Asymmetric transfer hydrogenation of ketones catalyzed by rhodium complexes containing amino acid triazole ligands

Article abstract of DOI:10.1039/c0ob00400f

Active and selective catalysts for the asymmetric reduction of ketones, under transfer hydrogenation conditions, were obtained by combining [RhCl ₂Cp*]₂, with a series of l-amino acid thioamide ligands functionalized with 1,2,3-triazoles. The obtained secondary alcohol products were formed with up to 93% ee.

Full text of DOI:10.1039/c0ob00400f

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Asymmetric transfer hydrogenation of ketones catalyzed by rhodium
complexes containing amino acid triazole ligands†
Fredrik Tinnis and Hans Adolfsson*
Received 8th July 2010, Accepted 6th August 2010
DOI: 10.1039/c0ob00400f
Active and selective catalysts for the asymmetric reduction
of ketones, under transfer hydrogenation conditions, were
obtained by combining [RhCl2Cp*]2, with a series of L-amino
acid thioamide ligands functionalized with 1,2,3-triazoles.
The obtained secondary alcohol products were formed with
up to 93% ee.
of compounds, possessing interesting coordination properties and
thus being suitable as ligands in asymmetric catalyzed reactions.
We,6 and others,7 have constructed active and selective ATH-
catalysts based on the use of a-amino acids. In one of our
previously described protocols for the rhodium-catalyzed ATH
reaction, we employed amino acid thioamide ligands (1).8 We
found that the activity and selectivity of the catalysts could be fine
tuned by structural variations of the ligands.8c Hence, variations
of the size of the aryl group (Ar) in 1 resulted in different steric
hindrance imposed by the ligand on the substrate during the
hydrogen transfer process. Herein, we report on a novel amino
acid-based ligand system (2), where the incorporation of 1,2,3-
triazoles allows for further fine tuning of the catalyst activity and
selectivity.
Asymmetric reductions of pro-chiral unsaturated compounds,
such as alkenes, ketimines and ketones, are important organic
transformations.1 The obtained enantiomerically enriched alka-
nes, amines, and alcohols are often employed as building blocks
incorporated into compounds possessing biological activity (e.g.
pharmaceuticals or agrochemicals). In most applications, catalytic
hydrogenations with transition metal catalysts in combination
with molecular hydrogen are being used. In contrast, asymmetric
transfer hydrogenation (ATH) represents a mild, practical and
safe alternative to catalytic hydrogenation.2 The ATH process
is characterized by the presence of a hydrogen donor, often the
solvent, and a transition metal catalyst with the ability to abstract
a hydride and a proton from the donor and deliver these atoms
to the substrate (the acceptor). Hydrogen donors such as 2-
propanol or formic acidare frequentlyemployed inATHreactions.
The substrates most suitable for transfer hydrogenations are
compounds possessing polarized unsaturation. The ATH reaction
is therefore mainly performed on ketones and ketimines, although
reductions of activated olefins can be accomplished. The classic ex-
ample of a hydrogen transfer process is the Meerwein–Ponndorf–
Verley reduction (MPV), in which aluminium isopropoxide is
used to mediate the transfer of a hydride from the isopropox-
ide to the coordinated ketone substrate.3 Today, most transfer
hydrogenations are performed using transition metal complexes,
predominantly based on ruthenium, rhodium or iridium.2 In the
An elegant method for the regioselective preparation of 1,4-
disubstituted 1,2,3-triazoles is the copper-catalyzed 1,3-dipolar
cycloaddition of organic azides and terminal alkynes.9 The
synthetic ease with which triazoles can be integrated into more
complex structures has led to numerous applications in quite
diverse areas.10 However, in catalysis there are only a few examples
of transition metal complexes where triazole-based ligands are
applied.11 In our previous studies of the ATH of acetophenone
in 2-propanol, using a rhodium-complex with ligand 1 (Ar = 2-
pyridyl) we obtained good catalytic activity and enantioselectivity
of the formed secondary alcohol.8c This observation indicates that
the presence of a potentially coordinating heteroatom in the aryl
group of the ligand did not diminish the catalytic activity of the
complex. To further our understanding on how other heterocycles
positioned at the C-terminal of the amino acid thioamide ligand
scaffold would affect the catalytic behaviour, we prepared a series
of compounds containing the 1,2,3-triazole unit. The preparative
route for the ligands with the common structure 2 is outlined
in Scheme 1. Activation of N-Boc-protected amino acids (3)
by isobutylchloroformate followed by treatment of the obtained
mixed anhydride with propargylamine led to the formation of
N-propargyl-substituted amino acid amides (4). Amino acid
amides containing 1,4-disubstituted 1,2,3-triazoles (5) were readily
obtained reacting 4 with various aryl and benzylic azides using the
Sharpless protocol.9a Treatment of the obtained amino acid amides
with Lawesson’s reagent resulted in the formation of the desired
ligand structures 2.
