Photocatalyst- And transition-metal-free α-allylation of: N -aryl tetrahydroisoquinolines mediated by visible light

Article abstract of DOI:10.1039/c9gc04191e

A convenient and efficient α-allylation of N-aryl tetrahydroisoquinolines has been achieved. This transformation can be realized under only visible light irradiation without the aid of transition metals or photocatalysts. The mechanism involves a novel in situ-generated electron-donor-acceptor (EDA) complex between the N-aryl tetrahydroisoquinolines and an allyl or a benzyl bromide. Irradiation with purple light triggered single-electron transfer (SET) from the N-aryl tetrahydroisoquinolines to the allyl or benzyl bromide of the EDA complex, inducing the formation of the corresponding allyl or benzyl radical and the subsequent radical-radical coupling. This approach represents the first example of a photocatalyst- and transition-metal-free α-allylic and benzylic functionalization of N-aryl tetrahydroisoquinolines.

Full text of DOI:10.1039/c9gc04191e

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DOI: 10.1039/C9GC04191E
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Photocatalyst- and transition-metal-free α-allylation of N-aryl
tetrahydroisoquinolines mediated by visible light
Received 00th January 20xx,
Accepted 00th January 20xx
Zhuohua Li, Pengju Ma, Yongzhu Tan, Yufei Liu, Min Gao,Yujun Zhang, Bo Yang,
Xuan Huang, Yuan Gao and Junmin Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract:
A
convenient and efficient α-allylation of N-aryl Lu,[5a,b] Hamashima,[5c] Rovis,[5d] and Breit[5e] research groups
tetrahydroisoquinolines has been achieved. This transformation successfully used allylic esters, allyl bromides, conjugated
Previous work: with transition metals and extra photocatalysts
can be realized under only visible light irradiation without the aid
a) Tunge et al.: Decarboxylative allylation of amino alkanoic esters[4]
of transition metals or photocatalysts. The mechanism involves a
novel in situ-generated electron-donor-acceptor (EDA) complex
between the N-aryl tetrahydroisoquinolines and an allyl or a
benzyl bromide. Irradiation with purple light triggered single-
electron transfer (SET) from the N-aryl tetrahydroisoquinolines to
the allyl or benzyl bromide of the EDA complex, inducing the
formation of the corresponding allyl or benzyl radical and the
subsequent radical-radical coupling. This approach represents the
first example of a photocatalyst- and transition-metal-free α-
R1
R2
N
R1
R2
[Ir]
/
Pd(PPh3)4
N
O
R3
White LED, Solvent, RT
R3
O
b) Xiao and Hamashima et al.: Radical cross coupling allylation[5a-c]
R1
R2
R1
R2
X
R5
N
[Ir]
Pd(PPh3)4 and/or
N
R5
+
R3
R4
Blue LED, Solvent, RT
R3
H
R4
R2
c) Rovis et al.: Hydroaminoalkylation of conjugated dienes[5d]
R1
R2
R1
allylic
and
benzylic
functionalization
of
N-aryl
[Ir]
CoBr2, dppp /
N
N
R
R
+
tetrahydroisoquinolines.
