An Extremely Facile Aza-Bergman Rearrangement of Sterically Unencumbered Acyclic 3-Aza-3-ene-1,5-diynes

The factors that affect the kinetics of the aza-Bergman cyclization of aza-enediynes (C,N-dialkynyl
imines) have not previously been elucidated. Here we report our kinetic studies of the aza-Bergman
reactions of a series of 6-triisopropylsilyl and 6-unsubstituted 1-phenyl-4-aryl-3-aza-3-ene-1,4-diynes
in which the aryl group is phenyl, o-(methoxy)phenyl, or p-(methoxy)phenyl. These aza-enediynes
are prepared as single isomers in modest yield from the corresponding 1-aryl-3-(triisopropylsilyl)propynone oximes. These aza-enediynes undergo aza-Bergman reaction followed by a rapid retro-aza-Bergman cyclization to afford β-alkynyl acrylonitrile products. In no case are products
corresponding to trapping the intermediate 2,5-didehydropyridine diradical isolated. While the rate
of aza-Bergman cyclization is not greatly affected by the nature of the 4-aryl substituent, the rate
is very dependent on the nature of the 6-substituent. 1-Phenyl-4-aryl-3-aza-3-ene-1,5-diynes that
lack a 6-substituent undergo aza-Bergman cyclization spontaneously at 20 °C with first-order half-lives of 36−78 min. The effect of solvent on the kinetics of aza-Bergman cyclization of 1,4-diphenyl-3-aza-3-ene-1,5-diyne was investigated. The rate of this cyclization is solvent dependent, proceeding
more rapidly in less polar solvents.