Iridium-catalyzed highly efficient chemoselective reduction of aldehydes in water using formic acid as the hydrogen source

Article abstract of DOI:10.1039/c7gc01289f

A water-soluble highly efficient iridium catalyst is developed for the chemoselective reduction of aldehydes to alcohols in water. The reduction uses formic acid as the traceless reducing agent and water as a solvent. It can be carried out in air without the need for inert atmosphere protection. The products can be purified by simple extraction without any column chromatography. The catalyst loading can be as low as 0.005 mol% and the turn-over frequency (TOF) is as high as 73 800 mol mol^-1 h^-1. A wide variety of functional groups, such as electron-rich or deficient (hetero)arenes and alkenes, alkyloxy groups, halogens, phenols, ketones, esters, carboxylic acids, cyano, and nitro groups, are all well tolerated, indicating excellent chemoselectivity.

Full text of DOI:10.1039/c7gc01289f

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Iridium-catalyzed highly efficient chemoselective
reduction of aldehydes in water using formic acid
as the hydrogen source†
Cite this: DOI: 10.1039/c7gc01289f
a,b
a,c
a,d
a
a
Zhanhui Yang,
Jing-Kui Yang
‡
Zhongpeng Zhu,‡ a,Re enshi Luo, Xiang Qiu, Ji-tian Liu,
and Weiping Tang
c
*
A water-soluble highly efficient iridium catalyst is developed for the chemoselective reduction of alde-
hydes to alcohols in water. The reduction uses formic acid as the traceless reducing agent and water as a
solvent. It can be carried out in air without the need for inert atmosphere protection. The products can
be purified by simple extraction without any column chromatography. The catalyst loading can be as low
as 0.005 mol% and the turn-over frequency (TOF) is as high as 73 800 mol mol− h−1. A wide variety of
functional groups, such as electron-rich or deficient (hetero)arenes and alkenes, alkyloxy groups, halo-
gens, phenols, ketones, esters, carboxylic acids, cyano, and nitro groups, are all well tolerated, indicating
excellent chemoselectivity.
1
Received 28th April 2017,
Accepted 27th May 2017
DOI: 10.1039/c7gc01289f
rsc.li/greenchem
neutral or basic conditions in organic solvents have been devel-
Introduction
6
7
8
oped, such as iso-propanol, 1,4-butanediol, hydrosilane and
9
The reduction of aldehydes to alcohols is a fundamentally ammonium formate (Scheme 1a). However, an operationally
important reaction in organic chemistry. For example, hydro- simple and green reduction method that works for chemists
10
formylation of alkenes followed by aldehyde reduction consti- in both academia and industry is still highly desirable. For
tutes one of the most important industrial processes for the example, organic solvents are employed in most of the above
1
10
manufacture of alcohols. Different strategies have been processes and water would be a more ideal solvent for TH.
developed for the reduction of aldehydes to alcohols, such as The waste generated from hydrogen donors can be further
2
electron-transfer reduction, reduction with metal hydrides reduced. Being able to conduct the reactions in air and
3
such as NaBH
4
and LiAlH
4
, transition-metal catalysed hydro-
4
5
genation, and transfer hydrogenation (TH). Among them,
transition metal-catalysed hydrogenation is the most atom-
economical and cleanest reduction method. However, it gener-
ally requires a high pressure of hydrogen gas, which causes
safety issues. The TH has the potential to become an ideal
green method for reduction. Various reducing agents for the
transition metal-catalysed TH reduction of aldehydes under
a
School of Pharmacy, University of Wisconsin–Madison, Madison, WI, 53705, USA.
E-mail: weiping.tang@wisc.edu
b
Faculty of Science, Beijing University of Chemical Technology, Beijing, 100029,
P. R. China
c
School of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing, P. R. China
d
School of Pharmacy, Gannan Medical University, Ganzhou, Jiangxi Province,
P. R. China
e
Department of Chemistry, University of Wisconsin–Madison, Madison, WI, 53706,
USA
†
Electronic supplementary information (ESI) available: Detailed procedure for
the synthesis of catalysts and reduction of aldehydes, characteristic data and
1
13
NMR spectra ( H and C) of products. See DOI: 10.1039/c7gc01289f
These two authors contributed equally.
Scheme 1 Reduction of aldehydes to alcohols by transfer hydrogen
(TH) reactions.
‡
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without complex purification procedures will deliver a greener Table 1 Optimization of reaction conditions
procedure.
