Asymmetric transfer hydrogenation of aryl ketones catalyzed by salt-free two samarium centers supported by a chiral multidentate alkoxy ligand

Article abstract of DOI:10.1021/ol048101f

(Chemical Equation Presented) We synthesized a chiral multidentate ligand, (R,R,R,R)-N,N,N′,N′-tetra(2-hydroxy-2-phenylethyl)-1,3-xylylene diamine [(R)-2], which can support two metals at adjacent positions. Asymmetric transfer hydrogenation of acetophenone and its derivatives was conducted by using salt-free bimetallic lanthanoid complexes of (R)-2, and the combination of two samarium atoms and (R)-2 was found to be the best catalyst system for asymmetric transfer hydrogenation of aryl ketones in high enantioselectivity (up to >99% ee).

Full text of DOI:10.1021/ol048101f

ORGANIC
LETTERS
2
004
Vol. 6, No. 25
695-4697
Asymmetric Transfer Hydrogenation of
Aryl Ketones Catalyzed by Salt-Free
Two Samarium Centers Supported by a
Chiral Multidentate Alkoxy Ligand
4
Kouji Ohno, Yasutaka Kataoka, and Kazushi Mashima*
Department of Chemistry, Graduate School of Engineering Science, Osaka UniVersity,
Toyonaka, Osaka 560-8531, Japan
mashima@chem.es.osaka-u.ac.jp
Received September 17, 2004
ABSTRACT
We synthesized a chiral multidentate ligand, (R,R,R,R)-N,N,N′,N′-tetra(2-hydroxy-2-phenylethyl)-1,3-xylylene diamine [(R)-2], which can support
two metals at adjacent positions. Asymmetric transfer hydrogenation of acetophenone and its derivatives was conducted by using salt-free
bimetallic lanthanoid complexes of (R)-2, and the combination of two samarium atoms and (R)-2 was found to be the best catalyst system for
asymmetric transfer hydrogenation of aryl ketones in high enantioselectivity (up to >99% ee).
Asymmetric transfer hydrogenation of ketonic substrates is
an intriguing and useful synthetic methodology to yield chiral
alcohols. Trivalent samarium compounds supported by bi-
catalytic activity of the samarium species were quite sensitive
to the purity of the lithiation reagent, such as n-BuLi.
Additionally, the samarium catalyst system based on (R)-1
exhibited a nonlinear amplification effect on the asymmetric
1
and tridentate chiral auxiliaries have been utilized as catalysts
2
2c
for this purpose. Among them, Evans et al. reported that a
transfer hydrogenation of o-chloroacetophenone, indicating
samarium species, which was in situ generated by treating
that the catalytically active mononuclear samarium species
was assumed to be in equilibrium with the dimmeric species.4
In contrast to the mononuclear metal complex, similar
efforts have not been made for bimetallic and multimetallic
3
the dilithio salt of a chiral tridentate ligand, (R)-1, with SmI ,
2
c
mediated the asymmetric reduction of aryl ketones. It is
assumed likely that the interaction of the lithium salt with
the catalytically active samarium center may play an im-
portant role in determining the activity and enantioselectivity
because some ate lanthanoid compounds have been reported
to show different catalytic performance compared with salt-
(3) (a) Fukuzawa, S.; Mutoh, K.; Tsuchimoto, T.; Hiyama, T. J. Org.
Chem. 1996, 61, 5400. (b) Hanamoto, T.; Sugimoto, Y.; Sugino, A.; Inanaga,
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Lett. 1993, 1981.
