Highly efficient reduction of aldehydes with silanes in water catalyzed by silver

Article abstract of DOI:10.1055/s-0033-1339660

A highly efficient silver-catalyzed chemoselective method for the reduction of aldehydes to their corresponding alcohols in water was developed by using hydrosilanes as reducing agents. The ketones remained essentially inert under the same reaction conditions, thereby providing an additional synthetically useful chemoselectivity. Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1339660

LETTER
▌2049
letter
Highly Efficient Reduction of Aldehydes with Silanes in Water Catalyzed by
Silver
Reduction of Aldehydes with Silanes in Water
Zhenhua Jia,a,b Mingxin Liu,a Xingshu Li,b Albert S. C. Chan,b Chao-Jun Li*a
a
Department of Chemistry and FQRNT Center for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke Street West, Montreal,
Quebec, H3A 0B8, Canada
Fax +1(514)3983797; E-mail: cj.li@mcgill.ca
b
Institute of Drug Synthesis and Pharmaceutical Process, School of Pharmaceutical Sciences, Sun Yat-Sen University, Guangzhou 510006,
P. R. of China
Received: 09.05.2013; Accepted after revision: 25.07.2013
tones with hydrosilanes used Rh,9 Ru,10 Pt,11 and Pd12 as
Abstract: A highly efficient silver-catalyzed chemoselective meth-
precious metal catalysts. Asymmetric reduction of ke-
od for the reduction of aldehydes to their corresponding alcohols in
tones with hydrosilanes was highly effective using chiral
water was developed by using hydrosilanes as reducing agents. The
ketones remained essentially inert under the same reaction condi-
Rh catalysts.13 Brønsted acids and Lewis acids were also
tions, thereby providing an additional synthetically useful chemo- used as activators for carbonyl reductions by hydrosi-
lanes;14 however, they often gave more complicated prod-
selectivity.
uct mixtures due to overreductions. It is particularly
notable that frustrated Lewis pairs have also been used for
such reductions.15 These reactions are all generally carried
out under anhydrous conditions, inert atmosphere, and in
the absence of unprotected acidic functional groups.
Key words: aldehyde, reduction, silver, water
The reduction of carbonyl compounds to alcohols is one
of the most widely used and fundamental reactions in or-
ganic chemistry.1 Several classical methods exist for such
commonly encountered transformations, including chem-
ical reductions, bioreductions, hydrogenations, and trans-
fer hydrogenations. Chemical reductions by LiAlH4,
nickel–aluminum alloy, and various borane reagents
(such as zinc borohydride, NaBH4, NaBH3CN, BH3–THF,
and amine boranes) have been the most widely utilized
methods to achieve the reduction of carbonyl groups.2
However, these reagents are generally hazardous to han-
dle and often do not show chemoselectivity between alde-
hydes and ketones.3 Biocatalytic reductions with baker’s
yeast reductase usually require a specific temperature,
pH value, and expensive enzymes.4 Transition-metal-
catalyzed hydrogenation and transfer hydrogenation often
rely on precious metal catalysts such as Pd, Pt, Rh, Ru,
and Ir.5 Recently, there have been important advances in
using cheaper iron6 and cobalt7 catalysts for hydrogena-
tions and transfer hydrogenations of carbonyl com-
pounds; however, they are often sensitive to moisture and
water.
Although the classical Grignard-type reactions are carried
out under strictly anhydrous conditions, over the past cou-
ple of decades, great advances have been made to carry
out such reactions in water.16 One particular example of
such reactions is the catalytic addition of terminal alkynes
to unsaturated electrophiles.17 These reactions can tolerate
various functional groups without the need of protection–
deprotection and thus greatly simplify synthetic steps. In
our previous work on the catalytic nucleophilic additions
of terminal alkynes to aldehydes/ketones in water cata-
lyzed by silver,18 the crucial step involved a C–H bond-
activation process of the terminal alkynes.
As a continuation of our interest in developing metal-
mediated or metal-catalyzed reactions in water,19 we hypo-
thesized that the Si–H bond may also be activated by sil-
ver catalyst, generating the corresponding silver–
hydrogen species, which could reduce unsaturated bonds
in water. In 2006, Stradiotto et al. described a rare silver-
catalyzed hydrosilylation of aldehydes in THF, however,
using their catalytic system failed to give the desired prod-
uct in water.20 Herein, we wish to report a highly efficient
and chemoselective catalytic reduction of aldehydes by
using simple silanes as reducing agents in water (Scheme
1). Furthermore, ketones are essentially inert under the
same reaction conditions. Therefore, this protocol pro-
vides an interesting chemoselective reduction of alde-
hydes in the presence of ketones.
