The (η⁶-benzene)Cr(CO)₃-Substituted Propargyl Cation: Spectroscopic Characterization and Reactions of an Ambident Electrophile

Abstract

An (arene)Cr(CO)3-stabilized propargyl cation can be easily generated upon ionization of the propargyl acetate Cr(CO)3(η6-C6H5)CH(OC(O)CH3)C⋮CPh with boron trifluoride. The structure investigation of this π-complex stabilized ambident electrophile by NMR spectroscopy and extended Hückel calculations reveals that the charge density in the propargylic position is higher than in the allenylic position. The rapid formation of the cation can be followed by measuring the UV/vis kinetics for the ionization of the acetate. The cationic species reacts with alcohols to give exclusively propargyl ethers Cr(CO)3(η6-C6H5)CH(OR)C⋮CPh and with thiols to give regioselectively allenyl thioethers Cr(CO)3(η6-C6H5)CHCC(SR)Ph.