Solvent/Base Effects in the Selective Domino Synthesis of Phenanthridinones That Involves High‐Valent Palladium Species: Experimental and Theoretical Studies

The domino reaction of o-bromobenzamides 1 a–m in the presence of K2CO3 and the [PdCl2(PPh3)2] catalyst granted a selective access to phenanthridinones 2 or to the new 1-carboxamide phenanthridinones 3 depending on the solvent, DMF or 1,4-dioxane, respectively. Investigations of the reaction parameters provided the first example of a direct correlation between the base dissociation and the solvent polarity on the selectivity observed. Moreover, mechanistic studies (NMR spectroscopy and ESI-MS monitoring) allowed us to characterize PdII palladacycle 4 and biaryl species as common intermediates for these two domino processes. On that basis, C(sp2)C(sp2) bond formation is envisaged by generation of a PdIV complex after oxidative addition of 1 into PdII palladacycle 4, a rationale that is supported by DFT calculations. A general catalytic cycle is proposed to account for these observations.