The mechanism of enantioselectivity in the asymmetric Ru-catalyzed metathesis of olefins is investigated with a theoretical approach. The models are based on the chiral N-heterocyclic (NHC)-based catalysts developed by Grubbs. Our analysis indicates that the origin of enantioselectivity in the ring-closing metathesis of achiral trienes is correlated to the chiral folding of the N-bonded aromatic groups, which is imposed by the Ph groups in positions 4 and 5 of the imidazole ring of the NHC ligand. This chiral folding of the catalyst imposes a chiral orientation around the RudC bond, which, in turn, selects between the two enantiofaces of the substrate. In the ring-closing transition state, the geometry in which additional groups on the forming ring are in pseudoequatorial positions is favored over transition states in which this additional group is in a pseudoaxial position. These combined effects rationalize the enantiomeric excesses experimentally obtained.

Origin of Enantioselectivity in the Asymmetric Ru-Catalyzed Metathesis of Olefins.

COSTABILE, Chiara;CAVALLO, LUIGI
2004

Abstract

The mechanism of enantioselectivity in the asymmetric Ru-catalyzed metathesis of olefins is investigated with a theoretical approach. The models are based on the chiral N-heterocyclic (NHC)-based catalysts developed by Grubbs. Our analysis indicates that the origin of enantioselectivity in the ring-closing metathesis of achiral trienes is correlated to the chiral folding of the N-bonded aromatic groups, which is imposed by the Ph groups in positions 4 and 5 of the imidazole ring of the NHC ligand. This chiral folding of the catalyst imposes a chiral orientation around the RudC bond, which, in turn, selects between the two enantiofaces of the substrate. In the ring-closing transition state, the geometry in which additional groups on the forming ring are in pseudoequatorial positions is favored over transition states in which this additional group is in a pseudoaxial position. These combined effects rationalize the enantiomeric excesses experimentally obtained.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/1058504
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