Aerobic α-Oxidation of N-Substituted Tetrahydroisoquinolines to Dihydroisoquinolones via Organo-photocatalysis

Article abstract of DOI:10.1002/adsc.201801301

An efficient visible-light-induced α-oxidation of N-substituted tetrahydroisoquinolines to dihydroisoquinolones has been developed using eosin Y as an organo-photocatalyst and oxygen as a green oxidant. The reactions were carried out under mild reaction conditions; the desired dihydroisoquinolones were obtained in up to 96% yield at room temperature under oxygen atmosphere. This transformation provides a convenient route to dihydroisoquinolones with a wide range of substrates. (Figure presented.).

Full text of DOI:10.1002/adsc.201801301

Title: Aerobic α-Oxidation of N-Substituted Tetrahydroisoquinolines to
Dihydroisoquinolones via Organo-photocatalysis
Authors: Kim Aganda, Boseok Hong, and Anna Lee
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801301
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Aerobic α-Oxidation of N-Substituted Tetrahydroisoquinolines
to Dihydroisoquinolones via Organo-photocatalysis
Kim Christopher C. Agandaa, Boseok Hongb and Anna Leea,b,*
a
Department of Energy Science and Technology, Myongji University, Yongin, 17058, Republic of Korea
Department of Chemistry, Myongji University, Yongin 17058, Republic of Korea
b
Fax: (+82)-31-335-7248
Phone: (+82)-31-330-6182
E-mail: annalee@mju.ac.kr
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
precursors of various organic molecules and bioactive
to compounds.[7] In addition, these compounds exhibit
numerous important biological activities such as
antimalarial,[8] antitussive,[9] and antiparkinsonian
properties,[10] and display a mitochondrial branched
chain aminotransferase (BCATm) inhibition effect
(Scheme 1).[11]
Abstract. An efficient visible-light-induced α-oxidation of
N-substituted tetrahydroisoquinolines
dihydroisoquinolones has been developed using eosin Y as
an organo-photocatalyst and oxygen as a green oxidant.
The reactions were carried out under mild reaction
conditions; the desired dihydroisoquinolones were obtained
in up to 96% yield at room temperature under oxygen
atmosphere. This transformation provides a convenient
route to dihydroisoquinolones with a wide range of
substrates.
Keywords: aerobic oxidation, dihydroisoquinolones;
lactams; photoredox catalysis
Oxidation is one of the most common and important
transformations in organic chemistry. Therefore,
numerous oxidation methods have been developed.
The most commonly employed conditions for this
process include the use of stoichiometric amounts of
oxidants or metal-mediated reactions.[1] Furthermore,
green activation modes have been explored in
response to the continuous interest in the
development of environmentally benign oxidation
processes. These include organo-oxidation methods
using DMSO[2] or hypervalent iodine compounds.
A more appealing method, aerobic oxidation, which
employs molecular oxygen (O2 or air), has also been
examined due to the abundant, inexpensive, and
environmentally friendly nature of oxygen.[4] In
addition, visible-light-induced processes have
recently emerged as efficient tools for various
oxidation reactions.[5] Given the importance of the
oxidation reaction in general, it is crucial to develop
new methods to this process.
Scheme 1. Bioactive compounds containing the
dihydroisoquinolone moiety.
[3]
Common methods for the synthesis of
dihydroisoquinolones rely on transition metal-
mediated reactions and/or multi-step processes.[7b, 12]
However, the α-oxidation of tetrahydroisoquinolines
is considered as the most efficient and direct
approach for their preparation.[6g] While the
significance of this transformation has increased, only
a limited number of reports of this method are
known.[13] For example, Zeng and Little examined the
electrochemical oxidative functionalization of
benzylic C-H bonds for the synthesis of N-substituted
The direct α-oxidation of amines to lactams have
proved to be a useful oxidative transformation.[6] This
tetrahydroisoquinolones.[13e] More recently, Wang
and Xiao reported on a direct oxidation procedure for
the synthesis of N-aryl dihydroisoquinolones from N-
approach
dihydroisoquinolones from tetrahydroisoquinolines
through an α-oxidation. In particular,
dihydroisoquinolones represent useful synthetic
allows
the
formation
of
1
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
aryl tetrahydroisoquinolines using a binuclear copper
complex. (Scheme 2, I).[6g]
the limited substrate scope and operational
difficulties of these methods imply the ongoing
demand for a novel and efficient approach. Herein,
we report an efficient aerobic oxidation of N-
substituted
tetrahydroisoquinolines
to
dihydroisoquinolones via organo-photocatalysis
under mild reaction conditions (Scheme 2, II).
