Organolanthanide‐Mediated Tandem Insertion/Cyclisation of Alkynylbenzonitriles with Secondary Amines Elucidated Through a Computational Approach.

A detailed mechanistic probe of the organolanthanide‐mediated tandem insertion/annulation of alkynylbenzonitriles with secondary amines by an archetypical homoleptic lanthanum silylamide starting material is presented. An in‐depth computational scrutiny of alternatively plausible pathways for relevant productive steps and also performance‐degrading pathways identified the pathway likely traversed in productive catalysis. It entails the transformation of the starting material into various of silylamide/amide compounds, of which the lanthanum bis‐silylamide/amide is thermodynamically prevalent, capable of promoting the process. Benzonitrile insertion is irreversible to readily afford the lanthanum amidinate, which can adopt various easily interconvertible ligation pattern. The rather rapid protonolysis of the La─N imine linkage would lead to undesirable aminoamidines, but its net endergonicity renders this performance‐degrading avenue nonviable. Instead, the lanthanum amidinate is converted back into catalytically competent lanthanum bis‐silylamide/amide with the release of the observed aminoisoindole product. This transformation favors a stepwise insertative cyclisation/La─C alkenyl protonolysis sequence over an otherwise kinetically noncompetitive proton‐triggered stepwise N─C/C─H bond forming process. The operative insertative pathway comprises turnover‐limiting and irreversible insertion of the alkyne C≡C tether into the La─N amidinate linkage followed by La─C alkenyl aminolysis at the intervening lanthanum alkenylisoindinyl intermediate. The DFT‐assessed barrier for turnover‐limiting insertative N─C ring closure favorably compares with reported performance data. [ABSTRACT FROM AUTHOR]