Stereoselective hydrogenation of aromatic alkynes using water, triethylsilane, or methanol, mediated and catalyzed by ni(0) complexes

Article abstract of DOI:10.1021/ic801823x

The use of complexes of the type [(P-P)Ni(n²-C,C-alkyne)] (P-P = 1, 2-bis(di-isopropyl-phosphinoethane or 1, 2- bis(diterbutylphosphino-ethane) in the presence of water, triethylsilane/water, or methanol as hydrogen sources yields the selective

Full text of DOI:10.1021/ic801823x

Inorg. Chem. 2009, 48, 386-393
Stereoselective Hydrogenation of Aromatic Alkynes Using Water,
Triethylsilane, or Methanol, Mediated and Catalyzed by Ni(0) Complexes
Rigoberto Barrios-Francisco and Juventino J. Garc´ıa*
Facultad de Qu´ımica, UniVersidad Nacional Auto´noma de Me´xico, Me´xico D. F. 04510, Me´xico
Received September 23, 2008
The use of complexes of the type [(P-P)Ni(η2-C,C-alkyne)] (P-P ) 1,2-bis(di-isopropyl-phosphinoethane or 1,2-
bis(diterbutylphosphino-ethane) in the presence of water, triethylsilane/water, or methanol as hydrogen sources
yields the selective production of E- or Z- aromatic alkenes from the corresponding alkynes. For instance, in the
case of diphenylacetylene (dpa) and water, a metal-mediated process was found to yield trans-stilbene
stoichiometrically, whereas in the case of triethylsilane/water and methanol, a catalytic system (1% mol) was found.
The catalytic systems gave >95% conversion to cis- or trans-stilbene, respectively. The use of a variety of substituents
on the aromatic ring was also assessed. Deuterium-labeling studies using D2O allowed the confirmation of water
as the hydrogen source for the alkyne reduction.
Introduction
perspective for conversion into potentially useful fuel
sources4 or, alternatively, for their use in synthetic work
where these molecules are incorporated into other substrates
by means of stoichiometric1a,b,5 or catalytically6 driven
reactions (e.g., hydrogen transfer).7 In the case of water,
The development of novel catalysts for the activation of
small molecules such as water,1 alcohols,2 or ammoniabo-
ranes3 generating or transferring dihydrogen to unsaturated
compounds is an important area in contemporary chemistry.
Inexpensive, environmentally friendly, and readily available
materials are particularly important features that are sought
and highlighted for such purposes, and this puts them in
traditional
splitting
methods
involve
photoche-
mical,8electrochemical,9 and combined photochemical/
electrochemical processes10 in the presence of a sacrificial
agent where dihydrogen is ultimately generated.
* Author to whom correspondence should be addressed. E-mail:
juvent@servidor.unam.mx.
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Hilmersson, G. Tetrahedron Lett. 2003, 44, 2661.
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386 Inorganic Chemistry, Vol. 48, No. 1, 2009
10.1021/ic801823x CCC: $40.75  2009 American Chemical Society
Published on Web 12/05/2008

StereoselectiWe Hydrogenation of Aromatic Alkynes
15
Scheme 1
Table 1.
ratio,
entrya ligand solvent T (°C) time (d)b conv. (%)c trans-stb:3ad
1
2
3
4
5
dippe benzene-d6 100
8
8
8
7
12
dippe toluene-d8
dippe THF-d8
dippe toluene-d8
dtbpe toluene-d8
100
100
140
140
40
100
100
100
20:1
12:1
15:1
only trans-stb(e)
To the best of our knowledge, little is known about the
use of transition metals in stoichiometric or catalytic reactions
that involve the splitting of water molecules, from which
the transfer of dihydrogen into organic molecules in solution
and under thermal conditions could be performed. Relevant
examples of metal-mediated hydrogen transfer to unsaturated
compounds such as alkenes or alkynes that yield the
corresponding alkanes or alkenes making use of water, a
transition metal (Co, Rh, and Pd), and a sacrificial agent (S)11
have been reported, although only in a handful of examples
(Scheme 1).
The use of 2 equiv of a Cp2Ti(Cl)(H2O) compound for
the reduction of alkenes using a second palladium or rhodium
compound in catalytic proportion has been reported.11a In
another report, a series of alkynes were reduced to olefins
displaying only the trans configuration using 5 mol% of the
palladium precursor compound [Pd(Cl)(η3-C3H5)]2, 10 mol%
of PPh3, D2O (10 equiv) or water (2.5 equiv), and hexam-
ethyldisilane (Me3Si-SiMe3, 1.5 equiv) as a sacrificial
agent.11b A variation of the latter methodologies that
consisted of the reductive coupling of activated alkenes in
the presence of alkynes to yield cross-coupled products when
using the cobalt catalyst, [Co(I)2(PPh3)2], and a mixture of
Zn and ZnI2 (as sacrificial agents) has also been reported.11c
The selective semihydrogenation of alkynes to yield cis-
alkenes selectively is a very important topic that still needs
to be developed in order to overcome several practical
inconveniences during the synthesis, along with the undesired
isomerization of the produced cis-alkene to the corresponding
trans one in the reaction mixture. A search for simple
preparative methods that may afford the cis-alkenes readily
and in high yield is badly needed in order to make the process
sustainable, and in this regard, some recent examples using
a zero-valent palladium catalyst reported by Elsevier and co-
workers can be highlighted.12 Other relevant examples
adressing the production of cis-alkenes through the use
hydrosilanes and palladium catalysts in acid media have also
been presented by Trost and Braslau.13
a All reactions were monitored by 1H and 31P{1H} NMR. A total of 350
equiv of water were used in all these reactions. b Heating was stopped when
the signals of the starting complex disappeared in the 31P{1H} NMR
spectrum. c Yields were primarily determined on the basis of the isolated
product and corroborated by H NMR and GC-MS. d Relative ratios were
1
determined by H NMR (see Supporting Information). e Only free trans-
1
stb was observed as a result of the hydrogen transfer reaction using 2b.
enantioselective preparation of alcohols and amines when
starting from the latter substrates,2i-k and also, examples that
address the reduction of alkenes or alkynes using methanol
and related media have been reported.2a-e In the case of
acetylenes, the search for improved systems that may drive
their selective and efficient reduction using water, hydrosi-
lanes/water, or methanol in the presence of an inexpensive
metal catalyst still represents a major challenge for sustain-
able chemistry.
