2004 Bader Award LectureMetal-ion-catalyzed acyl and phosphoryl transfer reactions to alcohols: La³⁺-promoted alcoholysis of activated amides, carboxylate esters, and neutral organophosphorus esters

Abstract

Unlike metal-ion-catalyzed hydrolysis processes, metal-ion-catalyzed methanolysis processes have received scant attention in the literature particularly from the standpoint of mechanistic studies. La 3+ , introduced into methanol solution as its triflate or perchlorate salt, is particularly effective in promoting methanolysis reactions of unactivated and activated esters, phosphate triesters, and activated amides such as acetyl imidazoles and lactams. Studies of the kinetics of methanolysis of these substrates as a function of solution pH and [La 3+ ] indicate that the solution comprises lanthanum dimers with one to five associated methoxides (La 2 3+ ( – OCH 3 ) 1–5 ), the most catalytically active form being La 2 3+ ( – OCH 3 ) 2 , which is produced at near neutral pH in methanol (8.4). Mechanisms for all the acyl and phosphoryl transfer reactions are proposed where the metal ion serves a dual role of acting as a Lewis acid to activate the C=O or P=O system to nucleophilic attack by a metal-coordinated methoxide nucleophile. In cases where direct comparisons can be made, the La 2 3+ catalyst system is more active for the methanolysis of nonactivated substrates than for activated substrates. Another general characteristic of this system is that the catalytic rate constant for the metal complex exceeds the second-order rate constant for free methoxide, in some cases by as much as 4600-fold. Overall the catalytic effects exhibited by the La 2 3+ system is spectacular for such substrates as paraoxon, where as little as 2 mmol L –1 La(OTf) 3 in the presence of equimolar NaOCH 3 accelerates the methanolysis by 10 9 -fold relative to the background reaction at neutral pH and ambient temperature.Key words: kinetics of methanolysis, metal ion catalysis, lanthanides, methanolysis of carboxylate esters and phosphate esters.