Visible Light-Driven α-Alkylation of N-Aryl tetrahydroisoquinolines Initiated by Electron Donor-Acceptor Complexes

Article abstract of DOI:10.1021/acs.orglett.0c02631

The visible light-driven α-alkylation of N-aryl tetrahydroisoquinolines was initiated through electron donor-acceptor complex photochemistry. The reaction can proceed smoothly without the addition of any photocatalysts, transition-metal catalysts, or additional oxidants. The proposed mechanism was supported by various mechanistic studies, and the reactive open-shell alkyl radicals were generally produced from an alkylamine and underwent radical coupling for alkylating a wide range of N-aryl tetrahydroisoquinolines.

Full text of DOI:10.1021/acs.orglett.0c02631

pubs.acs.org/OrgLett
Letter
Visible Light-Driven α‑Alkylation of N‑Aryl tetrahydroisoquinolines
Initiated by Electron Donor−Acceptor Complexes
Qing Xia, Yufei Li, Xinmin Wang, Peng Dai, Hongping Deng, and Wei-Hua Zhang*
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ABSTRACT: The visible light-driven α-alkylation of N-aryl tetrahydroisoquinolines was initiated through electron donor−acceptor
complex photochemistry. The reaction can proceed smoothly without the addition of any photocatalysts, transition-metal catalysts,
or additional oxidants. The proposed mechanism was supported by various mechanistic studies, and the reactive open-shell alkyl
radicals were generally produced from an alkylamine and underwent radical coupling for alkylating a wide range of N-aryl
tetrahydroisoquinolines.
1,2,3,4-Tetrahydroisoquinoline (THIQ) is one of the most
important scaffolds within natural products, and numerous
THIQ-containing alkaloids with a wide range of bioactivities
have been isolated (Figure 1a).1 Thus, THIQ-containing
molecules have attracted attention in the last few decades.2
Oxidation of a C(sp3)−H bond adjacent to the nitrogen atom
of amines is a convenient pathway to produce N-containing
organic scaffolds; therefore, the construction of 1-substituted
THIQ derivatives is critical during the synthesis of THIQ-
containing molecules.3 In general, THIQs are oxidized to
iminium ions4 and then attacked by various nucleophiles
including cyanides,5 aldehyde/ketones,6 electron-rich arenes,7
alkynes,8 and heteroatom nucleophiles9 to achieve cross-
dehydrogenative coupling (Figure 1b). As an alternative, the α-
amino radicals of THIQs have been employed in radical
Michael addition10 or cross-coupling (Figure 1b).11 Although
substantial advances on the C(sp3)−H functionalization of
THIQs have been achieved, the need for water-sensitive
nucleophiles, stoichiometric oxidants, and metal catalyst limits
its practical application value. Hence, an efficient method is
necessary.
Since 2008, photocatalysis has emerged as a powerful and
sustainable tool for catalytic organic synthesis under a mild
reaction condition;3f,12 however, it relies on the use of precious
metal complexes and elaborate organic dyes.13 Electron
donor−acceptor (EDA) complex photochemistry has received
increasing attention, since Chatani and Melchiorre reported
the seminal works in 2013.14 Under this reaction, an electron-
rich substrate (a donor) combines with an electron-deficient
molecule (an acceptor) to formulate a new complex in the
ground state called the EDA complex.15 Its light excitation
triggers an intramolecular single electron-transfer (SET) event,
which then generates a radical intermediate depending on the
presence of a suitable leaving group (LG).16 A number of
overarching and powerful strategies in a visible light-driven
radical synthesis have been developed based on this process
without using a photocatalyst.17 An example is the direct
coupling between donor and acceptor substrates after the
photoactivity of a stoichiometric EDA complex.18 Moreover
sacrificial donors and redox auxiliaries are used to expand the
scope of the substrates.19 More interestingly the catalytic EDA
complex is induced to expand the efficiency of its photo-
chemistry.20 In 2020, the Zhang group reported that the EDA
complex induced the α-allylation of N-aryl tetrahydroisoquino-
lines for the formation of an allyl or benzyl radical and
subsequent radical coupling (Figure 1c).21 Motivated by these
achievements, this work speculates that the electron-deficient
Katritzky salts,22 which can be readily prepared from amines,
can be installed as redox auxiliaries to generate unbiased alkyl
radical, and the α-alkylation of N-aryl tetrahydroisoquinolines
could be achieved under EDA complex photochemistry
without the need for oxidants and catalysts (Figure 1d).
Received: August 6, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02631
© XXXX American Chemical Society
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A

