Dearomatized BIAN alkaline-earth alkyl catalysts for the intramolecular hydroamination of hindered aminoalkenes

Reaction of a sterically encumbered bis(imino)acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae{CH(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatization of the aromatic ligand. The heteroleptic alkaline-earth alkyl species show enhanced stability toward Schlenk-type redistribution but undergo solution exchange when the bis(trimethylsilyl)methyl substituent is replaced by an anionic ligand of lower overall steric demands. In contrast, analogous reactions performed with [Ba{CH(SiMe₃)_{2} 2}(THF)] evidenced facile solution redistribution and resulted in an unusual C-C coupling reaction which is suggested to result from a sterically induced reductive process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species. Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at either of the alkenyl positions.