Synthesis of Ru-coordinating helical polymer and its utilization as a catalyst for asymmetric hydrogen-transfer reaction

Article abstract of DOI:10.1246/cl.2005.1642

An L-threonine-based helical poly(N-propargylamide)-Ru complex was synthesized, and used as a catalyst for hydrogen-transfer reaction of ketones. The enantiomeric excess (ee) values of the formed alcohols ranged from 12 to 36%. On the other hand, an Ru complex with a low-molecular-weight model ligand gave an alcohol with ee as low as 1.8%. Copyright

Full text of DOI:10.1246/cl.2005.1642

1
642
Chemistry Letters Vol.34, No.12 (2005)
Synthesis of Ru–Coordinating Helical Polymer and Its Utilization as a Catalyst
for Asymmetric Hydrogen-transfer Reaction
ꢀ
ꢀ
Fumio Sanda, Hitoshi Araki, and Toshio Masuda
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510
(Received September 8, 2005; CL-051151)
An L-threonine-based helical poly(N-propargylamide)–Ru
It was confirmed that poly(1) took a helical structure with pre-
dominantly one-handed screw sense from its large specific rota-
complex was synthesized, and used as a catalyst for hydrogen-
transfer reaction of ketones. The enantiomeric excess (ee) values
of the formed alcohols ranged from 12 to 36%. On the other
hand, an Ru complex with a low-molecular-weight model ligand
gave an alcohol with ee as low as 1.8%.
ꢃ
ꢃ
tion (½ꢀꢁ ¼ ꢂ218 ) as compared to that of 1 (½ꢀꢁ ¼ ꢂ12 ),
D
D
2
and intense Cotton effect (½ꢁꢁ ¼ ꢂ18;000 degꢄcm /dmol) at
350 nm, which arises from the conjugated polyacetylene back-
bone. The tert-butoxycarbonyl (BOC) group of poly(1) was re-
moved using HCl, and then the resulting polymer with ammoni-
um structure was neutralized with triethylamine to obtain poly-
0
16
Polymer-supported catalysts attract much attention due to
easy recoverability from reaction mixtures, recyclability, and
(1 ) carrying chiral ꢂ-amino alcohol moiety. This structure is
suitable to form a ruthenium complex with chiral N,O-ligand,
which has achieved excellent ee values in ketone reduction so
1
unusual activity due to polymer effect. Attempts have been
made to induce chirality into reaction products by immobilizing
optically active functional groups on crosslinked polystyrene
1
7
far. In the present study, the polymer–ruthenium complex
0
0
[poly(1 –Ru)] was obtained by the reaction of poly(1 ) with
1
8
(
phosphite–Rh complex for olefin hydroformylation, PS-sup-
PS) beads. Recent examples include PS-supported phosphine–
2
2 2
[RuCl (p-cymene)] . The content of incorporated ruthenium
was 45%, which was calculated from the weight of ash deter-
mined by elemental analysis of the polymer complex. It is con-
3
ported diamino–Ru complex for ketone hydrogenation, PS-sup-
4
0
ported diphenyl prolinol for ketone reduction, and PS-supported
cinchona ammonium salts for benzylation of glycine deriva-
sidered that poly(1 –Ru) carries ruthenium chelated with the
1
amino alcohol moiety as illustrated in Scheme 1. The H NMR
spectrum reasonably supported the formation of the polymer
complex. Figure 1 depicts the CD and UV–vis spectra of poly-
5
tives. Though there are several reports concerning polymer-
supported catalysts carrying chiral substituents in the side chain,
only a few attempts have been made to utilize the main chain
chirality of polymers. Takata and co-workers have synthesized
0
(1 –Ru). It exhibited intense CD signals at 270, 310, and
400 nm based on the conjugated polyacetylene backbone,
indicating that it takes a helical structure with predominantly
one-handed screw sense. Thus, we could synthesize a helical
polymer carrying ruthenium in the side chain.
