PdII Complexes with N-(Diadamantylphosphanyl)diaminocarbene and Related Ligands: Synthesis and Catalytic Applications in Intermolecular Alkyne Hydroaminations

Article abstract of DOI:10.1002/ejic.201701342

A new N-diadamantylphosphanyldiaminocarbene 5 is prepared, isolated, and characterized. The carbene appears to be much more stable than previously reported di-tert-butyl congeners. The molecular structure of the carbene is determined by X-ray diffraction

Full text of DOI:10.1002/ejic.201701342

A Journal of
Accepted Article
Title: Pd(II) complexes with a novel N-(diadamantylphosphanyl)-
diaminocarbene and related ligands: synthesis and catalytic
application in intermolecular alkyne hydroaminations
Authors: Anatoliy Marchenko, Georgyi Koidan, Anastasiya Hurieva,
Aleksandr Kostyuk, Dario Franco, Marco Baron, and Andrea
Biffis
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701342
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
Pd(II) complexes with a novel N-(diadamantylphosphanyl)-
diaminocarbene and related ligands: synthesis and catalytic
application in intermolecular alkyne hydroaminations
Anatoliy Marchenko,[a] Georgyi Koidan,[a] Anastasiya Hurieva [a] Aleksandr Kostyuk,*[a] Dario Franco,[b]
Marco Baron[b] and Andrea Biffis*[b]
Abstract: A novel N-diadamantylphosphanyl-diaminocarbene 5 was
prepared, isolated and characterised. The carbene appeared to be
much more stable compared to previously reported di-tert-butyl
congeners. The molecular structure of the carbene was determined
butylphosphanyl-N-heterocyclic carbenes; we have reported on
their preparation[5] and use as ligands in the synthesis of mono-
and dinuclear late transition metal complexes exhibiting
promising catalytic properties.[6] Very recently, we have extended
these investigations to novel acyclic N-di-tert-butylphosphanyl-
diaminocarbene ligands A,[8] their palladium(II) complexes B and
palladium(II) complexes of related iminophosphanes C – a
rearrangement product of ligand A which invariably occurs in the
preparation of type B complexes (Scheme 1).[9] We have tested
Pd complexes B as catalysts in Suzuki coupling reactions and
found that they exhibit a higher reactivity towards aryl chlorides
compared to acyclic diaminocarbene ligands not bearing the N-
di-tert-butylphosphanyl group,[10] but we have also recorded that
such catalytic performance is accompanied by complex
decomposition and by the appearance of reaction byproducts
deriving from Ullmann-type homocoupling of the aryl chloride,
which suggests in situ formation of catalytically competent Pd
colloids;[11] isomerized complexes C exhibit comparable or lower
reactivity, but without formation of the homocoupling product.
These observations have been tentatively attributed to the fact
that type B and C complexes feature small bite angle chelating
ligands, which produce stable square planar complexes with
palladium(II) but which, when the palladium is reduced to
palladium(0) in the course of the catalytic cycle, do not fit well in
the tetrahedral coordination geometry of palladium(0), leading to
ligand dissociation and catalyst decomposition, particularly in the
case of the diaminocarbene ligands.
by
X-ray
diffraction
analysis.
A
novel
also
(diisopropylamino)(diadamantylphosphanyl)carbene
8
was
prepared in situ but not isolated, since in this case the adamantyl
groups do not render the carbene more stable with respect to
previously known carbenes with di-tert-butyl substitution. Carbene 8
reacted in situ with phenylazide to give iminophosphane 9, which
was accessible also from carbene 5 upon rearrangement under
heating. Stable chelate Pd(II) complexes were synthesized using
both carbene 5 and iminophosphane 9 as ligand. The complex with
ligand 9 displayed a very promising catalytic performance in the
intermolecular hydroamination of alkynes with primary arylamines.
