Fast and chemoselective transfer hydrogenation of aldehydes catalyzed by a terdentate CNN ruthenium complex [RuCl(CNN)(dppb)]

Article abstract of DOI:10.1002/adsc.200700014

Aromatic, aliphatic and α,β-unsaturated aldehydes are quickly, quantitatively and chemoselectively reduced to primary alcohols with 2-propanol using 0.05-0.01 mol% of the terdentate CNN ruthenium complex RuCl(CNN)(dppb) (1) [HCNN = 6-(4′-methylphenyl)-2-pyridylmethylamine; dppb = Ph ₂P(CH₂)₄PPh₂] in the presence of potassium carbonate (K₂CO₃; 1-10 mol%) as a weak base, affording TOF values up to 5.0 × 10⁵ h^-1.

Full text of DOI:10.1002/adsc.200700014

COMMUNICATIONS
DOI: 10.1002/adsc.200700014
Fast and Chemoselective Transfer Hydrogenation of Aldehydes
Catalyzed by a Terdentate CNN Ruthenium Complex
[
RuCl
A
C
H
T
R
E
U
N
G
(CNN)(dppb)]
A
H
R
U
G
a,
a
a
Walter Baratta, * Katia Siega, and Pierluigi Rigo
a
Dipartimento di Scienze e Tecnologie Chimiche, Università di Udine, Via Cotonificio 108, 33100 Udine, Italy
Fax : (+39)-0432-558-803; e-mail: inorg@dstc.uniud.it
Received: January 8, 2007
Abstract: Aromatic, aliphatic and a,b-unsaturated
aldehydes are quickly, quantitatively and chemose-
lectively reduced to primary alcohols with 2-propa-
nol using 0.05–0.01 mol% of the terdentate CNN
lysts through coordination of carbon monoxide.
Therefore, in order to suppress these side reactions,
weak basic conditions and very short reaction time
are prerequisites to achieve efficient aldehyde reduc-
tion. One of the most active catalyst has been de-
scribed by Crabtree and co-workers using an iridium
NHC complex that catalyzes the reduction of alde-
ruthenium
HCNN=6-(4’-methylphenyl)-2-pyridylmethylam-
ine; dppb=Ph P(CH ) PPh ] in the presence of po-
complex
RuCl
A
H
R
U
G
A
H
R
U
G
(1)
[
AHCTREUNG
2
2
4
2
tassium carbonate (K CO ; 1–10 mol%) as a weak
base, affording TOF values up to 5.0”10 h .
hydes in 2-propanol with TOF values up to
2
3
5
À1
À1 [4b]
3000 h .
More recently, Xiao and co-workers re-
ported the most efficient system prepared by reacting
[Cp*IrCl(m-Cl)] with diamines and leading to TOF
Keywords: M-NH bifunctional effect; phosphanes;
pyridine; ruthenium; transfer hydrogenation
ACHTREUNG
2
À1
values up to 130,000 h (calculated on the conversion
after 5 min) for the reduction of benzaldehyde with
[
5]
sodium formate in the aqueous phase. Despite the
catalysts of choice for transfer hydrogenation of ke-
tones being ruthenium-based derivatives, only few
Catalytic transfer hydrogenation of carbonyl com- catalytic systems with this metal have been described
pounds mediated by transition metal complexes has for the transfer hydrogenation of aldehydes, namely
[
3a,c,6]
received increasing attention as a possible synthetic some ruthenium phosphane complexes,
and chiral
[
3g,7]
route for the production of a wide range of alcohols. arene-amino derivatives.
Excellent results have been obtained in the asymmet- In our recent study on catalytic transfer hydrogena-
ric transfer hydrogenation of ketones to optical active tion reactions, we have found that ruthenium phos-
secondary alcohols, which are an important class of phane complexes containing 2-(aminomethyl)pyridine
intermediates in the fine chemical industry, using 2- (ampy) are highly efficient catalysts for the reduction
[
8]
propanol or formic acid derivatives as hydrogen sour- of ketones, with the derivatives of general formula
[1]
[8d]
ces. It is worth noting that, because of the higher RuCl (PP)(ampy) (PP=chiral diphosphane) afford-
A
H
R
U
G
2
redox potentials of aldehydes compared to ketones, ing ee values up to 94%. An extremely active system
the equilibrium of the transfer hydrogenation reaction has subsequently been prepared using 6-(4’-methyl-
of aldehydes with 2-propanol is more shifted toward phenyl)-2-pyridylmethylamine (HCNN), leading to
the products, compared to the corresponding transfer the terdentate complex RuCl
A
H
R
U
G
A
H
R
U
G
[
2]
hydrogenation of ketones. By contrast, reduction of [dppb=Ph P
ACHTREUNG
2
2
4
2
aldehydes to primary alcohols via transfer hydrogena- tion of ketones with 2-propanol at low ruthenium
tion as well as the control of the chemoselectivity of loadings (0.05–0.001 mol%), in a very short reaction
6
À1
[9]
this reaction are considered rather difficult process- time and with high TOF values (10 h ) (Figure 1).
es. The difficulty for the catalytic reduction of alde-
[3]
In this paper we describe the use of complex 1 as a
hydes resides in the side reactions that may occur highly efficient catalyst for the chemoselective re-
during the catalytic reaction, usually performed in duction of aldehydes with 2-propanol as hydrogen
basic media. As a matter of fact, the hydrogens of the donor.
a-CH group are susceptible to deprotonation and can
When a 0.1M solution of benzaldehyde is refluxed
lead to aldol condensation. Furthermore, during catal- in 2-propanol with 1 (0.05 mol%) and K CO (1
2
3
ysis aldehydes may also undergo decarbonylation re- mol%), quantitative formation of benzyl alcohol has
[4]
actions, which may result in deactivation of the cata- been observed within 1 min, leading to a high turn-
Adv. Synth. Catal. 2007, 349, 1633 – 1636
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1633

