Desymmetrization of unactivated bis-alkenes via chiral Brønsted acid-catalysed hydroamination

3D-QSAR model-oriented optimization of Pyrazole β-Ketonitrile derivatives with diphenyl ether moiety as novel potent succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitor (SDHI) fungicides play important roles in the control of plant fungal diseases. However, they are facing serious challenges from issues with resistance and cross-resistance, primarily attributed to their frequent application and structural similarities. There is an urgent need to design and develop SDHI fungicides with novel structures.

RESULTS

Aiming to discover novel potent SDHI fungicides, 31 innovative pyrazole β-ketonitrile derivatives with diphenyl ether moiety were rationally designed and synthesized, which were guided by a 3D-QSAR model from our previous study. The optimal target compound A23 exhibited not only outstanding in vitro inhibitory activities against Rhizoctonia solani with a half-maximal effective concentration (EC50) value of 0.0398 μg mL-1 comparable to that for fluxapyroxad (EC50 = 0.0375 μg mL-1), but also a moderate protective efficacy in vivo against rice sheath blight. Porcine succinate dehydrogenase (SDH) enzymatic inhibitory assay revealed that A23 is a potent inhibitor of SDH, with a half-maximal inhibitory concentration of 0.0425 μm. Docking study within R. solani SDH indicated that A23 effectively binds into the ubiquinone site mainly through hydrogen-bonds, and cation-π and π-π interactions.

CONCLUSION

The identified β-ketonitrile compound A23 containing diphenyl ether moiety is a potent SDH inhibitor, which might be a good lead for novel fungicide research and optimization. © 2024 Society of Chemical Industry.

Enantioselective, Bro̷nsted Acid-Catalyzed Anti-selective Hydroamination of Alkenes

Chiral pyrrolidines are common structural motives in natural products as well as active pharmaceutical ingredients, explaining the need for methods for their enantioselective synthesis. While several, often metal-catalyzed, methods for their preparation do exist, the enantioselective synthesis of pyrrolidines containing quaternary stereocenters remains challenging. Herein, we report a Bro̷nsted acid-catalyzed intramolecular hydroamination that provides such pyrrolidines from simple starting materials in high yield and enantioselectivity. Key to an efficient reaction was the use of an electron-deficient protective group on nitrogen, the common nosyl-protecting group, to avoid deactivation of the Bro̷nsted acid by deprotonation. The reaction proceeds as a stereospecific anti-addition indicating a concerted reaction. Furthermore, kinetic studies show Michaelis-Menten behavior, suggesting the formation of a precomplex similar to those observed in enzymatic catalysis.

Rhodium-Catalyzed Highly Enantioselective Hydroboration of Acyclic Tetrasubstituted Alkenes Directed by an Amide

Although progress has been made in enantioselective hydroboration of di- and trisubstituted alkenes over the past decades, enantioselective hydroboration of tetrasubstituted alkenes with high diastereo- and enantioselectivities continues as an unmet challenge since the 1950s due to its extremely low reactivity and the difficulties to simultaneously control the regio- and stereoselectivity of a tetrasubstituted alkene. Here, we report highly regio-, diastereo-, and enantioselective catalytic hydroboration of diverse acyclic tetrasubstituted alkenes. The delicate interplay of an electron-rich rhodium complex and coordination-assistance forms a highly adaptive catalyst that effectively overcomes the low reactivity and controls the stereoselectivity. The generality of the catalyst system is exemplified by its efficacy across various tetrasubstituted alkenes with diverse steric and electronic properties.

Ligand effects, solvent cooperation, and large kinetic solvent deuterium isotope effects in gold(I)-catalyzed intramolecular alkene hydroamination

Kinetic studies on the intramolecular hydroamination of protected variants of 2,2-diphenylpent-4-en-1-amine were carried out under a variety of conditions with cationic gold catalysts supported by phosphine ligands. The impact of ligand on gold, protecting group on nitrogen, and solvent and additive on reaction rates was determined. The most effective reactions utilized more Lewis basic ureas, and more electron-withdrawing phosphines. A DCM/alcohol cooperative effect was quantified, and a continuum of isotope effects was measured with low KIE's in the absence of deuterated alcoholic solvent, increasing to large solvent KIE's when comparing reactions in pure MeOH to those in pure MeOH-d4. The effects are interpreted both within the context of a classic gold π-activation/protodeauration mechanism and a general acid-catalyzed mechanism without intermediate gold alkyls.

Catalytic enantioselective reductive Eschenmoser-Claisen rearrangements

An important challenge in enantioselective catalysis is developing strategies for the precise synthesis of neighboring congested all-carbon quaternary stereocenters. The well-defined transition states of [3,3]-sigmatropic rearrangements and their underlying stereospecificity render them powerful tools for the synthesis of such arrays. However, this type of pericyclic reaction remains notoriously difficult to catalyze, especially in an enantioselective fashion. Herein, we describe an enantioselective reductive Eschenmoser-Claisen rearrangement catalyzed by chiral 1,3,2-diazaphospholene-hydrides. This developed transformation enables full control of the two newly formed acyclic stereogenic centers, leading to amides with vicinal all-carbon quaternary-tertiary or quaternary-quaternary carbon atoms.

