Development of silica-supported frustrated Lewis pairs: Highly active transition metal-free catalysts for the Z-selective reduction of alkynes

Article abstract of DOI:10.1039/c5cy01372k

Supported Lewis acid/base systems based on a triphenyl phosphine fragment and Piers' reagent (HB(C₆F₅)₂) or BArF have been prepared and characterized. Both materials show unprecedented catalytic activity in the Z-selective hydrogenation of 3-hexyne to Z-3-hexene with a selectivity up to 87%. Other alkynes can also be hydrogenated Z-selectively, albeit with moderate yields. The activity of the supported phosphine/HB(C₆F₅)₂ adduct is similar to the only homogeneous example reported thus far based on bridged B/N frustrated Lewis pairs under high hydrogen pressure. Importantly, this transition metal-free supported catalyst was recycled five times in the challenging selective hydrogenation of a non-polar unactivated alkyne.

Full text of DOI:10.1039/c5cy01372k

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R. Gauvin and M. Taoufik, Catal. Sci. Technol., 2015, DOI: 10.1039/C5CY01372K.
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Development of Silica-Supported Frustrated Lewis Pairs: Highly
Active Transition Metal-Free Catalysts for the Z-Selective
Reduction of Alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Kai C. Szeto, Wissam Sahyoun, Nicolas Merle, Jessica Llop Castelbou, Nicolas Popoff, Frédéric
a
a
b
b
c
c
Lefebvre, Jean Raynaud, Cyril Godard, Carmen Claver, Laurent Delevoye, Régis M. Gauvin, and
Mostafa Taoufik*
www.rsc.org/
a
6 5 2
A supported Lewis acid/base systems based on a triphenyl phosphine fragment and Piers’ reagent (HB(C F ) ) or BArF have
been prepared and characterized. Both materials show unprecedented catalytic activity in Z-selective hydrogenation of 3-
hexyne to Z-3-hexene with a selectivity up to 87%. Other alkynes can also be hydrogenated Z-selectively, albeit with
6 5 2
moderate yields. The activity of the supported phosphine/HB(C F ) adduct is similar to the only homogeneous example
reported thus far based on bridged B/N frustrated Lewis pairs under high pressure of hydrogen. Importantly, this
transition metal-free supported catalyst was recycled five times in the challenging selective hydrogenation of a non-polar
unactivated alkyne.
organometallic Pd and Ni complexes as well as early transition
1
3,14
Introduction
metals (Ti, Nb).
The main issue for transition metal-based
catalysis, despite its high activity, is the possible contamination
of the final product by trace metals. For some products,
especially in fine chemistry and pharmaceutical industry, even
trace amounts of toxic or undesired metal must be avoided.
Hence, replacing the transition metal center by a metal-free
active phase while maintaining the catalytic property
Z-alkenes are very important starting materials for a large
1
-5
number of natural and industrially relevant compounds. Yet,
methods to selectively access Z-alkenes are less established
than those yielding the E-isomers due to thermodynamic
control favoring E-alkenes. A wide variety of methods are
amenable to Z-alkenes synthesis (albeit not always general
methods), including ubiquitous reactions in organic synthesis
such as Wittig olefination, cross-coupling, olefin metathesis
1
5
unchanged is of high interest. Only recently were highly
selective metal-free catalysts for alkyne hydrogenation
6
,16-20
reported.
quantities of sterically hindered Lewis acids-bases, a.k.a.
These catalysts are composed of equimolar
5
,6
and partial hydrogenation of alkynes. Among the mentioned
approaches, the most reliable method to obtain Z-alkene
selectively is the partial hydrogenation of the corresponding
alkynes.
2
1
frustrated Lewis pairs (FLP), as introduced by Stephan. FLP
comprise Lewis acidic sites (mainly based on boron) close to
Lewis bases (commonly amines or phosphines) without
creating Lewis adducts. The reactivity of FLP is reported for the
Different homogeneous and heterogeneous alkyne
-10
2
2
23
24
7
activation or fixation of small molecules such as CO2, CO,
2
hydrogenation catalysts have been reported.
The earliest
5
26
27
O, and H
28,29
.
