Catalytic asymmetric dihydroxylation by gold colloids functionalized with self-assembled monolayers

This Letter demonstrates the design and synthesis of functional nanoparticles based on the assembly of terminally substituted alkanethiols on the surface of colloidal gold. The colloids were modified with alkanethiolates presenting methyl groups and dihydroquinidine ligands (in a ratio of 3:1) for the Sharpless asymmetric dihydroxylation of olefins. Dihydroxylation reactions were performed at room temperature in tert-butyl alcohol/water with functionalized colloid (0.1 equiv based on immobilized ligand) OsO₄ (0.005 equiv) and K₃Fe(CN)₆ (3 equiv) as secondary oxidant. The dihydroxylation of trans-β-methylstyrene proceeded in 80% yield and gave diol with an enantiomeric excess of 90%. The dihydroxylation of trans-stilbene and methyl trans-cinnamate gave similar results and compare favorably with yields and selectivities obtained using polymer-supported alkaloid ligands. The colloids could be isolated from the reaction mixture by gel permeation chromatography and used again, but with a modest loss in efficiency. This report provides an early example of a functional monolayer protected colloid and, together with other examples to follow, will define the range of applications for which these nanoparticles will find use.