A mild and efficient rhenium-catalyzed transfer hydrogenation of terminal olefins using alcoholysis of amine-borane adducts as a reducing system

Article abstract of DOI:10.1016/j.jorganchem.2011.01.027

[ReBr₂(NO)(CH₃CN)(PTA)₂] (PTA = 1, 3, 5-triaza-7-phosphaadamantane) catalyzes the alcoholysis of ammonia-borane and amine-boranes and the catalytic transfer hydrogenations of various terminal olefins. Excellent yields were achieved at 70 °C in isopropanol using tBuOK as a co-catalyst affording TOF values up to 396 h^-1.

Full text of DOI:10.1016/j.jorganchem.2011.01.027

Journal of Organometallic Chemistry 696 (2011) 1803e1808
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A mild and efficient rhenium-catalyzed transfer hydrogenation of terminal olefins
using alcoholysis of amineeborane adducts as a reducing system
Hailin Dong, Heinz Berke*
Anorganisch-Chemisches Institut, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 November 2010
Received in revised form
[ReBr (NO)(CH CN)(PTA) ] (PTA ¼ 1, 3, 5-triaza-7-phosphaadamantane) catalyzes the alcoholysis of
2
3
2
ammoniaeborane and amineeboranes and the catalytic transfer hydrogenations of various terminal
ꢀ
olefins. Excellent yields were achieved at 70 C in isopropanol using tBuOK as a co-catalyst affording TOF
18 January 2011
ꢁ1
values up to 396 h
.
Accepted 25 January 2011
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Alcoholysis
Amineeborane
Homogeneous catalysis
Rhenium
Transfer hydrogenation
1
. Introduction
Amineeboranes were found by Couturier and co-workers to be
effective hydrogen transfer reagents in methanol requiring
however mediation by palladium centers [38e40]. Related to this
work we approached the ethanolysis of stable amineeborane
adducts proceeding under mild conditions using the rhenium
Amineeborane adducts have become promising candidates as
chemical hydrogen storage materials due to their stability and their
high H storage capacity [1e15]. In addition to this they are widely
2
applied as reagents in organic synthesis and industrial chemical
processes [12,13]. Respective use of amineeboranes was reported
for selective reductions and as hydroboration reagents [1,8]. Ami-
neeboranes are generally soluble in both protic and aprotic
solvents and possess considerable hydrolytic and thermal stability
especially in the solid state, which turned out to be of great prac-
tical advantage [13]. An additional prerequisite for their practical
application is their commercial bulk availability [14,15]. Transfer
hydrogenations [16e27], especially asymmetric transfer hydroge-
nations of organic carbonyl compounds, were found to operate on
the basis of palladium [18], ruthenium [16,17,19,27], rhodium
2 3 2
complex [ReBr (NO)(CH CN)(PTA) ]. In addition we report here
rhenium-catalyzed transfer hydrogenations of various terminal
olefins using amineeboranes as hydrogen donors in the presence of
the base tBuOK used as a co-catalyst.
2. Results and discussion
2.1. Non-catalyzed alcoholyses of amineeboranes
We started our study of transfer hydrogenations comparing the
catalyzed and non-catalyzed decomplexation and ethanolysis rates
[21,24] and iridium [20] catalysts. However, examples for transfer
of various amineeborane adducts in the presence and the absence
hydrogenation of olefins are relatively rare [28e32].
ꢀ
[
ReBr
2
(NO)(CH
3
CN)(PTA)
2
] at room temperature and at 50
C
Recent efforts of our group were devoted to the exploration and
exploitation of rhenium compounds in homogeneous catalysis. In
this context we discovered that rhenium compounds can quite
efficiently catalyze hydrogen or hydrogen related reactions, such as
hydrogenations, dehydrogenations, hydrosilylations, dehydrogen-
ative silylations and transfer hydrogenations [33e37] and several of
these catalyzes involved ammoniaeborane or amineeboranes.
according to Eq. (1). Even though decomplexation and alcoholysis
of amineeboranes are mechanistically distinct steps we will in the
following when appropriate use the respective alcoholysis as
a short notation for both subsequent steps.
