Access to functionalized isoquinoline N-oxides via sequential electrophilic cyclization/cross-coupling reactions

Article abstract of DOI:10.1002/adsc.200800301

Electrophilic cyclization of 2-alkynylbenzaldoximes with various electrophiles leads to the formation of 4-iodoisoquinoline N-oxides or 4-bromo-isoquinoline N-oxides under mild conditions. The reaction can be transferred to highly functionalized isoquinol

Full text of DOI:10.1002/adsc.200800301

FULL PAPERS
DOI: 10.1002/adsc.200800301
Access to Functionalized Isoquinoline N-Oxides via Sequential
Electrophilic Cyclization/Cross-Coupling Reactions
Qiuping Dinga,c and Jie Wua,b,*
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-216-510-2412; e-mail: jie_wu@fudan.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, 354 Fenglin Road, Shanghai 200032, Peopleꢀs Republic of China
Analytical Centre for Physics and Chemistry, Jiangxi Normal University, 437 West Beijing Road, Nanchang 330027,
c
Peopleꢀs Republic of China
Received: May 13, 2008; Published online: July 20, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Electrophilic cyclization of 2-alkynylbenz- isoquinoline N-oxides via palladium-catalyzed cross-
ACHTREUNG
ACHTREUNG
action can be transferred to highly functionalized reaction; electrophilic cyclization; isoquinoline N-
oxides
Introduction
variety of carbocycles and heterocycles.[7] Recently,
we found that o-alkynylbenzaldehyde was a versatile
The increasing significance of combinatorial chemistry building block in multicomponent reactions for con-
in pharmaceutical and material sciences demands the struction of the 1,2-dihydroisoquinoline skeleton.[8] In
development of new strategies to synthesize a collec- the reaction process, a 2-(1-alkynyl)arylaldimine[9,10]
tion of analogues of interesting compounds.[1] As a was believed to be the key intermediate for further
privileged fragment, the isoquinoline core is a ubiqui- transformation. Prompted by these results, we envi-
tous subunit in many alkaloids with remarkable bio- sioned that 2-alkynylbenzaldoximes could be also uti-
logical activities.[2] Among these, isoquinoline N-oxide lized as starting material for synthesis of N-heterocy-
is an important class of isoquinoline derivatives. cles due to the structural similarity with 2-(1-alkyn-
Moreover, they are also valuable Lewis basic organo-
ACHTREyUNG l)arylaldimines. Herein, we report a highly efficient
catalysis in organic synthesis.[3] In addition, applica- and general method for the generation of functional-
tions of these molecules have been discovered in ma- ized isoquinoline N-oxides via sequential electrophilic
terials science due to their interesting optochemical/ cyclization/cross-coupling reactions of 2-alkynylbenz-
physical properties, such as charge-transfer and metal
(Li+/Mg2+) sensor effects and use as a radical initiator
for atom-transfer radical polymerizations.[4] The im-
ACHTREaUGN ldoximes under mild conditions.
portance of isoquinoline N-oxides has stimulated the Results and Discussion
development of a number of approaches for the syn-
thesis of the isoquinoline N-oxide ring system.[5,6] As mentioned above, the electrophilic cyclization of
However, most of them are still conducted by oxida- heteroatomic nucleophiles such as oxygen, nitrogen,
tion of parent nitrogen heterocycles and the diversity sulfur, and phosphorus species with tethered alkynes
could not be easily introduced in the scaffold. Thus, it has proven to be an effective method for preparing a
is highly desired to develop efficient and general syn- large variety of heterocyclic ring systems.[11] The elec-
thetic methods for access to functionalized isoquino- trophiles such as iodine, bromine, ICl, and NBS were
line N-oxides in order to build up these molecules in commonly used in the reaction since the resulting
a combinatorial format.
iodo- or bromo-containing products are readily elabo-
The electrophilic annulation of alkynes has proven rated to more complex products by using known orga-
to be extremely effective for the synthesis of a wide nopalladium chemistry. In preliminary studies, we in-
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Access to Functionalized Isoquinoline N-Oxides
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Table 1. Electrophilic cyclization of 2-alkynylbenzaldoxime
with various electrophiles.
