Heteropoly acids: a green and efficient heterogeneous Br?nsted acidic catalyst for the intermolecular hydroamination of olefins

Article abstract of DOI:10.1016/j.tetlet.2008.03.034

Intermolecular hydroamination of non-activated olefins with amides and benzyl carbamate proceeds efficiently in the presence of environmentally benign silicotungstic acid (HSiW) catalyst under mild conditions in air to afford addition products in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2008.03.034

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2882–2885
Heteropoly acids: a green and efficient heterogeneous Brønsted
acidic catalyst for the intermolecular hydroamination of olefins
Lei Yang a, Li-Wen Xu a,b,*, Chun-Gu Xia a,*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
and Graduate School of the Chinese Academy of Sciences, Lanzhou 730000, PR China
b Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
Received 8 February 2008; revised 5 March 2008; accepted 6 March 2008
Available online 10 March 2008
Abstract
Intermolecular hydroamination of non-activated olefins with amides and benzyl carbamate proceeds efficiently in the presence of
environmentally benign silicotungstic acid (HSiW) catalyst under mild conditions in air to afford addition products in good to excellent
yields.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Heteropoly acids; Intermolecular hydroamination; Amides; Olefins
In recent years, Keggin type heteropoly acids (HPAs)
catalysts have received much attentions in both academic
and industrial applications due to their unique properties,
which offers several advantages in terms of catalytic perfor-
mance, strong acidic, and redox site and selectivity to par-
ticular reaction product by selective stabilization of
reaction intermediate.1 HPAs are non-corrosive, environ-
mentally benign, and economically feasible solid acid cata-
lysts compared to conventional homogeneous acids, such
as H2SO4 or TfOH. Furthermore, they can be reused and
recycled easily in most cases after the reaction and hence
they are regarded as green catalysts. As a consequence, a
variety of synthetically useful transformations have been
developed using HPAs as catalysts, such as oxidation of
alcohols,2 esterification,3 Friedel–Crafts reactions,4 Man-
nich reactions,5 cyanosilylation,6 ring-opening of epox-
ides,7 and dehydration.8
Hydroamination, the simple addition of an N–H bond
across C–C unsaturated organic fragment, has attracted
much attention in the past decades. Intermolecular hydro-
amination of olefins is one of the most important and chal-
lenging topics in this area.9 Despite significant efforts that
have been devoted into the intermolecular hydroamination
of olefins with alkylamines and arylamines, only a few
reports of the intermolecular hydroamination of non-acti-
vated alkenes with weakly basic amine nucleophiles such
as sulfonamides, carbamates, and carboxamides are known
(Scheme 1).
Recently, efficient platinum(II),10 gold(I),11 Cu(II),12
Fe(III),13 and other metal salts14 catalyzed hydroamina-
tions of amides and carbamates were reported. Along with
the metal catalysts, there also have been examples using
metal-free catalysts for the hydroaminations of olefins
and amides.15 Although some notable progress has been
made on the hydroamination reactions of alkenes with
NHR RHN
*
Transition Metal catalyst
Corresponding authors. Tel.: +86 0931 4968056; fax: +86 0931
+
NH2R
R1
R2
R1
R2
8277088 (L.-W.X.).
R1
R2
reflux
E-mail addresses: licpxulw@yahoo.com (L.-W. Xu), cgxia@lzb.ac.cn
Scheme 1. Hydroamination reaction.
(C.-G. Xia).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.034

L. Yang et al. / Tetrahedron Letters 49 (2008) 2882–2885
2883
Table 1
amides in the past two years, there are also some draw-
backs on the reported methods, such as using expensive
and toxic metals, higher reaction temperature, large excess
amounts of olefins, and tedious reaction procedures. Addi-
tionally, most of the reported methods must be carried out
under inert atmosphere. Therefore, the development of a
novel, green, and simple catalyst for addition reactions of
non-activated alkenes with sulfonamides, carboxamides
and carbamates remains an intriguing challenge.
Hydroamination between toluenesulfonamide and cyclohexene under
different reaction conditions
catalyst
NH2Ts
+
NHTs
Yieldb (%)
Entrya
Catalyst
Solvent
1
2
3
4
5
6
7
8
9
H3PW12O40ÁnH2O
H3PMo12O40ÁnH2O
H3SiW12O40ÁnH2O
Na3PMo12O40
(NH4)3PW12O40
Ag3PW12O40
H2MoO4
NH2SO3H
H3SiW12O40ÁnH2O
H3SiW12O40ÁnH2O
H3SiW12O40ÁnH2O
H3SiW12O40ÁnH2O
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH3CN
n-Heptane
CH3Ph
1,2-Dioxane
63
82
95
NR
Trace
Trace
NRc
NRc
Trace
86
Although silver-exchanged tungstophosphoric acid16 or
using HPAs as acid additive17 have been examined for
intermolecular hydroamination of alkynes with aromatic
amines, to the best of our knowledge, there are no studies
and reports of HPAs-catalyzed intermolecular hydroamin-
ation of olefins with amides. In the present letter, we report
an efficiently, easy operational, and reusable heteropoly
acids catalyst for the intermolecular hydroamination of
various olefins with both amides and carbamates.
In order to study the feasibility of HPAs-catalyzed
hydroamination of amides, hydroamination of cyclohexene
with p-toluenesulfonamide (TsNH2) was selected as a
model reaction (Table 1). Under our reaction conditions,
10
11
12
a
92
75
Reaction conditions: cyclohexene (2 mmol), NH2Ts (1 mmol), catalyst
(100 mg), solvent (2 ml), 85 °C, 18 h, sealed tube, in air.
b
GC yield.
c
Using 0.1 mmol of solid acid as catalyst.
Table 2
Hydroamination of unactivated olefins with nitrogen nucleophiles18
NHR
H SiW
O
12 40
·nH O
3
2
+
NH R
2
R
1
R
2
R
R
2
solvent
1
Entrya
1
Olefin
Nucleophile
NH2Ts
Time (h)/Temp (°C)
Solvent
DCE
Product
Yieldb(%)
78
NHTs
23/85
2
3
NH2Ts
18/85
18/85
DCE
DCE
91
54
NHTs
PhSO2NH2
NHSO2Ph
NHTs
4
5
NH2Ts
NH2Ts
24/45
24/rt
DCE
DCE
78
64
NHTs
NHTs
6
7
NH2Ts
NH2Ts
24/45
24/85
DCE
DCE
68
88
Cl
Cl
NHTs
(continued on next page)

