Branched-Selective Cross-Electrophile Coupling of 2-Alkyl Aziridines and (Hetero)aryl Iodides Using Ti/Ni Catalysis

The arylation of 2-alkyl aziridines by nucleophilic ring-opening or transition-metal-catalyzed cross-coupling enables facile access to biologically relevant β-phenethylamine derivatives. However, both approaches largely favor C-C bond formation at the less-substituted carbon of the aziridine, thus enabling access to only linear products. Consequently, despite the attractive bond disconnection that it poses, the synthesis of branched arylated products from 2-alkyl aziridines has remained inaccessible. Herein, we address this long-standing challenge and report the first branched-selective cross-coupling of 2-alkyl aziridines with aryl iodides. This unique selectivity is enabled by a Ti/Ni dual-catalytic system. We demonstrate the robustness of the method by a twofold approach: an additive screening campaign to probe functional group tolerance and a feature-driven substrate scope to study the effect of the local steric and electronic profile of each coupling partner on reactivity. Furthermore, the diversity of this feature-driven substrate scope enabled the generation of predictive reactivity models that guided mechanistic understanding. Mechanistic studies demonstrated that the branched selectivity arises from a TiIII-induced radical ring-opening of the aziridine.

Ni/Photoredox-Catalyzed C(sp3)-C(sp3) Coupling between Aziridines and Acetals as Alcohol-Derived Alkyl Radical Precursors

Aziridines are readily available C(sp³) precursors that afford valuable β-functionalized amines upon ring opening. In this article, we report a Ni/photoredox methodology for C(sp³)-C(sp³) cross-coupling between aziridines and methyl/1°/2° aliphatic alcohols activated as benzaldehyde dialkyl acetals. Orthogonal activation modes of each alkyl coupling partner facilitate cross-selectivity in the C(sp³)-C(sp³) bond-forming reaction: the benzaldehyde dialkyl acetal is activated via hydrogen atom abstraction and β-scission via a bromine radical (generated in situ from single-electron oxidation of bromide), whereas the aziridine is activated at the Ni center via reduction. We demonstrate that an Ni(II) azametallacycle, conventionally proposed in aziridine cross-coupling, is not an intermediate in the productive cross-coupling. Rather, stoichiometric organometallic and linear free energy relationship studies indicate that aziridine activation proceeds via Ni(I) oxidative addition, a previously unexplored elementary step.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides

We present a highly efficient convergent asymmetric hydrogenation of E/Z mixtures of enamides catalyzed by N,P-iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers (E and Z geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of E/Z mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ee). A detailed mechanistic study using deuterium labeling and kinetic experiments revealed two different pathways for the observed enantioconvergence. For α-aryl enamides, fast isomerization of the double bond takes place, and the overall process results in kinetic resolution of the two isomers. For α-alkyl enamides, no double bond isomerization is detected, and competition experiments suggested that substrate chelation is responsible for the enantioconvergent stereochemical outcome. DFT calculations were performed to predict the correct absolute configuration of the products and strengthen the proposed mechanism of the iridium-catalyzed isomerization pathway.

Asymmetric hydrogenation for the synthesis of 2-substituted chiral morpholines

Asymmetric hydrogenation of unsaturated morpholines has been developed by using a bisphosphine-rhodium catalyst bearing a large bite angle. With this approach, a variety of 2-substituted chiral morpholines could be obtained in quantitative yields and with excellent enantioselectivities (up to 99% ee). The hydrogenated products could be transformed into key intermediates for bioactive compounds.

2-Substituted chiral morpholines were synthesized via a newly developed asymmetric hydrogenation of dehydromorpholines catalyzed by a bisphosphine–rhodium complex bearing a large bite angle.

Scalable Asymmetric Synthesis of MK-8998, a T-Type Calcium Channel Antagonist

Two scalable and efficient synthetic routes for the synthesis of a T-type calcium channel antagonist MK-8998 were developed from a simple pyridine building block. The key step to set the stereochemistry relied on either chiral rhodium catalyst-mediated asymmetric hydrogenation of an enamide or transamination of an arylketone that provided the corresponding product in high enantioselectivity and high yield.

Recent Advances in Asymmetric Hydrogenation Catalysis Utilizing Spiro and Other Rigid C-Stereogenic Phosphine Ligands

Transition-metal-catalyzed asymmetric reactions have been a powerful tool in organic synthesis for many years. The design of chiral ligands with the right configuration is fundamental to induce high regio- and stereoselectivity to catalytic reactions and to achieve high turnover numbers and high yields. A challenge is the control of prochiral centers with similar electronic properties in a similar steric environment within the same molecule. Over the last 10 years, a range of novel rigid C-stereogenic chiral phosphine ligands has been developed and successfully applied in various types of asymmetric transformations. Many of these ligands are of a di-, tri-, or multidentate nature. The purpose of this Perspective is to highlight recent synthetic achievements (since 2010) with spiro-phosphines and other rigid phosphines and discuss some mechanistic aspects of the catalytic reactions.

