Visible-light-induced dearomative oxamination of indole derivatives and dearomative amidation of phenol derivatives

Article abstract of DOI:10.1039/d0cc03506h

Herein, we report a protocol for visible-light-induced dearomative oxamination reactions of indole derivatives to afford functionalized spirocyclic products. These step-economical reactions, which involve C-N and C-O bond formation, feature mild conditions, a broad substrate scope, high yields, exclusive diastereoselectivity and step-economy. In addition, a similar protocol could be used to synthesize spirolactams by dearomative amidation of phenol derivatives.

Full text of DOI:10.1039/d0cc03506h

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DOI: 10.1039/D0CC03506H
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Visible-Light-Induced Dearomative Oxamination of Indole
Derivatives and Dearomative Amidation of Phenol Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Lingang Wu a, Yanan Hao a, Yuxiu Liu a, Haibin Song a and Qingmin Wang a,b,
*
DOI: 10.1039/x0xx00000x
electron transfer.8 This strategy also provides efficient access to
nitrogen-centered radicals9 and has been used as a tactic for
discovering novel methods to form amidyl radicals.10 Herein, we
report a protocol for visible-light-induced dearomative oxamination
of indoles and dearomative amidation of phenol derivatives via
amidyl radicals.
Herein, we report
a
protocol for visible-light-induced
dearomative oxamination reactions of indole derivatives to afford
functionalized Spirocyclic products. These step-economical
reactions, which involve C–N and C–O bond formation, feature
mild conditions, a broad substrate scope, high yields，exclusive
diastereoselectivity and step-economy. In addition, a similar
protocol could be used to synthesize spirolactams by dearomative
amidation of phenol derivatives.
Scheme 1. Synthesis of spirocyclic compounds by dearomative
oxamination of indoles and dearomative amidation of phenol
derivatives
Dearomatized indole and phenol moieties are found in various
synthetic and naturally occurring bioactive molecules,1 and
therefore the development of dearomatization methods is an active
area of research in organic synthesis.2 N-Fused spirocyclic
derivatives are particularly important molecules of this type, and
their synthesis by means of intramolecular dearomative amination
has attracted considerable attention. Dearomative oxamination of
indoles via Rh-nitrenoid species has been reported separately by
the research groups of Stengel, Iwabuchi, and Che (Scheme 1a).3 In
addition, in an elegant study, Xu and co-workers efficiently
constructed spirocarbamates via Fe(II)-catalyzed intramolecular
oxamination of indoles (Scheme 1b).4 Compared with carbamates,
amides are more widely used for the synthesis of natural products
and drug molecules,5 but direct dearomative amidation of indoles
has rarely been explored. However, there has been considerable
work on dearomative amidation of phenols6 in natural product
synthesis.7 In previously reported work, we carried out dearomative
spirocyclization of silyl-protected phenol derivatives.6g However,
the need to use high temperature and excess oxidant encouraged
us to search for a more convenient and cost-effective method for
accessing spirolactam derivatives.
Oxyamination of indoles for spiro derivatives
a) Stengel et al., Iwabuchi et al., Che et al.
O
O
N
O
O
NH
OR
[Rh]
H2
N
N
Boc
Boc
b) Xu et al.
