Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C?N Bond Cleavage Reactions Induced by C?H Bond Activations

Article abstract of DOI:10.1002/asia.201601497

Five bis(quinolylmethyl)-(1H-indolylmethyl)amine (BQIA) compounds, that is, {(quinol-8-yl-CH₂)₂NCH₂(3-Br-1H-indol-2-yl)} (L¹H) and {[(8-R³-quinol-2-yl)CH₂]₂NCH(R²)[3-R¹-1H-indol-2-yl]} (L^2–5H) (L²H: R¹=Br, R²=H, R³=H; L³H: R¹=Br, R²=H, R³=iPr; L⁴H: R¹=H, R²=CH₃, R³=iPr; L⁵H: R¹=H, R²=nBu, R³=iPr) were synthesized and used to prepare calcium complexes. The reactions of L^1–5H with silylamido calcium precursors (Ca[N(SiMe₂R)₂]₂(THF)₂, R=Me or H) at room temperature gave heteroleptic products (L^{1, 2})CaN(SiMe₃)₂ (1, 2), (L^{3, 4})CaN(SiHMe₂)₂ (3 a, 4 a) and homoleptic complexes (L^{3, 5})₂Ca (D3, D5). NMR and X-ray analyses proved that these calcium complexes were stabilized through Ca???C?Si, Ca???H?Si or Ca???H?C agostic interactions. Unexpectedly, calcium complexes ((L^3–5)CaN(SiMe₃)₂) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C?N bond cleavage processes as a consequence of intramolecular C?H bond activation, leading to the exclusive formation of (E)-1,2-bis(8-isopropylquinol-2-yl)ethane.