vestigated the electrophilic cyclization between the 2-
alkynylbenzaldehyde oxime 1a and iodine and found
that the reaction took place in the presence of di-
chloromethane as solvent at room temperature
(Scheme 1). This reaction was highly effective and the
Entry Alkynylbenzaldehyde
Electro- Product Yield
oxime 1
phile
2 (X)
[%][a]
1
2
3
4
5
I2
2a (I)
2a (I)
2a (I)
2b (Br) 99
2b (Br) 75
92
93
32
ICl
NIS
Br2
NBS
1a
Scheme 1. Electrophilic cyclization of 2-alkynylbenzaldox-
ime 1a with iodine in different solvents.
6
7
I2
Br2
2c (I)
2d (Br) 25
88
desired product 2a was generated in 92% yield after
24 h. The structure of 2a was verified by 1H and
13C NMR, mass spectroscopy, as well as X-ray diffrac-
tion analysis (Figure 1, for details, see Supporting In-
formation). Other solvents were also screened, which
revealed that dichloromethane was the best choice.
1b
1c
1d
1e
1f
1g
8
9
I2
Br2
2e (I)
2f (Br) 99
92
10
11
I2
Br2
2g (I)
2h (Br) 99
91
12
13
I2
Br2
2i (I)
2j (Br) 94
89
14
15
Br2
Br2
2k (Br) 90
2l (Br) 85
Figure 1. ORTEP diagram of 4-iodoisoquinoline N-oxide 2a.
16
17
I2
Br2
2m (I) 90
2n (Br) 94
With this result in hand, the scope of this reaction
was then investigated, and the results are summarized
in Table 1. From Table 1, we found that the conditions
have proven to be useful for a range of 2-alkynylben-
zaldehyde oxime 1 and electrophiles. For instance, re-
action of 2-alkynylbenzaldehyde oxime 1a and ICl
was finished in 10 min and the expected product 2a
was obtained in 93% yield (Table 1, entry 2). An
almost quantitative yield of product 2b was obtained
when bromine was utilized as reactant (Table 1,
1h
1i
18
Br2
2o (Br) 64
[a]
Isolated yield based on 2-alkynylbenzaldoxime 1.
entry 4: 99% yield). A slightly lower yield was ob- 1b in the reaction of bromine since many by-products
served when NBS was used as replacement (Table 1, were observed during the reaction process. The yield
entry 5: 75% yield). Other 2-alkynylbenzaldehyde could be increased to 45% when the reaction was per-
oximes were also employed in the reaction of iodine formed at À108C (data not shown in Table 1). Reac-
or bromine. 2-Alkynylbenzaldehyde oxime 1b reacted tion of fluoro-substituted 2-alkynylbenzaldehyde
with iodine leading to the formation of isoquinoline- oxime 1c with iodine or bromine gave rise to the cor-
N-oxide 2c in 88% yield (Table 1, entry 6), while a responding product 2e or 2f in 92% and 99% yields,
25% yield of product 2d was generated when iodine respectively (Table 1, entries 8 and 9). When phenyl
was replaced by bromine (Table 1, entry 7). This may group attached on the triple bond was changed to a 4-
presumably be due to the high reactivity of substrate methACHTREoUNG xyphenyl group, the reactions also proceeded
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Qiuping Ding and Jie Wu
FULL PAPERS
smoothly to furnish the desired product in excellent
yields (Table 1, entries 10–14). 2-Alkynylbenzalde-
hyde oximes 1g and 1h were also examined and simi-
lar results were observed for reactions of iodine or
bromine (Table 1, entries 15–17). Interestingly, it was
found that a hydroxy group as substituent in the sub-
strate 1i was well tolerated under these conditions
and the 4-bromoisoquinoline N-oxide 2o could be iso-
lated in 64% yield (Table 1, entry 18).