The present work describes the reactivity of [(P-P)Ni(η2-
C,C-alkyne)] complexes (P-P ) 1,2-bis(diisopropyl-phos-
phinoethane, dippe, or 1,2-bis(diter-butyl-phosphinoethane,
dtbpe), in the presence of (a) water, (b) triethylsilane/water,
and (c) methanol, as hydrogen sources. The results show that
the selective production of the corresponding cis- or trans-
alkene from the corresponding alkyne is feasible under such
conditions and may give rise to high yields of products.
Results and Discussion
Compounds 2a and 2b were prepared following method-
ologies similar to the one reported originally by Jones et al.
using the nickel(I) hydride dimers, [(P-P)Ni(µ-H)]2 (P-P )
dippe, dtbpe; compounds 1a and 1b, respectively).14 The
formation of the nickel(0) compounds 2a and 2b was
confirmed by NMR using closed tubes equipped with J.
Young’s valves (see Supporting Information).
When reacted with water (eq 1), 2a evolves slowly to a
coordinated trans-stb analogue of the type [(dippe)Ni(η2-C-
C-{trans-C(H)(Ph)dC(Ph)(H)}] (3a), which after 8 days at
100 °C in THF-d8, toluene-d8, or benzene-d6 or 7 days at
140 °C in toluene-d8 (see Table 1) yields free trans-stb,
Ni(OH)2, and diphosphine monoxide, (iPr)2P-CH2-CH2-
P(dO) (iPr)2 (dippeO, see Supporting Information). Ni(OH)2
and dippeO correspond to the final fate of oxygen in this
reaction. In the case of 2b, its use also drives the formation
of the free trans-stb, although only under persistent heating
at a higher temperature (140 °C) and for a longer period of
time (12 days in toluene-d8). The formation of the nickel(0)
analogue bearing the coordinated trans-stb moiety, [(dtb-
pe)Ni(η2-(C,C)-{trans-C(H)(Ph)dC(Ph)(H)}] (3b), was elu-
sive under the reaction conditions used, and in this instance,
the stability of the resulting compound turned out to be very
In the case of alcohols, their use as hydrogen-transfer
agents has been found to be effective in the reduction of
unsaturated polar moieties such as carbonyls or imines, unlike
water. The use of methanol has afforded the catalytic and
(10) (a) Litch, S. J. Phys. Chem. B. 2003, 107, 4253. (b) Licht, S. Int. J.
Hydrogen Energy 2005, 30, 459. (c) Litch, S.; Halperin, L.; Kalina,
M.; Zidman, M.; Halperin, N. Chem. Commun. 2003, 3006. (d)
Fujihara, K.; Ohno, T.; Matsumura, M. J. Chem. Soc., Faraday Trans.
1998, 94, 3705.
(11) (a) Campan˜a, A. G.; Este´vez, R. E.; Fuentes, N.; Robles, F.; Cuerva,
J. M.; Bun˜uel, E.; Ca´rdenas, D.; Oltra, J. E. Org. Lett. 2007, 9, 2195.
(b) Shirakawa, E.; Otsuka, H.; Hayashi, T. Chem. Commun. 2005,
5885. (c) Chang, H.-T.; Jayanth, T. T.; Wang, C.-C.; Cheng, C.-H.
J. Am. Chem. Soc. 2007, 129, 12032.
(12) Hauwert, P.; Maestri, G.; Sprengers, J. W.; Catellani, M.; Elsevier,
C. J. Angew. Chem., Int. Ed. 2008, 47, 3223, and references therein.
(13) Trost, B. M.; Braslau, R. Tetrahedron Lett. 1989, 30, 4657.
(14) (a) Edelbach, B. L; Lachicotte, R. J.; Jones, W. D. Organometallics
1999, 18, 4040. (b) Waterman, R.; Hillhouse, G. L. Organometallics
2003, 22, 5182.
Inorganic Chemistry, Vol. 48, No. 1, 2009 387

Barrios-Francisco and Garc´ıa
low. A significant formation of Ni(OH)2 was formed over
time.
As indicated in Table 1, the overall processes exhibited a
strong dependence on temperature, solvent polarity, and steric
hindrance of the auxiliary ligands. Heating to 140 °C was
required to achieve complete reduction of dpa using 2b. In
the case of 2a, the reactions performed in nonpolar solvents
such as toluene-d8 or benzene-d6 (entries 1 and 2, Table 1)
resulted in poor to null conversions. Only the reaction using
THF reached completion, resulting in formation of a 12:1
ratio of free trans-stb-to-3a (entry 3). The immiscibility of
water in the systems was overcome by increasing the
temperature for the process from 100 to 140 °C (entries 2
and 4), which resulted in a significant improvement (entry
4).
Figure 1. ORTEP representation of complex 3a·0.5THF showing thermal
ellipsoids at the 50% probability level. Selected bond distances in angstroms:
Ni(1)-C(7) 1.979(4), Ni(1)-C(8) 1.983(4), Ni(1)-P(2) 2.1484(12),
Ni(1)-P(1) 2.1504(13), C(7)-C(8) 1.426(6), C(1)-C(7) 1.473(6), C(8)-C(9)
1.469(6).Selectedanglesindegrees:P(2)-Ni(1)-P(1)91.99(5),C(7)-Ni(1)-
C(8) 42.20(16), C(8)-Ni(1)-P(2) 112.66(13), C(7)-Ni(1)-P(1)
113.35(13).
