transformations, the oxidative cross dehydrogenative
coupling (CDC)5,6 of N-aryl a-amino carbonyl compounds such
as glycine derivatives has attracted signicant attention since
the pioneering study of Zhao and Li in 2008.6a In 2010, Huang
and coworkers developed a CDC reaction between a-amino acid
derivatives and ketones under the cooperative catalysis of a
copper salt and secondary amine.6b Recently, Wang reported an
asymmetric CDC reaction of N-substituted glycine esters with
a-substituted b-ketoesters by a chiral copper catalyst.6c More
recently, Huang's group developed a CDC reaction between
Initially, we hoped to access 2,3-diacylated indole 3a
(Scheme 1) with N-2-alkynylphenyl glycine ester 1a as
substrate via activated imine intermediate 4a followed by an
intramolecular nucleophilic attack with the alkyne as repor-
ted by Patel.16 Thus, the reaction was rstly carried out in
DMSO with aqueous TBHP as oxidant. Disappointingly, no
desired indole derivative 3a was obtained. The oxalic deriva-
tive 2a, an isomer of 3a, was isolated in 3% yield along with
the imine 4a and 2-ethynylaniline 5a as major products. To
gain an insight into this type of oxidative reaction, we were
aCo-Innovation Center of Hunan Province for Utilization of Functional Ingredients
from Botanicals, Hunan Agricultural University, Changsha, Hunan 410128, China.
E-mail: picheng55@126.com; Fax: +86 (731) 84686560
bPre-State Key Laboratory for Germplasm Innovation and Utilization of Crop, Hunan
Agricultural University, Changsha, Hunan 410128, China
† Electronic supplementary information (ESI) available. CCDC 1041450. For ESI
and crystallographic data in CIF or other electronic format. See DOI:
10.1039/c4ra17232a
‡ These authors contributed equally to this work and should be considered joint
rst authors.
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Scheme 1 Transformation of N-2-alkynylphenyl glycine ester 1a.
encouraged to optimize the reaction parameters to promote
We opted to investigate the photoredox transformation of
the yield of oxalic acid derivative 2a under more mild condi- module substrates N-2-alkynylphenyl glycine ester 1a to
tions using photocatalytic strategy.
oxalic amide 2a (Table 1). The ruthium complex
Table 1 Reaction conditions evaluationa
Entry
Oxidants
Solvent
Photocatalyst 2% mol
Metal salts 20% mol
Yieldb %
1
2
3
4
5
6
7
8
aq. TBHP
aq. TBHP
MeCN
DMSO
MeCN
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(phen)3Cl2
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
FeCl3
CuI
4
8
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
TBHP in decane
Open air
35
42
40
50
28
23
27
31
41
35
0
DMSO
MeCNc
DMSOc
DMSOc
DMSOc
Ir(ppy)3
9
DMSOc
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
Ru(bpy)3Cl2$6H2O
—
10
11d
12e
13
14
15f
16
17
18h
19
20i
DMSOc
DMSOc
CuBr
CuBr
CuBr
—
DMSOc
MeCN
MeCN
MeCN
Open air
Open air
0
0
—
TBHP in decane
K2S2O8
TBHP in decane
TBHP in decane
TBHP in decane
DMSO/MeCNg
DMSO/MeCNg
DMSO/MeCNg
DMSO/MeCNg
DMSO/MeCNg
CuBr
CuBr
CuBr
CuBr
CuBr
57
7
55
16
5
Ru(bpy)3Cl2$6H2O
a
Unless stated otherwise, the reaction was carried out with 1a (0.5 mmol), oxidants (4.0 eq.), metal salts (20 mol%), and photocatalyst (2 mol%) in
b
c
the indicated solvent (2 mL) and irradiated with 26 W compact uorescent lamp for 24 h at room temperature. Isolated yields. Super dried
solvents. 10 mol% CuBr was used. 40 mol% CuBr was used. 1.1 eq. of K2CO3 was added to the reaction system. Mixed solvents of super
dried MeCN and DMSO (VMeCN : VDMSO ¼ 4 : 1). 3.0 eq. of TBHP was used. Reaction was carried out in darkness.
