The Synthesis of a New Class of Chiral Pincer Ligands and Their Applications in Enantioselective Catalytic Fluorinations and the Nozaki–Hiyama–Kishi Reaction

Abstract

A new class of chiral tridentate N-donor pincer ligands, bis(oxazolinylmethylidene)isoindolines (boxmi), was synthesized in three steps starting from readily available phthalimides. Their reaction with ethyl (triphenylphosphoranylidene)acetate by means of a key-step Wittig reaction gave the ligand backbones, which were condensed with amino alcohols and then cyclized to obtain the corresponding ligands. These ligands were subsequently applied in the nickel(II)-catalyzed enantioselective fluorination of oxindoles and β-ketoesters to obtain the corresponding products with enantioselectivities of up to >99% ee and high yields. Application of the chiral pincer ligands in the chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi reaction of aldehydes gave the corresponding alcohols with an optimal enantioselectivity of 93%.

Structural Characterization of a Hydroperoxo Nickel Complex and Its Autoxidation: Mechanism of Interconversion between Peroxo, Superoxo, and Hydroperoxo Species

“…Riordan et al observed an interesting reactivity pattern in the reaction of oxygen with the nickel(I) tmc complex. In this system, the nickel(I) fragment reacts with a molecule of oxygen most likely via a superoxo intermediate to give the only peroxo-bridged nickel complex characterized to date.[12a]In the presence of O 2 , the peroxo complex is slowly converted into the mononuclear end-on superoxo species.[14]Recently, we developed a family of chiral pincer ligands, the iso-pmbox [15] and boxmi ligands, [16] which have displayed considerable potential in asymmetric catalysis [17] and the stabilization of reactive T-shaped nickel(I) complexes, in particular.[18] These iso-pmbox nickel(I) complexes 1 were found to react readily with molecular oxygen at low temperatures to form dinuclear 1,2-m-peroxo-bridged nickel(II) complexes 2 (Scheme 1), similar to Riordans previously described nickel tmc system. [14] For the latter system, the superoxo complex is the final thermodynamic product of the reaction of the Ni I complex with an excess of O 2 , whereas in our case, 1,2-m-peroxo complex 2 was found to be the major component in a dynamic, oxygen-pressure-dependent equilibrium with what appears to be an end-on coordinated superoxo species 3, as reported for palladium.…”

“…In a first experiment, 16 O 2 peroxo complex 2 b was stirred under an atmosphere of 18 O 2 , which led to a complete exchange of the dioxygen moiety at À15 8C (Figure 2 b). Repetition of this experiment with a mixture of 16 (Figure 2 c).…”

“…In a first experiment, 16 O 2 peroxo complex 2 b was stirred under an atmosphere of 18 O 2 , which led to a complete exchange of the dioxygen moiety at À15 8C (Figure 2 b). Repetition of this experiment with a mixture of 16 (Figure 2 c). [23] Furthermore, cross-over experiments were carried out at À40 8C by mixing two solutions that contained dinuclear 1,2-m-peroxo complexes with differently substituted pincer ligands (Ph and iPr; Scheme 2).…”

New Chemistry with Anionic NNN Pincer Ligands

“…The three-step synthesis starts with the Wittig reaction of commercially available phthalimides with ethyl (triphenylphosphoranyliden)acetate [39]. The resulting intermediate could then be reacted with chiral amino alcohols and subsequently cyclized yielding the chiral C 2 -symmetrical ligands (Scheme 6).…”

“…2) [39,46]. The BQA ligand is also known to bind to metal centers in a planar manner yielding complexes which are stable even in the presence of water or acid [47][48][49][50][51].…”