In 2000, Bryson reported an aqueous TH reduction of alde-
hydes with sodium formate at high temperature and high
1
1
pressure in low to moderate yields (Scheme 1b). In 2004,
Ajjou realized a Rh-catalyzed aqueous TH of aldehydes with
isopropanol as a hydrogen donor and 0.2 equivalents of
sodium carbonate as an additive under a nitrogen atmosphere
1
2
(
Scheme 1c). A breakthrough was made in 2006 by Xiao and
co-workers by using iridium catalysts (Ir-1 or Ir-2) with
1
3
N-sulfonyl ethylenediamine as the ligand (Scheme 1d). In
their work, the aldehydes were reduced in high efficiency (TOF
−1
−1
of up to 50 000 mol mol
h ) in water and in air under
neutral conditions by using 5 equivalents of sodium formate
as the hydrogen source. Imperfectly, the use of sodium
formate generates sodium bicarbonate salt as the waste.
We envision that if sodium formate can be replaced by
formic acid while retaining other advantages, it will yield a
greener procedure since formic acid is a traceless reducing agent
and no waste will be left in the reaction system after the
reduction. The development of a new catalyst is then necessary
because the activity of existing catalysts is decreased under
a
b
Conv.
(%)
Time
TOF
(mol mol−1 h−1
)
Entry
[Ir]
x
(min)
1
2
3
4
5
6
7
8
9
Ir-1
Ir-2
Ir-3
Ir-4
Ir-5
Ir-6
Ir-7
Ir-8
Ir-9
Ir-10
Ir-4
Ir-4
Ir-4
Ir-4
4
4
4
4
4
4
4
4
4
4
2
4
4
4
4.3
2.9
120
120
10
8
15
15
10
60
120
120
12
25
120
15
107
73
100
100
100
100
100
100
7.4
30 000
37 500
20 000
20 000
30 000
5000
185
143
25 000
12 000
1450
13
acidic conditions as noted by Xiao and co-workers. Numerous
catalysts have been developed for the decomposition of formic
14
acid to hydrogen and carbon dioxide. The decomposition
mechanism involves the generation of an iridium hydride inter-
1
1
0
1
5.7
14
mediate, which may be trapped by aldehydes before forming
100
100
58
c
hydrogen gas. Herein, we report that a greener reduction of alde- 12
d
e
1
1
3
4
hydes in air and water under acidic conditions can be realized
by employing novel catalysts Ir-3 or Ir-4 and formic acid as the
hydrogen source (Scheme 1e). The reduction with Ir-4 features
100
20 000
a
1
The conversions in entries 1, 2, 9, 10 and 13 were determined by H
NMR of the crude reaction mixture, while the rest were estimated by
TLC. The reactions were detected by TLC every 1 min within 10 min,
not only high efficiency (TOF of up to 73 800 mol mol−
1
−1
h
)
b
and low catalyst loading (0.005 mol%), but also the minimal every 5 min after 10 min, every 10 min after 0.5 h, and every 30 min
production of waste and excellent chemoselectivity.
c
d
after 1 h. The catalyst loading was 0.01 mol%. The catalyst loading
was 0.005 mol%. The temperature was 60 °C.
e
Results and discussion
conversion (entry 13). Lowering the temperature to 60 °C also
The reaction conditions were optimized by using 4-methoxy- decelerates the rate of the reaction (entry 14). The optimal con-
benzaldehyde 1a as the model substrate. Catalyst screening ditions are listed in entry 4. Notably, all catalysts from Ir-3 to
was first performed at a 0.02 mol% level with 4 equivalents of Ir-10 are well soluble in water.
formic acid at 80 °C. Although catalysts (Ir-1 and Ir-2) were
As presented in Fig. 1, the catalytic activity is closely associ-
very effective under neutral conditions, they gave low conver- ated with the pH values of the aqueous solution. Under acidic
1
3
sions under the current acidic conditions (pH = 2.2) (Table 1, conditions (pH < 4), the catalyst showed higher activity (TOF >
−
1
−1
entries 1 and 2). In contrast, 4-substituted pyridyl catalysts 25 000 mol mol
h ), and the highest activity (TOF =
Ir-3, Ir-4, Ir-5, and Ir-6 showed excellent activity toward the 30 000 h− ) was obtained when pH = 3.0. The catalytic activity
1
reduction, and complete conversion was obtained in less than decreased significantly as the pH increases from 4.0. We also
1
5 min (entries 3–6). 6-Substituted pyridyl catalysts Ir-7 and compared our catalyst Ir-4 with previously reported catalyst Ir-2
Ir-8 showed different activities; the former only needs 10 min at different pH values. It is clear that Ir-4 excelled Ir-2 under
while the latter requires 60 min to promote the reaction to more acidic conditions (pH < 5.0), while Ir-2 showed higher
completion (entries 7 and 8). The bipyridyl-based catalyst Ir-9 TOF under weakly acidic, neutral or basic conditions (pH > 5).
and phenanthroline-based catalyst Ir-10 exhibited poor activity
entries 9 and 10). Lowering the amount of formic acid to two highest TOF value was recorded as 73 800 mol mol−
equivalents and decreasing the catalyst loading to 0.01 mol% 2 min (Table 2).
only slightly decelerated the reaction rates (entries 11 and 12).
