Copper-Catalyzed Enantioselective C(sp3 )-SCF3 Coupling of Carbon-Centered Benzyl Radicals with (Me4 N)SCF3.

In contrast with the well-established C(sp2 )-SCF3 cross-coupling to forge the Ar-SCF3 bond, the corresponding enantioselective coupling of readily available alkyl electrophiles to forge chiral C(sp3 )-SCF3 bond has remained largely unexplored. We herein disclose a copper-catalyzed enantioselective radical C(sp3 )-SCF3 coupling of a range of secondary/tertiary benzyl radicals with the easily available (Me4 N)SCF3 reagent. The key to the success lies in the utilization of chiral phosphino-oxazoline-derived anionic N,N,P-ligands through tuning electronic and steric effects for the simultaneous control of the reaction initiation and enantioselectivity. This strategy can successfully realize two types of asymmetric radical reactions, including enantioconvergent C(sp3 )-SCF3 cross-coupling of racemic benzyl halides and three-component 1,2-carbotrifluoromethylthiolation of arylated alkenes under mild reaction conditions. It therefore provides a highly flexible platform for the rapid assembly of an array of enantioenriched SCF3 -containing molecules of interest in organic synthesis and medicinal chemistry.

A general copper-catalysed enantioconvergent C(sp3)-S cross-coupling via biomimetic radical homolytic substitution.

Although α-chiral C(sp3)-S bonds are of enormous importance in organic synthesis and related areas, the transition-metal-catalysed enantioselective C(sp3)-S bond construction still represents an underdeveloped domain probably due to the difficult heterolytic metal-sulfur bond cleavage and notorious catalyst-poisoning capability of sulfur nucleophiles. Here we demonstrate the use of chiral tridentate anionic ligands in combination with Cu(I) catalysts to enable a biomimetic enantioconvergent radical C(sp3)-S cross-coupling reaction of both racemic secondary and tertiary alkyl halides with highly transformable sulfur nucleophiles. This protocol not only exhibits a broad substrate scope with high enantioselectivity but also provides universal access to a range of useful α-chiral alkyl organosulfur compounds with different sulfur oxidation states, thus providing a complementary approach to known asymmetric C(sp3)-S bond formation methods. Mechanistic results support a biomimetic radical homolytic substitution pathway for the critical C(sp3)-S bond formation step.

Kinetics of H· Transfer from CpCr(CO)3H to Various Enamides: Application to Construction of Pyrrolidines.

The rate constants kH (kD) have been determined at 27 °C for H· (D·) transfer from CpCr(CO)3H(D) to the C=C bonds of various enamides. This process leads to the formation of α-amino radicals. Vinyl enamides with N-alkyl and N-phenyl substituents have proven to be good H· acceptors, with rate constants close to those of styrene and methyl methacrylate. A methyl substituent on the incipient radical site decreases kH by a factor of 4; a methyl substituent on the carbon that will receive the H· decreases kH by a factor of 380. The measured kH values indicate that these α-amino radicals can be used for the cyclization of enamides to pyrrolidines. A vanadium hydride, HV(CO)4(dppe), has proven more effective at the cyclization of enamides than Cr or Co hydrides-presumably because the weakness of the V-H bond leads to faster H· transfer. The use of the vanadium hydride is operationally simple, employs mild reaction conditions, and has a broad substrate scope. Calculations have confirmed that H· transfer is the slowest step in these cyclization reactions.

Cu(I)-Catalyzed Chemo- and Enantioselective Desymmetrizing C-O Bond Coupling of Acyl Radicals.

