A new and facile approach to 1,2-dihydroisoquinolin-3(4: H)-imines by the Cu(i)-catalyzed reaction of 2-ethynylbenzyl methanesulfonates, sulfonyl azides and amines

Article abstract of DOI:10.1039/c7ra12412k

A new, step-economical and operationally simple access to unsubstituted 1,2-dihydroisoquinolin-3(4H)-imines by Cu-catalyzed MCRs under mild conditions is described. In addition, selective hydrolysis of imines to the corresponding 1,2-dihydroisoquinolin-3(4H)-ones under refluxed conc. HCl has also been investigated.

Full text of DOI:10.1039/c7ra12412k

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A new and facile approach to 1,2-
dihydroisoquinolin-3(4H)-imines by the Cu(I)-
catalyzed reaction of 2-ethynylbenzyl
Cite this: RSC Adv., 2018, 8, 74
methanesulfonates, sulfonyl azides and amines†
*
*
Qihui Sun, Lirong Zhang and Fengping Yi
Ying Huang, Weiyin Yi,
Received 14th November 2017
Accepted 11th December 2017
A new, step-economical and operationally simple access to unsubstituted 1,2-dihydroisoquinolin-3(4H)-
imines by Cu-catalyzed MCRs under mild conditions is described. In addition, selective hydrolysis of
imines to the corresponding 1,2-dihydroisoquinolin-3(4H)-ones under refluxed conc. HCl has also been
investigated.
DOI: 10.1039/c7ra12412k
rsc.li/rsc-advances
skeleton and their derivatives, the synthetic pathways towards
1,2-dihydroisoquinolin-3(4H)-imines, which are a class of
Introduction
important members of THIQ family and have been wildly
applied in material sciences and biological research,10 are rarely
reported in the literature. A more careful literature survey
indicates that only Souaoui et al. in 1992 described their
synthesis via the 1,3-dipolar cycloaddition of arylazides with
1,2-dihydroisoquinoline derivatives followed by splitting
decomposition11 under heating (Scheme 1A) and Chang's group
Heterocycle skeletons are popular scaffolds in various phar-
maceuticals, biologically active compounds and functional
materials.1 Tetrahydroisoquinoline (THIQ), an important
nitrogen-containing heterocycle skeleton, is also found
frequently in naturally existing alkaloids which have been
shown to have a wide range of interesting biological activities
such as antimuor, antimicrobial activities, and other pharma-
cological properties.2 Due to these advantages, great efforts have
been made to develop various methodologies toward the
synthesis of the THIQ skeleton and its derivatives. In general,
traditional synthetic methods for the construction of this class
of compounds are the Bischler–Napieralski cyclization/reduc-
tion,3 the Pictet–Spengler reaction,3a,4 the noble-transition
metal-catalyzed asymmetric direct hydrogenation of isoquino-
line or dihydroisoquinoline derivatives5 and some other elegant
reactions.6 In recent years, in order to further expand the
desirable molecular library of the THIQ alkaloids for the
discovery of pharmaceutically active compounds, many excel-
lent approaches by directly using THIQ precursors to their
derivatives have also been developed. For example, currently,
the most popular strategies to construct the THIQ skeleton-
containing derivatives are the nucleophilic addition to the
preformed C]N+ bond of cyclic precursors offered from the
substrate of tetrahydroisoquinoline via the oxidative cross-
dehydrogenative coupling (CDC) reaction7 or the redox-neutral
reaction8 and those associated with free radical intermedi-
ates.9 Among the above-mentioned synthetic strategies,
although all approaches can provide access to the THIQ
School of Perfume and Aroma Technology, Shanghai Institute of Technology, Shanghai
201418, P. R. China. E-mail: yiwy@sit.edu.cn; yifengping@sit.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1574819. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra12412k
Scheme
droisoquinolin-3(4H)-imine.
1 Different strategies for the construction of 1,2-dihy-
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Scheme 2 Facile synthesis of 2-ethynylbenzyl methanesulfonates 4.
in 2006 reported a catalytic one-pot approach to the title methodology is also applied in the synthesis of 1,2-dihy-
product using aminoalkyne and azide as substrate under mild droisoquinolin-3(4H)-imines. For instance, Wu's group in 2011
condition12 (Scheme 1B). However, these methods to synthesize presented one type of 1,2-dihydroisoquinolin-3(4H)-imines via
1,2-dihydroisoquinolin-3(4H)-imine suffered from some disad- MCRs using 2-ethynylphenyl-chalcones, sulfonyl azides and
vantages including multiple reaction steps,11 harsh reaction amines as substrates under Cu(I) catalyst (Scheme 1C),13 in
conditions,11 few scope of substrate (one example only!)12 and which the strategy may experience the reaction process with the
the use of noble metal.12 Therefore, the search for a step- active ketenimine intermediate investigated widely by the
economical, more convenient and efficient catalytic system pioneers.14 It is well known that the pharmacological and bio-
with attractive features such as easily accessible starting mate- logical activity of molecule is shown to be dependent on the
rials, mild reaction conditions, and nontoxic side products to nature of substituents. However, most MCRs follow a single
generate structural diversity of 1,2-dihydroisoquinolin-3(4H)- reaction route to afford only one substituted type of molecular
imine is of great value.
