UC San Diego

Three diastereomers of a highly functionalized tetrasubstituted cyclopentadienylcobalt-diene complex, (Cp)Co[η4-CH(CO2Et)=C(SO2Ph)C(SiMe3)=CH(CO2Et)] (4-ZE, 4-EZ, 4-ZZ; Cp = C5H5), undergo oxidative demetallation by ceric ammonium nitrate (CAN) to give the corresponding metal-free dienes, CH(CO2Et)=C(SO2Ph)C(SiMe3)=CH(CO2Et) (5-ZE, 5-EZ, 5-ZZ), with retention of configuration. The oxidation of 4-ZE by iodine occurs with a different stereoselectivity than that observed with CAN, to provide the fourth diastereomer 5-EE in high yield. B97D/Def2-TZVPP(CH2Cl2) calculations predict the diene free-energy ordering as: 5-EE < 5-ZE < 5-EZ < 5-ZZ.