Redox Deracemization of Tertiary Stereocenters Adjacent to an Electron-Withdrawing Group

Influence of chemical structures on reduction rates and defluorination of fluoroarenes during catalytic reduction using a rhodium-based catalyst

Continuous growth in fluoroarene production has led to environmental pollution and health concerns owing to their persistence, which is attributed to the stable C-F bond in their structures. Herein, we investigated fluoroarene decomposition via hydrodefluorination using a rhodium-based catalyst, focusing on the effects of the chemical structure and functional group on the defluorination yield. Most compounds, except (pentafluoroethyl)benzene, exhibited full or partial reduction with pseudo-first-order rate constants in the range of 0.002-0.396 min-1 and defluorination yields of 0%-100%. Fluoroarenes with hydroxyl, methyl, and carboxylate groups were selected to elucidate how hydrocarbon and oxygen-containing functional groups influence the reaction rate and defluorination. Inhibition of the reaction rate and defluorination yield based on functional groups increased in the order of hydroxyl < methyl < carboxylate, which was identical to the order of the electron-withdrawing effect. Fluoroarenes with polyfluoro groups were also assessed; polyfluoro groups demonstrated a different influence on catalyst activity than non-fluorine functional groups because of fluorine atoms in the substituents undergoing defluorination. The reaction kinetics of (difluoromethyl)fluorobenzenes and their intermediates suggested that hydrogenation and defluorination occurred during degradation. Finally, the effects of the type and position of functional groups on the reaction rate and defluorination yield were investigated via multivariable linear regression analysis. Notably, the electron-withdrawing nature of functional groups appeared to have a greater impact on the defluorination yield of fluoroarenes than the calculated C-F bond dissociation energy.

Halogen-Atom Transfer Enabled Catalytic Enantioselective Coupling to Chiral Trifluoromethylated Alkynes via Dual Nickel and Photocatalysis

With halogen-atom transfer as an effective tool, a novel catalytic enantioselective protocol to generate chiral trifluoromethylated alkynes has been established by a cooperative photoredox and nickel catalysis system, providing a straightforward and modular route to access this type of product in good yields and enantioselectivities. The halogen-atom transfer process is essential for the reaction and this novel strategy offers another promising way to utilize alkyl halides with highly negative reduction potentials. It firstly expands nickel-catalyzed asymmetric reductive cross-couplings of organohalides from the traditional single-electron transfer to halogen-atom transfer.

Enantioselective synthesis of α,α-diarylketones by sequential visible light photoactivation and phosphoric acid catalysis

Chiral ketones and their derivatives are useful synthetic intermediates for the synthesis of biologically active natural products and medicinally relevant molecules. Nevertheless, general and broadly applicable methods for enantioenriched acyclic α,α-disubstituted ketones, especially α,α-diarylketones, remain largely underdeveloped, owing to the easy racemization. Here, we report a visible light photoactivation and phosphoric acid-catalyzed alkyne-carbonyl metathesis/transfer hydrogenation one-pot reaction using arylalkyne, benzoquinone, and Hantzsch ester for the expeditious synthesis of α,α-diarylketones with excellent yields and enantioselectivities. In the reaction, three chemical bonds, including C═O, C─C, and C─H, are formed, providing a de novo synthesis reaction for chiral α,α-diarylketones. Moreover, this protocol provides a convenient and practical method to synthesize or modify complex bioactive molecules, including efficient routes to florylpicoxamid and BRL-15572 analogs. Computational mechanistic studies revealed that C-H/π interactions, π-π interaction, and the substituents of Hantzsch ester all play crucial roles in the stereocontrol of the reaction.

Catalytic Deracemization Reactions

Deracemization, which converts a racemate into its single enantiomer without separation of the intermediate, has gained renewed interest in asymmetric synthesis with its inherent atomic economy and high efficiency. However, this ideal process requires selective energy input and delicate reaction design to surmount the thermodynamical and kinetical constraints. With the rapid development of asymmetric catalysis, many catalytic strategies in concert with exogenous energy input have been exploited to facilitate this nonspontaneous enantioenrichment. In this perspective, we will discuss the basic ideas to accomplish catalytic deracemization, categorized by the three major exogenous energy sources including chemical (redox)-, photo- and mechanical energy from attrition. Emphasis will be given to the catalytic features and the underlying deracemization mechanism together with perspectives on future development.

Light-driven redox deracemization of indolines and tetrahydroquinolines using a photocatalyst coupled with chiral phosphoric acid

The integration of oxidation and enantioselective reduction enables a redox deracemization to directly access enantioenriched products from their corresponding racemates. However, the solution of the kinetically microscopic reversibility of substrates used in this oxidation/reduction unidirectional event is a great challenge. To address this issue, we have developed a light-driven strategy to enable an efficient redox deracemization of cyclamines. The method combines a photocatalyst and a chiral phosphoric acid in a toluene/aqueous cyclodextrin emulsion biphasic co-solvent system to drive the cascade out-of-equilibrium. Systemic optimizations achieve a feasible oxidation/reduction cascade sequence, and mechanistic investigations demonstrate a unidirectional process. This single-operation cascade route, which involves initial photocatalyzed oxidation of achiral cyclamines to cyclimines and subsequent chiral phosphoric acid-catalyzed enantioselective reduction of cyclimines to chiral cyclamines, is suitable for constructing optically pure indolines and tetrahydroquinolines.