In order to verify the hypothesis of water being the
hydrogen source for the alkyne reduction, a deuterium
labeling experiment using deuterium oxide (D2O) instead of
water was undertaken at 140 °C. The experiment was
performed using toluene-d8 following the conditions de-
scribed in entry 4 of Table 1 (fully described in the
Supporting Information); 100% deuterium incorporation into
trans-stb was observed, also producing the d2-trans-stb
analogue. No d2-cis-stb was formed in the process. A
tentative mechanistic proposal for the reduction of dpa to
trans-stb starting with the nickel(0) compound 2a is shown
in Scheme S1 (Supporting Information).
Table 2. Summary of Crystallographic Data Obtained for 3a·0.5THF
empirical formula
fw
C30H48NiOP2
537.33
temperature
wavelength
cryst syst
space group
unit cell dimensions
100(2) K
0.71073 Å
monoclinic
P2(1)/c
a ) 11.2062(15) Å
b ) 15.874(2) Å
c ) 16.660(2) Å
R ) 90°
ꢀ ) 94.364(3)°
γ ) 90°
volume
Z
2955.0(7) Å3
4
density (calculated)
abs coeff
F(000)
1.208 Mg/m3
0.783 mm-1
1160
X-ray-quality crystals for compound 3a were found to
spontaneously crystallize from the H2O/THF reaction mixture
described in entry 3 of Table 1. An X-ray diffraction study
of these crystals16 allowed the confirmation of such an
intermediate, as produced from the catalytic process (Figure
1, Table 2).
An independent preparation of 3a using [(dippe)Ni(µ-H)2]
(1a) and trans-stb (described in the Supporting Information)
allowed full characterization of the former compound. A
reactivity study involving the replacement of the coordinated
trans-stb ligand in 3a by dpa was also addressed. The latter
study confirmed a partial substitution at room temperature
(24 h in THF), producing 2a. The reaction reached comple-
tion after heating the system to 50 °C for a period of 6 h in
THF, or 100 °C for 2 h in toluene. The observation of going
from 3a to 2a under heating is consistent with the proposal
illustrated in Scheme S1 (Supporting Information) for the
catalytic process, even if ultimately hampered by the
cryst size
0.23 × 0.21 × 0.16 mm3
θ range for data collection 1.77-25.00°
index ranges
reflns collected
ind reflns
-13 e h e 12, -18 e k e 18, -19 e l e 16
13604
5189 [R(int) ) 0.0590]
completeness to θ ) 25.00° 99.9%
refinement method
full-matrix least-squares on F2
5189/173/371
1.155
R1 ) 0.0688, wR2 ) 0.1310
R1 ) 0.0887, wR2 ) 0.1386
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole 0.557 and -0.487 e Å-3
oxidation of the catalyst intermediates (vide supra). The step,
in turn, is probably driven by the better π acceptance of the
dpa ligand versus trans-stb when coordinated to nickel(0),
provided that a considerable back-donation is excerpted by
the [(dippe)Ni0] moiety over these two.17,18
In addition to the latter points, studies were also undertaken
addressing the issue of selectivity toward trans-stb, whose
formation is indicative of a metal-mediated process as the
(15) Only minute amounts of cis-stilbene (cis-stb), apart from the main
product trans-stb, were observed in experiments described in entries
4 and 5. No traces of the phenylbencylketone expected from a
competing hydration reaction in these reactions was observed, all of
which confirms a preference for the selective reduction of dpe to yield
trans-stb.
(17) (a) Garc´ıa, J. J.; Are´valo, A.; Brunkan, N. M.; Jones, W. D.
Organometallics 2004, 23, 3997–4002. (b) Garcia, J. J.; Brunkan,
N. M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 9547–9555. (c)
Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544–5545.
(18) Schager, F.; Bonrath, W.; Po¨rschke, K.-R.; Kessler, M.; Kru¨ger, C.;
Seevogel, K. Organometallics, 1997, 16, 4276.
(16) Crystallographic data for 3a has been deposited at the Cambridge
Crystallographic Data Centre, CCDC 686568.
388 Inorganic Chemistry, Vol. 48, No. 1, 2009

StereoselectiWe Hydrogenation of Aromatic Alkynes
Scheme 2
Scheme 3
Scheme 4
direct addition of water to the coordinated dpa in 2a or 2bsif
occurring similarly to what has been speculated for the nitrile
hydration processes using nickel(0) compounds17,19 swould
be expected to result in the production of enols or ketones
but certainly not alkenes.20 The preference for the formation
of trans-stb over cis-stb or even a mixture of the two
suggested a thermodynamically driven process, as the
expected kinetic product, cis-stb, had not been observed at
this point. An experiment addressing the isomerization of
cis- to trans-stb in the presence of nickel(0) was undertaken
using a toluene-d8 mixture of the π-bound alkyne complex
2a in the presence of cis-stb and D2O.11b,21 The mixture was
heated to 140 °C for 7 days following the optimized
conditions described in entry 4 of Table 1. The incorporation
of proton residues over the final trans-stb proceeding from
2a, namely, trans-(D/H)(Ph)CdC(Ph)(D/H) instead of trans-
the complete tables for which are included in the Supporting
Information. A mechanistic proposal for the reduction of dpa
using Et3SiH/H2O is illustrated in Scheme S2 (Supporting
Information).
The use of methanol as a hydrogen source for the reduction
of dpa was also addressed as an alternative. The latter
substrate was expected to act as both a hydrogen source and
a sacrificial agent while readily oxidizing to formaldehyde.22
Scheme 4 summarizes our findings after 48 h at 140 °C,
under stoichiometric conditions.
