Enantioselective transfer hydrogenation of aliphatic ketones catalyzed by ruthenium complexes linked to the secondary face of β-cyclodextrin

Article abstract of DOI:10.1002/adsc.200700558

Ruthenium-η-arene complexes attached to the secondary face of β-cyclodextrin catalyze the enantioselective reduction (ee up to 98%) of aliphatic and aromatic ketones in aqueous medium in the presence of sodium formate (HCOONa).

Full text of DOI:10.1002/adsc.200700558

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DOI: 10.1002/adsc.200700558
Enantioselective Transfer Hydrogenation of Aliphatic Ketones
Catalyzed by Ruthenium Complexes Linked to the Secondary
Face of b-Cyclodextrin
Alain Schlattera and Wolf-D. Woggona,*
a
Department of Chemistry, University of Basel, St. Johanns Ring 19, CH-4056 Basel Switzerland
Fax : (+41)-61-267-1113; e-mail: wolf-d.woggon@unibas.ch
Received: November 27, 2007; Published online: March 18, 2008
Dedicated to Andreas Pfaltz on the occasion of his 60th anniversary.
Abstract: Ruthenium-h-arene complexes attached
to the secondary face of b-cyclodextrin catalyze the
enantioselective reduction (ee up to 98%) of ali-
phatic and aromatic ketones in aqueous medium in
the presence of sodium formate (HCOONa).
excesses (ee) have been accomplished for a number of
aromatic and aliphatic ketones in the presence of
HCOONa as the hydrogen donor in water and water/
DMF mixtures, respectively.[13] We convey here an al-
ternative strategy employing the modification of the
secondary face of b-cyclodextrin 1 (Figure 2).
Keywords: aliphatic ketones; b-cyclodextrin; enan-
tioselective reduction; ruthenium; transfer hydroge-
nation
Most of the systems which use the unique recogni-
tion/binding of hydrophobic substrates within the
cavity of b-cyclodextrins to create catalysts, often re-
sembling the reactivity of enzymes, were prepared by
derivatization of the primary face, that is, linking re-
active groups[14]/metal complexes[15,16] to C-6 of one of
the glucose units of the cyclic heptamer. In contrast,
Metalloenzymes serve as a paradigma for metalorgan- modification of the secondary face has been less fav-
ic, supramolecular catalysts for oxidations, reductions, oured in the past.[17] Based on a recent X-ray struc-
[1,2]
À
C C bond formation and cleavage.
Inter alia these ture of a b-cyclodextrin derivative we believed, how-
systems can be mimicked by a combination of metal ever, that it would be possible through secondary
complexes with cyclodextrins generating a center of face-modification of one sugar unit to change the size
reactivity and a hydrophobic binding site for sub- and shape of the cavity of b-cyclodextrin.[18] It was an-
strates, respectively. Beautiful examples have been re- ticipated that this would create distinctive features of
ported[3–4] which could be classified as systems in the corresponding ruthenium complex suitable for the
which the cyclodextrins act as first- and transient enantioface selective reduction of aliphatic ketones.
second-sphere ligand.[5]
We prepared the epoxide 2 in two steps from 3 ac-
We report here on a new supramolecular catalyst in cording to published procedures.[19] Addition of the
which a b-cyclodextrin derivative provides the first
ligand sphere for ruthenium, simultaneously binds lip-
ophilic molecules and participates in orientation and
activation of the substrates. Such a system could be
useful for the enantioselective reduction of non-con-
jugated ketones by ruthenium complex-catalyzed hy-
drogen transfer reactions which still remain a chal-
lenge.[6–12] Selective hydride addition to the re- or the
si-face of the carbonyl group could be accomplished
by artificially enlarging the difference of the two sub-
stituents at the carbonyl through encapsulation of one
group in the cavity of b-cyclodextrin (Figure 1).
For this purpose we have recently used a supra-
molecular system consisting of ruthenium coordinat-
ing to chiral amino alcohols attached to the primary
face of b-cyclodextrin 1 (Figure 2). High enantiomeric tive hydride addition to aliphatic ketones.
