Semihydrogenation of alkynes in the presence of Ni(0) catalyst using ammonia-borane and sodium borohydride as hydrogen sources

Article abstract of DOI:10.1016/j.apcata.2010.06.052

The selective semihydrogenation of terminal and internal alkynes was achieved using complexes of the type [(P-P)Ni(η²-C,C-alkyne)] (P-P = chelating diphospine ligands), which behave as the active catalysts and were generated in situ. Boron-hydride derivatives such as ammonia-borane (AB) or sodium boron hydride (SBH) were used as hydrogen sources. In the case of internal alkynes, the stereoselective formation of cis- or trans-alkenes was achieved in high yields and under mild reaction conditions.

Full text of DOI:10.1016/j.apcata.2010.06.052

Applied Catalysis A: General 385 (2010) 108–113
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Semihydrogenation of alkynes in the presence of Ni(0) catalyst using
ammonia-borane and sodium borohydride as hydrogen sources
∗
Rigoberto Barrios-Francisco, Juventino J. García
Facultad de Química, Universidad Nacional Autónoma de México, México D.F. 04510, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The selective semihydrogenation of terminal and internal alkynes was achieved using complexes of
the type [(P-P)Ni(␩ -C,C-alkyne)] (P-P = chelating diphospine ligands), which behave as the active cat-
alysts and were generated in situ. Boron-hydride derivatives such as ammonia-borane (AB) or sodium
boron hydride (SBH) were used as hydrogen sources. In the case of internal alkynes, the stereoselective
formation of cis- or trans-alkenes was achieved in high yields and under mild reaction conditions.
Received 18 March 2010
Received in revised form 29 June 2010
Accepted 30 June 2010
2
Available online 7 July 2010
©
2010 Elsevier B.V. All rights reserved.
Keywords:
Alkyne semihydrogenation
Nickel complexes
Ammonia-borane
Sodium borohydride
Hydrogen sources
1
. Introduction
made selective for alkenes, inevitably over-reducing the alkynes
to their corresponding alkanes [6c]. Although, ammonia-borane
(AB) adduct has been less used due to its high cost, the devel-
opment of new methodologies for its synthesis have gradually
compensated this inconvenience [2]. AB has shown high activity
as a reductant in the presence of polar unsaturations and only a
handful of examples that address the reduction of nonpolar moi-
eties have been reported [3e]. Recently, our group disclosed the
stereoselective reduction of aromatic alkynes by hydrogen transfer,
The interest in the chemistry of boron hydrides has increased
considerably due to some appealing and interesting features as
is for instance, their high hydrogen content, their stability, safe-
handling, environmentally benign properties and the possibility to
use them as hydrogen storage vessels for onboard engines [1]. Their
study has been classified in three main areas: (i) large-scale synthe-
ses and re-use of waste products [2], (ii) reactivity in the presence
of heterogeneous and homogeneous catalysts for the release of
hydrogen [3], and (iii) use in synthesis [4]. Historically, sodium
borohydride (SBH) has represented a valuable tool in organic syn-
thesis as a reducing agent, affording high activity and selectivity
over polar moieties; a characteristic that has not been successfully
applied over nonpolar unsaturations such as those of alkenes and
alkynes [5]. On this regard, a variety of systems based on transi-
tion metals catalysts have been developed. For example, the use of
SBH and electric reduction with a cathodic voltage in the presence
of acetylene [6a] or dithiolate–ferrous chloride complexes with an
excess of SBH (5.0 equiv. per alkyne) obtaining cis-alkenes in mod-
erate to good yields but with low selectivity [6b]. Recently, Cordes
et al. reported a rather simple methodology for the hydrogenation
of alkenes and alkynes to the corresponding alkanes, catalyzed by
metallic palladium using 4 equiv. of SBH and 2 equiv. of AcOH. In
the case of alkynes however, the reduction process could not be
2
making use of diphosphine complexes of the type [(P-P)Ni(␩ -C,C-
alkyne)] (P-P = 1,2-bis(di-isopropyl-phosphinoethane, 2, or 1,2-bis
(di-terbutylphosphinoethane, 3) generated in situ. The reduction
occurred stoichiometrically in water (metal-mediated process) and
could be made catalytically in a mixture of Et SiH/water or using
3
methanol (metal-catalyzed processes). In all these cases, the sub-
strates acted as the hydrogen sources and led to the formation of
cis- or trans-alkenes in high yields [7]. With the aim of developing
novel methodologies for the selective semihydrogenation of inter-
nal and terminal alkynes, the reactivities of 2 and 3 were assessed in
stoichiometric and catalytic proportions, using SBH or AB as hydro-
gen transfer agents under relatively mild conditions; the results of
which studies are disclosed herein.