As in the case in which Et3SiH was used, the reaction
time required to ensure complete conversion of dpa was
found to be shorter than with all of the previous experiments
using water (see Table 1). No catalyst decomposition took
place. The reaction was addressed in catalysis, which yielded
99.5% of trans-stb by GC-MS (100% conversion of dpa;
Table S3, Supporting Information), and no oxidation of the
[(dippe)Ni0] catalyst was observed at the 1 mol % catalytic
proportion, after 24 h. Analysis of the GC profile of the crude
reaction mixture confirmed the additional formation of cis-
stb and the enolether,20 (MeO)(Ph)CdC(H)(Ph), in minute
amounts of 0.49 and 0.01%, respectively. As a result of this,
an extremely selective reductive process to form trans-stb
when using the [Ni(0)] catalyst was devised. A proposal of
a competitive mechanism for the formation of the enolether
operating at a much slower rate than the one leading to trans-
stb is presented in Scheme S3 (Supporting Information). The
formation of trans-stb using 1 mol % of the nickel(0) catalyst
was achieved in 88.6% yield (95% overall conversion) at a
time as short as 12 h.
1
(D)(Ph)CdC(Ph)(D), was confirmed by H NMR. A sharp
singlet at δ 7.12, slightly deshielded from the original one
at δ 6.51, which is related to cis-stb, gradually appeared in
1
the H spectrum of the heated solution, confirming the
presence of the protiated trans-(D/H)(Ph)CdC(Ph)(D/H)
analogue. Its presence validated the reduction of dpa as a
thermodynamically driven process. Scheme 2 summarizes
these findings.
With the aim of optimizing the reduction of dpa using
nickel catalysts, it became clear that the use of an additional
sacrificial agent would be required, as in fact the P-P ligand
had been acting as a sacrificial agent in all the experiments
using water as the hydrogen source. Consequently, the use
of a silane as a sacrificial agent was envisaged as a useful
option and was explored. Scheme 3 summarizes the results
obtained under catalytic conditions.
The use of Et3SiH/H2O in THF allowed the selective
preparation of cis-stb in 98% yield at 180 °C, within 48 h.
GC-MS analysis of the crude reaction allowed confirmation
of the presence of (Et3Si)2O, and a 31P{1H} NMR followup
allowed for ruling out the presence of phosphine oxide. A
variety of reaction conditions was addressed after this result,
With the aim of extending the scope of this reaction to
other alkynes, the use of a variety of substituted aromatic
alkynes was assessed under catalytic conditions, using
Et3SiH/H2O and methanol. The results are sumarized in
Tables 3 and 4, respectively.
As shown, most of the systems gave high yields of the
corresponding alkene under similar reaction conditions. In
general, the use of Et3SiH/H2O selectively favors the
production of cis-alkenes, and the use of MeOH favors the
formation of trans-alkenes, with exception of 1-chloro-4-
(19) (a) Crestani, M. G.; Are´valo, A.; Garc´ıa, J. J. AdV. Synth. Catal. 2006,
348, 732–742. (b) Criso´stomo, C.; Crestani, M. G.; Garc´ıa, J. J. J.
Mol. Catal. A: Chem. 2007, 266, 139–148.
(20) A reference addressing water and metanol addition onto triple bonds:
Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. ReV. 2004, 104, 3079.
(21) (a) Lo´pez-Serrano, J.; Duckett, S. B.; Aiken, S.; Almeida-Len˜ero,
K. Q.; Drent, E.; Dunne, J. P.; Konya, D.; Whitwood, A. C. J. Am.
Chem. Soc. 2007, 129, 6517. (b) Lo´pez Serrano, J.; Lledo´s, A.;
Duckett, S. B. Organometallics, 2008, 27, 43.
(22) A positive chromotropic acid test for formaldehyde presence was
observed: Zhang, J.; Thickett, D.; Green, L. J. Am. Inst. ConserVation
1994, 33, 1,–47.
Inorganic Chemistry, Vol. 48, No. 1, 2009 389

Barrios-Francisco and Garc´ıa
Table 3. Results for Substituted Aromatic Alkynes Used under
trans-stb), were purchased from Aldrich and were stored in a
glovebox for their use. All water used was distilled. Toluene and
methanol were dried by standard methods and stored over 4 Å
molecular sieves, under argon. Protiated solvents (THF and hexanes,
J.T. Baker) were reagent-grade and were dried and deoxygenated
by distillation from purple benzophenone ketyl solutions, under
argon. Toluene was refluxed over sodium for 1 day under an argon
atmosphere to ensure complete dryness and was distilled and stored
in the glovebox. Deuterated solvents for NMR experiments were
purchased from Cambridge Isotope Laboratories and were stored
over 3 Å molecular sieves in the glovebox for at least 24 h, prior
to their use. The chelating bisphosphine ligand, dippe,23 was
synthesized from 1,2-bis(dichlorophosphino)ethane (Aldrich) and
the corresponding isopropylmagnesium chloride solution (2.0 M)
in THF (Aldrich). The nickel(I) dimer, [(dippe)NiH]2 (1), was
prepared from a hexane slurry of [(dippe)NiCl2]24 using superhy-
dride (LiHBEt3), similarly to the literature procedure.25 Neutral
alumina was used for the preparation of complex 1 and was first
dried under a vacuum at 200 °C for a period of 48 h. All other
substances, filters, and chromatographic materials were reagent-
grade and were used as received. The organometallic complexes
and organics produced in this work were purified either by
crystallization or by column chromatography. Reactor vessels for
catalysis were charged in the glovebox. The crude reaction mixtures
were recovered from these and were immediately analyzed by GC-
MS. The catalytic conversions of substrates were determined by
the integration of areas in the respective chromatograms. The
retention times of all products were compared against pure standards
of the same substances. NMR spectra of complexes and products
in this work were recorded at ambient temperature using a 300
Catalytic Conditions, Using Et3SiH/H2Oa
entry
X
conv. (%) yield, alkyne/trans-alkene/cis-alkeneb
1
2
3
4
CHO
p-F-C6H4-CO
100
95
20
100
91
0:0:100
5:0:95
80:0:20
0:99:0
9:3:88
OMe
Cl
5c OMe
a All reactions were performed in stainless reactor vessels (Parr) equipped
with internal magnetic drive stirring using 20 mL of THF at 180°C, over
48 h, using ratios of [(dippe)NiH]2/alkyne/Et3SiH/H2O of 1:200:268:60 000,
respectively. b Yields determined by GC-MS and 1H NMR. c Reaction was
performed at 200 °C.
