Exceptionally selective catalytic hydrogenation of alkene with pinacolborane

Article abstract of DOI:10.2174/157017812800233723

Abstract: The catalytic hydrogenation of model alkene - cyclohexene with the use of pinacolborane is reported along with the characterization of various side products. The non-standard hydrogenation product was obtained in high yield and purity under very

Full text of DOI:10.2174/157017812800233723

Letters in Organic Chemistry, 2012, 9, 257-262
257
Exceptionally Selective Catalytic Hydrogenation of Alkene with
Pinacolborane
Joanna Nizioł and Tomasz Ruman*
Rzeszów University of Technology, Faculty of Chemistry, 6 Powstańców Warszawy Ave. 35-959 Rzeszów, Poland
Received August 07, 2011: Revised January 23, 2012: Accepted February 01, 2012
Abstract: The catalytic hydrogenation of model alkene – cyclohexene with the use of pinacolborane is reported along
with the characterization of various side products. The non-standard hydrogenation product was obtained in high yield and
purity under very mild reaction conditions with the use of iridium catalyst. The exceptionally high selectivity of the
hydrogenation process is described for the first time in literature.
Keywords: Borane, Catalysis, Hydrogenation, Iridium, Pinacolborane.
1. INTRODUCTION
2. MATERIAL AND METHODS
1H NMR spectra were obtained with Bruker Avance II
spectrometer, operating in the quadrature mode at 500MHz.
The residual peaks of deuterated solvents were used as
internal standards. GC-MS analysis was carried out on
Agilent Technologies 6890N apparatus with 5973-Network
mass detector. HRMS spectra were obtained with the use of
Bruker MicroTOF-Q (Ag+ Ionspray). All other reagents and
deuterated solvents of the highest commercially available
grade were purchased from Aldrich and used without further
purification. Rubber septa joints were purchased from
Aldrich.
The olefin hydrogenation is an extremely important
chemical modification used in organic synthesis both on the
small and industrial scale. Most of the hydrogenation
processes utilize catalytic systems based on transition metal
complexes and gasous hydrogen at moderate to high
pressures and also often at high temperatures [1].
There are only few reports on the hydrogenation
processes conducted in mild conditions (atmospheric
pressure, room temperature) that use various boranes as
hydrogen sources [2]. The first reports of hydrogenation
process presented high-pressure hydrogen addition catalyzed
by simple boranes such as triisobutylborane at rather
demanding conditions (48h, 180-235oC, 6-48 hours) [3]. The
Kochi group described original hydrogenation method with
the use of borane-methyl sulfide complex acting as hydrogen
donor. The mentioned reaction requires additional mild one-
electron oxidant such as Orange-CRET•+ and was proved to
be effective for sterically demanding olefins [4]. The later
reports demonstrated effective transfer-hydrogenation of
olefins catalyzed by ReBr2(NO)(PR3)2L (where L = H2,
CH3CN and ethylene, R = iPr and Cy) employing
dimethylamine-borane complex as a hydrogen source [5].
There were also reports of hydrogenation by
dehydrocoupling of amine-boranes catalyzed by Rh colloids
[6], Ti- and Ni-complexes [7, 8] and also by iridium-pincer
complexes [9].
2.1. Reaction A
The catalytic hydrogenation process in CDCl3
environment was performed by mixing cyclohexene (0.1
mmol) in CDCl3 (0.6 ml) with iridium catalyst ([Ir(cod)Cl]2
1.5 µmol) and dppp (3 µmol) followed by PinBH addition
(0.12 mmol). The reaction mixture in NMR tube was then
vigorously shaken and inserted into NMR apparatus.
Analytical data presented below were collected after 370 min
1
(NMR) or 1-2 days (MS). H NMR (CDCl3) δ [ppm]: 8.02 -
7.15 (m, 20H, Ph, dppp); 5.62-5.72 (m, 2H, 1); 1.93-2.04 (m,
4H, 1); 1.72 - 1.66 (m, 4H, 3); 1.55-1.66 (m, 6H, 1); 1.52
(Cyclooctane) 1.42 (s, 12H, 2); 1.40 - 1.27 (m, 6H, 3); 1.26
(s, 12H, PinBOH) 1.24 (s, 12H, 3); 1.23 (s, 12H, (PinB)2O);
1.04 - 0.93 (m, 1H, 3). 13C NMR (CDCl3) δ [ppm]: 127.2 (1,
=CH); 83.1, ((PinB)2O, -C-CH3); 82.7 (3, -C-CH3); 27.9 (3);
27.1 (3); 26.9 (2, -CH2-); 26.8 (3); 25.1 (1, -CH2CH=); 24.7
(3, -CH3); 24.5 ((PinB)2O, -CH3); 22.6 (1, -CH2CH2-). 11B
NMR (CDCl3) δ [ppm]: 33.9 (3), 22.3 (PinBOH), 21.1
((PinB)2O). GC-MS (ES) m/z: 270 ((PinB)2O); 210 (3); 208
(4); 112 (Cyclooctane); 84 (2); 82 (1).
The present work describes unusual activity found in
applied catalytic system that is very similar to the one
reported by Miyaura group [10]. Mentioned work reported
mainly hydroboration processes taking place in the reaction
mixture. Our experimental data suggest that the main
reaction in applied catalytic systems is hydrogenation of
cycloalkene to cycloalkane (Scheme 1).
2.2. Reaction B
The catalytic hydrogenation process in benzene-d6
environment was performed by mixing cyclohexene (0.1
mmol) in benzene-d6 (0.6 ml) with iridium catalyst
([Ir(cod)Cl]2 1.5 µmol) and dppp (3 µmol) followed by
*Address correspondence to this author at the Rzeszów University of
Technology, Faculty of Chemistry, 6 Powstańców Warszawy Ave. 35-959
Rzeszów, Poland; Tel: +48178651896; Fax: +48178543655;
E-mail: tomruman@prz.edu.pl
1875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