Transition-metal-catalyzed enantioselective functionalization of acyl radicals has so far not been realized, probably due to their relatively high reactivity, which renders the chemo- and stereocontrol challenging. Herein, we describe Cu(I)-catalyzed enantioselective desymmetrizing C-O bond coupling of acyl radicals. This reaction is compatible with (hetero)aryl and alkyl aldehydes and, more importantly, displays a very broad scope of challenging alcohol substrates, such as 2,2-disubstituted 1,3-diols, 2-substituted-2-chloro-1,3-diols, 2-substituted 1,2,3-triols, 2-substituted serinols, and meso primary 1,4-diols, providing enantioenriched esters characterized by challenging acyclic tetrasubstituted carbon stereocenters. Partnered by one- or two-step follow-up transformations, this reaction provides a convenient and practical strategy for the rapid preparation of chiral C3 building blocks from readily available alcohols, particularly the industrially relevant glycerol. Mechanistic studies supported the proposed C-O bond coupling of acyl radicals.

Enantioconvergent Cu-catalysed N-alkylation of aliphatic amines.

Chiral amines are commonly used in the pharmaceutical and agrochemical industries1. The strong demand for unnatural chiral amines has driven the development of catalytic asymmetric methods1,2. Although the N-alkylation of aliphatic amines with alkyl halides has been widely adopted for over 100 years, catalyst poisoning and unfettered reactivity have been preventing the development of a catalyst-controlled enantioselective version3-5. Here we report the use of chiral tridentate anionic ligands to enable the copper-catalysed chemoselective and enantioconvergent N-alkylation of aliphatic amines with α-carbonyl alkyl chlorides. This method can directly convert feedstock chemicals, including ammonia and pharmaceutically relevant amines, into unnatural chiral α-amino amides under mild and robust conditions. Excellent enantioselectivity and functional-group tolerance were observed. The power of the method is demonstrated in a number of complex settings, including late-stage functionalization and in the expedited synthesis of diverse amine drug molecules. The current method indicates that multidentate anionic ligands are a general solution for overcoming transition-metal-catalyst poisoning.

Cu-catalysed enantioselective radical heteroatomic S-O cross-coupling.

The transition-metal-catalysed cross-coupling reaction has established itself as one of the most reliable and practical synthetic tools for the efficient construction of carbon-carbon/heteroatom (p-block elements other than carbon) bonds in both racemic and enantioselective manners. In contrast, development of the corresponding heteroatom-heteroatom cross-couplings has so far remained elusive, probably due to the under-investigated and often challenging heteroatom-heteroatom reductive elimination. Here we demonstrate the use of single-electron reductive elimination as a strategy for developing enantioselective S-O coupling under Cu catalysis, based on both experimental and theoretical results. The reaction manifests its synthetic potential by the ready preparation of challenging chiral alcohols featuring congested stereocentres, the expedient valorization of the biomass-derived feedstock glycerol, and the remarkable catalytic 4,6-desymmetrization of inositol. These results demonstrate the potential of enantioselective radical heteroatomic cross-coupling as a general chiral heteroatom-heteroatom formation strategy.

Mechanism-based ligand design for copper-catalysed enantioconvergent C(sp3)-C(sp) cross-coupling of tertiary electrophiles with alkynes.

In contrast with the well-established enantioconvergent radical C(sp3)-C cross-coupling of racemic secondary alkyl electrophiles, the corresponding coupling of tertiary electrophiles to forge all-carbon quaternary stereocentres remains underexplored. The major challenge arises from the steric hindrance and the difficult enantio-differentiation of three distinct carbon substituents of prochiral tertiary radicals. Here we demonstrate a general copper-catalysed enantioconvergent C(sp3)-C(sp) cross-coupling of diverse racemic tertiary alkyl halides with terminal alkynes (87 examples). Key to the success is the rational design of chiral anionic N,N,N-ligands tailor-made for the computationally predicted outer-sphere radical group transfer pathway. This protocol provides a practical platform for the construction of chiral C(sp3)-C(sp/sp2/sp3) bonds, allowing for expedient access to an array of synthetically challenging quaternary carbon building blocks of interest in organic synthesis and related areas.

Design of Hemilabile N,N,N-Ligands in Copper-Catalyzed Enantioconvergent Radical Cross-Coupling of Benzyl/Propargyl Halides with Alkenylboronate Esters.

