Theoretical investigation on the mechanisms of palladium-catalyzed benzannulations of 1-(indol-2-yl)but-3-yn-1-ols to access to functionalized carbazoles

Abstract

The detailed mechanisms of Pd(bpy)Cl2-catalyzed cascade reaction between 1-(indol-2-yl)but-3-yn-1-ols with aldehydes and Pd(OAc)2bpy-catalyzed self-cyclization of 1-(indol-2-yl)but-3-yn-1-ols were investigated by density functional theory (DFT) calculations. The results indicate that a novel “double cycle” mechanism involving d-CSA- and Pd-cycle is applicable for the Pd(bpy)Cl2-catalyzed system. The d-CSA-cycle achieves coupling cyclization between two reactants, while the aromatization leading to the carbazoles product proceeds through subsequent Pd-cycle. For the Pd(OAc)2bpy-catalyzed reaction, the whole mechanistic pathway involves coordination of alkynyl moiety to Pd(OAc)2bpy, intramolecular cyclization, OAc−-assisted deprotonation, d-CSA-assisted dehydration and the final protonation. The absence of the self-cyclized product in Pd(bpy)Cl2-catalyzed reaction is due to high energy barrier of Cl−-assisted deprotonation. The difference between the alkalinity of Cl− and OAc− is mainly responsible for the observed chemoselectivity. Meanwhile, d-CSA plays an important role in the selectivity of the reaction.