Control of the steric and electronic environments about a metal center are keys to develop efficient and selective catalyst systems for use in enantioselective homogeneous catalysis. Since the first boom of reports in the late 1960s, the design of new ligand architectures has been highly successful in modulating the reactivity and selectivity of transition metal catalysts in homogeneous catalysis as most notably recognized by the pioneering 2001 Nobel Prize award work of Knowles, Noyori, and Sharpless. To this day, new ligand architectures are necessary to increase the overall efficiency and selectivity of developed catalysis reactions as well as for the development of new catalytic reactions. Of the ligand systems developed, the mer-coordinating pincer-type ligand systems have become increasingly impactful in recent years, including those developed by Bröring, Gade, Nakada, and Nishiyama, with Nishiyama's ‘pybox’ ligand being a standard in the chiral “tool kit” of stereodirecting ligands. Our group developed the class of chiral mer-coordinating ligands, substituted 1,3-bis(4,5-dihydrooxazol-2-ylimino)isoindolines (L3H), for potential use in enantioselective catalysis. Our original report detailed the in-situ formation of the ligands in neutral Zn(L3)2 complexes, having two equivalents of the ligand coordinated in mer-fashion in its deprotonated anionic form in low-to-modest yields. Since that report, optimization of the synthesis and isolation of the ligands has occurred wherein the ligands are now prepared in a two-step process (i) in-situ as neutral Cd(L3)2 complexes, analogous to the Zn complexes, in high yield via microwave synthesis, and (ii) in free form, via a thiol-based ligand substitution reaction. These synthetic optimization details are reported herein. 1:1 Metal:ligand complexes (e.g., Pd(L3)(OAc)) are also reported as "proof-of-concept" of the synthesis of potential catalyst systems. Finally, preliminary enantioselective homogeneous catalysis studies are reported using a few of the developed 1:1 metal:ligand complexes.

Studies of a chiral, mer-coordinating, ligand as a potential chiral transfer agent in metal-mediated enantioselective catalysis

ACS Fall 2020 Virtual Meeting

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