Nickel-catalyzed alkylation and transfer hydrogenation of α,β-unsaturated enones with methanol

Article abstract of DOI:10.1021/om2010222

Complexes of the type [{(dippe)Ni} ( 2-Cα,Cβ- 1,4-dien-3-one)] (dippe = 1,2-bis(diisopropylphosphino)- ethane); n= 1, 2; enone = aromatic 1,4-pentadien-3-ones) were synthesized. The "[(dippe)Ni]" moiety derived from [(dippe)Ni(-H)]2 2-coordinated to the C,C double bonds of the corresponding α,β-unsaturated enone and was fully characterized using a variety of spectroscopic techniques, for instance, single-crystal X-ray diffraction, nuclear magnetic resonance (NMR), and mass spectrometry. The complexes were assessed in a catalytic transfer hydrogenation process using methanol (CH3OH) as a hydrogen donor. This alcohol turned out to be a very efficient reducing and alkylating agent of 1,4- pentadien-3-ones, under neat conditions. The current methodology allowed the selective reduction of C=C bonds in α,β- unsaturated enones to yield enones and saturated ketones by a homogeneous catalytic pathway, whereas by a heterogeneous pathway, the process leads to the formation of mono- and dimethylated ketones. In the latter case, the occurrence of nickel nanoparticles in the reaction media was found to participate in the catalytic alkylation of such dienones.

Full text of DOI:10.1021/om2010222

Article
pubs.acs.org/Organometallics
Nickel-Catalyzed Alkylation and Transfer Hydrogenation of α,β-
Unsaturated Enones with Methanol
Nahury Castellanos-Blanco, Marcos Flores-Alamo, and Juventino J. García*
́ ́
Facultad de Química, Universidad Nacional Autonoma de Mexico, Circuito Interior, Ciudad Universitaria, Mexico City 04510,
Mexico
S
* Supporting Information
ABSTRACT: Complexes of the type [{(dippe)Ni}n(η2-Cα,Cβ-
1,4-dien-3-one)] (dippe = 1,2-bis(diisopropylphosphino)-
ethane); n= 1, 2; enone = aromatic 1,4-pentadien-3-ones)
were synthesized. The “[(dippe)Ni]” moiety derived from
[(dippe)Ni(μ-H)]2 η2-coordinated to the C,C double bonds of
the corresponding α,β-unsaturated enone and was fully
characterized using a variety of spectroscopic techniques, for
instance, single-crystal X-ray diffraction, nuclear magnetic
resonance (NMR), and mass spectrometry. The complexes were assessed in a catalytic transfer hydrogenation process using
methanol (CH3OH) as a hydrogen donor. This alcohol turned out to be a very efficient reducing and alkylating agent of 1,4-
pentadien-3-ones, under neat conditions. The current methodology allowed the selective reduction of CC bonds in α,β-
unsaturated enones to yield enones and saturated ketones by a homogeneous catalytic pathway, whereas by a heterogeneous
pathway, the process leads to the formation of mono- and dimethylated ketones. In the latter case, the occurrence of nickel
nanoparticles in the reaction media was found to participate in the catalytic alkylation of such dienones.
contains an important amount of potentially usable hydrogen,
and its use as a reducing agent is attractive in some atom
economical processes.9a−d These alcohols and some other
methanol derivatives have been efficiently used in the reduction
of dienones in the presence of homogeneous catalysts
containing Ru,3a Ir,3b and Pd,3c or heterogeneous catalysts
using nickel nanoparticles.11a
1. INTRODUCTION
The catalytic transfer hydrogenation of α,β-unsaturated carbon-
yl compounds is an important methodology in synthetic
organic chemistry. This allows functionalization of the
conjugated system present in these entities, and modification
of some of its properties, for instance, the potential Michael
acceptor activity that promotes the formation of toxic
compounds with mutagenic and carcinogenic activity.1a,b In
the second half of the 20th century, new protocols have been
established for the hydrogenation of α,β-unsaturated ketones,
esters, or aldehydes,2a,b with the aim of avoiding the use of
hazardous hydrogen gas and employing some simple molecules
as hydrogen sources instead; the use of alcohols,3a−e,6b
hydrosilanes,4a,c,5b Et3N·HCl,6a or HCOOH8b is oftentimes
preferred. This type of transformation has been possible
through the use of catalysts based on transition metals, such as
iridium,3b palladium,5a,b titanium,6a,b rhodium,7a,b or rutheniu-
m,8a−c and a variety of ancillary ligands, including phosphine-
s,3a−c,5,7b,8a cyclopentadienyl,6a,b,7a or TsEN (TsEN = N-(p-
toluenesulfonyl)-1,2-ethylenediamine).7a The reduction using
simple organic molecules as hydrogen donors in the presence
of a catalyst makes the hydrogenation process safer and
environmentally friendly.
Among the reported methods using other simple molecules
for dienone reduction, worthy of mention is the work by
Hayashi and co-workers,12 who have reported the use of the
complex [{Ir(OH)(COD)}2] (COD: cyclooctadiene) in H2O,
to efficiently yield the addition of 1,6-aryl boronic acids to
dienones, with the further use of a palladium-catalyzed
hydrogenation being necessary to give saturated ketones.