A number of structurally diverse aldehydes were examined
At 0.005 mol% catalyst loading, it took 120 min to obtain 58% under optimal conditions (Table 3). In most cases they were
We also evaluated the catalyst reactivity with time and the
1
−1
(
h
after
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Table 3 Scope of substrates
Fig. 1 Activity comparison between our catalyst (Ir-4, a) and known
catalyst (Ir-2, b). (a) The TOF against the initial solution pH values in the
complete reduction: p-methoxybenzaldehyde (2 mmol), Ir-4 catalyst
(
0.02 mol%), HCOOH (4 equiv.), water (2 mL), 60 °C. The initial pH
value was determined by varying the HCOOH/NaOH molar ratios.
b) See ref. 13.
(
a
Table 2 TOF against reaction time
Time (min)
1
2
4
6
b
Conversion (%)
21.9
63 500
49.2
73 800
87.1
65 325
99.4
49 500
TOF (mol mol h−1
−1
)
a
The reduction of p-methoxybenzaldehyde (2 mmol) with Ir-4 catalyst
(
0.02 mol%) and HCOOH (4 equiv.) in water (2 mL) at 80 °C.
Determined by H NMR of the crude reaction mixture.
b
1
reduced to alcohols in excellent yields (91–99%) within
3
0 min. Upon completion, simple extraction with a green-
1
0d,g
chemistry-preferred solvent such as ethyl acetate
followed
by concentration under vacuum delivered pure alcohol pro-
ducts. Column chromatography is not necessary for purifi-
cation. Moreover, the reduction displayed very high efficiency
and functionality tolerance.
In addition to 4-methoxybenzaldehyde (1a), other electron-
rich aldehydes such as 4-hexyloxybenzaldehyde (1b), 4-allyloxy-
benzaldehyde (1c), 2,5-dimethoxybenzaldehyde (1d), and 2,4,6-
trimethoxy-benzaldehyde (1e), were readily reduced to the
corresponding alcohols efficiently (entries 1–5). Notably, the
allylic group in 2c and the very electron-rich phenyl rings in 2d
and 2e survived from the reduction under acidic conditions.
Benzaldehyde (1f) and its alkyl-substituted homologues
a
The reactions were detected by TLC every 5 min within 0.5 h, every
0 min after 0.5 h, and every 30 min after 1 h. Isolated yields by
b
1
extracting with ethyl acetate, drying over Na
2 4
SO , and concentrating
(
1g–1j) also underwent reduction readily and delivered excel-
lent yields of alcohols (entries 6–9). It was notable that at solvents. Catalyst loading at 0.05 mol%.
.05 mol% catalyst loading, the reduction of benzaldehyde 1f
was completed in only 2 min, with TOF as high as 60 000
c
under vacuum. 1.5 mL of water and 0.5 mL of ethanol were used as
d
0
mol mol−
1
h
−1
(entry 6). Halogen atoms were well tolerated, as simultaneously (entry 15), while only the aldehyde carbonyl
demonstrated by high yields of the fluoro-, bromo-, and group of 4-acetylbenzaldehyde (1p) was reduced (entry 16),
chloro-substituted aromatic alcohols (2j–2m, entries 10–13). demonstrating excellent chemoselectivity. Although it was
For the reaction of 3-cyanobenzaldehyde, the cyano group reported that the ketone groups could be reduced under
1
5
remained intact in 2n (entry 14). No product derived from the similar iridium-catalyzed hydrogen transfer reduction, we
reduction or hydrolysis of the cyano group was detected. The are glad that the ketone moiety of 2p was not affected under
two carbonyl groups in 1,4-phthalaldehyde (1o) were reduced our conditions (entry 16). The ester group in aldehyde 1q
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(entry 17), the nitro groups in aldehydes 1r and 1s (entries 18 was completely reduced within 10 minutes; however, reduced
and 19), and the trifluoromethyl group in 1t (entry 20) were product 2af and intramolecularly esterified product 3af were
immune to the reduction. Fused aryl aldehyde 1u (entry 21) both observed. Heating for additional 2 hours afforded 3af
and heteroaryl aldehydes 1v, 1w and 1x (entries 22–24) also exclusively in 91% isolated yield (entry 6).
underwent the reduction to yield the desired products in more
Under Xiao’s basic conditions, aliphatic aldehydes with
than 95% yields. The electron-deficient CvC double bonds in acidic α-protons were prone to undergo aldol condensation.
