Asymmetric reduction and hydrogenation over heterogeneous catalysts prepared by reacting nickel-boride with norephedrine

Article abstract of DOI:10.1016/S1381-1169(99)00212-5

Nanoparticles of NiB₂ were prepared by reducing nickel iodide with lithium borohydride in anhydrous oxygen-free THF. The hydroxy group and the amino-group of norephedrine reacted with boron atoms, evolving 1 mol of hydrogen for each mole of amino alcohol to afford a chiral oxazaborolidine anchored at the surface of the particles. The reduction of acetophenone utilizing borane-THF as reducing agent in the presence of 1 equivalent of NiB₂ bound oxazaborolidine afforded 1-phenylethanol was excellent enantioselectivity (ee). The oxazaborolidine was strongly bound to the particles since the catalyst could be recycled at least two times with no performance loss. The catalyst hydrogenated 4-methylpentan-2-one, isophorone, and acetophenone, into 4-methyl-pentan-2-ol, dihydroisophorone, and 1-phenylethanol, respectively, in any case the S configuration predominated slightly. The ee was constant when the catalyst was reused. The catalytic hydrogenation over an oxazaborolidine boron bound to Ni particles was not useful in the enantioselective synthesis of alcohol.

Full text of DOI:10.1016/S1381-1169(99)00212-5

Ž
.
Journal of Molecular Catalysis A: Chemical 150 1999 267–273
www.elsevier.comrlocatermolcata
Asymmetric reduction and hydrogenation over heterogenous
catalysts prepared by reacting nickel-boride with norephedrine
)
K. Molvinger, M. Lopez, J. Court
UniÕersite J. Fourier, CNRS-UMR 5616, BP 53X, 38041 Grenoble Cedex 9, France
´
Received 4 February 1999; accepted 8 April 1999
Abstract
The reduction of acetophenone with borane–THF over heterogeneous catalysts prepared by reacting nickel-boride with
. Ž . Ž
Ž
.
Ž
.
Ž
.
1R,2S - y -norephedrine, afforded R - q -1-phenylethanol with high enantioselectivity ees90% . The catalysts can be
recycled two times with little or no loss of performance. The excellent enantioselective properties resulted from the
formation of 1,3,2-oxazaborolidine which is strongly anchored at the surface of the nickel-boride. Used for the hydrogena-
tion of acetophenone, 4-methylpentan-2-one and isophorone, the catalyst hydrogenated with a slight predominance of the S
configuration. The ee was poor but it remained constant when the catalyst was reused. q 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Asymmetric reduction; Nickel-boride; Norephedrine; Acetophenone; 4-Methylpentan-2-one; Isophorone
1. Introduction
the case of hydrogenation of a-ketoesters over
cinchona-PtrAl2O3, the aromatic p-system of
the quinoline ring serves as a binding site for
the catalyst surface, and the enantioselectivity is
determined mainly by the interaction of the
Two enantioselective heterogenous systems:
w x
NirtartraterNaBr 1 and cinchona-PtrAl2O3
w
x
2–6 catalyse highly enantioselective hydro-
genation of b-ketoesters and a-ketoesters, re-
spectively. In both cases, the stereochemical
control is achieved by adding a chiral modifier
to the metal surface and only certain types of
substrate are suited. There is a general consent
that tartaric acid is chemically bonded to nickel
and that during hydrogenation the chiral
molecule does not leave the metal surface. In
w
x
quinuclidine N-atom with the substrate 7–10 .