Introduction
Strongly
electron-donating,
sterically
encumbered
trialkylphosphanes represent a class of compounds that has
explosively grown in number and importance in the course of the
last decades, mainly in relation to their application as ligands in
the preparation of late transition metal complexes exhibiting
outstanding performances in many catalytic processes.[1] The
most commonly employed ligands of this kind are the parent
tricyclohexylphosphane and tri-tert-butylphosphane, as well as
several related mono- and polytopic ligands featuring di-
cyclohexylphosphano or di-tert-butylphosphano moieties. The
resulting catalytic complexes are usually stable, as such ligands
provide good steric protection towards catalyst poisoning and
decomposition. In some cases, for even better steric protection
the diadamantylphosphano group instead of the di-tert-
butylphosphano one has been employed.[2–4]
In the course of the last years we[5,6] and others[7] have been
interested in novel ditopic ligands of this kind, namely N-di-tert-
[a]
Prof. A. Marchenko, Dr. G. Koidan, Dr. A. Hurieva, Prof. A. Kostyuk
Institute of Organic Chenistry
Scheme 1. Structure of the ligands and complexes investigated by our group.
National Academy of Sciences of Ukraine
Murmanska 5, Kyiv-94, 02660, Ukraine
E-mail: a. kostyuk@yahoo.com
D. Franco, Dr. M. Baron, Prof. A. Biffis
Dipartimento di Sciene Chimiche,
Università di Padova
Consequently, in the frame of the present work, we aimed at
directing further investigation on the catalytic potential of
complexes of this kind towards reactions that are generally
believed to be redox-neutral, such as alkyne hydroaminations, in
which we expected our complexes to be more stable and
consequently more productive as catalysts. Furthermore, we
also aimed at extending our library of complexes to include
[b]
Via Marzolo 1, 35131 Padova, Italy
E-mail: andrea.biffis@unipd.it
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
ligands with the diadamantylphosphano group, in order to study
whether replacing tert-butyl groups with 1-adamantyl ones
renders these complexes more productive catalysts.
and 1.308(3)Å in 4 instead of 1.7950(14), 1.333(2) and 1.305(2).
The angles P1-N1-C1 and N1-C1-N2 are 120.74(16) and
132.0(2)°in 4 instead of 122.29(13) and 131.79(18).
Deprotonation of salt
4 was achieved by treatment with
potassium hexamethyldisilazide in THF to give carbene 5.
Carbene 5 appeared indeed to be more stable compared to
analogous carbenes featuring tert-butyl groups instead of 1-
adamantyl ones: its lifetime (t1/2) measured in solution at 20 ˚C is
60 days, that is, more than 4 times longer.
Results and Discussion
The synthesis of carbene 5 (Scheme 2) was accomplished using
the procedure developed by us earlier.[8] Salt 3 was prepared by
treatment of P,P-di(adamantan-1-yl)phosphanoselenoic amide 1
with Alder’s dimer 2. It is a hydrolytically stable crystalline
substance that was reduced with hexamethylphosphorus
triamide to the highly moisture-sensitive PIII salt 4.
The molecular structure of 5 was determined by single crystal X-
ray diffraction (Figure 2). The N1-C1-N2 angle of 116.6(2)°of 5
is significantly smaller than the same angle in a related type A
carbene with R = t-Bu and Ar = 4-methoxy-phenyl[8] (121.1(2)°).
The same is true for the dihedral angle P1-N1-C1-N2, which is
157.08(17) in 5 compared to 176.10(15)°. The N1–C1 and N2-
C1 bond lengths are instead 1.410(3) and 1.324(3) for 5,
compared to 1.380(3) Å and 1.325(3) Å for the reference
compound; it is worth to remark the double bond character of the
N2-C1 bond, evidenced by its shorter length. The N1 and N2
atoms of 5 both have a trigonal-planar bond configuration (the
sums of the bond angles are 357.2(6)°for N1 and 35 9.3(6)°for
N2).