d
e
f
g
h
i
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Ru(bpy)3Cl2$6H2O and aqueous TBHP were chosen as the yield, entry 3) or (4-methoxylphenyl)ethynyl (2d, 46% yield,
photocatalyst and oxidant respectively. When a solution of entry 4), yields of the desired compounds were slightly lower
1a, aqueous TBHP and another 20 mol% of CuBr in MeCN than the model substrate (1a, 57% yield, entry 1). When an
was irradiated with a household 26 W compact uorescent electron-withdrawing uorine atom was substituted on the
bulb in the presence of 2 mol% of Ru(bpy)3Cl2$6H2O for 24 phenyl ring of phenylethynyl motif, moderate yields of
hours, the desired oxalic derivative 3a was obtained only in desired compounds (2e, 50% yield, entry 5; 2f, 54% yield,
4% isolated yield (Table 1, entry 1). When DMSO was used as entry 6) were achieved. Meanwhile, the optimized conditions
solvent instead of MeCN in the reaction, the yield of 3a could be applied to synthesize oxalic derivatives 2g (43%
slightly increased to 8% (Table 1, entry 2). Under the above yield, entry 7) and 2h (41% yield, entry 8) that possessed tert-
conditions, 2-ethynylaniline (5a) was isolated as major butylethynyl and 1-pentyl adjacent to the nitrogen atom
product. Thus, we considered that water from aqueous TBHP respectively. At this point, we thought that substituent group
solution had a signicant inuence on the reaction outcome R1 that directly connected on the N-phenyl ring possibly had
because of the side hydrolysis reaction. Next, a 6 M solution more obvious effect on the yields of target compounds. Thus,
of TBHP in decane was used as oxidant instead of aqueous we next examined the scope of R1. As outlined in Table 2,
TBHP. Fortunately, the change of oxidant resulted in signif- when the substituent at the N-phenyl ring was an electron-
icantly increased yield of 2a to 35% and 42% in MeCN (Table donating methyl group, a deleterious effect on overall reac-
1, entry 3) and DMSO (Table 1, entry 4) respectively. Based on tion yield was observed (entries 9–13). Meanwhile, 2-ethynyl
the above results, the reaction was then carried out in super anilines 5, the byproduct from imine hydrolysis reaction,
dried solvent and led to slightly increased yield of 2a in MeCN could be isolated with increased yields. To further verify the
(40% yield, entry 5) and DMSO (50% yield, entry 6) respec- inuence of R1, uoro (R1 ¼ F, entries 14–16) and chloro (R1
tively as we expected. In contrast, replacement of Ru(bpy)3- ¼ Cl, entries 17 and 18) substituted substrates were chosen
Cl2$6H2O with the complex Ru(phen)3Cl2 or Ir(ppy)3 resulted for the oxidation experiments. As we expected, the yields of
in decreased yield of 3a (entries 7 and 8). Notably, replace- target compounds 2n–2p (R1 ¼ F, entries 14–16) increased
ment of CuBr with FeCl3 or CuI provided 3a in decreased obviously compared with corresponding methyl substituted
yields respectively (27% and 31% yield, entries 9 and 10). substrates (R1 ¼ Me) due to the strong electron-withdrawing
Furthermore, it should be noted that when CuBr was reduced ability of uorine atom. With respect to the chloro substrates
to 10% mol or increased to 40% mol, only 41% and 35% yield (R1 ¼ Cl, entries 17 and 18), target compounds 2q–2r were
of 3a were achieved respectively (entries 11 and 12). In provided in 49% and 38% yields which slightly decreased
contrast, no desired product was detected when the oxidation from the corresponding uoro substrates (2j–2k). In most
reaction was carried out under open air condition in MeCN cases as shown in Table 2, 2-ethynyl anilines 5 could be iso-
with or without Cu salts (entries 13 and 14). Furthermore, lated as the main byproducts. Furthermore, the structure of
when 1.1 eq. of K2CO3 was added to reaction system under compound 2q was further established by X-ray diffraction
open air condition, no transformation of the substrate was study (Fig. 1).