On the other hand, it is known that alcohols, such as ethanol
and isopropanol, can also participate as alkylating agents by a
heterogeneous catalytic process involving nickel nanoparticle-
s.11a,b Recently, such nanoparticles have been used in the α-
alkylation of ketones, producing water as the only byproduct of
the reaction.19 However, these processes have not been yet
documented for the alkylation of α,β-unsaturated enones.
Feringa and co-workers reported recently a homogeneous
system involving the conjugate addition of diethylzinc to
symmetrical dienones, catalyzed by an in situ generated
copper−phosphoramidite complex13 to yield the selective
activation of the CC moiety instead of the CO bond.
According to the authors, the dienone functionalization
Recent reports on the use of short-chain alcohols for the
reduction of α,β-unsaturated ketones have been documented.
Regularly, ethanol and isopropanol are preferred because these
alcohols are an easily accessible, economic, clean, and safe
source of hydrogen.3a,c,10 Comparatively, methanol is less
frequently used in this kind of process, despite being cheap and
more readily available than the others mentioned above. It also
Received: October 23, 2011
Published: January 10, 2012
© 2012 American Chemical Society
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Scheme 1. Formation of Mono- and Dinuclear Complexes, [{(dippe)Ni}n(η2-Cα,Cβ-enone)]
Figure 1. ORTEP drawing of 3a, [{(dippe)Ni}2(η2-C ,C −C19H18O)], showing 30% probability ellipsoids. Selected bond distances (Å): C(19)−
α
β
C(18) 1.425(6), C(16)−C(15) 1.404(6), Ni(2)−C(15) 2.011(4), Ni(2)−C(16) 1.986(4), Ni(1)−C(18) 1.975(4), Ni(1)−C(19) 1.984(6).
Selected bond angles (deg): C(18)−Ni(1)−C(19) 42.20(18), C(18)−Ni(1)−P(4) 152.26(14), C(19)−Ni(1)−P(4) 110.43(14), C(18)−Ni(1)−
P(3) 115.21(14), C(19)−Ni(1)−P(3) 157.12(14), P(4)−Ni(1)−P(3) 92.36(5), C(16)−Ni(2)−C(15) 41.12(18), C(16)−Ni(2)−P(1) 150.69(14),
C(15)−Ni(2)−P(1) 109.88(13), C(16)−Ni(2)−P(2) 117.18(13), C(15)−Ni(2)−P(2) 157.70(14), P(1)−Ni(2)−P(2) 92.08(6).
resulted in the 1,4-addition of the alkyl-organometallic reagent
to produce monoalkyl-enones in good yields and with high
enantioselectivities.
Herein, we would like to report the use of a catalytic
precursor using a nonexpensive metal operating in two catalytic
processes, homogeneous and heterogeneous, and the use of a
primary alcohol as a hydrogen source, solvent, and alkylating
agent.
ligands.14a,b The 13C{1H} NMR spectra are in agreement
with η2-Cα,Cβ coordination of enones to Ni0; key signals for the
coordinated carbons are displayed between 61 and 49 ppm, at
high field from their positions in the corresponding free ligand
(originally, as singlets in the range of 130−150 ppm), with 2JC−
β
2
= 13.5 and JC−P = 19.5 Hz. The carbonyl signals for
P
α
compounds 2a−4a are slightly shifted to low field with respect
to the signals for the free dienones (δ 187−188), showing
1
singlets in the region of 190−193 ppm. The H NMR spectra
2. RESULTS AND DISCUSSION
of dinuclear complexes 2a−4a are also in agreement with the
η2-Cα,Cβ coordination to nickel(0), since resonances for the
olefinic protons in the complexes 2a−4a appear shifted to high
field in the range of 4.35−4.50 ppm, while the olefinic protons
of free dienones resonate between 7.1 and 7.7 ppm.
2.1. Reactivity of [(dippe)Ni(μ-H)]2 1 with Symmetrical
Dienones. The reaction of the hydride dimer [(dippe)Ni(μ-
H)]2 with symmetrical dienones in THF-d8 was immediate at
room temperature, and a color change from dark red to brown
along with the evolution of H2 was always observed (Scheme
1).
A 1:1 mixture of dienone 2 and complex 1 in toluene-d8 was
heated gradually, increasing the temperature from 25 to 120 °C
for 10 days. The 31P{1H} NMR spectrum at 120 °C shows the
signals assigned to the dinuclear complex 2a and two small
singlets assigned to [(dippe)2Ni], δ 50.2 and free diphosphine
dippe, δ 7.3.14a,16 This behavior is similar for dienones 3 and 4.