SO
2
,
NO,
N
2
2
In particular, the activation of
selective heterogeneous catalyst was developed by Lindlar and
1
1
H2 has attracted attention to hydrogenation reactions of
consists in Pd supported on CaCO3. Later on, Brown and
Ahuja reported an alternative catalyst based on nickel in the
1
9,28,30-33
imines, carbonyls, nitriles,
substrates such as alkenes, polyaromatics and alkynes.
In the latter case, FLP yield the Z-isomer of the corresponding
and far more challenging
1
6,19,20,34
1
2
presence of ethylenediamine.
Homogeneous partial
hydrogenation of alkyne can obviously be catalyzed by
1
6,19,20
alkenes.
Moreover, H
2
activation was also achieved by
3
5-38
several Lewis acids or bases, for example alanes,
3
,
47-
9-41
42
pyridines, amines,
43-46
borenium cations,
and carbenes.
4
9
However, most FLP-catalyzed hydrogenations employ
amines or phosphines as Lewis bases in combination with
boranes such as B(C as Lewis acids.
6 5 3
F )
Nevertheless, to the best of our knowledge, the concept of
FLP is unknown in transition metal-free heterogeneous
catalysis with a set of Lewis acid/base pair bearing a suitable
anchoring site. Generally, heterogeneous catalysts offer
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several advantages such as easy separation of the catalyst silica-grafted counterpart, and exhibited drastic improvements
from the product, catalyst recycling and the possibility to work in terms of activity, stereoselectivity and stability. These
under continuous flow conditions. In the continuity of our combined results validate our strategy toward both reduced
efforts in the preparation of transition metal-free surface interactions and improved electrophilic effects at the
heterogeneous hydrogenation catalysts prepared by surface active center. Our current strategy to access supported FLPs is
5
0
organometallic chemistry, we have turned our interest to the therefore based on the latter approach, via substitution of the
heterogenization of FLP based on boranes as Lewis acids and hydroquinone ligand by the commercially available (4-
phosphines as Lewis bases. Two strategies can be envisaged: i) hydroxyphenyl)diphenyl phosphine, followed by treatment
6 5 2 6 5
grafting of the Lewis acid on a conventional support, such as with B(C F ) R (R = C F : BArF, R=H: Piers’ reagent). The
silica, followed by coordination of phosphine; ii) grafting of the resulting Lewis acid/base adducts have been characterized and
Lewis base, followed by addition of the boron based Lewis evaluated in the hydrogenation of disubstitued alkynes
acid. Grafting methods of Lewis acids on silica, such as BArF or (Scheme 1).
5
1,52
6 5 2
Piers’ reagent (HB(C F ) ) have already been reported.
However, the resulting surface species undergo interactions
with nearby siloxane ligands due to their high Lewis acidity.
Similar interactions between oxophilic elements or Lewis acids
on silica surface have been described in the literature as
Scheme 1. Selective hydrogenation of alkynes to Z-alkenes.
1
7
53-55
observed by EXAFS and O NMR.
Hence, the grafted Lewis Experimental
acidic center will undergo coordinative competition between
General considerations. All experiments were carried out
under a controlled atmosphere, using Schlenk and glove box
techniques for organometallic syntheses. For the synthesis and
treatments of the surface species, reactions were carried out
using high-vacuum lines (ca. 1 mPa) and glove boxes. Benzene,
toluene and pentane were distilled on NaK-benzophenone and
degassed through freeze-pump-thaw cycles. Alkynes such as 3-
surface siloxane moieties and the chosen Lewis base. In
-
addition, no example of FLP based on borate ( BHR
3
) and
3
borate esters (B(OR) ) has been reported to be active in
catalytic hydrogenation due to continuous redistribution
5
6
between the B-H and B-OR bonds. Surface interactions can
be avoided by introducing a spacer between the surface and
the active center. However, this approach requires the
utilization of non-commercial products and thus needs a multi-
step synthesis of the spacer incorporating the Lewis acid. The
second approach (grafting of phosphine on silica) can be
implemented easily by employing bifunctional phosphines. The
most common ligand types comprise phosphines bearing
alkoxysilane moieties, tri-ethoxysilane being the most popular.
