DFT Studies on the Reaction Mechanisms of 1,4-Dilithio 1,3-Dienes with Nitriles

Mechanisms for the reactions of 1,4-dilithio-1,3-butadienes and nitriles are explored through both experiments and DFT calculations. The computational results suggest that the selectivity of these reaction systems is strongly affected by the structures of the substrates. As the first step of all reaction pathways, the addition intermediate of one C–Li bond to the nitrile is formed. When tetraalkyl-substituted 1,4-dilithio 1,3-dienes and 2-cyanopyridine are used, the intermediate gives the cyclopentadienyl amine product as the kinetic product because of the coordination of the pyridyl N atom to the lithium atoms (system B). This addition intermediate also undergoes a second nitrile insertion into the C–Li bond, giving the dilithio ketimine intermediate. When tetraalkyl-substituted 1,4-dilithio 1,3-dienes and aryl nitriles are used, the dilithio ketimine intermediate undergoes a 1,6-cyclization, generating pyridine and triazine products through thermodynamically favored pathways (systems A and B). When cyclic 1,4-dilithiobutadiene and tertiary aliphatic nitriles are used, the dilithio ketimine intermediate undergoes two sequential 1,5-cyclization steps with a lower energy barrier, generating tricyclic Δ 1 -bipyrrolines (system C). Experimentally, the lithium-containing triazine intermediate 10 and Δ 1 -bipyrroline intermediate 19 have been isolated and their structures investigated through NMR and quenching experiments. The calculation results clearly show the mechanism details of these reactions and are in good agreement with the experimental observations.