AN UNUSUAL EXAMPLE OF ENDO-STEREOSELECTIVITY IN THE LIGAND-EXCHANGE REACTIONS OF RHODIUM(I)DIETHYLENE DERIVATIVES WITH PARA-SEMIQUINOID LIGANDS

The ligand exchange reactions of [(C2H4)(2)Rh(acac)] in benzene and [( C2H4)(2)RhCl](2) in CH2Cl2 with -methyl-4-trichloromethyl-2,5-cyclohex adiene-1-one occur with 100% exo-stereoselectivity. The similar proces s with ethyl-2,6-dioxocyclohexylidene)-2,5-cyclohexadiene (trienedione ) is strictly exo-stereospecific only if [(C2H4)(2)Rh(acac)] in benzen e is used, while in the case of [(C2H4)(2)RhCl](2) in CH2Cl2, it proce eds with an endo-stereoselectivity of 43%. An explanation for these fa cts has been suggested that assumes that the metal atom initially atta cks the central double bond in the trienedione, which is removed from the area of main steric hindrance. The subsequent metallotropic rearra ngement of the resulting ethylene-triene intermediate gives rise to th e final eta(4)-coordinated pi-diene structures.