Rhodium-NHC hybrid silica materials as recyclable catalysts for [2+2+2] cycloaddition reactions of alkynes

Bis-silylated dihydroimidazolium salt 1 and monosilylated imidazolium salt 2 are transformed to (NHC)RhCl(COD) complexes 3 and 4, allowing the preparation of hybrid silica materials either by sol-gel or grafting processes. Full characterization of the materials by means of solid state NMR, N₂-sorption measurements, thermogravimetric analysis (TGA) and elemental analysis was followed by evaluation of catalytic activity in the [2+2+2] cycloaddition of alkynes. Excellent yields of the cycloadducts are obtained for up to six consecutive cycles with the grafted material, using simple filtration to recover the catalyst. Both conventional and microwave heating prove effective for the process described.