1 mol% Cat:
A ꢁ BH þ 3EtOH
!
r:t:or50 C
A þ BðOEtÞ þ3H
3
3
2
ꢀ
(
1)
A ¼ NH ; NH Me; NHMe ; NMe ; NEt ; MeCN
3
2
2
3
3
*
Corresponding author. Tel.: þ41 1 635 46 80; fax: þ41 1 635 68 02.
E-mail address: hberke@aci.uzh.ch (H. Berke).
Â
Ã
Cat: ¼ ReBr ðNOÞðCH CNÞðPTAÞ
2
3
2
0
022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.027

1804
H. Dong, H. Berke / Journal of Organometallic Chemistry 696 (2011) 1803e1808
The reactions were carried out in ethanol acting as a solvent and
eH
2
EtOHeðEtOÞ3ꢁnBH
n
#ðEtOÞ4ꢁnBðH ÞH
#ðEtOÞ BH
4ꢁn
nꢁ1 nꢁ1
H
2
2
reagent in excess. The evolution of dihydrogen gas was pursued
during the reactions furnishing volumetric yields of 91e97%
measured by with a gas burette [41]. In fact, this catalysis turned
out to be generally applicable, since a variety of amineeborane
adducts, like ammoniaeborane and primary, secondary, tertiary
and aromatic amineeborane adducts, showed catalytic enhance-
(
2)
The conversion of the amineeborane adducts was considerably
faster in the presence of the [ReBr (NO)(CH CN)(PTA) ] complex for
both in the absence and in the presence of the rhenium catalyst
ReBr (NO)(CH CN)(PTA) ] (entry 11).
2
3
2
2 3 2
ment in the presence of [ReBr (NO)(CH CN)(PTA) ].
[
2
3
2
Semi-logarithmic plots (activities vs time) of the conversions of
several representative amineeborane examples at two tempera-
tures are presented in Fig. 1. The overall reaction of decomplexation
and ethanolysis of the amineeboranes was found to follow pseudo-
first-order kinetics. The observed rate constants (kobs) determined
from the corresponding plots are listed in Table 1. Some of the
2.2. Catalyzed alcoholyses of amineeboranes
The rates were improved when the reactions were carried out at
room temperature in the presence of 1 mol% rhenium catalyst
according to Eq. (1). The combined reactions of decomplexations
and ethanolyses of ammoniaeborane, dimethylamineeborane,
triethylamineeborane and 4-methylmorpholineeborane were
finished in 15 h or less (entries 1, 3, 5 and 10). The catalysis with
ꢀ
uncatalyzed reactions did not finish after 7 days at 50 C (entries 2, 4,
6
and 8). The decomplexation and ethanolysis of 4-methyl-
morpholineeborane exhibited the highest activity in the absence of
ꢁ
4 ꢁ1
the catalyst with rate constant kobs of 0.5 ꢂ10
s
(entry 10). Other
adducts, such as ammoniaeborane, dimethylamineeborane, trie-
thylamineeborane, pyridineeborane and 2-methylpyridineeborane
completed the reactions with the rate constants kobs of 0.2, 0.1, 0.17,
3
MeCNeBH showed moderate performance at room temperature.
However, only in the case of trimethylamineeborane, the reaction
was still incomplete after 3 days, presumably due to blockage of the
rhenium center by the relatively strong trimethylamine ligand
formed during the reaction course (entry 4). Low oxidation state
rhenium complexes of trimethylamine are known with various
ancillary ligand environments, which makes the existence of amine
ꢁ
4 ꢁ1
0
.32 and 0.25 ꢂ 10
s , respectively (entries 1, 3, 5, 7 and 9). The
ethanolysis of acetonitrileeborane [42] was also tested showing only
medium activity The pseudo-first-order behavior points to a kinetics
with rate determining dissociative replacement of the amines of the
amineeboranes by ethanol (first two steps of Scheme 1).
