Transition-metal-free direct arylation: synthesis of halogenated 2-amino-2'-hydroxy-1,1'-biaryls and mechanism by DFT calculations.

A transition-metal-free, regioselective direct aryl-aryl bond-forming process for the synthesis of halogenated 2-amino-2'-hydroxy-1,1'-biaryls that are currently either inaccessible or challenging to prepare using conventional methods is disclosed. The addition of ArMgX to an o-halonitrobenzene at low temperature generates a transient N,O-biarylhydroxylamine that rapidly undergoes either [3,3]- or [5,5]-sigmatropic rearrangement in one-pot to form a 2-amino-2'-hydroxy-1,1'-biaryl or 1-amino-1'-hydroxy-4,4'-biaryl, respectively. The periselectivity is controlled by the choice of solvent and temperature. This direct arylation process is also readily scalable (1-10 mmol). DFT calculations suggest that from the N,O-biarylhydroxylamine intermediate there is a low-energy stepwise pathway that involves initial Mg-mediated N-O bond cleavage followed by pathway branching toward either [3,3]- or [5,5]-rearrangement products via C-C bond formation and rearomatization.