(iPr2N=CH)2O
_
_
TfO
_
Se
Se Ph
TfO
Ph
2
(Me2N)3P
Ad
Ad
Ad
Ad
Ad
Ad
TfO
PNHPh
P
N
2
P
N
N+Pr2-i
N+Pr2-i
-(Me2N)3PSe
1
4
CF3SO3 = TfO
3
Ph
KN(SiMe3)2
i-Pr2N
N
4
C
PAd2
5
Scheme 2. Synthesis of salts 3, 4 and of carbene 5.
Figure 2. ORTEP view of 5, hydrogen atoms and solvent molecules are
omitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected
bond distances (Å) and angles (°):N1-P1 1.742(2), N 1-C1 1.410(3), N2-C1
1.324(3), N1-C28 1.445(3), N1-C1-N2 116.6(2), C1-N1-P1 118.10(16), C1-N1-
C28 123.7(2), P1-N1-C28 115.45(16).
Figure 1. ORTEP view of the molecule 4, hydrogen atoms and triflate anion
are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
Selected bond distances (Å) and angles (°): N1-P1 1 .792(2), N1-C1 1.341(3),
N2-C1 1.308(3), N1-C28 1.455(3), N1-C1-N2 132.0(2), C1-N1-P1 120.74(16),
C1-N1-C28 121.7(2), P1-N1-C28 116.62(15).
As substitution of tert-butyl groups with 1-adamantyl ones made
type A carbenes more stable we also expected that the same
would be true for closely related (diisopropylamino)(phosphano)-
carbenes. In fact, Bertrand has shown that (diisopropylamino)-
(phosphano)carbenes can be separated as individual
compounds,[12,13] and that their stability depends on the
substituents at the phosphorus atom, the most stable compound
being (diisopropylamino)(di-tert-butylphosphano)carbene. It was
also shown that the phosphano group acts as a spectator group
and reactions can proceed either at the carbene atom or at
phosphorus. We assumed that the introduction of 1-adamantyl
groups at phosphorus increases the stability of these carbenes.
The 31P NMR spectrum of PV formamidinium salt 3 exhibited two
broad signals at δp = 139.0 and 142.6 ppm with an integral ratio
3:2 that can be attributed to hindered rotation about the P-N
bond. At the same time, the 31P NMR spectrum of PIII
formamidinium salt 4 exhibited one signal at δp = 130.9.
Compound
4 was studied by X-Ray analysis (Figure 1).
Structural parameters for 4 are very close to those reported by
us for the corresponding compound with tert-butyl instead of 1-
adamantyl substituents at the phosphorous atom.[8] In particular,
the distances P1-N1, N1-C1 and C1-N2 are 1.792(2), 1.341(3)
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
With this aim in mind, phosphanoiminium salt 7 was prepared
(Scheme 3). Previously, iminium salts of type 7 were prepared
starting from stannylphosphanes.[14] We propose here a new
approach, i.e. treatment of diadamantylphosphane with Alder’s
dimer. Deprotonation of phosphanoiminium salt 7 was carried
out with potassium hexamethyldisilazide in THF. Formation of
carbene 8 was recorded by 31P NMR spectroscopy (δp = 5 ppm).
Contrary to our expectation, though, its lifetime (t1/2) in solution
at 22 ˚C appeared to be only 30 min. An analogous carbene
featuring tert-butyl groups instead of 1-adamantyl was reported
to have a half-life of 12 h at room temperature.[8] The reaction of
8 with phenylazide proceeded at the carbene carbon atom
leaving intact the phosphane center. The moderate yield (50 %)
of phosphane 9 was attributed to facile decomposition of the
intermediate carbene.
Figure 3. ORTEP view of 9, hydrogen atoms are omitted for clarity. Ellipsoids
are drawn at the 50% probability level. Selected bond distances (Å) and
angles (°):P1-C1 1.879(3), N1-C1 1.279(3), N2-C1 1. 386(3), N1-C28 1.401(3),
N1-C1-N2 127.1(2), N1-C1-P1 115.41(19), N2-C1-P1 117.17(19), C1-N1-C28
127.1(2).