skeleton and their derivatives. Therefore, development of novel
As an ideal synthetic tool, multicomponent reactions (MCRs) MCRs for the selective formation of diverse substituted mole-
involving a tandem reaction process with three or more cules containing the same molecular skeleton is a worthy and
reagents in a single process under mild conditions have challenging goal. As a continuation of our interest in keteni-
emerged as a powerful and attractive strategy for the synthesis mine chemistry and THIQ skeleton, we herein report a conve-
of structurally diverse organic molecules. Recently, the nient and efficient synthetic strategy for the synthesis of 1,2-
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Base
Solvent
Temp. (ꢀC)
Yieldb (%)
1c
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18d
19e
CuI
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
CHCl3
MeCN
THF
Dioxane
DMF
DCM
DCM
DCM
DCM
DCM
DCM
rt
rt
rt
rt
40
nd
47
54
57
60
77
50
64
35
67
50
35
Trace
70
nd
37
82
80
Cu2O
CuCl
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
40
50
60
70
60
60
60
60
60
60
60
60
60
60
60
Et3N
K2CO3
(iPr2)NEt
DBU
DABCO
Et3N
Et3N
a
Reaction conditions: toluenesulfonyl azide (0.36 mmol), 2-ethynylbenzyl methanesulfonate (0.3 mmol), phenylamine (0.36 mmol), base (0.45
b
c
d
mmol), catalyst (0.03 mmol),solvent (1 mL). Isolated yields. nd ¼ not detected. Reaction carried out for 12 h. Reaction carried out for 0.5 h.
e
Reaction carried out for 1 h.
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dihydroisoquinolin-3(4H)-imines via novel MCRs from 2-ethy- conformed by X-ray crystallographic analysis (showed in Table
nylbenzyl methanesulfonates, sulfonyl azides and amines 1). Inspired by the result of reaction, we continue to optimize
(Scheme 1, this work).
the reaction condition of model reaction to improve the yield of
7a to the highest. The results for various optimization experi-
ments are summarized in Table 1. Subsequently, other copper
salt were then screened under the reaction condition just
mentioned. The results indicated that, in comparison to the
potential copper catalyst, only CuBr was proven to be more
effective one (54%), whereas Cu2O even can not lead to the
formation of 7a (entries 1–4). When the reaction temperature
under CuBr as catalyst was gradually increased from rt to 60 ꢀC,
the yield of 7a increases to 77%. However, it was found that
further elevating the reaction temperature, in contrast, made
the yield decrease (entries 5–8). We next examined the effect of
solvent in the MCRs. The results revealed that dichloromethane
(77%) was the most suitable solvent for the reaction (entry 7,
Results and discussion
Our studies started with the synthesis of variously substituted 2-
ethynylbenzyl methanesulfonates 4 from substituted 2-bromo-
benzaldehyde 1 according to the literature procedures15 as
indicated in Scheme 2.
And then, under N2 and sealed tube, we carried out a model
MCRs of 2-ethynylbenzyl methanesulfonate 4a with TsN3 5a and
phenylamine 6a in dichloromethane at room temperature in
the presence of CuI and Et3N for 12 h (Table 1). To our delight,
the desired product (entry 1), 2-dihydroisoquinolin-3(4H)-imine
of 7a was isolated in 40% yield, and its structure was further
Table 2 Substrate scope of MCRs for the formation of 7a
Entry
R1
R2
R3
Product
Yieldb [%]
1
2
3
4
5
6
7
8
H(4a)
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4-F(4b)
4b
5-F(4c)
4c
4c
4c
4c
6-F(4d)
4d
4d
4-Cl(4e)
4e
4-CH3C6H4(5a)
5a
5a
5a
5a
5a
5a
5a
5a
5a
C6H5(5b)
5b
C6H5(6a)
4-CH3C6H4(6b)
4-FC6H4(6c)
4-ClC6H4(6d)
2-CH3OC6H4(6e)
2-FC6H4(6f)
2,6-EtC6H3(6g)
2,4,6-CH3C6H2(6h)
Bn(6i)
tBu(6j)
6a
6e
6h
6a
6c
6a
6c
6a
6b
6c
6c
6a
6a
6c
6a
6a
6c
6c
6a
6b
6c
7a
7b
7c
7d
7e
82
78
84
77
68
76
65
74
35
0
80
78
74
85
84
51
83
89
77
84
81
80
75
88
71
77
87
72
77
73
51
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
7r
7s
7t
7u
7v
7w
7x
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
5b
2,4,6-CH3C6H2(5c)
5c
5a
5a
5a
5a
5a
5b
5c
5a
5a
5b
5a
5a
5b
5a
5a
5a
7y
7z
7aa
7ab
7ac
7ad
7ae
4e
4-CH3(4f)
4f
4f
a
Optimal reaction conditions: 4 (0.2 mmol), CuBr (0.02 mmol), Et3N (0.3 mmol) in DCM (1 mL), sealed and then heated at 60 ꢀC for 0.5 h under N2.