We envisioned that the α-oxidation of N-
substituted
tetrahydroisoquinolines
might
be
accomplished via a visible-light-induced process
using oxygen as a green oxidant. In an effort to test
this hypothesis, we employed 2-phenyl-1,2,3,4-
tetrahydroisoquinoline 1aa as a starting material and
tested various reaction conditions (selected examples
are summarized in Table 1). To achieve a green
reaction mode, organic dyes such as eosin Y, rose
bengal, and rhodamine B were examined. While low
yields were detected in the cases of rose bengal (33%,
not shown) and rhodamine B (40%, not shown), eosin
Y was found to be more effective for this
transformation, giving the desired product 2aa in
53% yield (entry 1). Furthermore, a noticeable
improvement in the yield was observed with the
addition of K2CO3 to the reaction (entries 2–5). In
fact, varying the amount of K2CO3 affected the
outcome of the reaction, with a high yield being
obtained in the presence of 1.5 equivalents of K2CO3
(73%, entry 4). Although other bases such as Na2CO3,
Cs2CO3 and pyridine were also tested (entries 6–8),
K2CO3 gave a higher reaction yield (entry 4). Solvent
screening experiments revealed that diminished
yields were obtained in DMF (44%, entry 9) and
acetonitrile (27%, entry 10). Notably, varying the
amount of eosin Y did not improve the yield (entries
11–13), even though the reaction still progressed in
the presence of 0.5 mol% of eosin Y (61% yield,
entry 13). However, eosin Y was deemed necessary
for the reaction to proceed, since almost no
transformation took place in its absence. (entry 14).
Moreover, changing the light source diminished the
reaction yield, giving the desired product 2aa in 43%
and 67% yields when blue or white LED light,
respectively, was employed (entries 15 and 16).
Importantly, no desired product was observed when
the reaction was performed in the dark (entry 17).
Furthermore, the reaction yield decreased when the
transformation was performed under air (entry 18),
and expectedly, no reaction was observed under
Scheme 2. Synthetic approaches to dihydroisoquinolones
via an α-oxidation of amines
However, these studies require specific reaction
techniques such as cyclic voltammetry and
electrolysis or the use of metal-based catalysts.
Metal-free procedures have also been reported by the
groups of Zhang,[13d] and Burley and Talbot[13g] with
the use of stoichiometric amounts of oxidants and/or
heating. Very recently, the Das group reported an
efficient α-oxygenation of tertiary amines to amides
via organo-photocatalysis (Scheme 2, I).[14]
Furthermore, additional studies report the formation
argon atmosphere (entry 19). Interestingly,
a
significant improvement in the yield was obtained
with the addition of NaN3, which is known to be a
strong singlet oxygen (1O2) quencher (entry 20). We
assumed that NaN3 would quench the singlet oxygen,
thereby preventing the formation of additional side
products during the reaction. Based on the results
from the optimization studies, we concluded that a
photocatalyst, visible light, and oxygen are necessary
for this transformation to take place efficiently.