However, this methodology suffers from formation of
electrostatic bonding to the support through the phosphonium
formed, as revealed by classical CP/MAS and HRMAS NMR
hexyne,
1,2-diphenylethyne,
and
hex-2-yn-1-
yloxy)trimethylsilane were purchased from Aldrich and
purified by distillation (or dried under vacuum for 1,2-
diphenylethyne (solid)).The alkyne derived from oleic methyl
ester: methyl octadec-9-ynoate was purchased from Alfa
Aesar© and distilled prior to use. B(C F ) (Strem Chemicals >
6
5 3
9
7%) was dried on Me SiCl, purified by vacuum sublimation
3
19
and controlled by F NMR before use. Piers reagent HB(C F )
6
5 2
70
was synthesized following the literature procedure. (4-
hydroxyphenyl)biphenylphosphine was purchased from ABCR.
SiO2-(700) was prepared from Aerosil silica (Degussa, specific
5
7
techniques.
In addition, preparation of the tethered
phosphine ligand requires a multi-step procedure. One general
strategy to overcome these drawbacks is to treat the surface
silanols with a reactive compound to quantitatively consume
them and at the same time create sufficiently reactive sites to
further react with the spacer. Alkyl aluminum compounds are
very attractive to reach this goal, yet to afford selective
catalytic systems, this strategy requires a single type of
2
-1
surface area of 200 m
⋅
C under high vacuum (about 1 mPa) for 15 h to give a white
g ), which was dehydroxylated at 700
°
2
-1
solid having a specific surface area of 190 m
⋅
g
and 0.7 OH
-2
nm . Gas-phase analyses were performed on a Hewlett-
Packard 5890 series II gas chromatograph equipped with a
flame ionization detector and an Al O /KCl on fused silica
2
3
column (50
performed at the Mikroanalytisches Labor Pascher, Remagen
Germany). It is noteworthy that, during the elemental-analysis
m x 0.32 mm). Elemental analyses were
5
8-65
reactive site, which has been difficult to secure in the past.
We have recently developed the synthesis of new and easily
accessible organic/inorganic hybrid supports involving first
(
procedures, molecules of diethyl ether were released in some
cases, especially if the aluminum atom was bonded to three
oxygen atoms. DRIFT spectra were recorded on a Nicolet 6700
66
grafting of tri-isobutyl aluminum on dehydroxylated silica at
00 °C (to consume all silanol groups), followed by reaction
7
67
with hydroquinone, to selectively afford the phenolic
material [( SiO) (AlO-C -OH)(Et O)] in two simple steps. A
1
13
FT-IR by using airtight cells. Solid-state H MAS, C CP/MAS
19
and F MAS NMR spectra were recorded on a Bruker Avance
≡
2
6
H
4
2
great advantage with this methodology is that all starting
materials are cheap and commercially available. This strategy
has previously been used for the grafting of zirconium,
molybdenum and tungsten complexes through surface
5
00 spectrometer with a conventional double-resonance 4 mm
CP-MAS probe at C2P2 – ESCPE Lyon. The samples were loaded
under argon in a zirconia rotor, which was then tightly closed.
Chemical shifts were given with respect to TMS as external
6
7-69
organometallic chemistry.
The catalytic performance of the
1
13
11
11
B
reference for H and C NMR, and to BF .OEt for B. The
3
31
and P solid-state NMR spectra were acquired on Bruker
2
hybrid material in olefin polymerization and metathesis
reactions was compared to those of the previously described
2
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1
Avance III 800 ( B, 256.71 MHz,
1
31
P, 323.89 MHz)
Results and discussion
spectrometers at a MAS frequency of 20 kHz at UCCS Lille. The
1
1
Silica was first dehydroxylated under high vacuum at 700 °C for
2 h to afford SiO2-700 which only presents isolated silanols.
Treatment of this silica with AliBu in diethylether afforded
bipodal surface species [( SiO) Al(iBu)(Et O)] with
concomitant transfer of isobutyl ligand onto an adjacent silicon
D-HMQC experiments were set up with a B spin echo
selective to the central transition, with pulses of 10 and 20 μs,
1
1
with H π/2 pulse of 4.1 μs on either side of the B π pulse.
11
3
2
71
)
≡
2
2
1,
a
The dipolar recoupling scheme (SR4
μs. The number of scans for each t increment was set to 3072.
The bipopodal species [( SiO) Al(iBu(Et O)], , was prepared
according to literature procedure.
Preparation of benzene solution of (4-
hydroxyphenyl)biphenylphosphine (0.180 g, 0.647 mmol) and
SiO) Al(iBu(Et O)] , , (2 g) was stirred at 25 °C for 12 h.