Suitable crystals for single-crystal X-ray studies for
compounds 3a and 4a were grown from a concentrated
According to Scheme 1, complexes 2a−4a were produced as
the main products on using 1 equiv of dienone per 1 equiv of
complex 1. The 31P{1H} NMR spectrum displays two broad
systems of doublet of doublets between 72 and 75 ppm with
P−P coupling constants in the range of 56−59 Hz, character-
2
istic of JP−P in nickel(0) complexes with diphosphine
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Scheme 2. Formation of Mononuclear [(dippe)Ni(η2-Cα,Cβ-1,4-pentadien-3-one)] Complexes
Figure 2. ORTEP drawing of 2b, [(dippe)Ni(η2-Cα,C −C17H14O)], showing 30% probability ellipsoids. Selected bond distances (Å): C(13)−C(14)
β
1.417(3), C(14)−C(15) 1.436(3), C(16)−C(17) 1.323(3), C(14)−Ni(1) 1.982(2), Ni(1)−P(1) 2.1538(6), Ni(1)−P(2) 2.1726(6). Selected bond
angles (deg): C(14)−Ni(1)−P(1) 148.06(7), C(13)−Ni(1)−P(1) 107.25(8), C(14)−Ni(1)−P(2) 119.67(7), C(13)−Ni(1)−P(2) 159.72(8),
P(1)−Ni(1)−P(2) 92.11(2).
solution in THF-d8 at low temperature. The ORTEP
representations for the dinuclear complexes 3a and 4a are
shown in Figure 1 and the Supporting Information,
respectively; these confirmed the structure proposed for
symmetrical dienone complexes. For 3a, in the asymmetric
unit, each of the two metal nickel centers is coordinated to two
phosphorus of the dippe ligand and η2-CαCβ bonds C15−
C16 and C18−C19 as expected, compared to the free dienone
(1.332(3) Å).15 The two metal moieties, [(dippe)Ni], are
arranged in a mutually trans position with respect to the plane
of ligand; each metal center has a slightly distorted trigonal-
planar geometry with an rms deviation of 0.0051 and 0.0058 Å
in the square plane P(3)/P(4)/centroid:C(18)−C(19) with
deviations Ni(1) −0.008(3), P(3) −0.002(1), P(4) 0.002(1),
centroid:C(18)−C(19) 0.003(1)° from the least-squares plane
[10.958(13)x − 7.651(24)y − 9.529(54)z = 4.076(11)] and
P(1)/P(2)/centroid:C(15)−C(16) with deviations Ni(2)
−0.010(3), P(1) 0.002(1), P(2) 0.002(1), centroid:C(15)−
C(16) 0.004(1)° from the least-squares plane [13.118(18)x +
5.142(24)y − 5.830(59)z = 9.162(26)] for Ni(1) and Ni(2)
metallic centers, respectively. The planarity of the η2-Cα,Cβ-
bis(4-methyl styryl)ketone ligand (rms deviation of fitted atoms
= 0.2046 Å) is significantly affected by the coordination to
nickel(0) atoms. In the supramolecular network, the
intermolecular contacts of van der Waals type C−H···O and
2a−4a were observed along with the mononuclear complexes
2b−4b depicted also in Scheme 1. Complexes 2a−4a can be
converted completely to 2b−4b by heating at 80 °C for 1 day
in THF-d8 (Scheme 2).
Similar to complexes 2a−4a, the 1H NMR spectra for
mononuclear 2b−4b complexes display a set of olefinic protons
located at δ 5.9 and 5.6, shifted to high field with respect to the
signals for free ligands (δ 7.1−7.7); these signals integrate for
two protons and were assigned to the η2-CαCβ coordination
in mononuclear complexes. The 31P{1H} characteristic signals
for complexes 2b−4b appear as two doublets located at δ 75.5
and 77.5 generated by two nonequivalent phosphorus
coordinated to a nickel center, with JP−P = 42−44 Hz.
Complexes 2b−4b were unequivocally characterized by
single-crystal X-ray diffraction, and the corresponding
ORTEP representation for the mononuclear complex 2b is
shown in Figure 2. The asymmetric unit of 2b consists of the
η2-dibenzylideneacetone coordinated through one CαCβ
bond to the Ni(0) fragment, [(dippe)Ni]. As expected, the
structure shows a lengthening for the η2-coordinated CαCβ
bond (1.417(3) Å) compared with the free ligand CαCβ bond
(1.323(3) Å) due to the bonding to nickel, with a dihedral
angle of 80.62° between the dienone ligand and the NiP2
planes. The tetracoordinate complex 2b shows a slightly
distorted trigonal-planar geometry with an rms deviation of
fitted atoms = 0.0061 Å in the square plane P(1)/P(2)/
centroid:C(13)−C(14) with deviations Ni(1) −0.010(4), P(1)
0.002(1), P(2) 0.003(1), centroid:C(13)−C(14) 0.004(1)°
from the least-squares plane [10.163(39)x − 3.324(17)y −
13.513(33)z = 6.688(54)]. The planarity of the η2-Cα,Cβ-bis(4-
methyl styryl)ketone ligand (rms deviation of fitted atoms =
0.2011 Å) is slightly affected by the coordination to the
nickel(0) atom. In the crystal packing, there is one
intermolecular contact of the type C−H···O [2.34(3) Å]
1
C−H···C with R2 (7) motif for C24−H···O(17)···H−C25 and
D motif for C28−H···C5 mainly lead to infinite chains aligned
with the a axis.