of
dihydroisoquinolones
from
N-substituted
tetrahydroisoquinolines as by-products in cross
dehydrogenative coupling (CDC) reactions.[15]
However, there are several limitations connected to
the aforementioned methods. More specifically, the
use of stoichiometric amounts of oxidants and metal-
containing catalysts or reagents is cause for both
environmental and economic concerns. In addition,
Table 1. Optimization of the reaction conditions.[a]
2
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
Table 2. Substrate scope.[a]
Entry
mol%
Base (equiv)
Solvent
Yield
(%)[b]
of eosin Y
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
none
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
53
64
68
73
69
70
52
no rxn
44
27
61
62
61
trace
43
67
no rxn
31
no rxn
K2CO3 (0.5)
K2CO3 (1.0)
K2CO3 (1.5)
K2CO3 (3.0)
Na2CO3 (1.5)
Cs2CO3 (1.5)
Pyridine (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
9
5
5
10
11
12
13
14
15[c]
16[d]
17[e]
18[f]
19[g]
20[h]
21[i]
2.5
1.0
0.5
none
5
5
5
5
5
5
5
95
74
[a]Reactions were conducted on a 0.5 mmol scale.
[b]Yields are of the isolated products after column
chromatography.
[c]Under blue LED light irradiation
[d]Under white LED light irradiation
[e]In the dark
[f]Under air atmosphere
[g]Under Ar atmosphere
[h]NaN3 (2.0 equiv) was added.
[i]NaN3 (1.0 equiv) was added.
With the optimized reaction conditions in hand, we
next explored the scope of the synthesis of
dihydroisoquinolones (Table 2). Notably, the reaction
was
tolerant
to
various
N-substituted
tetrahydroisoquinolines. The desired α-oxidation
products were obtained in moderate to high yields
(50–96%).
tetrahydroisoquinolines provided
Various
N-aryl
substituted
desired
the
dihydroisoquinolones in the presence of both
electron-donating (2ab–2af) and electron-
withdrawing (2ag–2an) substituents (58–96%).
Moreover, the products derived from the ortho-,
meta-,
tetrahydroisoquinolines
successfully (2ad–2al,
and
para-substituted
were also
58–96%).
N-aryl
obtained
However,
[a]Reactions were conducted on a 0.5 mmol scale. Yields
diminished reaction yields were observed with
electron-withdrawing substituents at the ortho-, and
para-positions (2ad vs 2ag, 2aj, and 2af vs 2ai, 2al,
2an). In contrast, a noticeable electronic effect of
meta-substituents was not observed in terms of the
reaction yield (2ae vs 2ah, 2ak). Nevertheless, longer
reaction times were required for fluoro- and
trifluoromethyl-substituents (36 h: 2aj–2al, 2an).
are
of
the
isolated
products
after
column
chromatography.[b]Reaction was conducted on a 5 mmol
scale [c]Performed for 36 h. For details, see the Supporting
Information.
3
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
‧‒
In addition, decreased reaction yields were observed
with sterically demanding substituents containing
methyl (2ab) and isopropyl (2ac) groups at the para-
position of the benzene ring. Similarly, sterically
hindered naphthyl substituted tetrahydroisoquinolines
provided the desired products in moderate yields
under the reaction conditions (2ao: 50%, 2ap: 63%).
Predominantly, these bulky substrates also required
longer reaction times (36 h: 2ac, 2ao, 2ap). We also
examined other amine protecting groups such as
methyl, acetyl, Boc, Cbz and benzyl groups (2ar,
2at–2aw, 2ba, 2bb). The results showed that these
protecting groups were all suitable under the reaction
conditions, giving the desired products in moderate to
high yields (61–91%). Furthermore, this method
could be applied to the five-membered ring system,
the desired isoindolinone 2ay was obtained in good
yield (69%).
In order to gain a mechanistic insight into this
transformation, we performed various control
experiments (Scheme 3). First, TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) was employed as a
radical inhibitor, which did not afford the desired
product, thereby indicating that the reaction
mechanism includes a radical pathway. Additionally,
the fact that no desired product was observed under
argon atmosphere and the yield slightly decreased in
air (open flask) prove that oxygen is the key
oxidation source in this reaction.
superoxide radical anion (O2 ) was observed by EPR
spectra (see the Supporting Information).[16] In
addition, no desired product was observed when the
reaction was performed in the presence of 2.0
equivalents of FeSO4 which could react with the
hydroperoxyl radical (HOO‧).[15g]
Figure 1. Catalyst quenching experiment. Fluorescence
titration of 20 μM eosin Y (DMSO) while increasing the
concentration of 1aa. I0 and I are the fluorescence
intensities before and after the addition of 1aa.
