First example of selective hydrogenation of unconstrained α,β-unsaturated ketone to α,β-unsaturated alcohol by molecular hydrogen

Article abstract of DOI:10.1039/b212441f

Unprecedent hydrogenation of unhindered α,β-unsaturated ketones to unsaturated alcohols by molecular H₂ on gold supported on Fe₂O₃ catalysts.

Full text of DOI:10.1039/b212441f

First example of selective hydrogenation of unconstrained
a,b-unsaturated ketone to a,b-unsaturated alcohol by molecular
hydrogen
C. Milone, R. Ingoglia, M. L. Tropeano, G. Neri and S. Galvagno*
Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italia.
E-mail: galvagno@ingegneria.unime.it
Received 2nd January 2003, Accepted 19th February 2003
First published as an Advance Article on the web 5th March 2003
Selective hydrogenation of unconstrained a,b-unsaturated ke-
tones to the corresponding unsaturated alcohols has been
achieved on Au supported on Fe2O3 catalysts.
The catalytic activity of Au supported on Fe2O3 and Al2O3 in
the liquid phase hydrogenation of trans-3-buten-4-phenyl-
2-one (benzalacetone) has been investigated. Gold catalysts
have been prepared by classical co-precipitation and deposi-
tion–precipitation methods. The co-precipitation was carried
out by adding an aqueous solution of HAuCl4 (Fluka) and
Fe(NO3)3·9H2O(Fluka) to a solution of Na2CO3 1 M (pH =
11.9) kept at 80 °C, under vigorous stirring. The solid was
digested overnight at room temperature and then washed with
water until free of chloride ions (AgNO3 test). The deposition–
precipitation method, described elsewhere,13 consists of the
addition of the support to a solution of HAuCl4 previously
adjusted at pH 8–9 with NaOH and kept at 70 °C. The slurry was
vigorously stirred for two hours then filtered off and washed
with water until chloride free. Catalysts were dried under
vacuum (p = 1022 mbar) at 80 °C for one day.
Catalytic experiments were carried out in a batch reactor at
atmospheric pressure under H2 flow, at 60 °C, using a four-
necked flask fitted with a reflux condenser, dropping funnel,
thermocouple and a stirrer head. Benzalacetone (Aldrich) and
solvent (ethanol) were commercial analytical grade products
and were used without further purification. The catalyst was
added to 25 ml of solvent and reduced “in situ” at 70 °C for 1 h.
After cooling at reaction temperature the substrate (6 3 1024
mol) was injected through one arm of the flask. The reaction
mixture was stirred at 700 rpm. The progress of the reaction was
followed by analysis of a sufficient number of microsamples by
means of GC-MS equipped with a EC-WAX capillary column
(60 m, 0.25 mm id).
Benzalacetone (trans-4-phenyl-3-buten-2-one) has been hy-
drogenated to unsaturated alcohol (4-phenyl-3-buten-2-ol) with
a selectivity higher than 60% up to 100% conversion.
The catalytic hydrogenation of a,b-unsaturated ketones to the
corresponding unsaturated alcohols by molecular hydrogen is a
very intriguing challenge for people working in catalysis. Up to
now studies on this topic have been almost unsuccessful. The
main routes for the syntheses of unsaturated alcohols from a,b-
unsaturated ketones are the classical homogeneous reduction
with LiAlH4 or reduction by hydride transfer from isopropanol
to the enone CNO group (Meervin–Pondorf–Verley reduc-
tion).1,2
In contrast to the a,b-unsaturated aldehydes, for which the
selective hydrogenation of the CNO bond can be achieved by
promoting noble metal catalysts with s,p non-transition ele-
ments such as Sn(II), Ge(IV), Ga(III) or transition metal elements
such as Fe(III), the hydrogenation of unsaturated ketones always
leads to the formation of the saturated ketone.3 Ponec et al. have
