Development of Manganese Catalysts and Their Application for Transfer (De)Hydrogenation Reactions

Jana, Akash (2021) Development of Manganese Catalysts and Their Application for Transfer (De)Hydrogenation Reactions. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Akash Jana (16RS031)) 16RS031_DCS.pdf - Submitted Version Restricted to Repository staff only Download (13MB)

Abstract

Designing sustainable synthetic protocols is a coercive component of coincidental organic chemistry as such environmentally benign methodologies favor conclusive applications in pharmaceuticals, agrochemicals, medicinal sciences, and functional organic materials. In this context, transition metal-catalyzed cross dehydrogenative coupling reactions (CDC) to construct C–C or C–heteroatom is highly alluring. Borrowing hydrogenation (BH) or hydrogen auto-transfer and acceptorless dehydrogenative coupling (ADC) strategies are the most powerful declensions of CDC. The BH precept is a potent approach that combines transfer hydrogenation (using alkanes, amines, or alcohols as hydrogen sources) with one or more intermediate reactions to synthesize more intricate molecules without the use of a direct hydrogen source or tedious separation processes. Similarly, in the ADC approach, the stored hydrogen by a catalytic metal fragment from a donor molecule is released to the environment in the form of molecular hydrogen rather than transferring to the intermediate. Over the decades, such types of CDC reactions were the traditional realm of precious transition metal catalysis. Therefore, the development of base metal (3d) catalysts for such hydrogenation reactions are desirable in terms of sustainable chemistry as they are less toxic and have higher contamination levels in pharmaceuticals than their heavier homologous. Furthermore, 3d metals are highly abundant in the earth's crust than 4d and 5d transition metals. In this regard, manganese is the 3rd most abundant transition metal (after iron and titanium), less toxic, less expensive, and found in various metalloproteins, and has recently been surfaced as a promising candidate for the ADC reactions. The thesis's essence is the development of bench stable manganese catalysts and their application in transfer hydrogenation and dehydrogenation reactions. With this objective, we have successfully developed a library of phosphine-free manganese complexes and showcase their potential activity towards various coupling reactions, which can be carried out in a sustainable and environmentally friendly manner. Manganese complexes bearing hydrazone-based ligand have been developed and well-characterized (Scheme 1). The mechanistic hypothesis for designing these catalysts are: 1) These hydrazone ligands could behave like tridentate or bidentate ligand systems and could form robust manganese complexes like A. 2) The presence of N-H proton, which undergoes deprotonation under the basic condition to form active catalytic species like B assisting in catalysis through the de-aromatization and re-aromatization of pyridine arm. 3) Alcohol could be converted to the corresponding aldehyde after reacting with the dearomatized form of the catalysts via an outer-sphere mechanism. The manganese (I) complexes, containing NNS-amine-based ligands were also subsequently developed and characterized. The significant features prompting the development of these complexes are: 1) The presence of weak coordinating sulfur or oxygen atoms which could act as a hemilabile ligand and result in coordination/ decordination to the metal center depending on the demand in a bond activation process. 2) Involvement of N-H proton can occur through the deprotonation under basic conditions to form the active catalytic amido species like B’ for the dehydrogenation of alcohol to the corresponding aldehyde. Thus, these types of amine-based ligands could show multifunctional behaviour during the bond activation process.

Actions (login required)

View Item