1
was applied for 800
1
6
6
atom, to yield an adjacent [≡Si-iBu] fragment, as confirmed
by DRIFT (Figure 1, a), elemental analysis (0.8 wt% Al) and H
≡
2
2
1
1
6
6
1
3
MAS NMR (Figure S1a), C CP/MAS NMR (Figure 2a). This
material was then treated with (4-
2:
A
hydroxyphenyl)diphenylphosphine (1.3 eq. per grafted
aluminum) in diethylether for 12 h at room temperature.
Repeated washings with diethylether, followed by evacuation
of the volatiles, afforded a white powder (Scheme 2).
[(≡
2
2
1
After filtration, the solid was washed three times with
benzene. The resulting white powder was dried under vacuum
(
1.34 Pa) and stored in the glovebox.
Preparation of 3a: A benzene solution of HB(C
mmol, 1.5 eq.) and (1g) were stirred at 25 °C for 3 h in
The DRIFT spectrum of the resulting material,
2
, displays
6 5 2
F ) (0.405
-1
new bands at 1501 and 1591 cm , as well as in the 3000-3035
2
-1
cm range assigned to the aromatic ring stretchings
2
double Schlenk. After filtration, the solid was washed three
times with benzene. The resulting white powder was dried
under vacuum (1.34 Pa) and stored in the glovebox.
2
2
ν(Csp =Csp ) and ν(Csp -H), respectively (Figure 1,b) and
attributed to the (4-hydroxyphenyl)diphenylphosphine
3
fragment. The presence of residual alkyls (ν(Csp -H) at 2800-
Preparation of 3b: A benzene solution of B(C
6 5 3
F ) (0.405 mmol,
-1
3
000 cm ) is in agreement with the presence of the inert Si-
iBu fragment. The aluminum content displays minor change
0.75 wt%) after the grafting. The protonolysis of the
aluminum isobutyl fragment by (4-
1.5 eq.) and 2 (1g) were stirred at 25 °C for 3 h in double
Schlenk . After filtration, the solid was washed three times
with benzene. The resulting white powder was dried under
vacuum (1.34 Pa) and stored in the glovebox.
(
hydroxyphenyl)diphenylphosphine ligand is evidenced by the
near quantitative amount of isobutane evolved (0.9 eq. per
grafted aluminum). This is confirmed by the elemental analysis
of carbon (7.0 wt%) and phosphorus (0.9 wt%) (21 C/Al; 1.04
P/Al) which is consistent with one molecule of (4-
hydroxyphenyl)diphenylphosphine grafted on an aluminum
Procedure for the hydrogenation experiments. Typically, an
accurate amount of the catalyst (around 65 mg) was loaded
into a 21 mL Parr autoclave within the glove box. Then 1 ml of
pentane, previously distilled over NaK, further purified by
freezed-pump-thaw cycles, was introduced. 3-hexyne,
previously distilled and purified by freezed-pump-thaw cycle
was then added. The desired amount of hydrogen purified
over deoxo catalyst and zeolite was added to the autoclave
before heating to 80 °C. After stirring for appropriate time via
a magnetic stirrer, all hydrocarbons were distilled to a liquid
nitrogen cooled schlenk flask under high vacuum (about 1
mPa, gentle heating was applied to the autoclave to ensure
complete evaporation) and analyzed by GC (HP 5890
atom, alongside
a Si-iBu fragment (th. 22, without
diethylether, see experimental section). Although not
accounted for in the mass balance analysis, the diethylether
molecule is still present at the aluminum center, as seen by
NMR (vide infra). The surface species on this material,
therefore be described as [( SiO) (AlO-C -P(C
Scheme 2).
2
, can
≡
2
6
H
4
6
H
5
)
2
(Et O)]
2
(
1
The H MAS NMR spectrum of 2 (Figure S1b) displays a new
2 3
chromatograph fitted with an Al O /KCl (50 m x 0.32 mm)
capillary column, FID detector for hydrocarbons). Conversion
and selectivity were calculated with respect to carbon number.
The recycling experiments were performed by adding 3-
hexyne and pentane directly to the autoclave in the glovebox
after having all hydrocarbons distilled and the solid catalyst
remained in the autoclave. When 1 equivalent (with respect to
phosphine content) of Piers’ reagent was employed, the Lewis
acid was first dissolved in toluene and mixed with the catalyst
in the autoclave. After 20 minutes of stirring, toluene was first
evaporated, then 3-hexyne and pentane were added to the
dried solid in the glovebox. Hydrogenation of the other
substrates was performed in the same manner in benzene.
