On water and in air: Fast and highly chemoselective transfer hydrogenation of aldehydes with iridium catalysts

Article abstract of DOI:10.1002/anie.200602122

(Chemical Equation Presented) Water as solvent: A fast, selective, and high-yielding transfer hydrogenation of a wide range of aldehydes is achieved using Ir^III catalysts containing simple ethylene-diamine (en) ligands (see scheme; Ts = p-toluenesulfonyl, TOF = turnover frequency). This procedure is suitable for aldehydes with a wide range of functional groups.

Full text of DOI:10.1002/anie.200602122

Communications
Transfer Hydrogenation
DOI: 10.1002/anie.200602122
On Water and in Air: Fast and Highly
Chemoselective Transfer Hydrogenation of
Aldehydes with Iridium Catalysts**
Xiaofeng Wu, Jianke Liu, Xiaohong Li,
Antonio Zanotti-Gerosa, Fred Hancock, Daniele Vinci,
Jiwu Ruan, and Jianliang Xiao*
The use of water as a solvent for organic reactions has
emerged as one of the most interesting fields for organic
chemists, both in the laboratory and in industry, because of
substantial economical and ecological gains.[1] Recently,
Sharpless and co-workers demonstrated the benefits of
[2]
À
performing C Cbond-formation reactions “on water”. We
recently discovered that water is an excellent solvent for the
asymmetric transfer hydrogenation of ketones.[3,4] Herein, we
disclose that [(Cp*IrCl2)2] (Cp* = C5Me5) in combination
with monotosylated ethylenediamine is a good catalyst system
for highly chemoselective transfer hydrogenation (TH) of
aldehydes.[5] The reduction works in air and appears to occur
on water.
TH reactions have been studied for more than one
century. The reduction of ketones and aldehydes by the
Meerwein–Ponndorf–Verley reaction was first described in
1925,[6] and since then a great deal of progress has been made
in TH chemistry.[7–10] However, aldehydes are difficult to
reduce by catalysts commonly used for TH, and controlling
the chemoselectivity of the reaction presents a further
challenge.[7b,11,12] To date, the most efficient TH catalyst
appears to be the iridium complex of an N-heterocyclic
carbene, which affords a turnover frequency (TOF) of up to
3000 hÀ1 in refluxing 2-propanol.[12a] With most other metal
catalysts, the TOFs range from a few to several hundred per
hour.[7–12] In fact, few catalysts have been reported that enable
fast, selective, and productive TH of aldehydes with inex-
pensive, eco-friendly reductants, and that can tolerate the
presence of synthetically useful functional groups.[7]
In continuing our investigation into aqueous TH reac-
tions,[3] we examined the TH of benzaldehyde with HCOONa
[*] X. Wu, Dr. J. Liu, Dr. X. Li, Dr. D. Vinci, Dr. J. Ruan, Prof. J. Xiao
Liverpool Centre for Materials and Catalysis
Department of Chemistry
University of Liverpool
Liverpool L697ZD (UK)
Fax: (+44)151-794-3589
E-mail: j.xiao@liv.ac.uk
Dr. A. Zanotti-Gerosa, Dr. F. Hancock
Johnson Matthey
Unit 28 Cambridge Science Park, Cambridge CB40FP (UK)
[**] We gratefully acknowledge the DTI MMI project and its industrial/
academic partners (Prof. R. Cartlow, Royal Institution; Dr. A.
Danopoulos, University of Southampton; Dr. A. Pettman, Pfizer; Dr.
P. Hogan and Dr. M. Purdie, AstraZeneca; Dr. P. Ravenscroft,
GlaxoSmithKline) for financial support and valuable suggestions.
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in water, initially using metal complexes with no additional
ligands (Scheme 1). After the complexes had been stirred in
water at 658Cfor 1 h, HOCONa and benzaldehyde
(10.0 mmol) were introduced to start the reduction. As
same reaction was carried out in 2-propanol or the azeotropic
formic acid/triethylamine mixture, which are the two most
commonly used solvents for TH reactions, the conversion was
less than 3% (Table 1, entries 12 and 13). These results show
the remarkable benefit of carrying out the reduction in the
aqueous phase.