Catalytic Desymmetric Dicarbofunctionalization of Unactivated Alkenes

The construction of multi-stereocenters by a transition metal-catalyzed cross-coupling reaction is a major challenge. The catalytic desymmetric functionalization of unactivated alkenes remains largely unexplored. Herein, we disclose -a desymmetric dicarbofunctionalization of 1,6-dienes via a nickel-catalyzed reductive cross-coupling reaction. The leverage of the underdeveloped chiral 8-Quinox enables the Ni-catalyzed desymmetric carbamoylalkylation of both unactivated mono- and disubstituted alkenes to form pyrrolidinone bearing two nonadjacent stereogenic centers in high enantio- and stereoselectivitives with broad functional-group tolerance. The synthetic application of pyrrolidinones allows the rapid access to complex chiral fused-heterocycles.

Dicoordinate Au(I)-Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)-ethylene π-complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)-ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed the best results reaching full conversion to the hydroaminated products under notably mild conditions (1 bar of ethylene pressure at 60 °C). Kinetic analysis together with density functional theory calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex. In addition, we have measured a strong primary kinetic isotopic effect that is consistent with the involvement of X-H bond-breaking events in the protodeauration turnover-limiting step.

A Powerful Chiral Super Brønsted C-H Acid for Asymmetric Catalysis

A new type of chiral super Brønsted C-H acids, BINOL-derived phosphoryl bis((trifluoromethyl)sulfonyl) methanes (BPTMs), were developed. As compared to widely utilized BINOL-derived chiral phosphoric acids (BPAs) and N-triflyl phosphoramides (NTPAs), BPTMs displayed much higher Brønsted acidity, resulting in dramatically improved activity and excellent enantioselectivity as demonstrated in catalytic asymmetric Mukaiyama-Mannich reaction, allylic amination, three-component coupling of allyltrimethylsilane with 9-fluorenylmethyl carbamate and aldehydes, and protonation of silyl enol ether. These new strong Brønsted C-H acids have provided a platform for expanding the chemistry of asymmetric Brønsted acid catalysis.

Chiral N-Triflylphosphoramide-Catalyzed Asymmetric Hydroamination of Unactivated Alkenes: A Hetero-Ene Reaction Mechanism

A highly enantioselective intramolecular hydroamination reaction catalyzed by chiral N-triflylphosphoramide (NTPA) that covers an exceptionally broad substrate scope of isolated unactivated alkenes was recently reported by some of us. Herein...

Computational study on N-triflylphosphoramide-catalyzed enantioselective hydroamination of alkenyl thiourea

The mechanism of the enantioselective intramolecular hydroamination of alkenyl thiourea catalyzed by chiral binaphthol N-triflylphosphoramide (NPTA) was investigated using density functional theory calculations. This study reveals the details of the hydrogen bonding mode between NPTA and the substrate and indicates the importance of the dual hydrogen binding properties of the thiourea moiety for the reactivity and stereoselectivity of the hydroamination.

Organocatalytic enantioselective reactions involving prochiral carbocationic intermediates

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

This review summarizes the most noteworthy achievements in the field of C–O and C–N bond formation by hydroalkoxylation and hydroamination reactions on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by earth-abundant 3d transition metal catalysts based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2012 until early 2021 has been covered.

Insights into the Chiral Phosphoric Acid-Catalyzed Dynamic Kinetic Asymmetric Hydroamination of Racemic Allenes: An Allyl Carbocation/Phosphate Pair Mechanism

Computational studies of chiral phosphoric acid (CPA)-catalyzed dynamic kinetic asymmetric hydroamination (DyKAH) of racemic allenes show that the reaction proceeds through a catalytic asymmetric model involving a highly reactive π-allylic carbocationic intermediate, generated from a racemic allene through an intermolecular proton transfer mediated by CPA, which also results in a high E/Z selectivity. Moreover, the distortion-interaction, atom in molecule, and electrostatic interaction analyses and space-filling models are employed on the basis of the DyKAH catalyzed by (S)-A5 (reaction 1) or (R)-A2 (reaction 2) to explain the high enantioselectivity and the controlling effects of SPINOL scaffolds on the signs of enantioselectivity. Our calculations indicate that the enantioselectivity of reactions 1 and 2 can be mainly ascribed to the favorable noncovalent interactions within the stronger chiral electrostatic environment created by the phosphoric acid in the preferential transition states. Finally, the effect of (S/R)-SPINOL-based CPAs on the signs of enantioselectivity can be explained by the different combination modes of substrates into the chiral binding pocket of the catalyst controlled by the chirality of SPINOL backbones. Overall, the new insights into the reaction rationalize the outcome and these key factors that affect the product enantioselectivity are important to guide the DyKAHs.