Ad
Ad
PH
(iPr2N=CH)2O
_
6
TfO
2
2
_
TfO
N+Pr2-i
Ad
Ad
Se
P
PAd2
Se
LiN(SiMe3)2
THF, -80 ο
Ph
Synthesis of palladium(II) complexes starting from carbene 5
resulted in a mixture of the expected palladium complex 12 and
palladium complex of phosphane 9 – compound 13 (Scheme 4).
Partial ligand isomerisation upon 1,2-shift of the phosphanyl
group in the course of complex formation reactions has been
previously observed by us also with di-tert-butylphosphanyl
substituted acyclic diaminocarbene ligands.[9] In order to prepare
carbene complex 12, it is more convenient to generate carbene
5 in situ just before addition of the palladium source; conversely,
complex 13 is best prepared starting directly from
iminophosphane 9. Both palladium complexes could be obtained
as pure compounds and their molecular structures were proved
by X-ray diffraction study (Figures 4 and 5).
5
N
7
3
:
C
C
NPr2-i
11
KN(SiMe3)2
THF, -90 ο
C
Ph
N
NPr2-i
(Me2N)3P
PhN3
NPr2-i
N
Ph
Ad
Ad
P
C
:
P
Ad
Ad
Ad2P
NPr2-i
Se
Se
9
10
8
Scheme 3. Attempted syntheses of carbenes 8 and 11.
We also attempted the synthesis of PV carbene 11 using two
approaches (Scheme 3). At first we deprotonated salt 3 with
lithium hexamethyldisilazide in THF at –90 ˚C. In the second
approach, PIII carbene 5 was oxidized with selenium. We
assume that in both cases the reaction indeed results in the
transient PV carbene 11, that however spontaneously undergoes
1,2-P migration to afford phosphaneselenide 10. The latter was
reduced to phosphane 9, whose structure was also proved by X-
ray diffraction study (Figure 3).
Ph
N
Ph
N
(PhCN)2PdCl2
Cl
Cl
iPr2N
C
P Ad2
Cl
iPr2N
+
Pd
5
Pd
P
Cl
Ad
Ad
12
13
The structure of 9 represents the first example of structure of a
phosphane obtained by 1,2-P migration form the corresponding
carbene. In the structure of 9 the distances N1-C1 and N2-C1
are 1.279(3) and 1.386(3) Å, respectively. The distance N1-C1 is
shorter than in 5, and this indicates a higher double bond
character; conversely, the distance N2-C1 is longer than in 5.
Scheme 4. Synthsis of the palladium(II) complexes 12 and 13.
In both the complexes the Pd(II) centers are in a distorted
square planar coordination environment. The bite angle of the P-
C donor ligand in 12 is slightly smaller than the bite angle of the
P-N donor ligand in 13 with P1-Pd1-C1 and P1-Pd1-N1 angles
being 68.71(18) and 70.44(6)°respectively. The fou r membered
metallacycle present in both structures deviates from planarity
and is slightly folded with a fold angle between the P1-Pd1-C1
and P1-N1-C1 planes of 10.6(4)°in 12 and a fold angle between
the P1-Pd1-N1 and P1-C1-N1 planes of 6.40(16)°in 13. In 12 a
hydrogen bond is present between the isopropyl CH proton (C5)
The N1-C1-N2 angle is wider in 9 (127.1(2)°) than in
5
(116.6(2)°); the same trend is observed for the C1- N1-C28
angle, 127.1(2)°in 9 and 123.7(2)°in 5.
