Electron Transfer Activation in the Thermal and Photochemical Osmylation of Aromatic EDA Complexes with Osmium(VIII) Tetroxide

Article abstract of DOI:10.1021/ja00232a039

Various types of arenes (Ar) spontaneously form with osmium tetroxide a series of highly colored solutions of electron donor-acceptor or EDA complexes such as in nonpolar solvents.Charge-transfer or CT osmylation is simply effected by the actinic irradiation of the absorption bands (hν_CT), and the molecular structures of the OsO4 adducts of benzene (B) and anthracene (A) are elucidated by X-ray crystallography.The metastable ion pair is established as the seminal intermediate in CT osmylation by time-resolved picosecond spectroscopy attendant upon the specific excitation of the EDA complex.According to Scheme II, the rapid collapse of the ion pair (eq 16) with a rate constant k ca. 1E9 s^-1 represents the critical transformation in adduct formation.Importantly, this ion-pair mechanism accomodates (a) the osmylation of a wide range of arene donors from the mononuclear benzenes to the electron-rich polycyclic arenes (Table V) under the common umbrella of photoexcitation and (b) the profound effect on the regiochemistry of anthracene (Table VI) by subtle variations in solvent polarity.In the absence of deliberate irradiation (i.e., in the dark), the EDA complexes of OsO4 with electron-rich arenes, especially the polynuclear naphthalene, anthracene and phenanthrene, slowly undergo the direct thermal or DT osmylation to yield the same series of adducts.As such, there is a close relationship between the photoexcited state leading to CT osmylation and the activated complex in DT osmylation.Indeed the formation of the highly unusual adduct A by OsO4 addition to the terminal ring of anthracene binds in common the transition state for DT osmylation and the ion-pair collapse in CT osmylation.Thus the electron-transfer mechanism in Scheme IV employs the adiabatic ion pair (eq 20) to account for the same regiospecificities in DT osmylations.Such a unified view of arene osmylation can be extended to the promoted thermal or PT osmylation via the five-coordinate OsO4(py), as a commonly practiced for bis-hydroxylation of alkenes.