Annuloselectivity of Dibenzoxazepines and Arynes in the Presence of CO2: [2+2] vs [2+2+2] Annulation

The annulation of arynes with dibenzo[b,f][1,4]oxazepines (N-aryl cyclic imines) in the presence of CO2 has been studied in comparison with 3,4-dihydroisoquinolines (N-alkyl cyclic imines). For the reaction with dibenzo[b,f][1,4]oxazepines, [2+2] annulation takes place specifically with arynes, with CO2 acting only as the protecting gas. 3,4-dihydroisoquinolines, however, exclusively underwent [2+2+2] annulation with arynes and CO2, with CO2 as a reactant. Our findings offer insights into imine-aryne annulation under CO2 and suggest that steric and nucleophilic factors control the annuloselectivity. Reactivity analysis revealed that electron-rich and less steric N-alkyl cyclic imines and electron-deficient benzynes favor the [2+2+2] annulation, while electron-deficient and bulky N-aryl cyclic imines prefer the [2+2] annulation. In the [2+2] annulation, the Hammett analyses reveal that both the nucleophilic attack of imines to benzynes (the first step, negative rho) and the attack of phenyl anions on the iminium species (the second step, positive rho) affect the reaction rate.