Based on the results from the present study and
previous reports,[15g, 17] a proposed reaction pathway
is shown in Scheme 4. First, eosin Y (EY) is excited
under visible light irradiation to generate an excited
state species (EY*). A single electron transfer from 1
to the excited state of eosin Y (EY*) then generates
aminyl cation radical I and EY‧‒. Subsequently, the
reaction between oxygen and EY ‧ ‒ results in the
‧‒
formation of a superoxide radical anion (O2 ), which
could then react with aminyl cation radical I to afford
intermediate II. Followed by a reaction with the
hydroperoxyl radical (HOO‧), a proton abstraction by
a base (B:) would provide the desired product 2. In
addition, EY* could also interact with O2 to generate
a singlet oxygen (1O2) via an energy transfer.[18]
However, we assume that NaN3 would quench the 1O2
to suppress the formation of additional side products
during the reaction.
Scheme 3. Control experiments.
To further probe the reaction mechanism, a Stern-
Volmer quenching study was performed. The results
showed that the luminescence of eosin Y could be
readily
quenched
by
2-phenyl-1,2,3,4-
tetrahydroisoquinoline (1aa) (Figure 1). In contrast,
no effect was observed with the addition of NaN3 or
K2CO3 (see the Supporting Information for details).
Next, to determine the active species of oxygen in
this transformation, an electron paramagnetic
resonance (EPR) study was carried out. When we
employed DMPO (5,5-dimethyl-1-pyrroline N-oxide)
as the radical trapping agent, a characteristic signal of
Scheme 4. Proposed reaction pathway.
In summary, we have developed an efficient α-
oxidation
reaction
of
N-substituted
tetrahydroisoquinolines to dihydroisoquinolones. The
transformation features visible-light-induced
a
organo-photo catalyzed reaction with oxygen acting
4
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
as a green oxidant. Notably, this transformation did
not require any metal catalysts or reagents. This
method would provide a straightforward access to
dihydroisoquinolones, which are important scaffolds
of various bioactive compounds and other key
synthetic intermediates.
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Experimental Section
Typical Procedure for the α-oxidation reaction of
tetrahydroisoquinolines
N-Substituted tetrahydroisoquinoline 1 (0.5 mmol, 1.0
equiv), eosin Y (0.025 mmol, 5 mol %), K2CO3 (0.75
mmol, 1.5 equiv), and NaN3 (1.5 mmol, 2.0 equiv) were
dissolved in dry DMSO (0.25 M). The reaction mixture
was stirred and irradiated by 5 W green LEDs (530 nm) at
room temperature under O2 atmosphere for 16-36 h. After
the reaction was completed, water (10 mL) and EtOAc (10
mL) were added, and the mixture was extracted by EtOAc
(3 x 10 mL). The combined organic layer was dried over
anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. Purification by flash chromatography
afforded the corresponding product.
[11] J. A. Borthwick, N. Ancellin, S. M. Bertrand, R. P.
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Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (2016R1D1A1B03931335)
and the Korea Institute of Energy Technology Evaluation and
Planning (KETEP) and the Ministry of Trade, Industry & Energy
(MOTIE) of the Republic of Korea (No. 20174010201160). We
thank Prof. Wook-Jin Chung and Dr. Kris Niño G. Valdehuesa
(Myongji Univ.) for GC/MS analysis.
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10.1002/adsc.201801301
Advanced Synthesis & Catalysis
UPDATE
Aerobic α-Oxidation of N-Substituted
Tetrahydroisoquinolines to Dihydroisoquinolones
via Organo-photocatalysis
Adv. Synth. Catal. Year, Volume, Page – Page
Kim Christopher C. Aganda, Boseok Hong and
Anna Lee*
7
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