N-heterocyclic carbene promoted one-pot synthesis of 1,2-diols and ɑ-Hydroxyketones from simple aldehydes

The α-hydroxyketones and 1,2-diols are versatile building blocks for the preparation of many natural and synthetic bioactive compounds. In this study, N-heterocyclic carbon (NHC) promoted coupling of several aldehydes with formaldehyde enabled the synthesis of 1,2-diols in moderate to high yields. The highly selective conversion of several aldehydes and formaldehyde into 1,2-diols and α-hydroxyketones was achieved in a one-pot two-step process via [BHMIm][OAc] as a key intermediate. In the first step, formaldehyde was quantitatively converted to [BHMIm][OAc] through the carbene addition. In the second step, 1,2-diol was produced through the intermediate-promoted aldol reaction. The results showed that benzaldehyde was capable to react with [BHMIm][OAc] efficiently, with a 54.1% yield of 1,2-diol after the 3-hour reaction at 85 °C. Among the tested benzaldehyde derivatives, the 4-nitrobenzaldehyde showed the highest yield of 1,2-diol (73.3%) under the optimized reaction conditions. Incorporation of an electron rich group on benzaldehyde promoted carbene regeneration. With 4-methoxybenzaldehyde as the substrate, the yield of α-hydroxyketone reached a maximum of 30.6%. Additionally, dihydroxyacetone derivatives were detected in the system. In situ NMR technique confirmed that it was generated from the further reaction of the carbene/1,2-diol complex with another aldehyde. Based on the results of NMR spectroscopy and mass spectrometry, a possible reaction pathway was also proposed.