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)2] 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.