(4) (a) Ohkuma, T.; Doucet, H.; Pham, T.; Mikami, K.; Korenaga, T.;
3
free lanthanoid compounds. The enantioselectivity and
Terada, M.; Noyori, R. J. Am. Chem. Soc. 1998, 120, 1086. (b) Kitamura,
M.; Suga, S.; Oka, H.; Noyori, R. J. Am. Chem. Soc. 1998, 120, 9800. (c)
Kitamura, M.; Suga, S.; Niwa, M.; Noyori, R. J. Am. Chem. Soc. 1995,
117, 4832. (d) Yamakawa, M.; Noyori, R. J. Am. Chem. Soc. 1995, 117,
6327. (e) Kitamura, M.; Suga, S.; Niwa, M,; Noyori, R.; Zhai, Z.-X.; Suga,
H. J. Phys. Chem. 1994, 98, 12776. (f) Guillaneux, D.; Zhao, S.; Samuel,
O.; Rainford, D.; Kagan, H. B. J. Am. Chem. Soc. 1994, 116, 9430. (g)
Kitamura, M.; Okada, S.; Suga, S.; Noyori, R. J. Am. Chem. Soc. 1989,
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(
1) (a) Ojima, I. Catalytic Asymmetric Synthesis; Wiley-VCH: New
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051.
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1
(
Yang, W.-B.; Fang, J.-M. J. Org. Chem. 1998, 63, 1356. (c) Evans, D. A.;
Nelson, S. G.; Gagn e´ , M. R.; Muci, A. R. J. Am. Chem. Soc. 1993, 115,
9
3
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2, 2355.
1
0.1021/ol048101f CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/17/2004

5
catalyst systems, in which each metal center acts coopera-
tively and mutually not only to activate substrates but also
to enhance any selectivities including enantioselectivity.
Recent achievements were that bimetallic catalyst systems
based on zinc, aluminum, and titanium with chiral auxiliary
ligands catalyzed some asymmetric reactions such as aldol
6
7
reaction and carbonyl reduction.
On the basis of these precedents, we could point out that
two factors, presence or absence of salt and mononuclear or
dinuclear status, are mainly responsible to the catalytic
performance of lanthanoid compounds, and thus we antici-
pated that a salt-free, dinuclear compound of samarium
bearing a new chiral multidentate ligand, (R,R,R,R)-N,N,N′,N′-
tetra(2-hydroxy-2-phenylethyl)-1,3-xylylene diamine [(R)-
Figure 1. Chiral multidentate ligands.
reaction of 3e with the alcoholic parts of (R)-2 resulted in
the formation of the salt-free complex 1,3-C
CHPhO) SmI] (4e) along with release of cyclooctatetraene.
Titration monitored by absorption spectroscopy indicated that
equiv of 3e were consumed in the reaction with (R)-2 to
6 4 2 2
H [CH N(CH -
2], could exhibit superior catalytic performance for asym-
2
2
metric transfer hydrogenation of aryl ketones.
The multidentate ligand (R)-2 was obtained as a pale
yellow oil in 50% yield by the reaction of m-xylylenediamine
2
give the salt-free dinuclear samarium species 4e.
When the catalyst 4e (5 mol %) was used for asymmetric
transfer hydrogenation of acetophenone in 2-propanol at 25
8
with excess amounts of (R)-(+)-styrene oxide. For preparing
a salt-free samarium complex of (R)-2, we used the precursor
8
9
SmI(η -cyclooctatetraene)(thf) (3e), in which the cyclooc-
°
C for 24 h, (R)-1-phenylethanol was obtained in 95% ee
tatetraene ligand acts as a dianion. Thus, the simple one-pot
and quantitative yield (Table 1, run 5). We found that the
(
5) (a) Ma, J.-A.; Cahard, D. Angew. Chem., Int. Ed. 2004, 43, 4566.
