Communication
and py3 and trpy2 with py4, py5, and py6).[29] The map clearly
indicates the almost C2 symmetric folding of the two trpy li-
gands, to form a narrow groove hosting the two O atoms of
the RuꢀO moieties. The py1 and py4 pyridyl rings of trpy1 and
trpy2 flank the RuꢀO1 and RuꢀO2 groups, limiting substrate
accessibility to the RuꢀO functionality. Py2 and py5 offer some
additional shielding to RuꢀO1 and RuꢀO2 respectively, where-
as py3 and py6 are oriented downwards. Selectivity in favor of
the cis or trans isomer of the substrate is of course determined
by the ability of the alkene isomers to adapt better to the reac-
tive pocket of the catalyst. A van der Waals representation of
the six key transition states is reported in Figure 3 (right
panel). In all cases, the substrate is attacking the O atom of the
RuꢀO1 metal bonded to the trpy1 moiety whereas the RuꢀO2
moiety bonded to tpry2 is responsible for hydrogen interaction
with the substrate only. For cis-2-octene, both the Me and the
n-pentyl groups of the alkene are pointing away from the
trpy1 ligand, with the n-pentyl protruding above py5 of trpy2.
Differently, for trans-2-octene the transition state presents the
n-pentyl tail protruding away from the catalyst right above
py3. The main difference between the two transition states is
in the orientation of the C=C double bond of the substrate. As
a consequence of this, the alkene C-atom forming the CꢀO
bond is oriented away from the discriminating RuꢀO2 unit in
the trans transition state, while it is oriented inwards in the fa-
vored cis transition state. In the latter case, the distance be-
tween the CꢀO forming C atom and the O2 atom is only
3.11 ꢁ. Considering the high electrophilicity of the RuꢀO bond,
ence between these two transition states is in the relative dis-
position of the H and Me groups at the C-b atom. In the cis
transition state is oriented towards the py4 unit, whereas in
the trans transition state it is oriented towards the py3 unit.
However, in both transition states the Me group is located
quite away from them, with minimum distance around 3.4 ꢁ,
so that the Me group is unable to generate any selectivity. As
a consequence of all this the cis and trans isomers have com-
parable epoxidation rates.
In conclusion, we have prepared a new dinuclear RuꢀOH2
complex, 2+, that thanks to the trianionic ligand backbone
(pyr-dc3ꢀ) can easily reach the IV,IV high oxidation state, which
is extremely powerful for the epoxidation of a variety of al-
kenes. The oxidized dinuclear complex 2+ in oxidation state
IV,IV, behaves in a stereoselective manner thanks to the differ-
ent role of the two Ru=O groups. While one of them is respon-
sible for oxygen transfer, the second one is partly responsible
for a supramolecular interaction. The latter is also influenced
by the ligand architecture that generates a discriminating
pocket for the incoming substrate. The combination of these
factors enables oxidized 2+ to behave as a stereoselective
supramolecular oxidation catalyst without the need to use spe-
cific modifications of the substrates. The present work consti-
tutes the first example of this new paradigm in supramolecular
oxidation catalysis and could be extendable to a large variety
of instances.
the higher stability of the cis-transition state is also improved Acknowledgements
by a favorable interaction between this CꢀO forming C atom
and the RuꢀO1 bond, which works in a synergic manner with
the previously mentioned H-bond between the substrate and
the RuꢀO moiety.
This research was supported financially by MINECO (CTQ2011-
23156/BQU, CTQ2010-21497 and PRI-PIBIN-2011-1278), ICIQ,
FEDER fund (UNGI08-4E-003), ICREA Academia, Generalitat de
Catalunya (2009SGR637); Ramꢃn y Cajal contract (RYC-2009-
05226) of MINECO, and Career Integration Grant (CIG09-GA-
2011-293900) of the European Commission. COST Actions
CM1205 and CM1202 are also gratefully acknowledged.
For stilbene, the Ph ring bound to the C atom of the form-
ing CꢀO bond is involved in a p-stacking interaction with the
aromatic plane of the trpy1 ligand in both the cis and the
trans transition states, with a rather similar orientation. This
suggests that the selectivity between cis and trans isomers is
related to the significantly different interaction of the second
Ph group attached to the other C atom (see Figure 3). The
main difference is in the Ph ring attached to the C-atom of the
non-forming CꢀO bond. In the cis transition state, this ring has
a p-stacking interaction with py4, while in the favored trans
transition state it has an interaction through the O2 atom
(3.03 ꢁ distance between the C-ipso atoms of the Ph ring and
the O2 atom). It is worth mentioning here that in both transi-
tion states, the C atom forming the initial CꢀO bond is located
quite away from the O2 atom, 3.30 and 3.56 ꢁ in cis and trans
transition states, respectively. Overall a common feature for the
favored transition states for 2-octene and stilbene is the inter-
action of the substrate with the O-atom of the Ru-O2 bond,
thus pointing out that the selectivity is basically dictated by
this supramolecular interaction.
Keywords: density functional calculations · electrochemistry ·
epoxidation · ruthenium · supramolecular redox catalysis
ˇ
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Finally, for cis- and trans-b-Me-styrene, the Ph ring is in-
volved in a p-stacking interaction with the trpy1 unit in both
transition states, and the relative orientation of the two moiet-
ies is relatively similar in both transition states. The only differ-
Chem. Eur. J. 2014, 20, 3898 – 3902
3901
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