Walter Baratta et al.
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1
of aldol condensation, as inferred from H NMR anal-
ysis. Also for these aliphatic aldehydes an increase of
the base concentration results in shorter reaction
time, with quantitative alcohol formation even at low
catalyst loading (0.01 mol%). These results can be
compared with those reported for the Cp*Ir-diamine
system which requires longer reaction time and drop-
wise aldehyde addition to prevent aldol condensa-
[
5]
tion.
High chemoselectivity has been observed with 1 in
the transfer hydrogenation of a,b-unsaturated alde-
hydes leading to fast reduction at the carbonyl group.
Thus, using 1 at 0.05 mol% and K CO (1 mol%) the
Figure 1.
2
3
5
À1
over frequency value of 1.3”10 h calculated at substrate trans-cinnamaldehyde is quantitatively con-
0% conversion [Eq. (1) and Table 1]. verted into cinnamyl alcohol after 2 min (TOF=1.8”
5
5
À1
1
0 h ). By increasing the base concentration (2, 5
and 10 mol%), the reaction occurs at a higher rate,
5
À1
affording TOF values up to 3.4”10 h (Table 1).
Under refluxing conditions in 2-propanol, the analysis
of the mixture reveals slow reduction of the C=C
double bond, leading to 20% of 3-phenyl-1-propanol
By increasing the base concentration to 5 and 10 after 1 h. Fast and chemoselective reduction has been
mol%, the reaction is complete in 30 and 20 s, respec- observed for tiglic aldehyde which gives quantitative
5
tively, affording TOF values of 3.0”10 and of 5.0” formation of trans-2-methyl-but-2-en-1-ol in 30 s with
5
À1
[5]
5
À1
1
0 h , which are the highest values reported to a TOF of 2.5”10 h . These results on the reduction
date. The effectiveness of using 1 in synthesis has of a,b-unsaturated aldehydes can be compared with
been tested by carrying out the reaction at a lower those reported for the few systems that enable the
[
3f,k,5]
loading of 1. Thus, benzaldehyde is quantitatively re- chemoselective reduction at the C=O bond.
duced using 0.01 mol% of 1 with a rate that progres- As regards the mechanism of the transfer hydroge-
sively increases at higher base concentrations with nation, in the presence of K CO , complex 1 in 2-
[
10]
2
3
4
4
5
TOF values of of 3.2”10 , 3.6”10 , 1.6”10 and 2.2” propanol solution is converted into the corresponding
5
À1
1
0 h with 1, 2, 5 and 10 mol% of K CO , respec- ruthenium isopropoxide species Ru
A
H
R
U
G
ACHTREUNG
2
3
tively, without deactivation of the catalyst. Because of
the high performance of 1 we extended this reduction elimination reaction,
procedure to a wider range of aldehyde substrates. dride complex RuH(CNN)(dppb). The subsequent
ACHTREUNG
[
11]
[
9]
A
H
R
U
G
A
H
R
U
G
Methyl-substituted benzaldehydes are rapidly and insertion of aldehyde into the RuÀH bond leads to a
completely reduced to the corresponding alcohols in a new ruthenium alkoxide that rapidly reacts with 2-
few minutes using 0.05 mol% of 1 in refluxing 2-prop- propanol, affording the primary alcohol product and
anol. Thus, for 4-methylbenzaldehyde the TOF value the ruhenium isopropoxide that closes the catalytic
4
À1
is 3.0”10 h when the base is 1 mol% and increases cycle. Without base, compound 1 displays no catalytic
4
À1
to 8.0”10 h when the base is 5 mol%. A similar activity in the reduction of both aldehydes and ke-
trend has been observed for 2,4-dimethylbenzalde- tones, in agreement with the necessity to generate a
hyde which is reduced in 5 min, displaying a TOF ruthenium hydride species which is more active by in-
4
À1
[9a]
value of 3.6”10 h with 1 mol% of K CO , whereas creasing the base concentration. On the other hand,
2
3
at 5 mol% of base quantitative conversion is achieved by performing the reaction with K CO (5 mol%) in
2
3
5
À1
in 20 s, affording a TOF of 4.5”10 h (Table 1).
the absence of 1, very little conversion of trans-cinna-
Complex 1 has been found to be extremely efficient maldehyde (<1%) was observed after 20 min. This
also for the reduction of aliphatic aldehydes. Thus, result is consistent with the slow reduction of 2-naph-
under the aforementioned experimental conditions, 2- thaldehyde (6% conversion after 6 h) using 50 mol%
[4b]
methylbutanal, hexanal and cyclohexanecarboxalde- of K CO , reported by Crabtree and co-workers.
2
3
hyde are completely reduced within a few minutes
We have to point out that with 1 the hydrogen
using 1 (0.05–0.01 mol%), affording TOF values of transfer takes place using freshly distilled aldehydes,
5
5
5
À1
2
.4”10 , 3.0”10 and 2.0”10 h with 5 mol% of since the presence of traces of aldehyde oxidation
K CO (Table 1). The enolizable 2-phenylpropanal is products inhibits the activity. For example, addition of
2
3
also quickly converted into alcohol with 0.05 mol% benzoic acid at 0.5 and 1 mol% to the solution con-
4
of 1 and 1 and 5 mol% of base (TOF=1.6”10 and taining benzaldehyde with 1 (0.01 mol%) and K CO3
9
2
4
À1
.0”10 h ), without apparent formation of products (10 mol%) leads to a decrease of the rate, with TOF
1634
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1633 – 1636