b) Sammis, G. M.; Danjo, H.; Jacobsen, E. N. J. Am. Chem. Soc. 2004,
26, 9928. (c) Somei, H.; Asano, Y.; Yoshida, T.; Takizawa, S.; Yamataka,
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Imajima, H.; Onodera, G.; Hidai, M.; Uemura, S. Organometallics 2004,
(
1
Table 1. Asymmetric Transfer Hydrogenation of
Acetophenone Using Catalyst Systems Derived from (R)-2 and 2
8
Equiv of Lanthanide Complexes, LnI(η -cyclooctatetraene)(thf)
n
23, 26. (f) Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Hidai, M.; Uemura,
3a-e)a
(
S. J. Org. Chem. 2004, 69, 3408. (g) Tosaki, S.; Nemoto, T.; Ohshima, T.;
Shibasaki, M. Org. Lett. 2003, 5, 495. (h) Nishibayashi, Y.; Onodera, G.;
Inada, Y.; Hidai, M.; Uemura, S. Organometallics 2003, 22, 873. (i)
Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Milton, M. D.; Hidai, M.;
Uemura, S. Angew. Chem., Int. Ed. 2003, 42, 2681. (j) Nishibayashi, Y.;
Yamauchi, A.; Onodera, G.; Uemura, S. J. Org. Chem. 2003, 68, 5875. (k)
Nishibayashi, Y.; Inada, Y.; Hidai, M.; Uemura, S. J. Am. Chem. Soc. 2003,
lanthanoid
complexes
conversion
(%)b
1
25, 6060. (l) Majima, K.; Takita, R.; Okada, A.; Ohshima, T.; Shibasaki,
run
ligand
ee (%) (config)c
M. J. Am. Chem. Soc. 2003, 125, 15837. (m) Yamagiwa, N.; Matsunaga,
S.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125, 16178. (n) Nishibayashi,
Y.; Inada, Y.; Hidai, M.; Uemura, S. J. Am. Chem. Soc. 2002, 124, 7900.
1
2
3
4
5
6
(R)-2
(R)-2
(R)-2
(R)-2
(R)-2
(R)-1
3a
3b
3c
3d
3e
3e
5
9
46
43
>99
77
18 (R)
22 (R)
79 (R)
77 (R)
95 (R)
82 (R)
(
o) Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Hidai, M.; Uemura, S. J.
Am. Chem. Soc. 2002, 124, 11846. (p) Inada, Y.; Nishibayashi, Y.; Hidai,
M.; Uemura, S. J. Am. Chem. Soc. 2002, 124, 15172. (q) Nishibayashi, Y.;
Wakiji, I.; Ishii, Y.; Uemura, S.; Hidai, M. J. Am. Chem. Soc. 2001, 123,
3
393. (r) Nishibayashi, Y.; Yamanashi, M.; Wakiji, I.; Hidai, M. Angew.
Chem., Int. Ed. 2000, 39, 2909. (s) Nishibayashi, Y.; Wakiji, I.; Hidai, M.
J. Am. Chem. Soc. 2000, 122, 11019.
a
(6) (a) Gnanadesikan, V.; Horiuchi, Y.; Ohshima, T.; Shibasaki, M. J.
Reaction temperature, 25 °C; reaction time, 24 h; S/C ) 20; 25 equiv
b 1
Am. Chem. Soc. 2004, 126, 7782. (b) Trost, B. M.; Mino, T. J. Am. Chem.
Soc. 2003, 125, 2410. (c) Trost, B. M.; Terrell, L. R. J. Am. Chem. Soc.
of 2-propanol for substrate. Conversion was determined by H NMR
spectroscopy. c Enantiomeric purity was determined by chiral HPLC.
Absolute configuration was assigned by comparison of product rotations
to the literature value.11
2003, 125, 338. (d) Trost, B. M.; Yeh, V. S. C. Angew. Chem., Int. Ed.
2002, 41, 861. (e) Trost, B. M.; Ito, H.; Silcoff, E. R. J. Am. Chem. Soc.
2001, 123, 3367. (f) Trost, B. M.; Yeh, V. S. C.; Ito, H.; Bremeyer, N.
Org. Lett. 2002, 4, 2621. (g) Trost, B. M.; Silcoff, E. R.; Ito, H. Org. Lett.
001, 3, 2497. (h) Yoshikawa, N.; Shibasaki, M. Tetrahedron 2001, 57,
catalytic activity and enantioselectivity depended highly on
the central metals, and the samarium catalyst 4e was the best
(
7) (a) Ooi, T.; Takahashi, M.; Yamada, M.; Tayama,; E.; Omoto, K.;
Maruoka, K. J. Am. Chem. Soc. 2004, 126, 1150. (b) Hanawa, H.;
Hashimoto, T.; Maruoka, K. J. Am. Chem. Soc. 2003, 125, 1708. (c) Kii,
S.; Maruoka, K. Tetrahedron Lett. 2001, 42, 1935. (d) Maruoka, K. Catal.