observed (entry 15). Interestingly, however, we found that a
To the best of our knowledge, the mechanism of TBHP
mixed solvents of MeCN and DMSO (VMeCN : VDMSO ¼ 4 : 1) mediated oxidation of a-amino carbonyl to oxalic derivatives
could slightly improve the yield of 3a (57% yield, entry 16). was not clear. Based on our experimental results and
Furthermore, when 4.0 eq. of K2S2O8 was used as oxidant previous available literature,8,9 a plausible catalytic mecha-
instead of TBHP, 100% conversion of substrate was observed, nism for the visible light mediated oxidation process was
but only 7% yield of desired compound 3a could be isolated proposed. As shown in Scheme 2, visible light excited Ru
with unidentied side products (entry 17). When 3.0 eq. of complex (*Ru(bpy)32+) was a strong reductant (E
¼ ꢀ 0.81
*II/III
1/2
3+
TBHP was used as oxidant, the desired compound 3a was still V vs. SCE),8a which is oxidized by TBHP to generate Ru(bpy)3
obtained in 55% yield (entry 18). Finally, to verify the role of through a single electron transfer (SET) process. Meanwhile,
photocatalyst and visible light in this type of oxidation Ru(bpy)33+, a powerful oxidant (E
¼ 1.29 V vs. SCE),8a is
III/II
1/2
reaction, control experiments were conducted. As outlined in able to oxidize substrate 1 to generate the corresponding
2+
Table 1, the requirement of a photocatalyst and visible light amino radical cation 6 and Ru(bpy)3 to start a new photo-
was veried, as signicantly decreased outcome of 3a was catalytic cycle. The relatively acidic tertiary amine radical
observed without photocatalyst or visible light irradiation cation 6 is deprotonated by a strongly basic species –OH and
respectively (entries 19 and 20).
then further oxidized by an SET process resulting in the
With the optimal oxidative conditions in hand, we next reactive imine intermediate, which coordinates with Cu+ to
sought to determine the scope of N-2-alkynylphenyl glycine provide a more reactive intermediate 7. At this point, the
esters that can be employed in this photocatalytic oxidation activated imine carbon of intermediate
7 accepts the
protocol. As shown in Table 2, the scope of alkyne motif nucleophilic attack from TBHP or t-BuOH and H2O generated
adjacent to the nitrogen atom was examined rstly. Gener- from the reaction system to give N-hybrid acetal peroxide 8
ally, the electron-donating group on the phenyl ring of phe- and byproduct aniline respectively. Intermediate 8 attends
nylacetylene motif decreased the oxidation efficiency. For the second photocatalytic cycle and is then oxidized by
3+
example, when group R2 was (4-methylphenyl)ethynyl (2b, Ru(bpy)3
to give amino radical cation 9, which is
52% yield, Table 2, entry 2), (4-ethylphenyl)ethynyl (2c, 49% deprotonated by basic species t-BuOꢀ or OHꢀ followed by
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Table 2 Visible light promoted oxidation of N-2-alkynylphenyl glycine esters to oxalic amidesa
Entry
R1
R2
Compounds 2
Yield of 2b (%)
Compounds 5
Yield of 5b (%)
1
2
3
4
H
H
H
H
H
H
H
H
4-Me
4-Me
4-Me
4-Me
4-Me
4-F
4-F
4-F
4-Cl
4-Cl
Phenyl
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
57
52
49
46
50
54
43
41
33
37
42
39
32
69
55
56
49
38
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
11
19
18
7
4-Me phenyl
4-Et phenyl
4-MeO phenyl
4-F phenyl
2-F phenyl
t-Butyl
5
15
14
—
—
35
31
28
14
23
5
6
7c
8c
9
n-Propyl
Phenyl
10
11
12
13
14
15
16
17
18
4-Me phenyl
4-MeO phenyl
4-F phenyl
2-F phenyl
Phenyl
4-Me phenyl
2-F phenyl
4-F phenyl
2-F phenyl
8
9
14
12
a
Conditions: compounds 2 (1.0 mmol), Ru(bpy)3Cl2$6H2O (2 mol%), CuBr (20 mol%), TBHP (4.0 eq.) in solvent (4 mL). b Isolated yield. c Hydrolysis
products 5 were not isolated.
