Kinetics and Mechanism of the Protodeauration of Gold(I) β-Styrenyl Complexes

Treatment of gold β-styrenyl complexes (E)-(PPh₃)AuC(H)═C(H)(4-C₆H₄X) [X = H (1a), CN (1b), CF₃ (1c), Me (1d), OMe (1e), and NMe₂ (1f)] with excess acetic acid in CD₂Cl₂ containing PPh₃ at 4 °C led to quantitative protodeauration to form the corresponding vinyl arene as the exclusive organic product. Kinetic analysis of the protodeauration of 1a-1e under these conditions established the rate law for the protodeauration of 1a: rate = k[1a][AcOH]_mon, where [AcOH]_mon = monomeric acetic acid and k = (4.0 ± 0.2) × 10^–2 M^–1 s^–1 (ΔG^‡ = 17.9 kcal/mol), and established the superior correlation of log(k_obs) with the Hammett σ parameter (ρ = −1.26; R² > 0.99) vis-a-vis the Hammett/Brown σ⁺ parameter (R² = 0.90). These and additional observations are consistent with a mechanism for the protodeauration of complexes 1a–1e involving transfer of proton from monomeric acetic acid to the Au–C bond with concerted C–H bond formation/Au–C(σ) bond cleavage. Protodeauration of complex 1f bearing a p-NMe₂ substituent was at least an order of magnitude faster than anticipated by the log(k_obs)/Hammett σ relationship established for the protodeauration of complexes 1a–1e, pointing to a substitution-dependent change in the mechanism of protodeauration to a pathway initiated by the protonation of the Cα═Cβ π-bond.