A comparative kinetic study of anomerization between the four main forms (a/b-pyranosides anda/b-furanosides) of D-galacturonic acid (GalAH2, 1) and tin(IV) 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 ratedue to kinetic template assistance of the carboxylic tail holding the metal in close proximity to thereactive centre and ii) a change in the dependence of the rate on the pD. Entropic and enthalpic factorscontribute to the huge activation energy decrease in 2. Anomerization rates of the complexed ligand aredetermined by 2D-EXSY cross-peak integration. Analysis of up to twelve direct and reverse rateconstants in the pD range 7.0–2.5 evidences ring size memory, and at least one reaction path whichbypasses 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-assistednucleophilic attack of water on the anomeric carbon atom cannot significantly contribute tomutarotation rates that are thousands of times faster.
|Numero di pagine||6|
|Stato di pubblicazione||Published - 2008|
All Science Journal Classification (ASJC) codes