Synthesis of vicinal dimethyl chirons by asymmetric hydrogenation of trisubstituted alkenes

Electrocatalytic Hydrogenation Using Palladium Membrane Reactors

Hydrogenation is a crucial chemical process employed in a myriad of industries, often facilitated by metals such as Pd, Pt, and Ni as catalysts. Traditional thermocatalytic hydrogenation usually necessitates high temperature and elevated pressure, making the process energy intensive. Electrocatalytic hydrogenation offers an alternative but suffers from issues such as competing H2 evolution, electrolyte separation, and limited solvent selection. This Perspective introduces the evolution and advantages of the electrocatalytic Pd membrane reactor (ePMR) as a solution to these challenges. ePMR utilizes a Pd membrane to physically separate the electrochemical chamber from the hydrogenation chamber, permitting the use of water as the hydrogen source and eliminating the need for H2 gas. This setup allows for greater control over reaction conditions, such as solvent and electrolyte selection, while mitigating issues such as low Faradaic efficiency and complex product separation. Several representative hydrogenation reactions (e.g., hydrogenation of C=C, C≡C, C=O, C≡N, and O=O bonds) achieved via ePMR over the past 30 years were concisely discussed to highlight the unique advantages of ePMR. Promising research directions along with the advancement of ePMR for more challenging hydrogenation reactions are also proposed. Finally, we provide a prospect for future development of this distinctive hydrogenation strategy using hydrogen-permeable membrane electrodes.

Reducing Challenges in Organic Synthesis with Stereoselective Hydrogenation and Tandem Catalysis

Tandem catalysis enables the rapid construction of complex architectures from simple building blocks. This Perspective shares our interest in combining stereoselective hydrogenation with transformations such as isomerization, oxidation, and epimerization to solve diverse challenges. We highlight the use of tandem hydrogenation for preparing complex natural products from simple prochiral building blocks and present tandem catalysis involving transfer hydrogenation and dynamic kinetic resolution. Finally, we underline recent breakthroughs and opportunities for asymmetric hydrogenation.

Nickel-Catalyzed Asymmetric Hydrogenation of Cyclic Alkenyl Sulfones, Benzo[b]thiophene 1,1-Dioxides, with Mechanistic Studies

A highly efficient catalytic system based on the cheap transition metal nickel for the asymmetric hydrogenation of challenging cyclic alkenyl sulfones, 3-substituted benzo[b]thiophene 1,1-dioxides, was first successfully developed. A series of hydrogenation products, chiral 2,3-dihydrobenzo[b]thiophene 1,1-dioxides, were obtained in high yields (95-99%) with excellent enantioselectivities (90-99% ee). According to the results of nonlinear effect studies, deuterium-labeling experiments, and DFT calculation investigations, a reasonable catalytic mechanism for this nickel-catalyzed asymmetric hydrogenation was provided, which displayed that the two added hydrogen atoms of the hydrogenation products could be from H₂ through the insertion of Ni-H and subsequent hydrogenolysis.

Catalytic Reductive Aldol and Mannich Reactions of Enone, Acrylate, and Vinyl Heteroaromatic Pronucleophiles

Catalytic reductive coupling of enone, acrylate, or vinyl heteroaromatic pronucleophiles with carbonyl or imine partners offers an alternative to base-mediated enolization in aldol- and Mannich-type reactions. In this review, direct catalytic reductive aldol and Mannich reactions are exhaustively catalogued on the basis of metal or organocatalyst. Stepwise processes involving enone conjugate reduction to form discrete enol or (metallo)enolate derivatives followed by introduction of carbonyl or imine electrophiles and aldol reactions initiated via enone conjugate addition are not covered.

Remarkably Facile Borane-Promoted, Rhodium-Catalyzed Asymmetric Hydrogenation of Tri- and Tetrasubstituted Alkenes

Oxime-directed catalytic asymmetric hydroboration is diverted to catalytic asymmetric hydrogenation (CAH) upon the addition of a proton source, such as MeOH, or by running the reaction under a hydrogen atmosphere. A borane (e.g., pinacolborane) is required to promote CAH. Tri- and tetrasubstituted alkenes, including the challenging all-alkyl tetrasubstituted alkenes, undergo CAH with enantiomer ratios (er) as high as 99:1. The mild reaction conditions, i.e., ambient temperature, moderate reaction times, and the need for only a slight excess of H₂, contrast those used in most state-of-the-art catalysts for related substrates.

