Rapid access to pyrazolo[3,4-c]pyridines via alkyne annulation: limitations of steric control in nickel-catalyzed alkyne insertions.

Polyfunctionalized pyrazolo[3,4-c]pyridines were readily prepared by the annulation of alkynes with tert-butyl 4-iodopyrazolocarboximines. The reaction was found to be catalyzed by both NiBr2(PPh3)2/Zn or PdCl2(PhCN)2 to yield complex heterocycles in good to moderate yields. Annulation using nickel catalysis was found to be regio-random, implying that steric control in nickel-catalyzed alkyne insertion has limitations based on the character of the Ni-C bond in the pre-insertion complex.