Additionally, dinuclear complexes 2a−4a were characterized
by EI+-MS, and in all cases, the corresponding molecular ion
was observed in agreement with the expected m/z ratio (see the
Experimental Section).
On using 0.5 equiv of complex 1 per 1 equiv of α,β-
unsaturated enones (2−4), the expected dinuclear complexes
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a
Table 1. Catalytic Hydrogenation of Di(p-methoxybenzylidene) Acetone Using MeOH as a Hydrogen Donor
b
yield (%)
entry
solvent
conv (%)
ivb
ivc
ivd
1
2
3
toluene
THF
0
5
5
0
3
0
methanol
47
31
13
a
All reactions were made in a stainless steel Parr reactor using 10 mL of solvent and 5 mL of methanol. A molar proportion of 1:100 of
b
[(dippe)Ni(μ-H)]2 and di(p-methoxybenzylidene) acetone, respectively, was used. All yields were measured by GC-MS.
Table 2. Effect of the Temperature in the Catalytic Hydrogenation of Di(p-methoxybenzylidene) Acetone Using MeOH as a
a
Hydrogen Source
b
yield (%)
entry
T (°C)
conv (%)
ivb
ivc
ivd
ivf
ivg
1
2
3
4
90
120
150
180
0.7
47
0.7
31
47
0
0
3
0
13
27
0
0
0
0
0
0
6
93
19
18
0
100
76
a
All reactions were carried out in a stainless steel Parr reactor using 15 mL of methanol. A molar proportion of 1:100 of [(dippe)Ni(μ-H)]2 and
b
di(p-methoxybenzylidene) acetone, respectively, was used. All yields were quantified by GC-MS.
a
Table 3. Mercury Drop Experiment: Heterogeneous Process
b
yield (%)
entry
conv (%)
Hg (drops)
ivb
ivc
ivd
ivf
ivg
1
2
100
100
0
3
0
0
0
18
6
76
40
6
0
54
a
All reactions were carried out in a stainless steel Parr reactor using 15 mL of methanol. A molar proportion of 1:100 of [(dippe)Ni(μ-H)]2 and
b
di(p-methoxybenzylidene) acetone, respectively was used. All yields were quantified by GC-MS.
mainly. This interaction leads to infinite chains aligned with the
a axis.
different α,β-unsaturated enones. These experiments were
assessed using catalyst precursor 1 in low loads (1% mol),
dienone 4, and MeOH as a hydrogen source. Selected results
are summarized in Table 1. The use of solvents, such as toluene
and THF (entries 1 and 2), showed a poor conversion.
However, conversion was improved with the use of methanol as
a reagent and solvent (entry 3), since 4 was partially reduced to
the corresponding dienone (ivb) and completely reduced to
2.2. Catalytic Transfer Hydrogenation of Symmetrical
Dienones Using Methanol as a Hydrogen Donor.
Considering that dinuclear complexes 2a−4a and mononuclear
2b−4b exhibited η2-coordination of CαCβ bonds to nickel(0)
and that the resulting complexes turned out to be thermally
stable, some catalytic transfer hydrogenation experiments were
assayed using a variety of solvents and temperatures for the
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Figure 3. TEM image for the sample of entry 1 (Table 3) and size distribution of the Ni nanoparticles.
a
Table 4. Catalytic Hydrogenation of Aromatic Dienones Using MeOH
b
yield (%)
entry
R
conv (%)
c
f
g
1
2
3
Ph
100
100
100
0
100
82
0
Ph(p-CH3)
0
18
6
Ph(p-OCH3)
18
76
a
All reactions were carried out in a stainless steel Parr reactor using 15 mL of methanol. A molar proportion of 1:100 of [(dippe)Ni(μ-H)]2 and
b
di(p-methoxybenzylidene) acetone, respectively, was used. All yields were quantified by GC-MS.
the saturated ketone (ivc). Noteworthy, a monomethylated
dienone (ivd) was detected by GC-MS.
heterogeneous Ni(0) was involved in the production of
alkylated products. Further studies on this reaction allowed
us to establish the formation and participation of Ni
nanoparticles, which were characterized by TEM (Figure 3).
Here, an average size of 5 nm for the nanoparticles was found
in a typical sample (from entry 1, Table 3).
To extend the observed reactivity between methanol and
α,β-unsaturated enones, the methylation process was applied to
other aromatic dienones also used in this work (see Table 4).
As found for 4, the monomethylation product f was preferred
in all the reactions shown in Table 4, being the only product in
the case in which R = Ph. Noteworthy, the substitution in the
aromatic ring with electron-releasing groups (entries 2 and 3)
yielded mixtures of mono- and dimethylated ketones.
