The rhodium meso-tetraphenylporphyrins catalysed cyclopropanation reactions of styrene, cyclohexene and norbornene by ethyldiazoacetate (EOA), have been investigated. The electronic effects of the bromine groups in the beta-positions of the porphyrins can be correlated to the syn/anti ratio obtained for the reaction products in the case of styrene, showing a good linear correlation between the sum of the Hammett's sigma(p) and the logarithm of the obtained selectivity. A decrease in the stereochemical ratio on increasing the number of halogen atoms was observed. This is not the case for cyclohexene and norbornene which show a different behavior and do not give good linear correlation. Furthermore, when the rhodium porphyrins have bulky methoxy substituents in the ortho-positions of the phenyl rings, comparing with the simple rhodium meso-tetraphenylporphyrin, the isomeric ratios decrease dramatically for styrene whilst for the other substrates remain almost in the same range. On the contrary, using a rhodium porphyrin bearing chlorine groups in the ortho-positions of the phenyl rings, the ratios increase, reaching higher values. These observations suggest the presence of different factors involved in determining the isomeric ratios. The electron-withdrawing effects of the substituents in the beta-positions seem to influence the stereo selectivity giving the less hindered isomer while the chlorine bulky substituents in the ortho-phenyl positions can compensate this trend. (C) 2002 Elsevier Science B.V. All rights reserved.
Tagliatesta, P., Pastorini, A. (2002). Electronic and steric effects on the stereoselectivity of cyclopropanation reactions catalysed by rhodium meso-tetraphenylporphyrins. JOURNAL OF MOLECULAR CATALYSIS. A: CHEMICAL, 185(2009/02/01 00:00:00.000), 127-133 [10.1016/S1381-1169(02)00080-8].
Electronic and steric effects on the stereoselectivity of cyclopropanation reactions catalysed by rhodium meso-tetraphenylporphyrins
TAGLIATESTA, PIETRO;
2002-01-01
Abstract
The rhodium meso-tetraphenylporphyrins catalysed cyclopropanation reactions of styrene, cyclohexene and norbornene by ethyldiazoacetate (EOA), have been investigated. The electronic effects of the bromine groups in the beta-positions of the porphyrins can be correlated to the syn/anti ratio obtained for the reaction products in the case of styrene, showing a good linear correlation between the sum of the Hammett's sigma(p) and the logarithm of the obtained selectivity. A decrease in the stereochemical ratio on increasing the number of halogen atoms was observed. This is not the case for cyclohexene and norbornene which show a different behavior and do not give good linear correlation. Furthermore, when the rhodium porphyrins have bulky methoxy substituents in the ortho-positions of the phenyl rings, comparing with the simple rhodium meso-tetraphenylporphyrin, the isomeric ratios decrease dramatically for styrene whilst for the other substrates remain almost in the same range. On the contrary, using a rhodium porphyrin bearing chlorine groups in the ortho-positions of the phenyl rings, the ratios increase, reaching higher values. These observations suggest the presence of different factors involved in determining the isomeric ratios. The electron-withdrawing effects of the substituents in the beta-positions seem to influence the stereo selectivity giving the less hindered isomer while the chlorine bulky substituents in the ortho-phenyl positions can compensate this trend. (C) 2002 Elsevier Science B.V. All rights reserved.Questo articolo è pubblicato sotto una Licenza Licenza Creative Commons
