Intramolecular hydroamination catalysed by Ag complexes stabilised in situ by bidentate ligands

Article abstract of DOI:10.1016/j.jorganchem.2008.10.038

A series of silver complexes generated in situ from AgOSO₂CF₃ (AgOTf) and a range of bidentate ligands were investigated as catalysts for the intramolecular hydroamination of 4-pentyn-1-amine. A variety of P- and N-donor ligands were tested including the novel pyrazole-phosphine ligand 1-(2-(diphenylphosphino)phenyl)pyrazole. The best catalyst was formed from equimolar amounts of the P,N-donor ligand 1-(2-(diphenylphosphino)ethyl)pyrazole and AgOTf, which achieved a turnover rate of 129 h^-1 for the cyclisation of 4-pentyn-1-amine.

Full text of DOI:10.1016/j.jorganchem.2008.10.038

Journal of Organometallic Chemistry 694 (2009) 309–312
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Intramolecular hydroamination catalysed by Ag complexes stabilised
in situ by bidentate ligands
*
Sophie R. Beeren, Serin L. Dabb, Barbara A. Messerle
School of Chemistry, The University of New South Wales, NSW, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of silver complexes generated in situ from AgOSO2CF3 (AgOTf) and a range of bidentate ligands
were investigated as catalysts for the intramolecular hydroamination of 4-pentyn-1-amine. A variety
of P- and N-donor ligands were tested including the novel pyrazole-phosphine ligand 1-(2-(diphenyl-
phosphino)phenyl)pyrazole. The best catalyst was formed from equimolar amounts of the P,N-donor
ligand 1-(2-(diphenylphosphino)ethyl)pyrazole and AgOTf, which achieved a turnover rate of 129 hꢀ1
for the cyclisation of 4-pentyn-1-amine.
Received 5 September 2008
Received in revised form 17 October 2008
Accepted 22 October 2008
Available online 30 October 2008
Keywords:
Ó 2008 Elsevier B.V. All rights reserved.
Hydroamination
Silver catalyst
Bidentate ligand
Alkynylamine
Triflate
Cyclisation
1. Introduction
[7,9,11]. Most recently, Ag(phen)(OSO2CF3) (10 mol%), which con-
tains the N,N-donor ligand phenanthroline (phen), has been shown
Nitrogen containing heterocycles are sub-units in biologically
active compounds which are important in the pharmaceutical
and agrochemical industries. Their synthesis via convenient and
direct approaches is therefore highly desirable. Catalysed cyclisa-
tion of alkynyl- and alkenylamines via intramolecular hydroamin-
ation has shown great potential for the energy efficient synthesis of
N-heterocycles [1]. Lanthanide [2], early [3] and late [4–10] transi-
tion metal complexes have been shown to successfully catalyse
this reaction. The advantage of late transition metals is their lower
oxophilicity (compared to early transition metal or lanthanide me-
tal catalysts), which leads to higher functional group tolerance and
less sensitivity to water or oxygen exposure.
to promote a conversion of 95% of 4-pentyn-1-amine (1) to 2-
methylpyrroline (2) after 4 h at 70 °C, whereas AgOSO2CF3 (AgOTf)
alone only promotes a conversion of 55% after the same time [9].
In this paper, we report the use of silver complexes generated
in situ from AgOTf and a series of bidentate N,N-, P,N- and P,P-do-
nor ligands as intramolecular hydroamination catalysts. Of partic-
ular interest for the development of efficient metal centred
catalysts are mixed P,N-donor ligands which combine hard (nitro-
gen) and soft (phosphorus) centres [13]. Rhodium(I) and iridium(I)
complexes containing the P,N-donor ligand 1-(2-(diphenylphos-
phino)ethyl)pyrazole (3) have shown high activity as hydrothiola-
tion [14], hydroalkoxylation [15] and hydroamination [8] catalysts.
