Ruthenium-catalyzed conjugate hydrogenation of ?±,?2-enones by in situ generated dihydrogen from paraformaldehyde and water

Article abstract of DOI:10.1002/ejoc.201403359

Notwithstanding that the highly selective hydrogenation of ?±,?2-enones to allylic alcohols can be realized by using Noyori's Ru bifunctional system, the selective reduction of the C=C bonds in ?±,?2-enones without touching the C=O bonds still lacks a general, simple, and efficient procedure. Ruthenium-catalyzed conjugate hydrogenation of various ?±,?2-enones to saturated ketones with high selectivity was investigated. The most important feature of this procedure was that hydrogen in situ generated from paraformaldehyde (or formalin) and water was employed as the reductant.

Full text of DOI:10.1002/ejoc.201403359

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DOI: 10.1002/ejoc.201403359
Ruthenium-Catalyzed Conjugate Hydrogenation of α,β-Enones by in situ
Generated Dihydrogen from Paraformaldehyde and Water
Wanfang Li[a] and Xiao-Feng Wu*[a]
Keywords: Ruthenium / Water chemistry / Hydrogenation / Enones / Paraformaldehyde
Notwithstanding that the highly selective hydrogenation of
α,β-enones to allylic alcohols can be realized by using
Noyori’s Ru bifunctional system, the selective reduction of
the C=C bonds in α,β-enones without touching the C=O
bonds still lacks a general, simple, and efficient procedure.
Ruthenium-catalyzed conjugate hydrogenation of various
α,β-enones to saturated ketones with high selectivity was in-
vestigated. The most important feature of this procedure was
that hydrogen in situ generated from paraformaldehyde (or
formalin) and water was employed as the reductant.
reaction conditions or special catalysts are often needed.[7]
Likely, heterogeneous hydrogenation (such as Pd/C) often
leads to the over-reduction or even hydrogenolysis of the
C=O groups in benzyl positions. Besides, some easily
reducible groups such as aryl halogens and nitro and nitrile
groups may not be tolerated.
Introduction
The conjugate reduction of α,β-enones is a very impor-
tant synthetic transformation because[1] α,β-enones can be
easily prepared by many simple procedures[2] and because
the corresponding reduced products are interesting sub-
structures for many pharmaceutically active molecules (Fig-
ure 1).[3] Reduction systems such as dissolving metals[1a]
and metal hydrides[1b] require cumbersome reagents, which
has limited their synthetic applications. Over the past
decades, conjugate transfer hydrogenations catalyzed by
transition-metal complexes[4] and organocatalysis[5] have
been continuously explored.
Ruthenium-catalyzed homogeneous hydrogenation of
alkenes and ketones is known for its high reactivity and
selectivity.[8] For safety and operation reasons, the use of
in situ generated hydrogen without an autoclave device is a
very promising alternative method for small-scale reactions
in organic laboratories. Nowadays, hydrogen production
from simple molecules such as water, formic acid, and
alcohols is an ever-growing topic from the point of view of
new, clean energy.[9] More recently, Prechtl and co-workers
reported the selective and mild production of hydrogen
from water and formaldehyde with the use of [RuCl2(p-
cymene)]2 as the catalyst.[10] They proposed that the ruth-
enium species could catalyze H/D scrambling between D2O
and the C–H bonds in HOCH2OH, and in addition the
dehydrogenation of deuterated methanediol would produce
a mixture of D2, H2, and HD through the deuterated formic
acids. Their procedure provided an elegant way to produce
hydrogen from small molecules. However, the controlled
generation of hydrogen from paraformaldehyde and water
has seldom been applied in hydrogenation reactions.
Paraformaldehyde is an inexpensive and readily available
C1 block that has been used in many organic synthesis[11]
and transition-metal-catalyzed transformations.[12] For ex-
ample, it has been used as a syngas equivalent in rhodium-
catalyzed olefin/alkyne hydroformylations[13] and in palla-
dium-catalyzed carbonylations.[14] To the best our knowl-
edge, very few examples have been reported in which para-
formaldehyde is used as the hydrogen donor in transition-
metal-catalyzed transfer hydrogenations.[15] In comparison
to formic acid, which is a pungent and corrosive liquid that
is usually used in large excess amounts, paraformaldehyde
Figure 1. Selected bioactive molecules with the 1,4-diaryl ketone
structures.