The effectiveness of using the IrIII catalysts in synthesis
was tested by carrying out the TH reaction at higher S/
Cratios. At an S/Cratio of 1 ” 10 4:1, Ir–Ts(en) led to a TOF of
28800 hÀ1 at 808C(Table 1, entry 15), and with the more
electron-deficient Ir–CF3Ts(en), a higher TOF of 42000 hÀ1
was achieved (Table 1, entry 16). Using Ir–CF3Ts(en), the S/
Cratio could be increased to 5 ” 10 4:1 and a TOF as high as
1.3 ” 105 hÀ1 was attained. Under these conditions, benzalde-
hyde (5.30 g) was reduced to give phenylmethanol in 98%
yield (5.28 g after 1 h using 0.4 mg of [(Cp*IrCl2)2]), demon-
strating the superior activity,[7] robustness, and easy scalability
of the aqueous IrIII catalytic system. However, as seen with
the TH of ketones using ruthenium catalysts,[1c,3b,14] the
reduction is also pH dependent, with neutral conditions
being most favorable (Figure 1).
Scheme 1. Metal precursors and ligands screened in this study.
Ts =toluene-4-sulfonyl.
shown in Table 1, the reaction was slow for all four complexes.
However, the introduction of the readily available ethyl-
enediamine (en) brought about a significant ligand-acceler-
ation effect on the rates and, most remarkably, the mono-
tosylated analogue Ts(en) led to around a 1000-fold increase
in rate (Table 1, entries 3, 11, and 14).[13]
Table 1: Optimization of the TH reaction of benzaldehyde by HCOONa
in water.[a]
Entry
Catalysts
t [h]
Conv. [%][b]
TOF[c]
[molmolÀ1 hÀ1
]
1
2
3
4
5
RuI
Rh
25
25
25
25
1
32
35
70
20
1.2
74
2
3
20
Ir
RuII
0.6
RuI–en
12[d]
6
Rh–en
1
900
7
8
9
Ir–en
1
1
2
0.33
0.08
1
1
1.5
0.9
0.3
1
99
0.8
99
1800
Figure 1. Plot showing the TOF against the initial solution pH values
in the reduction of benzaldehyde (10 mmol), using Ir–Ts(en) and
HCOONa (5 equiv) in water (10 mL) at 808C and S/C 5000:1. The
initial pH value was determined by varying the HCOOH/NaOH molar
ratios.
RuII–en
8[d]
RuI–Ts(en)
Rh–Ts(en)
Ir–Ts(en)
Ir–Ts(en), IPA[e]
Ir–Ts(en), F/T[f]
Ir–Ts(en)[g]
Ir–Ts(en)[h]
Ir–CF3Ts(en)[h]
Ir–CF3Ts(en)[i]
1000
6000
12000
26[d]
10
11
12
13
14
15
16
17
99
>99
2.6
1.5
>99
>99
>99
98
15[d]
With these findings in hand, we were interested in
extending this reduction system to a wider range of aldehydes.
The results on the TH of aromatic aldehydes with Ir–Ts(en) at
S/C5000:1 and with Ir–CF 3Ts(en) at S/C10000:1 are shown
in Table 2. To our delight, most of the reactions were
complete in a short time, and gave high yields. For example,
the reductions of 4-halo- and 4-methoxybenzaldehyde with
the Ir–Ts(en) catalyst both achieved over 99% conversion
within 1 h (Table 2, entries 3, 4, 5, 7, and 10). With the more
active Ir–CF3Ts(en) catalyst, similar results were obtained at
a higher S/Cratio (10000:1; Table 2, entries 6, 8, and 11). An
exception to these results is the reduction of sulfur-containing
substrates, probably because of coordination between sulfur
and iridium. However, reduction of these substrates can be
performed using a lower S/Cratio (1000:1; Table 2, entries 12,
25, and 26). The reduction also works well for sterically
demanding substrates such as trisubstituted benzaldehydes
(Table 2, entries 29 and 30) and significantly, the reduction
20400
28800
42000
132000
[a] 658C, HCOONa (5 equiv) at S/C 1000:1 in water (10 mL). [b] Deter-
mined by GC. [c] Based on conversion after 5 min. [d] Based on
conversion after 1 h. [e] 2-propanol (IPA) was used as hydrogen source
and solvent, 3.2% conversion after 8 h. [f] HCOOH/Et3N (F/T) azeo-
trope used, 9% conversion after 8 h. [g] S/C 1”104:1. [h] 808C, S/C 1”
104:1. [i] 808C, S/C 5”104:1.
