Zinc-Catalyzed Enantioselective Hydrosilylation of Ketones and Imines under Solvent-Free Conditions

Article abstract of DOI:10.1002/cctc.201601140

The zinc acetate promoted asymmetric hydrosilylation of various ketones and imines under solvent-free conditions was examined by using an unprecedented low catalyst loading. Exposure of ketones to 0.05 mol % Zn-based chiral diamine complex in the presence of triethoxysilane afforded enantioenriched alcohols in excellent yields (up to 98 %) and enantioselectivities (up to 97 % ee). This methodology also allowed for the chemoselective 1,2-reduction of α,β-unsaturated ketones and imines.

Full text of DOI:10.1002/cctc.201601140

Accepted Article
Title: Zinc-Catalyzed Enantioselective Hydrosilylation of Ketones and
Imines Under Solvent-Free Conditions
Authors: Marcin Szewczyk, Agata Bezłada, and Jacek Mlynarski
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10.1002/cctc.201601140
ChemCatChem
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Zinc-Catalyzed Enantioselective Hydrosilylation of Ketones and
Imines Under Solvent-Free Conditions
Marcin Szewczyk,[a] Agata Bezłada,[a] and Jacek Mlynarski*[a,b]
Dedication ((optional))
Abstract: Zinc acetate-promoted asymmetric hydrosilylation of
various ketones and imines under solvent-free conditions is reported
with unprecedented low catalyst loading. Exposure of ketones to 0.05
mol% Zn-based chiral diamine complex in the presence of
triethoxysilane affords enantioenriched alcohols in excellent yields of
isolated products (up to 98%) and ee (up to 97%). This methodology
also allowed for chemoselective 1,2-reduction of α,β-unsaturated
ketones and imines.
Catalytic reduction of prochiral ketones and imines is a
valuable attempt to obtain enantiopure alcohols and amines.
Among many various methods for the reduction of carbon-
heteroatom unsaturated bond, hydrosilylation promoted by chiral
metal-based catalysts is especially attractive, due to its mild
conditions and relatively low loadings of the chiral agent.[1]
Platinum group metals-based catalysts are considered to be
particularly effective for the stereoselective reduction of the
carbonyl compounds even at low catalyst loading,[1,2] yet high
toxicity and cost of platinum group metals seem to be substantial
limitations, especially in the large-scale synthesis of bioactive
compounds.
In respect to meet green chemistry approaches,
consideration of environmental concerns must be addressed at
early stages of product development. This is also important in the
synthesis of optically pure alcohols and amines which are crucial
building blocks for the synthesis of complex molecules. By doing
Scheme 1. Development of non-noble metal-based catalysis at low catalyst
loadings.
so, firstly an application of highly efficient catalytic systems based
on inexpensive, safer, non-noble metal sources and secondly
as efficient catalysts for the asymmetric hydrosilylation of
excluding of unnecessary reagents and solvents should constitute
carbonyl substrates, recently.
the crucial development priorities. To meet these criteria, a
number of iron[3,4] and zinc[5-6] complexes have been developed
The first example of zinc-based asymmetric hydrosilylation
of ketones was reported by Mimoun in 1999 by using
polymethylhydrosiloxane (Scheme 1a).[6a] Following this
[a]
[b]
Faculty of Chemistry
Jagiellonian University
Ingardena 3, 30-060 Krakow, Poland
Phone: (+48)-12-663-2035; E-mail: jacek.mlynarski@gmail.com
Homepage: www.jacekmlynarski.pl
Institute of Organic Chemistry
achievement, a number of protocols based on diethylzinc
complexes was then developed,[6] but the most efficient examples
require 1-2 mol% of a catalyst.[6a-f,h] Recent progress in this field
concerns application of cheap, stable and non-hazardous zinc
acetate[7,8] successfully replacing previously used diethylzinc
complexes.[6] However, known Zn-based catalysts are still less
efficient in comparison with platinum-group metals. In particular,
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw, Poland
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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hydrosilylation of carbonyl compounds employing low catalyst
loadings (≤1%) are fairly limited.