The use of methanol as a hydrogen source is well-known,17
being oxidized to formaldehyde (ive) during the reaction.9d
However, its use as an alkylating agent is less common;
therefore, ivd is an unexpected product for this homogeneous
catalytic process. Closely related reactions have been
documented by Yus et al., in which primary alcohols, such as
ethanol and isopropanol, can be used as alkylating agents in a
heterogeneous catalytic reaction using nickel nanoparticles.11a,b
Also, these nanoparticles have been used in the α-alkylation of
ketones recently, producing water as the only byproduct of the
reaction.19 These results prompted us to screen the observed
reactivity toward nickel nanoparticle formation (vide infra).
On raising the temperature for the same reaction to 150 °C,
an improvement in conversion was obtained (Table 2), and
consequently, an increase in yield for the hydrogenation and
alkylation products was observed. With a further rise to 180 °C,
two new alkylation products were detected in good yields by
GS-MS: the monomethylated (76%, ivf) and the dimethylated
ketone (6%, ivg) (see Table 2).
3. CONCLUSIONS
The selective CαCβ bond coordination and activation in α,β-
unsaturated enones, using a Ni(0) complex with electon-rich P-
donor ancillary ligands was found. The reactions that took place
yielded mononuclear and dinuclear complexes of the general
formula [{(dippe)Ni}n(η2-CαCβ-enone)]. A tandem catalytic
hydrogenation and alkylation of α,β-unsaturated enones with
methanol was found in a homogeneous/heterogeneous process
involving homogeneous nickel(0) complexes and nickel
nanoparticles, respectively. The current report offers the use
of CH3OH also as an alkylating agent, using a nonexpensive
metal, such as nickel, as a catalyst. Current studies are underway
To investigate the occurrence of nickel nanoparticles in the
reaction media, a mercury drop test was made, adding three
drops of elemental mercury into the reaction vessel. A
significant decrease in the formation of alkylation products
(ivf−ivg) and an increase of the hydrogenation product (ivc)
were observed (see Table 3) as compared with the mercury-
free reaction (entry 1), allowing us to conclude that
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[{(dippe)Ni}2(η2-C ,C -bis(4-methyl styryl)ketone)] (3a) complex.
Slow evaporation of THF-d8 at −26 °C in the drybox allowed the
crystallization of the pure product. The resulting solution was analyzed
by NMR spectroscopy: 31P{1H} NMR (121.32 MHz, THF-d8, r.t.): δ
to extend the scope of this reaction to other alcohols and P-
donor ligands.
α
β
4. EXPERIMENTAL SECTION
= 74.6 (dd, JP−P = 57.87 Hz, JC−P = 8.33 Hz), δ = 72.6 (dd, JP−P
=
Unless otherwise noted, all manipulations were performed using
standard Schlenk techniques in an inert-gas/vacuum double manifold
or under an argon atmosphere (Praxair 99.998) in an MBraun
glovebox (<1 ppm H2O and O2). All liquid reagents were purchased in
reagent grade and were degassed before use. All α,β-unsaturated
enones were purchased from Aldrich and were stored in a glovebox for
their further use. The nickel(I) dimer, [(dippe)Ni(μ-H)]2 (1), was
prepared from an n-hexanes slurry of [(dippe)NiCl2]18a using Super-
Hydride (LiHBEt3), according to the reported procedure.18b The
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. NMR spectra were recorded at room
temperature on a 300 MHz Varian Unity spectrometer unless
58.11 Hz, JC−P = 8.13 Hz). 13C{1H} NMR (75.36 MHz, THF-d8, r.t.):
δ = 190.57 (s, 9C, 13CO), 147.073 (s, 3C, C-aromatic), 130.37 (s,
6C, C-aromatic), 129.51 (s, 1C, 5C, CH-aromatic), 125.49 (s, 2C, 4C,
CH-aromatic), 60.68 (d, JC−P = 13.11 Hz, 7C, CH-olefinic), 50.4 (d,
JC−P = 19.44 Hz, 8C, CH-olefinic), 21.65 (s, p-CH3,10C), 28.0−17.9
(m, iPr CH, CH2, CH3). 1H NMR (300 MHz, THF-d8, r.t.): δ = 7.28
(d, JH−H = 8.1 Hz, 4H, 2H, aromatic), 6.812 (d, JH−H = 7.8 Hz, 5H,
1H, aromatic), 4.49 (m, JH−P = 3 Hz, 7H, CH-olefinic), 4.29 (m, JH−P
= 3 Hz, 8H, CH-olefinic), 2.12 (s, 10H, H-p-CH3), 2.28−0.334 (m, iPr
CH, CH2, CH3). [MS-EI+, m/z (%) 904(5.0%)].
4.4. Preparation of [(dippe)Ni(η2-Cα,C -Bis(4-methyl styryl)-
ketone)], (3b). The reaction of bis(4-methyβl styryl) ketone 3 (0.114
mmol, 0.030 g) with 1 (0.114 mmol, 0.0184 g) in 1 mL of THF-d8
yielded a mononuclear nickel(0) complex: [(dippe)Ni(η2-C ,C -bis(4-
α
β
1
otherwise noted. H NMR spectra (δ, parts per million) are reported
methyl styryl)ketone)], (3b). Slow evaporation of THF-d8 at −26 °C
in the drybox allowed the crystallization of the pure product. The
resulting solution was analyzed by NMR spectroscopy: 31P{1H} NMR
(121.32 MHz, THF-d8, r.t.): δ = 77.52, 75.35 (dd, JP−P = 42.46 Hz).
