Lithium diisopropylamide-mediated ortholithiation and anionic Fries rearrangement of aryl carbamates: Role of aggregates and mixed aggregates

Structural and mechanistic studies of the lithium diisopropylamide (LDA)-mediated anionic Fries rearrangements of aryl carbamates are described. Substituents at the meta position of the arene (H, OMe, F) and the dialkylamino moiety of the carbamate (Me2N, Et2N, and i-Pr2N) markedly influence the relative rates of ortholithiation and subsequent Fries rearrangement. Structural studies using Li-6 and N-15 NMR spectroscopies on samples derived from [Li-6, N-15] LDA reveal an LDA dimer, LDA dimer-arene complexes, an aryllithium monomer, LDA-aryllithium mixed dimers, an LDA-lithium phenolate mixed dimer, and homoaggregated lithium phenolates. The highly insoluble phenolate was characterized as a dimer by X-ray crystallography. Rate studies show monomer- and dimer-based ortholithiations as well as monomer- and mixed dimer-based Fries rearrangements. Density functional theory computational studies probe experimentally elusive structural and mechanistic details.