Shvo’s Catalyst in Hydrogen Transfer Reactions

Abstract

This chapter reviews the use of Shvo's catalyst in various hydrogen transfer reactions and also discusses the mechanism of the hydrogen transfer. The Shvo catalyst is very mild to use since no activation by base is required in the transfer hydrogenation of ketones or imines or in the transfer dehydrogenation of alcohols and amines. The Shvo catalyst has also been used as an efficient racemization catalyst for alcohols and amines. Many applications of the racemization reaction are found in the combination with enzymatic resolution leading to a dynamic kinetic resolution (DKR). In these dynamic resolutions, the yield based on the starting material can theoretically reach 100%. The mechanism of the hydrogen transfer from the Shvo catalyst to ketones (aldehydes) and imines as well as the dehydrogenation of alcohols and amines has been studied in detail over the past decade. It has been found that for ketones (aldehydes) and alcohols, there is a concerted transfer of the two hydrogens involved, whereas for typical amines and imines, there is a stepwise transfer of the two hydrogens. One important question is whether the substrate is coordinated to the metal or not in the hydrogen transfer step(s). The pathway involving coordination to activate the substrate is called the inner-sphere mechanism, whereas transfer of hydrogen without coordination is called the outer-sphere mechanism. These mechanistic proposals together with experimental and theoretical studies are discussed.

Efficient Ruthenium-Catalyzed N-Methylation of Amines Using Methanol

“…As shown in Scheme 2, the intermediate 26 45 was readily prepared by condensation of 1-naphthylmethylamine with cinnamaldehyde, followed by reduction. The potentially reducible double bond in 26 is well-tolerated, and Naftifine was isolated in 84% yield after the standard N-methylation reaction at 100°C.…”

Catalytic Hydrogenation of Arenes in Water Over In Situ Generated Ruthenium Nanoparticles Immobilized on Carbon

“…These results showed that it is not just the Ru metal but also the overall structure of the components of [( η 6 ‐benzene)Ru(en)Cl]PF 6 that play a crucial role in the observed high catalytic activity towards 1 . Notably, in transfer hydrogenation catalysts, ligands that have N−H bonds play a determining role particularly to help in the activation of the H source . It is evident from previous studies that among the Ru‐based catalysts explored for HCOOH to H 2 conversion or hydrogenation using HCOOH, the most active catalysts are those that have N‐based ligands .…”

“…have reported the selective hydrogenation of the aromatic ring of ketones and phenols using cyclic(amino)alkyl carbene rhodium complexes under H 2 (5 bar) . Notably, ruthenium arene complexes based on N‐donor ligands, such as ethylenediamine‐derived ligands have been identified as transfer hydrogenation catalysts, and therefore, we anticipated that these complexes could be used to achieve arene ring hydrogenation under precisely tuned reaction conditions. The reaction of the precursor, a ruthenium arene dimer, with ethylenediamine (en) in a suitable solvent led to the straightforward synthesis of cationic [( η 6 ‐arene)Ru(en)Cl] + complex .…”

“…have also explored analogous complexes for the enantioselective hydrogenation of naphthalene derivatives in the presence of H 2 gas . Notably, these and plenty of other transfer hydrogenation/hydrogenation reactions catalysed by (arene)Ru‐en‐based complexes target mainly C=O or C=N bonds . However, at an elevated temperature (60–100 °C) using high‐pressure H 2 gas (10–60 bar), the hydrogenation of C=C bonds (aromatic and others) catalysed by Ru complexes was also successful …”

Deracemization By Simultaneous Bio‐oxidative Kinetic Resolution and Stereoinversion

“…However, this strategy proved problematic for the compounds of interest: Among the published procedures for amine racemization, which rely on homogeneous[8] or heterogeneous[9] metal catalysis, or thiyl radical-mediated reversible H-abstraction,[10] no method was suitable to act exclusively on the benzylisoquinoline substrates whilst leaving the berbine products untouched. For example, Shvo’s diruthenium complex[11] racemized benzylisoquinolines and berbines alike[7a] when employed under previously described reaction conditions. [8g] Raney nickel led to degradation of the complex alkaloids, whereas palladium on charcoal did not show any detectable racemization activity at all.…”