Heteropolyacid catalyzed O-alkylation of oximes with alcohols via a carbocation in dimethyl carbonate and mechanism insight

Abstract

The C–O bond as a crucial structural skeleton exhibits high bioactivities in organic chemistry. There are few reports about the direct construction of C–O bonds by O-alkylation of oximes with alcohols, particularly the development of catalytic systems. Herein, a simple catalytic system only including H₃PW₁₂O₄₀·xH₂O was established for the O-alkylation of oximes with alcohols in dimethyl carbonate as the green solvent, which simplified the reaction systems, realized catalytic transformation, and improved the yields and selectivities. The developed system has broad substrate scope and good functional group tolerance. It is suitable for the O-alkylation of aromatic or aliphatic ketoximes and aldoximes, and oximes bearing heteroatoms with benzyl alcohols, aliphatic alcohols, allylic alcohols, and so on, affording up to 98% yield. And the gram-scale reaction and further transformations of oxime ethers have also been carried out well. Triphenylmethyl tetrafluoroborate instead of triphenylmethanol still generated the desired product, which indicated that the Ph₃C⁺ would be captured by the oxime and the reaction went through a carbocation process. The appearance and weakness of a new band at 404 and 433 nm in UV-Vis spectroscopy further proved a carbocation formation during the reaction. The changes of the absorption peak in FT-IR spectra and control experiments of different acids demonstrated the hydrogen bonding effect between the catalyst and the hydroxyl of triphenylmethanol.