Table 1. Asymmetric hydrosilylation of 1a under various conditions.[a]
Noteworthy progress have been observed in non-
asymmetric hydrosilylation of ketones promoted by iron
complexes.[4d,9] Basing on Tilley’s work,[9a] Turculet reported the
use of N-phosphinoamidine-based catalyst (Scheme 1b) at
remarkably low loadings (down to 0.01 mol%).[9b] Recently, iron
complexes with BIAN ligands were tested by Findlater.[9c]
Interestingly, various ketones were reduced by using 1 mol% of
catalyst under solvent-free conditions at elevated temperature
(Scheme 1c). More efficient catalyst was presented by Chirik and
co-workers who investigated the iron dialkyl complexes of pybox
ligands for the hydrosilylation of acetophenone derivatives
(Scheme 1d). The reactions were carried out under mild
conditions with only 0.3 mol% of catalyst providing corresponding
alcohols with moderate enantioselectivities (up to 50%),
Cat.
loading
[mol%]
Conv.
[%][b]
ee
Entry
Silane
Solvent
Temp.
[%][b]
DEMS
TES
TES
TES
TES
TES
TES
THF
THF
THF
THF
-
rt
rt
rt
1
-
-
1
2
1
99
99
-
93
94
-
0.3
0.3
0.3
0.05
0.05
3
-30 °C
4
5[c,d]
6[c,e]
7
rt
rt
99
99
-
89
94
-
-
-
-30 °C
however.[4d]
A significant example of more reactive and
[a] Reactions were carried out by stirring zinc acetate with L1 (1:2 molar
ratio) in THF (1 mL) at room temperature under argon. Silane (1 mmol) and
acetophenone (0.5 mmol) were added and the reaction mixture was stirred
at the mentioned temperature for 24 h. [b] Conversions and ees were
determined by HPLC methods. [c] Reaction was carried out by stirring
acetophenone (5 mmol) with zinc acetate and L1 (1:2 molar ratio) in TES
(10 mmol) at room temperature under argon. [d] Reaction was completed
after 40 min. [e] Reaction was completed after 6 h.
enantioselective hydrosilylation of ketones has been presented
recently by Gade.[4k] The reaction scope was, however,
demonstrated by using 5 mol% of iron complex with boxmi ligand.
Recently, we elaborated
a convenient protocol for
enantioselective hydrosilylation of prochiral ketones[8a,b] and
imines[8c] by using readily available chiral Zn(OAc)2 complexes.
However, the relatively high catalyst loading (5 mol%) was
considered to be a potential limitation for industrial application of
our methods.
(2a) as a model product. The reaction carried out in THF with 5
mol% of the catalyst resulted in the formation of desired alcohol
with 94% ee when DEMS was used as a hydrogen source and
93% ee in the case of TES. Further reduction of the catalyst
loading was unfortunately not possible in the case of DEMS
(Table 1, entry 1), but to our delight TES remained a useful
hydrogen source even at low catalyst loading (entries 2 and 3).
Given the considerable efficiency of Fe-based catalytic system
under solvent-free conditions reported previously[9c,11] we decided
to exclude solvent from the reaction. Such protocol resulted in
dramatic reduction of the reaction time from about 24 hours (entry
3) to 40 minutes (entry 5). High concentration of the reagents,
however, caused heat evolution and the resulting product was
obtained with lower enantioselectivity. Further decrease of the
amount of the catalyst to 0.05 mol% provided the desired product
with 94% ee and quantitative conversion of the substrate (entry
6). The catalytic activity of our system declines at low
temperatures (entries 4 and 7) while an excellent catalytic activity
was maintained for the reaction carried out at room temperature.