The precatalyst (M–ligand) was generated by simply
reacting the precursor complex with the ligand in water at
658Cfor 1 h. It is apparent from Table 1 that the Ir III catalysts
with en and Ts(en) ligands proved to be the most active. For
instance, benzaldehyde was reduced with almost complete
conversion in 5 min using Ir–Ts(en) with a substrate/catalyst
(S/C) ratio of 1000:1 at 658C(Table 1, entry 11). When the
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Table 2: The TH reaction of aromatic aldehydes with HCOONa.[a]
tolerates a variety of functionalities. Of
particular note is the reduction of 4-acetyl-
benzaldehyde, proceeding in 95% yield
after 0.5 h, with no sign of ketone reduction
(Table 2, entry 32). The results shown in
Table 3 also prove that the aqueous IrIII
system is the most efficient, chemoselective
catalyst for aldehyde reduction thus far
reported. In addition, these TH reactions
can be carried out in air without inert gas
protection (Table 3, entries 2, 5, and 22).
Interestingly, under the same conditions,
neither the water-soluble 4-carboxybenzal-
dehyde nor its sodium salt could be reduced.
However, the ester analogue, methyl 4-
formylbenzoate, was reduced with > 99%
conversion in 40 min (compare entries 33
and 34, Table 2). These results suggest that
the catalysis takes place on water rather
than in water in these biphasic reactions.
This supposition is supported by the obser-
vation that the catalyst is more soluble in the
organic phase and that the aqueous phase
remains almost colorless during the reduc-
tion.
Entry
1
Substr.
t [h]
Conv. [%][b]
Entry
Substr.
t [h]
Conv. [%][b]
99
0.6
>99 (98)
18
1.5
2
3
4
5
6
0.6[c]
0.5
>99
19
20
21
22
23
1.8[d]
99 (97)
99
>99
7
0.6
>99
1.5
1.5
1.5
>99
>99
>99
0.67[c]
0.9[d]
>99 (98)
>99
7
8
9
0.67
1.2[d]
3
99
99
98
24
25
26
0.25
0.5[f]
2[g]
98
A further test of the chemoselectivity of
the catalyst system is the TH of a,b-unsatu-
rated aldehydes. Few catalysts are available
that enable highly chemoselective TH of the
>99
99
=
carbonyl groups without reduction of the C
Cbonds.
[5,9]
As shown in Table 3, various
a,b-unsaturated aldehydes were selectively
reduced on the formyl group. Notably, these
include the acetylated cinnamaldehydes, in
which both the alkenyl and acetyl groups
remained intact during the reduction
(Table 3, entries 7 and 8). To the best of
our knowledge, no catalysts have been
reported for the TH of these substrates.[15]
Surprisingly, when aliphatic aldehydes
(for example, octanal) were subjected to TH
under the conditions reported in Table 2,
conversions were very low, even after pro-
longed reaction times. This reduction in the
rate does not appear to result from product
inhibition, as the TH of benzaldehyde was
not affected by the presence of 1-octanol or
1-butanol as additives. Rather it is more
likely related to the presence of a protons in
these substrates. Thus, trimethylacetalde-
hyde was reduced in 3 h [Eq. (1)]. In sharp
contrast, under the same conditions, the
reduction of octanal proceeded in only 3%
10
11
12
13
0.5
>99
>99
>99
>99
27
28
29
30
2
98 (97)
96 (94)
99 (97)
99 (97)
0.6[d]
0.5[e]
0.8
3
1.5[h]
0.7[i]
14
1
>99
31
1.5
99 (98)
15
16
0.5
0.5
98
91
32
33
0.5
0.6
99 (95)
>99
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Table 2: (Continued)
arising from the reduction of octa-
nal indeed show the presence of
aldol and enol products.
