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

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 v similar to k(3) [Ln(3+)](1)[acetic anhydride](1)[anisole](1). Eyring and Arrhenius analyses yield Delta H-double dagger = 12.9 (4) kcal.mol(-1), Delta S-double dagger = -44.8 (13) e.u., and E-a = 13.1 (4) kcal.mol(-1) for Ln = Yb, with the negative Delta S-double dagger 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 Ln(3+)-anisole pi-complexation, substrate-electrophile sigma-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-Ln(3+) interactions using paramagnetic Gd3+ and Yb3+ NMR probes and factors affecting reaction rates such as arene substituent and added LiClO4 cocatalyst are also explored.