Kinetische und mechanistische Untersuchungen von Uebergangsmetall-Komplex-Reactionen.XVII. Untersuchungenen zur Substituentenabhaengigkeit der cis/trans-Cyclopropan-Produktherhaeltnisse bei der Uebertragung von Benzylidenliganden auf Styrole

Article abstract of DOI:10.1016/0022-328X(93)80427-D

The benzylidene complexes (CO)5M=C(C6H4R-p)H react with styrenes, H2C=C(C6H4R-p)H (R = F, Cl, Br, H, Me, OMe), α-methylstyrene, vinylcyclohexane and 2-methyl-2-butene, respectively, by transfer of the benzylidene ligand to the C=C bond and formation of cis- and trans-cyclopropanes.From the temperature dependence of the cis/trans isomer ratios the differences in the activation enthalpies ΔΔH^* = Δ(H_cis)^* - Δ(H_trans)^* and activation entropies ΔΔS^* = Δ(S_cis)^* - Δ(S_trans)^* weredetermined.With all reactions the formation of the cis-isomer is favoured by the lower activation enthalpy.The formation of the trans-isomers, however, is generally favoured by the lower activation entropy.Exceptions are the reactions of 1a with styrene and of 1c with 2-methyl-2-butene.These two opposing influences determine the cis/trans isomer ratio observed.The results are discussed on the basis of stabilizing electronic and destabilizing steric interactions.