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

Abstract

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, iBu 2 AlH) versus ditert-butylaluminum hydride (tBu 2 AlH)) 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.

Al/P‐ and Ga/P‐Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C−C and C−N Bond Formation

“…Electronically unsaturated compounds with conjugated π‐bonds seem to be appropriate for such transformations. In a recent publication, we reported on the reaction of H−Al i Bu 2 with an aryl‐substituted ynimine, which had conjugated C≡C and C=N bonds . Heating to 55 °C for two days and hydrolytic workup afforded 6‐methyl‐4‐phenylquinoline probably by hydroalumination, C−H bond activation, and electrophilic aromatic substitution at the N‐bound aryl group.…”

“…IR spectra were recorded as KBr pellets on a Shimadzu Prestige 21 spectrometer, EI mass spectra on a Varian mass spectrometer (only the most intense peak of the correct isotopic pattern). The FLPs 1 , 2 , and 4 , diphenylazirine, 1‐isobutyl‐ and 1‐ p ‐tolyl‐4‐(1‐cyclohexenyl)‐1‐aza‐but‐1‐en‐3‐yne, diphenyl‐thiocyclopropenone, and N,2,3‐triphenyl‐cyclopropenimine were obtained according to literature procedures. Diphenylcyclopropenone was used as purchased.…”

“…In ar ecent publication, we reported on the reaction of HÀAliBu 2 with an aryl-substituted ynimine, which had conjugated CCa nd C=Nb onds. [17] Heating to 55 8Cf or two days and hydrolytic workup afforded 6-methyl-4-phenylquinoline probablyb yh ydroalumination,C ÀHb ond activation,a nd electrophilica romatic substitution at the N-bound aryl group. We modifiedt he starting ynimine by am ore extended delocalized electronic system and appliede asily available 4-(1-cyclohexenyl)-1-aza-but-1-en-3-ynes that had an isobutyl or a p-tolyl group attached to N( Scheme 3).…”