After choosing the best silane and with the reaction
conditions optimized, several alkyl-aryl ketones have been
investigated as useful carbonyl substrates. In all cases, the
Although significant steps toward sustainability can be
made by decreasing of the catalyst loading, another
environmentally benign strategy is to conduct the reactions under
solvent-free conditions.[10] To our best knowledge such a protocol
has never been used for Zn-catalyzed enantioselective
hydrosilylation,[9,11] and some previously reported studies required
application of protic solvent as a crucial reaction component.[6j]
Now, we present an improved protocol for the asymmetric
hydrosilylation where only 0.05-0.1 mol% of zinc acetate
complexes has been applied for the reduction of various ketones
and imines. Moreover, presented stable zinc complexes bearing
convenient
(R,R)-1,2-diphenylethylene-1,2-diamine-derived
ligands can be employed at room temperature not only with
unprecedented activity but also selectivity for the 1,2-reduction of
α,β-unsaturated ketones under solvent-free conditions.
Based on previous studies in our group[8] we selected
(EtO)2MeSiH (DEMS) and (EtO)3SiH (TES) as the most promising
silanes allowing to obtain enantioenriched (S)-1-phenylethanol
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slight increase of ee value was observed, whereas 2k with an
electron-withdrawing substituent at para position was reduced
with a low level of enantioselectivity (81%). The hydrosilylation of
2-acetonaphthone was also carried out with very good ee value
(2l).
Table 2. Yields and ee values of acetophenone derivatives hydrosilylation
promoted by zinc acetate-L1 complex.[a]
Based on the literature and our own studies, we postulate a
tentative mechanism for the hydrosilylation of acetophenone by
using [Zn]-DPEDA catalytic system as depicted in Scheme 2.
Previously, Kwit[12] suggested that formation of a hydrogen bond
between the proton of the amino group and the oxygen atom of
the carbonyl group is crucial for spatial arrangement of both a
catalyst and a ketone. Moreover, the pathway assuming formation
of a Zn-hydride species was reported to be the most favorable.
We indeed observed a Zn-H at δ = 4.55 ppm (see Supporting
Information). Thus, based on 1H NMR spectra analysis, we
suggested two possible transition states explaining observed
enantioselectivity (Scheme 2). The key step of the reaction
involves formation of a hydrogen bond between the amino group
[a] Reactions were carried out by stirring ketone (5 mmol) with zinc acetate
and L1 (1:2 molar ratio) in TES (10 mmol) at room temperature under argon
atmosphere for 6 h. The resulting silyl ethers were hydrolyzed with TBAF.
After purification by column chromatography ee values were determined by
HPLC methods. [b] 0.3 mol% of catalyst were used. [c] Reaction was
completed after 10 h.
Scheme 2. Proposed transition states explaining observed
stereoselevtivity.
reactions proceeded efficiently with only 2 equivalents of the
silane. The results are summarized in Table 2 with given yields of
the final products isolated by column chromatographic purification.
An increase of steric bulkiness of an alkyl chain heightened ee
values to 96% (2b, 2c), however, these substrates required
increasing catalyst loading to 0.3 mol% for the reactions to
proceed.
of the activated Zn-H catalyst and the carbonyl group of the
ketone. Relative orientation of the substrate is determined by the
steric repulsion between bulky 3,5-di-tert-butylbenzene moiety
and larger substituent adjacent to the carbonyl group. In the
favored approach zinc hydride attacks acetophenone from the Re
face resulting in the formation of (S)-configuration product.
Having in hand this efficient catalyst, we decided to test also
its applicability to the hydrosilylation of unsaturated ketones and
imines. In contrast to hydrosilylation of alkyl-aryl ketones and
imines relatively few reports were quoted involving selective 1,2-
reduction of α,β-unsaturated compounds promoted by non-noble
metals, in most cases focusing only on reduction of 4-phenylbut-
3-en-2-one.[4a,4f-g,6a,6d,6n,7a-b,13] However, the reaction deserves
special attention since so-obtained allyl alcohols and amines may
The hydrosilylation reactions of acetophenone derivatives
bearing various electron donating (2d-2i) and electron-
withdrawing groups (2j-2k) were also carried out with very good
yields. While for most substrates the efficiency of our catalytic
system did not depend on the type of the substituent and alcohols
were obtained with slightly lower enantioselectivities compared
with acetophenone (about 90%), for 2e bearing m-MeO group a
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Table 4. Conversion and ee values of various ketimines hydrosilylation
promoted by zinc acetate-L2 complex.[a]
be used for a wide range of subsequent diastereoselective
transformations.[14] Recently, Lipshutz provided protocols for the
CuH-catalyzed 1,2-reduction of β,β- and α-substituted enones
leading to secondary allyl alcohols (Scheme 1c).[13b,c] High
enantioselectivities (up to 95%) were induced by expensive and
difficult to obtain SEGPHOS and BIPHEP ligands.
