Silver triflate and triphenylphosphine co-catalyzed reactions of 2-alkynylbenzaldehyde, amine, and α,β-unsaturated ketone

Article abstract of DOI:10.1016/j.tetlet.2009.09.005

A two-component activation system that combines metal catalysis (AgOTf) and the employment of catalytic amount of organocatalyst (PPh₃) has been successfully employed in the three-component reaction of 2-alkynylbenzaldehyde, amine, and α,β-unsa

Full text of DOI:10.1016/j.tetlet.2009.09.005

Tetrahedron Letters 50 (2009) 6273–6275
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Silver triflate and triphenylphosphine co-catalyzed reactions
of 2-alkynylbenzaldehyde, amine, and a,b-unsaturated ketone
Shengqing Ye a, Jie Wu a,b,
*
a Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2009
Revised 26 August 2009
Accepted 1 September 2009
Available online 4 September 2009
A two-component activation system that combines metal catalysis (AgOTf) and the employment of cat-
alytic amount of organocatalyst (PPh3) has been successfully employed in the three-component reaction
of 2-alkynylbenzaldehyde, amine, and
a,b-unsaturated ketone. This reaction proceeds smoothly in THF
under mild conditions leading to the functionalized 1,2-dihydroisoquinolines in moderate to good yields.
Ó 2009 Elsevier Ltd. All rights reserved.
This Letter is dedicated to Professor Li-Xin
Dai on the occasion of his 85th birthday
Keywords:
2-Alkynylbenzaldehyde
Amine
a,b-Unsaturated ketone
1,2-Dihydroisoquinoline
Silver triflate
Triphenylphosphine
Recently, a combination of metal catalysis and organocatalyst1
as two-component activation system has been successfully em-
ployed in organic synthesis.2,3 For example, Córdova2a developed
the synthesis of diverse 1,2-dihydroisoquinolines via tandem reac-
tions starting from 2-alkynylbenzaldehyde and related com-
pounds.2i,9 For instance, we realized that copper(I), silver(I), or
palladium(II) salts were effective catalysts in the three-component
reactions of 2-alkynylbenzaldehyde, amine, and diethyl phos-
phite.9j,k In the reaction process, an isoquinolinium intermediate
might be involved. Encouraged by these results and the advance-
ment of Baylis–Hillman reaction, we conceived that under suitable
catalytic conditions, three-component reactions of 2-alkynylbenz-
direct catalytic a-allylic alkylation of aldehydes and cyclic ketones
via a combination of palladium and enamine catalysis. We also
realized that one-pot combination of silver triflate and proline
catalysis was highly effective for the synthesis of 1,2-dihydroiso-
quinoline derivatives via multicomponent reaction of 2-alkynyl-
benzaldehyde, amine, and ketones.2i Prompted by these results
and our interests for accessing privileged scaffolds, we became
interested in exploring new two-component activation system
(co-catalyzed by metal and organocatalyst) to facilitate the natural
product-like compounds generation.
aldehyde, amine, and
we disclosed our recent efforts for the three-component reaction
of 2-alkynylbenzaldehyde, amine, and ,b-unsaturated ketone co-
a,b-unsaturated ketone might occur. Herein,
a
catalyzed by AgOTf and PPh3, which afforded the expected 1,2-
dihydroisoquinoline derivatives in moderate to good yields.
Our studies commenced with the reaction of 2-alkynylbenzal-
dehyde 1a, p-anisidine 2a, and but-3-en-2-one 3a (Table 1). In
our previous report,9 we have demonstrated silver triflate as an
effective catalyst for the isoquinolinium intermediate generation.
