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 nu approximately k 3 [Ln (3+)] (1)[acetic anhydride] (1)[anisole] (1). Eyring and Arrhenius analyses yield Delta H++ = 12.9 (4) kcal.mol (-1), Delta S++ = -44.8 (1.3) e.u., and E a = 13.1 (4) kcal.mol (-1) for Ln = Yb, with the negative Delta S++ 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 Gd (3+) and Yb (3+) NMR probes and factors affecting reaction rates such as arene substituent and added LiClO 4 cocatalyst are also explored.