Thus, the Zn-L1 catalyzed enantioselective hydrosilylation
was extended to various enones and biaryl ketones (Table 3). It
should be noted that an optimal catalyst loading strongly depends
on type of substrate and was carefully determined for each
example. We started our substrate screening with 4-phenylbut-3-
en-2-one (3a) which resulted in the chemoselective 1,2-reduction
providing corresponding product (4a) with moderate yield and
Table 3. Yields and ee values of various ketones hydrosilylation promoted
by zinc acetate-L1 complex.[a]
[a] Reaction was carried out by stirring N-phosphinylimine (0.44 mmol) with
zinc acetate (0.022 mmol) and L2 (0.022 mmol) in TES (0.88 mmol) at room
temperature under argon atmosphere for 24 h. The reactions were then
quenched by addition of NaOH in MeOH. After purification by column
chromatography ee values were determined by HPLC methods.
81% ee. Then we tested the impact of further changes of α-
substitution on the outcome of the reaction. α-Substitution with a
methyl (4b) or a benzyl group (4d) led to the desired products with
very good enantioselectivities (89% for both examples), whereas
phenyl group (4c) decreased enantioselectivity down to 75%. This
tendency was previously reported by Lipshutz.[13b] Modification of
the length of an alkyl chain at β (4e) or α’ position (4f) did not
affect the stereoselectivity of the reaction leading to the
corresponding alcohols with 81% and 88% ee respectively. The
hydrosilylation reactions of substrates with two sp2 carbon atoms
adjacent to carbonyl group (4g-i) resulted in either obtaining
products with good yield but poor, close-to-racemic
enantioselectivity (4g, 4h) or no conversion in case of substrate
3i. The reductions of substrates with various conjugated systems
were also carried out. Interestingly, the reduction of the substrate
bearing ynone moiety gave a corresponding propargyl alcohol (4j)
with excellent enantioselectivity. Finally, α,β,γ,δ-unstaturated
systems were tested leading to quantitative conversion of dienons
[a] Reactions were carried out by stirring ketone (0.5 mmol) with zinc acetate
and L1 (1:2 molar ratio) in TES (10 mmol) at room temperature under argon
atmosphere for 6 h. The resulting silyl ethers were hydrolyzed with TBAF.
After purification by column chromatography ee values were determined by
HPLC methods. [b] Hydrolysis was carried out using K2CO3 in MeOH. [c] 0.3
mol% cat. [d] Reaction time 8 h. [e] 0.1 mol% cat. [f] 0.2 mol% cat. [g]
Starting material was retrieved. [h] 1M HCl was used for hydrolysis of silyl
ether.
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after only 6 hours with 0.1 mol% catalyst loading providing
products of 1,2-addition with good ees. Noteworthy, α-substitution
slightly increased stereoselectivity (4l) compared with
unsubstituted product (4k).
Keywords: asymmetric synthesis • solvent-free conditions •
hydrosilylation • reduction • zinc acetate
[1]
[2]
[3]
B. Marciniec, (Ed.), in: Hydrosilylation: A Comprehensive Review on
Recent Advances, Springer, Netherlands, 2009.
S. Díez-González, S. P. Nolan, Org. Prep. Proced. Int. 2007, 39, 523-
559.
Lastly, we applied our methodology for the reduction of
various N-phosphinylimines. This requited however 5 mol% of
For reviews on asymmetric iron catalysis see: a) K. Gopalaiah, Chem.
