Regioselective Silylation of Sugars through Palladium Nanoparticle-Catalyzed Silane Alcoholysis

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• 作者：Mee-Kyung Chung ; Galina Orlova ; John D. Goddard ; Marcel Schlaf ; Robert Harris ; Terrance J. Beveridge ; Gisele White ; and F. Ross Hallett
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：September 4, 2002
• 年：2002
• 卷：124
• 期：35
• 页码：10508 - 10518
• 全文大小：381K
• 年卷期：v.124,no.35(September 4, 2002)
• ISSN：1520-5126

文摘

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 hasbeen characterized by dynamic light scattering and transmission electron microscopy and consists ofcatalytically highly active nanoparticles of ~2 nm diameter. The silane alcoholysis reaction is an effectivemethod for the regioselective silylation of methyl and phenyl glycosides and generates hydrogen gas asthe only side product. For many of the sugar substrates investigated, the distribution of regioisomers obtainedis complementary to that of the traditional R₃SiCl/base (base = pyridine, imidazole) methodology and givesconvenient access to the 3,6- rather than the 2,6-silylated pyranosides, obtained as the main product bythe 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 thatthe observed regioselectivities do not reflect a kinetic vs thermodynamic product distribution but are inducedby the silylation agent employed. Models for the possible origin of the observed regioselectivity in bothsilylation methods (silane- and silyl chloride-based) are discussed.