Entry
Substr.
t [h]
Conv. [%][b]
Entry
34
Substr.
t [h]
Conv. [%][b]
na[j]
If one assumes that the reduc-
tion is retarded by the aldol con-
densation,[1c,12a] lowering the con-
centration of substrates should dis-
favor the aldol reaction and enable
TH of the aldehydes. Indeed, vari-
ous aldehydes with a-hydrogen
atoms could be reduced at S/
C2000:1 when the aldehyde was
slowly added portionwise over the
times shown in Table 4. Under such
17
1.3
>99
1
[a] 808C, HCOONa (5 equiv) at S/C 5000:1 in water (15 mL). [b] Numbers in brackets refer to yields of
isolated products. [c] The reaction was performed in air. [d] Ir–CF3Ts(en), S/C 1”104:1. [e] S/C 1000:1;
48% conversion after 8 h at S/C 5000:1. [f] S/C 1000:1; 30% conversion after 5 h at S/C 5000:1. [g] S/
C 1000:1; 32% conversion after 19 h at S/C 5000:1. [h] S/C 1000:1; 98% conversion after 12 h at S/
C 5000:1. [i] S/C 1000:1; 99% conversion after 8 h at S/C 5000:1. [j] Not applicable (na); initial pH 6.8;
no product was detected by NMR spectroscopy after 1 h.
conditions, however, these aldehydes are still more difficult to
reduce than aryl aldehydes. Thus, a competitive reduction of a
mixture of benzaldehyde (30 mmol) and phenylacetaldehyde
Table 3: The TH reaction of a,b-unsaturated aldehydes with HCOONa.[a]
Entry Aldehyde
Product
t [h] Conv. [%][b]
1
2
3
3
99 (97)
>99
98
3[c]
3[d]
Table 4: The TH reaction of aliphatic aldehydes using Ir–CF3Ts(en) with
HCOONa.[a]
Entry
Substrate
t [h]
13[b]
7
Conv. [%]
0.2
0.3
>99 (98)
>99 (98)
4
5
1
2
95
97
3
4
4
95
96
6
7
0.5
1.5
99 (98)
98 (96)
3.7
5
6
3.6
5[c]
98
90
8
9
3
99 (97)
0.4
>99 (98)
[a] HCOONa (5 equiv) at 808C in water (10 mL). The aldehyde
(20.0 mmol) was added dropwise in ten portions; 1-octanol
(2.0 mmol) as a diluting agent was added initially; HCOOH was
added to keepthe pH neutral after the fifth addition. [b] Without diluting
reagent. [c] S/C 1600:1.
10
0.7
99 (98)
11
12
13
4
98 (97)
97
(6 mmol) with Ir–Ts(en) (0.01 mmol) gave a 98% conversion
in 20 min for the former but required 5 h for the latter; as
expected, when the concentration of phenylacetaldehyde was
increased, the reduction rate decreased for both (equal molar
mixture and total S/C5000:1 gave 70% conversion for the
former and 16% for the latter after 24 h).
4.5
1
>99
14
9
98
In summary, we have demonstrated that Ir–Ts(en) and Ir–
CF3Ts(en) are highly active and chemoselective catalysts for
the aqueous-phase TH of aldehydes. The catalytic reduction
can be carried out with S/Cratios as high as 5 ” 10 4:1, the
initial TOFs near 1.3 ” 105 hÀ1, and an inert atmosphere is not
required. Of significant practical importance is that the
catalyst tolerates a wide variety of synthetically useful
functional groups including nitro groups, halogens, ketones,
esters, and olefins. To the best of our knowledge, this on-water
IrIII–diamine catalysis represents the most efficient, simple,
[a] 808C, HCOONa (5 equiv) at S/C 1000:1 in water (10 mL). [b] Num-
bers in brackets refer to yields of isolated products. [c] The reaction was
performed in air. [d] Ir–CF3Ts(en) used at S/C 5000:1.
yield after 18 h. One explanation is that under the reaction
conditions, the a position in the aldehydes is deprotonated,
leading to aldol products that might inhibit the iridium
catalysis. The NMR and mass spectra of the reaction mixture
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[9] For examples of the TH of a,b-unsaturated aldehydes, see: a) P.
Selvam, S. U. Sonavane, S. K. Mohapatra, R. V. Jayaram, Adv.
