Nickel-catalyzed transfer semihydrogenation and hydroamination of aromatic alkynes using amines as hydrogen donors

Article abstract of DOI:10.1021/om200233x

The transfer hydrogenation of diphenylacetylene to yield cis- and trans-stilbenes was achieved using a variety of amines as hydrogen donors and the complex 1 ([(dippe)Ni(μ-H)]₂) in catalytic amounts (0.5% mol). The use of nucleophilic amines such as pyrrolidine in neat conditions afforded the hydroamination of diphenylacetylene, in moderate to high yields. Cyclization of 2-ethynylaniline also was carried out under similar conditions, with 1 in catalytic amounts, but in low yield, mainly due to the formation of homocoupling products of the starting material. The hydrogenation of diphenylacetylene by using other nitrogenated compounds such as aromatic N-heterocycles was addressed to give a metal-mediated process, using 1 in stoichiometric amounts.

Full text of DOI:10.1021/om200233x

ARTICLE
pubs.acs.org/Organometallics
Nickel-Catalyzed Transfer Semihydrogenation and Hydroamination of
Aromatic Alkynes Using Amines As Hydrogen Donors
Adꢀan Reyes-Sꢀanchez, Farrah Ca~navera-Buelvas, Rigoberto Barrios-Francisco, Olga L. Cifuentes-Vaca,
Marcos Flores-Alamo, and Juventino J. García*
Facultad de Química, Universidad Nacional Autꢀonoma de Mꢀexico, Circuito Interior, Ciudad Universitaria, Mexico City 04510, Mexico
S Supporting Information
b
ABSTRACT: The transfer hydrogenation of diphenylacetylene to yield cis-
and trans-stilbenes was achieved using a variety of amines as hydrogen
donors and the complex 1 ([(dippe)Ni(μ-H)]2) in catalytic amounts (0.5%
mol). The use of nucleophilic amines such as pyrrolidine in neat conditions
afforded the hydroamination of diphenylacetylene, in moderate to high
yields. Cyclization of 2-ethynylaniline also was carried out under similar
conditions, with 1 in catalytic amounts, but in low yield, mainly due to the
formation of homocoupling products of the starting material. The hydro-
genation of diphenylacetylene by using other nitrogenated compounds such as aromatic N-heterocycles was addressed to give a
metal-mediated process, using 1 in stoichiometric amounts.
of interest due to the relative availability of the starting materials
and the occurrence of derivatives of the synthesized com-
pounds in many biological systems or their vast applications in
pharmaceutical11 and agrochemical products.10a,12 Most of the
developed work in the hydroamination of alkynes has largely
been achieved by using palladium complexes,10b,13 even though
therearemanyreportsin which other metals such astitanium,10b,14
rare earth metals,15 and some late transition metals16 have been
widely employed. However, despite the variety of developed
organometallic catalysts, to date, few are examples of the use of
complexes of nonexpensive metals such as nickel in the hydro-
amination of alkynes.16e,17
Herein we wish to report the semihydrogenation and hydro-
amination of diphenylacetylene and 2-ethynylaniline using com-
plex 1 ([(dippe)Ni(μ-H)]2) along with amines and aromatic
N-heterocyles as hydrogen sources. The relatively scarce use of
amines as hydrogen sources as well as the nonexpensive metal
based catalyst is spotlighted.