After the given reaction time, the autoclave was quenched and
gently depressurized. The solution was analyzed by GC (HP
5
890 equipped with HP5 (30 m x 0.32 mm) capillary column
Scheme 2. Formation of surface phosphine 2 and supported frustrated Lewis
pairs (FLP), 3a and 3b.
and FID).
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The reaction of material
6 5 2
2 with the Lewis acid HB(C F ) or
tris(pentafluorophenyl)borane (1.3 eq.) was carried out in
benzene for 3 h at room temperature. After multiple washings,
materials 3a and 3b were obtained respectively, as white
powders. These new materials were first analyzed by DRIFT
spectroscopy. The DRIFT spectrum of 3a (Figure 1c) is
-1
moderately altered in the 2800-3200 cm range but displays
-1
new bands at 1645, 1515, and 1472 cm , characteristic of
5
ν(C=C) vibrations from the pentafluorophenyl rings.
1,52,72
-1
Importantly, a band at 2405 cm appeared. This frequency is
in line with ν(BH) of HB(C coordinated to a Lewis base,
3b features similar characteristic bands in the
6 5 2
F )
7
3,74
such as THF.
DRIFT spectrum, apart the absence of the peak at 2405 cm
Figure S2).
The boron (0.27 wt% on 3a and 0.26 wt% on 3b), and
signal at 7.1 ppm, characteristic of the aromatic protons of the aluminium (0.72 wt% on 3a and 0.7 wt% on 3b) loadings on
4-hydroxyphenyl)diphenylphosphine. The signal at 0.14 ppm both materials have been determined by elemental analysis
in the spectrum of (Figure S1a), assigned to the CH
protons (B/Al = 0.92 on 3a and B/Al = 0.88 on 3b). The amount of
-1
(
(
1
2
of the isobutyl aluminum fragment is no longer present. The carbon was found to be 10.56 and 12.16 wt% for 3a and 3b
protonolysis of the isobutyl aluminum fragment is further respectively, corresponding to 33 and 38 C/Al. These data are
1
3
evidenced in the C CP/MAS spectrum by the disappearance in agreement with the expected coordination of Piers’ reagent
of the signal at 20.2 ppm and the decrease in intensity of the and BArF on , and support the formation of proposed surface
signal at 25.6 ppm (Figure 2b), which now corresponds solely species 3a and 3b in Scheme 2.
2
1
13
to the isobutyl silicon carbons. The intense signals at 120-140
ppm are assigned to the CAr-H carbons and the broad and weak not changed compared to that of precursor
The H MAS-NMR and C CP/MAS spectra of 3a and 3b are
(provided in
2
signal at 161.3 ppm correspond to the quaternary CAr-O Figures S3 and S4), as the new species do not comprise
carbons of the (4-hydroxyphenyl)diphenylphosphine moiety. additional protons and no significant shift is induced by borane
The presence of the diethylether molecule coordinated to the coordination (there is indeed a boron hydride in 3a that could
1
11
aluminum center is evidenced by the signals at 13 and 67 not be detected by solid state NMR, even on a H-{ B}-J-
6
7-69
31
ppm.
signal at δ = -6 ppm (Figure 2c), similar to the chemical shift of observe by liquid state NMR). The C CP/MAS spectra
p-hydroxyphenyl)diphenylphosphine in C
(-6.3 ppm). These comprise a broad signal in the aromatic region due to multiple
combined elements demonstrate that
(4- C-F coupling. The presence of the Lewis acid on 3a and 3b can
Material 2
shows in the P MAS NMR spectrum a HMQC MAS spectrum: such signals are notoriously difficult to
7
4
13
(
6 6
D
1
1
19
19
hydroxyphenyl)diphenylphosphine reacts selectively through be confirmed by B and F solid-state NMR. The F solid-
protonolysis of supported Al-iBu fragment by the phenolic state NMR spectrum of 3a displays three large signals around -
moiety to achieve formation on the targeted phosphine 126, -158 and -165 ppm, as defined by deconvolution of the
species.
spectrum (Figure S5a). The intensities and chemical shifts of
these three signals are characteristic of the ortho, para and
meta fluorine atoms, respectively, on the coordinated
HB(C F ) with triphenylphosphine in agreement with
6
5 2
7
5
19
literature data. The solid state F NMR spectrum of 3b
Figure S5b) shows three large signals (-134, -160, -167 ppm)
having the comparable chemical shifts as reported for the
(
7
6
molecular adduct [PPh •BArF].