2. Results and discussion
2
The
reactivity
of
[(dippe)Ni(␩ -C,C-dpa)]
(dpa =
diphenylacetylene; 2a), was first addressed in the presence of AB
at stoichiometric level. The complex was prepared following the
methodology reported by Jones et al. using the Ni(I) hydride dimer
∗ Corresponding author. Tel.: +52 55 56223514; fax: +52 55 56162010.
E-mail address: juvent@servidor.unam.mx (J.J. García).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.052

R. Barrios-Francisco, J.J. García / Applied Catalysis A: General 385 (2010) 108–113
109
Scheme 1.
[
(dippe)Ni(␮-H)]2 (1a), and 2 equiv. of dpa in THF-d8 [8]. The reac-
respectively. The complete tables that summarize the optimization
of this reaction are included in S.I. section (see tables S1 and S2).
The reaction is highly sensitive to solvent interactions and thus,
significant differences in selectivity to the isomeric products were
obtained in different solvents (Table 1).
tion resulted in formation of 2a, whose presence was confirmed by
1
31
1
H and P{ H} NMR spectroscopies. AB (1 mol.equiv.) was added
◦
to the solution and the latter was heated to 65 C for 11 h, resulting
in complete consumption of AB as deemed by 11B{ H} NMR. That
1
2
leads to the formation of the alkene complexes [(dippe)Ni(␩ -C,C-
As shown in the table, a high selectivity to trans-stb (86% yield;
entry 4, Table 1) was found in methanol, which is consistent with
our previous finding that the latter solvent allows the isomeriza-
tion of produced cis- to trans-alkenes [7]. Currently we have found
that the use of coordinating solvents completely reverses the selec-
tivity to the trans-alkene, regardless of the polarity of the solvent.
As such, the ether-type solvents, THF or 1,4-dioxane, resulted in
formation of cis-stb in 75 and 83% yield, respectively (entries 1
and 2, Table 1). Acetonitrile, which is both polar and coordinat-
ing resulted in formation of cis-stb in 90% yield (entry 5, Table 1).
The latter is highlighted provided the selectivity that the nickel
catalyst apparently has for the alkyne function of the substrate
over the CN of the nitrile, that is well established to coordinate
to nickel(0) [9]. This preference however, is not be expected to
be maintained for the alkenes as no isomerization take place to
any significant extent under these reaction conditions. Overall, we
rationalize these findings in the light of the competence that the
coordinating solvents effect over the available sites in the unsatu-
2
cis-stb)] (cis-stb = cis-stilbene; 4a), and [(dippe)Ni(␩ -C,C-trans-
stb)] (trans-stb = trans-stilbene; 5a), which were also corroborated
by multinuclear NMR. The additional presence of other species in
solution such as the alkyne-bridged complex {[(dippe)Ni] (␮-C,C-
2
dpa)} (6a), and free dibenzyl (Ph–CH –CH –Ph; see Supplementary
2
2
Information, S.I. section) was also determined during the reaction
follow-up. The presence of dibenzyl in particular is important as
it evidences partial over-reduction of dpa. Worth to note, under
the above conditions the starting material, 2a, was not consumed
completely and hence, a second equivalent of AB was required.
Complexes 2a and 6a were consumed completely after 19 h yield-
ing 4a (26%), 5a (42%) and dibenzyl (32%) as the final products.