6
7
helical polymers of binaphthyl salen–Zn and Mn complexes,
and examined asymmetric addition and olefin epoxidation using
the polymer complexes as catalysts. Yashima et al. have utilized
helical poly[N-(4-ethynylbenzyl)ephedrine] as a polymeric cata-
lyst for enantioselective addition of dialkylzincs to benzalde-
+
(nbd)Rh
[
η6-C H B C H ) ]
6
5
-
6 5 3
CH
C
O
8
hyde. We have reported that poly(N-propargylamides) carrying
n
H
(2 mol%)
H
11 M HCl
AcOH
Et3N
appropriate chiral substituents take a helical structure with
predominantly one-handed screw sense. This helix is stabilized
by intramolecular hydrogen bonding between the amide groups
N
N
NHBOC
OH
THF
NHBOC
OH
MeOH
O
9
in the side chains along with steric repulsion. We have also syn-
1
poly(1)
thesized poly(N-propargylamides) derived from some amino
acids to find that they transform the structure from helix to ran-
dom coil, or from helix to helix with opposite helical senses
CH
C
O
CH
C
O
n
n
1
0
11
H
H
caused by external stimuli such as heat, photoirradiation, sol-
0,12
[RuCl2(p-cymene)]2
N
N
Et3N
H2
N
vent,1
and pH. It is expected to utilize amino acid-based
13
NH2
OH
Cl
i -PrOH/MeOH = 90/10
Ru
helical poly(N-propargylamides) as reactive and functional poly-
mers. In this article, we wish to report the synthesis of a helical
poly(N-propargylamide–Ru complex), and evaluation of its cat-
alytic activity in the asymmetric hydrogen-transfer reaction of
ketones to produce chiral alcohols, which are important inter-
mediates in pharmaceutical, agrochemical, flavor, and fragrance
industries.14
Scheme 1 illustrates the synthetic route for the poly(N-prop-
argylamide–Ru complex). L-Threonine-based N-propargylamide
1 was synthesized by the condensation of N-tert-butoxycarbon-
yl-L-threonine with propargylamine, and it was polymerized
O
poly(1')
poly(1'−Ru)
0
Scheme 1. Synthesis of poly(1 –Ru).
Table 1 summarizes the results of asymmetric hydrogen-
0
transfer reaction of ketones using poly(1 –Ru) as a catalyst.
The corresponding alcohols were obtained in 23–48% yields.
The ee values of the alcohols were 12–36%, which were
determined by chiral HPLC. In the reaction of acetophenone, the
predominant enantiomer was (R)-2-phenylethanol, which was
confirmed by comparison with the authentic sample. The highest
ee was achieved in the reaction of 2-acetonaphthone. We also
with a rhodium zwitterion catalyst in THF to give poly(1) with
1
5
number-average molecular weight of 10,000 quantitatively.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.12 (2005)
1643
8
ligand structure improves the synergistic effect of chiralities of
the side chain and helical main chain of the polymer, results in
higher efficiency of chiral induction.
This work was partly supported by Izumi Science and Tech-
nology Foundation, and Iketani Science and Technology Foun-
dation. FS appreciates Dr. Daisuke Takeuchi at Tokyo Institute
of Technology who kindly assisted us to perform elemental anal-
ysis.
CD
6
4
2
θ
0
2
4
6
-
-
-
References and Notes
1
1
0
0
.2
.0
.8
.6
1
2
3
4
5
6
7
8
9
1
1
M. Benaglia, A. Puglisi, and F. Cozzi, Chem. Rev., 103, 3401
2003).
F. Shibahara, K. Nozaki, and T. Hiyama, J. Am. Chem. Soc.,
125, 8555 (2003).
S. Itsuno, A. Tsuji, and M. Takahashi, J. Polym. Sci., Part A:
Polym. Chem., 42, 4556 (2004).
R. J. Kell, P. Hodge, P. Snedden, and D. Watson, Org. Biomol.
Chem., 1, 3238 (2003).
R. Chinchilla, P. Mazon, and C. Najera, Adv. Synth. Catal.,
UV−vis
(
0
0
.4
.2
0
3
46, 1186 (2004).
2
00
300
400
500
Y. Furusho, T. Maeda, T. Takeuchi, N. Makino, and T. Takata,
Chem. Lett., 2001, 1020.
T. Maeda, Y. Furusho, and T. Takata, Chirality, 14, 587
Wavelength / nm
0
Figure 1. CD and UV–vis spectra of poly(1 –Ru) measured
in i-propanol/methanol = 95/5 in the presence of 10 equiv. of
t-BuOK.
(
2002).
E. Yashima, Y. Maeda, and Y. Okamoto, Polym. J., 31, 1033
1999).
R. Nomura, J. Tabei, and T. Masuda, J. Am. Chem. Soc., 123,
430 (2001).