3
1
The more informative B spectrum of 3a displays a single
1
signal at -24 ppm, previously attributed in homogeneous
studies to the tetrahedral boron from [PPh
3
•HB((C
6 5
F )] adduct
7
figure S6). The P NMR signal has shifted from -6.3 ppm in
5
31
(
2
1
to 10.1 ppm in 3a. The B- P correlation spectrum in Figure 3
1
31
was acquired using the D-HMQC pulse sequence, which
7
7,78
correlates nuclei through their dipolar coupling.
The signal
3
1
at 10.1 ppm in the P dimension correlates with the signal at -
1
4 ppm in the B dimension.
1
2
Furthermore, using scalar coupling filtering, namely
31
B{ P}-J-HMQC 1D spectrum, the signal at -24 ppm is
1
1
observed, unambiguoulsy confirming the presence of a P-B
bond, and thus formation of an adduct. This definitively rules
4
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6
(
2-[bis(pentafluorophenyl)boryl]-N,N-dialkylanilines),
7
that
9
requires
a
multiple-step synthesis.
Furthermore, its
homogeneous nature also prevents recycling of the catalyst.
Stereoselective heterogeneous catalytic hydrogenation of
3
-hexyne has been demonstrated with 3a and 3b at 80 °C in a
batch reactor. Already after 2 hours (10 bar H , 2 mol% catalyst
2
in pentane), 3a converts 3-hexyne to mainly Z-3-hexene (86%
selectivity) with a conversion of 13% (Table 1, Entry 1). The
minor products are hexane (about 4%), E-3-hexene (about
4.5%), E-2-hexene (about 3%) and Z-2-hexene (about 2.5%).
Formation of 2-hexenes stems from isomerization of Z-3-
hexene through hydroboration/retrohydroboration reactions,
6
as observed in homogeneous catalysis. Interestingly, the
substrate scope can be extended to other alkynes with some
restrictions (Table 2). If 1,2-diphenylethyne can be
advantageously converted to Z-1,2-diphenylethylene albeit
with a lower reaction rate probably due to steric hindrance,
both (hex-2-yn-1-yloxy)trimethylsilane and methyl octadec-9-
1
1
31
Figure 3. MAS-D-HMQC experiment of 3a to a pair of nuclei { B, P}.
out the assignment of the signal to the [HB(C
1
6
F
5
)
4
] anion, ynoate yield corresponding Z-alkenes very selectively (>99%)
which also features a B chemical shift of -25.5 ppm, close to though in modest quantities. We suspect a strong interaction
1
2
9
that of 3a
Figure S7), leading to a correlation between signals δ( B)= -9 substrates. Remarkably, the molecular analogous FLP:
.
Similarly the same experiment preformed on 3b of the Lewis pair with the polar moieties of these particular
1
1
(
3
1
ppm and δ( P)= 17 ppm confirmed the coordination of BArF HB(C
F
)
/PPh
system is inactive in hydrogenation of 3-
3
6
5
2
7
6
to
surface organometallic chemistry two new supported using Piers’ reagent along with different Lewis bases. The
materials based on
tethered phosphine (Lewis base) reason is due to protonation of B-C from the
combined with Piers’ reagent or BArF (Lewis acid). These well- phosphonium ion, obtained after activation on the
defined materials were evaluated in the highly challenging Frustrated Lewis pair, leading to release of
heterogeneous hydrogenation of 3-hexyne to Z-3-hexene.
pentafluorobenzene and loss of the Lewis acidity (see Scheme
2
as reported in the literature. We have thus prepared by hexyne. Such observation has previously been observed when
6
a
a
F
6 5
H
2
8
0
FLP are known to catalyze hydrogenation of organic 3). Such bi-molecular decomposition process is common in
substrates containing double or triple bonds. Substrates homogeneous catalysis. Advantages of a heterogenized system
containing heteroatoms are generally fairly easy to reduce due (beside already mentioned reasons, vide supra) are fixing the
to the polarity of the molecules which facilitates the transfer active sites on the surface and consequently avoid bi-
of the Lewis acidic proton. However, hydrogenation of molecular decomposition processes. Such phenomenon has
unactivated alkynes is more challenging. Only one recent already been observed in homogeneous catalysis for example
example reported by Papai and Repo of a homogeneous in alkene and alkyne metathesis reaction with molecular
8
1,82
system demonstrates hydrogenation of 3-hexyne yielding complexes based on Mo and W.