The borane reduction of ketones catalysed by
1,3,2-oxazaborolidine is a very efficient homo-
geneous process for the preparation of sec-
ondary alcohols with high optical yields. Our
goal is to anchor a 1,3,2-oxazaborolidine to a
metal surface, hoping that interactions between
this metal surface, the heterocycle and the
prochiral ketone might induce enantioselective
hydrogenation. The oxazaborolidine ring can be
prepared by reacting a b-amino alcohol either
)
Corresponding author. Tel.: q33-76-514600; Fax: q33-76-
514382
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 99 00212-5

(
)
268
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
w
x
Ž
.
with borane 11 or with a substituted boronic
tion of lithium borohydride 9 ml, 18 mmol
w
x
acid 12 . Water is formed in the second proce-
dure and has to be eliminated to prevent hydrol-
ysis of the heterocycle. Therefore, we decided
to explore the reaction of a b-amino alcohol
with a nickel-boride. The black solid thus ob-
was then added: gas evolution was observed
immediately together with the decoloration of
the dark solution and the precipitation of a black
solid. After 10 min, the black suspension was
heated to reflux during 2 h. After cooling down
to room temperature, the solvent was removed
by decantation through a transfer tube by means
of a pressure differential. THF 80 ml was
syringed into the flask and the mixture stirred
while refluxing for half an hour. After cooling
down and allowing to settle, the THF was re-
moved. This washing of the catalyst was re-
peated twice to eliminate lithium iodide.
Ž
. Ž
.
tained has been reacted with 1R,2S - y -2-
amino-1-phenylpropan-1-ol, norephedrine, and
used in the reduction of acetophenone with
borane–THF complex. This nickel-boride bound
b-amino alcohol has also been used for the
hydrogenation of 4-methylpentan-2-one, aceto-
phenone and isophorone.
Ž
.
2. Experimental
2.3. NiB2 treated with NaOH
2.1. Materials
An oxygen-free solution of sodium hydroxide
Ž
.
Ž
.
2.0 M, 70 ml was added to NiB2 9 mmol , an
Ž
. Ž
.
Nickel iodide NiI2 P6H2O Alfa was dried
in a desiccator for 48 h at room temperature,
under vacuum over zeolite 4 A freshly dehy-
drated at 600 K. Lithium borohydride 2.0 M
immediate and vigourous gas evolution was ob-
served. After 10 min, the black suspension was
heated to reflux for 2 h. After cooling down, the
aqueous solution was removed by decantation,
the catalyst was washed twice with water and
three times with THF.
˚
Ž
solution in tetrahydrofuran packaged under ni-
. Ž
.
Ž
trogen Aldrich 4-methylpentan-2-one 99.5%,
. Ž
.
Ž
.
HPLC grade Aldrich norephedrine Aldrich
isophorone 97% Aldrich acetophenone Al-
drich were used as received.
Ž
. Ž
.
Ž
2.4. Characterizations
.
Ž
.
THF Carlo Erba was distilled under nitro-
The overall composition of each catalyst was
determined by chemical analysis of the samples
dissolved in nitric acid.
gen immediately prior to use from sodium ketyl
of benzophenone; cyclohexane 99.5% Al-
Ž
. Ž
.
drich was refluxed for 2 h with Raney nickel
The total surface areas were measured by
and distilled under nitrogen.
nitrogen adsorption at the temperature of liquid
Ž
.
nitrogen BET method . Before measuring the
total surface, the catalyst was desorbed at 413 K
for 4 h under high vacuum.
2.2. Preparation of NiB2
Reactions were carried out in an oven dried
three-necked round bottom flask equipped with
a magnetic stirrer and connected through a con-
denser to a vacuum line. The flask was charged
The nickel surface area was determined from
hydrodesulfurization of 3-methylthiophene in the
liquid phase. The stoichiometry of the reaction
w
x
was taken as SrNis1r2 13 . The reaction
was performed at 363 K under a hydrogen
pressure of 1.1 MPa, using 0.3–0.6 g of the
catalyst and 1.85 mmol of 3-methylthiophene in
150 ml of cyclohexane. After 2 h, which is
sufficient for complete reactive adsorption of
3-methylthiophene on the catalyst surface, the
Ž
.
with anhydrous NiI2 2.813 g, 9 mmol . The
flask was evacuated and backfilled with nitro-
gen three times and a nitrogen flow was main-
tained during the reaction. Dry oxygen-free THF
Ž
.
80 ml was syringed in, stirring at room tem-
perature dissolving partially the NiI2. A solu-

(
)
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
269
Ž
the mixture was treated with acetophenone 1.5
mmol and stirring was continued at room tem-
amount of 3-methylthiophene remaining in the
reaction mixture was determined by gas chro-
.
