3
8
V. Vetere et al.
The presence of SnBux fragments blocks some active sites
for the hydrogenation on the Pt surface and, as a conse-
quence, the catalytic activity is reduced. The situation is
quite different for the chiral systems; Fig. 3 and Table 1
show that there is a significant increase in activity when the
monometallic catalyst is modified with the chiral inductor
Sn Men . The presence of (-)-menthyl chiral fragments on
catalyst proposed in this work has another advantage,
which is the possibility of being reused, keeping both its
activity and chemo- and enantioselectivity in several suc-
cessive cycles of hydrogenation, as has been previously
reported for the enantioselective hydrogenation of other
prochiral carbonyl compounds [12].
2
6
the surface might be responsible for some kind of geo-
metric interaction with the substrate, which would cause a
rate acceleration in the key step of enantiodifferentiation.
This phenomenon has already been reported in previous
studies for the hydrogenation of 3,4-dimethoxyacetophe-
none using the same chiral inductor [11] and goes in the
same direction as the commonly observed effect in the
enantioselective hydrogenation using classical Pt-cincho-
nidine systems [3, 28]. The rate-promoting effect of tin on
the enantioselective hydrogenation of 3,4-hexanedione
over polymer-stabilized platinum nanoclusters has been
investigated by Zhang et al. These authors also found that
the enantioselectivity could be enhanced by the addition of
4 Conclusions
The main findings of this study are summarized below:
–
–
–
–
It has been possible to prepare organobimetallic
catalysts by the selective reaction of an organotin
compound (SnBu4 and Sn Men ) and a Pt-based
2
6
catalyst. These systems have been obtained through
controlled surface reactions, using techniques derived
from SOMC/M.
These solids proved to be active catalysts in the
hydrogenation of 3,4-hexanedione. The achiral Pt–
SnBu catalyst presented a slightly lower activity than
4
2
?
2?
Sn , probably due to the electrophilic nature of Sn [29].
Table 1 reports the selectivity to the different products
from the hydrogenation of 3,4-hexanedione. As already
mentioned, the reaction proceeds in two stages. In a first
step the hydrogenation of one of the C=O groups takes
place, yielding 4-hydroxyhexan-3-one, and then the
hydrogenation of the other C=O group occurs, producing
the corresponding monometallic catalyst, possibly due
to a dilution effect of the active sites on the Pt surface,
due to the presence of Sn.
For the chiral system Pt–Sn Men , there was a
2
6
significant increase in the hydrogenation rate, which
could be due to the presence of (-)-menthyl groups on
the surface that would cause some specific interaction
with the substrate, accelerating the reaction rate in the
key step of enantiodifferentiation.
3
,4-hexanediol. As shown in Table 1, when using the
monometallic catalyst, 4-hydroxyhexan-3-one is the main
product obtained, and to a lesser extent, 3,4-hexanediol.
The addition of Sn changes the catalytic behavior in terms
of selectivity, significantly increasing the amount of diols
obtained. Ionic Sn species on the surface of the base metal
act as electrophiles or Lewis acid sites for the adsorption
and activation of the C=O group via the free electron pair
of the oxygen atom, promoting the attack of the carbonyl
group by the chemisorbed hydrogen and favoring the sec-
ond stage of hydrogenation, i.e., the production of 3,4-
hexanediol from 4-hydroxyhexan-3-one.
The Pt–Sn Men catalyst proved to be enantioselective
2
6
in the two steps of the hydrogenation, reaching values
of enantiomeric excess of 17 and 5% for 4-hydroxyh-
exan-3-one and 3,4-hexanediol, respectively. The
enantiomeric excess obtained for the hydroxy ketone
is comparable to those achieved with classical enan-
tioselective systems.
Acknowledgements This work has been sponsored by UNLP
Project X487, Argentina) and ANPCyT (PICT 14-11243, Argentina).
(
It is interesting to note that, for none of the catalytic
systems used in this work, the formation of a hemiketal
between the solvent (2-propanol) and 3,4-hexanedione or
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