Donor-Functionalized Cyclic(Alkyl)(Amino)Carbene (CAAC) Complexes with Transition Metals: Synthesis and Their Catalytic Applications

Kumar, Ankit (2025) Donor-Functionalized Cyclic(Alkyl)(Amino)Carbene (CAAC) Complexes with Transition Metals: Synthesis and Their Catalytic Applications. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Ankit Kumar (20MS081)) 20MS081_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (1MB)

Abstract

The design of electronically tunable and sustainable ligands is at the core of advancing cutting-edge catalysis. Cyclic (Amino)(Alkyl)Carbenes (CAACs) have been shown to be efficient ligands owing to their high σ-donating and π-accepting capability, which stabilizes metals more efficiently than NHCs. This thesis focuses on the synthesis and application of donor-functionalized precursors of CAACs, which are meant to stabilize transition metal complexes in catalysis, particularly those containing earth-abundant and non-toxic metals. One of the key enhancements presented here is the addition of a phenyl spacer between the donor arm and the pyrrolinium unit, significantly reducing the acidity of the benzylic proton. The structure is open to clean deprotonation by KHMDS and inhibits undesirable cyclopropane formation. The resulting free carbene was successfully trapped as a CAAC–Cu(I) complex, which confirms the effectiveness of this method. The compounds were all well characterized by multinuclear NMR and HRMS. The thesis further outlines the synthesis of a donor-functionalized bis-pyrrolinium salt, which was the precursor to bis-CAAC ligands—a family of ligands that were not reported until 2024. The ligands are particularly good at stabilizing low-valent metal centers, and their bite angles lower the activation barrier for oxidative addition, enhancing catalytic performance. The ligand also possesses a reversible nitrogen donor, offering hemilabile coordination to satisfy the electronic needs of the metal center. Based on Bertrand's imine-functionalized CAACs, an asymmetric donor-functionalized pyrrolinium salt was also assembled as a highly promising scaffold for enantioselective catalysis. Even though it is under development, this avenue promises the potential of the realization of asymmetric transformations with tailor-designed CAAC ligands. Finally, the CAAC–PdCl₂ complex of the donor-functionalized ligand was utilized in Suzuki–Miyaura cross-coupling, in which it exhibited better catalytic activity with a very low 0.1 mol% catalyst loading, being more efficient than most of the existing systems with higher yield.

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