On the Ability of the N-O Bond to Support a Stable Stereogenic Axis: Peptide-Catalyzed Atroposelective N-Oxidation

Catalysis of Free C-C Bond Rotation: C-F---H-X H-Bonds Find a Catalytic Role

There are few dynamic processes in organic chemistry that are more central to the molecular structure than C-C bond rotation. It is notable, however, that there exist few (if any) cases in which its hindered variants can actually be catalyzed. In this communication, we report a unique model system for the clear documentation of the catalysis of 360° C-C bond rotation that employs a transient but key N-H---F-C hydrogen bond as a linchpin and secondary "dual" charge-induced n → π* interactions and ion pairing effects that bolster catalysis.

Catalyst-Substrate Pairings for Carbocyclic and Heterocyclic Systems in Atroposelective Quinazolinone Synthesis

Asymmetric catalytic reaction development depends critically on the matching of an appropriate catalytic scaffold with a substrate of interest. In many cases, a catalyst will be discovered to be quite selective for a given substrate, and that same catalyst is then evaluated for its scope with respect to alterations of the substrate. In the context of a catalytic atroposelective cyclocondensation, we discovered that a chiral phosphoric acid catalyst (CPA), (R)-TCYP, mediated these processes with up to 98:2 enantiomeric ratio (er) and in 95% yield. Yet, when the same reaction was attempted in the presence of a basic nitrogen heteroatom within the substrate, enantioselectivity was significantly reduced (73:27 er). In these instances, a different catalyst scaffold based on phosphothreonine (pThr), while ineffective for the carbocyclic substrate (53:47 er), was found to be quite selective (90:10 er) for its pyridyl analog. Mechanistic studies exploring this divergence in reactivity unveiled that the 8-carbocyclic substrate (using (R)-TCYP) displayed a positive nonlinear effect (NLE), whereas the 8-heterocyclic substrate (using a pThr-based catalyst) displayed no NLE at all. The mechanistic distinctions between these two scenarios suggest significant differences in the nature of the noncovalent interactions that operate to deliver high enantioselectivity.

Maleic anhydride derivatives as catalysts for N-oxidation of pyridine using hydrogen peroxide

Maleic anhydride derivatives were evaluated as catalysts in N-oxidation of various pyridine substrates using hydrogen peroxide (H2O2). Depending on the electronic properties of the pyridine substrates, pyridines with electron-donating groups reacted well with 2,3-dimethylmaleic anhydride (DMMA). In contrast, 1-cyclohexene-1, 2-dicarboxylic anhydride (CHMA) was most effective for electron-deficient pyridines. The different performance of these two anhydrides is attributed to the diacid-anhydride equilibrium, which is crucial for regenerating the peracid oxidant through an anhydride intermediate in the catalytic cycle. This approach using a catalytic amount of anhydride with H2O2 has the potential to replace stoichiometric amounts of percarboxylic acid as an oxidant for a broader range of organic substrates.

Atroposelective Synthesis of Biaryl N-Oxides via Cu-Catalyzed De Novo Heteroaromatic N-Oxide Ring Formation

Heteroaromatic N-oxides, renowned for their highly polar N─O bond and robust structure, exhibit significant bioactivities and have played a pivotal role in various drug development projects since the discovery of Minoxidil. Moreover, heteroaromatic N-oxides, featuring axially chiral biaryl frameworks, are indispensable as Lewis base catalysts and ligands in organic synthesis. Despite their importance, synthesizing these chiral compounds is challenging, necessitating chiral starting materials or resolution processes. Catalytic strategies rely on the functionalization of heteroaromatic N-oxide compounds, leading to products with a relatively limited skeletal diversity. This study introduces a Cu-catalyzed atroposelective method for synthesizing biaryl N-oxides via de novo heteroaromatic N-oxide ring formation. This mild and efficient approach achieves excellent stereoselectivities (up to 99:1 er), enabling the production of a wide array of N-oxides with novel heteroaromatic scaffolds. The axially chiral N-oxide product 3f demonstrates high stereoselectivity and recyclability as a Lewis base catalyst. Additionally, product 3e exhibits promising therapeutic efficacy against triple-negative breast cancer, with IC50 values of 4.8 and 5.2 µm in MDA-MB-231 and MDA-MB-468 cells, respectively. This research not only advances the synthesis of challenging chiral heteroaromatic N-oxides but also encourages further exploration of N-oxide entities in the discovery of bioactive small molecules.

Enantioconvergent synthesis of axially chiral amides enabled by Pd-catalyzed dynamic kinetic asymmetric aminocarbonylation

Atropisomeric biaryls bearing carbonyl groups have attracted increasing attention due to their prevalence in diverse bioactive molecules and crucial role as efficient organo-catalysts or ligands in asymmetric transformations. However, their preparation often involves tedious multiple steps, and the direct synthesis via asymmetric carbonylation has scarcely been investigated. Herein, we report an efficient palladium-catalyzed enantioconvergent aminocarbonylation of racemic heterobiaryl triflates with amines via dynamic kinetic asymmetric transformation (DyKAT). This protocol features a broad substrate scope and excellent compatibility for rapid construction of axially chiral amides in good to high yields with excellent enantioselectivities. Detailed mechanistic investigations discover that the base can impede the intramolecular hydrogen bond-assisted axis rotation of the products, thus allowing for the success to achieve high enantioselectivity. Moreover, the achieved axially chiral heterobiaryl amides can be directly utilized as N,N,N-pincer ligands in copper-catalyzed enantioselective formation of C(sp3)-N and C(sp3)-P bonds.

Catalytic Enantioselective Sulfoxidation of Functionalized Thioethers Mediated by Aspartic Acid-Containing Peptides

A peptide-catalyzed enantioselective oxidation of sulfides to yield pharmaceutically relevant chiral sulfoxides is reported. Experimental evidence suggesting that a hydrogen bond-donating moiety must be present in the substrate to achieve high levels of enantioinduction is supported by computational modeling of transition states. These models also indicate that dual points of contact between the peptidic catalyst and substrate are likely responsible for the formation of one desired sulfoxide in 94:6 er.

Rotational Behavior about the N3-Pyridyl Bond in 3-(Pyridin-2-yl)quinazolin-4-one and 4-Thione Derivatives

The rotational barriers about the N3-(2-pyridyl) bond in 2-iso-propyl-3-(pyridin-2-yl)quinazolin-4-one and the thione analogue were evaluated though VT-NMR measurement of a diastereotopic iso-propyl group followed by a line-shape simulation. In 3-(pyridin-2-yl)quinazoline-4-thione bearing a chiral center as the C2 substituent, the formation of dynamic diastereomers was detected by NMR. The rotational pathway about the N3-(2-pyridyl) bond and the stereochemistries of dynamic diastereomers were revealed through a computational study.

Organocatalytic activation of hydrogen peroxide: towards green and sustainable oxidations

The advent of organocatalysis provided an additional option in every researcher's arsenal, towards the development of elegant and sustainable protocols for various organic transformations. Oxidation reactions are considered to be key in organic synthesis since oxygenated functionalities appear in many natural products. Hydrogen peroxide is categorized as a green oxidant, since its only by-product is water, offering novel opportunities for the development of green and sustainable protocols. In this review article, we intend to present recent developments in the field of the organocatalytic activation of hydrogen peroxide, providing useful insight into the applied oxidative protocols. At the same time, we will present some interesting mechanistic studies, providing information on the oxygen transfer processes.