Scheme 1 summarizes these findings.
As shown in Scheme 1, an excess of AB leads to the
formation of dibenzyl and the final fates of the remaining
fragment [(dippe)Ni ] (7a) are the trihydride bridged complex
0
[
[
(dippe) Ni (␮-H) ][BH ] (8a), phosphine-borane adduct namely
2 2 3 4
0
(BH )(i-Pr) P–CH CH –P(i-Pr) (BH )] (9), and metallic nickel.
rated 14e catalyst of [Ni ] fragment, 7a, that ultimately block the
3
2
2
2
2
3
1
Noteworthy, no dihydrogen formation was detected in the
H
subsequent isomerization reaction (see Scheme 1). When the com-
2
NMR spectra of the reaction follow-up at any time, probably due
to a more favorable hydrogen transfer process as opposed to
dehydrocoupling–hydrogenation, reported to occur in a separate
alkene reduction system that also uses AB [3e]. The final fate of AB
products was confirmed to be as previously reported by Baker and
co-workers [3j], to yield a variety of cyclic B–N products (see S.I.
section, Fig. S6).
On using complex 2a at a catalytic proportion (1 mol.% gener-
ated in situ), complete conversion of dpa in the presence of AB was
observed after 6 h at 80 C, in THF. NMR follow-up of the reduc-
plex [(dtbpe)Ni(␩ -C,C-dpa)] (3a) was used as the catalyst under
the conditions described for entry 1 of Table 1, a complete con-
version of dpa was observed. The selectivities to cis- and trans-stb
were determined to be 91 and 9% respectively, which are higher
than those obtained on making use of THF only. A plausible expla-
nation for these results is proposed on the basis of the large steric
hindrance that the bound diphosphine ligand possibly effects over
the generated alkenes, which in this case results in a higher selec-
tivity to the cis-stb. Table 2 summarizes the main results for the
reduction of a variety of internal and terminal alkynes employing
AB as hydrogen source and complex 2a as catalyst.
◦
tion reaction verified formation of cis-stb (94%) and trans-stb (6%),

1
10
R. Barrios-Francisco, J.J. García / Applied Catalysis A: General 385 (2010) 108–113
Table 1
Entrya
Solvent
Conv. (%)b
100
100
100
100
96
dpa:cis-stb:trans-stb
1
2
3
4
5
THF
1,4-Dioxane
Toluene
Methanol
Acetonitrile
0:75:25
0:83:17
0:82:18
0:14:86
4:90:6
a
◦
All reactions were performed under argon atmosphere for 72 h at 80 C, maintaining the ratios of dpa:AB:1a at 200:200:1.
Conversions were determined on the basis of H NMR and GC–MS.
b
1
Table 2
Entrya
Substrate
Conditions
Conv. (%)b
alkyne:cis-alkene:trans-alkene
◦
◦
1
2
3
4
R1 = 4-MeO–C6H4R2 = C6H5
MeOH, 72 h, 80
C
C
100
81
99
0:2:98
19:72:9
1:77:22
6:92:2
0:100:0
0:0:100
0:100
3:97
0:100
0:100
1
1
1
1
1
1
1
1
1
2
◦
R
R
R
R
R
R
R
R
R
= 4-MeO–C6H4R = C6H5
THF, 24 h, 80
C
2
= 2-F–C6H4
= 2-F–C6H4
= 4-CHO–C6H4 R = C6H5
= C6H5
= C6H5
= C6H5
= 4-MeO–C6H4
= 4-MeO–C6H4
R
R
= C6H5
MeOH, 72 h, 80
2
◦
= C6H5
THF, 24 h, 80
C
94
c
2
◦
◦
◦
5
MeOH, 72 h, 80
MeOH, 72 h, 80
C
C
C
100
100
100
97
100
100
2
6
7
8
9
R = Me
R2 = H
MeOH, 24 h, 80
2
◦
R
= H
THF, 24 h, 80
C
2
◦
R
R
= H
= H
MeOH, 24 h, 80
C
2
◦
1
0
THF, 24 h, 80
C
a
b
c
Reactions were performed under argon atmosphere using ratios of alkyne:AB:1a of 200:200:1.