2 3 2
complexes of the type [ReBr (NO)(NR )(PTA) ] quite plausible
The evolution of hydrogen is thought to proceed via internal
[46,47]. Ammonia stemming from ammoniaeborane could lead
also to a relatively strong ammonia complex. It is anticipated that
all the other amines being constituents of the amineeboranes will
coordinate to the rhenium center with varying strength and
proton transfer in the ethanol adducts to form H
43e45] at the boron centers as sketched below in a general form
for any stage of the ethanolysis of the BeH bonds:
2
complexes
[
ꢀ
Fig. 1. Representative pseudo-first-order plots of the decomplexation of the amineeboranes. C indicates the uncatalyzed reactions at 50 C, - indicates the catalyzed reactions at
ꢀ
room temperature, : indicates the catalyzed reactions at 50 C. Note: amineeBH
3
/ amine þ BH
3
(decomplexation); BH
3
þ 3HOEt / B(OEt)
3
2
þ 3H (ethanolysis).

H. Dong, H. Berke / Journal of Organometallic Chemistry 696 (2011) 1803e1808
1805
Table 1
Ethanolysis of various amineeboranes using [ReBr
in ethanol according to Eq. (1).
2
(NO)(CH
3
CN)(PTA)
2
] as a catalyst
a
obs ( ꢂ 10ꢁ
4 ꢁ1
s )
Entry
Substrate
k
Uncatalyzed
1 mol% cat.
(r.t.)
1 mol%
cat. (50 C)
ꢀ
ꢀ
(50 C)
1
2
3
4
5
6
H
MeH
(Me)
Me
Et NeBH
3
NeBH
3
0.2
1.6 [10]
0.1
0.26
0.17
1.3
0.3
10.7
3.2
5.3
0.55
2.0
3.21
b
2
NeBH
3
2
HNeBH
3
1.3
b
0.17c
1.1
3
NeBH
3
3
3
b
tBuH
2
NeBH
3
0.24
0.8
7
8
9
N BH3
H
0.32
0.54
0.38
0.85
1.6
0.33b [48]
0.25
0.56
4.0
O
N
BH3
N
BH3
Scheme 2. Suggested pathway for the rhenium complex mediated exchange of the
amine with ethanol of amineeboranes.
H
1
0
0.5
1.6
0.9
8.0
2.0
O
N
BH3
2 3 2
suggested that in this process the [ReBr (NO)(CH CN)(PTA) ] cata-
11
MeCNeBH
3
0.28
lyst could act as a Lewis acid with ligand exchange competing for the
amine of amineeborane adducts assisting formation of the crucial
primary intermediate ethanol$BH as shown in Scheme 1.
3
A possible pathway for the rhenium complex mediated exchange
of the amine with ethanol is sketched in Scheme 2. According to this,
2 containing coordinated ethanol could be formed from 1 by
replacement of the acetonitrile ligand with ethanol. The coordinated
ethanol is then substituted by the amine of the amineeborane
a
All catalyzed reactions were carried out with 0.5 mmol of the substrate and
(NO)(CH CN)(PTA) ] in ethanol (0.8e1.0 ml) at
1
mol% rhenium catalyst [ReBr
2
3
2
proper temperature.
b
Percentage of triethyl borate formed after 7 days, based on integration of the 11
B
B
NMR spectrum.
c
Percentage of triethyl borate formed after 3 days, based on integration of the 11
NMR spectrum.
adduct to form the amine complex 3 and the crucial ethanol$BH
3
kinetics. To prove our anticipation a variety of amines such as
dimethylamine, trimethylamine, pyridine and morpholine were
reacted with [ReBr (NO)(NR )(PTA) ]. The reactions did proceed for
2 3 2
intermediate. Complex 3 can be converted back to 2 by replacement
of the coordinated amine with free ethanol so that a catalytic cycle
for the amine substitution in amineeboranes is established.
the cases of pyridine and morpholine presumably due to the effect
of the strong steric hindrance. For the cases of dimethylamine and
trimethylamine, the evidence of the reactions could be obtained
2.3. Hydrogenation of terminal olefins utilizing alcoholysis of
1
31
from the H NMR and P NMR, however, the attempts to isolate the
amine-coordinated rhenium complexes were unsuccessful.