mainly Z-3-hexene. Their system is based on a bridged B/N FLP
Table 1. Catalytic hydrogenation of 3-hexyne and other alkynes at 80 °C by 3a and 3b.
Selectivity (%)
E-2-hexene
Time
(h)
Conversion
(%)
Pressure
Catalyst
Entry
Catalyst
H
2
(bar)
Z-3-
hexene
(mol%)
E-3-hexene
Hexane
Z-2-hexene
1
2
3
4
5
6
7
8
2
4
10
10
10
10
20
40
40
40
13.2
34.7
99.2
99.6
74.8
98.7
80.1
9.0
86.9
85.3
83.8
83.4
86.8
78.5
85.1
81.7
4.3
5.2
5.6
6.2
4.3
6.8
4.5
5.0
2.6
3.1
3.5
3.6
2.7
4.0
2.8
2.7
3.7
4.3
4.2
4.2
3.7
8.0
5.3
6.0
2.5
2.1
2.9
2.6
2.5
2.6
2.3
4.6
3a
3a
3a
3a
3a
3a
3a
3b
2
2
2
2
2
2
1
1
8
16
4
4
4
4
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Table 2. Catalytic hydrogenation of selected alkynes by 2 mol% 3a at 80 °C,
1
6 h, 10 bar hydrogen.
Substrate
Conversion
%)
Z-alkene
(%)
E-alkene
(%)
(
1
,2-diphenylethyne
25.1
15.2
5.0
88
12
(
hex-2-yn-1-yloxy)trimethylsilane
>99
trace
trace
methyl octadec-9-ynoate
>99
Kinetic studies reveal that an increase of reaction time also
increases the conversion (Figure S8a), and full conversion is
reached after 8 hours (Table 1, Entries 2-4). The selectivity is
virtually unaffected by the reaction time (Figure S8b).
Furthermore, hydrogen pressure increase has a beneficial
influence on conversion (Table 1, Entries 2, 5 and 6). The
selectivity remains unchanged (around 85% Z-3-hexene)
except for the experiment at 40 bar with full conversion, in
which the selectivity of Z-3-hexene decreases to 78% and the
amount of hexane (around 8%) and other hexenes increases
slightly. Higher yield of Z-3-hexene is obtained (80%
2
conversion at 40 bar H , 1 mol% catalyst in pentane, 4 hours)
with lower catalyst loading (table 1, entry 7). Under the same
conditions, 3b gives low conversion (9%) of 3-hexyne with a
selectivity of 82% toward Z-3-hexene (Table 1, Entry 8).
These results have shown that the heterogeneous FLP 3a
catalytically reduces 3-hexyne into Z-3-hexene with high
selectivity. The activity is comparable to the only example
6
reported under homogeneous conditions and high pressure.
In order to take advantage of the heterogeneous nature of this
new catalyst, its recycling was studied at 40 bar, 80 °C, 1 mol%
of 3a, 4 hours reaction time. The first cycle showed a drastic
drop in conversion (26%), most probably due to the loss of the
6 5 2
Lewis acid (HB(C F ) ) during the extraction of the products
under high vacuum. In these reactions, the Piers’ reagent
content is about 7 mg and is therefore easily stripped off with
the volatile products. The selectivity remains high (86%).
Nevertheless, the catalytic activity was restored, even after 5
6 5 2
cycles, when adding 1 equivalent of HB(C F ) in the recycling
process (Figure 4), with retention of similar high selectivity
toward Z-3-hexene. In a continuous process, rather than
subsequent batch cycles, recycling would likely improve. The
catalytic mechanism may be described by two pathways: i)
traditional approach comprising heterolytic hydrogen splitting depicted in Scheme 4). Further experimental and theoretical
followed by protonation of hexyne, or ii) 3-hexyne activation studies may provide further mechanistic insights of this
via hydroboration accompanied by heterolytic hydrogen system. In particular, the substrate scope of this
splitting and protonation, recently proposed by Papai and hydrogenation method could be extended by appropriately
Repo based on experimental and theoretical data, and further selecting the Lewis pair and surface tether to better
6
,16,20
reported more recently.