Blue LED, Solvent, RT
R3
H
R3
d) Breit et al.: Hydroaminoalkylation of alkynes and allenes[5e]
The allyl moiety is an important group capable of
undergoing various synthetic functional group conversions
because it possesses a carbon-carbon double bond; as a result,
reactions for the introduction of allyl moieties into organic
molecules are highly desirable.[1] Among reactions, α-allylic
alkylations of tertiary amines represent particularly useful and
challenging reactions for preparing homoallylic amines, which
are frequently employed as intermediates for the synthesis of
pharmaceuticals and natural products.[2] Traditional methods
for the direct α-allylation of tertiary amines are mainly based
on cross-dehydrogenative coupling (CDC) strategies.[3] In
contrast, recently developed visible light-mediated
photoredox-catalysed α-allylic alkylations of tertiary amines
that do not require water-sensitive nucleophiles or
stoichiometric oxidants have received substantial attention. In
2014, the Tunge group reported a decarboxylative allylation of
α-amino allyl esters by combining Iridium-based photoredox
catalysis and palladium catalysis.[4] Thereafter, the Xiao and
Me
R1
R2
R
R
R1
R3
R2
H
N
[Ir]
[Rh(cod)Cl]2, L /
N
R
+
or
•
Blue LED, Solvent, RT
R3
R
This work: no transition metal and no extra photocatalyst
R1
R2
R1
R2
Purple LED, base
DMF, RT
N
N
+
Br
R3
H
R3
(1) transition-metal and photocatalyst free
(2) readily available, abundant and cheap allylation reagents
(3) mild and operationally simple conditions
Scheme 1. α-Allylations of tertiary amines mediated by visible light
dienes, and alkynes as allylation reagents for the α-allylic
alkylation of tertiary amines under a combination of transition-
metal catalysis with iridium-based photoredox catalysis, also
called metallaphotoredox dual catalysis.[6] Although significant
advances have been achieved, noble metal-based
photocatalysts and transition metals are usually necessary in
such approaches. The pursuit of a greener and more efficient
method is of great interest. Here, we report a novel, visible
light-induced, photocatalyst- and transition-metal-free, sp3 C-H
α-allylic alkylation of N-aryl tetrahydroisoquinolines.
a. International Joint Research Center for Molecular Science, College of Chemistry
and Environmental Engineering, Shenzhen University, Shenzhen, 518060, P. R.
China. Email: zhangjm@szu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
DMF was found to be the optimal solvenVtiew(eArntitcrleyOn4lin)e.
Considering that one equivalent of HBr isDgOeIn: 1e0r.a10te39d/Cin9GthCi0s4191E
Table 1. Optimization of the Reaction Conditions.[a]
Br
light, additive
solvent
N
Table 2. Reaction Scope[a]
N
+
R1
Br
R1
N
400 nm, Cs2CO3
DMF, RT
1a
2a
3a
N
+
Entry
1
Solvent
MeCN
MeCN
DMSO
DMF
Light
Additive
Yield (%)[b]
Trace
25
R2
H
R2
1
2
3
450 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
400 nm
dark
–
2
–
N
N
N
3
–
n.d.
4
–
36
3a
, 68% yield[b] (<10% yield)[a]
, 83% yield
N
3b
[c]
3c
, 76% yield
(18% yield)[a]
5
EtOH
–
Trace
Trace
21
N
6
1,4-Dioxane
DMF
–
7
Et3N
[b]
[c]
3d
3e
, 86% yield
, 42% yield
5
8
DMF
DBU
Trace
48
9
DMF
2,4,6-collidine
K2CO3
Cs2CO3
TFA
N
N
N
10
11
12
13[d]
DMF
70
DMF
85 (83)[c]
n.d.
3f
3g
3h
, X = H, 36% yield
, X = Et, 46% yield
, X = t-Bu, 52% yield
X
3j
, 71% yield
(dr = 1:1)
3i
, 86% yield (dr = 1:1)
CF3
DMF
DMF
Cs2CO3
n.d.
N
N
N
[a] Reaction conditions: 1a (0.20 mmol), 2a (0.60 mmol), additive (0.4 mmol) and
solvent (1 mL) at room temperature under a nitrogen atmosphere irradiated with a 3 W
LED for 15 h. [b] Yield determined by GC using n-dodecane as an internal standard. [c]
Yield of isolated product. [d] Reaction was carried out at 100°C for 24 h. DMSO =
dimethyl sulfoxide, DMF = N,N-dimethyl formamide, TFA = trifluoroacetic acid.
