Alkylations of Chiral Nickel(II) Complexes of Glycine under Phase-Transfer Conditions

Alkylation reactions of nickel(II) complex 6 derived from glycine and 2-[(1-benzyl-L-prolyl)amino]benzophenone (BPBP) were studied under phase-transfer-catalysis (PTC) conditions. The goal of this work was to find an alternative suitable solvent for these reactions to replace the commonly used CH2Cl2 which leads to the formation of several by-products, thus lowering the yield of target compounds. We demonstrate that 1,2-dichloroethane is a markedly better solvent providing higher yields (7599%) of the desired products 10 with 3688% diastereoisomer purity (Scheme 3 and Table). Furthermore, we show that the stereochemical outcome of these PTC reactions (kinetic control) can be easily improved to >95% de by treatment of the PTC products with MeONa/MeOH. The scope of these reactions includes alkylations with methyl iodide as well as activated halides such as benzyl, allyl or propargyl, bromides and most notably ethyl 2-bromoacetate (Table).