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.