X
X
[d]
3k
, X = Me, 81% yield
[d]
3o
, 86% yield
[d]
[c]
[c]
3l
3p
3q
, X = Et, 77% yield
, X = Cl, 41% yield
[c]
3m
, X = Cl, 48% yield
, X = Br, 71% yield
[c]
3n
, X = Br, 75% yield
N
Photoabsorbing EDA complexes generated by noncovalent
molecular interactions between electron donors and acceptors
can mediate photocatalyst-free transformations upon
irradiation with visible light via the generation of radical
species.[7,8] Currently, this strategy has become a powerful
approach for building C-C and C-X (X = hetero atoms)
bonds.[9,10] Melchiorre and co-workers reported a series of
pioneering works in this area.[9] In 2013, their research group
skilfully utilized chiral enamine intermediates and alkyl halides
to form EDA complexes and achieved the α-alkylation of
aldehydes under visible light irradiation.[9a] They also
elucidated the key mechanistic aspects of this process through
a series of verification experiments.[9b-9e] Inspired by their
N
N
N
O
N
OMe
[b]
3t
, 60 %
3r
, 32 %
3s
, 51 %
[a] Reaction conditions: 1 (0.20 mmol), 2 (0.60 mmol), Cs2CO3 (0.40 mmol) and
DMF (1.0 mL) at room temperature under a nitrogen atmosphere irradiated
with a 3 W LED for 10-15 h. Yields of isolated products are reported. [b] 2,4,6-
Collidine instead of Cs2CO3. [c] 1 (0.20 mmol), 2 (0.70 mmol), Cs2CO3 (0.40
mmol) and DMF (1.0 mL) at room temperature under a nitrogen atmosphere
irradiated with a 10 W LED for 25-52 h. [d] 0.40 mmol 2a was used.
reaction, a series of bases were then introduced into the
reaction system to promote this transformation. Triethylamine
and diazabicycloundecene (DBU) significantly reduced the
yield of this reaction, which may be due to the structural
similarity between theses additive and the reactants
interfering with the reaction (entries 7 and 8).[12] A survey of
inorganic bases revealed Cs2CO3 to be most effective, as it
provided the desired product in the highest yield (entries 10
and 11). When trifluoroacetic acid was used in place of the
base, the reaction was completely inhibited, which may
indicate that deprotonation is essential (entry 12). Even when
heated to 100 °C, the desired reaction did not proceed in the
dark, which confirmed that the reaction was a photochemical
rather than a thermodynamic process (entry 13).
excellent
works,
we
speculated
that
N-aryl
tetrahydroisoquinolines, which are somewhat structurally
similar structures to enamines, might form EDA complexes
with allyl bromide in the same way, and the α-allylic alkylation
of N-aryl tetrahydroisoquinolines could thus be achieved
under irradiation with visible light.
To validate the feasibility of this idea, we began our
investigations by using N-phenyltetrahydroisoquinoline and 3-
bromo-2-methylpropene as substrates (Table 1). Initially,
acetonitrile was chosen as the solvent, and the reaction was
irradiated with a 450-nm blue LED. However, only trace
product was obtained (entry 1). Varying the light source to a
400-nm purple LED proved to be effective,[11] and the α-allylic
alkylation product was obtained in 25% yield (entry 2). Then,
various solvents of different polarities were screened, and
2 | J. Name., 2012, 00, 1-3
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a)
a) Radical trapping experiment
Cs2CO3 (3.25 g 10 mmol)
View Article Online
DOI: 10.1039/C9GC04191E
N
Br
N
+
DMF 25 ml, 400 nm LED (24 W)
RT, 32 h
2a
2.16 g (16 mmol)
TEMPO (0.6 mmol)
Cs2CO3 (0.4 mmol)
Br
O
N
N
+
Ph
1a
1.05 g (5 mmol)
+
N
Ph
3a
0.89 g (68 % yield)
400 nm, DMF, RT, 10 h
+
HRMS
Calcd
Found
: (M+H )
2a
(0.6 mmol)
1a
(0.2 mmol)
3a
n.d.
