II International Symposium on Carbon for Catalysis ABSTRACTS

PP-<strong>II</strong>-17
with existing X-ray data the most stable structures in type I and <strong>II</strong> catalysts have a carbene
unit perpendicular to the L-Ru-C plane independent on the ligands X and L. After dissociation
of phosphane, a 14-electron intermediate is formed with the carbene unit perpendicular to the
L-Ru-C plane, too. However, this 14-electron intermediate RuX 2 (L)(=CH 2 ) is only a local
minimum and can transform due to a rather small Ru=CH 2 rotation barrier into a more stable
one with the carbene unit parallel to the L-Ru-C plane.
Cl
L
Cl
Ru
PCy 3
+ NBE
Cl
H
Ru
H
Cl
L = PCy 3
- PCy 3
L
L = H 2 IMes
H
H
L
Cl
Ru
regeneration
of precatalyst
Cl
H
H
+ NBE
Cl
L
Ru
Cl
Cl
H
H
L
Ru
Cl
H
H
Cl
L
Ru
Cl
H
H
metallacyclobutane
intermediate
The analysis of geometries and frontier molecular orbitals of both conformers of the 14-
electron intermediate shows that whereas the conformer with the carbene unit parallel to the
L-Ru-C plane leads to a stable π-complex with NBE, the conformer with a carbene unit
perpendicular to the L-Ru-C plane is prone to the direct formation of a metallacyclobutane
intermediate. Calculations show that the relative stability of these two conformers depends on
both ligands L and X. Moreover, the energy difference between two rotational conformations
is systematically larger for all calculated 14-electron reactive intermediate of type I catalysts
than for type <strong>II</strong> catalysts. Consequently, the 14-electron reactive intermediate of type <strong>II</strong>
catalysts with the carbene unit perpendicular to the L-Ru-C plane leading to a favourable
formation of the metallacyclobutane should be more stable than those of I. That should in due
consequence lead to much better partitioning into the product direction in type <strong>II</strong> catalysts
compared to type I catalysts.
References
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