Asymmetric hydrogenation of 1-phenylethenylboronic acid and esters for the synthesis of chiral organoboron compounds

Article abstract of DOI:10.1016/S0022-328X(01)01239-6

The hydrogenation of ethanediol 1-phenylethenylboronic ester was carried out at -20 °C under a hydrogen atmosphere (9 atm) in the presence of [Rh(cod)₂]BF₄-(R)-BINAP (3 mol%). After alkaline hydrogen peroxide oxidation, the reaction gave 1-phenylethanol with 80% e.e.

Full text of DOI:10.1016/S0022-328X(01)01239-6

Journal of Organometallic Chemistry 642 (2002) 145–147
www.elsevier.com/locate/jorganchem
Asymmetric hydrogenation of 1-phenylethenylboronic acid and
esters for the synthesis of chiral organoboron compounds
Masato Ueda, Atsushi Saitoh, Norio Miyaura *
Faculty of Engineering, Di6ision of Molecular Chemistry, Graduate School of Engineering, Hokkaido Uni6ersity, Sapporo 060-8628, Japan
Received 29 June 2001; received in revised form 20 August 2001; accepted 8 September 2001
Abstract
The hydrogenation of ethanediol 1-phenylethenylboronic ester was carried out at −20 °C under a hydrogen atmosphere (9
atm) in the presence of [Rh(cod)2]BF4–(R)-BINAP (3 mol%). After alkaline hydrogen peroxide oxidation, the reaction gave
1-phenylethanol with 80% e.e. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Hydrogenation; Asymmetric synthesis; Boron and compounds
1. Introduction
achieve high enantioselectivity for ethanediol 1-
phenylethenylboronic ester (1b), which was first synthe-
sized by resolution of the bisdemethylbrucine derivative
[8] and was recently studied extensively by the catalyzed
hydroboration of styrene with catecholborane [5].
Chiral organoboron compounds [1] are available
from the hydroboration of internal alkenes with chiral
hydroboration reagents including di(isopinocam-
pheyl)borane [2], mono-isopinocampheylborane [2], and
2,5-dimethylboronane [3]. The discovery of the in-
tramolecular SN2 substitution reaction of chiral (a-
2. Results and discussion
haloalkyl)boronic
esters
with
carbon-
or
hetero-nucleophiles has resulted in an additional and
practical method of broad scope [4]. Although such
synthesis using a stoichiometric chiral auxiliary has its
own excellence, the catalyzed hydroboration of alkenes
with metal–chiral phosphine complexes significantly
overcame the limitations of the cost/availability of the
chiral borane reagents [5]. On the other hand, the
asymmetric hydrogenation of prochiral functionalized
alkenes with a metal–chiral phosphine complex is a
promising and reliable method for the synthesis of
optically active compounds, which has not yet been
studied in regard to 1-alkenylboron compounds [6].
Here, we wish to report the preliminary results for the
asymmetric hydrogenation of 1-phenylethenylboronic
acid and its esters (1) with a rhodium–chiral phosphine
complex (Scheme 1). A combination of [Rh(cod)2]BF4
and BINAP [7] was found to be most effective to
Hydrogenation of 1 with a rhodium–chiral phos-
phine complex was carried out at room temperature
under a hydrogen atmosphere (9 atm). After the alka-
* Corresponding author. Tel./fax: +81-11-7066561.
E-mail address: miyaura@org-mc.eng.hokudai.ac.jp (N. Miyaura).
Scheme 1. Asymmetric hydrogenation of 1-phenylethenylboronic acid
and esters.
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01239-6

