Proline-Mediated Transition Metal-Free Access to 1H-Indazolones from 2-Halobenzohydrazides.

Transition metal-free access to 1H-indazolones 1 has been achieved on the basis of base-mediated intramolecular C-N bond formation. Reactions of 2-halobenzohydrazides 2 with potassium tert-butoxide in the presence of catalytic proline provided indazolones 1 in moderate to good yields. This transformation may proceed primarily via the radical pathway according to the control experiment with a radical scavenger.