Protonation of Cobalt−Allene Constitutional Isomers: Highly Selective Formation of Cobalt−Allyl and Oxacobaltacyclopentadiene Complexes

The first reactivity studies of mononuclear cobalt−allene complexes are reported. The isomeric cobalt−allene complexes (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2-CH(SO2Ph)═C═CH(CO2Et))] (2-Z) and (SR,RS,E)-[(η5-C5H5)Co(2,3-η2-CH(SO2Ph)═C═CH(CO2Et))] (3-E) undergo protonation with HBF4 to give the cationic π-allyl complex (η5-C5H5)Co(PPh3)[exo-η3-CH(CO2Et)CHCH(SO2Ph)](BF4) (4) and the first structurally characterized oxacobaltacyclopentadiene complex, (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CH═C(CH2SO2Ph)](BF4) (5-BF4), respectively. Deprotonation of 5-BF4 regenerates cobalt−allene complex 3-E, as well as 2-Z.