Cycloaddition Reactions of 1-Lithio-1,3-dienes with Aromatic Nitriles Affording Multiply Substituted Pyridines, Pyrroles, and Linear Butadienylimines

Fully or partially substituted 1-iodo- or 1-bromo-1,3-dienes could be readily lithiated using t-BuLi or n-BuLi to afford their corresponding 1-lithio-1,3-diene derivatives in quantitative yields. When these in situ generated lithium reagents were treated with organonitriles, depending on the substitution patterns of the butadienyl skeletons, substituted pyridines, pyrroles, and/or linear butadienyl imines were formed in good to excellent yields via N-lithioketimine intermediates. In the cases of 1,2,3,4-tetrasubstituted and 2,3-disubstituted 1-lithio-1,3-dienes, pyridine derivatives or linear butadienyl imines were generally formed depending on the reaction temperatures. When 1,2,3,4-tetrasubstituted 4-halo-1-lithio-1,3-dienes and 1,2-disubstituted 1-lithio-1,3-dienes were treated with organonitriles, pyrrole derivatives or linear butadienyl imines were obtained. Competition between 5-exo and 6-endo cyclization was found to be responsible for the formation of either pyrroles or pyridines. Selective elimination of RLi from the lithiated cyclic N-containing intermediates was observed. The order of elimination was found to be LiCl > Me₃SiLi > LiH.