Bis(oxazolinylmethyl) Derivatives of C₄H₄E Heterocycles (E = NH, O, S) as C₂‐Chiral Meridionally Coordinating Ligands for Nickel and Chromium

Abstract

The synthesis of the three ligands employed in this study is based on the condensation of two molar equivalents of (S)-valinol with the diester precursors pyrrole-2,5-bis(ethyl)acetate (2a), furan-2,5-bis(ethyl)acetate (2b) and thiophene-2,5-bis(ethyl)acetate (2c). This gave the corresponding bis(oxazolinylmethyl)pyrrole (iPrL N H, 4a), bis(oxazolinylmethyl)-furan (iPrL O , 4b) and bis(oxazolinylmethyl)thiophene (iPrL S , 4c) ligands. Stirring 4a in MeOD at ambient temperature in the presence of a catalytic amount of acetic acid (1 mol-%) led to complete hydrogen/deuterium exchange in the two bridging methylene groups of the ligand. This behaviour is explained by an acetate-mediated reversible proton transfer between the oxazoline N atom and the methylene bridge, a conjecture which was supported by a DFT study of the process. Deprotonation of iPrL N H (4a) with tBuLi at -78°C and subsequent stirring with NiCl 2 yielded the square planar nickel(II) complex [Ni(iPrL N )Cl] (5). However, on stirring

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

“…The backbones of the pincer ligands (4 a-d) were prepared according to improved Wittig procedures [16] (combined with a subsequent Suzuki coupling [17] for 4 d). Compounds 5 a-j were synthesized in high yields by melting 4 with the corresponding amino alcohols in the presence of a catalytic amount of NaH, [18] and the desired ligands 1 a-j were obtained from 5 using a Ph 3 P/CCl 4 /Et 3 N cyclization protocol [19] (for the detailed screening conditions of the ligand synthesis, see the Supporting Information) Deprotonation of 1 a with two molar equivalents of NaH and subsequent stirring with NiCl 2 yielded the nickel(II) complex 10 a as a black red solid (Scheme 3, reaction a). The alternative methods to obtain nickel complex, direct complexation of 1 a with NiA C H T U N G T R E N N U N G (OAc) 2 in methanol at room temperature, gave the corresponding nickel(II)-acetato complex 10 b (Scheme 3, reaction b).…”

Stereoselective Hydrodehalogenation via a Radical‐Based Mechanism Involving T‐Shaped Chiral Nickel(I) Pincer Complexes

“…The enantioselectivities of the products obtained from the catalytic hydrodehalogenation were measured on a Finnigan Focus GC device using a Chiraldex G‐TA column. The phenyl‐, isopropyl‐, tert ‐butyl‐ and indandiyl‐substituted bis(oxazolinylmethyl)pyrrole protioligands ( Lig ph H , Lig i Pr H , Lig t Bu H , Lig ind H ) and complexes 1 b , 2 b and the corresponding acetato complex ( Lig i Pr (iso)NiOAc ) were synthesized according to literature 5b,c. All other reagents were obtained from commercial sources and used as received unless explicitly stated otherwise.…”

Electronic structure and reactivity of nickel(i) pincer complexes: their aerobic transformation to peroxo species and site selective C–H oxygenation

“…Both ligand systems contain an almost identical delocalized 10-electron-π-system involving three nitrogen donor atoms and unsaturated carbon-linkers which interacts with the central metal ion upon its coordination. 36 …”