Isolated yields are given.
b
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Table 3 Hydrolysis of imines 7 to 1,2-dihydroisoquinolin-3(4H)-ones
entries 9–13). As showed in Table 1, different base also affected
the nal desired outcome. The reaction was carried out effi-
ciently when triethylamine acted as the base, furnishing the
desired product in 77% yield, and the corresponding product
was not almost afforded in the presence of K2CO3 or DBU (entry
7, entries 14–17). Additionally, it should be noted that the
reaction time also played the important role in improving the
yield of target product. The experimental results indicated the
shorter reaction time would be benecial to the formation of 7a
(entry 7, entries 18–19). Hence, aer screening the reaction
condition of MCRs, the optimized conditions to selectively
obtain the desired product 7a, under N2 and sealed tube, are
identied as the employment of Et3N as base and CuBr as
8a
Entry
R1
R2
R3
Product
Yieldb [%]
1
2
3
4
H(a)
4-CH3C6H4(b)
C6H5
8a
8b
8c
8d
66
74
59
69
a
a
a
b
b
b
4-CH3C6H4
4-FC6H4
4-ClC6H4
ꢀ
catalyst in dichloromethane for 0.5 h at 60 C.
With the optimized reaction conditions secured, we subse-
quently applied the optimized conditions for the formation of 7a
to explore the scope and generality of the MCRs. The results in
this section, as summarized in Table 2, demonstrated that the
present method is generally applicable in the formation of target
products depending on the employment of the corresponding
starting materials. It was found that all aromatic substrates
bearing various substutution such as electron donating and
withdrawing group underwent smoothly this transformation
generating the corresponding products 7 with moderate to
excellent yields. However, we also found that in addition to the
substrates with withdrawing group such as F at the para position
of phenyl ring giving the lower yields (51% in both) (entries 16,
31), the property of other substituents on phenyl ring showed no
obvious impact on the yield of corresponding products. Intrigu-
ingly, when the substrates are aliphatic amines, the methodology
only furnish either the low corresponding yield (entry 9, 35%) or
no desired product (entry 10, 0%). The result may be caused by
the fact that in contrast to aromatic amines, aliphatic amines
with stronger nucleophilic have higher activity and lower selec-
tivity. Therefore, when aliphatic amines reacted with other
substrates, the product in a low or no yield will be achieved.
Based on the above results and the chemistry related to
ketenimine, a plausible mechanism of this MCRs is proposed
and shown in Scheme 3. Taking the formation of 7a as an
example, rst, the ketenimine intermediate A is formed via the
reaction of 2-ethynylbenzyl methanesulfonate (4a) with TsN3
under the optimized conditions. Subsequently, when the
anilines added into the reaction, the double nucleophilic
addition would happen to afford the intermediate B. And then,
intermediate B will tautomerize into the intermediate C. Finally,
a
Reaction conditions: 7 (0.5 mmol), conc HCl (10 mL), reactions
reuxed for 12 h. b Isolated yields.
the intermediate C, which undergo sequential proton transfer,
will transform into the desired product 7a.
Subsequently, to further take advantage of the synthetic
application of this methodology, we then explored the synthesis
of 1,2-dihydroisoquinolin-3(4H)-ones 8 by hydrolysis of the
resulting products 7. To the best of our knowledge, 8 were
considered as important building block in organic synthesis16
and have good physiological activities in medicinal chemistry.
For example, more recently, the researchers found that the
compounds containing the skeleton of 1,2-dihydroisoquinolin-
3(4H)-one were acetylcholinesterase and b-secretase dual
inhibitors in treatment with alzheimer's disease.17 Therefore,
we proceeded to treat the dihydroisoquinolin-3(4H)-imines 7
with concentrated HCl under reux for 12 h. To our delight, the
corresponding title products 8 were achieved in good yields
(Table 3). The results indicate that 7 can be considered as a class
of important precursor of 1,2-dihydroisoquinolin-3(4H)-ones 8.
In summary, a new and facile MCRs for the formation of
unsubstituted 1,2-dihydroisoquinolin-3(4H)-imines in good yields
has been developed under mild condition. The experimental
results indicated that various aromatic substrates could be well
tolerated in this reaction. Meanwhile, selective hydrolysis of
imines to the corresponding 1,2-dihydroisoquinolin-3(4H)-ones
has also been investigated under reuxed conc. HCl. In addition,
some 2-ethynylbenzyl methanesulfonates that had not previously
been reported were also synthesized and characterized. The
methodology provides a step-economical and operationally simple
access to functionalized desired product and can further enrich
the compound library of 1,2-dihydroisoquinolin-3(4H)-imine.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are grateful for nancial support by the Science Foundation
Scheme 3 Possible reaction mechanism for the formation of 7a.
of Shanghai Institute of Technology (grant number YJ2013-61).
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