Spectroscopic Characterization, Reactivity, and Reactions of (Arene)Cr(CO)₃-Stabilized γ-Propargyl−Allenyl Cations

(Arene)Cr(CO)₃-stabilized γ-propargyl−allenyl cations Cr(CO)₃(η⁶-R¹C₆H₄)C⋮CC⁺R²R³ (2) are reactive intermediates in the synthesis of arene-complex-substituted allenes or propargyl derivatives using mild in situ ionizations of the corresponding propargyl derivatives Cr(CO)₃(η⁶-R¹C₆H₄)C⋮CCXR²R³ (X = OH, OAc). The regioselectivity of the nucleophilic attack is strongly dependent on the substitution pattern on the propargyl side chain, i.e., steric and electronic factors. In a representative case of a stable cation (2a; R¹ = H, R² = R³ = Ph) the structure investigation of this ambident electrophile by NMR spectroscopy and UV/vis spectroscopy reveals that a considerable contribution of the allenylium resonance structure accounts for a strong participation of the chromium carbonyl fragment in the efficient stabilization. The reactivity of 2a was studied by measuring the UV/vis kinetics for the nucleophilic trapping reactions revealing that the complexed cation 2a is by 2 orders of magnitude less reactive than the corresponding free ligand.