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(p z)(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 repor ted. The catalytic activity is very similar to that observed under homogene ous 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 n itrogen atoms. The strength of such interactions allows the recovery and re cycling of the supported catalysts for a number of cycles.