Thus, initial efforts were performed in the presence of AgOTf and
PBu3 as catalysts in THF at 70 °C (Table 1, entry 1). However, only
trace amount of the product was detected after 24 h under the
above-mentioned conditions. Similar results were observed when
TFP or P(o-Tol)3 was employed as a replacement (Table 1, entries
2 and 3). Gratifyingly, the result was dramatically improved when
The significance of Baylis–Hillman reaction4–6 in organic synthe-
sis has been recognized. Usually, a Lewis base, such as phosphine,
tertiary amine, or sulfide is utilized as catalyst or promoter to initi-
ate the reaction. Recently, 2-alkynylbenzaldehyde was identified as
a versatile building block for the construction of N-heterocycles.7,8
We also involved in this field and developed efficient methods for
* Corresponding author. Address: Department of Chemistry, Fudan University,
220 Handan Road, Shanghai 200433, China. Tel.: +86 21 5566 4619; fax: +86 21
6510 2412.
E-mail address: jie_wu@fudan.edu.cn (J. Wu).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.005

6274
S. Ye, J. Wu / Tetrahedron Letters 50 (2009) 6273–6275
Table 1
choice for this transformation (Table 1, entries 6–11). Decreasing
the amount of PPh3 diminished the product yield (Table 1, entries
12 and 13). A similar result was obtained when the amount of PPh3
was increased to 50 mol % (70% yield, Table 1, entry 14). We also
examined other Lewis acids in the reaction. Thus, copper salts or
Dy(OTf)3 was tested, however, no better results were displayed
(Table 1, entries 15–17). Addition of molecular sieves could not im-
prove the final outcome as well (Table 1, entry 19). Moreover, we
tried to add additive [proline, Zn(OTf)2, Mg(ClO4)2, and urea] in
the reaction based on the advancement of Baylis–Hillman reac-
tion4 (data not shown in Table 1). Again, low yields of product 4a
were isolated.
With this preliminary optimized conditions in hand, the scope
of this reaction was then investigated [AgOTf (10 mol %), PPh3
(20 mol %), THF, 70 °C], and the results are summarized in Table
2. From Table 2, we noticed that, for most cases, the silver triflate
and triphenylphosphine co-catalyzed reactions of 2-alkynylbenzal-
Conditions optimization for the reaction of 2-alkynylbenzaldehyde, p-anisidine, and
but-3-en-2-one
O
NH2
CHO
Lewis acid
(10 mol %)
Me
2a
MeO
+
PMP
Ph
N
O
phosphine
(20 mol %)
Ph
3a
Me
solvent, 70 ο
C
1a
4a
Entry
Lewis acid
Phosphine
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
CuI
PBu3
TFP
THF
THF
THF
THF
Trace
Trace
Trace
70
24
54
49
53
41
33
Trace
54
52
70
11
13
NR
Trace
47
P(o-Tol)3
PPh3
PCy3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
THF
MeCN
DCE
DMF
Toluene
DME
EtOH
THF
THF
THF
THF
THF
9
dehyde 1, amine 2, and
a,b-unsaturated ketone 3 proceeded
10
11
12b
13c
14d
15
16
17
18
19e
smoothly leading to the corresponding products 4 in moderate to
good yields. Usually, 12–24 h was needed for the completion of
the reaction. We found that the conditions have proven to be use-
ful for various aromatic amines. As expected, both electron-rich
and electron-poor anilines are suitable partners in the reaction of
2-alkynylbenzaldehyde 1a and but-3-en-2-one 3a. However, no
reactions occurred when aliphatic amines were employed in the
above reactions (data not shown in Table 2). We reasoned that it
might be due to the stability issue of the imine generated. With re-
spect to the R2 group attached on the triple bond in substrate 1, it
seemed that the n-butyl group was not suitable in this process. We
observed the product formation during the reaction process, how-
ever, the product obtained was not stable and easy to be decom-
posed (Table 2, entry 7). Product 4h was generated with 40%
yield when cyclopropyl group was used as a replacement (Table
Cu(OTf)2
Dy(OTf)3
AgOAc
AgOTf
THF
THF
THF
a
b
c
Isolated yield based on 2-alkynylbenzaldehyde.
In the presence of 10 mol % of PPh3.
In the presence of 5 mol % of PPh3.
In the presence of 50 mol % of PPh3.