Ž
.
Ž
matography with a 20% Hallcomid on Chro-
mosorb W column and n-octane as internal
standard.
perature until the reaction was complete g.l.c.
.
analysis, usually 2–2.5 h . The solids were al-
lowed to settle and the liquid phase was re-
moved through a transfer tube by means of a
pressure differential, the catalyst being left in
the flask for reuse. The liquid phase was diluted
with 2 M HCl and extracted with ethyl acetate,
which was then washed with saturated aqueous
NaCl, dried, and evaporated. The enantiomeric
purity of the product was determined by capil-
Infrared spectra were obtained with a Nicolet
impact 400 FTIR spectrometer in the range
400–4000 cmy1. The samples of NiB2 were
Ž
.
diluted with KBr 1:25 .
2.5. Reaction of NiB2 with norephedrine: prepa-
(
)
ration of NiB2
oxaza
y x
x
Ž
lary GC with a chiral column hydrodex-b-
Ž
.
Norephedrine 0.9 mmol in 10 ml THF was
Ž
cyclodextrin, 25 m = 0.25 mm Machery–
Ž
added to the stirred suspension of NiB2
9
..
Nagel .
.
Ž
.
mmol in THF 80 ml . After 12 h at room
temperature and 4 h at reflux, the THF was
decanted and concentrated to 5 ml. Analysis of
the condensate by t.l.c. showed that no amino
alcohol remained. Furthermore, repeating the
reaction using a graduated tube previously filled
with mineral oil placed over the open end of a
delivery tube showed that 0.45 mmol of hydro-
gen was formed at room temperature. Heating
of the reaction mixture for 8 h at 558C afforded
the evolution of the same amount of hydrogen
2.7. Hydrogenation conditions
The hydrogenation of 4-methylpentan-2-one,
acetophenone and of isophorone upon the pre-
pared catalyst, using cyclohexane as solvent,
was carried out in a 250-ml static reactor, under
Ž
.
constant hydrogen pressure 0.9 MPa , with a
Ž
.
stirring speed of 660 rpm at 298 K isophorone
Ž
and at 333 K 4-methylpentan-2-one and aceto-
phenone . Before introduction of the substrate,
the reactor containing a suspension of the cata-
lyst in cyclohexane 140 ml was purged with a
flow of hydrogen. The catalyst 9 mmol of Ni
was pretreated at room temperature for 1 h at a
.
Ž
.
0.45 mmol . Thus, the 0.9 mmol of amino
alcohol reacted with boron and the catalyst is
referred to as NiB2 oxaza , where x is the
ratio amino alcoholrboron and oxaza stands for
Ž
.
Ž
.
yx
x
Ž
.
pressure of 0.9 MPa. Temperature was then
Ž
raised to 298 or 333 K and the substrate 80
.
mmol was introduced.
The end of the reaction conversion G98%
Ž
.
was controlled by gas chromatographic analysis
of samples withdrawn from the reaction mix-
ture. After cooling down and settling, the reac-
tor was depressurized, the liquid phase was
removed through a transfer tube by means of a
pressure differential, leaving catalyst behind for
reuse. After removal of cyclohexane by evapo-
ration, the solution was distilled under reduced
pressure. This distillation gave 4-methylpentan-
2-ol, 1-phenylethanol and dihydroisophorone
with a chemical purity of 99.5%, 94% and 98%,
respectively.
the reduction of acetophenone by borane affords
evidence for the formation of an oxazaborolid-
Ž
.
ine vide supra .
2.6. Reduction of acetophenone using borane–
(
)
THF and NiB2
oxaza 0.05
y 0.05
Ž
.
A total of 1.05 ml of borane–THF 1 M was
added to the stirred suspension of NiB2
-
y0.05
Ž
.