Hydrosilanes are less polar and are inert towards common
functional groups as well as water in the absence of acti-
vators. These attributes make them easy to handle and
store. Furthermore, hydrosilanes can be activated readily
with various metal complexes, which provides opportuni-
ties as powerful reducing reagents for chemoselective re-
actions through the changing of catalyst and/or ligands.8
In early studies, catalytic reductions of aldehydes and ke-
SYNLETT 2013, 24, 2049–2056
Advanced online publication: 02.09.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
Scheme 1 Silver-catalyzed reduction of aldehyde by silanes in water
DOI: 10.1055/s-0033-1339660; Art ID: ST-2013-U0435-L
© Georg Thieme Verlag Stuttgart · New York

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LETTER
Table 1 Optimization of the Reaction Conditions for the Reduction of Benzaldehyde Using Tripropylsilane in Watera
n-Pr
n-Pr
Si
OH
O
O
n-Pr
H
H
Ag cat., ligand
H
H
n-Pr
Si n-Pr
n-Pr
H
+
H
+
base
H2O
1a
100 °C, 24 h
2a
3a
3a'
O
PCy2
PCy2
O
P
i-Pr
i-Pr
O
O
i-Pr
i-Pr
O
i-Pr
L2
L1
L3
F
F
PPh2
Fe
PPh2
P
Ph2P
PPh2
F
L4
L5
L6
Entry
Ag cat. (mol%)
Ligand (mol%)
L1 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L2 (20)
L3 (20)
L4 (20)
L5 (20)
L6 (20)
L6 (15)
L6 (10)
L6 (2)
Base (mol%)
Yield (%)b
3a/3a′
1
2
AgCl (10)
AgCl (10)
AgBr (10)
AgI (10)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
no base
5
11
9
no 3a′
no 3a′
trace 3a′
no 3a′
no 3a′
trace 3a′
12:1
3
4
5
5
AgF (10)
11
6
6
AgOTf (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (10)
AgPF6 (7.5)
AgPF6 (5)
AgPF6 (1)
AgPF6 (5)
AgPF6 (5)
7
18
trace
18
14
5
8
trace 3a′
1:3.4
9
K2CO3 (1 equiv)
DBU (1 equiv)
10
11
12
13
14
15
16
17
18
19
20
21
1:1
KOt-Bu (1 equiv)
DIPEA (1 equiv)
DIPEA (20)
1:7.3
29
n.r.c
2
10:1
–
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
trace 3a′
trace 3a′
24:1
5
96
95
95 (90)d
12
94
77e
19:1
19:1
no 3a′
17:1
L6 (7.5)
L6 (7.5)
12.7:1
Synlett 2013, 24, 2049–2056
© Georg Thieme Verlag Stuttgart · New York

LETTER
Reduction of Aldehydes with Silanes in Water
2051
Table 1 Optimization of the Reaction Conditions for the Reduction of Benzaldehyde Using Tripropylsilane in Watera (continued)
n-Pr
n-Pr
Si
OH
O
O
n-Pr
H
H
Ag cat., ligand
H
H
n-Pr
Si n-Pr
n-Pr
H
+
H
+
base
H2O
1a
100 °C, 24 h
2a
3a
3a'
O
PCy2
PCy2
O
P
i-Pr
i-Pr
O
O
i-Pr
i-Pr
O
i-Pr
L2
L1
L3
F
F
PPh2
Fe
PPh2
P
Ph2P
PPh2
F
L4
L5
L6
Entry
Ag cat. (mol%)
Ligand (mol%)
L6 (7.5)
Base (mol%)
Yield (%)b
3a/3a′
22
23
24
25
AgPF6 (5)
AgPF6 (5)
AgPF6 (5)
–
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
DIPEA (20 mol%)
45f
6.6:1
5.7:1
no 3a′
–
85g
L6 (7.5)
–
trace
n.r.
L6 (7.5)
a Reaction conditions: 1a (0.1 mmol), 2a (0.3 mmol).
b Yield based on 1a and determined by 1H NMR analysis by using mesitylene as an internal standard.
c n.r. = no reaction.
d Isolated yield of 3a.
e The reaction was conducted at 80 °C.
f The reaction was conducted at 60 °C.
g The reaction was carried out in air.
