Catalysis of transesterification reactions by lanthanides – Unprecedented acceleration of methanolysis of aryl and alkyl esters promoted by La(OTf)3 at neutral sspH and ambient temperatures

Abstract

La3+ catalysis of the methanolysis of the esters p-nitrophenyl, 2,4-dinitrophenyl, and phenyl acetate (1–3), phenyl benzoate (4), and ethyl, i-propyl, cyclohexyl, and tert-butyl acetate (5, 6a, 6b, 7) was studied at 25°C as a function of sspH and [La(OTf)3]. The active form of the catalyst is attributed to a dimethoxy-bridged dimer of stoichiometry (La3+)2(–OCH3)2, having maximum activity at sspH 8 to 9. For preparative reactions, the active catalyst can be made in situ simply by adding 0.01 equiv of La(OTf)3, and 0.01 equiv of NaOCH3 to a methanol solution containing the ester (1 M). Strong catalysis of methanolysis of both aryl and alkyl esters was observed, although tert-butyl acetate was inert. At sspH 8.5, where the catalyst is maximally active, the transesterification reactions are accelerated by 40 000-fold to 18 000 000-fold in the presence of as little as 5 mM catalyst relative to the background reaction depending on the ester structure. A mechanism for catalysis of transesterification is presented wherein the reactive species is generated by breaking a single La—OCH3 bond of the dimethoxy-bridged dimer to reveal a nucleophilic metal-bound methoxide – Lewis acid La3+ electrophilic pair.Key words: lanthanides, transesterification, methanolysis, catalysis, kinetics.