Reduction of the Diazo Functionality of α-Diazocarbonyl Compounds into a Methylene Group by NHBH or NaBH Catalyzed by Au Nanoparticles.

Supported Au nanoparticles on TiO (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group [C=(N) → CH] by NHBH or NaBH 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/CeO-catalyzed reaction of α-diazocarbonyl compounds with NHBH 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.