From homogeneous to heterogeneous catalysis:: Novel anchoring of polypyrazolylborate copper(I) complexes on silica gel through classical and nonclassical hydrogen bonds.: Use as catalysts of the olefin cyclopropanation reaction

The fixation of the preformed polypyrazolylborate copper(I) complexes BpCu (1), Tp*Cu (2), and [B(pz)(4)]Cu (3) (Bp = dihydrobispyrazolylborate, H2B(pz)(2); Tp* = hydrotris(3,5-dimethyl)pyrazolylborate, HB(pz*)(3); B(pz)(4) = tetrakispyrazolylborate) on silica gel and their use, under heterogeneous conditions, as catalysts for the olefin cyclopropanation reaction are reported. The catalytic activity is very similar to that observed under homogeneous conditions, as a consequence, according to the studies presented herein, of a Ligand-to-support interaction that would involve the hydroxyl groups of the silica surface and the borohydride B-H group and/or the pyrazolyl nitrogen atoms. The strength of such interactions allows the recovery and recycling of the supported catalysts for a number of cycles.