Regioselective Silylation of Sugars through Palladium Nanoparticle-Catalyzed Silane Alcoholysis

• Published in: Journal of the American Chemical Society 2002-09, Vol.124 (35), p.10508-10518
• Main Authors: Chung, Mee-Kyung, Orlova, Galina, Goddard, John D, Schlaf, Marcel, Harris, Robert, Beveridge, Terrance J, White, Gisele, Hallett, F. Ross
• Format: Article
• Language: English
• Subjects: Carbohydrates - chemistry; Catalysis; Chemistry; Exact sciences and technology; Galactose - chemistry; Glucose - chemistry; Hexoses - chemistry; Kinetics and mechanisms; Mannose - chemistry; Nanotechnology; Organic chemistry; Palladium - chemistry; Particle Size; Pentoses - chemistry; Pyrans - chemistry; Reactivity and mechanisms; Silanes - chemical synthesis; Silanes - chemistry; Substrate Specificity
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja026723v

Palladium(0)-catalyzed silane alcoholysis was applied to sugars for the first time using tert-butyldimethylsilane (TBDMS-H) and Ph3SiH as the silanes. The catalyst is a colloidal solution of Pd(0) generated in situ from PdX2 (X = Cl-, OAc-) and TBDMS-H in N,N-dimethylacetamide. The colloid has been characterized by dynamic light scattering and transmission electron microscopy and consists of catalytically highly active nanoparticles of ∼2 nm diameter. The silane alcoholysis reaction is an effective method for the regioselective silylation of methyl and phenyl glycosides and generates hydrogen gas as the only side product. For many of the sugar substrates investigated, the distribution of regioisomers obtained is complementary to that of the traditional R3SiCl/base (base = pyridine, imidazole) methodology and gives convenient access to the 3,6- rather than the 2,6-silylated pyranosides, obtained as the main product by the silyl chloride method. The method also allows a selective axial silylation of levoglucosan and 1,3,5-O-methylidene-myo-inositol. In an attempt to rationalize the observed regioselectivities, ab initio predictions (HF/3-21G*) have been made on the relative energies of some of the silylated products. They suggest that the observed regioselectivities do not reflect a kinetic vs thermodynamic product distribution but are induced by the silylation agent employed. Models for the possible origin of the observed regioselectivity in both silylation methods (silane- and silyl chloride-based) are discussed.