Nickel-catalyzed asymmetric transfer hydrogenation of conjugated olefins

A nickel catalyst is used for asymmetric hydrogenation of electron-deficient olefins using formic acid as hydrogen source.

Filling gaps in asymmetric hydrogenation methods for acyclic stereocontrol: application to chirons for polyketide-derived natural products

The large volume of research studying hydrogenation catalysis might suggest that stereoselective hydrogenation of alkenes is a solved problem, but we believe the most important parts of asymmetric hydrogenation methodology remain unmastered. The most popular chiral catalysts, Rh- and Ir-diphosphine complexes, do not hydrogenate the largest categories of prochiral alkenes, hindered tri- and tetra-substituted ones, at useful rates unless the substrate has a "classical" coordinating functional group (CFG), for example, amides or homoallylic alcohols, to anchor the substrate to the metal. Therefore, while many methods are available for the asymmetric hydrogenation of alkenes with appropriate CFGs, synthetic chemistry would benefit from chiral hydrogenations of substrates with functional groups that typically do not coordinate in Rh- and Ir-diphosphine complexes. In this Account, we demonstrate the application of chiral analogues of Crabtree's catalyst to asymmetric hydrogenations of coordinating unfunctionalized, trisubstituted alkenes. Crabtree's catalyst, a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine, differs from Rh- and Ir-diphosphine complexes, which we broadly refer to as "chiral analogues of Wilkinson's catalyst." Crabtree's catalyst analogues hydrogenate substrates that do not contain functionalities generally recognized as CFGs, and we propose reasons for this chemistry based on the catalytic mechanisms. Thus, chiral analogues of Crabtree's catalyst facilitate many hydrogenations that would not be possible using Rh- or Ir-diphosphine complexes. Directed hydrogenations have been used in acyclic stereocontrol for decades, but the realization that these catalysts can be used for acyclic stereocontrol without the types of directing groups that are necessary for other hydrogenations significantly broadens the scope of hydrogenations for this purpose. Recently, we have prepared chirons for polyketide-derived natural products using an N,carbene-Ir complex (1). This approach has led to catalytic syntheses of several important chirons to facilitate preparations of these ubiquitous natural products.

Synthesis of the Prelog-Djerassi lactone via an asymmetric hydroformylation/crotylation tandem sequence

A synthesis of the Prelog-Djerassi lactone [(+)-1] has been accomplished in three isolations and 57% overall yield from the known vinyl ortho ester 2. A Rh(I)-catalyzed asymmetric hydroformylation/crotylation tandem sequence has been developed and used to set the C2-C4 stereochemistry. A Rh(I)-catalyzed asymmetric hydrogenation was employed to set the C6 sterechemistry, resulting in an unusually short and efficient enantioselective synthesis of this touchstone molecule from achiral starting material.

An asymmetric hydrogenation route to (-)-spongidepsin

(-)-Spongidepsin 1, a cytotoxic marine natural product, was prepared via two iridium-catalyzed hydrogenation reactions; both were highly stereoselective, giving convenient access to pivotal intermediates. This synthesis was modified to give several spongidepsin analogues, and their cytotoxicities were compared with those of the natural product.

Chiral N-heterocyclic carbene-catalyzed generation of ester enolate equivalents from {alpha},{beta}-unsaturated aldehydes for enantioselective Diels-Alder reactions

The catalytic generation of chiral ester enolate equivalents from α,β-unsaturated aldehydes with chiral N-hetereocyclic carbene catalysts makes possible highly enantioselective hetero-Diels-Alder reactions. The reactions proceed under simple, mild conditions with both aliphatic and aromatic substituted enals as substrates. Previous attempts to employ these starting materials as enolate precursors gave structurally different products via catalytically generated homoenolate equivalents. Critical to the success of the enolate generation was the strength of the catalytic base used to generate the active N-heterocyclic carbene catalyst. To complement these studies, we have investigated the enolate structure using computational methods and find that it prefers conformations perpendicular to the triazolium core.

Phosphine-free chiral metal catalysts for highly effective asymmetric catalytic hydrogenation

In addition to the brilliant chiral phosphorus metal catalysts, chiral phosphine-free metal complexes find increasing application as catalysts for asymmetric hydrogenation. In this account, two types of chiral phosphine-free ligands, N-heterocyclic carbene-based C,N-ligands and diamine-based N,N-ligands, in the homogeneous asymmetric hydrogenation of prochiral ketones, imines and quinolines are reviewed.