Conversions were determined on the basis of H NMR and GC–MS.
CHO group was reduced to CH2–OH.
1
No over-reductions of the starting alkynes to the corresponding
the reaction media took place under such conditions and the forma-
tion of cis- and trans-stb was verified. Table 3 summarizes the series
of experiments which lead to the optimization of this reaction.
The results obtained suggest that the selectivity to either cis- or
trans-stb is dependent on the amount of SBH added to the mix-
ture, relative to dpa. As examples, cis-stb in the ratio 77:3 was
preferentially obtained when 1.0 equiv. of SBH was added whereas
formation of trans-stb in a ratio 4:96 was favored when 2 equiv.
were used instead (entries 1 and 5). The greater selectivity to trans-
stb in the presence of excess of SBH can be explained on the basis
alkanes were observed in any of these reactions and hence they
show that the stereoselective semihydrogenation of the former can
be achieved in catalysis with high conversions and with a wide
variety of acetylenes. Noteworthy, when a potentially active moiety
for reduction such as a formyl group was present in the starting
acetylene (entry 5, Table 2), both the triple bond and the carbonyl
were reduced and an alcohol was produced.
When SBH was used in the reduction of dpa using the in situ gen-
erated complex 2a (1 mol.%) as catalyst in THF, no conversion or H2
evolution was detected. The reaction was performed in a closed
of a higher H pressure generated in this reaction, which assists the
2
◦
flask heated at 80 C for 72 h. As encountered, the reaction is also
isomerization of cis-stb. The rationale is consistent with a model of
kinetic-to-thermodynamic olefin isomerization. A related example
of this type of isomerization has been reported for the catalytic
solvent sensitive and the reduction of dpa could be achieved using
a 1:1%v/v mixture of MeOH/THF. On using SBH evolution of H from
2
Table 3
Entrya
dpa:SBH
Conversion (%)
dpa:cis-stb:trans-stbb
1
2
3
4
5
1.0:1.0
1.0:1.2
1.0:1.4
1.0:1.7
1.0:2.0
1.0:2.0
80
87
98
100
100
0.8
20:77:3
13:82:5
2:79:19
0:32:68
0:4:96
c
6
99:0.5:0.3
a
All reactions were performed in a closed flask equipped with Rotaflo® high-vacuum stopcock.
Yields were determined on the basis of H NMR and GC–MS.
Reaction carried out in the absence of metal catalyst.
b
c
1

R. Barrios-Francisco, J.J. García / Applied Catalysis A: General 385 (2010) 108–113
111
reduction of alkynes using palladium complexes in the presence of
dihydrogen pressure [10]. Complete tables for the optimization of
this reaction are included in S.I. section.
−44 ppm. All coupling constants (J values) are given in Hz. Melting
points of purified organics were determined using an Electrother-
mal Digital Melting Point Apparatus by capillary method.
3
. Experimental
3.2. Hydrogenation of alkynes using ammonium-borane as
reductant
3.1. General considerations
2
3
.2.1. Preparation of [(dippe)Ni(ꢀ -C,C-dpa)] (2a)
Unless otherwise noted, all manipulations were performed
using standard Schlenk and glovebox techniques under high purity
argon (Praxair, 99.998%). An MBraun glovebox operating under
To a dark red solution of [(dippe)Ni(␮-H)] , 1a (0.120 g,
.138 mmol) in toluene (5.0 mL) was added dpa (0.064 g,
.372 mmol). The mixture was stirred at room temperature allow-
2
0
0
1
ppm of H O or O was used. Liquid substrates were reagent
2 2
ing all the hydrogen gas generated from the reaction to vent
into the glovebox. After 30 min, a yellow solution was obtained.
The solvent was removed in vacuo using a Schlenk line and the
remaining residue was dried under vacuum for 3 h. The residue
was re-dissolved in hexane and the solution transferred by can-
ula to a second flask from which the solvent was again evaporated.