Catalytic hydrogenation is always reckoned as the most
clean and efficient reduction method for modern organic
synthesis. The selective hydrogenation of either the olefin or
the ketone group in α,β-enone motifs can be challenging in
some situations. In the 1990s, Noyori et al. selectively
hydrogenated the carbonyl group in α,β-enones by using
Ru–diphosphine–diamine bifunctional catalysts.[6] Al-
though the hydrogenation of C=C bonds is usually thermo-
dynamically favored for α,β-enones, careful control over the
[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
E-mail: xiao-feng.wu@catalysis.de
http://www.catalysis.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403359.
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W. Li, X.-F. Wu
SHORT COMMUNICATION
is a more stable and easier-to-handle solid. Herein, we wish
With the optimized conditions in hand, we next extended
to report the ruthenium-catalyzed selective hydrogenation the substrates to other substituted α,β-enones. Various sub-
of the C=C bonds in α,β-enones by using paraformaldehyde stituted chalcones were smoothly reduced to their corre-
and water as the in situ hydrogen generator.
sponding saturated ketones in good yields (Table 2).
Notably, aryl halides such as chloro (see compound 1e),
iodo (see compound 1d), and bromo (see compound 1f)
were well tolerated and no dehalogenation was observed in
these cases. The aryl carbon halides in these products were
further converted into some versatile drug-like molecules
through palladium-catalyzed couplings. Furthermore, nitro
(see compound 1c) and nitrile (see compound 1i) groups,
which are easily reduced under heterogeneous hydrogena-
tion conditions, remained untouched with this procedure.
Substituents ortho to either the keto group or the alkene
had no clear effect on the efficiency of the reaction (see
compound 1g). The ferrocenyl chalcone (see compound 1j)
and pentamethyl chalcone (see compound 1k) were also re-
duced to their saturated forms in good yields. α,β-Enones
with heterocycles such as furan, (benzo)thiophene, indole,
and pyrrole were also reduced by this method without diffi-
culty. If R1 was an alkyl groups such as cyclopropanyl (see
compound 1q), 2q was obtained in 87% yield. Cyclic enones
such as chromone (1r) was reduced to chromane (2r) in
excellent yield. The two conjugated C=C bonds in dibenz-
ylideneacetone (1s) were selectively saturated in 84% yield.
If there were two cumulative C=C bonds in the enone
substrate, as in 1t, no conversion was observed upon using
RuCl2(PPh3)3 as the catalyst. However, upon using
[Ru(p-cymene)Cl2]2 as the catalyst, a mixture of two pos-
sible products was obtained. Moreover, the trisubstituted
alkenes in 1u and 1v were also reduced but in lower yields.
The α,β-unsaturated ester methyl cinnamate (1w) was re-
duced to the saturated ester under the same conditions.
Results and Discussion
At the outset, we performed the reaction with chalcone
(1a) as the standard substrate and paraformaldehyde as the
reductant to optimize the reaction conditions (Table 1).
Upon heating a mixture of 1a, paraformaldehyde (5 equiv.),
K2CO3 (1 equiv.), and RuCl2(PPh3)3 (2 mol-%) at 120 °C
under an atmosphere of argon in H2O, the conversion was
greater than 99% but the yield of 2a was only 70% (Table 1,
entry 1). Next, we performed the reaction in mixed toluene/
water solvents with different ratios (Table 1, entries 2–5). To
our satisfaction, 2a was obtained in almost quantitative
yield in the toluene/H2O = 10 system (Table 1, entry 4).