Our studies commenced with the reaction of benzalde- Using other bases, such as DBU and KOt-Bu, the reaction
hyde (1a) and tripropylsilane (2a) in the presence of also gave the hydrosilylation product 3a′ (Table 1, entries
AgCl, RuPhos (L1), and N,N-diisopropylethylamine 10 and 11). Increasing the amount of DIPEA to one equiv-
(DIPEA) in water at 100 °C. To our delight, the benzylal- alent, the reaction afforded alcohol 3a as major product in
cohol (3a)21 was generated, albeit only in 5% yield after 29% (Table 1, entry 12). Further screening of different
24 hours (Table 1, entry 1). The use of XPhos (L2) slightly ligands indicated that dppf (L6) was the most effective
increased the yield of 3a (Table 1, entry 2). Other silver ligand, affording 3a in 96% yield (Table 1, entries 13–16).
catalysts were then tested by using the XPhos (L2) as the Subsequently, the efforts were made to decrease the load-
ligand (Table 1, entries 3–7). AgPF6 was the most prom- ing of catalyst and ligand. In the case of 5 mol% of cata-
ising, and the reaction gave the expected product in 18% lyst, we obtained 3a in 90% isolated yield (Table 1, entries
yield together with a trace amount of hydrosilylation 17–19). When the amount of ligand was adjusted to 7.5
product 3a′. To our surprise, we can tune the ratio of hy- mol%, we also obtained 3a in 94% yield (Table 1, entry
drosilylation product 3a′ and alcohol 3a by using different 20). Lowering the reaction temperature resulted in lower
bases. Under base-free conditions, we observed a trace efficiency (Table 1, entries 21 and 22). Notably, this reac-
amount of the desired product (Table 1, entry 8). While tion also worked well in air to give the desired product in
one equivalent of K2CO3 was utilized, the hydrosilylation 85% yield (Table 1, entry 23). In the absence of ligand,
product 3a′ was obtained in 61% yield (Table 1, entry 9).
© Georg Thieme Verlag Stuttgart · New York
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2052
Z. Jia et al.
LETTER
only a trace amount of the expected product was detected heptanone, even when we prolonged the reaction time to
(Table 1, entry 24).
48 hours (Table 3, entry 13). We then attempted to expand
the scope of the reaction to more complicated aldehydes.
Under the same conditions, the natural product perillalde-
hyde gives a moderate 35% yield with no C=C reduction
detected (Table 3, entry 14). When the same conditions
were applied to the substrate 2-(4-chlorophenoxy)benzal-
dehyde, it afforded the reduced product in 72% yield (Ta-
ble 3, entry 15).
Other silanes were then examined under the same reaction
conditions (Table 2). When we used triethoxysilane and
PMHS [poly(methylhydrosiloxane)], we observed only a
trace amount of the product or no reaction occurred (Table
2, entries 1 and 2). The use of phenylsilane generated 36%
yield and no hydrosilylation product was detected (Table
2, entry 3). Furthermore, the use of diethylsilane provided
the expected product in 80% yield (Table 2, entry 4). Control experiments were then carried out to get insight
When dimethylphenylsilane was employed, a full conver- into this transformation (Scheme 2). In the absence of si-
sion with 48% yield was observed and only a trace amount lane, there was no reaction under the optimized conditions
of hydrosilylation byproduct was detected (Table 2, entry (Scheme 2, eq. 1). When the reaction was conducted in
5).
anhydrous N,N-dimethylformamide (DMF), we observed
3a and 3a′ (with a ratio of 3a/3a′ = 0.72:1, Scheme 2, eq.
2). When D2O was employed as solvent, we observed a
similar result as in H2O (Scheme 2, eq. 3). There exists a
competition in this transformation to generate the H2 gas
via the oxidation of silane by water (Scheme 2, eq. 4);
however, under H2 atmosphere (1.0133 bar) we observed
no reaction (Scheme 2, eq. 5). All of these experiments
strongly supported the mechanism that the silane mediat-
ed this reduction in water. To further verify this interest-
ing chemoselectivity between aldehyde with ketone,
heptanal (1j) and 2-heptanone (1m; 1j/1m = 1:1) was
chosen as the starting materials and under the same reac-
tion conditions only product 3j was detected from the
crude reaction mixture (Scheme 2, eq. 6). This was further
confirmed by using deuterated silane, which provided
cleanly the deuterated product (Scheme 2, eq. 7).
