derivatives under mild conditions. For the first time, we report a
nanoheterogeneous system stabilized by various methylated
cyclodextrins which have proved to be a discriminating tool for
aromatic ring hydrogenation. The selective hydrogenation between
reducible exocyclic functions such as CLC double bonds and
aromatic groups was easily controlled by the choice of the
substitution degree. These preliminary results may be extended to
other substrates and under hydrogen pressure to increase turnover
activities.
Notes and references
Scheme 2 Proposed selective mechanism of styrene hydrogenation.
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more efficiently the aromatic rings and avoid their hydrogenation.
Moreover, the group of Fenyvesi has recently established the
association constants of Me-b-CD (with SD = 1.9) for various
benzene derivatives.15 This work reports that the stability of the
complex increases with the carbon number of the substituent on
the aromatic ring, namely benzene (110 M21) , toluene (144 M21
)
, ethylbenzene (320 M21), thus showing a significant value for the
ethyl group.
Finally, we could presume that Me-CDs play several essential
roles in arene hydrogenation by ruthenium nanocatalysts. Firstly,
CDs act as a steric stabilizer of the colloidal suspension via
hydrophobic–hydrophobic interactions between Ru nanoparticles
and cyclodextrins16 or via interactions between Ru nanoparticles
and hydroxyl groups of native cyclodextrins, as previously
reported for gold nanoparticles.17 In our catalytic system, we have
established that 3 equivalents of Me-CDs per mole of metal were
enough to stabilize the colloidal suspension. Secondly, cyclodex-
trins act also as a supramolecular shuttle between the organic
phase and the surface of the water-soluble nanoparticles by
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inclusion complex formation are the different types of interactions
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sensitive to the shapes, sizes and polarities of the substituent
groups. The transport function of CDs justifies the necessity to
have an excess of CDs in the media. In our case, 7 equivalents of
Me-CDs could play this role and no good catalytic activity was
observed when only 3 equivalents of CDs were used. This
phenomenon has been proved by additive studies. Firstly, we have
shown no hydrogenation reaction with ortho-xylene by Ru–Me-
a-CD (R = 10), a substrate known to interact very weakly with the
a-CD.19 Secondly, we added 5 eq. Me-b-CD (1.8) to a previously
stabilized Ru–Me-a-CD (R = 5) catalytic system (in previous
experiments, we have observed that stabilization with Me-a-CD
was better with R = 5, this kind of CD being less hydrophilic). In
that case, the hydrogenation of ortho-xylene was carried out with
100% conversion in 1,2-dimethylcyclohexane in 15 hours. We
could undoubtly presume that only the Me-b-CD plays the role of
supramolecular carrier as the substrate does not fit tightly within
Me-a-CD. The proposed mechanism is presented in Scheme 2.
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reduction of RuCl3. These nanoparticles present interesting activity
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298 | Chem. Commun., 2006, 296–298
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