In the presence of 4 Å molecular sieves.
d
e
PPh3 was used as an organocatalyst in the reaction, and the desired
product 4a was isolated in 70% yield (Table 1, entry 4). Inferior
yield was displayed when PCy3 was utilized (24% yield, Table 1,
entry 5). Blank experiment indicated that no desired product was
detected in the absence of phosphine (data not shown in Table
1). Further screening of the solvents revealed that THF was the best
2, entry 8). Fluoro-substituted 2-alkynylbenzaldehyde
1 also
worked well, which afforded the desired product 4i or 4j in 60%
or 40% yields, respectively (Table 2, entries 9–10). However, only
trace amount of product 4k was detected when 2-alkynylbenzal-
Table 2
Silver triflate and triphenylphosphine co-catalyzed reactions of 2-alkynylbenzaldehyde 1, amine 2, and
a
,b-unsaturated ketone 310
O
CHO
AgOTf (10 mol %)
PPh3 (20 mol %)
THF, 70 οC
R4
O
R1
R3 NH2
R3
R2
+
+
R4
N
R1
2
R2
3
1
4
Entry
R1
R2
R3
R4
Product
Yielda (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
4-MeOC6H4 (2a)
4-MeC6H4 (2b)
3-NO2C6H4 (2c)
4-FC6H4 (2d)
4-CF3C6H4 (2e)
C6H5 (2f)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeC6H4 (2b)
4-FC6H4 (2d)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Me (3a)
Et (3b)
Et (3b)
Et (3b)
Et (3b)
Et (3b)
Et (3b)
Et (3b)
OMe (3c)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
70
63
60
70
65
70
Trace
40
60
40
Trace
56
60
63
67
54
45
48
n-Bu
Cyclopropyl
Ph
Ph
Ph
Ph
Ph
Ph
5-F
4-F
4,5-(OMe)2
H
H
H
H
H
Ph
Ph
4-CF3C6H4 (2e)
C6H5 (2f)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
4-MeOC6H4 (2a)
H
4-F
H
Cyclopropyl
Ph
Ph
NR
a
Isolated yield based on 2-alkynylbenzaldehyde 1.

S. Ye, J. Wu / Tetrahedron Letters 50 (2009) 6273–6275
6275
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O
O
Me
3
Me
PPh3
Ph3P
C
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CHO R3 NH2
R3
R3
R2
N
2
R1
R1
N
AgOTf
R1
A
R2
R2
H2O
TfOAg
[Ag]
1
B
5. For recent selected examples, see: (a) Schwartz, B. D.; Porzelle, A.; Jack, K. S.;
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72, 3326.
O
PPh3
O
Me
Me
H
R3
N
R3
R2
N
R1
R1
R2
AgOTf
PPh3
D
4
[Ag]
Scheme 1. Possible mechanism for the silver triflate and triphenylphosphine
co-catalyzed three-component reaction.
6. For recent selected examples, see: (a) Berkessel, A.; Roland, K.; Neudoerfl, J. M.
Org. Lett. 2006, 8, 4195; (b) Rodgen, S. A.; Schaus, S. E. Angew. Chem., Int. Ed.
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dehyde 1 with electron-rich group attached on the aromatic ring
was employed (Table 2, entry 11). It might be due to the lower
electrophilicity of the in situ-generated iminium. Pent-1-en-3-
one 2b was also a suitable partner in this reaction (Table 2, entries
12–18). However, no product was detected when methyl acrylate
2c was employed as a substrate in the reaction (Table 2, entry
19). The possible mechanism was proposed as well (Scheme 1).
We reasoned that in the presence of AgOTf, the triple bond might
coordinate to the silver salt, and subsequently, the nitrogen atom
of imine A could attack the triple bond via 6-endo-cyclization to af-
ford an isoquinolinium intermediate B. Meanwhile, triphenylphos-
phine as a nucleophilic catalyst attacked the
a,b-unsaturated
ketone 3 leading to the enolate C, which then underwent intermo-
lecular attack of the isoquinolinium intermediate B to generate
phosphinium D. Finally, elimination of phosphine gave rise to the
desired product 4.