With the optimized conditions in hand, we then investi-
gated the scope of silver-catalyzed reduction in water by
using the tripropylsilane (Table 3). Substituents of both
electron-donating and electron-withdrawing groups on
the aromatic ring were well tolerated, providing the corre-
sponding products in moderate to excellent yields (Table
3, entries 1–5). It should be mentioned that electron-with-
drawing groups such as trifluoromethyl somewhat retard-
ed the reaction (Table 3, entry 5). The 1-naphthaldehyde
proceeded smoothly to give the corresponding 1-naphthyl-
methanol in 75% yield (Table 3, entry 6). Heterocyclic al-
dehydes, such as furfural and 3-pyridinecarboxaldehyde,
were also suitable substrates for this reduction process in
good yields (Table 3, entries 7 and 8). We also accom-
plished this transformation with the use of aliphatic alde-
hydes, giving 3-phenyl-1-propanol and 1-heptanol in 75%
and 61% yield, respectively (Table 3, entries 9 and 10). In summary, we have developed a novel silver-catalyzed
Interestingly, almost no reaction occurred with simple aldehyde reduction in water based on Si–H bond-activa-
ketones and only acetophenone gave the corresponding tion strategy. The reaction is operationally simple, conve-
1-phenylethanol in 6% yield (Table 3, entry 11). Cyclo- nient, and highly chemoselective towards aldehydes in the
hexanone gave a trace amount of the expected product presence of ketones. Further studies on the application of
(Table 3, entry 12) and no reaction was observed with 2- this type of silver catalyst are under way.
Table 2 Screening of Silanesa
R
R
OH
Si
O
O
R
H
H
AgPF6 (5 mol%)
6 (7.5 mol%)
H
H
L
H
silane
+
+
DIPEA (20 mol%)
H2O
100 °C, 24 h
1a
2
3a
3'
Entry
2
Yield (%)b
3a/3′
1
2
3
4
5
(EtO)3SiH
PMHS
n.r.c
2
–
–
PhSiH3
36
80
48
–
Et2SiH2
–
PhMe2SiH
trace 3′
a Reaction conditions: 1a (0.1 mmol), 2 (0.3 mmol), AgPF6 (5 mol%), L6 (7.5 mol%), DIPEA (20 mol%) in water at 100 °C for 24 h.
b Yield based on 1a and determined by 1H NMR analysis by using mesitylene as an internal standard.
c n.r. = no reaction.
Synlett 2013, 24, 2049–2056
© Georg Thieme Verlag Stuttgart · New York

LETTER
Reduction of Aldehydes with Silanes in Water
2053
Table 3 Substrate Scope of Reduction of Aldehydes and Ketones in Water Catalyzed by Silvera
AgPF6 (5 mol%)
L6 (7.5 mol%)
O
OH
n-Pr
Si n-Pr
n-Pr
+
H
R1
R2
R1
R2
DIPEA (20 mol%)
H2O
100 °C
1
2
3
Entry
Substrate 1
Product 3
Yield (%)b
O
OH
H
1
90
3a
1a
O
OH
H
2
3
4
5
91
89
90
78
Me
Me
1b
3b
O
OH
H
Br
Br
1c
3c
O
OH
H
MeO
MeO
3d
1d
O
OH
H
F3C
F3C
1e
3e
O
H
HO
6
75
3f
1f
O
OH
O
O
7
8
61
69
H
3g
1g
OH
O
H
N
N
3h
3i
1h
O
OH
H
9
75
1i
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2049–2056

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Z. Jia et al.
LETTER
Table 3 Substrate Scope of Reduction of Aldehydes and Ketones in Water Catalyzed by Silvera (continued)
AgPF6 (5 mol%)
L6 (7.5 mol%)
O
OH
n-Pr
Si n-Pr
n-Pr
+
H
R1
R2
R1
R2
DIPEA (20 mol%)
H2O
100 °C
1
2
3
Entry
Substrate 1
Product 3
Yield (%)b
61
O
OH
Me
Me
10
H
3j
1j
O
OH
Me
Me
11
6c
1k
1l
3k
3l
O
OH
12
13
trace
n.r.d
OH
Me
O
Me
Me
Me
1m
3m
O
H
OH
14
35
1n
3n
O
O
OH
H
O
15
72
Cl
Cl
3o
1o
a Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), AgPF6 (5 mol%), L6 (7.5 mol%), DIPEA (20 mol%) in water at 100 °C for 24 h.
b Isolated yield.
c 1H NMR yield analysis by using mesitylene as an internal standard.
d n.r. = no reaction.