1H NMR (300 MHz, THF-d8, r.t.): δ = 7.58 (d, JH−H = 8.1 Hz, 4H,
relative to the residual protio-solvent. 13C{1H} spectra are referred to
the characteristic carbon signal of each solvent. 31P{1H} NMR
chemical shifts (δ, parts per million) are reported relative to external
85% H3PO4. 1H and 13C{1H} NMR spectra of the reduction products
were obtained in CDCl3. An Oxford Diffraction Gemini “A”
diffractometer with a CCD area detector (λMo Kα = 0.71073 Å) was
used for X-ray structure determinations. Catalytic experiments were
carried out in a 100 mL stainless steel Parr, T315SS reactor. Elemental
analyses (EAs) were also performed by USAI-UNAM using a
PerkinElmer microanalyzer 2400. EAs of pure compounds showed
variable inconsistencies due to their high oxygen sensitivity and were
not reported; however, all of them display satisfactory MS-EI+. Mass
spectrometry determinations (MS-EI+) of pure compounds were
performed by USAI-UNAM using a Thermo-Electron DFS.
2H, aromatic), 6.81 (d, JH−H = 8.1 Hz, 5H, 1H, aromatic), 5.906 (d,
JH−P = 11.4 Hz, 7H, CH-olefinic), 5.704 (m, JH−P = 13.2 Hz, 8H, CH-
olefinic), 2.118 (s, 10H, H-p-CH3), 2.009−0.328 (m, iPr CH, CH2,
CH3). [MS-EI+, m/z (%) 584 (1.7%)].
4.5. Preparation of [{(dippe)Ni}2 (η2 -Cα ,Cβ -Bis(4-
methoxybenzylidene)acetone)], (4a). Similar to the preparation
of compounds 2a and 3a (vide supra), the reaction between 1 (0.104
mmol, 0.0689 g) and bis(4-methoxybenzylidene)acetone) 4 (0.105
mmol, 0.030 g) in THF-d8 (1 mL) yielded the [{(dippe)Ni}2(η2-
Cα,C -bis(4-methoxybenzylidene)acetone)] (4a) complex. Slow evap-
4.1. Preparation of [{(dippe)Ni}2(η2-Cα,C -Dibenzylideneace-
β
β
oration of THF-d8 at −26 °C in the drybox allowed the crystallization
of the pure product. The resulting solution was analyzed by NMR
spectroscopy: 31P{1H} NMR (121.32 MHz, THF-d8, r.t.): δ = 74.0
(dd, JP−P = 59.28 Hz, JC−P = 8.00 Hz, JC−P = 7.15 Hz), δ = 72.3 (dd,
JP−P = 59.44 Hz, JC−P = 8.12 Hz, JC−P = 8.25 Hz). 13C{1H} NMR
(75.36 MHz, THF-d8, r.t.): δ = 190.161 (s, 1C, 13CO), 156.081 (s,
7C, C-aromatic), 142.525 (s, 4C, C-aromatic), 126.183 (s, 6C, 8C,
tone)], (2a). The reaction of dibenzylideneacetone 2 (0.047 mmol,
0.0109 g) with 1 (0.046 mmol, 0.030 g) in THF-d8 (1 mL) yielded a
dinuclear nickel(0) complex [{(dippe)Ni}2(η2-C ,C -dibenzylidenea-
α
β
cetone)] (2a). Immediate effervescence due to reductive elimination
of H2 was observed during mixing. The resulting solution was analyzed
by NMR spectroscopy: 31P{1H} NMR (121.32 MHz, THF-d8, r.t.): δ
= 75.0 (dd, JP−P = 56.5 Hz, JC−P = 8.3 Hz), δ = 72.4 (dd, JP−P = 57.02
Hz, JC−P = 8.28 Hz). 13C NMR (75.36 MHz, THF-d8, r.t.): δ = 190.86
(s, 1C, 13CO), 150.211 (s, 4C, C-aromatic), 128.59 (s, 5C, 9C, CH-
aromatic), 125.57 (s, 6C, 8C, CH-aromatic), 121.65 (s, 7C, C-
CH-aromatic), 114.296 (s, 5C, 9C, CH-aromatic), 61.083 (d, JC−P
=
12.81 Hz, 3C, CH-olefinic), 49.978 (d, JC−P = 20.12 Hz, 2C, CH-
olefinic), 55.37 (s, p-O-CH3, 10C), 26.88−17.47 (m, iPr CH, CH2,
CH3). 1H NMR (300 MHz, THF-d8, r.t.): δ = 6.98 (d, JH−H = 7.8 Hz,
5H, 9H, aromatic), 6.53 (d, JH−H = 7.8 Hz, 6H, 8H, aromatic), 4.36
(m, JH−P = 2.4 Hz, 3H, CH-olefinic), 4.25 (m, JH−P = 2.4 Hz, 2H, CH-
olefinic), 3.50 (s, 10H, H-p-O-CH3), 2.20−0.327 (m, iPr CH, CH2,
CH3). [MS-EI+, m/z (%) 935(5.5%)].