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a
Table 1. Optimization of the Reaction Conditions
b
entry
variations from standard conditions
none
3aa (%)
1
2
3
4
5
6
7
8
9
72
62
trace
22−68
0−67
70
58
trace
43
450−465 nm blue LEDs
365−375 nm purple LEDs
bases other than K2CO3
solvents other than DCM
DCM [0.1 M], instead of DCM [0.2 M]
DCM [0.05 M], instead of DCM [0.2 M]
in the dark
without K2CO3
10
air condition
53
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol, 1.5 equiv),
K2CO3 (0.2 mmol, 1.0 equiv), DCM [0.2 M], room temperature,
b
argon condition, 390−400 nm purple LEDs, 24 h. Isolated yield
based on 1a.
a
Scheme 1. Substrate Scope with Respect to Katritzky Salts
Figure 1. Synthesis strategies for α-functionalized N-aryl-tetrahy-
droisoquinolines and representative compounds with crucial bio-
logical activities.
The model reaction between 2-phenyl-1,2,3,4-tetrahydroi-
soquinoline 1a and Katritzky salt 2a was investigated under
various reaction conditions. Upon extensive reaction opti-
mization, at room temperature under the irradiation of 390−
400 nm purple light-emitting diodes (LEDs) with K2CO3 as
the base and dichloromethane (DCM) as the solvent (15 W)
(see the Supporting Information for details), the desired
product 3aa was obtained in 72% isolated yield (Table 1, entry
1). For light sources, 390−400 nm purple LEDs were observed
to be optimal, and a sharp decrease in efficiency was found
when employing 365−375 nm purple LEDs (Table 1, entry 3).
Replacing K2CO3 with other inorganic bases or organic bases
did not improve the yield (Table 1, entry 4; see Supporting
Information for details). Other solvents such as dichloro-
ethane, tetrahydrofuran, ethyl acetate, and dimethylacetamide
influenced the yield (Table 1, entry 5), and 0.2 M 1a was
determined to be optimal (Table 1, entries 6 and 7). Control
experiments revealed that visible light is necessary for this
reaction (Table 1, entry 8). In the absence of the base and
argon condition, the reaction efficiencies were also lowered
(Table 1, entries 9 and 10).
a
Reaction conditions: see Table 1, entry 1. Isolated yields are
b
provided. The homocoupling product 2y′ was generated in 78%
yield; see the Supporting Information for details.
excellent yields. The electronic property of benzylamine
remarkably influences the outcome of this radical coupling
reaction. The electron-withdrawing group (EWG) trifluor-
omethyl substituting benzylamine showed only 40% yield for
2g. Moreover, the ortho and meta position substituents were
inconsequential for the reaction efficiency and generated
moderate yields of the corresponding products (3bj−3bo). α-
Naphthylmethylamine (2p) and 2-aminomethylpyridin (2q)
tolerated the reaction conditions and afforded 3bp and 3bq in
43% and 37% yields, respectively. However, there was no
desired coupling product 3by when para-methoxycarbonyl-
substituted benzylamine Katritzky salt 2y was employed, while
The applicability of the EDA complex photochemistry for
Katritzky salts was explored under optimal conditions (Scheme
1). The reactivity of various benzylamines was first tested.
Fluoro (3bc), chloro (3bd), bromo (3be), iodo (3bf), methyl
(3bi), trifluoromethyl (3bg), and trifluoromethoxy (3bh) at
the para position of benzylamine produced moderate to
B
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the homocoupling product 2y′ was observed in 78% yield (see
the Supporting Information). The low yields of some Katritzky
salts may be attributed to a side homocoupling reaction.23 In
addition to the primary amines, various inactivated cyclic and
heterocyclic secondary amines were prepared as Katritzky salts
and smoothly underwent this reaction with good yields for the
α-cycloalkyl substituted THIQ (3br−3bx, 37%−83%). This
scope highlights the utility of the developed method in
synthesizing valuable THIQ-containing building blocks, and it
had an essential improvement when compared with the recent
similar work.23
EDA complex photochemistry (see the Supporting Informa-
tion for details).25 As shown in Figure 2, 1a and 2a efficiently
The variety of THIQs for this protocol was subsequently
evaluated (Scheme 2). Different kinds of groups at the para
Figure 2. Visible light-driven α-alkylation of N-aryl tetrahydroisoqui-
nolines on an SFMT reactor.
Scheme 2. Substrate Scope with Respect to N-Aryl
tetrahydroisoquinolines
a
triggered the EDA complex reaction with the presence of 1,4-
diazabicyclo[2.2.2]octane (DABCO) under blue LED irradi-
ation in the SFMT reactor and generated the target product
3aa in 79% yield within 10 h. This method represents a
practical protocol for the oxidant and catalyst-free radical α-
alkylation of N-aryl tetrahydroisoquinolines under visible light.
A series of experiments was performed to gain insights into
the reactivity between THIQs and Katritzky salts. When
2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and butylated
hydroxytoluene (BHT) were added into the standard reaction
systems (Table 1, entry 1), trace amount of the corresponding
product 3aa was produced (Figure 3a,b). However, the radical
generated from the Katritzky salt 2a could be trapped by
TEMPO, and the adduct 4 was yielded at 66% (Figure 3a).
a