cinnamaldehydes 1y and 1z (entries 25 and 26) remained Therefore, their reduction was drastically inhibited. For
untouched during the reduction, indicating the highly chemo- example, under their standard conditions, octanal was
1
3
selective reduction of aldehyde groups.
reduced to only 3% yield after 18 h. Lowering the aldehyde
By using Xiao’s catalyst, the reduction of aldehydes contain- concentration and slow addition of aldehydes were required to
1
3
ing acidic protons such as carboxylic acid 1ad (Table 4) was overcome the inhibition. In contrast, under our acidic con-
1
3
not successful. Xiao and co-workers did not report the ditions, aliphatic aldehydes can be easily reduced. Alcohols
reductions of 2-, 3-, or 4-hydroxybenzaldehydes under their 2ag–2ai were prepared in good to excellent yields (entries 7–9)
1
3
conditions. When we tried to reduce aldehydes 1aa, 1ac, and in a short period of time. In addition, the reductions of alde-
-hydroxy benzaldehydes on a 1 mmol scale under their stan- hyde 1b (entry 2, Table 3) as well as 1ai (entry 9, Table 4) took
4
dard conditions (0.02 mol% Ir-2, 1 mL of deionized water, in a longer time than others, presumably because of their lower
air, 80 °C) for 1 h, no reduction products were detected by solubility in reaction media. The success of these two
TLC. Similarly, under the same conditions, 2- and 3-carboxy- examples and those substrates bearing acidic protons also
benzaldehydes 1ae and 1af could not be reduced. Gratifyingly, indicates that our reduction occurs on water, while Xiao’s
1
3
by using our catalyst under acidic conditions, most of these reduction appeared to occur in water.
aldehydes could be reduced to alcohols in high yields The current reduction can be easily scaled up. As shown in
Table 4). For instance, ortho- and meta-hydroxy benzaldehydes Scheme 2, 13.6 grams of 4-methoxybenzaldehyde (1a) are con-
(
1
aa, 1ab and 1ac were readily reduced in excellent yields verted to (4-methoxyphenyl)methanol (2a) in >99% yields
(entries 1–3). To our surprise, although 4-hydroxy benz- (13.8 g) at 0.005 mol% catalyst loading. Again, the product was
aldehydes were completely consumed, only complex mixtures obtained in high purity by simple extraction.
were obtained. More acidic para- and meta-carboxybenzalde-
hydes 1ad and 1ae also showed excellent reactivity toward the
reduction (entries 4 and 5). ortho-Carboxybenzaldehyde 1af
Table 4 Reduction of basicity-sensitive aldehydes
Scheme 2 Gram-scale reduction.
a
The reactions were detected by TLC every 5 min within 0.5 h, every
0 min after 0.5 h, and every 30 min after 1 h. Isolated yields by
b
1
2 4
extracting with ethyl acetate, drying over Na SO , and concentrating
c
under vacuum. 1.5 mL of water and 0.5 mL of ethanol were used as
solvents.
Fig. 2 Proposed catalytic mechanism.
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The proposed mechanism for the reduction is shown in 21602010) for financial support. We thank the China
Fig. 2. The formate anion exchanged with the chloride ligand Scholarship Council (to Z. Yang), the University of Chinese
on iridium catalyst Ir-4 to yield active catalyst A, which Academy of Sciences (to Z. Zhu), and the Gannan Medical
1
4
extruded carbon dioxide to give rise to iridium hydride B.
University (to R. Luo) for their financial support for visiting
Hydride B could be trapped by aldehydes 1, possibly via tran- scholar and visiting student positions in the University of
sition state C, to deliver alkoxide D. Protonation then afforded Wisconsin–Madison.
alcohol 2 and the active catalyst A was regenerated. Hydride B
1
4
could also be protonated to release hydrogen gas directly.
Under our conditions, it appeared that the addition of
Ir-hydride to aldehydes was preferred over direct protonation.
Notes and references
1
4b–d
14e
Kinetic studies from Himeda’s
DFT calculations from Himeda’s group
and Li’s
groups and
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Conclusions
We have developed an efficient iridium catalyst for the chemo-
selective reduction of aldehydes in water under acidic con-
ditions. The reduction uses formic acid as the traceless redu-
cing agent. It does not require inert atmosphere protection or
purification by column chromatography, offering an operation-
ally simple and green procedure. The catalyst efficiency is
extremely high, and the instant TOF value can be as high as
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−1
7
3 800 mol mol−
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(
2 mmol) in deionized water (2 mL) at 80 °C was added the
−
1
aqueous solution of catalyst Ir-4 (80 μL, 0.005 mol L). For
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.5 mL of water and 0.5 mL of ethanol were used. The resulting
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