Hydride-Abstraction-Initiated Catalytic Stereoselective Intermolecular Bond-Forming Processes

The stereoselective intermolecular bond-forming reactions through the direct manipulation of ubiquitous yet inert C(sp³)-H bonds represent an important and long-standing goal in chemistry. In particular, developing such a stereoselective bimolecular transformation involving carbocation intermediates generated via site-selective hydride abstraction or formal hydride abstraction by organic oxidants would avoid the preinstallation of directing groups and is therefore attractive. Hydride-abstraction-initiated bimolecular transformations have received considerable attention, but existing examples lack stereoselective studies. Prevalent stereoselective studies typically suffer from the narrow substrate scope of specific and highly reactive N-aryl amines and diarylmethanes together with limited synthetic utility. This Account describes our recent advances in the development and synthetic application of hydride-abstraction-initiated stereoselective intermolecular C-C and C-H bond-forming processes with significantly expanded scopes involving structurally diverse N-acyl amines and ethers together with nitriles, esters, and perfluoroalkyl moieties.We first explored hydride-abstraction-initiated stereoselective intermolecular C-C bond-forming processes. Utilizing triarylmethyl cations or oxoammonium ions as hydride abstractors, we accomplished the diastereoselective oxidative C-H functionalization of structurally diverse N-acyl amines and ethers with a range of organoboranes and C-H components, efficiently installing a series of alkyl, alkenyl, aryl, and alkynyl species into the α-position of heteroatoms with good levels of diastereocontrol. Subsequently, we developed an "acetal pool" strategy as the toolbox to regulate the stability of cationic intermediates and the compatibility of organic oxidants with a delicate asymmetric catalysis system. Utilizing this strategy, we achieved the catalytic enantioselective oxidative C-H alkenylation, arylation, alkynylation, and alkylation of diverse N-acyl heterocycles with a range of boronates and C-H components. Simultaneously, we extended this strategy to the asymmetric oxidative C-H alkylation of ethers. Notably, the method allows solvents that are used daily, such as tetrahydrofuran, tetrahydropyran, and diethyl ether, to be facilely transformed to high-value-added optically pure bioactive molecules. We further expanded the scope of this challenging area from the C(sp³)-H bond adjacent to electron-donating heteroatoms to valuable electron-withdrawing functional groups including nitriles, esters, and perfluoroalkyl moieties for the stereoselective construction of single and vicinal quaternary carbon stereocenters, respectively.We studied hydride-abstraction-initiated catalytic asymmetric intermolecular C-H bond-forming processes, known as redox deracemization. Utilizing the acetal pool strategy, we reported the first redox deracemization of cyclic benzylic ethers. Later, we disclosed an aerobic one-pot deracemization of diverse α-amino acid derivatives with excellent functional group compatibility. We further achieved the deracemization of the tertiary stereogenic center adjacent to electron-withdrawing groups including perfluoroalkyl, cyano, and ester moieties, which are otherwise difficult to construct.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Asymmetric Brønsted base-catalyzed aza-Michael addition and [3 + 2] cycloaddition reactions of N-ester acylhydrazones and enones

We reported the first enantioselective Brønsted base-catalyzed aza-Michael addition and stepwise cascade [3+2] cycloaddition between acylhydrazones and unsaturated ketones. The enantio-enriched aza-Michael adducts can be transformed into 2-pyrazolines.

Sythesis of γ-hydroxy-γ-perfluoroalkyl butenolides and exocyclic double bond butanolides via regioselective allylic alkylations of MBH carbonates with 2-perfluoroalkyl-oxazol-5(2H)-ones

Herein, two general one-pot two-step methods for the preparation of γ-hydroxyl-γ-perfluoromethyl butenolides and γ- hydroxyl-γ-perfluoromethyl exocyclic double bond butanolides from Morita−Baylis−Hillman (MBH) carbonates with 2-perfluoroalkyl-oxazol-5(2H)-ones have been reported for the...

Copper(i)-catalyzed asymmetric [3 + 2] cycloaddition of N-ester acylhydrazones and β-trifluoromethyl-α,β-unsaturated ketones

The first copper(i)-catalyzed asymmetric [3 + 2] cycloaddition of N-ester acylhydrazones and ketones has been developed, affording an array of chiral pyrazolidines containing a trifluoromethyl moiety bearing three vicinal stereocenters.

Organocatalytic asymmetric formal oxidative coupling for the construction of all-aryl quaternary stereocenters

A new catalytic asymmetric formal cross dehydrogenative coupling process for the construction of all-aryl quaternary stereocenters is disclosed, which provides access to rarely explored chiral tetraarylmethanes with excellent enantioselectivity. The suitable oxidation conditions and the hydrogen-bond-based organocatalysis have enabled efficient intermolecular C–C bond formation in an overwhelmingly crowded environment under mild conditions. para-Quinone methides bearing an ortho-directing group serve as the key intermediate. The precise loading of DDQ is critical to the high enantioselectivity. The chiral products have also been demonstrated as promising antiviral agents.

A one-pot oxidation of racemic triarylmethanes to form para-quinone methides followed by enantioselective construction of all-aryl quaternary stereocenters has been developed.

Redox deracemization of diarylmethyl alkynes

A highly effective one-pot redox deracemization of diarylmethyl alkynes has been disclosed.