Synth. Catal. 2004, 346, 542 – 544; b) T. Mizugaki, Y. Kanayama,
K. Ebitani, K. Kaneda, J. Org. Chem. 1998, 63, 2378 – 2381, and
references therein.
[10] For a recent review including hydrogenation of aldehydes (as
well as a,b-unsaturated aldehydes), see: B. Chen, U. Dingerdis-
sen, J. G. E. Krauter, H. G. J. Lansink Rotgerink, K. Mꢁbus, D. J.
Ostgard, P. Panster, T. H. Riermeier, S. Seebald, T. Tacke, H.
Trauthwein, Appl. Catal. A 2005, 280, 17 – 46.
and environmentally friendly catalytic system for the reduc-
tion of aldehydes to date.
Experimental Section
[(Cp*IrCl2)2] (1.6 mg, 0.002 mmol) and Ts(en) (1.2 mg, 0.0048 mmol)
were suspended in degassed distilled water (15 mL). After the
[16]
reaction mixture had been stirred at 808Cfor 1 h,
HCOONa
(6.8 g, 0.1 mol) and benzaldehyde (2.1 g, 20 mmol) were added to the
resulting solution. The reaction mixture was rapidly degassed three
times through vacuum-nitrogen cycles and then heated at 808Cfor
0.6 h. The workup and analysis were conducted as previously
reported.[3a]
[11] For reviews, see: a) K. Nomura, J. Mol. Catal. A 1998, 130, 1 – 28;
b) F. Joó, ꢂ. Kathó, J. Mol. Catal. 1997, 116, 3 – 26.
[12] a) J. R. Miecznikowski, R. H. Crabtree, Organometallics 2004,
23, 629 – 631; b) J. W. Yang, M. T. Hechavarria Fonseca, N.
Viganola, B. List, Angew. Chem. 2004, 116, 6829 – 6832;
Angew. Chem. Int. Ed. 2004, 43, 6660 – 6662; c) A. N. Ajjou, J.-
L. Pinet, J. Mol. Catal. A 2004, 214, 203 – 206; d) S. Naskar, M.
Bhattacharjee, J. Organomet. Chem. 2005, 690, 5006 – 5010.
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7931 – 7944.
Representative of a TH reaction (S/Cratio of 5 ” 10 4:1): An
aqueous solution of the catalyst Ir–CF3Ts(en) (0.001m, 1 mL) was
added to degassed distilled water (30 mL), and the mixture was
stirred at 808Cfor 10 min. HCOONa (17 g, 0.25 mol) and benzalde-
hyde (5.30 g, 50.0 mmol) were then introduced. A conversion of 98%
was achieved after 1 h, and following workup, phenylmethanol was
obtained as a colorless liquid (5.28 g, 98% yield).
[14] a) J. Carnivet, L. Karmazin-Brelot, G. Sꢀss-Fink, J. Organomet.
Chem. 2005, 690, 3209 – 3211; b) T. Abura, S. Ogo, Y. Watanabe,
S. Fukuzumi, J. Am. Chem. Soc. 2003, 125, 4149 – 4154.
[15] We thank one referee for suggesting these substrates.
[16] The iridium compound isolated is consistent with [Cp*IrClL]
(L = Ts(en) or CF3Ts(en) without the amido hydrogen atom)
with a structure probably similar to the related, structurally
characterized Ir–Ts(dpen) compound (dpen = 1,2-diphenylethy-
lenediamine).[17] Analytical data for the Ir–CF3Ts(en) catalyst:
1H NMR (400 MHz, CDCl3/TMS): d = 8.04 ppm (d, J = 8.1 Hz,
2H) 7.63 (d, J = 8.1 Hz, 2H), 4.07 (brs, 2H), 2.74 (2H, m), 2.66
(m, 2H), 1.75 (s, 15H). HRMS (ES) for C19H26ClF3IrN2O2S
([M+H]+): calcd: 631.0932 (Ir191Cl37) and 631.0985 (Ir193Cl35);
found: 631.0950; for C19H25F3IrN2O2S ([MÀCl]+): calcd:
Received: May 27, 2006
Published online: September 8, 2006
Keywords: aldehydes · chemoselectivity · iridium ·
.
transfer hydrogenation · water
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)
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