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
1.319(3), N2-C1 1.351(3), Cl2-Pd1-Cl1 92.00(2), P1-Pd1-Cl2 99.46(2), P1-
Pd1-Cl1 168.47(2), N1-Pd1-Cl2 169.81(5).
and the chlorine Cl1, the distance C5-Cl1 is 3.266(7) Å. The
presence of this interaction is confirmed also in the 1H NMR
spectrum of 12. The two isopropyl groups give two set of signals
in the 1H and 13C NMR spectra, as for the free carbene 5.
1
The performance of Pd complexes 12 and 13, as well as of other
complexes of type B and C bearing di-tert-butylphosphano
moieties, has been screened in the catalysis of a technologically
relevant, redox neutral synthetic reaction such as the
intermolecular catalytic hydroamination of alkynes.[15] which is a
synthetically useful reaction that can be catalyzed by
palladium(II) species,[16] including NHC complexes of Pd.[16e-g] as
well as Pd complexes with iminophosphanes.[16d] Catalytic tests
were run on the hydroamination of phenylacetylene with an
aromatic primary amine such as mesitylamine (Scheme 5),
which is notoriously a rather difficult amine substrate for
hydroamination reactions.[16f] Two equivalents with respect to Pd
of a silver salt were employed as cocatalyst, which removes the
halide ligands thereupon activating the Pd center for
reaction.[15b,16]
Moreover, in the H NMR spectrum, two CH proton signals are
found at 4.25 and 6.16 ppm, the proton at lower fields is the one
involved in the hydrogen bond interaction. In 13, due to the
different connectivity of the ligand the two isopropyl CH protons
are found both at 3.92 ppm. In a recent work from our group, we
have reported the structure of complexes of type B (R = tert-
butyl and Ar = 4-X-phenyl (X = H, CH3, OMe and CF3)) and C (R
= tert-butyl and Ar = 4-OMe-phenyl).[9] Bond distances and
angles of 12 and 13 are fully consistent with those previously
reported for the other complexes of type B and C.
Scheme 5. The employed standard hydroamination reaction.
NPr2i
N
Cl
tBu
tBu
Pd
tBu
tBu
N
N
Cl
N
OMe
Figure 4. ORTEP view of complex 12, hydrogen atoms are omitted for clarity.
Ellipsoids are drawn at the 50% probability level. Selected bond distances (Å)
and angles (°):Pd1-Cl1 2.3733(9), Pd1-Cl2 2.3552(8) , Pd1-C1 2.038(3), Pd1-
P1 2.2060(9), P1-N1 1.730(3), N1-C1 1.387(4), N2-C1 1.323(8), Cl2-Pd1-Cl1
87.38(3), P1-Pd1-Cl2 97.06(3), P1-Pd1-Cl1 173.92(3), C1-Pd1-Cl2 165.75(9).
P
iPr2N
P
P
tBu
Pd
Pd
tBu
Cl
Cl
Cl
Cl
16
15
14
Scheme 6. Other catalysts employed in the hydroamination reactions.
The catalytic performance of compounds 12 and 13 in the
hydroamination reaction was compared with that of related
complexes (Scheme 6), namely complexes 14 and 15, similar to
12 and 13, respectively, but bearing tert-butyl substituents at
phosphorus instead of 1-adamantyl substituents,[9] and complex
16 with an N-heterocyclic 1,3-iminophosphane ligand.[17] The
results are reported in Table 1. Blank tests were performed
before starting the investigation, in order to ascertain whether
the Ag salt cocatalyst could promote the reaction by itself, since
there have been reports in the literature that hydroamination can
be also catalyzed by Ag salts.[18] Under the reaction conditions
employed herein, though, the Ag salts AgPF6 and AgOTf
displayed only low catalytic efficiency towards hydroamination
and produced mostly alkyne hydration and/or alkyne oligo- and
polymerization products (Table 1, first two entries); the higher
incidence of alkyne hydration in the test with AgPF6 can be
explained in terms of the much higher hygroscopicity.of this salt
compared to AgOTf (see also below).