Enantioselective Iridium-Catalyzed Allylation of Nitroalkanes: Entry to β-Stereogenic α-Quaternary Primary Amines

The first systematic study of simple nitronate nucleophiles in iridium-catalyzed allylic alkylation is described. Using a tol-BINAP-modified π-allyliridium C,O-benzoate catalyst, α,α-disubstituted nitronates substitute racemic branched alkyl-substituted allylic acetates, thus providing entry to β-stereogenic α-quaternary primary amines. DFT calculations reveal early transition states that render the reaction less sensitive to steric effects and distinct trans-effects of diastereomeric chiral-at-iridium π-allyl complexes that facilitate formation of congested tertiary-quaternary C-C bonds.

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.

Recent Developments in Transition-Metal-Catalyzed Asymmetric Hydrogenation of Enamides

The catalytic asymmetric hydrogenation of prochiral olefins is one of the most widely studied and utilized transformations in asymmetric synthesis. This straightforward, atom economical, inherently direct and sustainable strategy induces chirality in a broad range of substrates and is widely relevant for both industrial applications and academic research. In addition, the asymmetric hydrogenation of enamides has been widely used for the synthesis of chiral amines and their derivatives. In this review, we summarize the recent work in this field, focusing on the development of new catalytic systems and on the extension of these asymmetric reductions to new classes of enamides.

1 Introduction

2 Asymmetric Hydrogenation of Trisubstituted Enamides

2.1 Ruthenium Catalysts

2.2 Rhodium Catalysts

2.3 Iridium Catalysts

2.4 Nickel Catalysts

2.5 Cobalt Catalysts

3 Asymmetric Hydrogenation of Tetrasubstituted Enamides

3.1 Ruthenium Catalysts

3.2 Rhodium Catalysts

3.3 Nickel Catalysts

4 Asymmetric Hydrogenation of Terminal Enamides

4.1 Rhodium Catalysts

4.2 Cobalt Catalysts

5 Rhodium-Catalyzed Asymmetric Hydrogenation of Miscellaneous Enamides

6 Conclusions

Enantioconvergent and enantiodivergent catalytic hydrogenation of isomeric olefins

The asymmetric catalytic hydrogenation of olefins is one of the most widely studied and utilised transformations in asymmetric synthesis. This straightforward and atom-economical strategy can provide excellent enantioselectivity for a broad variety of substrates and is widely relevant for both industrial applications and academic research. In many instances the hydrogenation is stereospecific in the regard that the E-Z-geometry of the olefin governs the stereochemistry of the hydrogenation, producing an enantiodivergent outcome. Interestingly, the possibility to hydrogenate E- and Z-isomer mixtures to a single stereoisomer in an enantioconvergent manner has been reported. This avoids the need for synthesis of geometrically pure alkene starting materials and therefore constitutes a significant practical advantage. This review article aims to provide an overview of the different stereochemical outcomes in the hydrogenation of olefins. Although the field is well developed and selectivity models have been proposed for a number of catalytic systems, an organized collection of enantioconvergent results, as opposed to the more common enantiodivergent case, might promote new investigation into these phenomena.

Cobalt-Catalyzed Asymmetric Hydrogenation of C=N Bonds Enabled by Assisted Coordination and Nonbonding Interactions

An efficient cobalt-catalyzed asymmetric hydrogenation of C=N bonds has been realized. Chiral hydrazines were obtained in high yields and with excellent enantioselectivities (95-98 % ee). The hydrogenation went smoothly at up to 2000 substrate/catalyst and on a gram scale. The success of this reaction relies on the presence of an NHBz group in the substrates, with the reactivity and enantioselectivity improved by an assisted coordination to the cobalt atom and a nonbonding interaction with the ligand. Furthermore, this reaction has practical applications for the synthesis of several useful chiral nitrogen-containing compounds.

Synthesis of Difluoroalkyl Unsaturated β-Amino Acid Derivatives Exclusively through Alkyne Difunctionalization

Alkynes difunctionalization is a powerful strategy in organic synthesis that provides a convenient synthetic entry for internal alkenes. The main challenge in this field was considered to be the geometry control of the newly formed double bond (thermodynamically controlled or kinetically controlled). Herein, we report a novel procedure (through the cyclic compounds broken) to completely control the regioselectivity of olefins. The products, difluoroalkyl unsaturated β-amino acid derivatives, have potential applications in some important pharmaceuticals on account of the special nature of fluorine atoms.