1. INTRODUCTION
Hydrogenation of alkynes to afford alkenes can be carried out
by using hydrogen and a homogeneous or heterogeneous
catalyst, such as the one developed by Lindar.1 Transfer hydro-
genation has became an alternative to the use of dihydrogen on
the small and middle scale, due to its simplicity and reduction
of risks and operational difficulties associated with the use of
gaseous hydrogen, along with the environmentally friendly
properties of some hydrogen donors.2 Synthesis of alkenes
through the semihydrogenation of alkynes is an important
process since alkene derivatives are valuable building blocks
for both academia and industry. In recent years, the research
on homogeneous transfer semihydrogenation of alkynes has
been focused on the use of complexes of noble metals such as
iridium,3 ruthenium,4 and palladium,5 with methanol, 2-propa-
nol, or the azeotropic mixture of HCOOH and NEt3 being the
most common hydrogen sources. In this context, although the
catalyzed dehydrogenation of amines is a well-known topic,6
their use in the reduction of alkynes has been scarcely studied,7
and one of the closest approaches to this process is the use of
alkenes such as tert-butylethylene as hydrogen acceptors in the
dehydrogenation of amines.6d Regarding the latter transforma-
tion, the hydrogen abstraction can be attained by the amino or
aliphatic dehydrogenation8 depending on the substitution of the
amine, resulting in the formation of imines or enamines as
oxidation products, respectively. A further study on the hydrogen
transfer from amines to alkynes would lead to a better under-
standing of their potential use as hydrogen donors as well as
contribute to the implementation of synthetic methods for the
synthesis of imines through the dehydrogenation of amines.9
On the other hand, a second pathway for the activation of
alkynes is to achieve their hydroamination aimed to obtain
imines, amines, and N-heterocycles.10 This has been a process
2. RESULTS AND DISCUSSION
In order to assess the general reactivity of complex 1 toward
the transfer hydrogenation and/or the hydroamination of diphe-
nylacetylene (DPA), complex [(dippe)Ni(η2-C,C-DPA)] was
synthesized according to the reported methodology18 using 1
and diphenylacetylene. Reaction of [(dippe)Ni(η2-C,C-DPA)]
with 4 equiv of cyclohexylamine at 140 °C yielded the corre-
sponding hydrogenation products. NMR spectra of the reaction
mixture are evidence for the presence of complex [(dippe)Ni(η2-
C,C-trans-stilbene)]. Key signals for this complex were assigned
in the 31P{1H} NMR at 66.94 ppm (s) and in the 1H NMR at
Received: March 16, 2011
Published: June 03, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 2
Table 1. Catalytic Hydrogenation of Diphenylacetylene
Using Cyclohexylamine As Hydrogen Sourcea
entry T (°C)
solvent
THF
1b (equiv) yield (%)b 1c (%) 1d (%)
1
2
3
4
5
6
7
100
140
180
180
180
180
180
400
400
2
4
0
0
2
4
THF
THF
1000
2000
2000
2000
2000
15
11
100
2
5
10
6
THF
5
dioxane
acetonitrile
toluene
15
2
85
0
69
10
59
a All reactions were carried out in a stainless steel Parr reactor using
15 mL of solvent. A molar proportion of 1:200 of [(dippe)Ni(μ-H)]2
and diphenylacetylene, respectively, was employed. b Chromatographic
yields.
Table 2. Catalytic Hydrogenation of Diphenylacetylene
Using Pyrrolidine As Hydrogen Sourcea
1
4.49 ppm (s, br); also, based on the H NMR spectra, free cis-
(s, 6.40 ppm) and trans-stilbene (s, 6.95 ppm) were formed as
represented in Scheme 1 (see Scheme 2 and Scheme S1, SI, for
mechanistic proposals). After 77 h, the reaction was completed
and the crude was analyzed by GC-MS. Chromatographic data
showed complete hydrogenation of diphenylacetylene and the
formation of cis- (17.5%) and trans-stilbene (82.5%) (Figure S3).
To note, no hydroamination products were detected.
Considering the above, experiments under catalytic condi-
tions were assayed using a variety of solvents, temperatures, and
proportions of cyclohexylamine, in order to find the optimized
conditions for the hydrogenation of diphenylacetylene. The main
results for these experiments are summarized in Table 1.
According to these results, there is a low conversion when a
coordinating solvent such as THF was used despite the increase
in the quantity of the amine or temperature (entries 1 to 4).
Catalytic conversion is enhanced when low coordinating solvents
entry
solvent
yield (%)b
1c (%)
1d (%)
2e (%)
1
2
3
4
THF
46
91
28
51
26
16
16
37
71
6
2
3
toluene
dioxane
neat
100
3
59
37
a All reactions were carried out in a stainless steel Parr reactor using
15 mL of solvent. A molar proportion of 1:200:1000 of [(dippe)Niμ-
H]2, diphenylacetylene, and amine, respectively, was used. b Chromato-
graphic yields.
such as toluene or dioxane are used (entries 5 and 7). Imine 1e
is a characteristic dehydrogenationÀtransamination product
from the transfer hydrogenation reaction,6a detected by GC-MS
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Table 3. Catalytic Hydrogenation of Diphenylacetylene Using Different Amines As Hydrogen Sourcesa,b,c,d
a All reactions were carried out in a stainless steel Parr reactor using 15 mL of dioxane at 180 °C. A molar proportion of 1:200:1000 of [(dippe)Ni(μ-
H)]2, diphenylacetylene, and amine, respectively, was used. ND = not determined. b Chromatographic yields. c Reaction at 160 °C. d 6.5 g of
methylamine. One percent of the hydroamination product was also formed.
