BpCu-catalyzed cyclopropanation of olefins: A simple system that operates under homogeneous and heterogeneous conditions (Bp=dihydridobis(pyrazolyl)borate)

The complexes BpCu (1), BpCuL (L = bipy, 2; L = Ph2PCH2CH2PPh2, 3; L = PCy3, 4) and BpCuL(2) (L = py, 5; L = PPh3, 6) catalyze the cyclopropanation of olefins in moderate to high yields in the homogeneous phase. These compounds can also be used under heterogeneous conditions, when supported on silica gel. Complex 1 converts an equimolar mixture of an olefin (styrene, cis-cyclooctene,1-hexene) and ethyl diazoacetate in the corresponding cyclopropanes in high yield with no excess of olefin employed. For the heterogeneous system, the catalysts can be recovered and reused several times (6-12 cycles) up to ca. 1000 turnovers. The data available suggest the existence of a common active species: the 14-electron BpCu fragment 1. These data consist of competition experiments with para-substituted styrenes as well as kinetic studies carried out in the presence and in the absence of the alkene.