A yellow solid remained and this was further dried in vacuo for
grade and were degassed by three consecutive freeze-pump-thaw
cycles as per standard procedure, prior to their use. All alkynes,
standard cis- and trans-stilbene (cis-stb and trans-stb) were pur-
chased from Aldrich and were stored in a glovebox. Acetonitrile
and methanol were dried by standard methods and stored over 4 Å
molecular sieves, under argon. Regular solvents (THF, 1,4-dioxane
and hexanes, J.T. Baker) were reagent grade and were dried and
deoxygenated by distillation from purple benzophenone ketyl solu-
tions, under argon. Toluene was refluxed under argon over sodium
for 1d and then distilled and stored in the glovebox. Deuterated
solvents for NMR experiments were purchased from Cambridge
Isotope Laboratories (CIL) and were stored over 3 Å molecular
sieves in the glovebox for at least 24 h, prior to usage. Ammonium-
borane (AB, 97% purity) and sodium borohydride (SBH, >98% purity)
were purchased from Aldrich. The chelating bisphosphine lig-
ands (a) dippe (1,2-bis-di-iso-propylphosphinoethane) [11] and
h. Yield of 2a: 95% (0.176 g). NMR data for 2a: 1H NMR (THF-
d8): ı 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, CHMe ),
4
(
1
2
.65 (d, JH–P = 9.2 Hz, 4H, PCH CH P), 1.08 (quintet, JH–H = 7.2 Hz,
2 2
13
1
JH–P = 7.2 Hz, 24H, CHMe ).
C{ H} NMR (THF-d ): ı 141.3 (t,
2
8
2
2
JC–P = 7.1 Hz, C–Ph), 132.1 (s, Ar CH), 128.3 (d,
C–Ph), 127.3 (s, Ar CH), 124.9 (s, Ar CH), 26.7 (t, J
2.2 (t, JC–P = 19.1 Hz), 20.4 (t, JC–P = 4.0 Hz), 19.1 (s). P{ H} NMR
J
= 8.6 Hz,
C–P
= 10.4 Hz),
C–P
31
1
2
(
THF-d ): ı 80.9 (s).
8
(
b) dtbpe (1,2-bis-di-tert-butylphosphinoethane) [12] were syn-
thesized according to the reported methods. The nickel(I) dimer
2
3
.2.2. Reduction of dpa with [(dippe)Ni(ꢀ -C,C-dpa)] (2a) using
[
(dippe)Ni(␮-H)]2 (1a), was prepared from an hexane slurry of
(dippe)NiCl ] [13] using LiHBEt3 (Super-Hydride), similarly to the
AB as hydrogen source (stoichiometric experiment)
[
2
Into a NMR tube equipped with a Young’s valve was charged a
solution of 2a (45 mg, 0.09 mmol) in 0.7 mL of deuterated solvent
literature procedure [14]. The analogous dimer [(dtbpe)Ni(␮-H)]2
1b) was prepared by a similar procedure using [(dtbpe)NiCl ].