Figure 5. ORTEP view of complex 13, hydrogen atoms and solvent molecules
are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
Selected bond distances (Å) and angles (°):Pd1-Cl1 2.3657(6), Pd1-Cl2
2.3027(6), Pd1-N1 2.0368(18), Pd1-P1 2.2244(6), P1-C1 1.888(2), N1-C1
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
The results indicate that complexes based on iminophosphane
ligands 13 and 15 perform much better than complexes with N-
phosphanyl diaminocarbene ligands 12 and 14, allowing to
reach greater conversions of phenylacetylene and higher yields
in hydroamination products. We believe that this is due to the
lower stability of the carbene complexes under the reaction
conditions, leading to catalyst decomposition and formation of
Pd black in the course of the reaction. Furthermore, complex 16
with an iminophosphane ligand derived from an N-heterocyclic
moiety is much less efficient compared to acyclic ligands of the
same kind. We were able to determine a low quality crystal
structure of 16 (Supporting Information, Figure S1), which
reveals a rather distorted square planar geometry for this
complex: both Pd-N1 and Pd-P bond distances are significantly
longer than in complexes 13 and 15 (Pd-N 2.09 vs 2.04 Å; Pd-P
2.25 vs. 2.22 Å) and the two angles N1-C1-P1 and N2-C1-P1,
which should ideally have similar amplitude, are instead widely
different (103.82° and 140.44°, respectively). Thus , it appears
that the more rigid N-heterocyclic structure of the ligand in
complex 16 negatively impacts on the stability of the complex
and consequently on its catalytic performance. Finally, slightly
better results were indeed obtained as expected with the more
sterically protecting adamantyl-substituted ligand compared to
the tert-butyl substituted one.
compound were also obtained, but they were not of sufficient
quality to allow the calculation of a reliable structure, though the
results of the diffraction experiment confirmed the postulated
connectivity of the complex. Use of 17 as preformed catalyst
without AgPF6 addition caused as expected a drastic decrease
of the hydration product, but not a corresponding increase in
hydroamination product, since alkyne polymerization was
promoted as well (Table 2, entry 2).
Scheme 7. Preparation of complex 17.
Luckily, the extent of alkyne polymerization could be limited by
running the reaction under neat conditions: the hydroamination
yield with catalyst 17 increased in this case from 51 to 87%
(Table 2, entry 3). Furthermore, under these conditions it was no
longer necessary to employ a preformed dicationic catalyst for
high chemoselectivity: use of the parent catalyst 13 together with
less hygroscopic AgOTf and a slight excess (2 equivalents) of
the alkyne as both reagent and reaction solvent turned into an
almost quantitative hydroamination yield with respect to the
amine (Table 2, entry 5). Finally, higher yields with respect to
acetonitrile as solvent (but lower than under neat conditions)
could also be obtained using as solvent a relatively hydrophobic
Table 1. Catalyst screening for the hydroamination reaction
Catalyst
Cocatalyst
Alkyne
conversion
(%)
Hydroamination
yield (%)
Hydration
yield (%)
ionic
liquid
such
as
1-butyl-2-methylimidazolium
-
AgPF6
AgOTf
AgPF6
AgPF6
AgPF6
AgPF6
AgPF6
20
39
77
100
73
96
64
4
10
1
bis(trifluoromethylsulfonyl)imide (Table 2, entry 4). The reason
for this remarkable dependence of the reaction chemoselectivity
on the nature of the solvent is still unclear and is currently being
investigated by tracing it back to the mechanistic differences
between the various reactions that can take place.
-
4
12
13
14
15
16
20
59
15
55
16
20
27
32
34
16
Table 2. Screening of the reaction conditions for the hydroamination
reaction
Entry
Catalyst/
Cocatalyst
Solvent
Alkyne
conversion
(%)
Hydro-
amination
yield (%)
Hydration
yield (%)
Reaction conditions: 1 mmol mesitylamine, 1 mmol phenylacetylene, 1 mol%
Pd catalyst, 2 mol% silver salt cocatalyst, 1mL acetonitrile, 80°C, 25h.
