RESUMO
The gas-phase thermal isomerizations of 1-13C-2,2,3,3-d4-cyclopropane lead to isotopically labeled propenes characteristic of both the traditional reaction mechanism involving a trimethylene diradical intermediate and a previously predicted, but never observed, path involving rate-limiting conversion of the cyclopropane to singlet 1-propylidenes, followed by a [1,2]-deuterium shift. The isomerizations give mixtures of both 1-13C-2,3,3,3-d4-propene and 1-13C-1,2,3,3-d4-propene, products characteristic of the two mechanisms that are clearly observable by 13C{1H} NMR spectroscopy.
RESUMO
Certain sugars react readily with basic silicic acid to form soluble 2/1 (sugar/silicic acid) silicate complexes. Failure of monohydroxy compounds to give soluble products under these conditions indicates that the sugar silicates are chelates: five-membered diolato rings. Only furanose forms react. Pyranose sugars are stable under these conditions. Because all glycosides fail to react with silicic acid under these conditions, reaction appears to involve the anomeric position (C1 in aldoses, C2 in ketoses), which has a more acidic hydroxy group. Reaction is completed only when the anomeric hydroxy group is cis to an adjacent hydroxy group. The appropriate furanose form must have sufficient natural abundance and solubility to provide an observable product, as measured by (29)Si and (13)C NMR spectroscopy. These structural and practical constraints rationalize the successful reaction of the monosaccharides ribose, xylose, lyxose, talose, psicose, fructose, sorbose, and tagatose and the disaccharides lactulose, maltulose, and palatinose. Glucose, mannose, galactose, and sucrose, among others, failed to form complexes. This high selectivity for formation of sugar silicates may have ramifications in prebiotic chemistry.