There are a number of examples of silver catalysed hydroamin-
ation reactions [5,7,9–12], however, there are only a few examples
of the use of silver as a catalyst for the cyclisation of alkynylamines
to form N-heterocycles [5,7,9,10]. These previous studies have pri-
marily focussed on the use of silver salts rather than silver com-
plexes as catalysts, and it has been found that these salts have a
tendency to decompose at high temperatures and become inactive
before the complete conversion of substrate is reached. However, a
more active Ag(I) catalyst may be obtained by complexation of the
silver centre with a suitable ligand, where the ligand stabilises and/
or improves the efficiency of the catalytically active Ag centre
2. Results and discussion
2.1. Synthesis of 1-(2-(diphenylphosphino)phenyl)pyrazole (4)
To further investigate the use of P,N-donor ligands in hydroamin-
ation catalysis the novel pyrazole-phosphine ligand 1-(2-(diphenyl-
phosphino)phenyl)pyrazole (4) was synthesised (Scheme 1). In this
ligand, modelled on the previously reported ligand 3 [14], a phenyl
bridge replaces the ethyl backbone. By thus expanding the range of
pyrazole-phosphine ligands available we can consider the impor-
tance of the relative flexibility of the ligand backbone in catalysis.
1-Phenylpyrazole was synthesised via a condensation reaction be-
tween 1,1,3,3-tetraethoxypropane and phenylhydrazine hydrochl-
* Corresponding author. Tel.: +61 2 9385 4653; fax: +61 2 9385 6141.
E-mail address: b.messerle@unsw.edu.au (B.A. Messerle).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.038

310
S.R. Beeren et al. / Journal of Organometallic Chemistry 694 (2009) 309–312
Scheme 1. Synthesis of ligand 4.
oride [16,17]. Ethyl magnesium bromide was used to deprotonate 1-
phenylpyrazole at the ortho-position, to form 2-(1-pyrazolyl)phen-
ylmagnesiumbromide in situ. Addition of chlorodiphenylphosphine
to the reaction mixture afforded the air stable ligand 4 cleanly, after
the crude product was recrystallised from hot methanol. Ethyl mag-
nesium bromide was used in place of n-butyl lithium, which has pre-
viously been used in the synthesis of 3,5-dimethyl(1-(2-
(diphenylphosphino)phenyl))pyrazole [18], due to the preference
of n-butyl lithium to deprotonate the 3 or 5 position of the pyrazole
ring over the phenyl protons [19,20].
Fig. 1. Bidentate P- and N-donor ligands reacted with AgOTf to generate hydro-
amination catalysts in situ.
Table 2
2.2. Catalysis
Hydroamination of 4-pentyn-1-amine (1): relative efficiencies of Ag+ catalysts formed
in situ from AgOTf and various bidentate P and N-donor ligands.a
The efficiency of a range of Ag catalysts generated in situ from
AgOSO2CF3 (AgOTf) and a series of bidentate ligands, including 3
and the new ligand 4, was investigated using the intramolecular
hydroamination of 4-pentyn-1-amine (1) to form 2-methylpyrro-
line (2) (Scheme 2). The catalysis was carried out at 60 °C in
THF-d8 under a N2 atmosphere. Reaction progress was monitored
by the acquisition of 1H NMR spectra at regular intervals and %con-
version was determined by integration of relevant peaks in the
NMR spectra.
As a control experiment the reaction was first carried out in the
presence of AgOTf with no added ligand at different loadings –
2 mol%, 5 mol% and 10 mol% (Table 1). At each of these concentra-
tions the catalyst was initially highly active, as indicated by the
consistently high turnover rates measured at 50% conversion.
When using only 2 mol% AgOTf the catalytic activity dropped be-
fore complete conversion of the substrate, with no more than
90% conversion reached even after 7 days. In the presence of
5 mol% and 20 mol% of AgOTf, the rate of reaction was sufficiently
fast that >98% conversion was reached quickly, before the com-
plete decomposition of the catalyst.