258 Letters in Organic Chemistry, 2012, Vol. 9, No. 4
Nizioł and Ruman
O
O
O
O
PinBH, cat.
B
B
+
+
4
1
2
3
Cl
Ir
CH3
CH3
P
cat. =
P
CH4
Scheme 1. The hydrogenation (2), hydroboration (3) and dehydrogenative borylation (4) of cyclohexene catalyzed by iridium(I) complex.
Fig. (1). Catalytical hydrogenation of cyclohexene in benzene-d6. Spectra from bottom to top were recorded after 5, 30, 55, 455 and 3120
min respectively.
PinBH addition (0.22 mmol). The reaction mixture in NMR
tube was then vigorously shaken and inserted into NMR
apparatus. Analytical data presented below were collected
after ca. 3100-3200 min (NMR) or following 1-2 days (MS).
1H NMR (C6D6) δ [ppm]: 8.50 - 6.50 (m, 20H, Ph, dppp);
5.67-7.70 (m, 2H, 1); 1.88-1.93 (m, 4H, 1); 1.47-1.53
(Cyclooctane); 1.50 (m, 6H, 1); 1.40 (s, 12H, 2); 1.04 - 0.93
(m, 1H, 3). DEPT 135 (C6D6) δ [ppm]: 127.6; 127.1 (1,
=CH-); 27.0 (2, CH2); 25.2 (1, CH2CH=); 24.7; 24.6; 24.4
((PinB)2O, CH3); 22.7 (1, CH2CH2). 11B NMR (C6D6) δ
[ppm]: 28.6 (PinBH); 21.9 ((PinB)2O).
2.3. Reaction C and D
The catalytic hydrogenation process (Reaction C) in
CDCl3 environment was performed similarly to Reaction A
with the larger amount of catalyst ([Ir(cod)Cl]2 - 15 µmol
and dppp - 30 µmol). Reaction D was similar to A with the
PinBH amount of 1.2 mmol.
3. RESULTS AND DISCUSSION
The first experiment that was conducted in benzene-d6
with 3% molar ratio of catalyst and 2 eq. of pinacolborane in

