Mechanistic analysis of the transition metal-catalyzed hydrogenation of imines and functionalized enamines

The asymmetric hydrogenation of prochiral imines has been mainly studied on cationic rhodium or iridium complexes and involves the coordination of the nitrogen atom on the metal centre. Oxidative addition of dihydrogen followed by one hydride transfer onto the carbon atom produces a σ-bonded nitrogen-metal intermediate from which reductive elimination occurs with the second hydride to produce the amine. From all these reactivity studies we propose that in an early step a hydrogen atom transfer could occur on the nitrogen atom to generate an iminium species which coordinates the metal centre through the CN double bond. The Shvo catalyst, an active neutral ruthenium(II) complex, presents the particularity to transfer almost simultaneously a proton from the hydroxycyclopentadienyl ligand and a hydride bonded to the metal center. In this case too, we suggest the first proton transfer generates an iminium species.Morever, a few titanium complexes have been shown to catalyze the hydrogenation of imines.Concerning enamines, literature is rather scarce on catalytic success. On the contrary, many studies focused on hydrogenation of enamides, which represent functionalized enamines, and enantiomeric excesses very close to 100% have been obtained. Extensive studies of the effect of the (chiraldiphosphine)Rh⁺ framework on the asymmetric induction have shown that several reversible steps can occur related to the oxidative addition of dihydrogen before or after the coordination of the enamide; the irreversible step directly connected to the asymmetric induction is the formation of the chiral alkyl-rhodium species.     

Highly enantioselective Ir-catalyzed hydrogenation of exocyclic enamines

[Ir(COD)Cl]₂/MeO-BiPhep/I₂ catalyst system is highly effective for the asymmetric hydrogenation of exocyclic enamines with high enantioselectivities (up to 96% ee).     

Highly efficient synthesis of beta-amino acid derivatives via asymmetric hydrogenation of unprotected enamines

A direct asymmetric hydrogenation of unprotected enamino esters and amides is described. Catalyzed by Rh complexes with Josiphos-type chiral ligands, this method gives beta-amino esters and amides in high yield and high ee (93-97% ee). No acyl protection/deprotection is required.     

Recent advances in transition metal-catalyzed sp3 C-H amination adjacent to double bonds and carbonyl groups

Transition metal-catalyzed C-H amination at positions adjacent to double bonds and carbonyl groups is discussed in this critical review. While the focus will center on the recent developments of α-oxidative amination, some historical developments and mutually beneficial reports in the broader field of C-H amination will be discussed. C-H amination has become a viable method for the efficient installation of nitrogen atoms en route to target molecules (89 references).     

Recent advances in the transition metal-catalyzed twofold oxidative C-H bond activation strategy for C-C and C-N bond formation

The direct functionalization of heterocyclic compounds has emerged as one of the most important topics in the field of metal-catalyzed C-H bond activation due to the fact that products are an important synthetic motif in organic synthesis, the pharmaceutical industry, and materials science. This critical review covers the recent progresses on the regioselective dehydrogenative direct coupling reaction of heteroarenes, including arylation, olefination, alkynylation, and amination/amidation mainly utilizing transition metal catalysts (113 references).     

Recent advances in catalytic hydrogenation of carbon dioxide

Owing to the increasing emissions of carbon dioxide (CO(2)), human life and the ecological environment have been affected by global warming and climate changes. To mitigate the concentration of CO(2) in the atmosphere various strategies have been implemented such as separation, storage, and utilization of CO(2). Although it has been explored for many years, hydrogenation reaction, an important representative among chemical conversions of CO(2), offers challenging opportunities for sustainable development in energy and the environment. Indeed, the hydrogenation of CO(2) not only reduces the increasing CO(2) buildup but also produces fuels and chemicals. In this critical review we discuss recent developments in this area, with emphases on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field (319 references).     

Recent advances in the electrochemical hydrogenation of unsaturated hydrocarbons

Electrochemical hydrogenation of unsaturated organic compounds is emerging as a very promising alternative to the conventional transition-metal-catalyzed transformation as it avoids the preparation, transportation, and storage of pressured molecule hydrogen (H₂). Besides the direct usage of the on-site-generated H₂ on the cathode surface, the electrochemical means also provide opportunities for novel hydrogenation reactions of unsaturated hydrocarbons, especially electrochemical Birch reduction. Another attractive aspect of such an approach is the possibility of highly selective deuteration of unsaturated hydrocarbons with the readily available, cost-effective deuterium source, D₂O.     

Recent advances in iridium-catalysed asymmetric hydrogenation of unfunctionalised olefins

In this account, the recent advances which have been made in asymmetric iridium-catalysed hydrogenation are reviewed. The first part focuses on our own studies of bicyclic pyridine–phosphinite ligands. These ligands have greatly enhanced the application range of asymmetric hydrogenation and, for the first time, have allowed highly enantioselective hydrogenation of simple, alkyl-substituted olefins and substituted furans. In the second part of this account, experimental and computational mechanistic studies are discussed. Whether the catalytic cycle proceeds via Ir(I)–Ir(III) intermediates or via Ir(III)–Ir(V) intermediates is still the subject of debate.     

Facile access to chiral 1-pyrrolines through Rh-catalyzed enantioselective partial hydrogenation of unprotected simple pyrroles

Highly enantioselective Rh-catalyzed partial hydrogenation of unprotected simple 2-alkyl-5-aryl-disubstituted pyrroles has been successfully developed, generating a series of chiral 1-pyrroline derivatives generally with excellent results (95%–99% yields, 91%–96% ee). Moreover, 2,5-aryl-1H-pyrroles were hydrogenated well in high yields and good enantioselectivities. This efficient protocol features easily accessible substrates, wide substrate scope, well functional group compatibility, commercially available rhodium precursor and chiral ligand. It provides a versatile route to access chiral 1-pyrroline derivatives that are of great importance in organic synthesis and pharmaceutical chemistry.     

Recent advances into the enantioselective protonation of prostereogenic enol derivatives

We discuss the recent developments into the enantioselective protonation of substituted enol derivatives and comment on the effect that reaction parameters (such as solvent, temperature and salt additive) have on the facial preference.     

Recent advances in enantioselective [2 + 2 + 2] cycloaddition

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.     

Recent advances in the enantioselective 1,3-dipolar cycloaddition of azomethine ylides and dipolarophiles

The synthesis of diastereo- and enantiopure heterocyclic molecules via catalytic asymmetric 1,3-dipolar cycloaddition reaction between azomethine ylides, generated in situ from α-amino acid-derived iminoesters and dipolarophiles is considered one of the most powerful and versatile techniques. In this review, we make a detailed overview of the latest developments in this area since 2014 and highlight the recent improvements in the structural scope of dipolarophiles, azomethine ylide precursors, and chiral ligands.