Catalyst
Conversion at Nt (hꢀ1
)
2 h (%)
AgOTf + 3
AgOTf + 9
AgOTf
AgOTf + 5
AgOTf + 7
AgOTf + 4
AgOTf + 8
AgOTf + 6
AgOTf + 10
>98
90
82
43
40
37
37
28
12
129
64
62
–
–
–
–
–
–
a
Reactions were performed using 2 mol% AgOTf and 2 mol% of ligand.
activity for the intramolecular hydroamination reaction (Table 2).
Specifically, AgOTf was treated with each of the three bidentate
P,N-donor ligands; 1-(2-(diphenylphosphino)ethyl)pyrazole (3),
1-(2-(diphenylphosphino)phenyl)pyrazole (4) or 2-(2-(diphenyl-
phosphino)ethyl)-1-methylimidazole (5) [21], to form catalysts
in situ. Ligands 3 and 5 were first synthesised within our research
group and Ir and Rh complexes incorporating 3 and 5 have previ-
ously shown catalytic efficiency for the hydroamination reaction
[14,21]. When 2 mol% of AgOTf and 2 mol% of ligand 3 were com-
bined, a highly active and stable catalyst was generated which cat-
alysed the complete conversion of 1 in 1.7 h with a turnover rate of
129 hꢀ1. This is the best reported result to date for the cyclisation
of 1 using a silver catalyst [9]. In particular the turnover rate ex-
ceeded that of AgOTf alone (62 hꢀ1). Both the more rigid pyra-
zole-phosphine ligand 4 and the imidazole-phosphine ligand 5
generated poor catalysts which did not promote more than 30%
conversion of substrate to product.
With the aim of generating reactive, but stable, Ag catalysts
AgOTf was reacted with each of the ligands shown in Fig. 1 in solu-
tion. The complexes thus generated in situ were tested for catalytic
Scheme 2. Hydroamination of 4-pentyn-1-amine (1) to 2-methylpyrroline (2).
Three bidentate N-donor ligands were combined with AgOTf to
form complexes in situ which were tested as catalysts for hydroam-
ination. The ligands studied were bis(pyrazol-1-yl)methane (6)
[22], bis(N-methylimidazol-2-yl)methane (7) [23] and glyoxal
bis(4-methylphenylimine) (8) [24] (Fig. 1). Ligands 6, 7, and 8 all
formed catalysts that were initially highly active and promoted
the conversion of 25% of the substrate within 15 min. The reaction
rate then slowed significantly and catalysis dropped below detect-
able amounts, most likely due to decomposition of the Ag+
complexes.
Table 1
Catalytic efficiency of AgOTf for the hydroamination of 4-pentyn-1-amine (1).
AgOTf
(mol%)
Time at 80% conversion
(h)
Time at >98% conversion
(h)
Nt
a
(hꢀ1
)
2
5
20
1.5
0.6
1.5
164 (90%b)
11.5
1.25
62
48
50
a
Turnover rates (Nt, hꢀ1) = (moles of product)/(moles of catalyst)/hour, calcu-
lated at 50% coversion.
b
The activity of in situ silver catalysts formed from AgOTf and
two bidentate phosphorus donor ligands, 1,2-bis(diphenylphos-
Complete conversion of substrate in the presence of 2 mol% AgOTf was not
observed.

S.R. Beeren et al. / Journal of Organometallic Chemistry 694 (2009) 309–312
311
when the ligand introduced provides the correct balance between
reactivity and stabilisation of the Ag(I) metal centre, the best
example studied here being 1-(2-(diphenylphosphino)ethyl)pyra-
zole (3). In the presence of equimolar concentrations of AgOTf
and 3, the hydroamination of 4-pentyn-1-amine (1) was catalysed
to completion with a turnover rate of 129 hꢀ1
.