(
2
(
THF-d ), followed by the addition of 1.0 equiv. of AB (0.003 g,
Neutral alumina was used for the purification of 1, previously
8
◦
0.09 mmol). The tube was closed in the glovebox, taken out and
dried at 200 C under vacuum, for 48 h. All other substances, fil-
◦
1
heated to 65 C for 11 h in a thermostated silicon oil bath. H and
ters and chromatographic materials were reagent grade and were
used as received. The organometallic complexes and organics pro-
duced in this work were purified either by crystallization or column
chromatography. Only Schlenk flasks equipped with Rotaflo high-
vacuum stopcocks were used for catalysis experiments, all of which
were charged in the glovebox. The crude reaction mixtures recov-
ered from these were analyzed immediately by GC–MS. GC–MS
determinations were performed using a Varian Saturn 3 equipped
with a 30 m DB-5MS capillary column. The conversions resulting
from the catalytic runs were determined considering the inte-
grated chromatographic peaks of unreacted materials and their
products. The retention times of all products were established
against pure standards. Multinuclear NMR spectra of organometal-
lic complexes and organics were recorded at ambient temperature
31
1
P{ H} NMR spectra of the heated mixture were periodically
collected. A second equivalent of AB was added and resulting
mixture heated at the same temperature for a period of 8 h,
after which time the 1H and P{ H} peaks of complex 2a and
31
1
of AB were no longer observable in the spectra. The formation
2
2
of [(dippe)Ni(␩ -(C–C)-trans-stb] (5a) [(dippe)Ni(␩ -(C–C)-cis-
stb] (4a) and dibenzyl (Ph–CH –CH –Ph) in ratios of 1.6:1.0:1.2
respectively, were determined on the basis of H NMR comparing
vinylic protons of 5a, 4a and the methylene protons of dibenzyl
with signals at ı = 4.23, 3.95 and 2.86 ppm. The presence of addi-
2
2
1
i
i
tional complexes such as [(BH )( Pr) P–CH –CH –P( Pr) (BH )]
3
2
2
2
2
3
(
[
9), {[(dippe)Ni] (␮-C–C-dpa)} (6a) (temporarily formed) and
2
(dippe) Ni (␮-H) ][BH ] (8a) was also established from the NMR
2
2
3
4
1
1
13
1
follow-ups; see Figs. S1–S6 (vide infra). H NMR data [7,8,15]
using a 300 MHz Varian Unity spectrometer. The H, C{ H} and
3
1
1
11
1
of the mixture after reaction completion (THF-d , 300 MHz): ı
P{ H} and B{ H} NMR spectra of the organometallic com-
plexes were obtained using concentrated solutions of the pure
8
2
(
ppm) 4.23 {s, [(dippe)Ni(␩ -(C–C)-{trans-C(H)(Ph) C(Ph)(H)}]},
2
1
13
1
3.94 {s,[(dippe)Ni(␩ -(C–C)-{cis-C(H)(Ph) C(Ph)(H)}]}, 2.86 [s,
compounds in THF-d8 or toluene-d . The H and C{ H} NMR
8
(
Ph–CH –CH –Ph)], -13.34 {quintet, JH–P = 23.7 Hz [(dippe) Ni (␮-
spectra of the purified reduction products were obtained from
concentrated CDCl3 or DMSO-d6 solutions. All air and moisture
sensitive samples were handled under inert atmosphere using thin
wall (0.38 mm) WILMAD NMR tubes equipped with Young’s valves.
The samples were heated using stirred thermostated silicon oil
2
2
2
2
13 1
H) ][BH ]}.
C{ H} of the mixture after reaction completion
3
4
2
(
(
{
THF-d , 75 MHz): ı (ppm) 52.7 {t, J
= 8.62 Hz [(dippe)Ni(␩ -
8
P–C
50.7
C–C)-{trans-C(H)(Ph) = C(Ph)(H)}]},
{t,
JP–C = 9.22 Hz,
39.0 (s,
2
s,[(dippe)Ni(␩ -(C–C)-{cis-C(H)(Ph) C(Ph)(H)}]},
3
1
1
1
Ph–CH –CH –Ph).
P{ H} NMR data for the mixture after
baths. H chemical shifts (ı, ppm) are reported relative to residual
2
2
13
1
reaction completion (THF-d , 121.3 MHz): ı (ppm) 97.7 (s, 8a),
proton resonances in the deuterated solvent. C{ H} NMR spec-
8
11
1
6
9.5 (s, 5a), 66.1 (s, 4a), 37.3 (br. d, 9). B{ H} of the mixture
after reaction completion (THF-d , 159.1 MHz): ı (ppm) 29.4 (br.
tra reported relative to the reference signal of the corresponding
deuterated solvent. 3 P{ H} NMR spectra are reported relative to
1
1
8
1
1
1
s, c-[BH NH] ), 24.7 (br. s, c-[BH NH] ), −12.8 (s, c-[BH –NH ] )
external 85% H PO . B{ H} NMR spectra are reported relative to
5
3
2
2 3
3
4
i
i
−
46.4 [d, JB–P = (BH )( Pr) P–CH –CH –P ( Pr) (BH )].
