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(kobs) 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(kobs)/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.