Today 2001, 66, 33. (e) Hanawa, H.; Kii, S.; Asao, N.; Maruoka, K.
Tetrahedron Lett. 2000, 41, 5543. (f) Ooi, T.; Itagaki, Y.; Miura, T.;
Maruoka, K. Tetrahedron Lett. 1999, 40, 2137. (g) Ooi, T.; Miura, T.;
Takaya, K.; Maruoka, K. Tetrahedron Lett. 1999, 40, 7695. (h) Ooi, T.;
Miura, T.; Maruoka, K. Angew. Chem., Int. Ed. 1998, 37, 2347. (i) Ooi,
T.; Takahashi, M.; Maruoka, K. Angew. Chem., Int. Ed. 1998, 37, 835. (j)
Ooi, T.; Tayama, E.; Takahashi, M.; Maruoka, K. Tetrahedorn Lett. 1997,
6 4 2 2 2 2
among the tested catalysts 1,3-C H [CH N(CH CHPhO) LnI]
(
4a-e) (Table 1), which were derived by mixing (R)-2 and
8
2 equiv of LnI(η -cyclooctatetraene)(thf)
n
(3a: Ln ) La, n
)
3; 3b: Ln ) Ce, n ) 3; 3c: Ln ) Pr, n ) 3; 3d: Ln )
9
Nd, n ) 2; 3e: Ln ) Sm, n ) 1). The enantioselectivity of
the product decreased in the order of the increased atomic
radii, La ≈ Ce < Nd < Pr , Sm, such a tendency being
3
1
8, 7403. (k) Ooi, T.; Takahashi, M.; Maruoka, K. J. Am. Chem. Soc. 1996,
18, 11307.
(
8) (a) Manickam, G.; Sundararajan, G. Tetrahedron: Asymmetry 1997,
(9) (a) Mashima, K.; Nakayama, Y.; Nakamura, A.; Kanehisa, N.; Kai,
Y.; Takaya, H. J. Organomet. Chem. 1994, 473, 85. (b) Mashima, K.;
Takaya, H. Tetrahedron Lett. 1989, 30, 3697.
8
, 2271. (b) Trost, B. M.; Van Vranken, D. L.; Bingl, C. J. Am. Chem.
Soc. 1992, 114, 9327.
4696
Org. Lett., Vol. 6, No. 25, 2004

Table 2. Asymmetric Transfer Hydrogenation of Aryl Ketones
by the Catalyst System Derived from (R)-2 and
8
a
SmI(η -cyclooctatetraene)(thf) (3e)
run substrate time (h) conversion (%)b ee (%) (config)c
1
2
3
4
5
6
7
8
9
0
1
2
3
4
6
24
48
24
24
24
24
24
24
24
48
24
48
24
48
22
47
>99
77
90
>99
83
98
10
40
29
68 (R)
68 (R)
>99 (R)
93 (R)
87 (R)
89 (R)
78 (R)
>99 (R)
>99 (R)
>99 (R)
45 (R)
45 (R)
65 (R)
65 (R)
7
8
9
10
11
12
13
1
1
1
1
1
Figure 2. Ketonic substrates.
14
35
17
21
15
16
that ortho-substituted derivatives were readily reduced
compared with the corresponding meta- and para-substituted
derivatives (Table 2, runs 3-7) with higher enantioselec-
tivities (Table 2, runs 3 and 6). Such ortho-halogen sub-
a
Reaction temperature, 25 °C; reaction time, 24 h; S/C ) 20; 25 equiv
of 2-propanol for substrate. Conversion was determined by 1H NMR
spectroscopy. Enantiomeric purity was determined by chiral HPLC.
b
c
Absolute configuration was assigned by comparison of product rotations
to the literature value.
stituent effects have been reported for asymmetric hydro-
1
1
2b,10
genation by using samarium reagents.
