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Keywords:

  • β-alkylation of secondary alcohols;
  • immobilization;
  • iridium;
  • N-alkylation of amines;
  • N-heterocyclic carbenes

Abstract

A mesoporous silica (SBA-15)-supported pyrimidine-substituted N-heterocyclic carbene iridium complex was prepared and used as a catalyst for both environmentally friendly N-alkylation of amines and β-alkylation of secondary alcohols with primary alcohols. The structure of the supported iridium catalyst was characterized by Fourier transform infrared (FT-IR), 13C and 29Si solid-state nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), iridium K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses which demonstrated that the coordination environment of the iridium centre and the 3-dimensional-hexagonal pore structure of SBA-15 were retained after the immobilization. The catalyst was found to be highly efficient for both kinds of reaction on a wide range of substrates under mild conditions. Moreover, the supported iridium catalyst was obviously superior to the unsupported one in the N-alkylation of aniline and β-alkylation of 1-phenylethanol with benzyl alcohol as substrate, which indicated that not only the iridium complex moiety but also the support material contributed to the catalytic activity of the supported iridium catalyst in these reactions. The supported iridium catalyst can be easily recycled by simple washing without chemical treatment, and exhibited excellent recycling performance without notable decrease in catalytic efficiency even after twelve test cycles for N-alkylation of aniline with benzyl alcohol, nine cycles for N-alkylation of different amines with different alcohols, and eight cycles for β-alkylation of 1-phenylethanol with benzyl alcohol, respectively.