The enantioconvergent radical C(sp3)-C(sp2) cross-coupling of alkyl halides with alkenylboronate esters is an appealing tool in the assembly of synthetically valuable enantioenriched alkenes owing to the ready availability, low toxicity, and air/moisture stability of alkenylboronate esters. Here, we report a copper/chiral N,N,N-ligand catalytic system for the enantioconvergent cross-coupling of benzyl/propargyl halides with alkenylboronate esters (>80 examples) with good functional group tolerance. The key to the success is the rational design of hemilabile N,N,N-ligands by mounting steric hindrance at the ortho position of one coordinating quinoline ring. Thus, the newly designed ligand could not only promote the radical cross-coupling process in the tridentate form but also deliver enantiocontrol over highly reactive alkyl radicals in the bidentate form. Facile follow-up transformations highlight its potential utility in the synthesis of various enantioenriched building blocks as well as in the late-stage functionalization for drug discovery.

An Exception to the Carothers Equation Caused by the Accelerated Chain Extension in a Pd/Ag Cocatalyzed Cross Dehydrogenative Coupling Polymerization.

The Carothers equation is often used to predict the utility of a small molecule reaction in a polymerization. In this study, we present the mechanistic study of Pd/Ag cocatalyzed cross dehydrogenative coupling (CDC) polymerization to synthesize a donor-acceptor (D-A) polymer of 3,3'-dihexyl-2,2'-bithiophene and 2,2',3,3',5,5',6,6'-octafluorobiphenyl, which go counter to the Carothers equation. It is uncovered that the second chain extension cross-coupling proceeds much more efficiently than the first cross-coupling and the homocoupling side reaction (at least 1 order of magnitude faster) leading to unexpectedly low homocoupling defects and high molecular weight polymers. Kinetic analyses show that C-H bond activation is rate-determining in the first cross-coupling but not in the second cross-coupling. Based on DFT calculations, the high cross-coupling rate in the second cross-coupling was ascribed to the strong Pd-thiophene interaction in the Pd-mediated C-H bond activation transition state, which decreases the energy barrier of the Pd-mediated C-H bond activation. These results have implications beyond polymerizations and can be used to ease the synthesis of a wide range of molecules where C-H bond activation may be the limiting factor.

Asymmetric dearomatization catalysed by chiral Brønsted acids via activation of ynamides.

Chiral Brønsted acid-catalysed asymmetric synthesis has received tremendous interest over the past decades, and numerous efficient synthetic methods have been developed based on this approach. However, the use of chiral Brønsted acids in these reactions is mostly limited to the activation of imine and carbonyl moieties, and the direct activation of carbon-carbon triple bonds has so far not been invoked. Here we show that chiral Brønsted acids enable the catalytic asymmetric dearomatization reactions of naphthol-, phenol- and pyrrole-ynamides by the direct activation of alkynes. This method leads to the practical and atom-economic construction of various valuable spirocyclic enones and 2H-pyrroles that bear a chiral quaternary carbon stereocentre in generally good-to-excellent yields with excellent chemo-, regio- and enantioselectivities. The activation mode of chiral Brønsted acid catalysis revealed in this study is expected to be of broad utility in catalytic asymmetric reactions that involve ynamides and the related heteroatom-substituted alkynes.

Carboxylate breaks the arene C-H bond via a hydrogen-atom-transfer mechanism in electrochemical cobalt catalysis.