In summary, we have described a three-component reaction of
2-alkynylbenzaldehyde, amine, and
a,b-unsaturated ketone cata-
lyzed by the combination of AgOTf and PPh3 as an activation sys-
tem. This reaction proceeds smoothly under mild conditions,
giving rise to the functionalized 1,2-dihydroisoquinolines in mod-
erate to good yields. Further transformation combining metal
catalysis and organocatalysis is under investigation in our labora-
tory, and the results will be reported in due course.
Acknowledgments
9. (a) Chen, Z.; Yang, X.; Wu, J. Chem. Commun. 2009, 3469; (b) Ding, Q.; Wang, Z.;
Wu, J. J. Org. Chem. 2009, 74, 921; (c) Yu, X.; Ding, Q.; Chen, Z.; Wu, J.
Tetrahedron Lett. 2009, 50, 4279; (d) Ding, Q.; Wang, Z.; Wu, J. Tetrahedron Lett.
2009, 50, 198; (e) Gao, K.; Wu, J. Org. Lett. 2008, 10, 2251; (f) Ding, Q.; Wu, J.
Adv. Synth. Catal. 2008, 350, 1850; (g) Yu, X.; Ding, Q.; Wang, W.; Wu, J.
Tetrahedron Lett. 2008, 49, 4390; (h) Gao, K.; Wu, J. J. Org. Chem. 2007, 72, 8611;
(i) Ding, Q.; Ye, Y.; Fan, R.; Wu, J. J. Org. Chem. 2007, 72, 5439; (j) Sun, W.; Ding,
Q.; Sun, X.; Fan, R.; Wu, J. J. Comb. Chem. 2007, 9, 690; (k) Ding, Q.; Wang, B.;
Wu, J. Tetrahedron 2007, 63, 12166.
Financial support from National Natural Science Foundation of
China (20772018), the Science and Technology Commission of
Shanghai Municipality (09JC14049), and Program for New Century
Excellent Talents in University (NCET-07-0208) is gratefully
acknowledged.
10. General procedure for the silver triflate and triphenylphosphine co-catalyzed
Supplementary data
reactions of 2-alkynylbenzaldehyde 1, amine 2, and a,b-unsaturated ketone 3: a,b-
unsaturated ketone 3 (0.4 mmol 2.0 equiv) was added to a solution of 2-
alkynylbenzaldehyde 1 (0.20 mmol), amine 2 (0.22 mmol, 1.1 equiv), AgOTf
(0.02 mmol, 10 mol %), and PPh3 (0.04 mmol 20 mol %) in THF (2.0 mL). The
solution was then stirred at 70 °C. After completion of reaction as indicated by
TLC, the solvent was evaporated. The residue was purified by flash
chromatography on silica gel to provide the desired product 4. Data of
selected example: 3-(2-(4-Methoxyphenyl)-3-phenyl-1,2-dihydroisoquinolin-
1-yl)but-3-en-2-one 4a. 1H NMR (400 MHz, CDCl3) d 2.37 (s, 3H), 3.64 (s, 3H),
5.41 (s, 1H), 5.98 (s, 1H), 6.08 (s, 1H), 6.62 (d, J = 9.2 Hz, 2H), 6.64 (s, 1H), 7.03–
7.06 (m, 3H), 7.11–7.26 (m, 6H), 7.48 (d, J = 6.8 Hz, 2H); 13C NMR (100 MHz,
CDCl3) d 49.4, 77.4, 84.8, 133.7, 135.9, 146.1, 146.4, 147.9, 148.8, 148.9, 149.2,
149.7, 150.1, 150.5, 151.5, 154.7, 159.5, 163.0, 163.3, 170.0, 177.2, 222.6;
HRMS calcd for C26H24NO2 (M++H): 382.1807, found: 382.1824. (For details,
please see Supplementary data).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.09.005.
References and notes
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