Control experiments showed that 1 equivalent of K2CO3
was necessary for the reaction (Table 1, entries 6–9). Other
solvents such as acetonitrile, dioxane, and DMF were much
less efficient for the selective reduction of 1a to 2a (Table 1,
entries 10–12). The catalyst formed in situ from RuCl3 and
PPh3 also proved effective for this reduction (Table 1,
entry 13). Interestingly, the reaction also produced 2a in
95% yield in the presence of [Ru(p-cymene)Cl2]2 (2 mol-%)
under phosphine-free conditions (Table 1, entry 14). Upon
replacing the ruthenium catalyst with Wilkinson catalyst or
PdCl2(PPh3)3, the conversion of 1a was rather low (Table 1,
entries 15 and 16). Finally, the optimized conditions in-
cluded the use of RuCl2(PPh3)3 as the catalyst, K2CO3 as
the base, and toluene/H2O = 10 as the solvent at a reaction
temperature of 120 °C.
Table 1. Optimization of the conjugate reduction of chalcone (1a).[a]
Entry
Catalyst[b] (mol-%)
Base (equiv.)
Solvent
Conversion[c] [%]
Yield[c] [%]
1
2
3
4
5
6
7
8
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
A (2)
C (2)
D (2)
E (2)
F (2)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (0.4)
KOH (1)
H2O
Ͼ99
Ͼ99
Ͼ99
Ͼ99
)
Ͼ99
Ͼ99
23
49
32
41
Ͼ99
96
70
86
toluene/H2O = 2
toluene/H2O = 5
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
MeCN/H2O = 10
dioxane/H2O = 10
DMF/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
toluene/H2O = 10
91 (13[d]
)
97 (95[e])
Ͼ99[f] (3[g]
95[e]
88
89
4
Na2CO3 (1)
Et3N (1)
9
32
20
36
81
91
95
22
–
10
11
12
13
14
15
16
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
Ͼ99
24
Ͻ1
[a] Reaction conditions: 1a (52.1 mg, 0.25 mmol), (CH2O)n (37.5 mg, 1.25 mmol) in solvent (1.8 mL) for 16 h at 120 °C. [b] Catalyst: A
= RuCl2(PPh3)3; B = Ru3(CO)12 (3%) + PPh3 (10%); C = RuCl3·3H2O (2%) + PPh3 (6%); D = [Ru(p-cymene)Cl2]2; E = RhCl(PPh3)3;
F = PdCl2(PPh3)2. [c] Determined by GC by using n-C16H34 as the internal standard. [d] In air. [e] Yield of isolated product. [f] 37%
Aqueous formalin solution (92 μL, 1.25 mmol) was used. [g] Trioxin (37.5 mg, 1.25 mmol) was used.
2
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Ruthenium-Catalyzed Conjugate Hydrogenation of α,β-Enones
Table 2. Substrate scope for the conjugate reduction of α,β-enones.[a]
[a] All reactions were performed with 0.25 or 0.3 mmol of the substrate. [b] (CH2O)n (4 equiv.) was added. [c] [Ru(p-cymene)Cl2]2 was
used as the catalyst and an approximately 2:1 mixture of 1,5-diphenylpent-4-en-1-one/1,5-diphenylpentan-1-one was obtained. [d] GC
yield. [e] Determined by GC, 60% conversion.
However, the reduction of unfunctionalized alkenes such 4- decreasing abundance.[10] The final decomposition product
phenylbutene (1x) proceeded much more slowly, which indi- of paraformaldehyde was CO2, which could be detected by
cated that the carbonyl group probably participated in co- bubbling gas into lime water (see Figure S1 in Supporting
ordination to the ruthenium atom during the hydrogena- Information for details).
tion.
Next, we tried to gain some insight into the mechanism
of this conjugate reduction process. First, a mixture of
chalcone and deuterated paraformaldehyde was heated
under the standard conditions in toluene/D2O (10:1); 62%
deuteration in the α position of 2a was observed, which
was due to the acidity of the proton α to the ketone group.
