Sequential Electrophile/Nucleophile Additions for η2-Cyclopentadiene Complexes of Osmium(II), Ruthenium(II), and Rhenium(I)

A series of d⁶ transition-metal complexes of the type ML₅(η²-CpH), where ML₅ = [Os^II(NH₃)₅]²⁺, [Ru^II(NH₃)₅]²⁺, and [Re^I(PPh₃)(PF₃)(dien)]⁺, were synthesized as their triflate salts and combined with electrophiles (HOTf, CH₂(OMe)₂) to form η³-allyl complexes. Treatment of these π-allyl complexes with the mild carbon nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)propene (MMTP) followed by decomplexation affords substituted η²-cyclopentene derivatives with excellent regio- and stereocontrol. Deuteration and NOE studies for the π-allyl complexes along with stereochemical analysis of the organic products confirm that both electrophilic and nucleophilic addition occurs exclusively from the exo face of the ring (opposite to metal coordination) for all three systems.