Rev. 2013, 113, 3248-3296; b) B. Plietker, (Ed.), Iron Catalysis in
Organic Chemistry, Wiley-VCH, Weinheim, 2008.
zinc
acetate-(1R,2R)-N,N’-dibenzyl-1,2-diphenylethane-1,2-
diamine (L2) with (1:1) [Zn]:L2 molar ratio (Table 4). As collected
data show solvent-free conditions had only a slight influence on
enantioselectivity of preformed reactions (6a-6e) compared with
those carried out in dry THF.[8f] Conjugated N-phosphinylenimines
gave corresponding products with very good ees (6f, 6g), which
are similar to the values obtained when ZnEt2 was used as a zinc
source.[6n]
[4]
For prominent examples see: a) H. Nishiyama, A. Furuta, Chem.
Commun. 2007, 760-762; b) N. S. Shaikh, S. Enthaler, K. Junge, M.
Beller, Angew. Chem. 2008, 120, 2531-2535; Angew. Chem. Int. Ed.
2008, 47, 2497-2501; c) B. K. Langlotz, H. Wadepohl, L. H. Gade, Angew.
Chem. 2008, 120, 4748-4752; Angew. Chem. Int. Ed. 2008, 47, 4670-
4674; d) A. M. Tondreau, J. M. Darmon, B. M. Wile, S. K. Floyd, E.
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Hosokawa, J. I. Ito, H. Nishiyama, Organometallics 2010, 29, 5773-5775;
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J. 2010, 5, 1687-1691; g) T. Inagaki, L. T. Phong, A. Furuta, J. I. Ito, H.
Nishiyama, Chem. Eur. J. 2010, 16, 3090-3096; h) T. Inagaki, A. Ito, J. I.
Ito, H. Nishiyama, Angew. Chem. 2010, 122, 9574-9577; Angew. Chem.
Int. Ed. 2010, 49, 9384-9387; i) M. Flückiger, A. Togni, Eur. J. Org. Chem.
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Am. Chem. Soc. 2015, 137, 2456-2459; l) Z. Zuo, L. Zhang, X. Leng, Z.
Huang, Chem. Commun. 2015, 51, 5073-5076.
In summary, a zinc acetate-diamine complex has been
proven to catalyze hydrosilylation of various ketones under
solvent-free conditions with remarkably low catalyst loadings
(0.05-0.3 mol%). The reductions of substrates with various
conjugated systems resulted in formation of products of 1,2-
addition. The enantioenriched alcohols were obtained with very
good yields (up to 98% of isolated product) and stereoselectivities
(ees up to 97%). The mechanistic aspect of our method has been
studied by 1H NMR spectra analysis proving formation of the Zn-
H species. In general, enantioenriched N-Dpp amines were also
obtained with good yields and very good enantioselectivities,
although the catalyst loading necessary for the reactions to
proceed still remains relatively high (5 mol%).
[5]
[6]
For reviews on asymmetric zinc catalysis see: a) D. Łowicki, S. Baś, J.
Mlynarski, Tetrahedron 2015, 71, 1339-1394; b) S. Enthaler, ACS Catal.
2013, 3, 150-158.
For examples of diethylzinc-based asymmetric hydrosilylation see: a) H.
Mimoun, J. Org. Chem. 1999, 64, 2582-2589; b) H. Mimoun, J. Y. De
Saint Laumer, L. Giannini, R. Scopelliti, C. Floriani, J. Am. Chem. Soc.
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Martelli, D. Savoia, J. F. Carpentier, Eur. J. Org. Chem. 2004, 3040-
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45, 4383-4387; h) B. A. Mortreux, D. Savoia, J. F. Carpentier, Adv. Synth.
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Gawroński, M. Kwit, Org. Biomol. Chem. 2011, 9, 3863-3870.
Experimental Section
General procedure for asymmetric hydrosilylation of ketones
Zn(OAc)2 and L1 samples were prepared using microbalance. To
a 5 ml vial equipped with a magnetic stir bar were subsequently added:
0.46 mg (0.0025 mmol) Zn(OAc)2, 3.06 mg (0.005 mmol) L1, ketone
(0.833–5 mmol, 1 equiv.) and triethoxysilane (1.67–10 mmol, 2 equiv.).