After the successful generation of 4-bromo- or 4-io-
doisoquinoline N-oxide 2, we considered to introduce
more diversity into the isoquinoline N-oxide scaffold
via palladium-catalyzed cross-coupling reactions.
Thus, 4-bromoisoquinoline N-oxide 2b or 2n was re-
acted with sodium tetraphenylborate to give the de-
sired 3,4-disubstituted isoquinoline N-oxide 3a or 3b
Scheme 3. Synthesis of 3,4-disubstituted isoquinoline N-
oxides via palladium-catalyzed Sonogashira coupling.
in 93% or 83% yield, respectively [Scheme 2, Eq. Conclusions
(1)]. Similarly, the reaction of 4-bromoisoquinoline
N-oxide 2b with phenylboronic acid furnished the ex- In summary, we have described a novel and efficient
pected product 3a in 99% yield [Scheme 2, Eq. (2)]. method for the synthesis of highly functionalized iso-
Isoquinoline N-oxide 2j reacted smoothly with 4- quinoline N-oxides via sequential electrophilc cycliza-
methylphenylboronic acid leading to the correspond- tion/cross-coupling reactions of 2-alkynylbenzaldox-
ing product 3c in 78% yield [Scheme 2 Eq. (3)]. imes. Application of 2-alkynylbenzaldoximes in other
Fluoro-substituted isoquinoline N-oxide 2f was also a transformations is currently under investigation, and
good partner in the cross-coupling reaction with 4- the results will be reported in due course.
methylphenylboronic acid. As expected, the corre-
sponding 3,4-disubstituted isoquinoline N-oxide 3d
was generated in 95% yield [Scheme 2, Eq. (4)].
Experimental Section
The Sonogashira coupling reactions of 4-iodoisoqui-
noline N-oxide 2 with 4-methoxyphenylacetylene
were also examined (Scheme 3). Under standard con-
ditions, the coupling reactions proceeded well to gen-
erate the desired product 4 in moderate to good yield.
General Procedure for Preparation of 2-Alkynylbenz-
aldoxime 1[12]
A solution of 2-alkynylbenzaldehyde (3.0 mmol), hydroxyl-
AHCTREaUNG mine hydrochloride (6 mmol, 2.0 equiv.), pyridine
Scheme 2. Synthesis of 3,4-disubstituted isoquinoline N-oxides via palladium-catalyzed Suzuki–Miyaura reactions.
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Access to Functionalized Isoquinoline N-Oxides
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(6.0 mmol, 2.0 equiv.) in C2H5OH (15 mL) was stirred under
refluxed for 2 h. After completion of reaction as indicated
by TLC, the solvent was evaporated and the reaction resi-
due then quenched with water (10 mL), extracted with
EtOAc (2”30 mL), dried by anhydrous Na2SO4. Evapora-
tion of the solvent followed by purification on silica gel pro-
vided the corresponding 2-alkynylbenzaldoxime 1.
stirred at 808C under N2 for 24 h. After completion of reac-
tion as indicated by TLC, the solvent was evaporated and
the residue quenched with water (10 mL), extracted with
EtOAc (2”10 mL), dried by anhydrous Na2SO4. Evapora-
tion of the solvent followed by purification on silica gel pro-
vided the corresponding 3,4-disubstituted isoquinoline N-
oxide 4.
Data of a selected example: 4-[2-(4-ethoxyphenyl)ethynyl]-
3-phenylisoquinoline N-oxide 4a: 1H NMR(400 MHz,
CDCl3): d=3.81 (s, 3H), 6.83 (d, J=8.8 Hz, 2H), 7.22 (d,
J=8.8 Hz, 2H), 7.48–7.57 (m, 3H), 7.61–7.75 (m, 5H), 8.30
(d, J=8.8 Hz, 1H), 8.87 (s, 1H); 13C NMR(100 MHz,
CDCl3): d=55.3, 82.2, 101.4, 114.1, 120.0, 124.7, 125.9, 127.9,
128.5, 129.2, 129.3, 129.4, 130.6, 132.2, 133.2, 136.3, 148.9,
160.4; MS (ESI): m/z=352 (M+ +H); HR-MS: m/z=
352.1339, calcd. for C24H18NO2 (M+ +H): 352.1338. (For de-
tails, please see Supporting Information.)