O
O
N
O
O
[Fe]
NH
OR
HN
OR
N
SO2Ph
SO2Ph
This work:
O
N
Ru(bpy)3Cl26H2O (2.0 mol %)
BI-OAc (1.1 equiv)
CONHAr
Ar
R'
R'
OCOR
RCO2H (15 equiv)
N
N
MeCN
blue LED, 12 h
COtBu
COtBu
O
Ru(bpy)3Cl26H2O (2.0 mol %)
BI-OAc (1.2 equiv)
Ar
O
N
RO
HN
Ar
O
MeCN
blue LED, 48 h
We began our study by exploring photocatalytic dearomative
oxamination reactions of N-phenyl-3-(1-pivaloyl-1H-indol-3-
yl)propanamide (1a) with acetic acid (2a) as an oxygen source
(Table 1). We were pleased to find that in the presence of 2.0 mol%
of Ru(bpy)3Cl2·6H2O as a photocatalyst and acetoxybenziodoxole
(BI-OAc) as an oxidant, irradiation of a solution of 1a in acetonitrile
with a 5 W blue LED smoothly gave spirocyclic product 3a in 73%
yield (entry 1). Owing to steric effects, only one diastereomer was
generated. The structure of 3a was unambiguously confirmed by X-
ray crystallography, which indicated that the two new chemical
bonds formed on opposite faces of the molecule.11 Along with 3a,
we also isolated small amounts of 3a’ and o-iodobenzoic acid,
which was generated by decomposition of BI-OAc. In order to
inhibit the influence of acetic acid and o-iodobenzoic acid
originated from BI-OAc, 15 equiv carboxylic acid, as nucleophile
Visible light photocatalysis is a powerful strategy for activating
of organic molecules and has been used to generate a diverse range
of radical intermediates under mild reaction conditions via single
a. State Key Laboratory of Elemento-Organic Chemistry, Research Institute of
Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic
of China. E-mail: wangqm@nankai.edu.cn
b. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, People’s Republic of China
†
Electronic Supplementary Information (ESI) available: Experimental procedures
and compound characterization data. CCDC 1966018. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
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reagent, are used. Screening of various solvents (DCE, DMSO,
Having found suitable conditions for the desired dearomative
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MeOH, acetone, and AcOH) revealed that MeCN was the most oxamination of indoles (Table 1, entry 12),DwOeI:e10v.a1l0u3a9t/eDd0CthCe03s5c0o6pHe
effective (entries 1–6). The other two oxidants that we tested (BI- of this reaction with a variety of indoles 1 and acids 2 (Table 2). The
OH and BI(OAC)2; entries 7 and 8) did not give better results than influence of the substituent on the phenyl group attached to the
BI-OAc, and neither did the other two photocatalysts we evaluated amide was studied first. Substrates bearing para-substituents with
(entries 9 and 10). When the amount of BI-OAc was reduced to 1.1 different electronic properties were tolerated well, affording 3a–3f
equiv and the volume of MeCN was increased to 4 mL, the yield in good yields. Substrates with a meta- or ortho-methyl group gave
increased to 81% (entries 11 and 12). Finally, in the absence of an good yields of spirocyclic products 3g and 3h, respectively. A
oxidant, a photocatalyst, or light, no reaction occurred (entries 13– polysubstituted phenylamide and a naphthylamide (1i and 1j) were
15), which indicates that all three components were necessary.
also compatible with this protocol. Next, we evaluated the
influence of substituents on the indole ring. Substrates bearing a
methyl group, a methoxy group, or a chlorine atom produced the
corresponding products (3k–3o) in good yields. Notably, the
protecting group on the nitrogen atom of the indole nucleus played
an important role in this reaction. When the pivaloyl group was
replaced with another protecting group, such as benzoyl or Boc, the
desired product were not generated (see the Supporting
Information for details). Presumably, the appropriate electron-
withdrawing ability and steric effect of pivaloyl group are essential
for oxamination of the indole derivative. In addition to acetic acid,
other alkyl or aryl carboxylic acids (2b–2f) could be used as
nucleophiles to complete the cyclization reaction of 1a. A methyl-
substituted substrate (1p) failed to provide the desired product (3u),
perhaps owing to steric hindrance. Note that when methanol was
used as a solvent for the reaction of 1a and 2a (Table 1, entry 4),
methoxylated product 3v was obtained in a moderate yield (53%),
along with a small amount of 3a.