All the substrates, except for trimethylamineeborane, were
completely cleaved by ethanolysis within 10 h when the reactions
amineeborane adducts as reducing systems
The previously described work made plausible that alcoholeBH
3
adducts are crucially involved in the alcoholysis reaction of ami-
neeboranes. Alcohol adducts of BH are however unstable with
respect to release of H . If one envisaged the utilization of such
ꢀ
were carried out at 50 C with a catalyst loading of 1 mol%. For the
3
adducts of ammoniaeborane, dimethylamineeborane, 2-methyl-
pyridineeborane and 4-methylmorpholineeborane, the comple-
2
adducts as hydrogen donors in hydrogenations, the alcohol adduct is
expected to appear in low concentrations generated from a suitable
source, which via equilibria would provide the right concentration
tion of the ethanolysis required 1.5, 3, 4 and 2 h, respectively
ꢀ
(
entries 1, 3, 9 and 10). A comparison of the reaction rates at 50 C
with and without the catalyst furnished acceleration factors
between 2 and 50 in favor of the catalyzed cases.
for stability of the alcoholeBH
3
adduct and its further trans-
formations. We had seen that a major influence in these reactions
came from the kind of the starting amineeborane adduct. Therefore
a screening of various amineeboranes for catalytic transfer hydro-
As expected the quantitative data revealed also significant
dependence of the ethanolysis rate on the type of the amineeborane
adduct. Therefore we presumed that the rate determining step or
steps involved dissociation of the amineeborane and/or the ami-
neerhenium complex and to a lesser extent or not at all the etha-
nolysis steps of the BeH bonds. High overall reaction rates can be
expected when both dissociations are kinetically facile avoiding
blocking of the reactive sites. We reckoned that the dissociation of
the amine/acetonitrileeborane adducts was rate determining to
2 3 2
genation of various terminal olefins with [ReBr (NO)(CH CN)(PTA) ]
as a catalyst was initiated. Not only the amineeboranes in which
amines contain hydrogen but also the ones in which amines possess
no hydrogen like trimethylamineeborane, pyridineeborane and
morpholineeborane worked for the transfer hydrogenation which
exclude the possibility that the amineeboranes can directly act as
a hydrogen donor via the dehydrogenation. Dimethylamineeborane
was chosen as the candidate due to the highest activity it presented
eventually form the kinetically crucial EtOH$BH
3
species. It is
within all the amineeboranes. It was found that terminal alkenes
ꢀ
can be hydrogenated to the corresponding alkanes at 70
C
according to Eq. (2) by transfer of hydrogen during the ethanolysis
reaction of the amineeboranes.
Representative examples of catalytic transfer hydrogenations of
terminal olefins are presented in Table 2. The reactions were carried
ꢀ
out in ethanol at 70 C with 1 mol% of the rhenium catalyst. Except
for 4-bromostyrene and 3-bromostyrene (entries 15 and 16), all the
reactions provided the excellent performance with yields greater
Scheme 1. Uncatalyzed decomplexation and ethanolysis of the amineeborane
adducts.

1806
H. Dong, H. Berke / Journal of Organometallic Chemistry 696 (2011) 1803e1808
than 95% after given reaction times. Among these conversions the
reduction of 1-octene required the shortest reaction time (3 h) to
obtain octane almost quantitatively (entry 1). For other substrates,
such as allylbenzene, allylcyclohexane, vinylcyclohexane, 3-bute-
nenitrile, allylcyclopentane and 3-methylstyrene, less than 10 h
were required to reach excellent yields (entries 1e4, 10 and 12).