aromatic), 60.4 (d, JC−P = 10.70 Hz, 3C, CH-olefinic), 50.4 (d, JC−P
=
19.59 Hz, 2C, CH-olefinic), 26.90−17.44 (m, iPr CH, CH2, CH3). 1H
NMR (300 MHz, THF-d8, r.t.): δ = 7.13 (d, JH−H = 7.2 Hz, 5H, 9H,
aromatics), 6.98 (t, JH−H = 7.2 Hz, 6H, 8H, aromatics), 6.75 (t, JH−H
=
7.2 Hz, 7H, aromatics), 4.48 (m, JH−P = 3 Hz, 3H, CH-olefinic), 4.48
(m, JH−P = 3 Hz, 2H, CH-olefinic), 2.29−0.34 (m, iPr CH, CH2, CH3).
Molecular ion mass spectrometry: [MS-EI+, m/z (%) 874 (2.2%)].
4.2. Preparation of [(dippe)Ni(η2-Cα,Cβ-Dibenzylideneace-
tone)], (2b). The reaction of dibenzylideneacetone 2 (0.0427
mmol, 0.010 g) with 1 (0.0213 mmol, 0.030 g) in THF-d8 (1 mL)
yielded a mononuclear nickel(0) complex [(dippe)Ni(η2-Cα,Cβ-
dibenzylideneacetone)] (2b). The reaction mixture was prepared in
an NMR tube equipped with a J. Young valve and was heated in an oil
bath at 80 °C during 21 h. Slow evaporation of THF-d8 at −26 °C in
the drybox allowed the crystallization of the pure product. The
resulting solution was analyzed by NMR spectroscopy: 31P{1H} NMR
(121.32 MHz, THF-d8, r.t.): δ = 77.54, 75.5 (dd, JP−P = 42.49 Hz). 1H
NMR (300 MHz, THF-d8, r.t.): δ = 7.39 (d, JH−H = 7.2 Hz, 5H, 9H,
4.6. Preparation of [(dippe)Ni(η2 -Cα ,Cβ -Bis(4-
methoxybenzylidene)acetone)], (4b). The reaction between
bis(4-methoxybenzylidene)acetone) 4 (0.105 mmol, 0.030 g) and 1
(0.053 mmol, 0.0344 g) in THF-d8 (1 mL) yielded the mononuclear
nickel(0) complex [(dippe)Ni(η2-C ,C -bis(4-methoxybenzylidene)-
α
β
acetone)], (4b). Slow evaporation of THF-d8 at −26 °C in the
drybox allowed the crystallization of the pure product. The resulting
solution was analyzed by NMR spectroscopy: 31P{1H} NMR (121.32
1
MHz, THF-d8, r.t.): δ = 75.0 (dd, JP−P = 44.03 Hz). H RMN (300
MHz, THF-d8, r.t.): δ = 7.318 (d, JH−H = 8.7 Hz, 4H, 2H, aromatic),
6.78 (d, JH−H = 8.7 Hz, 5H, 1H, aromatic), 5.89 (d, JH−P = 10.5 Hz,
7H, CH-olefinic), 5.643 (m, JH−P = 12.6 Hz, 8H, CH-olefinic), 3.727
(s, 10H, H-p-O-CH3), 2.033−0.799 (m, iPr CH, CH2, CH3). [MS-EI+,
m/z (%) 615 (4.5%)].
aromatic), 7.16 (t, JH−H = 7.2 Hz, 6H, 8H, aromatic), 6.93 (t, JH−H
=
4.7. Catalytic Transfer Hydrogenation. A 100 mL Parr reactor
was typically charged with a constant amount of 1 (1.55 × 10−3 mmol,
0.001 g), and the corresponding α,β-unsaturated ketone:bis(4-
methoxybenzylidene)acetone) (0.155 mmol, 0.0456 g), dissolved in
methanol (15 mL). The resulting solutions were heated with vigorous
stirring at the temperatures and times indicated in Tables 1 and 2, and
after that time, the reactor was opened in a well-vented hood prior to
7.2 Hz, 7H, aromatic), 5.89 (d, JH−P = 3 Hz, 3H, CH-olefinic), 5.74 (d,
JH−P = 3 Hz, 2H, CH-olefinic), 2.30−0.34 (m, iPr CH, CH2, CH3).
[MS-EI+, m/z (%) 554 (0.6%)].
4.3. Preparation of [{(dippe)Ni}2(η2-Cα,C -Bis(4-methyl styryl)-
ketone)], (3a). Similar to the procedure descrβibed for complex 2a, the
reaction between 1 (0.114 mmol, 0.0737 g) and bis(4-methyl styryl)
ketone 3 (0.114 mmol, 0.030 g) in THF-d8 (1 mL) yielded the
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dx.doi.org/10.1021/om2010222 | Organometallics 2012, 31, 680−686

Organometallics
Article
workup. Orange or yellow solutions were formed. These were filtered
over Celite and analyzed by GC-MS.