2884
L. Yang et al. / Tetrahedron Letters 49 (2008) 2882–2885
Table 2 (continued)
Entrya
Olefin
Nucleophile
CH3NHTs
Time (h)/Temp (°C)
Solvent
DCE
Product
Yieldb(%)
60
NMeTs
8
9
24/85
NHCOPh
NHNs
PhCONH2
NH2Ns
28/85
28/85
28/85
Dioxane
Dioxane
DCE
67
85
86
10
11
NHSO2Ph
PhSO2NH2
NHTs
12
13
NH2Ts
24/50
DCE
62
NHSO2Ph
NHCbz
PhSO2NH2
24/50
26/50
DCE
DCE
48
71
14
NH2COOBn
a
Reaction conditions: olefin (2 mmol), NH2Ts (1 mmol), H3SiW12O40ÁnH2O (100 mg), solvent (2 ml), sealed tube, in air.
b
Isolated yield.
among the HPAs tested, H3SiW12O40ÁnH2O was found to
be the most efficient catalyst compared to
NH2Cbz. All the nitrogen donors underwent hydroamina-
tion with good yields at 50 °C within 24–26 h. The possibil-
ity of the recycling of the catalyst was also examined. For
this reaction, the hydroamination of NH2Ts and styrene in
DCE at 45 °C in the presence of catalytic H3SiW12O40Á
nH2O was selected as a model. When the reaction com-
pleted, the catalyst could be recycled after simple filtration
and washed with DCE. The obtained powder was dried
under vacuum, and then directly reused in subsequently
recycled reaction. After two runs, the yield of the corre-
sponding addition compound was somewhat decreased
(74%, 62%).
In conclusion, we have developed a commercially avail-
able and reusable HPAs catalyzed intermolecular hydro-
amination of unactivated alkenes with weakly basic
amine nucleophiles such as sulfonamides, benzyl carba-
mate, and benzamide, which can be used successfully and
environmental friendly to afford good to excellent yields
of addition products. Furthermore, this methodology offers
significant improvements with regard to the scope of this
transformation, simplicity in operation, and green aspects
by avoiding toxic metals or corrosive catalysts.
H3PW12O40ÁnH2O and H3PMo12O40ÁnH2O. Solid acids
such as NH2SO3H and H2MoO4 could not furnish the
desired products. No or only trace products were found
when using the HPAs salts such as Na3PMo12O40,
(NH4)3PW12O40, and Ag3PW12O40 as catalysts. Using
H3SiW12O40ÁnH2O as catalyst, various solvents were then
examined. Among the solvents examined, non-polar DCE
was found to give better results in terms of chemical yields.
Polar solvent 1,4-dioxane tended to shut down the reacting
system and afforded the addition product in lower yield.
To investigate the scope of the H3SiW12O40ÁnH2O cata-
lyzed hydroamination reaction, several selected olefins and
amides were then examined under given reaction condi-
tions and the results are summarized in Table 2.17 Cyclo-
pentene and cyclohexene were successfully converted to
the corresponding N-cyclopentenyl p-toluenesulfonamide
and N-cyclohexyl p-toluenesulfonamide. PhSO2NH2 gave
lower yield of the addition products. The hydroamination
reactions of NH2Ts with various vinyl arenes gave fast
and high-yielding reactions under mild reaction conditions.
Interestingly, addition of NH2Ts to p-methylphenylene cat-
alyzed by H3SiW12O40ÁnH2O occurred even at room tem-
perature in good yield. Similarly, the additions of various
amides to the more strained norbornene furnished the
hydroamination products in good to excellent yields under
modified reaction conditions.
Acknowledgments
This study was supported by the National Natural
Science Founder of China (Project No. 20572114) and
National Natural Science Funds for Distinguished Young
Scholar, PR China (Project No.20625308).
To further extend the scope of the substrate, 1,3-cyclo-
hexadiene was investigated in the reaction of amides and

L. Yang et al. / Tetrahedron Letters 49 (2008) 2882–2885
2885
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C
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