(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 Ea = 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/kD = 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.
ASJC Scopus subject areas
- Organic Chemistry