4. Experimental section
Chemicals were purchased from Aldrich or Lancaster and used
without further purification. Nitrogen (>99.5%) was obtained from
Linde Gas Pty. Ltd. and used as supplied. THF-d8 was purchased
from Cambridge Isotopes and dried over sodium prior to use. 4-
Pentyn-1-amine (1) was synthesised by the Organic Synthesis Cen-
tre, School of Chemistry, University of Sydney and was dried over
calcium hydride prior to use. All spectra were obtained at 298 K
unless otherwise reported and 1H and 13C spectra are referenced
to internal solvent references. 31P NMR chemical shifts were exter-
nally referenced to H3PO4 (85% in D2O) at 0 ppm. 1-Phenylpyrazole
[16,17], 1-(2-(diphenylphosphino)ethyl)pyrazole (3) [14], 2-(2-
(diphenylphosphino)ethyl)-1-methylimidazole (5) [21], bis(pyra-
zol-1-yl)methane (6) [22], bis(N-methylimidazol-2-yl)methane
(7) [23], and glyoxal bis(4-methylphenylimine) (8) [24] were syn-
thesised by literature procedures.
Fig. 2. Reaction profile for the cyclisation of 4-pentyn-1-amine (1) catalysed by
5 mol% AgOTf ( ) and 5 mol%. AgOTf in the presence of 2 mol% (h), 5 mol% (N) and
10 mol% (j) of 3.
D
Fig. 3. Numbering system for 4 for reporting of NMR spectroscopic data.
4.1. Synthesis of 1-(2-(diphenylphosphino)phenyl)pyrazole (4)
Ethyl magnesium bromide (14 mL, 1 M in THF) was added to a
solution of 1-phenylpyrazole (2.00 g, 13.9 mmol) in THF (5 mL) un-
der an atmosphere of nitrogen. This mixture was refluxed over-
night in an oil bath at 100 °C. The red/brown mixture that
formed was cooled to room temperature and then on ice before
the slow addition of diphenylphosphine chloride (3.07 g, 139
mmol). After stirring for 3 h at room temperature, water was added
in air to quench the reaction. The product was extracted into ethyl
acetate (3 ꢁ 15 mL), dried over magnesium sulfate and the solvent
evaporated in vacuo to yield a red solid. Recrystallisation from hot
methanol yielded 4 as a yellow crystalline solid (1.67 g, 37%) m.p.
104–105 °C.
phino)ethane (9) and 1,3-bis(diphenylphosphino)propane (10) was
also investigated. Ligand 9 generated an effective catalyst that pro-
moted 90% conversion of 4-pentyn-1-amine (1) in 2.1 h with a
turnover rate of 64 hꢀ1. The same reactivity was not observed in
the case of ligand 10, which contains a longer alkyl chain between
phosphine donors. Instead, AgOTf and 10 formed a complex that
catalysed only 12% conversion of 1 in 2 h.
Having established that the most efficient catalyst of those
tested here for the hydroamination of 1 was formed in situ from
2 mol% of AgOTf and 2 mol% of P,N-donor ligand 3, a series of
experiments were performed to establish which stoichiometric ra-
tio of AgOTf and ligand would generate the most effective catalyst.
The reaction was carried out in the presence of 5 mol% of AgOTf in
combination with 2, 5 and 10 mol% of 3 (Fig. 2). A turnover rate of
130 hꢀ1 was achieved using 5 mol% of AgOTf and 5 mol% of 3 and
complete conversion to imine 2 was reached in 50 min. By compar-
ison, turnover rates of 48 hꢀ1, 66 hꢀ1 and 38 hꢀ1 were observed
when 0, 2 and 10 mol% of 3 were used, respectively. It was con-
cluded, therefore, that the most efficient catalyst is formed from
equimolar amounts of AgOTf and 3.
The catalysed cyclisation of 5-hexyn-1-amine (11) to form 2-
methyl-3,4,5,6-tetrahydropyridine (12) was also investigated
using the catalyst generated in situ from AgOTf and 3. Previous re-
sults have shown that the cyclisation of alkynylamines to form six-
membered rings is typically harder to achieve than five-membered
rings [4,9], which was also shown to be the case here. The combi-
nation of AgOTf (2 mol%) and 3 (2 mol%) promoted a conversion of
11 to 12 of only 34% after 21 h, with no significant improvement
after this time.