Reaction conditions: see Table 1, entry 1. Isolated yields are
provided.
and ortho position on the N-benzene ring were tolerated (3aa,
3ca−3ea, 3ja). The yield of trifluoromethyl-substituted N-
phenyl tetrahydroisoquinolines (3da) was also reduced to
48%, which may be attributed to the reduced electronic cloud
density of the central nitrogen atom. This method could also
be extended to N-biphenyl- (3fa), N-β-naphthyl- (3ga), N-α-
pyridyl- (3hb), and N-1,4-benzodioxin (3ia and 3ib)-
substituted THIQs, thus highlighting the application value
for constructing complex skeleton structures. 6,7-Dimethoxyl
THIQs are ubiquitous in natural THIQ-containing alkaloids;
hence, their applicability in EDA complex-induced radical
coupling was examined. Several kinds of N-benzene ring-
substituted 6,7-dimethoxyl THIQs reacted successfully with
the benzyl Katritzky salts and delivered the desired products in
moderate yields (3ba, 3kb−3nb, 48%−70%). However, the
reaction of unprotected THIQ 1o and N-alkyl-substituted
THIQs (1p−1r) could not give the desired products. It was
reasoned that N-aryl or N-heteroaryl might be essential for the
formation of the EDA complex and the following electron
transfer.21a,24
Figure 3. Mechanistic studies. (a, b) Radical-trapping experiment. (c)
UV−vis absorption spectra. (d) Job’s plot between 1a and 2a. (e)
NMR titration experiments between 1a and 2a. (f) Light on−off
experiment.
The reaction was conducted on a stop-flow microtubing
(SFMT) reactor to determine the synthesis applicability of this
C
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The radical from THIQs 1a can be identified through high-
resolution mass spectrometry (Figure 3b). Given that this
photochemistry is suggested to be enabled by an EDA complex
between THIQ and the Katritzky salt, UV−visible spectro-
scopic measurements experiments were subsequently con-
ducted (see the Supporting Information for details). When a
solution of 1a was mixed with 2a, a bathochromic shift was
observed and indicated the formation of an EDA complex
(Figure 3c). Moreover, a Job’s plot disclosed the stoichio-
metric ratio of the EDA complex between 1a and 2a to be 1:1
(Figure 3d), implying that THIQ and the Katritzky salt can
form the charge−transfer (CT) complex through SET. Further
AUTHOR INFORMATION
■
Corresponding Author
Wei-Hua Zhang − Jiangsu Key Laboratory of Pesticide Science,
College of Sciences, Nanjing Agricultural University, Nanjing
210095, China; Email: zhangwh@nju.edu.cn
Authors
Qing Xia − Jiangsu Key Laboratory of Pesticide Science, College
of Sciences, Nanjing Agricultural University, Nanjing 210095,
China; orcid.org/0000-0003-0831-025X
Yufei Li − Jiangsu Key Laboratory of Pesticide Science, College of
Sciences, Nanjing Agricultural University, Nanjing 210095,
China
Xinmin Wang − Jiangsu Key Laboratory of Pesticide Science,
College of Sciences, Nanjing Agricultural University, Nanjing
210095, China
1
NMR titration experiments revealed that the H NMR signal
of THIQ 1a shifted upfield with the addition of the Katritzky
salt 2a (Figure 3e). Finally, results of the light on−off
experiment of the standard reaction between 1a and 2a (Figure
3f) excluded the possibility of a radical chain process in this
EDA complex system.
The above experimental results suggested a mechanism
shown in Figure 4. An EDA complex between electron-rich
Peng Dai − Jiangsu Key Laboratory of Pesticide Science, College
of Sciences, Nanjing Agricultural University, Nanjing 210095,
China
Hongping Deng − Jiangsu Key Laboratory of Pesticide Science,
College of Sciences, Nanjing Agricultural University, Nanjing
210095, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02631
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21907052), National Key R&D Program of China
(2018YFD0200100), and the Fundamental Research Funds for
the Central Universities (KJQN202057 and KYTZ201604) for
generous financial support for our programs.
Figure 4. Proposed mechanism.
THIQs 1 and electron-deficient Katritzky salts 2 was initially
formed. With visible-light irradiation, an SET occurred within
the EDA complex and generated THIQ radical cation 7 and
pyridinyl radical 6, which irreversibly fragmented to afford
alkyl radical 8 and triphenylpyridine. Meanwhile, α-amino
radical 9 was produced through a base-promoted deprotona-
tion of 7. Finally, the alkyl radical 8 rapidly combined with the
α-amino radical 9 to provide the desired α-alkylation product
3.
In summary, the visible-light-induced α-alkylation of N-aryl
tetrahydroisoquinolines was presented. A colored EDA
complex was generated and triggered by visible light to release
reactive open-shell radical intermediates. This process does not
require photocatalysts, transition-metal catalysts, or additional
oxidants and exemplifies a promising method for the α-
alkylation of N-aryl tetrahydroisoquinolines by using easily
available benzylamine and cycloalkylamine as alkylation
reagent. Further exploration for the scopes and limits of this
reaction are ongoing in our laboratory.
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Experimental details, procedures, and characterization
data for all products (PDF)
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