Acknowledgment
References and Notes
We are grateful to the Canada Research Chair Foundation (C.J.L.),
the Canada Foundation for Innovation (CFI), NSERC, and FQRNT
for support of our research. Z.H.J. acknowledges the Oversea Study
Program of Guangzhou Elite Project for financial support.
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A: New York, 1972. (b) Hudlicky, M. Reductions in
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Synlett 2013, 24, 2049–2056
© Georg Thieme Verlag Stuttgart · New York

LETTER
Reduction of Aldehydes with Silanes in Water
2055
AgPF6 (5 mol%)
dppf (7.5 mol%)
DIPEA (20 mol%)
O
+
no silane
H
no reaction
(1)
(2)
100 °C, H2O, 24 h
n-Pr
n-Pr
1a
Si
H
H
OH
O
n-Pr
AgPF6 (5 mol%)
dppf (7.5 mol%)
DIPEA (20 mol%)
O
H
H
n-Pr
H
+
+
H
Si n-Pr
n-Pr
100 °C, DMF, 24 h
1a
2a
3a
3a'
conv = 100%
NMR yield of 3a = 42%
NMR yield of 3a' = 58%
ratio of 3a/3a' = 0.72
n-Pr
n-Pr
Si
O
OD
AgPF6 (5 mol%)
dppf (7.5 mol%)
DIPEA (20 mol%)
O
n-Pr
n-Pr
Si n-Pr
n-Pr
H
H
H
H
H
+
H
+
(3)
100 °C, D2O, 24 h
1a
2a
3a
conv = 100%
3a'
NMR yield of 3a' = 8%
NMR yield of 3a = 92%
ratio of 3a/3a' = 12
AgPF6 (5 mol%)
n-Pr
dppf (7.5 mol%)
DIPEA (20 mol%)
n-Pr
H2O
H
Si n-Pr
n-Pr
2a
+
(4)
(5)
H2
HO Si n-Pr
n-Pr
+
100 °C, 24 h
O
AgPF6 (5 mol%)
dppf (7.5 mol%)
DIPEA (20 mol%)
H
H2 (1 atm)
+
no reaction
100 °C, H2O, 24 h
1a
OH
O
Me
Me
Me
H
AgPF6, dppf, DIPEA
silane
3j
1j
(6)
+
+
100 °C, H2O, 24 h
Me
Me
3m
OH
1m
O
no 3m
(1j/1m = 1:1)
i-Pr
i-Pr
i-Pr
AgPF6 (5 mol%)
dppf (7.5 mol%)
DIPEA (20 mol%)
OH
Si
O
O
D
H
i-Pr
Si
i-Pr
D
H
H
(7)
+
D
+
Me
100 °C, H2O, 24 h
i-Pr
Me
Me
1b
2b
3b*
3b'
conv. = 64%
isolated yield of 3b* = 23%
NMR yield of 3b': not determined
Scheme 2 Investigation of the mechanism for the reduction
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Degassed CH2Cl2 (0.25 mL) was added to a microwave tube
containing the ligand dppf (8.3 mg, 0.015 mmol) and AgPF6
(2.5 mg, 0.01 mmol) under argon. The resulting suspension
was stirred at r.t., until a clear, colorless solution was
obtained; then the solvent was removed under high vacuum.
Benzaldehyde (1a; 20.3 μL, 0.2 mmol), tripropylsilane (2a;
125 μL, 0.6 mmol), DIPEA (6.9 μL, 0.04 mmol), and H2O
(0.5 mL) were subsequently added. The reaction mixture
was stirred for 24 h at 100 °C, then cooled to r.t. and
extracted with CH2Cl2 (3 × 10 mL). The combined organic
phase was concentrated and purified by flash column
chromatography on silica gel (hexane–EtOAc, 20:1) to give
the desired product 3a as a colorless oil (19.5 mg, 90%).
The NMR data are in full agreement with those previously
reported in the literature.22 1H NMR (400 MHz, CDCl3): δ =
7.38–7.35 (m, 4 H), 7.32–7.29 (m, 1 H), 4.69 (s, 2 H), 1.82
(s, 1 H). 13C NMR (75 MHz, CDCl3): δ = 140.8, 128.5,
127.6, 126.9, 65.3.
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(22) Ford, L.; Atefi, F.; Singer, R. D.; Scammells, P. J. Eur. J.
Org. Chem. 2011, 942.
Synlett 2013, 24, 2049–2056
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