C-H bond activation of benzene and thiophene by photochemically generated rhenocene cation

A cationic rhenocene-acetonitrile adduct [Cp₂Re(NCMe)](BF₄)(1) reacted with an excess of benzene, thiophene, 2-methylthiophene, and pyrrole under UV irradiation to afford the C - H bond activation products [Cp₂Re(H)R]BF₄ (R = phenyl, 2-thienyl, 2-(5-methylthienyl), 2-pyrrolyl) in high yields. In cases of thiophene derivatives and pyrrole, α-C - H bonds are selectively activated. A plausible mechanism involves the photodissociation of acetonitrile from 1 to generate a coordinatively unsaturated rhenocene cation [Cp₂Re]⁺. When 2,5-dimethylthiophene and dibenzothiophene, having no α-C - H bonds, were used as substrates, products of the activation of other C - H bonds were formed first, but they isomerized to thermodynamically more stable η¹-S-coordinated complexes in refluxing acetone. On the other hand, irradiation of the η¹-S-coordinated complexes reproduced the original C - H bond activation products. Because of the cationic character, [Cp₂Re(H)R]BF₄ were readily deprotonated by triethylamine to give neutral rhenocene derivatives Cp₂ReR. When R is thienyl or 2-(5-methylthienyl), treatment of Cp₂ReR with HBF₄ · Et₂O and MeI resulted in protonation and methylation to give [Cp₂Re(H)R]BF₄ and [Cp₂Re(Me)R]I. Thermolysis of [Cp₂Re(Me)R]I in the presence of PPh₃ unexpectedly resulted in migration of R to the Cp ring to give [(2-thienyl C₅H₄) CpRe (PPh₃)]I.