an external aqueous solution of sodium borohydride, calibrated to
3 2 2 2 2 3

1
12
R. Barrios-Francisco, J.J. García / Applied Catalysis A: General 385 (2010) 108–113
3
3
.3. Catalytic reduction of dpa using nickel(0) and AB
3.4.2. Catalysis assessments at different ratios of dpa:SBH
Schlenk flasks were charged in separate runs with dpa (0.16 g,
1 equiv. 0.92 mmol) and 1a (0.003 g, 0.005 equiv. 0.0045 mmol) and
a different amount of SBH: (a) 1 equiv. (0.035 g, 0.92 mmol), (b)
1.2 equiv. (0.042 g, 1.11 mmol), (c) 1.4 equiv. (0.049 g, 1.3 mmol),
(d) 1.7 equiv. (0.059 g, 1.58 mmol) and (e) 2.0 equiv. (0.071 g,
1.83 mmol). All the mixtures were dissolved in a 1:1%v/v mixture
.3.1. Catalysis assessments using different solvents
A series of Schlenk flasks (50 mL) equipped with inner mag-
netic stirring drive were charged in the glovebox with dpa (0.16 g,
equiv. 0.92 mmol), 1a (0.003 g, 0.005 equiv. 0.0045 mmol) and
1
AB (0.029 g, 1 equiv. 0.92 mmol). Each mixture was dissolved in a
different solvent (10 mL): THF, 1,4-dioxane, toluene, methanol or
acetonitrile. The flasks were closed, taken out of the glovebox and
◦
of THF:MeOH (10 mL). The flasks were heated at 80 C for 72 h.
Figs. S26–S30 show the chromatographic plots for these runs.
placed in thermostated oil baths. The solutions were then heated at
◦
8
0 C for 72 h. The vessels were opened to air and the crude reaction
3.4.3. Catalysis assessments at different reaction times using SBH
(Table S4)
Similar to previous experiments, flasks were charged with
dpa (0.16 g, 0.92 mmol), 1a (0.003 g, 0.0045 mmol) and SBH
mixtures analyzed by GC–MS. Figs. S7–S11 illustrate the chromato-
graphic plots of each system after reaction. The conversions of dpa
into cis- and/or trans-stb were determined by GC–MS.
(
0.071 g, 1.83 mmol), dissolved in 10 mL of THF:MeOH 1:1%v/v.
◦
The flasks were heated at 80 C for (a) 6 h, (b) 18 h and (c) 72 h.
Figs. S29, S31 and S32 show the GC plots obtained from these
experiments.
3
(
.3.2. Catalysis assessments at different reaction times using AB
Table S1)
Flasks were charged and handled as described in Section 3.1
with dpa (0.16 g, 1 equiv. 0.92 mmol), 1a (0.003 g, 0.005 equiv.
.0045 mmol) and AB (0.029 g, 1 equiv. 0.92 mmol). Each mixture
3
(
.4.4. Catalysis assessment at different temperatures using SBH
Table S5)
Flasks were charged with dpa (0.16 g, 0.92 mmol), 1a (0.003 g,
.0045 mmol) and SBH (0.071 g, 1.83 mmol) in 10 mL of THF:MeOH
0
was dissolved in THF (10 mL) and the closed flasks carefully heated
◦
to 80 C for (a) 12 h, (b) 16 h, (c) 24 h and (d) 72 h. Figs. S7, S12–S14
0
1
illustrate the relevant chromatographic plots for these experiments
after reactions.
◦
◦
◦
:1%v/v. Catalysis was evaluated at (a) 20 C, (b) 55 C and (c) 80 C
for 18 h. The GC plots from these experiments are included in
Figs. S32–S34.
3
(
.3.3. Catalysis assessments at different temperatures using AB
Table S2)
Flasks were charged as described above with dpa (0.16 g, 1 equiv.