(phenyl-ethynyl)benzene, the last giving in both systems the
corresponding trans-alkane, probably due to the formation
of minute amounts of HCl, allowing the cis to trans
isomerization of the olefin.11b
Conclusions
We have shown that the selective reduction of dpa to trans-
stb using nickel(0) complexes of the type [(P-P)Ni(η2-C,C-
dpa)] can be successfully achieved using water (metal-
mediated case) or methanol (metal-mediated and catalytically)
in high yields. The use of triethylsilane/water (catalytic)
allowed the selective preparation of cis-stb, also in high
yields. In the first case, the diphosphine is ultimately oxidized
in the process and acts as a sacrificial agent, whereas in the
case of triethylsilane, the decomposition is prevented as the
latter acts as both a hydrogen source and sacrificial agent
that traps oxygen as (Et3Si)2O. In the case of methanol, a
similar system was found in which it acted as a hydrogen
source and a reductive agent, the latter having been oxidized
to formaldehyde during the catalytic process. The use of
methanol allowed the reaction to occur at a much faster rate,
100% conversion of dpa into trans-stb being observed after
24 h of heating at 180 °C. Other aromatic alkynes can be
selectively reduced. The use of Et3SiH/H2O allows the
production of cis-alkenes, and the use of MeOH allows the
formation of trans-alkenes with the exception of 1-chloro-
4-(phenyl-ethynyl)benzene, the last giving in both systems
the corresponding trans-alkane. The use of nickel as an
inexpensive active catalyst for the reduction of dpa is
highlighted. Further studies are currently underway to fully
elucidate the mechanistic pathway that operates for this
process.
1
MHz Varian Unity spectrometer. H, 13C{1H}, and 31P{1H} NMR
spectra of the complexes were obtained from concentrated THF-
d8, benzene-d6, and toluene-d8 solutions of the pure compounds.
All air- and moisture-sensitive samples in this work were handled
under an inert atmosphere using thin-wall (0.38 mm) WILMAD
NMR tubes equipped with J. Young’s valves. Heating of the
charged NMR tubes was done using stirred thermostatted silicon
oil baths. The 1H and 13C{1H} NMR spectra of all purified products
were obtained at room temperature from concentrated CDCl3 or
CD2Cl2 solutions. 1H chemical shifts (δ, ppm) are reported relative
to residual proton resonances in the deuterated solvent. 13C{1H}
NMR spectra are reported relative to the reference signal of the
corresponding deuterated solvent. 31P{1H} NMR spectra are of
nickel compounds, and free phosphines are reported relative to
external 85% H3PO4. Coupling constants (J values) are given in
hertz. GC-MS determinations were performed using a Varian Saturn
3 equipped with a 30m DB-5MS capillary column. Melting points
of purified organics were determined in a capillary using an
electrothermal digital melting point apparatus. MS-EI+ analyses
were performed by USAI-UNAM using a Thermo-Electron DFS.
Mass spectrometric and elemental analyses of pure compounds 2a,
2b, and 3a showed variable inconsistencies due to their high oxygen
sensitivity and were not reported.
Experimental Section
Reduction of dpa with [(dippe)Ni(η2-C,C-dpa)] (2a) Using
Water As a Hydrogen Source. Into NMR tubes with Young’s
valves were charged solutions of complex 2a (45 mg, 0.09 mmol)
in 0.7 mL of deuterated solvents (toluene-d8 or THF-d8), and to
these were added 300 equiv of water (0.45 mL, 0.45 mg, 27 mmol).
General Considerations. Unless otherwise noted, all manipula-
tions were performed using standard Schlenk and glovebox
techniques under high-purity argon (Praxair, 99.998%) using an
MBraun glovebox (<1 ppm H2O and O2). Parr Series 4590 and
4561M stainless steel autoclaves (T316SS), bench top mini reactors
(100, 300 mL), and 4750 general purpose vessels (125 mL) were
used for catalysis experiments. Liquid substrates were reagent-grade
and were degassed using the freeze-thaw-pump method prior to
their use. All alkyne standards, cis and trans-stilbene (cis-stb and
(23) Cloke, F. G. N.; Gibson, V. C.; Green, M. L. H.; Mtetwa, V. S. B.;
Prout, K. J. Chem. Soc., Dalton Trans. 1988, 2227, 2229.
(24) Scott, F.; Kru¨ger, C.; Betz, P. J. Organomet. Chem. 1990, 387, 113.
(25) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855–
10856.
390 Inorganic Chemistry, Vol. 48, No. 1, 2009

StereoselectiWe Hydrogenation of Aromatic Alkynes
Table 4. Results for Substituted Aromatic Alkynes Used under Catalytic Conditions, Using Methanola
entry
X
conv. (%)
yield, alkyne/trans-alkene/cis-alkene/enoletherb
1
2
3
CHO
p-F-C6H4-CO
OMe
100
94
91
0:85:0:15
6:64:14:16
9:90:1:0
4
Cl
100
98
0:99:1:0
2:82:11:5
5c
p-F-C6H4-CO
a All reactions were performed in stainless reactor vessels (Parr) equipped with internal magnetic drive stirring using 30 mL of toluene/MeOH in a ratio
of 2:1 at 180 °C, over 24 h, using ratios of [(dippe)NiH]2/alkyne of 1:200. Yields determined by GC-MS and H-NMR. c Reaction was performed at 200
b
1
°C.
Everything was then heated in a thermostatted silicon oil bath. For
toluene-d8 at 100 °C, the reaction was heated and monitored over
8 days by NMR (Figures S1 and S2, Supporting Information).
