Radical-based epoxide opening by titanocenes.

The binding of 2,2-diphenyloxirane to Cp2TiCl is studied on the electronic level by magnetic resonance spectroscopy and quantum chemical calculations. The complexation of 2,2-diphenyloxirane is accompanied by dissociation of the chloride ligand, and thus, the epoxide binds to the cationic titanocene(III) complex. The titanocene(III)-epoxide species persists only for short periods of time (<5 min) even at 243 K, indicating that the ring-opening reaction is exothermic. A short-lived paramagnetic titanocene(IV)-epoxide radical species has not been directly observed. However, by a combination of isotope labeling and spin-trapping, evidence for the existence of such a species has been unequivocally demonstrated. The observation of a titanocene(III)-epoxide complex is unprecedented and provides direct evidence for inner-sphere electron transfer between epoxides and titanocenes, responsible for the high regioselectivity of ring-opening.