Abstract
Supported Au nanoparticles on TiO2 (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group [C=(N2) → CH2] by NH3BH3 or NaBH4 in methanol as solvent. The Au-catalyzed reduction that occurs within a few minutes at room temperature formally requires one hydride equivalent (B-H) and one proton that originates from the protic solvent. This pathway is in contrast to the Pt/CeO2-catalyzed reaction of α-diazocarbonyl compounds with NH3BH3 in methanol, which leads to the corresponding hydrazones instead. Under our stoichiometric Au-catalyzed reaction conditions, the ketone-type carbonyls remain intact, which is in contrast to the uncatalyzed conditions where they are selectively reduced by the boron hydride reagent. It is proposed that the transformation occurs via the formation of chemisorbed carbenes on Au nanoparticles, having proximally activated the boron hydride reagent. This protocol is the first general example of catalytic transfer hydrogenation of the carbene-like α -ketodiazo functionality.
Highlights
Supported Au nanoparticles on TiO2 (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group
One proton that originates from the protic solvent. This pathway is in contrast to the Pt/CeO2 catalyzed reaction of α-diazocarbonyl compounds with NH3 BH3 in methanol, which leads to the corresponding hydrazones instead
We presented a novel and useful transfer hydrogenation methodology for the reduction of the carbene moiety of α-diazocarbonyl compounds into a methylene group catalyzed by recyclable and reusable supported Au nanoparticles on TiO2
Summary
We presented the first example of Au-catalyzed hydrosilylation of α-diazocarbonyl compounds using recyclable and reusable Au nanoparticles on TiO2 as the active catalytic sites (Scheme 1) [8] This reaction proceeds in short reaction times in anhydrous 1,2-dichloroethane (DCE) affording α-silyl carbonyl compounds in high yields. The only side product of the hydrosilylation reaction is the replacement of the diazo group by two hydrogen atoms [C=(N2 ) → CH2 ] This side pathway depends on the amount of moisture in the solvent and apparently arises from the chemisorbed H species on the Au nanoparticle. This observation initiated our efforts to develop methodology for the reduction of the diazo functionality of α-diazocarbonyl comp. Organic transformations, despite the metallic character of Au ato green nanoparticles [10,11,12,13], including transfer hydrogenation reductive processes [
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