: 212.2009
: 212.2008
MeO
MeO
2,4,6-collidine (3.0 equiv)
MeO
MeO
b)
allyl bromide (3.5 equiv)
N
N
N
N
BHT (2 mmol)
Cs2CO3 (0.4 mmol)
400 nm LED (10 W), 15 h
DMF, RT
Br
O
O
N
Ph
+
N
1j
Ph
3u(58% yield)
400 nm, DMF, RT, 10 h
O
2a
(0.6 mmol)
1a
(0.2 mmol)
Trace
N
O
O
b) Spectroscopic studies: UV-Vis absorption spectra
Ref.[5a]
CO2Me
CO2Me
Scheme 2. Gram-scale reaction and α-allylation of N-heteroaryl-substituted
tetrahydroisoquinoline
After determining the optimal reaction conditions, we
proceeded to explore the scope of allylic bromides. As shown
in Table 2, this protocol was suitable for the α-allylic alkylation
of N-phenyl tetrahydroisoquinolines with allyl bromide (3a), as
well as short-chain, long-chain and polysubstituted allyl
bromides, affording the corresponding products in 42−86%
yields (3b-e). Due to the structural similarities between allyl
bromides and benzyl bromides, we next tried the reaction of
N-phenyltetrahydroisoquinoline with benzyl bromide under
the same conditions. As expected, this protocol is also
Optical absorption spectra recorded in dimethyl sulfoxide in 1-mm path quartz cuvettes
using a UV-visible spectrophotometer. [1a] = 0.2 M, [2a] = 0.6 M.
Scheme 3. Mechanistic studies.
applicable
to
benzyl
bromides
with
N-
phenyltetrahydroisoquinoline, affording the corresponding
products in 36−52% yields (3f-h). When a secondary benzyl
The utility and practicality of this α-allylation of N-aryl
tetrahydroisoquinolines were demonstrated by the gram-scale
preparation of 3a. As shown in Scheme 2a, 5 mmol of 1a could
be α-allylated with 2a under our established conditions, and
an isolated yield of 3a (68% yield, 0.89 g) comparable to that of
the small-scale reaction was obtained.
This α-allylation reaction is also suitable for 1,2,3,4-
tetrahydroisoquinoline with a nitrogen-substituted aromatic
heterocycle. The Xiao research group has shown that N-(2-
benzoxazolyl) tetrahydroisoquinoline can be successfully
incorporated into a dual catalytic allylation reaction and
bromide was used, the benzylation proceeded with
a
comparable yield and afforded the desired products in a 1:1
diastereomeric ratio (3i and 3j). Next, the scope of N-aryl
tetrahydroisoquinolines in this reaction was investigated. Both
electron-donating groups and electron-withdrawing groups on
N-benzene ring are tolerated, giving the corresponding
allylation products (3k-3q) in good yields. This method could
also be significantly extended to the allylation of tetrahydro-β-
carbolines (3r and 3s). N,N-Dimethyl anilines, N-phenyl
pyrrolidines, N-phenyl piperidines, N-benzyl-N-methylaniline
(it
is
closer
to
structure
with
N-
transformed to
a
key synthetic intermediate of 8-
phenyltetrahydroisoquinoline) and
2-benzyl-1,2,3,4-
oxoprotoberberine derivatives via sequential reductive
deprotection and SN2 substitution reactions.[5a] Here, this N-
aryl tetrahydroisoquinoline α-allylation transformation was
also explored using our optimized reaction conditions. To our
delight, the direct allylation product can be obtained in a
satisfactory yield without a photocatalyst or a palladium
catalyst.
To further elucidate the mechanism of this photoinduced
allylation reaction, several experiments and analyses were
conducted. When TEMPO was added to the optimal reaction
system, the reaction was completely suppressed, and TEMPO-
trapped product 5 was detected by high-resolution mass
spectrometry, which indicated that the reaction mechanism
involved a radical process (Scheme 3a).[13] To investigate
whether this photochemical reaction was mediated by the EDA
tetrahydroisoquinoline had also been tested as substrates for
the reaction, but couldn’t give the desired products. Other
activated alkyl bromides were also tested, such as
bromoacetonitrile,
propargyl
bromide,
methyl
α-
bromoisobutyrate, ethyl bromodifluoroacetate and cinnamyl
bromide, and we found that only methyl α-bromoisobutyrate
was suitable for this reaction with 60% yield (3t), but the
others couldn’t give the desired α-alkylation products.