Furthermore, no deuteration occurred at the much less
acidic β position [Scheme 1, Equation (1)]. Upon using
non-deuterated paraformaldehyde instead [Scheme 1,
Equation (2)], 20% deuteration at the β position was ob-
Scheme 1. Deuterated experiment (the reactions were performed
under the conditions of entry 5 in Table 1).
served. These results suggested that the hydrogen atom
transferred to 1a partially originated from paraformalde-
hyde and partially from water. Upon using deuterated para-
On the basis of the above analysis and experiments, we
formaldehyde and H2O, we found a much higher ratio of believe that the conjugate reduction is most probably a
hydrogen in both the α and β positions of the products ruthenium-catalyzed hydrogenation process. The hydrogen
[Scheme 1, Equation (3)]. This is also in agreement with the atom transferred to the products came from decomposition
observation from Prechtl and co-workers: the dehydrogena- of paraformaldehyde and water in the presence of the ruth-
tion of (CD2O)n in H2O produced mainly H2, and dehydro- enium catalyst; this was further verified by an experiment
genation of (CH2O)n in D2O produced D2, HD, and H2 in in a two-chamber reactor (Scheme 2, see Figure S2 in Sup-
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W. Li, X.-F. Wu
SHORT COMMUNICATION
support. General support from Professor Matthias Beller at
LIKAT is appreciated.
porting Information for details). In chamber B, 1a was still
reduced to 2a in 76% yield, so we confirmed that gaseous
hydrogen was produced in chamber A by considering that
RuCl2(PPh3)3 is a good catalyst for olefin hydrogenation.[16]
The high selectivity of our procedure lied in the fact that
only 10 equiv. of hydrogen was produced at most, and
accordingly, the partial pressure of H2 was estimated to be
no more than 2 kPa under our optimized conditions
(Table 1, entry 5).[17]
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Scheme 2. Reduction of 1a in a two-chamber reactor.
Conclusions
In conclusion, we realized the chemoselective hydrogena-
tion of α,β-enones by using a cheap and commercially
available ruthenium complex with the use of paraformalde-
hyde and water as the hydrogen source. In situ generated
hydrogen was experimentally validated through deuterated
experiments and a two-chamber reaction. With this new
method, we were able to realize the selective hydrogenation
of olefins in α,β-enones without the manipulation of haz-
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hydrogen donors such as the HCOOH–Et3N azeotrope, this
new combination is cheaper and easier to handle. The
employment of such in situ generated hydrogen in other
catalytic hydrogenation reactions is underway in our group.
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General Procedure for the Conjugate Hydrogenation Reaction: An
oven-dried pressure tube (10 mL) was charged with 1a (52 mg,
0.25 mmol), RuCl2(PPh3)3 (4.79 mg, 10 μmol), paraformaldehyde
(37.5 mg, 1.25 mmol), and K2CO3 (35 mg, 0.25 mmol). The pres-
sure tube was then transferred into a larger Schlenk tube and was
vacuumed and purged with argon (3ϫ). Next, toluene (1.6 mL) and
distilled water (0.16 mL) were added by syringe under argon flow
before the pressure tube was sealed with a Teflon cap. After that,
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in an alloy block for 18 h. The mixture was cooled to room tem-
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ethyl acetate = 30) and obtained in 95% yield.
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Acknowledgments
The authors thank the Federate State of Mecklenburg-Vorpom-
mern, the Bundesministerium für Bildung und Forschung (BMBF),
and the Deutsche Forschungsgemeinschaft (DFG) for financial
4
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[17] The theoretical partial pressure of P(H2) was calculated ac-
cording to the equation of state of an ideal gas P(H2) = nRT/
V, in which T = 393 K (120 °C), V = 20 cm3, n(H2) =
12.5ϫ10–3 mmol; therefore, P(H2) = 2042 Pa.
Received: October 16, 2014
Published Online: ᭿
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Hydrogenation
W. Li, X.-F. Wu* .............................. 1–6
Ruthenium-Catalyzed Conjugate Hydro-
genation of α,β-Enones by in situ Gener-
ated Dihydrogen from Paraformaldehyde
and Water
Dihydrogen (H2) is produced from para-
formaldehyde and water in the presence of
simple RuCl2(PPh3)3 as a catalyst. The
in situ generated hydrogen is utilized in the
selective hydrogenation of olefins in α,β-
enones catalyzed by the same catalyst. This
high-pressure-free procedure shows very
high selectivity and generality.
Keywords: Ruthenium / Water chemistry /
Hydrogenation / Enones / Paraformalde-
hyde
6
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