Recent advancement in oxidation or acceptorless dehydrogenation of alcohols to valorised products using manganese based catalysts

Abstract

Manganese based compounds have gained much interest as catalytic materials due to their natural abundance, biocompatibility, and multiple oxidation states. Manganese-based homogeneous and heterogeneous catalysts are key constituents in the fine chemical and pharmaceutical industries. Valorisation of alcohols (aliphatic, aromatic and bio-based alcohols) has revolutionized chemical manufacturing as it generates several key products like carbonyl and heterocyclic compounds. Valorisation of alcohols by manganese complexes is generally performed by using stoichiometric oxidants such as hydrogen peroxide, tert-butyl hydroperoxide (TBHP), air or molecular oxygen. The homogenous manganese-based metal catalysts have been applied for aerobic or peroxide mediated alcohol oxidation. However, these aforesaid homogeneous catalytic systems are non-recyclable and need additives such as radical initiator or organic acids, or corrosive peroxide oxidants which generate copious amounts of waste. Therefore, the focus has been shifted towards scrutinizing more atom economic, benign and green processes for alcohol oxidation reactions. Heterogeneous manganese oxides (MnOx) or manganese oxide-based octahedral molecular sieves have been used as catalyts for the alcohol oxidation under aerobic conditions. The advantages associated with these manganese-based heterogeneous catalysts are due its recyclability, additives free process and employing aerobic oxidants (such as air or molecular oxygen) instead of peroxide-based oxidant. Recently, acceptor-less dehydrogenation (AD) has become a vital alternative technique in the toolbox of the synthetic chemist for alcohol valorisation without generating a huge amount of waste. Several manganese-based pincer, salen and pyrazolyl – imidazolyl complexes have been used for the AD reactions of alcohols.