Formation of the free trans-stilbene and [(dippe)Ni(η2-(C-C)-{trans-
C(H)(Ph)dC(Ph)(H)}] (3a) in a ratio of 20:1 was found by integral
comparisons in the 1H spectrum, comparing vinylic protons of free
trans-stilbene (s, 7.05 ppm) and those coordinated to [(dippe)Ni0]
Preparation of [(dippe)Ni(η2-C,C-trans-stb)] (3a). To a dark
solution of 1a (0.030 g, 0.046 mmol) in 5 mL of toluene was
added trans-stb (0.016 g, 0.093 mmol). The mixture was allowed
to stir for a period of 8 h, during which time a yellow solution
was obtained. The solvent was evaporated and the remaining
1
residue dried in vacuo. Yield of 3a: 98% (0.045 g). H NMR
(toluene-d8): δ 7.25 (d, JH-H ) 7.8 H, 4H), 7.11 (t, JH-H ) 7.8
Hz, 4H), 6.89 (t, JH-H ) 7.8 Hz, 2H), 4.46 (s, 2H, vinylic
protons), 2.09 (septuplet, JH-H ) 7.2 Hz, 2H, CHMe2), 1.65 (d,
JH-P ) 9.2 Hz, 4H, PCH2CH2P), 1.05 (quintet, JH-H ) 7.2 Hz,
JH-P ) 7.2 Hz, 24H, CHMe2). 13C{1H} NMR (toluene-d8): 147.56
(s, Ar CH), 126.8 (s, Ar CH), 124.59 (s, Ar CH), 121.3 (s, Ar
1
(s, 4.5 ppm), respectively. H NMR of vinylic protons (toluene-
d8):
δ 7.05 (s, trans-C(H)(Ph)dC(Ph)(H)), 6.5 (s, cis-
C(H)(Ph)dC(Ph)(H)), 4.57 (s, [(dippe)Ni(η2-(C-C)-{trans-
(H)(Ph)dC(Ph)(H)}]). 31P{1H} NMR data for the mixture at the
end of the reaction (toluene-d8): δ 77.4 (s, 2a), 67.07 (s, 3a), 51.7
(d, 2JP-P ) 36.8 Hz, (iPr)2P-CH2-CH2-P(dO)(iPr)2), 8.35 (d, 2JP-P
) 36.8 Hz, (iPr)2P-CH2-CH2-P(dO) (iPr)2).
2
CH), 50.9 (t, JC-P ) 8.5 Hz, )C(H)(Ph)), 25.6 (t, JC-P ) 9.5
Hz), 23.8 (t, JC-P ) 9.52 Hz), 18.9 (t, JC-P ) 3.5 Hz), 17.1 (s).
31P{1H} NMR (toluene-d8): δ 65.8 (s).
Preparation of [(dippe)Ni(η2-C,C-dpa)] (2a). To a dark red
solution of [(dippe)Ni(µ-H)]2, 1a, (0.120 g, 0.138 mmol) in
toluene (5.0 mL) was added dpa (0.064 g, 0.372 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 in vacuo, and the remaining
residue was further dried under a vacuum for 3 h. The residue
was redissolved in hexane, and the solution was filtered through
a canula into a second Schlenk flask, from which the solvent
was again stripped. The remaining yellow solid was dried for
4 h under a vacuum. Yield of 2a: 95% (0.176 g). 1H NMR (THF-
d8): δ 7.35 (d, JH-H ) 7.4 Hz, 4H), 7.18 (t, JH-H ) 7.4 Hz, 4H),
7.01 (t, JH-H ) 7.3 Hz, 4H), 2.12 (septuplet, JH-H ) 7.2 Hz,
2H, CHMe2), 1.65 (d, JH-P ) 9.2 Hz, 4H, PCH2CH2P), 1.08
(quintet, JH-H ) 7.2 Hz, JH-P ) 7.2 Hz, 24H, CHMe2). 13C{1H}
Metal-Mediated Experiments for the Reduction of dpa
Using Water, Performed in Closed NMR Tubes. Reduction
of dpa with [(dippe)Ni(η2-C,C-dpa)] (2a) Using Water As a
Hydrogen Source. NMR tubes with J. Young′s valves were
charged in different runs with solutions of 2a (45 mg, 0.09 mmol)
using 0.7 mL of deuterated solvents (toluene-d8 or THF-d8), and
to these was added 300 equiv of water (0.45 mL, 0.45 mg, 27
mmol). The mixtures were then heated in a thermostatted silicon
oil bath.
For the reaction in toluene-d8 at 100 °C, the NMR tube was
heated and monitored over 8 days (Figures S1and S2, Supporting
Information). The formation of free trans-stb and [(dippe)Ni(η2-
(C-C)-{trans-C(H)(Ph)dC(Ph)(H)}] (3a) in a ratio of 20:1 was
determined by 1H NMR, comparing the integrals of the respective
vinylic protons of free trans-stb and those of the coordinated
ligand to [(dippe)Ni0] (7.04 and 4.5 ppm, respectively). 1H NMR
2
NMR (THF-d8): δ 141.3 (t, JC-P ) 7.1 Hz, C-Ph,), 132.1 (s,
2
Ar CH), 128.3 (d, JC-P ) 8.6 Hz, C-Ph), 127.3 (s, Ar CH),
of vinylic protons (toluene-d8):
δ
7.05 (s, trans-
124.9 (s, Ar CH), 26.7 (t, JC-P ) 10.4 Hz), 22.2 (t, JC-P ) 19.1
Hz), 20.4 (t, JC-P ) 4.0 Hz), 19.1 (s). 31P{1H} NMR (THF-d8):
δ 80.9 (s).
