Chemoselective Hydroalumination of 1‑Aza-but-1-en-3-ynes (<i>C</i>‑Iminoalkynes): Formation of Propargylamines by Imine Reduction and of 5‑Aluminazoles and 1‑Aza-butadienes by Anti-Michael Attack

Hydroalumination of 1-aza-but-1-en-ynes 1 provides facile access to propargylamines 4 by reduction of the CN bonds or alternatively to 1-aza-buta-1,3-dienes 6 by reduction of the triple bond. The chemoselectivity depends not only on the steric properties of both the hydroalumination agent (di-iso-butylaluminum (DIBAL-H, iBu2AlH) versus di-tert-butylaluminum hydride (tBu2AlH)) and the substrates but also on the reaction temperatures. In several cases, initial aluminum species of 5-aluminazole type 5 could be isolated and characterized by X-ray diffraction, indicating an “anti-Michael” addition of the hydride to the triple bond. Aqueous workup of those species led to 1-azabutadiene derivatives 6. High-level DFT calculations indicate that the observed chemoselectivity is only compatible with a dimeric nature of the hydroaluminating agent. Using such a dimer, the imine reduction corresponds to the kinetically controlled pathway, whereas the triple bond reduction leads to the thermodynamically much more stable 5-aluminazoles, 5.