Electronically tuned chiral ruthenium porphyrins: extremely stable and selective catalysts for asymmetric epoxidation and cyclopropanation.
We report the use of three enantiomerically pure and electronically tuned ruthenium carbonyl porphyrin catalysts for the asymmetric cyclopropanation and epoxidation of a variety of olefinic substrates. The D(4)-symmetric ligands carry a methoxy, a methyl or a trifluoromethyl group at the 10-position of each of the 9-[anti-(1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene)]-substituents at the meso-positions of the porphyrin. Introduction of a CF(3)-substituent in this remote position resulted in greatly improved catalyst stability, and turnover numbers of up to 7500 were achieved for cyclopropanation, and up to 14,200 for epoxidation, with ee values typically >90 % and approximately equal to 80 %, respectively. In one example, the axial CO ligand at the ruthenium was exchanged for PF(3), resulting in the first chiral ruthenium porphyrin with a PF(3) ligand reported to date. In cyclopropanations with ethyl diazoacetate, the latter catalyst performed exceedingly well, and gave a 95 % ee in the case of 1,1-diphenylethylene as substrate.
Berkessel
Albrecht
A
Institut für Organische Chemie der Universität zu Köln, Greinstrasse 4, 50939 Cologne, Germany. berkessel@uni-koeln.de
Kaiser
Patrick
P
Lex
Johann
J