Tin(IV) catalyzed D-galacturonic acid anomerization

摘要

A comparative kinetic study of anomerization between the four main forms (alpha/beta-pyranosides and a/beta-furanosides) of D-galacturonic acid (GalAH(2), 1) and tin(rv) complexed D-galacturonate ([Sn(CH3)(2)(GalA)(H2O)(2)], 2) in D2O, is reported. Important differences are shown by pD vs. mutarotation rate profiles of 1 and 2. Coordination, in fact, causes: i) a spectacular increase of the rate due to kinetic template assistance of the carboxylic tail holding the metal in close proximity to the reactive centre and ii) a change in the dependence of the rate on the pD. Entropic and enthalpic factors contribute to the huge activation energy decrease in 2. Anomerization rates of the complexed ligand are determined by 2D-EXSY cross-peak integration. Analysis of up to twelve direct and reverse rate constants in the pD range 7.0-2.5 evidences ring size memory, and at least one reaction path which bypasses the aldehydo open chain intermediate. Solvent oxygen atom incorporation into the ligand is, at pH 4.5, about ten fold faster when catalyzed by the metal, and proves that the metal-assisted nucleophilic attack of water on the anomeric carbon atom cannot significantly contribute to mutarotation rates that are thousands of times faster.