C(H)(Ph)dC(Ph)(H)), 6.5 (s, cis-C(H)(Ph)dC(Ph)(H)), 4.57 (s,
[(dippe)Ni(η2-(C-C)-{trans-(H)(Ph)dC(Ph)(H)}]). 31P{1H} NMR
(toluene-d8): δ (ppm) 77.4 (s, 2a), 67.07 (s, 3a), 51.7 (d, JP-P
)
Preparation of [(dtbpe)Ni(η2-C,C-dpa)] (2b). The preparation
of this compound was performed following the procedure
described above for 2a, using [(dtbpe)Ni(µ-H)]2 (1b; 0.045 g,
0.059 mmol) and dpa (0.021 g, 0.119 mmol) in 5.0 mL of
36.8 Hz, (iPr)2P-CH2-CH2-P(dO) (iPr)2), 8.35 (d, JP-P ) 36.8
Hz, (iPr)2P-CH2-CH2-P(dO) (iPr)2). After reaction completion,
the tube was opened in the air, and the remaining volatiles were
removed in vacuo using a Schlenk line. The corresponding free
olefin was purified by column chromatography in silica gel,
eluting with hexanes. Yield of trans-stb after workup: 40% of
white crystals (0.0065 g). Mp: 112 °C. MS-EI+, m/z (%): 180
1
toluene. Yield of 2b: 93% (0.613.0 g). H NMR (benzene-d6):
δ 7.58 (d, JH-H ) 7.4 Hz, 4H), 7.24 (t, JH-H ) 7.4 Hz, 4H),
7.06 (t, JH-H ) 7.3 Hz, 2H), 1.45 (m, 4H, PCH2CH2P), 1.18 (d,
2
CH3, 36H). 13C{1H} NMR (benzene-d6): δ 143.97 (t, JC-P
)
1
(M+). H (CDCl3): δ 7.49 (d, JH-H ) 6.9 H, 4H), 7.33 (t, JH-H
2
8.25 Hz, C-Ph), 138.89 (t, JC-P ) 19.95 Hz), 128.24 (s, Ar
) 7.2 Hz, 4H), 7.24 (t, JH-H ) 8.7 Hz, 2H), 7.09 (s, 2H, vinylic
protons). 13C{1H} NMR (CDCl3): 137.5 (s, Ar CH), 128.8 (s,
Ar CH), 127.8 (overlapping s, Ar CH and dC(Ph)(H)), 126.7
(s, Ar CH). Formation of a small amount of a green solid was
detected during the course of the reaction. The solid was
CH), 127.39 (s, Ar CH), 124.23 (s, Ar CH), 34.50 (t, JP-C
)
7.05 Hz, CMe3), 30.81 (t, JP-C ) 3.15 Hz, CMe3), 23.97 (t, JP-C
) 16.5 Hz, PCH2CH2P). 31P{1H} NMR (benzene-d6): δ 94.09
(s).
Inorganic Chemistry, Vol. 48, No. 1, 2009 391

Barrios-Francisco and Garc´ıa
identified as Ni(OH)2, characterized by solubility and reactivity
with ammonium hydroxide according to trials reported in the
existing literature26 and comparing it with an authentic sample
of this compound.
ment of trans-stb (Figure S4, Supporting Information). Similar
results were observed using THF-d8.
Proof of Isomerization of cis-stb into trans-stb. A toluene-d8
solution of 2a (45 mg, 0.09 mmol) was charged in a NMR tube
equipped with a Young’s valve, and to it were added 300 equiv
of D2O (0.3 mL, 0.45 mg, 27 mmol) and 1 equiv of cis-stb (15
µL, 0.016 g, 0.09 mmol). The reaction was heated to 140 °C
for a period of 7 days, after which time it was exposed to air,
For the reaction in toluene-d8 at 140 °C, a NMR tube was
charged in a glovebox using complex 2a (45 mg, 0.09 mmol) in
0.7 mL of deuterated solvents (toluene-d8 or THF-d8), and to it
was added 300 equiv of water (0.45 mL, 0.45 mg, 27 mmol),
following the procedure described above. The reaction was
heated and monitored for 7 days using NMR spectroscopy, after
which time, compound 2a had reacted completely. As in the
case of the system heated at 100 °C (vide supra), the formation
of free trans-stb and 3a were detected, however, in a ratio of
15:1, on the basis of 1H NMR analysis. Formation of the
phosphine monoxide, (iPr)2P-CH2-CH2-P(dO) (iPr)2 (Figure S1,
Supporting Information) and Ni(OH)2 were also observed.
For the reaction using THF-d8 at 100 °C, a reaction system
was set up following the same scheme described above, although
using THF-d8. The system was heated to 100 °C and monitored
over 8 days, after which time 2a reacted completely. Formation
of free trans-stb and 3a in a relative ratio of 12:1 was confirmed
1
and the crude mixture was analyzed by H (CDCl3). Figure S5
(Supporting Information) shows the corresponding spectrum. The
singlet at 7.09 ppm corresponds to the vinylic protons of trans-
stb.
Metal-Mediated Experiments for the Reduction of dpa
Using Water, Performed in Stainless Steel Reactor Vessels.
Catalysis Assessment Using a 10:1 Molar Ratio of dpa/
[Ni(0)]. A stainless steel autoclave (T316SS, 100 mL) equipped
with an inner magnetic stirring drive was charged in a glovebox
with a toluene solution (20 mL) of dpa (0.16 g, 0.92 mmol), 1a
(0.03 g, 0.045 mmol), and H2O (5.0 mL, 277 mmol). The vessel
was heated at 180 °C for 7 days, after which time heating was
resumed, and the vessel was allowed to cool down to room
temperature before its opening in a fume hood. The vessel was
then opened and its contents examined. The crude reaction
mixture was recovered from the vessel and analyzed by GC-
MS (chromatogram included as Figure S7, Supporting Informa-
tion). Conversion of dpa into trans-stb was determined directly
by GC-MS analysis. Yield of trans-stb after reaction: 20%.
Catalysis Assessment Using a 100:1 Molar Ratio of dpa/
[Ni(0)]. A vessel was charged with a toluene solution (20 mL)
of dpa (0.16 g, 0.92 mmol), 1a (0.003 g, 0.0045 mmol), and
H2O (5.0 mL, 277 mmol) and the system heated at 180 °C for
7 days following the reaction scheme described for the 1:10
molar ratio experiment. Yield of trans-stb after reaction: 5%.
1
by H NMR. The mentioned phosphine monoxide and Ni(OH)2
were also observed to form progressively in the same reaction
mixture. Incidentally, 3a was isolated from this mixture as
crystals suitable for X-ray diffraction.
