Acid-Induced Liberation of Polysubstituted Cyclopentadiene Ligands from Cyclopentadienyl Cobalt: A [2 + 2 + 1] Cycloaddition Route toward 1,2,4-Trisubstituted Cyclopentadienes

Here,
we report that trifluoroacetic acid (TFAH) induces demetallation
and protodesilylation of the cyclopentadiene ligand in cobalt−η4-cyclopentadiene complexes of general formula [(η5-C5H5)­Co­(η4-exo-C­(TMS)C­(SO2Ph)­CHCRCH­(CO2Et))] (1-Ph, R = Ph; 1-ArtBu, R = p-C6H4tBu; 1-ArNMe2, R = p-C6H4NMe2; and 1-Me, R = Me). The trisubstituted cyclopentadiene
products are isolated as a mixture of two tautomers, [(CH2C­(SO2Ph)CHC­(CO2Et)CR)] (8-R-A) and [(CHC­(SO2Ph)­CH2C­(CO2Et)CR)] (8-R-B). The endo isomer, [(η5-C5H5)­Co­(η4-endo-C­(TMS)C­(SO2Ph)­CHCPhCH­(CO2Et))] (1-Ph-endo), also undergoes
demetallation and protodesilylation to give 8-Ph-A and 8-Ph-B in excellent yield. The cobalt–cyclopentadiene
complex, [(η5-C5H5)­Co­(η4-exo-C­(TMS)C­(SO2Ph)­CHC­(CO2Me)­CH­(CO2Et))] (1-CO2Me),
undergoes demetallation and protodesilylation upon treatment with
TFAH to give a hydrogen-bonded fulvenol (8-CO2Me). Liberation of the ethoxy-substituted cyclopentadiene ligand
of [(η5-C5H5)­Co­(η4-exo-C­(TMS)C­(SO2Ph)­CHC­(OEt)­CH­(CO2Et))] (1-OEt) leads to formation of a cyclopentenone
derivative (11). Thermolysis of 8-Ph-A/8-Ph-B in the presence of maleimide leads to a highly functionalized
Diels–Alder adduct, whereas 8-Ph-A/8-Ph-B serves as precursors to trisubstituted ruthenocenes by in situ
deprotonation and reaction with [(η5-C5R5)­Ru­(NCMe)3]­PF6 (16-H, R = H; 16-Me, R = Me).