146
M. Ueda et al. / Journal of Organometallic Chemistry 642 (2002) 145–147
Table 1
Asymmetric hydrogenation of 1-phenylcthenylboronic acid and estersa
Entry
1
Ligand
Temperature (°C)
Yield (%) b
% ee
1
2
3
4
5
6
7
8
1b
(R,R)-DIOP
20
20
20
20
−20
20
72
4
19 (R)
16 (S)
42 (R)
61 (S)
80 (S)
25 (S)
53 (S)
33 (R)
(R,R)-CHIRAPHOS
(S,S)-Me–DuPHOS
(R)-BINAP
(R)-BINAP
(R)-BINAP
49
75
65 c
25
77
92
1a
1c
1d
(R)-BINAP
(R)-BINAP
20
20
a A mixture of [Rh(cod)2]BF4 (0.03 mmol), (R,R)-DIOP (0.036 mmol), 1 (1 mmol) in 1,2-dichloroethane was stirred for 24 h under atmosphere
of H2 (9 atm). The alkaline hydrogenperoxide oxidation gave 1-phenylethanol.
b Isolated yields of 1-phenylethanol.
c The reaction was carried out for 7 days.
A screening of the representative chiral ligands re-
vealed the efficiency of BINAP (entries 1–4). The reac-
tion employed (R)-BINAP gave 1-phenylethanol with
65% ee at room temperature. The enantioselectivity was
strongly dependent on the reaction temperature, higher
selectivity being observed at lower temperature. Al-
though the hydrogenation was slow at temperatures
lower than 0 °C, the enantioselectivity was improved to
80% ee at −20 °C (entry 5). The catalyst was in situ
prepared from [Rh(cod)2]BF4 and a chiral phosphine,
but the isolated [Rh(R-BINAP)]BF4 revealed essentially
the same enantioselectivity. The selectivity can be con-
trolled by the steric factor between the phenyl and
boryl groups because the 1,3-propanediol ester (1c)
resulted in a slightly lower enantioselectivity than that
of (1b) (entry 7) and the pinacol ester 1d reversed the
absolute configuration due to the greater bulkiness of
the boryl ring than that of the phenyl ring (entry 8).
Further studies are in progress to elucidate the hy-
drogenation of other 1-alkenylboronates and the possi-
ble synthetic applications.
line H2O2 oxidation of 2a–d, 1-phenylethanol (3) was
analyzed by a chiral stationary column (Dicel Chiralcel
OD-H) to evaluate the enantioselectivities of 2. High
enantioselectivity up to 80% e.e. was often observed in
ethanol or isopropanol, but the reactions in alcohols
suffered from low yields due to the CꢀB bond cleavage
of 2 giving ethylbenzene because of the sensitivity of
benzylboronates to the hydrolytic protodeboronation
with alcohols. Thus, the reactions in aprotic solvents
afforded good yields of 3, though the catalytic hy-
drogenolysis of the CꢀB bond still provided some ethyl-
benzene (B10%) [9]. Nonpolar solvents having a high
solubility of the catalyst such as CH2Cl2 or 1,2-
dichloroethane revealed higher enantioselectivity than
donating solvents. The effect of solvents on enantiose-
lectivity was observed as follows for the hydrogenation
of 1b with a [Rh(cod)(MeCN)2]BF4–(R)-BINAP cata-
lyst: CH2Cl2 (S-3, 64% ee), 1,2-dichloroethane (S-3,
65% ee), dimethoxyethane (S-3, 46% ee), THF (S-3,
43% ee), acetone (S-3, 31% ee) and DMF (R-3, 12%
ee). DMF reversed the absolute configuration of 3
presumably due to the coordination of the solvent to
the rhodium metal center because the neutral rhodium
complexes such as [Rh(cod)Cl]2–2(R)-BINAP also
yielded R-3 (46% yield, 21% ee). On the other hand, the
hydrogen pressure did not change the enantioselectivity
of 3 in a range of 1–20 atm [7b].
3. Experimental
3.1. Material and reagents
Hydrogenation was carried out in a thick-walled
glass bottle fitted with a Young valve. (R,R)-DIOP,
(R,R)-CHIRAPHOS, (S,S)-Me-DuPHOS and (R)-BI-
NAP were purchased from AZmax. [Rh(cod)2]BF4 was
prepared by the reported procedure [10].
The hydrogenation of 1 with a [Rh(cod)2]BF4–chiral
phosphine catalyst is summarized in Table 1, wherein
less effort was directed to optimization of chemical
yields.

M. Ueda et al. / Journal of Organometallic Chemistry 642 (2002) 145–147
147
3.2. 1-Phenylethenylboronic esters (1) [8]
EtOH). The absolute configuration was established to
the S-form by the rotation of an authentic sample,
[h]2D0+41.7 (c 1.00, EtOH) [11].
To a 500-ml flask charged with magnesium turnings
(1.46 g, 60 mmol) and THF (100 ml) were dropwise
added 1,2-dibromoethane (0.8 ml) followed by a-bro-
mostyrene (9.15 g, 50 mmol) in THF (80 ml). After
being stirred for 30 min at 80 °C, the mixture was
cooled to −78 °C. A solution of B(OMe)3 (5.6 ml) in
THF (80 ml) was dropwise added over 1 h to the
Grignard solution. The resulting mixture was stirred for
1 h at −78 °C and overnight at room temperature
(r.t.), and was then treated with (aq.) 6 M HCl (50 ml).
The mixture was extracted with ether and washed with
water and all volatile matters were evaporated in vacuo
to give a crude solid of 1-phenylethenylboronic acid.
The solid was dissolved in toluene (50 ml) and treated
with 1,3-propanediol (5.4 ml, 75 mmol) and MgSO4 (3
g) at r.t. overnight. After filtration of the drier, the
filtrate was washed with water to remove the excess diol
(30×3 ml) and dried again over MgSO4. Distillation
under reduced pressure gave 2-(1-phenylethenyl)-1,3,2-
dioxaborinane (1c). Yield 6.27 g (66%); b.p. 157 °C/14
mmHg.
References
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A mixture of [Rh(cod)2]BF4 (0.03 mmol), (R)-(+)-
BINAP (0.036 mmol), and ethanediol 1-phenylethenyl-
boronic ester (1b) [8] (1.0 mmol) in 1,2-dichloroethane
(6 ml) was stirred for 30 min in a glass pressure bottle.
After being cooled to −20 °C, H2 was introduced
from the gas cylinder and was then discarded in vacuo
(10 mmHg). After three repetitions, the bottle was
pressured to 9 atm. The resulting mixture was stirred
for 7 days at −20 °C. NaOAc (3 M, 1 ml) and H2O2
(30%, 1 ml) were added. After being stirred for 2 h at
r.t., 1-phenylethanol was isolated by chromatography
over silica gel with hexane–ether=10:1 in 65% yield.
HPLC analysis using a chiral stationary column (Dicel
Chiralcel OD-H) revealed 80% ee ([h]2D0 −31.9 (c 1.00,
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[11] (R)-(+)-1-Phenylethanol (\99%) purchased from Aldrich. See
also [5,8].