reported that the rate of hydrogenation of the CNO bond of
propanal is dramatically enhanced by addition of Sn(II), Ga(III
)
or Fe(III) to a Pt/SiO2 catalyst, whereas the rate of hydro-
genation of acetone is only slightly increased.4
A direct evidence of the low efficacy of the addition of
promoters to noble metal catalysts for the selective hydro-
genation of a,b-unsaturated ketones has also been obtained in
this present study.
The hydrogenation of benzalacetone, carried out on Ru/C and
Pt/C catalysts (Me = 2 wt.%) and on Sn(II) promoted Ru and Pt
catalysts, Ru = 2 wt.%; Sn = 1.56 wt.% and Pt = 2 wt.%; Sn
= 0.58 wt.%, led to saturated ketone as the main reaction
product, regardless of the presence of the promoter. It should to
be noted that the above catalysts are highly selective towards the
formation of the unsaturated alcohol in the hydrogenation of
a,b-unsaturated aldehydes as cinnamaldehyde and citral.5–7
Recently, an example has been reported of selective hydro-
genation of the CNO bond of ketoisophorone, an a,b-un-
saturated ketone having a sterically hindered carbonyl group, to
the corresponding unsaturated alcohol over Pd/Al2O3.8 How-
ever the authors have pointed out that when the catalyst has been
employed for the reduction of an a,b-unsaturated ketone having
unconstrained CNO bond, no unsaturated alcohol has been
obtained.
In the present work we demonstrate that gold supported on
Fe2O3 catalysts can be successfully used for the selective
hydrogenation of a,b-unsaturated ketones, having an un-
hindered CNO group, to the correspondent a,b-unsaturated
alcohols. It has already been reported that gold dispersed on a
nanometric scale shows a remarkable intrinsic selectivity, with
respect to the Group VIIIB metal catalysts, towards the
hydrogenation of the CNO bond in the hydrogenated a,b-
unsaturated aldehydes,9 and that the rate of hydrogenation of
CNO is strongly influenced by the nature of the support, Au/
Fe2O3 and Au/ZrO2 being much more selective than Au/
SiO2.9–12
The reaction products, 4-phenyl-butan-2-one (saturated ke-
tone) and 4-phenyl-butan-2-ol (saturated alcohol) were identi-
fied by comparison with commercial products. The unsaturated
alcohol, 4-phenyl-3-buten-2-ol, was identified by comparison
with a standard compound synthesized according to a method
reported in the literature14 and extensively characterized by
mass spectrometery and NMR.
Preliminary runs carried out at different stirring conditions,
loading and catalyst grain size demonstrated the absence of
external and internal diffusion limitations.
Fig. 1 shows a typical course of the hydrogenation of
benzalacetone on Au/Fe2O3 catalysts. It shows that the
formation of the saturated ketone and of the unsaturated alcohol
occurs through parallel reactions, whereas the saturated alcohol
is mainly obtained by the further hydrogenation of the CNO
bond of the saturated ketone. The hydrogenation of the
unsaturated alcohol is not observed up to 100% of conversion of
benzalacetone.
In order to rule out the possibility that the hydrogen atoms are
supplied by the alcoholic solvent, the reaction was carried out
under N2. After activation “in situ”, the system was flushed in
N2 at 60 °C for one night then the substrate was introduced into
the reaction vessel. Under these conditions, no conversion of
benzalacetone was observed up to 3 h of contact time thus
indicating that the reaction does not occur through a Meervin–
Pondorf–Verley reduction mechanism. Moreover, when H2 was
readmitted into the reaction vessel, the hydrogenation of
benzalacetone occured.
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CHEM. COMMUN., 2003, 868–869
This journal is © The Royal Society of Chemistry 2003

