Lanthanide triflate-catalyzed arene acylation. Relation to classical Friedel-Crafts acylation

(Chemical Equation Presented) Lanthanide trifluoromethanesulfonates, Ln(OTf)₃ (OTf^- = trifluoromethanesulfonate), serve as effective precatalysts for the rapid, regioselective, intermolecular acylation of activated arenes. This contribution probes mechanism and metal ionic radius effects in the catalytic lanthanide triflate-mediated acylation of anisole with acetic anhydride. Kinetic studies of Ln(OTf)₃ (Ln = La, Eu, Yb, Lu)-mediated anisole acylation with acetic anhydride in nitromethane reveal the rate law ν ∼ k₃ [Ln³⁺]¹[acetic anhydride]¹[anisole]¹. Eyring and Arrhenius analyses yield ΔH‡ = 12.9 (4) kcal·mol^-1, ΔS‡ = -44.8 (1.3) e.u., and E_a = 13.1 (4) kcal·mol^-1 for Ln = Yb, with the negative ΔS‡ implying a highly organized transition state. The observed primary kinetic isotope effect of k _H/k_D = 2.6 ± 0.15 is consistent with arene C-H bond scission in the turnover-limiting step. The proposed catalytic pathway involves precatalyst formation via interaction of Ln(OTf)₃ with acetic anhydride, followed by Ln³⁺-anisole π-complexation, substrate-electrophile σ-complex formation, and turnover-limiting C-H bond scission. Lanthanide size effects on turnover frequencies are consistent with a transition state lacking significant ionic radius-dependent steric constraints. Substrate-Ln³⁺ interactions using paramagnetic Gd³⁺ and Yb³⁺ NMR probes and factors affecting reaction rates such as arene substituent and added LiClO₄ cocatalyst are also explored.