The vial was flushed with Ar and sealed. The completion of the reaction
was monitored by TLC. After specified time the reaction was cooled to 0 °C,
quenched with tetrabutylammonium fluoride solution 1.0 M in THF. After
stirring for 5 minutes the mixture was then subjected to a silica gel column
and eluted with a mixture of hexane and ethyl acetate (6:1). The ee values
were then determined by HPLC methods.
[7]
[8]
[9]
For examples of zinc acetate-based asymmetric hydrosilylation see: a)
T. Inagaki, Y. Yamada, L. T. Phong, A. Furuta, J.-I. Ito, H. Nishiyama,
Synlett 2009, 253-256; b) K. Junge, K. Möller, B. Wendt, S. Das, D.
Gördes, K. Thurow, M. Beller, Chem. Asian J. 2012, 7, 314-320; c) S.
Pang, J. Peng, J. Li, Y. Bai, W. Xiao, G. Lai, Chirality 2013, 25, 275-280.
a) D. Łowicki, A. Bezłada, J. Mlynarski, Adv. Synth. Catal. 2014, 356,
591-595; b) M. Szewczyk, F. Stanek, A. Bezłada, J. Mlynarski, Adv.
Synth. Catal. 2015, 357, 3727-3731; c) A. Bezłada, M. Szewczyk, J.
Mlynarski, J. Org. Chem. 2016, 81, 336-342; d) A. Adamkiewicz, J.
Mlynarski, Eur. J. Org. Chem. 2016, 1060-1065.
Acknowledgements
This work was financed by Polish National Science Center (grant
number NCN 2012/07/B/ST5/00909).
a) J. Yang, T. Tilley, Angew. Chem. Int. Ed. 2010, 49, 10186-10188; b)
A. Ruddy, C. Kelly, S. Crawford, C. Wheaton, O. Sydora, B. Small, M.
Stradiotto, L. Turculet, Organometallics 2013, 32, 5581-5588; c) F.
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Wekesa, R. Arias-Ugarte, L. Kong, Z. Sumner, G. P. McGovern, M.
Findlater, Organometallics 2015, 34, 5051-5056.
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S. Varma, ChemSusChem 2014, 7, 24-44.
[11] For examples of non-asymmetric solvent-free hydrosilylation see also: a)
L. Chen, J. Peng, J. Li, Y. Bai, Y. Hu, H. Qiu, H. Wu, G. Lai, Lett. Org.
Chem. 2008, 5, 591-593; b) D. Bézier, F. Jiang, T. Roisnel, J.-B. Sortais,
C. Darcel, Eur. J. Inorg. Chem. 2012, 1333-1337; c) S. Demir, Y. Gökç,
N. Kaloğlu, J.-B. Sortais, C. Darcel, İ. Özdemir, Appl. Organometal.
Chem. 2013, 27, 459-464; d) M. Zhao, W. Xie, C. Cui, Chem. Eur. J.
2014, 20, 9259-9262.
[12] J. Gajewy, J. Gawronski, M. Kwit, Eur. J. Org. Chem. 2013, 307-318.
[13] a) K. Nolin, R. Ahn, F. Toste, J. Am. Chem. Soc. 2005, 127, 12462-
12462; b) R. Moser, Ž. Bošković, C. Crowe, B. Lipshutz, J. Am. Chem.
Soc. 2010, 132, 7852-7853; c) K. Voigtritter, N. Isley, R. Moser, D. Aue,
B. Lipshutz, Tetrahedron 2012, 68, 3410-3416.
[14] A. Lumbroso, M. L. Cooke, B. Breit, Angew. Chem. Int. Ed. 2013, 52,
1890-1932.
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Marcin Szewczyk, Agata Bezłada, Jacek
Mlynarski*
Page No. – Page No.
Zinc-Catalyzed Enantioselective
Hydrosilylation of Ketones and
Imines Under Solvent-Free
Conditions
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