Vanadate chelate esters of monoionized diols and carbohydrates

Abstract

The title diols are ethane-1,2-diol (H₂ed), propane-1,3-diol (H₂pd), catechols (H₂Rcat) as well as glycerol (H₃pt) which act essentially as a substituted diol. The carbohydrates are methylated β-D-galactopyranoside (β-D-H₂gp), α-D-mannopyranoside (α-D-H₂mp) and β-D -ribofuranoside (β-D -H₂rf) and 4,6-benzylidine-α -D-mannopyranoside (α-D -H₂bmp), each of which have two vicinal hydroxyl groups having cis disposition. To achieve mononuclearity blocking of three coordination positions by tridentate ONO coordinating salicylaldimines (H₂Asal) of α-amino acids and hydrazones (H₂Abh) of benzyolacetone and salicyldehyde have been employed. Esters of type [VO(Hed)(Asal)], [VO(Hpd)(Asal)], [VO(H₂pt)(Asal)], [VO(β-D-Hgp)(Asal)], [VO(α-D-Hmp)(Asal)], [VO(β-D-Hrf)(Asal)], [VO(Hed)(Abh)], [VO(Hpd)(Abh)], [VO(α-D-Hbmp)(Abh)], [VO(HRcat)(Abh)] have been isolated and characterized. In general, the monoionized diols and carbohydrates are O,O chelated to the metal and the systems generally display hydrogen bonded association in the crystalline state. Species with chiral Asal^2- ligands have exclusive endo configuration in the solid state, but in solution a sterically controlled endo-exo equilibrium prevails in the case of the aliphatic diol esters for which the ⁵¹V chemical shift is also an index of chelate ring size. The carbohydrate esters do not display endo-exo isomerism in solution. Unlike the aliphatic systems the aromatic diol esters undergo spontaneous catecholase reaction with oxygen in solution, quantitatively affording the corresponding quinone. A catalytic cycle has been constructed for the reaction.