Examination of Dianionic Diamide Ligand Structural Effects on Ti-Catalyzed Hydrohydrazination of Terminal Alkynes

Ti-catalyzed alkyne hydrohydrazination of terminal alkynes
using
1,1-disubstituted hydrazines is reported. Here, a variety of bi- or
tridentate dianionic diamide or diamidoamine ligated TiNNPh2 catalysts were examined, wherein fast and selective TiNNPh2 catalysts have ligands with a “goldilocks”
intermediate level of the steric environment. TiNNPh2 catalysts with ligands of high/medium steric bulk showed slower
reactivity compared to that of the optimal catalysts. Catalysis with
Ti complexes with sterically less encumbered ligands is extremely
slow (requiring 5–14 days to completion). Development of a
fast and selective diamidoamine catalyst with flanking N-SiMe2Ph groups led to an expansion of the hydrazine scope and a
structurally diverse set of hydrazones.