Combined computational and experimental studies elucidated the distinctive mechanistic features of electrochemical cobalt-catalyzed C-H oxygenation. A sequential electrochemical-chemical (EC) process was identified for the formation of an amidylcobalt(iii) intermediate. The synthesis, characterization, cyclic voltammetry studies, and stoichiometric reactions of the related amidylcobalt(iii) intermediate suggested that a second on-cycle electro-oxidation occurs on the amidylcobalt(iii) species, which leads to a formal Co(iv) intermediate. This amidylcobalt(iv) intermediate is essentially a cobalt(iii) complex with one additional single electron distributed on the coordinating heteroatoms. The radical nature of the coordinating pivalate allows the formal Co(iv) intermediate to undergo a novel carboxylate-assisted HAT mechanism to cleave the arene C-H bond, and a CMD mechanism could be excluded for a Co(iii/i) catalytic scenario. The mechanistic understanding of electrochemical cobalt-catalyzed C-H bond activation highlights the multi-tasking electro-oxidation and the underexplored reaction channels in electrochemical transition metal catalysis.

A diquat-containing macrocyclic anion acceptor in pure water.

By taking advantage of a newly developed self-assembly approach based on oxime condensation, we successfully obtained a tetracationic macrocycle bearing two diquats in acidic aqueous media in a high yield. The ring is able to accommodate HPO42- and HCO3- in pure water.

C-H Acidity and Arene Nucleophilicity as Orthogonal Control of Chemoselectivity in Dual C-H Bond Activation.

We discovered a cooperative gold/silver catalysis mechanism in the oxidative cross-coupling reaction between 1,2,4,5-tetrafluorobenzene and N-TIPS-indole, using DFT calculations. A silver(I)-catalyzed CMD mechanism is responsible for the C-H activation of 1,2,4,5-tetrafluorobenzene, and C-H acidity determines the chemoselectivity. A gold(III)-catalyzed SE2Ar mechanism is responsible for the C3-H activation of N-TIPS-indole, and arene nucleophilicity determines the chemo- and regioselectivity. The orthogonal chemoselectivity control provides a mechanistic guide for dual C-H activation reactions.

Cu/Chiral Phosphoric Acid-Catalyzed Asymmetric Three-Component Radical-Initiated 1,2-Dicarbofunctionalization of Alkenes.

An asymmetric intermolecular, three-component radical-initiated dicarbofunctionalization of 1,1-diarylalkenes with diverse carbon-centered radical precursors and electron-rich heteroaromatics by a copper(I) and chiral phosphoric acid cooperative catalysis strategy has been developed, providing straightforward access to chiral triarylmethanes bearing quaternary all-carbon stereocenters with high efficiency as well as excellent chemo- and enantioselectivity. The key to success is not only the introduction of a sterically demanding chiral phosphoric acid to favor radical difunctionalization over the otherwise remarkable side reactions but also the in situ generation of carbocation intermediates from benzylic radical to realize asymmetric induction with the aid of a removable hydroxy directing group via cooperative interactions with chiral phosphate. Density functional theory calculations elucidated the critical chiral environment created by the hydrogen-bonding and ion-pair interactions between the chiral phosphoric acid catalyst and substrates, which leads to the enantioselective C-C bond formation.

Temperature-dependent self-assembly of a purely organic cage in water.

Using a novel dynamic covalent approach relying on reversible hydrazone formation, a purely organic 3-dimensional prismatic cage was developed in water at elevated temperatures. By lowering the temperature, the hydrazone bond becomes kinetically inert. This self-assembled cage acts as an effective receptor for donor-acceptor pairs, whose interactions are weak in the absence of the cage.

[Harmine induces apoptosis in human SGC-7901 cells].

OBJECTIVE: [corrected] To investigate the effect of apoptosis induced in human SGC-7901 cells by Harmine. METHODS: The effect of Harmine on human SGC-7901 cell survival and apoptosis was determined by MTT assay, light microscopy and flow cytometry. Cell genomic DNA was detected by agarose electrophoresis. RESULTS: The survival of human SGC-7901 cells decreased; Apoptotic cells were observed by fluorescent microscope; FCM analysis showed that the peak of apoptosis increased. Typical DNA Ladder were detected in DNA agarose gel electrophoresis. CONCLUSION: HM can induce apoptosis in human SGC-7901 cells.