Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium

Metalation of the formamidine Dipp–NC(H)–N(H)–C₂H₄–Py (1a) and benzamidine Dipp–NC(Ph)–N(H)–C₂H₄–Py (1b) with [(thf)₂M{N(SiMe₃)₂}₂] (M = Ca, Sr) yields the corresponding homoleptic complexes [M{Dipp–NC(R)–N–C₂H₄–Py}₂] (M/R = Ca/H (2a), Ca/Ph (2b), and Sr/Ph (3b)) regardless of the applied stoichiometry. Only during calciation of Dipp–NC(H)–N(H)–C₂H₄–Py (1a), the heteroleptic intermediate [{(Me₃Si)₂N}Ca{Dipp–NC(R)–N–C₂H₄–Py}]₂ (2a′) has been observed. The formamidinate complex of strontium crystallizes as a tmeda adduct of the type [(tmeda)Sr{Dipp–NC(H)–N–C₂H₄–Py}₂] (3a). Metalation of the pivalamidine Dipp–NC(tBu)–N(H)–C₂H₄–Py (1c) leads to the formation of heteroleptic mononuclear [{(Me₃Si)₂N}M{Dipp–NC(tBu)–N(H)–C₂H₄–Py}] (M = Ca (2c) and Sr (3c)) with a side-on bonding of the Dipp group to the alkaline earth metals. Calciation of chiral Dipp–NC(tBu)–N(H)–CH₂CH(Me)–Py (R)-1d and (S)-1d with [(thf)₂Ca{N(SiMe₃)₂}₂] yields the homoleptic complexes [Ca{Dipp–NC(tBu)–N–CH₂CH(Me)–Py}₂] with the enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. The complexes 2c and 2d catalyze the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-methyl-4,4-diphenylpyrrolidine but the stereochemistry cannot be influenced by the chiral compounds (R,R)-2d and (S,S)-2d.