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4.8. Mercury Drop Tests. Following the procedure described in
section 4.7, three drops of elemental Hg were added to the reaction
mixture (entry 2, Table 3). At the end of each run, the corresponding
solution was filtered and analyzed by GC-MS. The nanoparticles were
characterized using a JEOL-2010 electron microscope, equipped with a
lanthanum hexaboride filament, operated at an acceleration voltage of
200 kV.
4.9. X-ray Structure Determination. The crystals for com-
pounds 3a−4a and 2b−4b were first cryoprotected using Paratone-N
and mounted on glass fibers; immediately, the crystals were cooled at
130 K using a Cryojet cryostream (Oxford Cryosystems device).
Diffraction data were collected on an Oxford Diffraction Gemini
diffractometer with a CCD-Atlas area detector using a radiation source
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Org. Chem. 1990, 55, 5679.
graphite monochromator, λMo
= 0.71073 Å. CrysAlisPro and
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(b) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. J. Am.
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The Methanol Economy; Wiley VCH: Weinheim, Germany, 2006; pp
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386.
Kα
CrysAlis RED software packages20a were used for data collection and
data integration. All data sets consisted of frames of intensity data
collected with a frame width of 1° in ω, a counting time of 27−30 s/
frame, and a crystal-to-detector distance of 55.00 mm. The double pass
method of scanning was used to exclude any noise. The collected
frames were integrated by using an orientation matrix determined from
the narrow frame scans. Final cell constants were determined by a
global refinement; collected data were corrected for absorbance by
using analytical numeric absorption correction20b using a multifaceted
crystal model based on expressions upon the Laue symmetry using
equivalent reflections.
(10) Tsuchiya, Y.; Hamashima, Y.; Sodeoka, M. Org. Lett. 2006, 8,
4851.
Structure solution and refinement were carried out with the
program(s) SHELXS97 and SHELXL9720c; for molecular graphics, the
program ORTEP-3 for Windows20d was used; and to prepare material
for publication, the software WinGX was used.20e
(11) (a) Alonso, F.; Riente, P.; Yus, M. Acc. Chem. Res. 2011, 44, 379.
(b) Alonso, F.; Riente, P.; Yus, M. Tetrahedron Lett. 2008, 64, 1847.
(12) Nishimura, T.; Yasuhara, Y.; Hayashi, T. Angew. Chem., Int. Ed.
2006, 45, 5164.
(13) Sevesta, R.; Pizzuri, G.; Minnaard, A.; Feringa, B. Adv. Synth.
Catal. 2007, 349, 1931.
Full-matrix least-squares refinement was carried out by minimizing
2
(Fo − 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 C−H =
0.93−1.00 Å and Uiso (H) = 1.2Ueq(C), or 1.5Ueq(C) for aromatic,
methylene, methyne, and methyl groups. Crystal data and
experimental details of the structure determination are listed in the
Supporting Information.
(14) (a) Edelbach, B. L.; Lachiotte, R. J.; Jones, W. D. Organo-
metallics 1999, 18, 4040. (b) Flores-Gaspar, A.; Pinedo, P.; Crestani,
M.; Munoz, M.; Morales, D.; Warsop, B.; Jones, W. D.; García, J. J.
̃
Mol. Catal. A: Chem. 2009, 309, 1.
(15) Complex 3b is taken as a reference for the CαC bond
β
distance in the free ligand, due to the presence of uncoordinated
olefinic bond C15−C16.
ASSOCIATED CONTENT
(16) (a) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem.
Soc. 1998, 120, 2843.
■
S
* Supporting Information
(17) (a) Tani, K.; Iseki, A.; Yamagata, T. Chem. Commun. 1999,
1821.
Complete experimental procedures, ORTEP drawings, NMR
spectra, and GC-MS determinations of all products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(18) (a) Scott, C.; Kruger, C.; Betz, P. Organomet. Chem. 1990, 387,
̈
113. (b) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855.
(19) Alonso, F.; Riente, P.; Yus, M. Synlett 2007, 12, 1877.
(20) (a) Oxford Diffraction CrysAlis CCD and CrysAlis RED; Oxford
Diffraction Ltd.: Abingdon, England, 2010. (b) Clark, C.; Ried, J. S.
Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 887.
(c) Sheldrick, G. M. SHELXS97 and SHELXL97; University of
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: juvent@servidor.unam.mx.
Gottingen: Gottingen, Germany, 2008. (d) Farrugia, L. J. J. Appl.
̈
̈
Crystallogr. 1997, 30, 565. (e) Farrugia, L. J. J. Appl. Crystallogr. 1999,
32, 837.
ACKNOWLEDGMENTS
■
We thank CONACyT (080606) and PAPIIT-DGAPA-UNAM
(IN-201010) for their financial support to this work. N.C.-B.
thanks CONACyT for a graduate studies grant. Also, we thank
́
Dr. Alma Arevalo for technical support.
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