Aryl to imidoyl palladium migration

Scheme 10 Aryl to imidoyl palladium migration in the synthesis of 9-fluorene-9-ones...

Scheme 13 Dual mechanisms for the aryl to imidoyl palladium migration in the synthesis of fluoren-9-ones [top dual C-H activation bottom single C-H activation]...

To date, palladium migration to vinylic positions remains unknown. Thus, palladium migration to sp2 carbons has been largely limited to aryl carbons. However, in addition to vinylic and aryl carbons, aldehydic and imidoyl carbons also possess sp2 hybridization and carry a C-H bond. This special type of sp2 C-H bond has also been subjected to palladium migration studies and preliminary success has been achieved. Currently, aryl to imidoyl [27] and aryl/alkyl to acyl [72] palladium migrations are known. Migrations from other types of carbons to imidoyl and acyl positions are... 

Other palladium-catalyzed 1,5-C-H activation processes reported are extensions of known work. In these examples, palladium activates the C-H bond, but whether the migration occurs or not is still debatable. An aryl to imidoyl C-H activation takes place in substrate 28 (7) under the same reaction conditions that promote the 1,4-palladium migration. However, this reaction affords a much lower yield (compare with Scheme 10), which implies a relatively low efficiency for this 1,5-C-H activation. Mechanistically, the reaction can either go through a direct C-H activation to form a six-membered palladacycle, followed by reductive elimination, or a proton channeling-based palladium migration, followed by an arylation with the original aromatic ring. The exact path has not been established experimentally or computationally. 

The mechanisms of these two cyclization reactions appear fairly different. While the xanthone synthesis would appear to follow a simple palladium migration/arylation mechanism, the fluoren-9-one synthesis has been subjected to isotope labeling experiments (Scheme 12), which indicate that more than one mechanistic pathway may apply. It turns out that the deuterium that replaces the iodide after palladium migration comes not only from the imidoyl position, but also from the pendent aromatic ring used to trap the migrated palladium via arylation. Such results can... 

The C-H transformation of imines via palladacycles has recently provided a novel synthesis of fluoren-9-ones [4], This high yielding synthesis is achieved by migration of palladium from an aryl position to an imidoyl position via C-H activation of the imine moiety, followed by intramolecular arylation (Scheme 2). 

---