Fast and Chemoselective Transfer Hydrogenation of Aldehydes
COMMUNICATIONS
[a]
Table 1. Catalytic transfer hydrogenation of aldehydes catalyzed by 1 (0.05 mol%).
[b]
À1 [c]
Aldehyde
Alcohol
K CO [mol%]
Time [min]
Conversion [%]
TOF [h ]
2
3
5
1
5
1
1
30 s
20 s
99
99
99
1.3”105
3.0”10
5
0
5.0”10
4
1
5
5
2
99
99
3.0”104
8.0”10
4
1
5
5
20 s
99
98
3.6”105
4.5”10
5
5
30 s
99
2.4”10
5
5
1
30 s
2
99
99
3.0”105
[d]
0
3.0”10
5[d]
5
5
99
2.0”10
4
1
5
5
2
99
99
1.6”104
9.0”10
5
1
2
5
1
2
1
30 s
30 s
99
99
99
99
1.8”105
2.0”10
5
3.3”105
0
3.4”10
5
5
30 s
99
2.5”10
[a]
[b]
[c]
[d]
The reactions were carried out at 828C with the aldehyde 0.1M in 2-propanol.
The conversion was determined by GC and for entry 3 also by NMR analyses.
TOF: turnover frequency (moles of aldehyde converted to alcohol per mole of catalyst per hour) at 50% conversion.
0.01 mol%.
1
5
4
À1
6
values of 1.2”10 and 9.3”10 h , respectively, com-
As observed for the h -arene amino ruthenium-
5
À1
[12]
pared to 2.2”10 h obtained in absence of benzoic based catalysts, the presence of the NH group in 1
2
acid. Under the typical catalytic conditions (1 0.05 is fundamental for obtaining high performance in
mol%, K CO 5 mol%), addition of a small amount transfer hydrogenation, and for 1 this can be ascribed
2
3
of acetic acid (1 mol%) to 2-phenylpropanal signifi- to the ability of the amino group to facilitate, via hy-
cantly retards the reduction, the conversion being drogen bond interactions, the formation of ruthenium
completed in 45 min at a considerably lower rate alkoxides which are in rapid equilibrium with the
3
À1
4
À1
[9]
(
(
TOF=4.0”10 h ), with respect to 9.0”10 h
Table 1). These data suggest that the carboxylates, of a robust framework built up by the association of a
ruthenium hydride. Furthermore, the presence in 1
which form by reaction of the acids with K CO , com- terdentate ligand with the diphosphane may hinder
2
3
À
pete with the i-PrO ligand, thus preventing the b-hy- the aldehyde decarbonylation side reaction.
drogen elimination from the ruthenium isopropoxide.
Adv. Synth. Catal. 2007, 349, 1633 – 1636
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1635