Substrates containing hetero-atoms such as 2-chlorostyrene, 2-
methyl-3-buten-2-ol, 3-methoxylstyrene, 4-bromostyrene, 4-fluo-
rostyrene and 2-vinylpyridine etc required 15e48 h to complete the
reactions with high yields indicating that hetero-atoms slowed
down the rate of the transfer hydrogenation presumably through
competition in the coordination to rhenium (entries 6, 8, 9 and
probably due to its high solubility in isopropanol. Several olefin
substrates were then examined in transfer hydrogenations in iso-
ꢀ
propanol at 70 C in the presence of 1 mol% of the rhenium complex
2 3 2
[ReBr (NO)(CH CN)(PTA) ] with addition of 10e15 equivalents of
tBuOK as a co-catalyst and with isopropanolysis of dimethylami-
neeborane as the reducing system [Eq. (3)]. It should be mentioned
at this point that no reaction occurred when tBuOK was added to
the reaction system as the sole catalyst.
Allylcyclohexane gave the best activity after 0.25 h with the
ꢁ1
highest TOF value of 396 h (entry 2). 1-Octene, 3-butenenitrile,
allylbenzene completed the transfer hydrogenation after 0.3, 0.67
and 0.5 h, respectively (entries 1, 3 and 4). For the substrates of 2-
methyl-3-buten-2-ol, 4-methylstyrene, more time (3.0 and 3.5 h,
respectively) was required (entries 5 and 6). Overall, it was
apparent that the reaction rates of the transfer hydrogenations of
terminal olefins were significantly improved by the addition of
tBuOK as a co-catalyst for the reactions comparing with those
13e15). The reduction revealed a great tolerance toward a variety of
functionalities, such as cyano-, methoxy-, fluoro-, chloro- and
bromo-substitution (entries 4, 6, 9, 14, 16, 18 and 19). Attempts to
carry out transfer hydrogenations of internal olefins were, however,
not successful.
Although the catalytic transfer hydrogenations of the unsatu-
rated compounds proceeded smoothly, the catalytic activities
reactions without tBuOK. It should also be mentioned that in the
absence of dimethylamineeborane no transfer hydrogenation
catalysis of terminal alkenes could be observed in presence of
(
TOF’s) were not as good as initially expected. Therefore, we
attempt optimization of these reactions and tested transfer
hydrogenations in the presence basic additives as co-catalysts, such
as tBuOK, KH, NaH, KOH and NaOH. Both the bases (tBuOK) and
2 3 2
[ReBr (NO)(CH CN)(PTA) ] and tBuOK as a base in isopropanol
under the same conditions.
(
KOH) led to improved rates of the transfer hydrogenation,
2.4. Investigation of the reaction mechanism
however, tBuOK worked much better than any other base most
We initially considered that the transfer hydrogenation process
Table 2
could follow the route of a “metal-free” reduction. “Metal-free”
Rhenium complex-catalyzed transferhydrogenation reactions of terminal olefinsusing
the ethanolysis of Me
would mean
a
reaction path related to the Meer-
a
2
HNeBH
3
as a reducing system The reactions refer to Eq. (2).
weinePonndorfeVerley reduction, which is considered to be
concerted during the reaction process. Then we tested the reactions
Entry Substrate
Time (h) Ethanolysisb Hydrogenationc TOFd
ꢁ
(h 1)
(%)
(%)
without the involvement of the rhenium complex [ReBr
(CH CN)(PTA) ] according to Eq. (4). However the reactions did not
proceed at all, which indicated that [ReBr (NO)(CH CN)(PTA) ] was
2
(NO)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1-Octene
3
3.2
8
9
3.9
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
99
99
99
99
99
98
99
99
98
99
99
99
99
95
80
90
97
99
98
33
31
12.4
11
3
2
Allylcyclohexane
Vinylcyclohexane
3-Butenenitrile
Allylbenzene
2-Chlorostyrene
4-Chlorostyrene
2
3
2
not only essential for the decomplexation and alcoholysis of the
amineeboranes but also for the transfer hydrogenations of the
terminal olefins.
