Stereospecific Oxidative Demetallation of Highly Functionalized CpCo(1,3-Diene) Complexes: An Experimental and Computational Study

Three diastereomers of a highly functionalized tetrasubstituted cyclopentadienylcobalt–diene complex, (Cp)Co[η 4 -CH(CO 2 Et)=C(SO 2 Ph)C(SiMe 3 )=CH(CO 2 Et)] ( 4 - ZE , 4 - EZ , 4 - ZZ ; Cp = C 5 H 5 ), undergo oxidative demetallation by ceric ammonium ­nitrate (CAN) to give the corresponding metal-free dienes, CH(CO 2 Et)=C(SO 2 Ph)C(SiMe 3 )=CH(CO 2 Et) ( 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(CH 2 Cl 2 ) calculations predict the diene free-energy ordering as: 5 - EE 5 - ZE 5 - EZ 5 - ZZ .