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J. Name., 2013, 00, 1-3 | 3
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complex of the N-aryl tetrahydroisoquinolines and allyl
bromide, we measured the UV-visible spectra of the different
reaction components in the allylation. Immediately after
mixing
Conclusions
View Article Online
DOI: 10.1039/C9GC04191E
In conclusion, we have developed a visible light-induced
α-allylation of N-aryl tetrahydroisoquinolines via the formation
of a novel, photoactive EDA complex. Compared with previous
reports, the present approach offers several advantages: 1) no
need for transition metals or photocatalysts; 2) inexpensive
and readily available allylation reagents; and 3) mild and
operationally simple reaction conditions. The reaction can
tolerate a series of allyl and benzyl bromides, which is of vital
importance for the preparation of homoallylic amines and β-
phenylethylamines.
Br
N
Cs2CO3
Ar
+
N
400 nm, DMF, RT
Ar
H
radical-radical
coupling
Br
+
N
Ar
N
B
D
hv
Br
Notes and references
A
EDA complex
base
- H
1
(a) K. C. Nicolaou, P. G. Bulger and D. Sarlah, Angew. Chem.
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Am. Chem. Soc., 2004, 126, 11810; (b) C.-J. Li, Acc. Chem.
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1368.
N
Ar
C
H
Scheme 4. Proposed mechanism
the two substrates, the optical absorption spectrum shifted to
the visible region (Scheme 3b).
2
3
Based on the above experiments and literature reports, a
possible mechanism for the visible light-mediated α-allylation
of N-aryl tetrahydroisoquinolines is proposed. Irradiation with
visible light triggers a single-electron transfer (SET) from EDA
complex A involving N-aryl tetrahydroisoquinolines and the
allyl
or
benzyl
bromide
to
form
N-
phenyltetrahydroisoquinoline radical cation C and allyl or
benzyl radical B and release a bromide anion. Deprotonation
of the radical cation resulted in the formation of α-amino
radical D. A radical-radical coupling reaction could then
provide the α-allylation product.
In conclusion, we have developed a visible light-induced α-
allylation of N-aryl tetrahydroisoquinolines via the formation
of a novel, photoactive EDA complex. Compared with previous
reports, the present approach offers several advantages: 1) no
need for transition metals or photocatalysts; 2) inexpensive
and readily available allylation reagents; and 3) mild and
operationally simple reaction conditions. The reaction can
tolerate a series of allyl and benzyl bromides, which is of vital
importance for the preparation of homoallylic amines and β-
phenylethylamines.
4
5
(a) S. B. Lang, K. M. ƠNele and J. A. Tunge, J. Am. Chem. Soc.,
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Chem. Rev., 2013, 113, 5322; (c) J. W. Beatty and C. R. J.
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and A. Studer, Chem. Soc. Rev., 2015, 44, 3505; (e) K. L.
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Acknowledgements
We are grateful to the National Natural Science Foundation of China
(21402124 and 21777106), the Training Foundation for Outstanding
Young Teachers in Higher Education Institutions of Guangdong
(YQ2015145), the Project of Featured Innovation in Higher Education
Institutions of Guangdong (2017KTSCX159), the Shenzhen Science and
Technology Foundation (KQJSCX20180328100401788) and the Principal
Foundation of SZU (8570700000307). We gratefully acknowledge the
support from the Instrumental Analysis Center of Shenzhen University.
We also appreciate the thoughtful suggestions from the referees.
6
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Green Chemistry
Page 6 of 6
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C9GC04191E
Graphical abstracts
Br
Cs2CO3
N
Ar
+
N
Purple LED, DMF, RT
Ar
H
(1) transition-metal and photocatalyst free
(2) readily available, abundant and cheap allylation reagents
(3) mild and operationally simple conditions
A
convenient and efficient α-allylation of N-aryl
tetrahydroisoquinolines has been achieved. This transformation
can be realized under only visible light irradiation without the
aid of transition metals or photocatalysts. The mechanism
involves a novel in situ-generated electron-donor-acceptor (EDA)
complex between N-aryl tetrahydroisoquinolines and allyl or
benzyl bromides
6 | J. Name., 2012, 00, 1-3
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