6
mid 1990’s, Noyori and co-workers found that ruthenium h -
arene complexes in combination with vicinal amino alcohol or
diamine ligands, respectively, served as efficient catalysts for the
reduction of ketones and ketimines under ATH-conditions.4 This
milestone discovery opened the field for further investigations,
and currently there are a number of different ligand classes
existing, that have been derived and employed in selective transfer
hydrogenations.5
An important feature when developing a new ligand for any
catalytic asymmetric application is the existence of modular
ligand building blocks. a-Amino acids are excellent examples of
building blocks which easily can be transformed into a variety
Department of Organic Chemistry, Stockholm University, The Arrhenius
Laboratory, SE-10691 Stockholm, Sweden. E-mail: hansa@organ.su.se;
Fax: (+) 46-8-154908; Tel: (+) 46-8-162484
† Electronic supplementary information (ESI) available: Experimental
procedures and spectral data. See DOI: 10.1039/c0ob00400f
With the novel amino acid derivatives 2a–i in hand, we set out
to investigate the catalytic activity of these compounds when used
as ligands in the rhodium-catalyzed ATH of acetophenone. The
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Table 1 Rhodium-catalyzed asymmetric transfer hydrogenation of
acetophenonea
Reaction
time/min
Entry
Ligand
Conv.(%)b
ee (%)c
TOF10/h-1d
456
1
2
3
4
5
6
7
8
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
10
120
10
120
10
120
10
120
10
120
10
120
10
120
10
76
88
62
81
79
90
67
87
68
79
68
84
76
91
73
88
68
90
90 (R)
89 (R)
89 (R)
87 (R)
71 (R)
66 (R)
89 (R)
86 (R)
88 (R)
88 (R)
72 (R)
67 (R)
93 (R)
90 (R)
93 (R)
89 (R)
92 (R)
88 (R)
372
474
402
9
408
10
11
12
13
14
15
16
17
18
Scheme 1 Synthetic method for the preparation of amino acid thioamides
functionalized with 1,2,3-triazoles For explanations of substituents R1 and
R2, see Scheme 2.
408
2f
2g
2g
2h
2h
2i
456
438
120
10
120
408
2i
a Reaction conditions: see Scheme 2. General procedure for the asymmetric
transfer hydrogenation of acetophenone using ligands 2a–i (Table 1);
{[RhCl2Cp*]2} (3.1 mg, 0.005 mmol), ligand (0.011 mmol) and LiCl
(4.2 mg, 0.1 mmol) were dried under vacuum in a dry Schlenk tube for
15 min. Distilled 2-propanol (4.5 mL), acetophenone (116 mL, 1 mmol)
and a solution of 0.1 M i-PrONa (1.0 mL, 0.1 mmol) were added under
nitrogen in sequential order. The reaction mixture was stirred at ambient
temperature. Aliquots were taken according to reaction times indicated in
Table 1, and were passed through a pad of silica using EtOAc as eluent.
The resulting solutions were analyzed by GLC (CP Chirasil DEXCB).
b Conversion determined by GLC. c Enantiomeric excess determined by
GLC (CP Chirasil DEXCB). d Turnover frequencies were calculated based
on the conversion obtained after 10 min.
The obtained conversions correspond to turn over frequencies
(TOF10) of >400 h-1. In comparison to other amino acid-based
catalyst systems used in the ATH of acetophenone, these triazole-
functionalized catalysts show significantly higher activity. The
enantioselectivity of the formed 1-phenylethanol ranged between
71–93%, with the best ees obtained using the catalysts derived
from the valine ligands 2a and 2g–i (entries 1, 13, 15 and 17).12
The use of the phenylalanine ligand 2b resulted in 89% ee (entry
3), whereas the catalytic reduction using the leucine-based ligand
2c only gave the product in 71% ee. Comparing the results after
two hours show that higher conversions were obtained in all cases,
however, no reaction went to completion.
