Direct Observation of Selective Migratory Insertion of Unactivated Terminal Alkenes into Ir-N Bonds

We report fast insertions of alkenes into the metal-nitrogen bonds of cationic bisphosphine-ligated hydridoiridium amides that are potential intermediates in previously reported enantioselective hydroaminations of unactivated alkenes with 2-aminopyridines. The amido complexes were synthesized and structurally characterized, shown to undergo insertion at temperatures as low as -80 °C, and shown to insert alkene into the metal-amide bond more rapidly than into the metal-hydride bond. The hydridoiridium alkyl complexes resulting from this insertion were unambiguously characterized by NMR spectroscopy (¹H, ¹³C, ³¹P, ¹⁵N) and shown to form the amine, with high enantiomeric excess matching that of the catalytic process, by reductive elimination at 25 °C. Reversible migratory insertion of terminal alkenes into the Ir-N bond was observed for amides derived from electron-rich 2-aminopyridines. The arrangement of ligands at iridium in the hydridoiridium(III) amides and precursor iridium(I) amides was elucidated by preparing ¹⁵N-enriched derivatives and was corroborated by matching experimental and computational coupling constants. DFT calculations suggest that chelation by the 2-pyridylamido ligand to iridium facilitates insertion of alkenes into the Ir-N bond and that the coordination geometry precludes insertion into the Ir-H bond. These studies clarify the origins of the favorable insertion of unactivated alkenes into the Ir-N bonds over the Ir-H bonds and show that late-transition-metal-amido complexes, even of third-row d⁶ metal centers, can insert alkenes with exceptionally high rates.