1
2
13/AgPF6
17/-
CH3CN
CH3CN
neat
100
100
100
94
59
51
87
74
96
27
1
During the screening of the various complexes it emerged that
the yields in hydroamination product was limited by at least two
side reactions, namely alkyne polymerization and hydration by
traces of water adventitiously introduced in the system.
Concerning hydration, we soon realized that the main source of
adventitious water was the employed AgPF6 salt, which is rather
hygroscopic. We tried to overcome this problem by separately
preparing and employing the dicationic complex 17 (Scheme 7).
3
17/-
0.5
6
4
13/AgOTf
13/AgOTf
IL[b]
5[a]
neat
63
4
Reaction conditions: 1 mmol mesitylamine, 1 mmol phenylacetylene,
1
mol% Pd catalyst, 2 mol% silver salt cocatalyst, 1mL solvent, 80°C, 25h. [a]
Reaction performed with 2 equivalents of the alkyne reagent; [b] IL= 1-butyl-
2-methylimidazolium bis(trifluoromethylsulfonyl)imide.
Complex 17 was isolated as an analytically pure, air-stable solid
upon removal of the chloride ligands with AgPF6. Crystals of the
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10.1002/ejic.201701342
European Journal of Inorganic Chemistry
FULL PAPER
mesitylamine, which is a sterically encumbered amine known to
be comparably unreactive. Work is currently in progress to
further optimize the catalytic system, to understand its
mechanistic features and to extend its application to other
alkyne hydrofunctionalization reactions.
Basing on the neat conditions, a substrate screening was
performed to determine the applicability of this catalytic
system. The results are reported in Table 3. It is apparent that
on the one hand the reaction is not limited to phenylacetylene
but can be extended to other terminal and internal alkynes as
substrates; on the other hand, primary aromatic amines appear
the only competent nucleophiles among those tested, and
moderate to good yields can be obtained irrespective of the
nature of the substituents on the aromatic amine. Furthermore,
the reaction time is unoptimised and can probably be reduced
in several instances without significantly affecting the yield.
These results compare favorably with those obtained with
other Pd-based catalytic systems previously reported in the
literature, which generally require higher Pd loadings, higher
reaction temperatures, a larger excess of alkyne or the
addition of an acid cocatalyst.[16]
Experimental Section
Details on the preparation of the various compounds and on
their characterization are reported in the Supporting Information.
General procedure for catalytic hydroaminations
In a Schlenk tube equipped with a magnetic stirring bar were placed
under an inert atmosphere 10 µmol Pd complex and 20 µmol silver salt
cocatalyst. The tube was degassed and put under an inert atmosphere.
1.00 mmol aniline, 1.00-2.00 mmol alkyne and optionally 1 mL dry
solvent were then injected into the Schlenk tube. The flask was
immediately placed in an oil bath preheated at 80 °C and the reaction
mixture was vigorously stirred for 25 hours. Conversions and yields were
determined by 1H NMR on a sample of the reaction mixture diluted in
CDCl3, after addition of 1,4-bis-trimethylsilylbenzene as an internal
standard.
Table 3. Substrate screening for the hydroamination reaction with catalyst
13
Amine
Alkyne
Amine
conversion (%)
Hydroaminati
on yield (%)
mesitylamine
mesitylamine
mesitylamine
4-chloroaniline
4-anisidine
phenylacetylene
1-phenylpropyne
1-octyne
96
63
59
91
75
90
20
0
96
63a
59
85
60
65
0
Keywords: N-heterocyclic carbenes • acyclic diaminocarbenes •
palladium • hydroamination • alkyne
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
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Text for Table of Contents Pd complexes with novel di-1-adamantylphosphanyl carbene and imino ligands have been
prepared and employed as catalysts for the efficient intermolecular hydroamination of alkynes with arylamines.
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