CHIRALITY 27:422–424 (2015)
Asymmetric Borane Reduction of Prochiral Ketones Catalyzed By
Helical Poly[(S)-3-vinyl-2,2’-dihydroxy-1,1’-binaphthyl]
*
GUOCHANG QIN, YEHUI CHEN, LIWEN YANG, NIANFA YANG, AND ZHUSHENG YANG
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University,
Xiangtan, Hunan, People’s Republic of China
ABSTRACT
The application of helical poly[(S)-3-vinyl-2,2’-dihydroxy-1, 1’-binaphthyl] (L*) in the
asymmetric borane reduction of prochiral ketones was studied. The results showed that L* had excel-
lent catalytic activity as well as enantioselectivity, giving up to 96% yield and up to 99% enantiomeric
excess (ee) of the corresponding secondary alcohol at 25 °C. Moreover, L* can be easily recovered
and reused without loss of catalytic activity. Chirality 27:422–424, 2015. © 2015 Wiley Periodicals, Inc.
KEY WORDS: helical polymer; enantioselective inductivity; borane; reduction; ketones
(Tokyo, Japan) system consisting of DG-1580-53 degasser, PU-980
Recently, a new design strategy to use chiral soluble mac-
romolecules as chiral ligands emerged.1–3 In this strategy,
chiral ligands were bonded to preferred-handed helical poly-
mers. The development of well-defined helical polymer cata-
lysts that can efficiently catalyze asymmetric organic
reactions and be separated easily from the product still re-
mains a challenge. The very first attempt to carry out asym-
metric reaction using helical polymer ligand was reported
by Yashima et al.4,5 Since then, helical polymer catalysts have
been attracting much attention in asymmetric organic synthe-
sis.6–11 For example, polyquinoxaline-based helically chiral
phosphine was prepared and used to catalyze the asymmetric
hydrosilylation of styrene, with 70–96% enantiomeric excess
(ee) of the products being achieved.12,13 Subsequently, Zhang
et al. reported asymmetric aldol reaction with ee up to 80%,
which was catalyzed by optically active helical substituted
polyacetylenes.14
HPLC pump, UV-970 UV/Vis detector, RI-930 RI detector, and CO-
2065-plus column oven (at 38 °C) using two connected Shodex GPC-
KF-804 L columns in THF (sample concentration = 1 wt%; flow
rate = 1.0 mL/min). The molecular weight was calibrated with
a
commercially available polystyrene. L* was prepared according to the
literature method.21
General procedure for asymmetric borane reduction of
prochiral ketone
Under an argon atmosphere, BH3-THF (1 mL, 1.0 mmol/L in THF )
was added to a solution of L* (0.13 g, 0.4 mmol) in dry toluene (4 mL).
The suspension was stirred at 0 °C for 2 h. Then a solution of ketone
(1.0 mmol) in toluene (1 mL) was added by syringe and the reaction
mixture was stirred at 25 °C for 12 h. The reaction was quenched with
water and the mixture was added to 30 mL MeOH. The precipitate
was isolated by filtration. The precipitate was dried at 45 °C under vac-
uum for 12 h and reused for the next asymmetric borane reduction of
prochiral ketones. After removal of the solvent of the filtrate, the residue
was purified by flash column chromatography (silica gel, 10% CH2Cl2/
hexane) to afford the corresponding secondary alcohol. The ee value
was determined by Daicel Chiralcel OD column (Tokyo, Japan) (eluent:
hexane/ 2-propanol).
As we know, asymmetric synthesis of enantiomerically
enriched secondary alcohols has been widely studied because
chiral secondary alcohols are important synthetic intermedi-
ates for various other functionalities such as halide, amine, es-
ter, and ether, which are involved in many natural products
and pharmaceuticals.15–17 Enantioselective borane reduction
of prochiral ketone is one of the useful methods to obtain chi-
ral secondary alcohols.18,19 To our knowledge, there has been
little information about the asymmetric borane reduction of ke-
tones catalyzed by helical polymers so far. More recently, our
group synthesized helical poly[(S)-3-vinyl-2,2’-bis(methoxy-
RESULTS AND DISCUSSION
As we know, the reaction of the asymmetric borane reduc-
tion of prochiral ketones is greatly affected by the amount of
the catalyst. In order to evaluate the efficiency of the catalyst
in the reaction of the asymmetric reduction, in preliminary ex-
periments we attempted to perform the asymmetric reduction
of acetophenone with BH3-THF and catalyst L* to explore the
optimum reaction conditions. As shown in Table 1, when
20 mol% L* was employed less than 15% ee of the product
was obtained; when 40 mol% L* was employed the reaction
gave 65% ee of the product (Table 1, Entry 3). When
50 mol% L* was employed the reaction gave 67% ee of the
product but yield decreased remarkably (55%).
20
methoxy)-1,1’-binaphthyl] and poly[(S)-3-vinyl-2,2’-dihydroxy-
1,1’-binaphthyl]21 (Scheme 1). Herein we report the application
of helical poly[(S)-3-vinyl- 2,2’-dihydroxy-1,1’-binaphthyl] (L*) in
the asymmetric borane reduction of prochiral ketones.
EXPERIMENTAL
General
All reagents were used as supplied commercially unless otherwise
noted. Tetrahydrofuran (THF) and toluene were distilled from sodium
under N2 before use. (S)-3-formyl-2,2’-bis(methoxymethoxy)-1,1’-binaphth-
yl 1 was prepared according to the literature method.22 1H and 13C NMR
spectra were performed on a Bruker ARX400 MHz spectrometer (Billerica,
MA) using tetramethylsilane(TMS) as internal standard.
Optical rotation data were measured on a Perkin Elmer Model 341 LC
Polarimeter (Boston, MA) at 365 nm. Elemental analyses were carried
out on Elementar Vario EL (Palo Alto, CA) instrument. Gel permeation
chromatography (GPC) analysis was performed with a JASCO-GPC
*Correspondence to: Nianfa Yang, Key Laboratory of Environmentally
Friendly Chemistry and Applications of Ministry of Education, College of
Chemistry, Xiangtan University, Xiangtan, Hunan 411105, People’s Republic
of China. E-mail: nfyang@xtu.edu.cn
Received for publication 14 November 2014; Accepted 4 April 2015
DOI: 10.1002/chir.22459
Published online 15 May 2015 in Wiley Online Library
(wileyonlinelibrary.com).
© 2015 Wiley Periodicals, Inc.