Ž
.
oxaza
15 mmol in 100 ml of THF at
0.05
room temperature under nitrogen. After 30 min,

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)
270
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
( )
2.8. Determination of enantiomeric excess ee
rate and hydrogen. Recently, the difference be-
tween nonaqueous and aqueous reduction of
Co2q by NaBH4 has been pointed out by Glavee
The enantiomeric excesses were determined
from measurements of optical rotation Perkin
w
x
Ž
et al. 20–22 ; in aqueous media, the primary
product is Co2 B, in nonaqueous media the pri-
mary product is Co, which catalyses the decom-
position of diborane to boron and hydrogen
affording ultra fine particles, with the bulk com-
position being CoB2 as was observed by Scha-
effer.
.
Elmer 241 polarimeter .
a
20 of hydrogenated product
D
ee % s
=100
Ž
.
a
20 of pure enantiomer
D
Ž .
The specific optical rotations used were: S -
20
D
w x Ž .
s q20.58 neat ,
20
4-methylpentan-2-ol
a
The reduction of nickel iodide with lithium
borohydride in oxygen-free dry THF, gives a
black precipitate containing 2 mol of boron for
Ž .
w x Ž .
s q298 neat ,
20
S -dihydroisophorone
a
D
Ž .
w
x
Ž .
s y428 neat .
20
S -1-phenylethanol
a
D
w x
Ž
. Ž
.
Ž
1R,2S - y -norephedrine a D sy478 Cs
Ž
Ž ..
each mole of nickel Table 1, Eq. 1
NiI2 q2LiBH4 THF anhydrous NiB s
.
7, 1 N HCl .
2Ž .
N2
q2LiIq4H2
1
Ž .
3. Results and discussion
NiB22 presents a very large BET surface area
Ž
.
120 m rg in comparison with the metal sur-
3.1. EÕidence for the formation of nickel-boride:
NiB2
2
Ž
.
face area 5 m rg . These results show that the
surface of the nanoscale particles is mainly
occupied by boron atoms. The large increase of
the nickel surface after a treatment with sodium
The bulk composition of the nickel-borides
prepared according to Brown and Brown 14–
w
Ž
.
hydroxide Table 1 sustains this assertion and
gives evidence for an intimate mixture of boron
and nickel.
The IR data of NiB2 are interpreted by com-
parison with those of bidentate mononuclear
MBH4 complex. For such complex
x
16 , by reducing nickel salts with NaBH4 either
in water or in 95% ethyl alcohol, is Ni2 B. The
surface of these solids which has been charac-
terized by Okamoto et al. 17 and Deng et al.
w
x
w
x
18 is relatively complex. From the XPS spec-
tra, it appeared that the nickel metal is partially
oxidized and that two kinds of boron species are
present, one is assigned to boron interacting
with nickel, the other one is assigned to oxi-
Ž
.
dized boron BO2 . The ratio of these two kinds
of boron depends on the preparation procedure,
this ratio is difficult to control and we believe
that only boron interacting with nickel would
act an anchor. Thus, we have decided to choose
another method of preparation.
the terminal hydrogen–boron stretching fre-
Ž
.
quency nB
and the bridge stretching fre-
.
are usually observed between
yH t
Ž
quency nB
yH b
Table 1
Physical properties of the catalyst prepared by reduction of NiI2
with LiBH4
It has been reported by Stewart and Schaeffer
w
x
Catalyst NirB atomic ratioUU SBET m rg
SNi m rg
2
2
19 that the reduction of cobalt bromide with
Ž
.
Ž
.
LiBH4, in dry ether under oxygen-free condi-
tions, affords a black pyrophoric material ex-
hibiting chemical behavior typical of the ele-
ments. For example, boron reacts with dry
methanol to give stoichiometrically methyl bo-
NiB2
NiBU2
0.5
4
120
80
5
80
UAfter treatment with sodium hydroxyde solution.
UU The atomic ratio: NiB was determined by chemical analysis of
the samples dissolved in nitric acid.

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)
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
271
2400–2600 and 1300–1500 cmy1, respectively
norephedrine, i.e, with reaction conditions lead-
ing to poor enantioselectivity, the amount of
borane being too low to form the oxazaboroli-
y
1
w
x
23 . Furthermore, a strong doublet, 50–80 cm
splitting is observed when one boron atom is
bonded to a hydrogen terminal pair, whereas the
spectrum exhibits only one frequency when a
single terminal hydrogen is attached to the boron
w
x
dine and to reduce the ketone 25 . As shown in
Ž
. Ž
.