A very effective
ortho-substituent influence was observed for methoxy ac-
etophenones 12 and 13 (Table 2, runs 8-10): 12 was almost
consumed within 24 h to give (R)-(+)-2-methoxy-R-meth-
ylbenzyl alcohol in >99% ee, whereas 13 was converted in
only 10%, though enantioselectivity was still very high
consistent with the catalysts derived from the reaction of a
2c
dilithium salt of (R)-1 ligand with LnI The salt-free
3
.
samarium species C CH N(CH CHPhO) SmI (5), in situ
6
H
5
2
2
2
derived by treating (R)-1 with 1 equiv of 3e, catalyzed the
asymmetric hydrogenation of acetophenone under the same
condition to afford the alcohol in 82% ee and 77% conver-
sion, whose values were less than the reported values (96%
ee and 83% conversion) when the samarium catalyst system
(>99% ee). Reduction of cyclic ketones 14 and 15 resulted
in slow conversion and low enantioselectivities (Table 2, runs
1
1-14).
In summary, we have prepared a new chiral multidentate
alkoxy ligand, (R)-2, capable of supporting two metal centers
at adjacent positions. Salt-free samarium catalyst mediated
asymmetric transfer hydrogenation of acetophenone and its
derivatives to give the corresponding alcohols in high
enantioselectivity (up to 99% ee).
derived from the reaction of the dilithium salt of (R)-1 with
c
1
equiv of SmI
3
was used.2
We measured pseudo-first-order rate constants for the
asymmetric transfer hydrogenation of acetophenone by using
the dinuclear samarium species 4e and the mononuclear
samarium species 5, both being salt-free catalysts, in the
temperature range of -10 to 20 °C. The rate constant kobs at
Acknowledgment. The present research was supported
in part by the grant-in-aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan
-
1
0
°C in the bimetallic catalyst system 4e (0.16 s ) was found
-
1
to be 10 times larger than that in 5 (0.012 s ), indicating
that two metal centers cooperatively accelerated the reaction.
Eyring plots afforded the thermodynamic parameters (4e:
Supporting Information Available: Experimental details
of the synthesis of chiral ligand (R)-2 and catalytic transfer
hydrogenation of ketonic substrates. This material is available
free of charge via the Internet at http://pubs.acs.org.
q
q
q
∆
8
(
H ) 34(1) kJ/mol, ∆S ) -170(1) J/mol‚K, and ∆G )
q q
1(2) kJ/mol (0 °C); 5: ∆H ) 71(6) kJ/mol, ∆S ) -55-
q
7) J/mol‚K, and ∆G ) 86(5) kJ/mol (0 °C)), the larger
q
negative value of ∆S found for 4e suggesting that acetophe-
none was easily trapped by the mutual two samarium centers.
Table 2 summarizes the asymmetric transfer hydrogenation
of various aryl ketones in 2-propanol at 25 °C. Catalytic
activity and enantioselectivity are sensitive to the substituents
on acetophenone. Enantioselectivity and conversion of the
reduction of propiophenone (6) decreased (Table 2, run 1).
For halogenated acetophenone derivatives 7-11, we found
OL048101F
(10) (a) Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. ReV. 1993, 93,
307. (b) Evans, D. A.; Kalor, S. W.; Jones, T. K.; Clardy, J.; Stout, T. J.
1
J. Am. Chem. Soc. 1990, 112, 7001.
11) For acetophenone, 9, 12, and 13: Hayashi, T.; Matsumoto, Y.; Ito,
(
Y. J. Am. Chem. Soc. 1989, 111, 3426. For 7 and 8: Pickard, S, T.; Smith,
H. E. J. Am. Chem. Soc. 1990, 112, 5741. For 10: Weibel, D. B.; Walker,
T. R.; Schroeder, F. C.; Meinwald, J. Org. Lett. 2000, 2, 2381. For 6, 14
and 15: Hu, Y.; Ziffer, H. J. Chromatogr. 1989, 482, 227.
Org. Lett., Vol. 6, No. 25, 2004
4697