According to previous literature, the protocol for transition
metal such as gold, palladium or copper catalyzed cyclo-
isomerization of 2-alkynylaniline to indole derivatives was well
established.17 However, when the nitrogen atom of 2-alkynyla-
niline was acylated, the acylated 2-alkynylaniline carried two
kind of nucleophiles, the N atom of amide and O atom of
enolated amide. Thus, the heterocyclization could take place
either through O- or N-ring closure processes.18 With
compounds 2 in hand, we next attempted to study the subse-
quent transformation of these oxalic amides. As shown in
Scheme 3, when a solution of compound 2, 20 mol% of AgNO3
and 2.0 eq. of K2CO3 in MeCN was heated at 70 ꢁC for 2 hours,
Fig. 1 X-ray derived ORTEP drawing (A) and hydrogen-bonded motif
the reaction proceeded through the favored 5-endo-dig N-cycli-
(B) of compound 2q. Crystal data for compound 2q: C18H13ClFNO3, M
zation process to provide 2-aryl indoles 12 in excellent isolated
¼ 345.74, monoclinic, a ¼ 14.191(2) A˚, b ¼ 4.4973(7) A˚, c ¼ 25.988(4) A˚,
a ¼ 90.00ꢁ, b ¼ 95.570(2)ꢁ, g ¼ 90.00ꢁ, V ¼ 1650.7(4) A˚3, T ¼ 100(2) K,
yield. Notably, no further step was needed to deacylate the N
space group P21/n, Z ¼ 4, m(MoKa) ¼ 0.257 mmꢀ1, 15 559 reflections
atom of indole. This transformation could be applied to
measured, 4056 independent reflections (Rint ¼ 0.0691). The final R1
synthesis of 3-aryl indole derivatives efficiently using cheap
values were 0.0596 (I > 2s(I)). The final wR(F2) values were 0.1059 (I >
AgNO3 as catalyst.
2s(I)). The final R1 values were 0.0986 (all data). The final wR(F2) values
were 0.1197 (all data). The goodness of fit on F2 was 1.049.
In conclusion, we have demonstrated
a visible light
promoted oxidation of N-2-alkynylphenyl glycine ester to oxalic
amides with Ru(bpy)3Cl2$6H2O as photocatalyst and TBHP as
oxidant respectively. The possible oxidation mechanism was
also proposed. Furthermore, the N-2-alkynylphenyl oxalic
amides can be used to synthesize 2-aryl indoles efficiently using
AgNO3 as catalyst without additional deacylation step.
another SET process to give imine peroxide 10. Intermediate
10 decomposes through a SET process to provide enolated
anion 11, which is nally protonated to afford the target
compound 2.
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Scheme 2 Possible mechanism.
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Scheme 3 Transformation of oxalic amides 2 to 2-aryl indoles 12.
Acknowledgements
This research was nancially supported by Natural Science
Foundation of China (no. 31402109), Natural Science Founda-
tion of Hunan Province (no. 13JJ5044), Research Fund for the
Doctoral Program of Higher Education, Ministry of Education of
China (no. 20124320120006).
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17388 | RSC Adv., 2015, 5, 17383–17388
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