Found: C, 76.90; H, 5.31; N, 8.68%. C21H17N2P requires C, 76.82;
H, 5.22; N, 8.53.
m
max/cmꢀ1 3064 (br), 1598, 1569, 1517, 1473,
1452, 1433, 1433, 1412, 1392, 1330, 1194, 1075, 1045, 1019,
934, 845, 767, 749, 697, 658, 620, 514, 501, 490, 420. (See Fig. 3
for numbering) dH (300 MHz; benzene-d6; Me4Si) 7.58 (1H, d,
3
3JH3ꢀH4 = 1.4 Hz, H3), 7.47 (1H, d, JH5ꢀH4 = 2.5 Hz, H5), 7.38 (1H,
3
3
4
ddd, JH3 –H4 = 7.9 Hz, JH3 –P = 4.0 Hz, JH3 –H5 = 1.3 Hz, H30), 7.35–
0
0
0
0
0
3
0
0
7.28 (4H, m, o-CH of PPh2), 7.14 (1H, ddd, JH6 –H5 = 7.0 Hz,
4
4
JH6 –P = 3.5 Hz, JH6 –H4 = 1.5 Hz, H60), 7.05–7.00 (6H, m, m-CH
0
0
0
and p-CH of PPh2), 6.96 (1H, ddd apparent dt, 3J = 7.6 Hz,
JH4 –H6 = 1.4 Hz, H40), 6.84 (1H, ddd apparent dt, 3J = 7.5 Hz,
4
0
0
JH5 –H3 = 1.3 Hz, H50), 6.04 (1H, dd apparent t, 3J = 2.0 Hz, H4)
4
0
0
ppm. dP{H} (121 MHz; benzene-d6; Me4Si) –14.0 ppm. dC{H}
1
(75 MHz; CDCl3; Me4Si) 144.8 (d, JC2 –P = 21.1 Hz, C20), 140.6
0
(C3), 136.7 (d,1JC–P = 11.6, i-C of PPh3), 134.9 (C60), 134.1 (d,
3
2
JC–P = 20.3 Hz, m-C of PPh2) 133.9 (d, JC1 –P = 20.3 Hz, C10) 131.3
0
4
(d, JC5–P = 5.8 Hz, C5), 129.8 (C40), 129.0 (p-C of PPh2), 128.7 (d,
2
2
JC–P = 7.3 Hz, o-C of PPh2), 128.4 (C50), 126.4 (d, JC3 ꢀP = 2.9 Hz,
0
C30), 106.4 (C4) ppm. ES-MS (MeOH) (ES+): m/z 330 ([4+H]+, 100%).
3. Conclusion
4.2. General procedure for catalytic reactions
The use of silver complexes generated in situ from AgOTf and a
series of bidentate ligands were investigated as catalysts for the
intramolecular hydroamination of 4-pentyn-1-amine (1). A variety
of P- and N-donor ligands were tested, including the novel pyra-
zole-phosphine ligand 1-(2-(diphenylphosphino)phenyl)pyrazole
(4). It was determined that an efficient catalyst is generated only
All catalytic reactions were performed on a small scale in NMR
tubes fitted with a Young’s concentric Teflon valve. All reactions
were carried out and samples prepared under an atmosphere of
nitrogen. Samples were prepared by addition of 4-pentyn-1-amine

312
S.R. Beeren et al. / Journal of Organometallic Chemistry 694 (2009) 309–312
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it was frozen in liquid nitrogen. The first spectrum was measured
when the sample had been thawed to room temperature and in-
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tive to the substrate resonances. The turnover rate (Nt, hꢀ1) was
calculated as the number of moles of product/moles of catalyst/
hour and was determined at the point of 50% conversion.
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Acknowledgements
Financial support from the Australian Research Council and The
University of New South Wales is gratefully acknowledged. We are
grateful to Dr. K. Vuong, Dr. D. Kennedy and Dr. S. Rumble for the
synthesis of ligands 5, 6 and 8, respectively.
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