Figure 1. Concept of cyclodextrin-supported, enantioselec-
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Alain Schlatter and Wolf-D. Woggon
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Figure 2. Structure of b-cyclodextrin 1.
achiral amino alcohol 4 gave a single product 5 in value observed for the di-equatorial situation (J1,2
=
1
4
which the new altrose unit adopts the C4 conforma- 2.3 Hz).[20] Accordingly, the initially formed C1 con-
1
1
tion. Evidence for the C4 conformation rests on the formation 6 flips over to the more stable C4 form 5
coupling constant of the two di-axial protons at C-1 changing the interior of b-cyclodextrin to a distorted
and C-2 (J1,2 =7.3 Hz) which is about three times the ellipsoid cavity (Scheme 1).[18]
Scheme 1. Synthesis of the b-cyclodextrin derivative 7 with a distorted cavity; a) NaH, 1-tosyl-1,2,4-triazole, THF; b)
NaOEt, EtOH; c) NH2CH2CH2OH, EtOH, reflux, 12 h; d) TBAF, THF.
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Enantioselective Transfer Hydrogenation of Aliphatic Ketones
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Table 1. Hydrogen transfer reactions catalyzed by 8 and
11.[a]
complexes of 7 were prepared in situ prior to catalytic
experiments by mixing 7 with [RuCl CHTREN(UG C6H6)]2. Surpris-
2A
1
ingly H NMR experiments of this Ru complex 8 re-
Entry
Substrate
Yield[b]
ee[c]
À À
À
À
À
vealed no chemical shifts of the O CH2 CH2 NH
unit as observed when this moiety was linked to C-6
at the primary face of b-cyclodextrin.[13] Nevertheless,
8 displayed very good reactivity and high enantiomer-
ic excess in the hydrogen transfer reaction to aliphatic
ketones (Table 1).
99%[8]
89%[8]
99%[11]
86%[11]
1
98%[8]
92%[8]
99%[11]
93%[11]
2
3
Hence, it was conlcuded that Ru does not coordi-
nate to the amino ethanol unit but to the amino-N
and the CD-oxygen at C-2, see 8 (Figure 3). Accord-
98%[11]
96%[11]
À
ingly, the N CH3 derivative 10 was prepared and the
corresponding Ru complex 11 generated. The
1H NMR spectrum of 11 shows considerable down-
field shifts of the protons of the glucose unit which
35%[8]
76%[11]
95%[8]
99%[11]
98%[8]
93%[11]
90%[8]
86%[11]
4
À
carries the N CH3 group and thus confirms that Ru
coordinates to N H and an adjacent OH group at C-
5
À
2 of the same sugar (e.g., 0.23 Dppm for H at C-2;
0.63 Dppm for H at C-3; 0.38 Dppm for N CH3
group) (Scheme 2).
6
60%[8]
89%[8]
À
72%[8]
98%[11]
80%[8]
99%[11]
55%[8]
99%[11]
56%[8]
80%[11]
70%[8]
74%[11]
89%[8]
89%[11]
91%[8]
93%[11]
91%[8]
93%[11]
7
The results of hydrogen transfer reactions using 8
and 11, HCOONa in water/DMF 3:1and various ke-
tones are shown in Table 1. Although the yields vary
to a certain extent the enantiomeric excesses are
quite consistent in the series around 90% ee in favour
of the S-configuration. Note that chiral amino alco-
hols such as ephedrine lacking a CD unit show only
8
9
10
11
37%[11]
93%[11] moderate ee values for various carbonyl compounds
under hydrogen transfer conditions.[12,21]
55%[11]
91%[11}
77%[11]
This is an intriguing result since for catalysts such
as 8 and 11 the b-cyclodextrin unit is the only chiral
moiety to induce enantioface selectivity. It is impor-
tant to note that selectivities relying on the enantio-
face differentiating capacity of b-cyclodextrin alone
rarely exceed 50% ee.[13,22,23]
Unfortunately, the metal complexes 8 and 11 re-
fused to gave crystals suitable for X-ray analysis, ac-
cordingly we have no precise information on the
structure of these pre-catalysts, the substrate inclusion
complexes and the corresponding catalytically active
hydride-ruthenium complexes. The NMR spectra of 8
and 11 provided no evidence that the phenyl ring co-
12
13
61%[11]
99%[11]
40%[11]
14
85%[11]
82%[12]
15
[a]
Reaction conditions: 8/substrate/HCOONa=1:10:100 in
water/DMF=3:1; 508C, 12 h; same conditions for 11
except T=258C .
Yields were determined by GC. No side products ob-
served in any catalytic reaction.
Absolute configurations and enantiomeric excesses were
determined by HPLC (entries 1–3), GC (entries 4 and 5)
and NMR analyses of the Mosher esters of products (en-
tries 6–15). All products possess the S configuration.