Table 2 the R - q -1-phenylethanol was ob-
tained in excellent optical yield.
w
x
atom 24 .
The oxazaborolidine ring has never before
been prepared by reacting an amino alcohol
1
We observed a medium band at 2500 cmy
Ž
.
w x
assigned to B–Ht and a strong broad band
with boron 27 ; the close agreement observed
1
centred at around 1350 cmy
in the borane reductions of acetophenone ho-
Ž
.
mogeneous and heterogeneous, Table 2 clearly
indicates, however, the formation of an oxaz-
aborolidine in the reaction of norephedrine with
nickel-boride.
The amino alcohol is strongly bound to the
nickel-boride nanoparticles, since by t.l.c. no
traces of amino alcohol could be detected in the
liquid phase after reduction. Furthermore, the
catalyst can be recycled two times with little or
no loss of performance. Our catalyst affords
higher enantiomeric excesses in the reduction of
acetophenone than polymer-bound oxazaboroli-
Ž
.
bridge stretching . From the band intensities,
one can estimate that the terminal hydrogen is
often missing on the bridge structure:
Ž
. Ž
.
dine catalysts prepared with 1R,2S - y -
w
x
norephedrine 26 , i.e., 90% as to compare with
74%.
Ž
Though the bulk composition was NiB2 Ta-
(
)
x
3.3. Hydrogenation oÕer NiB2 , NiB2
oxaza
y x
.
ble 1 , no boron was detected by STEM analy-
sis, however, two kinds of boron 169 and 179
eV were found by Auger spectroscopy. The
(
)
and Ni oxaza catalysts
x
Ž
.
3.3.1. Hydrogenation of 4-methylpentan-2-one
a Over NiB2. The hydrogenation was rather
slow and 30 h were necessary for total conver-
Ž
.
first one 169 eV could be assigned to boron
linked to nickel, the second one being elemen-
tary boron. Before the STEM analysis, the cata-
lyst NiB2 has been desorbed at 385 K for one
night under high vacuum, thus, during the des-
orption and according to the other physico-
chemical observations, the catalyst is decom-
posed and boron is pumped away.
Ž .
11
Ž1
.
sion. The NMR spectra H and B of the
hydrogenated products showed that 4-methyl-
Table 2
Enantioselectivity in the reduction of acetophenone by BH3 and
the nickel-oxazaborolidine catalyst and reuse of this catalyst
3.2. EnantioselectiÕe borane reduction of aceto-
phenone: eÕidence for the formation of
(
)
NiB2
oxaza
y x
x
U Using 30 mol% of catalyst prepared by reacting 1R,2S - y -
. Ž
The reduction of acetophenone has been car-
ried out with 0.7:1 molar ratio of borane and
Ž
.
w
x
2-amino-1-phenylpropanol with phenylboronic acid 26 .

(
)
272
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
pentan-2-ol and the corresponding borate were
obtained.
In this later case, the coordination of the
ketone oxygen to the boron atom of the oxaz-
aborolidine induces the enantioselective hydro-
genation of the C5C bond. Thus, we are of the
opinion that the binding of the oxazaborolidine
atom at the more sterically accessible electron
pair of the ketone oxygen atom, occurs as it is
proposed in homogeneous reduction. But the
next step of which the enantioselectivity de-
pends on, i.e., the face selective hydride transfer
via a six-membered transition state is of course
impossible, the hydrogen atom which instead
arises from the surface is not well suited, and
the ee is very poor. Thus, as shown by the
reduction with borane THF complex,
norephedrine reacts with elementary boron, but
the resulting oxazaborolidine cannot efficiently
induce the enantioselective hydrogenation of ke-
tone.
A reuse of the catalyst exhibited a different
behaviour: the hydrogenation of the same
amount of ketone in the same conditions was
completed in 1 h instead of 30 h and the alcohol
was the only product.