Current results support the latter accommodate polar moieties. Ongoing efforts are now
mechanism, as 3b is an order of magnitude less active than 3a dedicated to the generalization of this metal-free Z-selective
which already contains a B-H moiety that can directly undergo hydrogenation of alkynes.
hydroboration (first step in the proposed catalytic cycle
Conclusions
The first supported FLP systems, 3a and 3b, based on a
triphenyl phosphine fragment and either Piers’ reagent or BArF
have been prepared and characterized. Both materials show
unprecedented
catalytic
activity
in
stereoselective
hydrogenation of 3-hexyne to Z-3-hexene while the molecular
6 5 2 3
Scheme 3. Restructuration of [HB(C F ) ]⋅[PPh ] in solution after hydrogen
addition.
6
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counterpart is not active. Other internal alkynes can also be 16. D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2015, 54
,
6
1
400-6441.
7. B.-H. Xu, G. Kehr, R. Froehlich, B. Wibbeling, B. Schirmer, S.
hydrogenated selectively to the corresponding Z-alkene, albeit
in moderate yields. The obtained activity of 3a is similar to the
only homogeneous example reported catalyzed by a bridged
Grimme and G. Erker, Angew. Chem. Int. Ed., 2011, 50, 7183-
186.
7
B/N FLP. Importantly, 3a can be recycled when adding 1 18. J. Paradies, Angew. Chem. Int. Ed., 2014, 53, 3552-3557.
equivalent of the Lewis acid. A fully recyclable FLP could be 19. D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 10018-10032.
2
3
2
2
0. Y. Liu, L. Hu, H. Chen and H. Du, Chem. Eur. J., 2015, 21,
495-3501.
obtained by grafting a bridged frustrated B/P Lewis pair, which
will be the topic of future investigation. Current work has
demonstrated a new strategy to access a metal-free active
1. D. W. Stephan, Org. Biomol. Chem., 2008,
2. D. W. Stephan and G. Erker, Chem. Sci., 2014,
6
, 1535-1539.
, 2625-2641.
5
phase for the stereoselective hydrogenation of alkynes, an 23. C. M. Momming, E. Otten, G. Kehr, R. Frohlich, S. Grimme, D.
important step in the synthesis of natural and industrially W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2009, 48, 6643-
6
2
1
2
646.
relevant compounds.
4. M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2010,
32, 13559-13568.
5. K. V. Axenov, C. M. Momming, G. Kehr, R. Frohlich and G.
Erker, Chem. Eur. J., 2010, 16, 14069-14073.
26. A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme, A.
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Acknowledgements
This work was supported by the French Government through
the “MATCALCAT” ANR Project (contract ANR 07-CP2D-21-02)
2
2
011, 50, 7567-7571.
7. E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc.,
and the Competitiveness Cluster on Chemistry and the 2009, 131, 9918-9919.
2
8. G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124-1126.
Environment of Lyon 1 Rhône-Alpes (Axelera). We also thank
the CNRS and the French Ministry of Research and Higher
Education (Ph.D. grant to N.P.) for their financial support.
Financial support from the TGIR-RMN-THC Fr3050 CNRS for
2
1
3
9. G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129
880-1881.
0. T. Soos, Pure Appl. Chem., 2011, 83, 667-675.
,
,
conducting the research is gratefully acknowledged. Bertrand 31. D. W. Stephan, Org. Biomol. Chem., 2012, 10, 5740-5746.
3
4
3
2. D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2010, 49
6-76.
3. D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J.
Hastie, S. J. Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C.
Welch and M. Ullrich, Inorg. Chem., 2011, 50, 12338-12348.
Doumert is thanked for his assistance with solid-state NMR.
The URV team is grateful to the Spanish Ministerio de
Economía y Competitividad (Ramón y Cajal to C.G., CTQ2010-
15835) for financial support.
34. M. Lindqvist, K. Borre, K. Axenov, B. Kotai, M. Nieger, M.
Leskela, I. Papai and T. Repo, J. Am. Chem. Soc., 2015, 137, 4038-
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3
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041.
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