(Figures S4 and S5, SI). When the reaction is carried out in
acetonitrile(entry6), the yielddecreases dramatically probablydue
to the oxidative addition of this nitrile to nickel.19 As can be seen in
Table 1 and also considering the results from the use of stoichio-
metric quantities of 1 (vide supra), the formation of trans-stilbene
(thermodynamic product) suggests a Ni-catalyzed cisÀtrans iso-
merization of the initially formed cis-stilbene (Scheme S1, SI).20
On using a different hydrogen donor, such as pyrrolidine (2b),
rather similar results were obtained, as shown in Table 2. As seen
before, a high conversion of diphenylacetylene to its correspond-
ing hydrogenation products is attained by using low-coordinating
solvents (toluene and dioxane), unlike the use of solvents such
as THF. It is noticeable that in these experiments low amounts of
the hydroamination product could be obtained (entries 1 to 3).
Improved yields for the hydroamination process to produce
2e are obtained under neat conditions (entry 4, Table 2); this
is a good result considering the low favored intermolecular
hydroaminations.10 Clearly, the presence of these products shows
the effect of the increase in the nucleophilicity of the hydrogen
donor in the formation of condensation products.
The effect of structure and substitution of the hydrogen donor
for different amines was also addressed using the conditions for 2b,
summarized in Table 3. With the use of primary or secondary
amines with R-H to the nitrogen atom, such as in 6b, the catalytic
conversion can be successfully achieved as in the case of cyclohex-
ylamine or pyrrolidine. For diamines 7b and 8b the conversion was
decreased presumably because of the displacement of the substrate
1a from the metal center by the chelating diamines, resulting in the
formation of [(dippe)Ni (diamino)] complexes, inhibiting the
catalytic activity. Low to moderate yields are obtained on using
aromatic N-heterocylces, such as 5b (entry 3), probably due to
their low nucleophilicity. This effect also can be drastically noticed
when protonated amines are used (entry 1).
A mechanistic proposal for both catalytic hydroamination and
transfer hydrogenation of alkynes is depicted in Scheme 2.
A different set of hydrogen donors, such as tert-butylamine,
indole, carbazole, and pyrrole, was tested. Nevertheless, none of
these reagents afforded a catalytic reduction of 1a, and such
transformation could be achieved only with the use of complex 1
in stoichiometric amounts. The lack of catalytic activity of these
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the terminal alkyne (Figures S9 to S13, SI), along with
hydroamination products between the produced indole and
the starting material.
3. CONCLUSIONS
We have shown a new methodology for the transfer hydro-
genation of internal aromatic alkynes catalyzed by a nickel
complex exhibiting moderate to excellent yields depending on
the structure of the hydrogen donor. Transfer hydrogenation
under the described conditions is sensitive to the use of
coordinating solvents and hydrogen donors such as diamines,
N-heterocycles, and THF due to the deactivation of the catalyst
by those species linked to the formation of rather stable inter-
mediates. Hydroamination is performed with better yields on
using an excess of nucleophilic amine. The use of a substrate
containing both a NÀH donor moiety and a terminal alkyne
allowed cyclization, but there is a strong competence of
C(sp)ÀH bond activation by the metal center leading to
homocoupled products. Studies are underway to decrease or
avoid homocoupling.
Figure 1. ORTEP drawing (50% probability) of [(dippe)Ni (k1-
C8H6N)2]. Selected bond lengths (Å) and angles (deg): N(1)ÀNi(1)
1.9307(16), N(2)ÀNi(1) 1.9339(15), P(1)ÀNi(1) 2.1836(5), P(2)À
Ni(1) 2.1834(5), N(1)ÀNi(1)ÀN(2) 90.53(7), N(1)ÀNi(1)ÀP(2)
176.59(5), N(2)ÀNi(1)ÀP(2) 90.35(5), N(1)ÀNi(1)ÀP(1) 91.76(5),
N(2)ÀNi(1)ÀP(1) 175.94(5), P(2)ÀNi(1)ÀP(1) 87.55(2).