Table 1. Optimization of reaction conditions for dearomative
oxaminationa
O
O
AcOH (15 equiv) (2a)
photocatalyst (2.0 mol %)
oxidant (1.5 equiv)
O
O
Ph
N
Ph
N
CONHPh
OAc
COtBu
N
I
N
solvent (2 mL)
blue LED, 12 h
COtBu
N
COtBu
3a'
3a
1a
entry
1
photocatalyst
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
[Ir(dtbbpy)(ppy)2][PF6]
oxidant
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OH
solvent
MeCN
DCE
yield (%)b
73 (7c)
67
0
2
3
DMSO
MeOH
4
10
13
60
12
0
Table 2. Evaluation of substrate scope of dearomative oxamination
reaction of indolesa
acetone
AcOH
5
O
Ru(bpy)3Cl26H2O (2.0 mol %)
CONHAr
6
N
Ar
R'
BI-OAc (1.1 equiv)
R'
OCOR
COtBu
N
RCO2H (15 equiv)
N
COtBu
MeCN (4 mL)
blue LED, 12 h
2
7
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
1
3
O
O
N
O
8
BI(OAc)2
BI-OAc
BI-OAc
BI-OAc
BI-OAc
-
N
OMe
N
Me
Ph
OAc
OAc
N
COtBu
OAc
N
N
COtBu
COtBu
9
47
0
CCDC 1966018
3c, 76%
3b, 85%
3a, 81%
O
Me
O
N
O
-
Mes-Acr-Me+ClO4
O
10
11d
12de
13
14
N
N
N
Br
Cl
Ph
Cl
OAc
COtBu
OAc
N
COtBu
OAc
OAc
N
COtBu
N
N
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
-
78
81
0
COtBu
3g, 87%
3f, 80%
3d, 83%
3e, 77%
O
O
O
O
N
Me
Me
N
Ph
OAc
N
N
Cl
OAc
N
COtBu
OAc
N
OAc
N
COtBu
N
COtBu
COtBu
3k, 73%
3j, 65%
3i, 52%
3h, 64%
O
O
N
O
N
O
O
N
BI-OAc
BI-OAc
0
N
Ph
Ph
Ph
N
Ph
Cl
Ph Me
OAc
MeO
OCOEt
OAc
OAc
OAc
N
N
15f
Ru(bpy)3Cl2·6H2O
0
N
Me
O
N
COtBu
N
COtBu
3o, 83%
COtBu
3n, 70%
COtBu
COtBu
3p, 80%
3l, 84%
3m, 85%
O
O
a
O
N
General conditions, unless otherwise noted: a mixture of 1a (0.25
mmol), photocatalyst (0.005 mmol), oxidant (0.375 mmol), AcOH
(2a, 3.75 mmol), and solvent (2.0 mL) at room temperature was
irradiated with a 5 W blue LED under Ar atmosphere. DCE,
Me
N
N
Ph
OCOPh
Ph
N
Ph
Ph
O
OCOCy
O
N
N
N
N
t
O
COtBu
3r, 70%
I
CO Bu
t
COtBu
O
CO Bu
3s, 78%
3t, 72%
3q, 75%
b
dichloroethane; BI-OAC, acetoxybenziodoxole. Isolated yields are
O
N
O
c
d
given. The yield in parentheses is for 3a. The amount of oxidant
Ph
Ph
N
was reduced to 0.275 mmol (1.1 equiv). e The volume of MeCN was
increased to 4 mL. The reaction was performed in the absence of
OAc
Me
OMe
N
N
COtBu
f
COtBu
3v, 53%b
3u, 0%
light.
2 | J. Name., 2012, 00, 1-3
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a
Reactions were performed on a 0.25 mmol scale, and isolated Scheme 3. Proposed mechanism
View Article Online
DOI: 10.1039/D0CC03506H
yields are given. b The reaction was carried out in MeOH (4 mL).
O
When phenol derivatives
4
were used as substrates,
Ph
N
CONHPh
dearomative amidation afforded spirolactams in moderate to good
yields (Table 3). Specifically, various substituted 3-(4-
methoxyphenyl)-N-arylpropanamides 4a–4h were amenable to this
transformation. In addition, the presence of a Boc-protected amino
group in the α-position relative to the carbonyl group had little
influence on the reaction outcome; the corresponding product (5i)
was obtained in 56% yield. Moreover, TBS-protected phenol
derivative 4j could also afford spirocyclization product 5a. These
spiro derivatives are versatile class of building blocks that could
undergo hydrogenation7c and ozonization reaction7f affording
important synthetic intermediates.