(
Table 1. Asymmetric hydrogen-transfer reaction of ketones
0
8
a
catalyzed by poly(1 –Ru)
0 H. Zhao, F. Sanda, and T. Masuda, Macromolecules, 37, 8888
(2004).
1 F. Sanda, T. Teraura, and T. Masuda, J. Polym. Sci., Part A:
Polym. Chem., 42, 4641 (2004).
O
poly(1'−Ru) (cat.)
OH
Ar
R
i-PrOH/MeOH = 95/5 Ar ∗ R
b
c
Ketone
Acetophenone
Yield /%
ee /%
12 G. Gao, F. Sanda, and T. Masuda, Macromolecules, 36, 3932
2003).
13 F. Sanda, K. Terada, and T. Masuda, Macromolecules, 38,
149 (2005).
(
23
39
28
19
36
12
d
1
2
1
a
-Acetonaphthone
-Acetonaphthone
-Indanone
8
48
43
0
1
4 R. Noyori, ‘‘Asymmetric Catalysis in Organic Synthesis’’,
Wiley, New York (1994).
Conditions: [ketone]0 = 0.10 M, [poly(1 –Ru)] = 1 mM
repeating unit), [t-BuOK] = 10 mM, solvent i-PrOH/
MeOH = 95/5 (v/v), 50 C, 24 h, under N2. Determined
by GC. Determined by HPLC equipped with DAICEL
CHIRALPAK IA eluted with hexane/ethyl acetate = 98/
15 Details of the synthesis and polymerization of 1 have been
reported. See: F. Sanda, H. Araki, and T. Masuda, Macromole-
cules, 37, 8510 (2004).
16 Poly(1) (484 mg, 2 mmol–repeating unit) was dissolved in
acetic acid (15 mL). 11 M HCl (15 mL) was added to the
poly(1) solution, and the resulting mixture was stirred at room
temperature for 30 min. It was concentrated by a rotary evap-
orator, and the residue was dissolved in methanol. An excess
amount of triethylamine was added to the solution, and it
(
ꢃ
b
c
d
2
–95/5 (volume ratio). Isolated yield.
CH (CH )
H
3
2 17
N
0
1
NH2
OH
was poured into acetone to precipitate poly(1 ). H NMR
(
O
400 MHz, CD3OD): ꢃ 1.32 (broad s, -CH3, 3H), 3.5–4.4 (m,
-CH2-, >CH-O-, >CH-N<, 4H), 6.2 (broad s, -CH=C<, 1H).
7 a) D. A. Alonso, D. Guijarro, P. Pinho, O. Temme, and P. G.
Andersson, J. Org. Chem., 63, 2749 (1998). b) D. G. I. Petra,
P. C. J. Kamer, P. W. N. M. van Leeuwen, K. Goubitz,
A. M. van Loon, J. G. de Vries, and H. E. Schoemaker, Eur.
J. Inorg. Chem., 12, 2335 (1999).
1
2
Scheme 2.
examined the hydrogen-transfer reaction of acetophenone with
a chiral amino alcohol 2–Ru complex to find that it induced
chirality in the formed alcohol as low as 1.8% ee (Scheme 2).
1
8 A solution of triethylamine (101 mg, 1 mmol) in 2-propanol
0
(
9 mL) was added to a solution of poly(1 ) (30 mg, 0.2 mmol–
0
This result suggests that the helical structure of poly(1 –Ru) is
repeating unit) and [RuCl2(p-cymene)]2 (61 mg, 0.1 mmol) in
effective to enhance the catalytic ability for chiral induction.
In summary, we have demonstrated the synthesis of an L-
threonine-based helical poly(N-propargylamide)–ruthenium
complex, and its application as a catalyst for asymmetric hydro-
gen-transfer reaction of ketones. We believe that tuning the
ꢃ
methanol (1 mL). The resulting mixture was stirred at 50 C
overnight, then concentrated by a rotary evaporator. The resi-
0
1
due was poured into ether to precipitate poly(1 –Ru). H NMR
(400 MHz, D O): ꢃ 1.4 (broad s, -CH ), 2.4 (broad s, -CH ,
2
3
3
-CH(CH3)2, 3.5 (broad s), 5.2–6.2 (broad m).
Published on the web (Advance View) November 12, 2005; DOI 10.1246/cl.2005.1642