General Procedure for Electrophilic Cyclization of 2-
Alkynylbenzldoxime 1 with Various Electrophiles
The electrophile (1.2 equiv.) in 2 mL of CH2Cl2 was added
dropwise to
a
solution of 2-alkynylbenzaldoxime
(0.30 mmol) in 4 mL of CH2Cl2. The reaction mixture was
stirred at room temperature for a period of time (10 min:
Br2, ICl, or NIS; 24 h: NBS and I2). The reaction mixture
was then diluted with CH2Cl2 (50 mL), washed with saturat-
ed aqueous Na2S2O3 (25 mL), dried (Na2SO4) and filtered.
The solvent was evaporated under reduced pressure and the
product 2 was isolated by chromatography on a silica gel
column.
CCDC 690391 contains the supplementary crystallograph-
ic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Data of a selected example: 4-iodo-3-phenylisoquinoline
N-oxide 2a: 1H NMR(400 MHz, CDCl 3): d=7.38 (d, J=
6.8 Hz, 2H), 7.48–7.58 (m, 3H), 7.59–7.69 (m, 3H), 8.06 (d,
Supporting Information
Experimental procedures, characterization data, as well as
J=8.3 Hz, 1H), 8.85 (s, 1H); 13C NMR(100 MHz, CDCl ):
3
1
copies of H and 13C NMRof all compounds are available as
d=122.1, 124.9, 127.3, 128.5, 128.9, 129.4, 129.7, 129.8, 129.9,
133.4, 136.0, 147.7; MS (ESI): m/z=348 (M+ +H); elemental
analysis calcd. (%) for C15H10INO: C 51.90, H 2.90, N 4.03;
found: C 51.64, H 2.88, N 3.98.
Supporting Information.
Acknowledgements
General Procedure for Synthesis of 3,4-Disubstituted
Isoquinoline N-Oxide 3 via Palladium-Catalyzed
Suzuki–Miyaura Reaction
We thank Dr. Renhua Fan for his invaluable advice during
the course of this research. Financial support from National
Natural Science Foundation of China (20772018), Shanghai
Pujiang Program, and Program for New Century Excellent
Talents in University (NCET-07–0208) is gratefully acknowl-
edged.
A solution of 4-bromoisoquinoline N-oxide 2 (0.30 mmol),
sodium tetraphenylborate (0.15 mmol, 0.5 equiv.) or arylbor-
onic acid (0.45 mmol, 1.5 equiv.), palladium catalyst (5
mol%), base (1.2 mmol, 4.0 equiv.) in DMF/H2O (2.0 mL)
was stirred at 808C under N2 for 24 h. After completion of
reaction as indicated by TLC, the solvent was evaporated
and the residue quenched with water (10 mL), extracted
with EtOAc (2”10 mL), and dried by anhydrous Na2SO4.
Evaporation of the solvent followed by purification on silica
gel provided the corresponding 3,4-disubstituted isoquino-
line N-oxide 3.
Data of a selected example: 3,4-diphenylisoquinoline N-
oxide 3a: 1H NMR(400 MHz, CDCl 3): d=7.07–7.10 (m,
2H), 7.19–7.28 (m, 5H), 7.34–7.46 (m, 3H), 7.50 (dt, J=1.5,
6.8 Hz, 1H), 7.60 (t, J=6.8 Hz, 1H), 7.77 (d, J=8.3 Hz,
1H), 7.96 (t, J=6.8 Hz, 1H), 9.08 (s, 1H); 13C NMR
(100 MHz, CDCl3): d=124.9, 126.2, 127.7, 127.8, 128.0,
128.3, 128.6, 129.0, 129.1, 129.7, 130.3, 130.7, 131.9, 134.7,
136.5, 137.0, 146.0; MS (ESI): m/z=298 (M+ +H); HR-MS:
m/z=298.1221, calcd. for C21H16NO (M+ +H): 298.1232.
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