Abstract
Over the past few decades, Ru catalyzed transfer hydrogenation (TH) and asymmetric transfer hydrogenation (ATH) reactions of unsaturated hydrocarbons, imine, nitro and carbonyl compounds have emerged as economic and powerful tools in organic synthesis. These reactions are most preferred processes having applications in the synthesis of fine chemicals to pharmaceuticals due to safe handling as these do not require hazardous pressurized H2 gas. The catalytic activity and selectivity of Ru complexes were investigated with a variety of ligands based on pincer NHC, cyclophane, half-sandwich, organophosphine etc. These ligands coordinate to Ru center in a proper orientation with a labile group replaced by H-source (like methanol, isopropanol, formic acid, dioxane, THF), which facilitate the β-hydrogen transfer to generate metal hydride species (Ru-H) and produce desired reduced product. This chapter describes the recent advances in TH and ATH reactions with homogeneous and heterogeneous Ru catalysts having different ligand environments and mechanistic details leading to their sustainable industrial applications.
Highlights
Ruthenium was first discovered in 1844 by Karl Ernst Claus and he had named it as Ruthenia in the honor of his motherland
From the commercial point of view, ruthenium has been used in a variety of applications such as its alloy with other heavy metals used for voltage regulators, jewelry, fountain pen nibs and electromechanical devices etc
The simple operational procedure, the mild reaction conditions with high catalytic activity and selectivity make the transfer hydrogenation (TH) reactions an attractive alternative to direct hydrogenation using H2 gas
Summary
Ruthenium was first discovered in 1844 by Karl Ernst Claus and he had named it as Ruthenia (in Latin Russia) in the honor of his motherland. Like Diels–Alder, eco-friendly CO2 hydrogenation to hydrocarbon, transfer hydrogenation of unsaturated substrates, oxidation of alcohols, atom transfer radical addition (ATRA), metathesis (ring closing metathesis (RCM)/ring-opening polymerization (ROMP)) are catalyzed by inevitably the ruthenium complexes. Alternate strategy for the hydrogenation is the transfer hydrogenation (TH) and asymmetric transfer hydrogenation (ATH) reactions, which require sacrificial hydrogen donor These hydrogen donors include organic hydrogen source or different azeotropic mixtures with hydrogen acceptor substrates and catalysts in presence or absence of base promoters (NaOH, KOH, Et3N, Cs2CO3 etc.) (Figure 1). It was correlated that reactivity and enantioselectivity of complexes were found to depend on optimum steric and electronic properties of the arene and TsDPEN ligands Efficiency of this robust catalyst (R,R-1) can be seen by the reduction of multifunctional ketone, which gave R benzylic alcohol (5) in ca. The mechanistic details were discussed wherever possible with limitations as well as future perspectives in this important area
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