25.4
3.1
2.5
3.3
2
12.4
19.8
28.3
6.6
1.9
1.7
2.3
4.2
5.2
3.6
32
40
2-Methyl-3-buten-2-ol 30
4-Methoxylstyrene
Allylcyclopentane
4-Methylstyrene
3-Methylstyrene
3-Methoxylstyrene
4-Fluorostyrene
4-Bromostyrene
3-Bromostyrene
2-Vinylnaphthalene
4-Nitrostyrene
48
8
5
0
1
2
3
4
5
6
7
8
9
3.5
15
48
46
40
23
19
27
A possible mechanism for the alcoholysis of the amineeboranes
and transfer hydrogenation of the terminal olefins catalyzed by the
2-Vinylpyridine
[
ReBr
2
(NO)(CH
(NO)(CH
3
CN)(PTA)
CN)(PTA)
2
] complex is proposed in Scheme 3.
a
All reactions were carried out with 0.25 mmol of Me
2
HNeBH
3
, 0.005 mmol of
[ReBr
] is suggested to takes the role of a pre-
2
3
2
the rhenium catalyst [ReBr
2
(NO)(CH
3
CN)(PTA)
2
] and 0.5 mmol of olefin in ethanol
1
catalyst via primary replacement of the CH
3
CN ligand. In the
H
ꢀ
(
0.8e1.0 ml) at 70 C.
b
11
NMR spectrum this dissociation became evident by a singlet at
1.93 ppm assigned to the CH group of free CH CN. The alcoholysis
of amineeboranes is presented in cycle A of Scheme 3. BH eHOR
Yield on the basis of integration of the B NMR spectrum.
Yield on the basis of integration of the H NMR spectrum.
c
1
3
3
d
Defined as mol product per mol of catalyst per hour.
3

H. Dong, H. Berke / Journal of Organometallic Chemistry 696 (2011) 1803e1808
1807
2 3 2
Scheme 3. Proposed pathways for the alcoholysis of amineeboranes and transfer hydrogenations of olefins catalyzed by [ReBr (NO)(CH CN)(PTA) ].
coordinates to the rhenium center of 4 as a
forming 5. Then oxidative addition of the HOR$BH
rhenium center occurs with concomitant PTA dissociation to form
the Re(III) hydride species 6. Subsequently H reductively elimi-
h
2-BH
3
eHOR ligand
ligand to the
II) containing an atmosphere of purified nitrogen. Solvents were
initially distilled under N atmosphere using standard procedures
and were degassed by freezethaw cycles prior to use. CH CNeBH
[42] and [ReBr (NO)(CH CN)(PTA) ] [48] were prepared according
3
2
3
3
2
2
3
2
nates of with re-coordination of PTA to the rhenium center, re-
establishing the initial complex 4.
to known procedures. Except for the rhenium compounds all the
chemicals were purchased from Aldrich Chemical Co. or Fluka.
Unless otherwise noted all commercial materials (amineeborane
Similarly, the transfer hydrogenation of terminal olefins using
1
11
BH
coordinates to the vacant site of 4 forming the saturated species 7.
Then dissociation of PTA and coordination of BH eHOR to the
3
eHOR as a hydrogen donor is presented in cycle B. The olefin
adducts and olefins) were used without purification. H and
B
NMR spectra were recorded on a Varian Gemini 300 (299.78 MHz),
or on a Varian Mercury 200 (199.78 MHz) spectrometer.
3
rhenium center occurs with the formation of species 8, which
undergoes intramolecular insertion, leading to the Re-alkyl species
4
.2. General procedure for the ethanolysis of the various
9
2
. Then BH eOR dissociates from the rhenium center leading to the
amineeborane adducts according to Table 1
rhenium hydride species 10, which then undergoes reductive
elimination to re-form species 4, resulting in the alkane product
and closing up of the catalytic cycle. Attempts to isolate the hydride
complexes 6 and 10 were not successful. Cycles related to B are
thought to be in operation with BH (OR)eHOR and BH(OR) eHOR.
