In situ generated polypyrazolylborate–copper complexes as cyclopropanation catalysts

Polypyrazolylborate–copper complexes with electron withdrawing or donating groups on the pyrazoles were examined as catalysts for the cyclopropanation of alkenes with ethyl diazoacetate. The electron-deficient complexes were much more effective catalysts and afforded rapid consumption of the ethyl diazoacetate and generally favored cyclopropanation over the completing dimerization of the ethyl diazoacetate. Given the ease with which the starting materials can be prepared and the simplicity of in situ catalyst generation, these scorpionates afford a convenient new family of cyclopropanation catalysts.