[9-{2-(Dimethylamino)ethyl}fluorenyl] as a Ligand on Magnesium Leads to a THF Ring-Opening by the Fluorenyl Moiety

Abstract

[9-{2-(dimethylamino)ethyl}fluorenyl] (L²)⁻ is explored as a simple yet effective ancillary ligand for heteroleptic Mg chemistry. The proligand L²H reacts with [Mg(ⁿBu)₂(thf)_x] to give [(L²)Mg(ⁿBu)(thf)] (1), whose THF can be replaced by DMAP (4-dimethylaminopyridine) to give [(L²)Mg(ⁿBu)(dmap)] (2). 1 catalyzes the hydroamination/cyclization of a few benchmark primary aminoalkenes to prove the effectiveness of (L²)⁻. But a heteroleptic [(L²)MgH], another signature test for a ligand on Mg, is inaccessible either by β-H elimination from [(L²)Mg(NHⁱPr·BH₃)(L)] (L = THF (3); DMAP (4)) or by ligand exchange between [(L²)K] (6) or [(L²)₂Ca] (7) and MgBr₂. More interestingly, reacting 1 with PhSiH₃ gives a THF ring-opened complex [(L²)Mg{(μ-O-(CH₂)₄-^FLL²}]₂ (5₂), where the fluorenyl of a (L²)⁻ acts as the nucleophile. The homoleptic [(L²)₂Mg], though not isolated, is likely an intermediate toward the ring-opening, and a plausible mechanism is postulated by the help of density functional theory calculations. In addition, 7 readily gives the solvent-separated ion pair [Ca(thf)₆][L²]₂ (8) in THF to suggest the limitation of (L²)⁻ with the heavier alkaline earths. In this regard, (L²)⁻ is closely compared to an analogous [(Flu)-(CH₂)₂-NHC^Dipp]⁻((L¹)⁻; NHC = N-heterocyclic carbene, Dipp = 2,6-ⁱPr₂–C₆H₃) that has a bulky NHC in the place of NMe₂ in the chelating side arm.