.92 mmol), 1a (0.003 g, 0.005 equiv. 0.0045 mmol) and AB (0.029 g,
equiv. 0.92 mmol), and then dissolved in THF (10 mL). Heating of
3.4.5. Catalysis assessment using compound 1b as catalytic
precursor (Table S6)
0
1
dpa (0.141 g, 0.79 mmol), SBH (0.060 g, 1.58 mmol) and 1b
(
0.003 g, 0.005 equiv. 0.0039 mmol) were mixed following the
◦
◦
◦
the flasks was set at (a) 40 C, (b) 60 C and (c) 80 C for equal periods
of 72 h. Figs. S7, S15 and S16 illustrate the chromatographic plots
of these systems after reactions.
◦
above procedures and the mixture heated to 80 C for 72 h. Fig. S35
shows the GC plot for this experiment.
3.4.6. Catalytic reductions of other alkynes using SBH and
3.3.4. Catalysis using [(dtbpe)Ni(ꢁ-H)]2 (1b) as catalytic
nickel(0) (Table S7)
precursor (Table S3)
Catalytic runs were prepared using 1a (0.003 g, 0.0045 mmol),
SBH (0.071 g, 1.83 mmol) and the alkyne of interest (0.92 mmol).
As per standard procedure, reactions were performed in 10 mL of a
mixture of THF:MeOH 1:1%v/v.
A flask was charged with dpa (0.14 g, 0.79 mmol), AB (0.024 g,
0.79 mmol), and 1b (0.003 g, 0.0039 mmol; add mol%) in THF
◦
(
10 mL). The mixture was heated to 80 C for 72 h. Fig. S17 shows
the chromatographic plot for this experiment.
4. Conclusions
3
3
.3.5. Catalytic reductions of other alkynes using AB and nickel(0)
.3.5.1. Catalysis in methanol. Schlenk flasks (50 mL) were charged
We have shown that the stereoselective semihydrogenation of
internal and terminal alkynes is feasible using boron hydride’s
derivatives as hydrogen sources. The process can be made selec-
tively while avoiding the competing over-reduction process and is
catalyzed by Ni(0) complexes under mild reaction conditions that
give rise to alkenes in high yields. The best catalyst for the semi-
hydrogenation was found to be 2a to yield cis-stb in dioxane and
to yield trans-stb in methanol. On using SBH the choice of catalyst
precursor allowed the selective preparation of trans-stb for 1a or
cis-stb for 1b.
with 1a (0.003 g, 0.0045 mmol), AB (0.029 g, 0.92 mmol) and the
alkyne of interest (0.92 mmol), methanol (10 mL) was added. The
flasks were heated at 80 C for 72 h. Figs. S18–S21 show the chro-
matographic plots obtained from these experiments.
◦
3.3.5.2. Catalysis in THF. Schlenk flasks (50 mL) were charged with
1
a (0.003 g, 0.0045 mmol), AB (0.029 g, 0.92 mmol), and the alkyne
of interest (0.92 mmol) dissolved in THF (10 mL). The systems were
heated at 80 C for 24 h. Figs. S22–S24 show the chromatographic
◦
plots obtained from these experiments.
Acknowledgements
We thank PAPIIT-DGAPA-UNAM (IN-201010) and CONACYT
080606) for their financial support to this work. We also thank
to Dr. Alma Arévalo and Dr. Marco G. Crestani for their technical
support. RBF also thanks CONACYT for a Ph.D. grant.
3
3
.4. Catalytic reduction of dpa using nickel(0) and SBH
.4.1. Reaction of dpa with SBH in the absence of a nickel(0)
(
catalyst
A Schlenk flask was charged with dpa (0.16 g, 0.92 mmol) and
SBH (0.071 g, 1.83 mmol) and then dissolved in a 1:1%v/v mixture
of THF:MeOH (10 mL). The flask was heated at 80 C for 72 h. No
Appendix A. Supplementary data
◦
conversion was observed after reaction. Fig. S25 shows the chro-
matographic plot for this run.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.06.052.

R. Barrios-Francisco, J.J. García / Applied Catalysis A: General 385 (2010) 108–113
113
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