[b]
[c]
Removal of the protecting groups with TBAF in
THF gave the free b-cyclodextrin 7; its NMR spec-
trum in D2O revealed the same altrose conformation
as for 5 which is evident from J1,2 =6.7 Hz of the di-
axial protons at C-1and C-2 distinct from axial-equa-
torial protons (J1,2 between 3.8 and 4.1Hz), respec-
tively, of the glucose units of 7. The ruthenium arene Figure 3. Structure of 8 – coordination of Ru to ligand 7.
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Alain Schlatter and Wolf-D. Woggon
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Scheme 2. Structure of catalyst 11; a) NH2CH3, EtOH, reflux in autoclave; b) NH4F, MeOH; c) [RuCl2ACHTRE(UNG C6H6)]2, H2O.
ordinating to ruthenium is included into the cavity of selectivity of a catalyst in which b-cyclodextrin is the
b-cyclodextrin. This result is supported by molecular only chiral auxiliary. The origin of enantioselectivity,
modelling revealing that the attachment of Ru to O however, remains speculative to a certain extent.
at C-2 and N at C-3 is too rigid to allow for a flexibili- Through close attachment to the secondary face of b-
ty of the unit which could render the phenyl ring to cyclodextrin the ruthenium becomes most likely
be included into the cavity of b-cyclodextrin. Regard- chiral. In the catalytically active complex the hydride
ing substrate binding we have NOE evidence that, for is transferred preferentially to the re-face of the car-
example, 4-tert-butylacetophenone (entry 3, Table 1) bonyl group which is exposed through hydrogen
binds into the cavity of b-cyclodextrin derivatives.[24] bonding and orientation of the longer side chain in
The formation of substrate inclusion complexes is fur- the cavity of b-cyclodextrin (Figure 4). The situation
ther supported by the fact that the excellent results seems to be similar for both catalysts derived from 8
reported in Table 1were only obtained in water or
and 11 since entries 1–2, 4–5 and 7–10 in Table 1 show
water/DMF (3:1) whereas in apolar solvents such as similar ee values but significantly higher yields for the
dichloromethane only 6% yield and ee values as low hydride complex of 11.
À
as 21% were observed. The significance of the N H
proton for catalysis and substrate orientation has
been already discussed.[13]
In conclusion, aminoethanol and methylamine, re-
Experimental Section
spectively, were linked to the secondary face of b-cy-
clodextrin at C-3. Due to a chair flip of the modified
altrose unit the cyclodextrin cavity is elliptically dis-
torted and hence offers new binding properties for
guest molecules. The Ru complexes of the CD deriva-
tives were employed in hydrogen transfer reactions to
ketones. Enantioselectivities from 70% to 98% are
observed for a series of aliphatic ketones representing
the highest ee values for these challenging substrates
NMR spectra were recorded on a Broker 500 spectrometer
(1H NMR at 500 MHz, 13C NMR at 125 MHz) in CDCl3 or
D2O. Chemical shifts are reported in ppm and calibrated
with the solvent peak (CDCl3 =7.26 ppm, D2O=4.79 ppm).
TLC was visualized by spraying the plates with a solution of
250 mL ethanol, 6 g anisaldehyde and 2.5 mL concentrated
sulfuric acid. Mono(2A,3A-anhydro)-heptakis(6-O-tert-butyl-
dimethylsilyl)-b-CD (2) was synthesized according to a re-
to date and also show unprecedentedly high enantio- ported procedure.[14]
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Enantioselective Transfer Hydrogenation of Aliphatic Ketones
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Synthesis of Mono-3-deoxy-3-[N-methyl]heptakis(6-
O-tert-butyldimethylsilyl)-3-amino-b-CD (9)
Compound 2 (118 mg, 0.06 mmol, 1.0 equiv.) and 0.23 mL of
8M methylamine in EtOH (1.85 mmol, 30 equiv.) were
stirred in 0.75 mL EtOH in the microwave oven for 6 h at
828C. After removal of solvent and excess methylamine, the
residue was dissolved in DCM and washed with water. The
organic phase was dried over Na2SO4 and purified by
column chromatography (SiO2, EtOAc/EtOH/H2O/Et3N,
50:7:4:0.6), affording 9 as colourless crystals; yield: 77 mg
(64%). TLC (SiO2, EtOAc/EtOH/H2O/Et3N, 50:7:4:0.6) Rf =
0.17; 1H NMR (500 MHz, CDCl3): d=4.85–4.94 (m, 5H,
5H1), 4.81(d, J=3.6, 1H, H1), 4.48 (d, J=7.3, 1H, H1),
3.40–4.16 (m, 40H), 3.01–3.09 (m, 1H), 2.60–2.67 (m, 1H),
2.38 (broad, 3H, CH3), 0.84–0.90 (m, 63H, CH3C), 0.01–0.05
(m, 42H, CH3Si); ESI-MS (MeOH): positive ion mode:
m/z=1948 (100, [M+H]+)
Figure 4. Proposed interaction between aliphatic ketones
and the Ru hydride complex corresponding to 11.
