Calcium Complexes with the Bulky Tris((2-N-piperidinomethyl)phenyl)methoxide Ligand. Synthesis and Catalytic Activity in the Dearomatization of N-Heterocycles

Abstract

The reaction of equimolar amounts of tris((2-N-piperidinomethyl)phenyl)methanol (2‑C₅H₁₀NCH₂-o-C₆H₄)₃COH and [(Me₃Si)₂N]₂Ca(THF)₂ (toluene, 23^○C) resulted in the synthesis of mixed-ligand calcium alkoxide-amide complex [(2-C₅H₁₀NCH₂C₆H₄)₃CO]CaN(SiMe₃)₂ (I) in 70% yield. According to X-ray diffraction data, the monoanionic alkoxide ligand in I is coordinated to the metal in the κ³-O,N,N-mode. Complex I catalyzes the addition of PhSiH₃ to annulated N-containing heterocycles: quinoline, 4-methylquinoline, 6-methylquinoline, and isoquinoline. The reactions are chemo- and regioselective and proceed over periods of 24 to 120 h. The reaction of I with PhSiH₃ (molar ratio of 1 : 2) gives the bis(alkoxide) complex [κ²-O,N-(2-C₅H₁₀NCH₂C₆H₄)₃CO]₂Ca (II) and CaH₂ as a result of symmetrization of the intermediate calcium alkoxy hydride. In the crystalline state, complex II has a non-linear structure, with the OCa(1)O angle being 112.65(4)° and only one of the piperidine moieties in each ligand being involved in the coordination to the metal ion. The molecular structure of complexes I and II was determined using X-ray diffraction (CCDC nos. 2443979 (I), 2443980 (II)).