4. EXPERIMENTAL SECTION
Unless otherwise noted, all experiments were carried out using
standard Schlenk techniques in a double vacuum-argon manifold or in
a glovebox (MBraun Unilab) under high-purity argon (Praxair 99.998),
with controlled concentrations of water and oxygen (<1 ppm). Catalytic
experiments were carried out in a stainless steel Parr (T315SS) reactor.
All liquid reagents were purchased in reagent grade and were degassed
before use. Diphenylacetylene and alkene standards were purchased
from Aldrich and were stored in the glovebox for further use. Solvents
were dried using standard techniques and stored in the glovebox before
use. Deuterated solvents were purchased from Cambridge Isotope
Laboratories and were stored under 4 Å molecular sieves for 24 h before
use. Nickel dimer [(dippe)Ni(μ-H)]2 was synthesized according to
the procedure reported in the literature.21 NMR spectra of complexes
and organic products were acquired at room temperature using a
300 MHz Varian Unity Inova spectrometer. Samples and reactions were
manipulated under an inert atmosphere and charged in thin-wall (0.33
mm) Wilmad NMR tubes, equipped with J. Young valves, and were
heated in silicon oil baths at the desired temperature. Chemical shifts in
1H MNR spectra (δ, ppm) are reported according to the residual
protio-solvent. 13C{1H} NMR spectra are referenced to the carbon
signal of each solvent. 31P[1H} NMR spectra are referenced to external
85% H3PO4. GC-MS determinations were performed using an Agilent
5975C equipped with a 30 m DB-5MS capillary (0.32 mm i.d.) column.
Elemental analyses were also performed by USAI-UNAM using a
Perkin-Elmer 2400 microanalyzer.
Table 4. Catalytic Cyclization of 2-Ethynilanilinea
entry
solvent
yield (%)b
1
2
3
dioxane
toluene
neat
8
15
25
a All reactions were carried out in a stainless steel Parr reactor using
15 mL of dioxane, except for neat conditions, where 0.5 mL of 13b was
used. A molar proportion of 1:200 of [Ni(dippe)μ-H]2 and 2-ethynilani-
line was used. b Chromatographic yields.
systems is mainly due to the formation of rather stable
N-coordinated complexes, like the one isolated when indole
was employed as hydrogen source (Figure 1); here a Ni(II)
complex in a square-planar geometry was observed. This
complex was prepared independently in high yield reacting
1 with indole (see Experimental Section) and was highly
thermally stable under argon.
To further extend the reactivity found in the addition of
amines to alkynes, the intramolecular cyclization of 2-ethynylani-
line was explored in the conditions shown in Table 4. After 72 h,
there was a complete conversion of the starting material; how-
ever, depending on the solvent, only 8% to 25% yield corre-
sponds to the cyclization product and the remainder
corresponds to other byproducts. GC-MS determinations
are consistent with the formation of homocoupling products
derived from the starting material via CÀH bond activation in
Synthesis of [(dippe)Ni(η2-C,C-DPA)]. Following the reported
procedure by Jones et al,18 to a dark red solution of [(dippe)Ni(μ-H)]2
(0.053 g, 0.0822 mmol) in THF (5.0 mL) was added DPA (0.0292 g,
0.164 mmol). The mixture was stirred at room temperature for 15 min,
and all hydrogen gas produced was vented away from the system into the
glovebox. After 30 min of stirring, a yellow solution was obtained. The
solvent was removed under vacuum, and the remaining yellow residue
was further dried under vacuum for 4 h. 1H NMR (THF-d8): δ 7.35 (d,
JHÀH = 7.4 Hz, 4H), 7.18 (t, JHÀH = 7.4 Hz, 4H), 7.01 (t, JHÀH = 7.3 Hz,
4H), 2.12 (septuplet, JHÀH = 7.2 Hz, 2H, CHMe2), 1.65 (d, JHÀP = 9.2
Hz, 4H, PCH2CH2P), 1.08 (quintet, JHÀH = 7.2 Hz, JHÀP = 7.2 Hz, 24H,
CHMe2). 13C{1H} NMR (THF-d8): δ 141.3 (t, 2JCÀP = 7.1 Hz, C-Ph,),
132.1 (s, Ar CH), 128.3 (d, 2JCÀP = 8.6 Hz, C-Ph), 127.3 (s, Ar CH),
124.9 (s, Ar CH), 26.7 (t, JCÀP = 10.4 Hz), 22.2 (t, JCÀP = 19.1 Hz), 20.4
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(t, JCÀP = 4.0 Hz), 19.1 (s). 31P{1H} NMR (THF-d8): δ 80.9 (s). Yield:
90%. Satisfactory elemental analysis was also obtained for this sample.