Synthesis of Mono(3-deoxy-3-[N-(2-hydroxyethyl)]-
heptakis(6-O-tert-butyldimethylsilyl)-3-amino-b-CD
(5)
Synthesis of Mono-3-deoxy-3-[N-methyl]-3-amino-b-
CD (10)
Aminoethanol 4 (228 mg, 3.74 mmol, 10.0 equiv.) was added
to a solution of 736 mg mono(2A,3A-anhydro) heptakis(6-O-
tert-butyldimethylsilyl)-b-CD (2) (0.38 mmol, 1.0 equiv.) in
0.5 mL ethanol and refluxed overnight. After the addition of
2 mL of water, the mixture was extracted 3 times with 5 mL
of dichloromethane, dried over Na2SO4 and chromatograph-
ed over silica gel (ethyl acetate/ethanol/water, 20:4:2) to
give pure 5 as a colourless powder; yield: 620 mg (82%).
Compound 9 (77 mg, 0.04 mmol, 1.0 equiv.) and 51 mg am-
monium fluoride (1.38 mmol, 35 equiv.) were refluxed in
2 mL MeOH for 2 h. After removal of the solvent, the
crude product was purified by RP-18 chromatography (gra-
dient: water only to 25% MeOH in 30 mL fractions), afford-
ing 10 as colourless crystals; yield: 34 mg (73%). TLC (SiO2,
EtOAc/i-PrOH/NH4OH/H2O=7:7:5:4): Rf =0.15; 1H NMR
(500 MHz, D2O): d=5.12–5.18 (m, 2H, 2H1), 5.01–5.08 (m,
4H, 4H1), 4.89 (d, J=6.9, 1H, H1), 4.36 (m, 1H, H5), 4.17
(m, 1H, H4), 3.52–4.02 (m, 39H, 7H2, 6H3, 6H4, 6H5, and
MS (ESI): m/z=1978 (M+, 68), 1988.5 ([2M+Na]2+, 48),
ACHTREUNG
2000 (
ACHTREUNG
1
20:4:2); mp >2758C; H NMR (500 MHz, CDCl3): d=0.00–
0.10 (m, Me-Si, 42H), 0.80–0.90 (m, Me3-C, 63H), 2.67 (m,
HOCH2CH2NH-CD,
1H),
2.75
(m,
H-3A
and
14H ), 3.02 (broad, 1H, H3), 2.51(s, 3H, CH 3); ESI-MS
6
(MeOH): positive ion mode: m/z=585.5 (32, [M+H]2+),
HOCH2CH2NH-CD, 2H), 3.34–4.15 (heavily overlapped,
43H), 4.50 (d, J=7.3 Hz, H-1A, 1H), 4.82 (d, J=7.3 Hz,
1H), 4.86–4.94 (m, 5H); 13C NMR (125 MHz, CDCl3): d=
À5.0 (Me-Si), 18.4 (C-Me3), 26.1( Me3-C), 50.8
1148 (100, [M+H]+), 1170 (48, [M+Na]+)
General Procedure for Catalysis
(HOCH2CH2NH-CD),
60.6,
60.9
(C-3A),
61.6
(HOCH2CH2NH-CD), 61.7, 61.8, 62.0, 71.5, 71.9, 72.5 (C-
2A), 72.6, 73.7, 73.9, 74.0, 74.1, 74.3, 74.6, 78.4, 80.5, 80.9,
81.4, 81.5, 81.7, 82.2, 101.6, 102.1, 102.5, 102.7, 103.0, 105.7
(C-1A).
Ligand 8 (11) (0.005 mmol) was dissolved in H2O/DMF (3:1,
0.25 mL), [RuCl2ACHTRE(UGN C6H6)]2 (0.0025 mmol) was added, and the
resulting mixture was stirred for 1h at room temperature.