The enantioselectivity of the product decreased in most reac-
tions, indicating that the equilibrium between starting materials
(acetophenone and 2-propanol) and products (1-phenylethanol
and acetone) was reached. Ligands 2g–i, derived from aryl
azides possessing different electronic properties, were included
in the study to get information on possible coordination from
the triazole to the rhodium ion.13 Theoretically, the triazole in
2i would coordinate stronger due to the electron releasing 4-
methoxy substituent on the aryl ring. However, as seen in Table 1,
comparing the results obtained with ligands 2g–i, only small
differences were observed, and no conclusion regarding triazole
coordination can be drawn. Attempts were made to isolate the
catalyst formed between the rhodium–arene precursor and these
novel amino acid-based ligands; however, so far we have been
unsuccessful in obtaining material suitable for characterization.
Scheme 2 Conditions for the rhodium-catalyzed ATH of acetophenone.
reaction conditions for the catalytic reduction, along with the
structures of the investigated ligands, are shown in Scheme 2.
The active reduction catalyst was formed in situ from the
rhodium–arene precatalyst {[RhCl2Cp*]2} in combination with
the appropriate ligand (2a–i) in the presence of sodium iso-
propoxide and lithium chloride. The catalytic ATH reaction was
performed in 2-propanol with an acetophenone concentration
of 0.2 M. We have previously discovered that the addition of
lithium chloride has a beneficial effect on the catalytic activity and
selectivity of the ATH process.6f The reason for higher activity and
selectivity is attributed to a tighter transition state in the hydride
transfer step from the rhodium-catalyst to the substrate, where a
lithium ion plays a crucial role in coordinating both substrate and
catalyst. Hence, under the conditions described, acetophenone
was reduced and the results are presented in Table 1. In the ligand
screening process, all reactions were sampled after 10 min and
after 2 h. As presented in Table 1, the ATH reactions proceeded
smoothly with most of the catalysts, reaching conversions rang-
ing from 62–79% after 10 min with 1 mol% catalyst loading.
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an impressing 5820 h-1.14 The enantioselectivity of the products
was unfortunately only moderate to good, and clearly inferior in
comparison to reactions catalyzed by rhodium–arene complexes
combined with other amino acid thioamide ligands.
Conclusions
In conclusion, we have demonstrated that highly active catalysts
for the asymmetric reduction of aryl alkyl ketones can be formed
from amino acid thioamide triazole ligands. The ligands were
readily prepared in good yields, using a straightforward pro-
cedure where the copper-catalyzed [3+2] cycloaddition between
N-propargyl amino acid amides and aryl or benzylic azides
was the key step. The catalytic activity of the in situ formed
rhodium complexes was in general very good, and in the case of
the reduction of 4¢-trifluoromethylacetophenone we were able to
reduce the catalytic loading to only 0.1 mol%. The typical amount
of catalyst used in ATH processes is 0.5–1 mol%, and going
below this amount usually results in unproductive conversions.
The high activity demonstrated by catalysts based on the triazole-
functionalized amino acid derivatives presented here makes them
promising for further studies.
Fig. 1 Aryl alkyl ketones screened in the ATH catalyzed by Rh-2a.
Table 2 Rhodium-catalyzed asymmetric transfer hydrogenation of aryl
alkyl ketonesa
Reaction
time/min
Entry
Substrate
Conversion (%)b
ee (%)c
1
6
120
10
120
10
120
10
120
10
120
120
120
120
120
73
97
>99
81
91
97
99
77
91
51
89
90
85
79 (R)
87 (R)
86 (R)
81 (R)
79 (R)
87 (R)
86 (R)
92 (R)
89 (R)
73 (R)
84 (R)
89 (R)
90 (R)
Acknowledgements
2d
3d
4
7
7
This work was financially supported by the Swedish Research
Council, the Carl Trygger Foundation, the Wenner-Gren Founda-
tions, the K & A Wallenberg Foundation, and the Magn. Bergvall
Foundation.
8
5
6
7
8
8
9
9
10
10
11
12
13
14
9
10
11
12e
13
Notes and references
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12 ATH of acetophenone using the isoelectronic ruthenium catalyst
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conversion and 80% ee of 1-phenylethanol after 2 h reaction time.
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14 The turnover frequency calculated at 5 min reaction time was
8836 h-1.
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Org. Biomol. Chem., 2010, 8, 4536–4539 | 4539
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