2 2
Further detailed studies are currently underway in our group to
achieve full understanding of the mechanism.
A solution of the appropriate amineeborane (0.5 mmol) and
mol% of the rhenium catalyst in ethanol (0.8e1.0 ml) was stirred
1
for given time at proper temperature under nitrogen atmosphere.
Upon completion, the reaction mixture was filtered over Celite. The
resulting solution was analyzed B NMR spectroscopy. The yield is
based on integration of the B NMR spectrum.
11
11
3
. Conclusion
4.3. General procedure for the transfer hydrogenation of various
terminal olefins according to Tables 2 and 3
In summary, we have successfully developed the efficient
ethanolysis of various amineeborane adducts catalyzed by the
rhenium complex [ReBr (NO)(CH CN)(PTA) ] under mild condi-
A solution of the proper olefin substrates (0.5 mmol), dime-
thylamineeborane (0.25 mmol) and 1 mol% of the rhenium
2
3
2
tions. In addition, we have demonstrated that rhenium-catalyzed
transfer hydrogenation of various terminal olefins using alcoholysis
of amineeboranes as a reducing system. Transfer hydrogenation of
the olefins can be remarkably accelerated by the addition of tBuOK
Table 3
Rhenium complex-catalyzed transfer hydrogenation of terminal olefins using
tBuOK as a co-catalyst.b
a
ꢁ1
with the TOF values up to 396 h . It was also proven that the
rhenium complex [ReBr (NO)(CH CN)(PTA) ] was indispensable
Time (h) Hydrogenationc (%)
TOFd (hꢁ1
)
Entry Substrate
2
3
2
1
2
3
4
5
6
7
8
9
1-Octene
0.3
99
99
99
99
99
99
99
99
98
330
396
148
198
55
110
33
28.3
45
both for the alcoholysis of the amineeborane and the transfer
hydrogenation of the terminal olefins. We are currently investi-
gating the detailed mechanism for the reactions and the catalytic
transfer hydrogenations of the olefins in water.
Allylcyclohexane
3-Butenenitrile
Allylbenzene
0.25
0.67
0.5
2-Methyl-3-buten-2-ol 1.8
4-Methylstyrene
4-Chlorostyrene
3-Bromostyrene
2-Vinylpyridine
0.9
3.0
3.5
2.2
4
. Experimental section
a
1
0e15 equivalents to the rhenium catalyst.
All reactions were carried out with 0.25 mmol of BH
(NO)(CH CN)(PTA)
4.1. General comments
b
3
eNHMe
2
, 0.005 mmol of
ꢀ
[
ReBr
2
3
2
], and 0.5 mmolofolefinsinisopropanol(0.8e1.0ml)at70 C.
All manipulations were carried out using standard vacuum line,
Schlenk and cannula techniques or in a dry box (M. Braun 150B-G-
c
1
Yield on the basis of integration of the H NMR spectrum.
Defined as mol product per mol of catalyst per hour.
d

1808
H. Dong, H. Berke / Journal of Organometallic Chemistry 696 (2011) 1803e1808
complex (with 10e15 mol% of tBuOK according to Table 3) in the
given solvent (0.8e1.0 ml) was stirred for given time at 70 C under
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nitrogen atmosphere. Upon completion, the reaction mixture was
1
filtered over Celite. The resulting solution was analyzed by H NMR
1
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1
-octene, allylcyclohexane, 4-methylstyrene and 4-chlorostyrene
in isopropanol was treated with 1 mol% of the rhenium complex
and 10e15 mol% of tBuOK, no transfer hydrogenation was observed
at all under the same condition as described above.
[
[
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We gratefully acknowledge financial support from the Swiss
National Science Foundation and the funds of the University of
Zurich.
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