HCOONa (0.500 mmol) was then added, and after further
stirring for 10 min the ketone (0.050 mmol) was injected.
The reaction was then stirred for 12 h at 508C (258C). The
mixture was extracted three times with hexane (2 mL), the
combined hexane extracts were washed with water (6 mL)
and dried over Na2SO4. The yields were analyzed by GC.
The ee values of the products were determined by chiral
Synthesis of Mono-3-deoxy-3-[N-(2-hydroxyethyl)]-3-
amino-b-CD (7)
Compound 5 (525 mg, 0.27 mmol, 1.0 equiv.) and 2.66 mL
(2.66 mmol, 10.0 equiv.) of a 1M solution of TBAF in THF
were refluxed in additional 2 mL of THF for 4 h. THF was
removed and the purple residue was dissolved in 2 mL of
water. The water phase was washed 5 times with 5 mL of
chloroform and purified over Amberlite CG-50 (100–200
mesh) ion exchange resin (elution with water) to give pure 2
as a colourless powder; yield: 265 mg (85%). MS (ESI):
1
HPLC, chiral GC or H NMR/19F NMR spectroscopic stud-
ies of the corresponding Mosher esters: entry 1(HPLC):
Chiralcel OD-H, 0.46 cm, 25 cm, 0.5 mLminÀ1, heptane/
i.PrOH, 97:3, S-enantiomer 20.5 min, R-enantiomer
21.6 min; entry 2 (HPLC): Chiralcel OD-H, 0.46 cm, 25 cm,
0.5 mLminÀ1, heptane/i-PrOH. 95:5, S-enantiomer 22 min,
R-enantiomer 16 min; entry 3 (HPLC): Chiralcel OD-H,
0.46 cm, 25 cm, 0.5 mLminÀ1, heptane/i-PrOH 95:5, R-enan-
tiomer 11 min, S-enantiomer 12 min; entry 4 (GC): Hydro-
ACHTREUNG
m/z=1178 (M+, 100); Rf: 0.23 (RP-18, methanol/water, 1:1);
mp >2758C; 1H NMR (500 MHz, D2O): d=3.31–3.46 (m,
HOCH2CH2NH-CD, 2H), 3.54–3.73 (heavily overlapped,
14H), 3.79–4.04 (heavily overlapped, 31H), 4.31–4.39 (m,
2H), 5.01–5.07 (m, 4H), 5.11 (d, J=3.9 Hz, 1H), 5.14 (d, J=
3.9 Hz, 1H), 5.18 (d, J=4.0 Hz, 1H).
dex 3-b-P, 25 m, 0.25 mm, from 1008C to
1
18C 9
(0.28CminÀ1), S-enantiomer 75 min, R-enantiomer 77 min;
entry 5 (GC): Hydrodex 3-b-P, 25 m, 0.25 mm, from 1008C
to 1108C (0.28CminÀ1), S-enantiomer 32 min, R-enantiomer
35 min; entries 6–15 (Mosher ester): general procedure: The
reaction mixture was quenched with water (1mL) and ex-
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Alain Schlatter and Wolf-D. Woggon
COMMUNICATIONS
tracted with TBME (3 ” 2 mL). The organic phase was
dried over Na2SO4 and the alcohol separated from the rest
of the ketone (kieselgel, hexane/TBME, 3:1). The alcohol
was stirred with (R)-(+)-a-methoxy-a-trifluoromethylphen-
[9] T. Ohkuma, C. A. Sandoval, R. Srinivasan, Q. Lin, Y.
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[10] J. Mao, B. Wan, F. Wu, S. Lu, Tetrahedron Lett. 2005,
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ACHTREUNGylacetyl chloride in 0.25 mL of pyridine for 30 min. After
quenching with 1mL of water the solution was extracted
three times with 2 mL of TBME, the organic phase dried
over Na2SO4 and purified over kieselgel (hexane/TBME
10:1) to get the pure Mosher ester, which was analyzed by
1H NMR/19F NMR.
[11] X. Wu, X. Li, M. McConville, O. Saidi, J. Xiao, J. Mol.
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[14] R. Breslow, B. Zhang, Acc. Chem. Res. 1995, 28, 146–
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Acknowledgements
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We thank Dr. D. Häussinger for NMR-spectra and the Swiss
National Science Foundation for supporting this research.
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