Direct Synthesis of Pyrazolo[5,1-a]isoindoles via Intramolecular Palladium-Catalyzed CH Bond Activation

An efficient, direct synthesis of pyrazolo-[5,1-a]isoindoles employing a palladium-catalyzed intramolecular C-H bond activation of 1-(2-halobenzyl)pyrazoles has been developed. The use of lithium chloride (LiCl) was found to be essential in these reactions, to suppress further C-H bond activation at the C-3 position of pyrazolo[5,1-a]isoindole, when C-3 is unsubstituted. This protocol can be applied to the synthesis of a pyrazolo[5,1-a]isoquinoline possessing a six-membered central ring system and a fully substituted pyrazolo[5,1-a]isoindole using sequential intra- and intermolecular C—H bond activation.