Exceptionally Selective Catalytic Hydrogenation of Alkene with Pinacolborane
Letters in Organic Chemistry, 2012, Vol. 9, No. 4
259
A
O
Cl
O
P
P
P
Ir
Ir
O
P
O
Cl
B
Cl
HBPin
Cl
Ir
COD
O
COD
O
P
P
P
P
Ir
O
B
O
H
O
O
HBPin
1
HOBPin
O2
PinBOBPin + H2
CH4
CH4
Cl
Cl
P
P
P
O
O
CH4
CH4
Ir
Ir
CH4
CH4
O
CH
CH 4
CH
CH
4
4
COD
CH
CH
4
B
4
4
CH
CH44
P
CH
CH4
CH44
H
H2
HOBPin
COD
Cl
CH4
CH4
Cl
Ir
P
O
O
O
P
CH4
Ir
CH4
CH
CH44
O
CH
CH
CH
CH
CH44
CH
CH4
4
4
CH44
CH
CH
B
CH
CH44
4
4
CH4
O
B
P
4
CH4
CH44
CH44
CH
CH
4
P
CH4
CH
CH4
H
H
O
H
H
CH4
CH4
CH4
2
CH
CH
4
4
CH
CH
4
4
CH
CH4
CH44
CH4
CH4
CH4
CH4
CH4
Scheme 2. Oxygen-rich dimeric iridium complex (A) and catalytic mechanism for the hydrogenation of cyclohexene with pinacolborane (B).
the sealed NMR tube gave suprisingly clear 1H NMR
spectrum (Reaction B, Fig 1). The NMR-controlled
experiment clearly showed the diminishing of cyclohexene
(1) resonances (resonance centers at 5.69, 1.90, 1.50 ppm)
down to ca. 82.0% after 50 min and 9.6% of their initial
areas after 3120 minutes.
Moreover, the disappearance of 1 resonance was
accompanied with growing singlet resonance at 1.40 ppm.
The CH3 resonances of pinacol-related moieties also under
went visible changes where starting singlet resonance at 0.99
ppm disappeared during the reaction course while another
singlet had risen at 1.01 ppm. What is more, after 3120 min

260 Letters in Organic Chemistry, 2012, Vol. 9, No. 4
Nizioł and Ruman
of the reaction also the BH resonance (wide multiplet
resonance of visible quartet structure at 4.79 - 3.75 ppm) had
experiment) that formed during the reaction was assigned as
cyclooctane which must have been synthesized in the
hydrogenation of cyclooctadiene ligand. The 11B NMR
spectrum recorded after 370 min showed lack of
characteristic PinBH doublet but other interesting singlet
resonances were found – very small at 34.0 along with two
much larger at 22.3 and 21.1 ppm that most probably belong
to the hydroboration product 3, PinBOH and (PinB)2O
respectively [10,11]. The areas of the substrate 1 resonances
were also decreasing being ca. 78 % after 50 min and 60%
after 370 min of reaction time proving noticeably higher
reaction rate in CDCl3 solvent. What is more, we have
noticed that there were no PinBH resonances after 370 min
of reaction with no further reaction progress as it can be
judged from 1H NMR.
completely disappeared (Fig
1
and Supplementary
Materials). The various NMR data (1H NMR, DEPT135, 13
C
NMR, 13C-1H HSQC, COSY) and MS experiments (ESI-MS
and Ag+ ionspray MS) confirmed the identity of newly
formed product to be cyclohexane. The 11B NMR spectrum
of the reaction mixture after 7 min has shown doublet
resonance at 28.6 ppm (1JBH = 174 Hz) from pinacolborane,
disappearing wide singlet at 23.0 ppm, rising wide singlet
resonance at 21.9 ppm and very small resonance at 31.3 ppm
the last three being most probably the PinBOH, PinB2O and
3 respectively [10,11]. The most striking observation is that
we were unable to find expected substantial amounts (only
traces were found in GCMS analysis) of alternative products
of hydroboration (3) or dehydrogenative borylation (4)
The only reasonable catalytic mechanism for processes
observed should be based on the assumption that both
hydrogen atoms that are introduced by the catalyst to the
unsaturated moiety should originate from PinBH. Based on
this assumption it can be concluded that reaction should stop
after consumption of roughly 55-60% of the olefin 1 for the
applied PinBH:substrate molar ratio. Indeed, this hypothesis
was observed and confirmed in our borane-deficient CDCl3
experiment which thereby gave crucial mechanistic
information. Another experiment with 10:1 PinBH:1
(Reaction D) molar ratio applied has shown the reaction rate
to decrease. Similarly, the use of larger amount of catalyst
(Reaction C) also didn’t accelerate this reaction in a
significant manner. Moreover, two described modified
experiments did not show high hydrogenation selectivity that
was observed in two main experiments (Reaction A and B).
1
processes as judged from H NMR. It could be concluded
that the hydrogenation reaction was the main process in our
catalytic system.
The analogical NMR-controlled experiment was
conducted in the similar conditions (PinBH:1 molar ratio of
1.2:1) using chloroform-d environment (Reaction A, Fig. 2).
The observations were almost identical when compared to
benzene-d6 system (vide supra). The areas of resonances of
the 1 (5.67, 1.99, 1.61 ppm) were decreasing during the
reaction course with only 1.42 ppm singlet resonance rising.
The 13C NMR data also confirmed our observations. The
newly formed 1.42 ppm singlet belongs to the carbon atom
at 26.9 ppm, a value that is almost identical with literature or
reference NMR data for this environment. The resonances in
the pinacol-Me and BH region also behaved in the similar
manner compared to the C6D6 experiment. The small singlet
resonance at 1.52 ppm (CH2 as judged from DEPT135
The tested reaction conditions were almost exactly the
same as reported by Miyaura [10] both with and without
Fig. (2). Catalytical hydrogenation of cyclohexene in chloroform-d. Spectra from bottom to top were recorded after 5, 20, 30, 50 and 370
min respectively