OAc
N
N
COtBu
COtBu
3a
1a
-OAc
hv
RuII
RuII*
BI.
AcOH
RuIII
BIOAc
o-IC6H4CO2H
O
O
Ph
N
CONPh
Ph
N
N
N
COtBu
COtBu
N
C
COtBu
A
B
Table 3. Dearomative amidation of phenol derivativesa
On the basis of the above-described observations and literature
reports, we propose the mechanism shown in Scheme 3. The
transformation is initiated by single electron transfer from
photoexcited Ru(bpy)32+* to BI-OAc, and then Ru(bpy)33+ and BI. are
generated.12 Oxidation of 1a by BI. generates amidyl radical A,
which undergoes an intramolecular 5-exo cyclization to provide
O
Ru(bpy)3Cl2·6H2O (2.0 mol %)
BI-OAc (1.2 equiv)
Ar
RO
HN
Ar
O
N
MeCN (4 mL), 48 h
blue LED
O
4
5
Ph
Br
O
O
O
Me
O
3+
N
carbon radical B. Radical B is oxidized by Ru(bpy)3 to afford
N
N
N
O
O
O
carbocation intermediate C, and then nucleophilic attack by acetic
acid (or another carboxylic acid) from the less-hindered face of C
delivers target product 3a. The dearomative amidation of the
phenol derivatives proceeds via a similar radical process.
O
5a, 80%
5c, 76%
Me
5d, 62%
5b, 71%
O
OMe
CO2Et
O
O
O
N
O
N
N
N
O
O
O
5h, 43%
5g, 84%
5e, 80%
In conclusion, we have developed a novel, efficient protocol
for visible-light-induced dearomative oxamination of Indoles
and dearomative amidation of phenol derivatives. These step-
economical reactions exhibited exclusive diastereoselectivity,
broad substrate scope, and moderate to excellent yields.
Further studies aimed at evaluating the bioactivity of the
products and broadening the synthetic applications of the
protocol are in progress in our laboratory.
5f, 73%
O
O
Ph
O
N
N
O
5a, 83%b
BocHN
5i, 56%
a
Reactions were performed on a 0.25 mmol scale, and isolated
yields are given. R = MeO. b R = TBS.
Having evaluated the scope of these reactions, we turned our
attention to the mechanism (Scheme 2). Control experiments
showed that the dearomative oxamination of indole 1a was
impeded by the addition of a radical inhibitor, either 2,2,6,6-
tetramethyl-1-piperidinyloxy or butylatedhydroxytoluene; under
these conditions, only a trace of the desired product was obtained.
The former reaction generated byproducts 6 and 7, as indicated by
mass spectrometry, suggesting that an alkyl radical had been
generated (see the Supporting Information for details).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21732002, 21672117) for generous financial support for
our programs.
Scheme 2. Mechanistic experiments
O
Notes and references
Ph
O
N
N
[M + H]+
AcOH (15 equiv)
Ru(bpy)3Cl26H2O (2.0 mol %)
BI-OAc (1.1 equiv)
Calc. 504.3221
Found: 504.3222
CONHPh
Ph
1
2
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O
N
N
OAc
tBuOC
N
N
COtBu
MeCN (4 mL), TEMPO (2.0 equiv)
blue LED, 12 h
COtBu
1a
3a
, trace
6
, detected by HRMS
O
AcOH (15 equiv)
Ru(bpy)3Cl26H2O (2.0 mol %)
N
Ph
OAc
CONHPh
BI-OAc (1.1 equiv)
N
N
COtBu
COtBu
MeCN (4 mL), BHT (2.0 equiv)
blue LED, 12 h
1a
3a
, 0%
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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