NHC/NHSI-Ligated Transition Metal Complexes: Versatile Platforms for Catalysis and Small Molecule Activation

Abstract

NHC/NHSi-ligated Transition Metal Complexes: Versatile Platforms for Catalysis and Small Molecule Activation Sandeep Kaulage and Shabana Khan* The chemistry of N-heterocyclic carbenes (NHCs) and N-heterocyclic silylene (NHSis) has witnessed tremendous development in the past two decades. NHCs/NHSis have not only become versatile ligands for transition metals but have also emerged as powerful organic catalysts in molecular chemistry. While the NHC was found in bottleable form by Arduengo in 1991, West and Denk discovered the first NHSi in 1994. Since then, several research groups from both academia and industry have ardently examined their potential applications in catalysis, synthesis, and stoichiometric transformations, signalling the start of a burgeoning era in low-valent main-group chemistry. NHCs have shown greater potential than their heavier congeners in the fields. The chemistry of transition-metal complexes bearing isolable NHSi ligands has recently developed appealing and new synthetic methods with a wide range of properties that have significantly influenced organic methodologies, particularly small-molecule activations, and a very small number of organic transformation reactions. We have expanded our understanding of NHCs/NHSis adaptability in transition metal chemistry, catalysis, and small molecule activation by introducing the additional facets through this thesis. The first section of the thesis demonstrated a Silylene-Phosphine-based hybrid ligand with Pd(0)/Ni(II) source as a catalyst for sterically hindered C-N cross-coupling reactions. Further, we have shown that tuning the electronic and steric properties of ligands can be utilized for the C-N cross-coupling reaction of enantiopure anilines. It is worth noting that this system demonstrated excellent enantioretention, which is otherwise a challenge in Pd-catalyzed C–N coupling reactions. This work paves the way for the future use of silylene-based ligands in asymmetric catalysis. In the second section, we describe NHC copper complexes for regioselective protoboration and CO2 hydrogenation. The highly σ-donating nature of NHCs which can be tunned with ease, in conjunction with their sterics plays a crucial role in attaining regioselectivity. The low toxicity, cost-effectiveness, and earth abundance of copper make them an ideal choice. Here we control the regioselectivity in the borylation of alkynes. This process is scalable and some intermediates were also isolated to shed light on the mechanistic cycle. The synthesis of NHC copper(I) hydride complexes has attracted considerable interest. The conversion of CO₂ to value-added products like formic acid, methanol, methane (CH₄), etc. represents a promising strategy for mitigating greenhouse gases. Herein, we describe the synthesis and characterization of a dimeric monoamido-amino carbene (MAC) copper(I)-hydride and its utilization in the reduction of CO2. Here we showed a rationally designed synergistic cooperation between copper-hydride complexes and classical amine-borane Lewis pairs for CO2 hydrogenation. In the last section, we describe the protoboration of terminal alkynes by using NHSi-Cu(I) complexes. These methods offer a broad substrate scope, good functional-group compatibility, and gram-scale synthetic ability. Overall, given that main group ligands are unquestionably crucial in contemporary synthetic chemistry, this thesis work attempted to highlight the unique potentials of NHCs & particularly NHSis, towards a few real-time applications. We hope these studies will enhance the scope of homogeneous catalysis and applications of main group elements in the new organic transformations and small molecule activations.