Table 1 Catalytic activity and selectivity for the hydrogenation of benzalacetonea
Vi 3 108/mol
Selectivity (%) Selectivity (%) Selectivity (%)
Conversion (%) S.K. U.A. S.A.
Catalyst
Metal load (wt.%)Preparation method
gcat21 s21
Au/Fe2O3
Au/Fe2O3
Au/Fe2O3
5.3
16.6
4.2
Coprecipitation
1.6
3.5
10
28.7
99.1
26.5
90.5
27.7
70.2
26.7
36.3
92.1
35.8
1.5
57.3
6.9
42.3
7.4
21.2
3.3
9.8
3.8
0.8
3.8
56.2
64.2
67.5
66.6
65.8
10.9
1.7
Coprecipitation
28.4
11.2
30.1
24.4
85.3
97.5
94.8
Deposition–Precipitation
Au/Al2O3
Ru/Fe2O3
5.1
2.0
Deposition–Precipitation
Impregnation
0.2
20
1.4
a S.K. = saturated ketone, U.A. = unsaturated alcohol, S.A. = saturated alcohol.
the Au/Fe2O3 catalysts towards the hydrogenation of the CNO
bond for the hydrogenation of an unsaturated aldehyde agrees
with our previous findings on the hydrogenation of citral
(3,7-dimethyl-2,6-octadien-1-al) on similar catalysts.12
The enhancement of the selectivity towards the formation of
the cinnamyl alcohol on Au/Fe2O3 with respect to Au/Al2O3
can be explained by considering that iron sites on the surface of
the support act as promoters, activating the CNO bond of the
unsaturated aldehyde.12
By analogy, it can be speculated that for the hydrogenation of
benzalacetone, the higher selectivity towards the formation of
unsaturated alcohols on Au supported on Fe2O3 with respect to
Al2O3 is due to the activation of the CNO bond on the iron sites
of the support. However, it cannot be ruled out that the role of
the support is to modify the electronic properties of the metal
which in turn leads to an increase in the hydrogenation rate of
the CNO bond.
In conclusion, we would like to remark that the Au/Fe2O3
catalysts are the first examples of heterogeneous catalysts able
to selectively hydrogenate unconstrained unsaturated ketones to
unsaturated alcohols with a yield higher than 60%.
This work was performed with the financial support of
Ministero dell’Istruzione, Università e Ricerca (MIUR) through
a COFIN 2001 grant.
Fig. 1 Hydrogenation of benzalacetone on Au/Fe2O3 (Au = 16.6 wt.%).
(Reaction conditions: T = 60 °C, PH = 1 atm, ethanol = 25 ml,
benzalacetone = 6 3 1024 mol, catalyst2 = 0.500 g). Benzalacetone (/),
saturated ketone (:), unsaturated alcohol (-), saturated alcohol (D)
Table 1 reports the initial rate of disappearance of benzalace-
tone Vi (mol gcat21 s21) and the product distribution, at low and
high conversion, during the hydrogenation of the unsaturated
ketone.
On Au supported on Fe2O3 catalysts, the activity and
selectivity towards the formation of the unsaturated alcohol
depend on the gold content and on the preparation method. On
catalysts prepared by co-precipitation the catalytic activity and
the selectivity increases with the gold loading. Moreover, Au/
Fe2O3 prepared by deposition–precipitation is more active and
selective towards the formation of unsaturated alcohol than an
analogous sample prepared by co-precipitation.
The activity and selectivity towards the formation of the
unsaturated alcohol are strongly influenced by the nature of the
support, with Au supported on Fe2O3 being much more
selective than the analogous catalyst dispersed on Al2O3.
It should be noted, however, that on Ru/Fe2O3 the selectivity
to the unsaturated alcohol was less than 2% (Table 1), thus
indicating the high specificity of gold for this reaction.
Gold catalysts reported in Table 1 have also been used for the
hydrogenation of cinnamaldehyde (trans-3-phenyl-2-propen-
1-al), the a,b-unsaturated aldehyde having the same structure as
the benzalacetone. It has been found that on Au/Fe2O3 the
selectivity towards the formation of the cinnamyl alcohol is
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