Reduction of Diphenylacetylene in [(dippe)Ni(η2-C,C-
DPA)] Using Cyclohexyamine as Hydrogen Source. Into a
NMR tube with a Young’s valve was charged a solution of [(dippe)Ni-
(η2-C,C-DPA)] (45 mg, 0.09 mmol) in 0.7 mL of toluene-d8, and to this
was added 4 equiv of cyclohexylamine (0.046 mL, 0.360 mmol). The
tube was closed and heated at 140 °C. The formation of [(dippe)Ni-
(η2-C,C-trans-stilbene)] was detected by the 31P{1H} NMR spectrum
of the reaction mixture, as a singlet at 66.9 ppm (see Figure S1, SI). After
77.5 h the heating was stopped and the reaction mixture was analyzed by
GC-MS (cis-stilbene: 17.5, trans-stilbene: 82.5).
Catalytic Reduction of Diphenylacetylene Using Cyclo-
hexylamine As Hydrogen Source. A typical experiment for the
catalytic reduction of diphenylacetylene was performed as follows: A
stainless steel reactor (T316SS) with inner magnetic stirring was
charged in a glovebox with diphenylacetylene (0.166 g, 0.92 mmol,
200 equiv), 1 (0.0032 g, 0.0045 mmol, 1 equiv), and cyclohexylamine
(0.532 mL, 4.5 mmol for 1000 equiv) in 15 mL of solvent (THF, MeCN,
toluene, or 1,4-dioxane). The reactor was heated in an oil bath at
different temperatures (100, 140, 180 °C) for 72 h. At the end of that
reaction time, the reaction was analyzed by GC-MS.
Catalytic Hydroamination and Reduction of Diphenyla-
cetylene Using Pyrrolidine As Hydrogen Source. A stainless
steel reactor was charged with diphenylacetylene (0.166 g, 0.92 mmol,
200 equiv), 1 (0.0032 g, 0.0045 mmol, 1 equiv), and pyrrolidine
(0.389 mL, 4.5 mmol) in 15 mL of solvent (THF, toluene, 1,4-dioxane,
or cyclohexylamine). The reactor was heated in an oil bath at 180 °C for
72 h. At the end of that time, the reaction was analyzed by GC-MS.
Catalytic Hydroamination and Reduction of Diphenyla-
cetylene Using Different Amines As Hydrogen Source. A
stainless steel reactor was charged with diphenylacetylene (0.166 g, 0.92
mmol, 200 equiv), 1 (0.0032 g, 0.0045 mmol, 1 equiv), and 1000 equiv
of amine. The reactor was heated in an oil bath at 180 °C for 72 h. At the
end of that time, the reaction was analyzed by GC-MS.
Full-matrix least-squares refinement was carried out by minimizing
2
(Fo À Fc2)2. All non-hydrogen atoms were refined anisotropically. H
atoms attached to C atoms were placed in geometrically idealized
positions and refined as riding on their parent atoms, with CÀH
distances fixed to 0.95 (aromatic CH) with Uiso = 1.2Ueq(C), 0.98
(methyl CH3) with Uiso = 1.5Ueq(C), and 1.00 (methyne CH) with
Uiso = 1.2Ueq(C). Goodness-of-fit on F2 = 0.960.
’ ASSOCIATED CONTENT
S
Supporting Information. This material is available free
of charge via the Internet at http://pubs.acs.org.
b
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: juvent@servidor.unam.mx.
’ ACKNOWLEDGMENT
We thank PAPIIT-DGAPA-UNAM (IN-201010) and CON-
ACYT (080606) for their financial support to this work. Also we
thank DGAPA-UNAM for a postdoctoral grant to F.C.-B. and
Dr. A. Arꢀevalo for technical assistance.
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(0.060 g, 0.093 mmol) was mixed with indole (0.109 g, 0.933 mmol) and
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Organometallics
ARTICLE
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