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

Abstract

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 noα-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 stableη11-S-coordinated complexes in refluxing acetone. On the other hand, irradiation of theη1-S-coordinated complexes reproduced the original C–H bond activation products. Because of the cationic character,[Cp2Re(H)R]BF4were readily deprotonated by triethylamine to give neutral rhenocene derivativesCp2ReR. When R is thienyl or 2-(5-methylthienyl), treatment ofCp2ReRwithHBF4⋅Et2Oand MeI resulted in protonation and methylation to give[Cp2Re(H)R]BF4(R=and[Cp2Re(Me)R]I. Thermolysis of[Cp2Re(Me)R]Iin the presence ofPPh3unexpectedly resulted in migration of R to the Cp ring to give[(2 thienyl C5H4)CpRe(PPh3)]I.