Exceptionally Selective Catalytic Hydrogenation of Alkene with Pinacolborane
Letters in Organic Chemistry, 2012, Vol. 9, No. 4
261
Fig. (3). Hydride region in 1H NMR spectrum of the reaction mixture (CDCl3 environment) registered after 700 min of reaction.
oxygen in the system. The NMR spectra of the reaction
mixture in oxygen-free system (process conducted both in
the 10 ml reaction flask and NMR tubes with nitrogen
degassing) were similar to reported results [10]. The reaction
conducted with availability of molecular oxygen produced
mainly hydrogenation product. It should be noted that
Hadebe and Robinson reported failure in trans-4-octene
hydroboration with pinacolborane when RhCl(PPh3)3 was
used. The same group has found that this reaction was
catalyzed by O2-treated RhCl(PPh3)3, a new complex which
contains two rhodium ions bridged by two dioxygen
molecules. It is noteworthy to mention that the new complex
was formed only by aeration of the precursor’s solution [12].
The existence of the one of main products – (PinB)2O
suggests that oxygen may in fact be responsible for
unexpected catalytic activity [13]. The water as the
oxygen (both in the form of iridium complex and in the gas
phase) for the processes described.
The hydride region (Fig. 3) of the catalytic system could
bring crucial information to the discussion of the catalytic
mechanism. The most upfield shifted three triplets (2JPH = 11,
9 and 14 Hz for resonances at -16.3, -24.2 and -25.6 ppm
respectively) are in the typical positions for iridium
monohydride species where hydride is most probably axially
aligned rel. to two phosphorous donors [15]. Another
interesting doublet hydride resonance was found at -4.34
ppm (2JPH = 83 Hz) suggesting that phosphine ligand may
partially dissociate during the reaction. The large value of
2
coupling constant JHP may suggest that hydride is trans to
the phosphorus atom. The resonances in the -4 – -9 ppm
region may also originate from σ-borane complexes [16].
1
oxygen/hydrogen donor can be excluded on the basis of H
NMR spectra. The proposed reaction mechanism is shown in
Scheme 2 (B). As it can be concluded from catalytic
mechanism, the presence of oxygen may promote
PinBOBPin and also hydrogen formation, the latter being
most likely responsible for the alkene hydrogenation.
Moreover, singlet resonance (4.56 ppm) in 1H NMR
spectrum (Reaction B) is in perfect alignment with data for
o-H2 moiety coordinating to the iridium ion [14]. Ionspray-
MS (Ag+ ionization) experiments partially confirmed the
existence of IrH3Cl(1)(OBPin) which is one of the crucial
intermediates in catalytic cycle (sample from Reaction C).
The suggested catalyst dimeric iridium starting form
(Scheme 2, A) is very similar to the one found by Robinson
group [12]. The observed insignificant changes of the
reaction rate in the experiments with large amounts of
pinacolborane or catalyst may suggest that the reaction rate
could be dependent on the oxygen diffusion rate. The simple
calculations confirmed that reaction system contains enough
CONFLICT OF INTEREST
Declared none.
ACKNOWLEDGEMENTS
Supported by the Ministry of Science and Higher
Education of Poland, Grant No. N204 226940.
SUPPLEMENTARY MATERIALS
Supplementary material is available on the publishers
Web site along with the published article.
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