Walter Baratta et al.
COMMUNICATIONS
In conclusion, the terdentate ruthenium complex 1
represents one of the most active catalysts for the re-
duction of aldehydes reported to date, leading to
rapid and quantitative conversion of aldehydes with
low loading of catalyst (0.05 to 0.01 mol%), a small
amount of a weak base (K CO ), and affording TOF
[3] a) H. Imai, T. Nishiguchi, K. Fukuzumi, J. Org. Chem.
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976, 41, 665; b) B. R. James, R. H. Morris, J. Chem.
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2
3
5
À1
values up to 5.0”10 h . The very short reaction time
minutes) required for the complete reduction, limits
(
the side reactions, making this route a highly chemo-
selective transfer hydrogenation process for alde-
hydes.
Experimental Section
All reactions were carried out under an argon atmosphere
using standard Schlenk techniques. Aldehydes and 2-propa-
nol were purchased from Aldrich and distilled under argon
before use, whereas the complex 1 was prepared according
[
4] a) C. M. Beck, S. E. Rathmill, Y. J. Park, J. Chen, R. H.
Crabtree, L. M. Liable-Sands, A. L. Rheingold, Orga-
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R. H. Crabtree, Organometallics 2004, 23, 629.
[9a]
to literature procedure.
[
[
5] X. Wu, J. Liu, X. Li, A. Zanotti-Gerosa, F. Hancock, D.
Vinci, J. Ruan, J. Xiao, Angew. Chem. Int. Ed. 2006, 45,
Typical Procedure for the Catalytic Transfer
Hydrogenation of Aldehydes
6
718.
6] a) H. Imai, T. Nishiguchi, K. Fukuzumi, Chem. Lett.
975, 807; b) D. J. Darensbourg, F. Joo, M. Kannisto, A.
Katho, J. H. Reibenspies, Organometallics 1992, 11,
990; c) S. Naskar, M. Bhattacharjee, J. Organomet.
1
The ruthenium complex 1 (2.3 mg, 3.0 mmol) was dissolved
in 3 mL of 2-propanol. The aldehyde (2 mmol) was dissolved
in 19 mL of 2-propanol and K CO (2.8 mg, 0.02 mmol) was
1
2
3
Chem. 2005, 690, 5006; d) B. T. Kim, C. S. Cho, T. J.
Kim, S. C. Shim, J. Chem. Res.(S) 2003, 368; e) S. Bha-
duri, K. Sharma, D. Mukesh, J. Chem. Soc., Dalton
Trans. 1992, 77; f) S. Sabata, J. Vcelak, J. Hetflejs, Col-
lect. Czech. Chem. Commun. 1995, 60, 127.
added, obtaining a suspension which was rapidly heated to
reflux under argon. By addition of the solution containing
the ruthenium complex (1 mL) the reduction of the alde-
hyde starts immediately (1 0.05 mol% and K CO 1 mol%).
2
3
The reaction was sampled by removing an aliquot of the re-
action mixture, adding ether (1:1 in volume) and, after filtra-
tion over a short silica pad, the conversion was determined
by GC analysis.
[
[
7] I. Yamada, R. Noyori, Org. Lett. 2000, 2, 3425.
8] a) W. Baratta, P. Da Ros, A. Del Zotto, A. Sechi, E.
Zangrando, P. Rigo, Angew. Chem. Int. Ed. 2004, 43,
3
584; b) W. Baratta, A. Del Zotto, G. Esposito, A.
Sechi, M. Toniutti, E. Zangrando, P. Rigo, Organome-
tallics 2004, 23, 6264; c) W. Baratta, J. Schꢁtz, E. Herdt-
weck, W. A. Herrmann, P. Rigo, J. Organomet. Chem.
Acknowledgements
2
005, 690, 5570; d) W. Baratta, E. Herdtweck, K. Siega,
M. Toniutti, P. Rigo, Organometallics 2005, 24, 1660.
9] a) W. Baratta, G. Chelucci, S. Gladiali, K. Siega, M. To-
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G. Chelucci, A. Del Zotto, K. Siega, M. Toniutti, E.
Zangrando, P. Rigo, Organometallics 2006, 25, 4611.
This work was supported by the Ministero dell’Università e
della Ricerca (MUR). K.S. acknowledges support through a
grant from the Regione Friuli Venezia Giulia.
[
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1633 – 1636