Carbon-carbon and carbon-oxygen bond-forming reactions mediated by lanthanide triflates: Friedel-Crafts acylation and intramolecular hydroalkoxylation of unactivated alkenes.

摘要

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 ∼ k3 [Ln3+]1[acetic anhydride] 1[anisole]1. Eyring and Arrhenius analyses yield DeltaH ‡ = 12.9 (4) kcal mol-1, DeltaS‡ = --44.8 (1.3) e.u., and Ea = 13.1 (4) kcal mol-1 for Ln = Yb, with the negative DeltaS ‡ implying a highly organized transition state. The observed primary kinetic isotope effect of kH/ 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 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-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.;Ln(OTf)3-catalyzed processes typically require the use of polar, moderately-coordinating, solvents e.g. nitromethane, that reduce Ln 3+ Lewis acidity through coordination to the metal center. Using room temperature ionic liquids based on non-coordinating anions, in place of volatile organic solvents, will provide a more open Ln3+ coordination sphere, hence increased Lewis acidity of the Ln3+ center. Lanthanide triflate complexes of the general type Ln(OTf)3 (Ln = La, Sm, Nd, Yb, Lu) serve as effective, recyclable catalysts for the rapid intramolecular hydroalkoxylation (HO)/cyclization of primary/secondary and aliphatic/aromatic hydroxy alkenes in imidazolium-based room temperature ionic liquids (RTILs) to yield the corresponding furan, pyran, spirobicyclic furan, spirobicyclic furan/pyran, benzofuran, and isochroman derivatives. Products are straightforwardly isolated from the catalytic solution, and conversions exhibit Markovnikov regioselectivity, and turnover frequencies are as high as 47.0 h-1 at 120 °C. The ring size rate dependence of the present primary alkenol cyclizations is 5 > 6, consistent with a sterically-controlled transition state. The hydroalkoxylation/cyclization rates of terminal alkenyl alcohols are slightly more rapid than those of internal alkenyl alcohols, suggesting modest steric demands in the cyclic transition state. Cyclization rates of aryl-functionalized hydroxyalkenes, are more rapid than those of the linear hydroxyalkenes, while five- and five-six membered spirobicyclic skeletons are also regioselectively closed. Turnover frequency dependence on metal ionic radius decreases by ∼ 80x on going from La3+ (1.160 A) to Lu3+ (0.977 A), presumably reflecting steric impediments along reaction coordinate. The overall rate law for alkenyl alcohol hydroalkoxylation/cyclization is v ~ k[catalyst]1 [alkenol]1. An observed ROH/ROD kinetic isotope effect of 2.48(9) is suggestive of a catalytic pathaway involving intermolecular proton transfer. The present activation parameters DeltaH‡ = 18.2 (9) kcal·mol -1, DeltaS‡ = --17.0 (1.4), and Ea = 18.2 (8) kcal·mol-1, suggest a highly organized transition state. Proton scavenging and coordinative probing results suggest that lanthanide triflates do not serve as precursors for free triflic acid. Based on kinetic and mechanistic evidence, the proposed catalytic pathway invokes hydroxyl and olefin activation by the electron-deficient Ln 3+ center, and intermolecular H+ transfer, followed by alkoxide nucleophillic attack with ring closure.