The low catalytic activity of NiB2, in the
hydrogenation of 4-methylpentan-2-one com-
pared to its activity when it is reused, is in
accordance with a large concentration of boron
atoms at the surface, these atoms being progres-
sively removed from the surface as borate by
reacting with the alcohol formed.
Ž .
Ž
.
b Over NiB2 oxaza , xs0.05. The ac-
yx
x
tivity of this catalyst was the same of that of
NiB2, i.e., 30 h were necessary for completion
of the reaction. The borate was the major prod-
uct. After distillation under reduced pressure,
Ž
.
4-methylpentan-2-ol 12 mmol with a purity of
99.5% was obtained with 1% ee. Similarly, the
Ž
.
boron atoms of the NiB2 oxaza are re-
4. Conclusion
yx
x
moved during the hydrogenation of 4-methyl-
pentan-2-one, thus, reused these catalysts are
Ž
.
Nanoparticles of nickel-boride NiB2 have
been prepared by reducing nickel iodide with
lithium borohydride, in anhydrous oxygen-free
THF. The hydroxy group and the amino-group
of norephedrine react with boron atoms, evolv-
ing 1 mol of hydrogen for each mole of amino
alcohol to afford a chiral oxazaborolidine
strongly anchored at the surface of the particles.
The reduction of acetophenone using borane–
THF as reducing agent in the presence of 1
equivalent of nickel-boride bound oxazaboroli-
dine affords 1-phenylethanol with excellent ee.
The oxazaborolidine is strongly bound to the
particles since the catalyst can be recycled at
least two times with no loss of performance.
The catalyst hydrogenates 4-methylpentan-2-
one, acetophenone, isophorone into 4-methyl-
pentan-2-ol, 1-phenylethanol, dihydroiso-
phorone respectively in any case the S configu-
ration predominates slightly. The ee is poor but
as observed for the reduction it is constant when
the catalyst is reused. In our hands, the catalytic
hydrogenation over an oxazaborolidine boron-
Ž
.
x
referred to as Ni oxaza . As observed with
Ž
.
NiB2, Ni oxaza presented a higher activity
upon reuse: the ketone 80 mmol was reduced
in 1.5 h. After distillation, the S alcohol was
x
Ž
.
Ž .
isolated in 1% ee.
(
)
3.3.2. Hydrogenation oÕer Ni oxaza 0.05
The ee observed in hydrogenation over
Ž
.
Ni oxaza
catalyst is low, however, the fol-
0.05
lowing sequence of reactions diminishes
strongly the interference of side products and
Ž
.
artefacts. A catalyst prepared with 1R,2S -
Ž
.
y -norephedrine hydrogenates 4-methylpen-
tan-2-one leading to the alcohol of predomi-
Ž . Ž
.
Ž
nantly S - q configuration ees1% deter-
.
mined from optical rotation reused it hydro-
genates acetophenone leading to the alcohol
Ž . Ž . Ž
S - y ees1% from optical rotation and
.
capillary g.c. with a chiral column ; in a third
utilization dihydroisophorone S - q ees2%
from optical rotation predominates in the hy-
Ž . Ž . Ž
.
drogenation of isophorone.

(
)
K. MolÕinger et al.rJournal of Molecular Catalysis A: Chemical 150 1999 267–273
273
w
w
x
Ž
.
11 S. Itsuno, K. Ito, J. Org. Chem. 49 1984 555.
bound to nickel particles cannot be useful in
enantioselective synthesis of alcohol.
x
12 E.J. Corey, R.M. Bahshi, S. Shibata, C.P. Chen, V.K. Singh,
Ž
.
J. Am. Chem. Soc. 109 1987 7925.
w
w
w
w
w
w
w
w
w
w
x
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Ž
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x
Ž
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14 H.C. Brown, C.A. Brown, J. Am. Chem. Soc. 84 1962
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x
Ž
15 H.C. Brown, C.A. Brown, J. Am. Chem. Soc. 85 1963
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x
Ž . Ž
1 1966
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ˆ
x
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Ž
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.
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