Reduction of dpa with [(dtbpe)Ni(η2-C,C-dpa)] (2b) Using
Water. Compound 2b (0.03 g, 0.054 mmol) was reacted with
water (0.3 mL, 16 mmol) in a toluene-d8 solution (0.7 mL) at
140 °C following the methodology described for compound 2a.
Heating was resumed until complete consumption of 2b was
reached, according to 31P{1H} NMR followup, which occurred
after 12 days at that temperature. The reaction was quenched
by exposing the system to air, and all volatiles were removed
in a Schlenk line, under a vacuum. Produced trans-stb was
purified by column chromatography as described already. Yield
of trans-stb: 98% of white crystals.
Metal-Mediated Experiments for the Reduction of dpa
Using Methanol. Stoichiometric Reduction of dpa in
[(dippe)Ni(η2-C,C-dpa)] (2a), Using Methanol in a Closed
NMR Tube. Using a toluene-d8 solution of 2a (0.045 g, 0.09
mmol) and 80 equiv of MeOH (0.3 mL, 7 mmol) a NMR tube
equipped with a Young’s valve was charged, and the closed tube
was heated in an oil bath to 140 °C for 48 h. A complete
conversion of 2a into 3a was confirmed to take place after this
time.
Catalytic Reduction of Alkynes Using [Ni(0)] in the
Presence of Methanol, Using a Reactor Vessel. A typical
experiment for each alkyne was made as follows: A series of
reactor vessels were charged in separate runs with charges of
dpa (0.16 g, 0.92 mmol), 1a (0.003 g, 0.0045 mmol), and MeOH
(5.0 mL, 123 mmol) in 20 mL of toluene. The vessels were
heated to 180 °C for different periods of time (12, 24, and 72 h),
and their contents were analyzed by GC-MS after each comple-
tion of each reaction time. The evolution of each system is
illustrated in Figures S9-S11 (Supporting Information) by their
corresponding chromatograms. At the end of each reaction, three
products were observed, namely, trans- and cis-stb and minute
amounts of the enol-ether, illustrated in Figure S6 (Supporting
Information). Table S1 (Supporting Information) indicates the
respective ratios obtained for these products.
Deuterium Labeling Using D2O. An NMR tube fitted with a
J. Young’s valve was charged in a glovebox using a toluene-d8
(0.7 mL) solution of 2a (0.045 g, 0.09 mmol) and 300 equiv of
D2O (0.3 mL, 27 mmol). The mixture was heated to 140 °C
over 7 days, after which time it was exposed to air. The produced
volatiles were removed in vacuo, and the product was purified
by chromatography. Yield of trans-(Ph)(D)CdC(Ph)(D), after
work up: 100%. MS-EI+, m/z (%): 182 (100%).
Figure S3, Supporting Information, shows the corresponding
1H spectrum of this product. The signal that corresponds to the
vinyl protons is absent in the spectrum, which is a confirmation
of 100% incorporation of deuterium.
Displacement of trans-stb by dpa in Complex 3a. A solution
of 3a (0.03 g, 0.059 mmol) in toluene-d8 (0.7 mL) was charged
into a NMR tube equipped with a Young’s valve in a glovebox,
and to it was added 1 equiv of dpa (0.01 g, 0.059 mmol). The
tube was closed and stirred at room temperature for 1 day, during
which time only a very small amount of 2a was observed to be
formed by 31P{1H} NMR spectroscopy. The reaction was then
heated to 100 °C in a stirred oil bath, and progression of the
reaction was continuously monitored. After 3.5 h, 3a was found
to be converted entirely into 2a, confirming a complete displace-
Reduction of dpa using methanol and [(dtbpe)Ni] catalysts
in a reactor vessel. A reactor vessel was charged in a glovebox
with dpa (0.14 g, 0.79 mmol), 2a (0.003 g, 0.0039 mmol), and
MeOH (5 mL, 123 mmol), in 20 mL of toluene. The vessel was
heated to 180 °C for 12 h, and the crude mixture was analyzed
by GC-MS, as done for previous experiments. Conversion of
(26) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley Interscience: New York, 1988; p 743.
392 Inorganic Chemistry, Vol. 48, No. 1, 2009

StereoselectiWe Hydrogenation of Aromatic Alkynes
dpa after reaction: 1% (chromatogram in Figure S12, Supporting
Information) of a mixture of cis- and trans-stb.
illustrated in Figure S14 (Supporting Information), showing the
presence of cis-stb.
Isomerization of cis to trans-stilbene using methanol and
[(dippe)Ni]. A toluene solution (20 mL) of cis-stb (0.158 mL,
0.16 g, 0.90 mmol), 1a (0.003 g, 0.0045 mmol), and MeOH
(5.0 mL, 123 mmol) was heated to 180 °C, for 5 h. Conversion
of cis-stb into trans-stb, after reaction: 37% (figure S13,
Supporting Information).
Catalytic reduction of alkynes using Et3SiH/H2O. A typical
experiment for each alkyne was conducted as follows: dpa (0.165
g, 0.93 mmol), 1a (0.003 g, 0.0045 mmol), Et3SiH (0.297 mL,
1.86 mmol), and H2O (5.03 mL, 270 mmol) were dissolved in
20 mL of THF (a similar procedure described for methanol, vide
supra). At the end of the reaction, the crude mixture was analyzed
and quantified by GC-MS. A representative chromatogram is
Acknowledgment. We thank CONACyT (Grant F80606)
and DGAPA-UNAM (Grant IN202907-3) for the financial
support of this work. We also thank Dr. Miguel Mun˜oz-
Herna´ndez (X-ray), Dr. Alma Are´valo, and Marco G.
Crestani for their technical assistance. R.B.-F. also thanks
CONACyT for a Ph.D. grant.
Supporting Information Available: Complementary experi-
mental procedures, GC-MS determinations, and CIF file for
3a·0.5THF. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC801823X
Inorganic Chemistry, Vol. 48, No. 1, 2009 393