Enantioselective H-Atom Transfer Reaction: A Strategy to Synthesize Formaldehyde Aldol Products

Catalytic Asymmetric Hydrogen Atom Transfer Based on a Chiral Hydrogen Atom Donor Generated from TBADT and Chiral BINOL

Enantioselective radical reactions mediated by TBADT have seldom been seen due to the inherent challenges. Herein, we disclose a new chiral hydrogen atom transfer (HAT) reagent that was generated easily from 8H-BINOL, potassium carbonate, and TBADT under irradiation. The new complex 8H-BINOL/DTs could be used as a chiral H donor. A series of azaarenes could be converted into the corresponding chiral compounds via radical addition followed by enantioselective HAT.

A Type of Chiral C2-Symmetric Arylthiol Catalyst for Highly Enantioselective Anti-Markovnikov Hydroamination

The development of chiral hydrogen donor catalysts is fundamental in the expansion and innovation of asymmetric organocatalyzed reactions via an enantioselective hydrogen atom transfer (HAT) process. Herein, an unprecedented type of chiral C2-symmetric arylthiol catalysts derived from readily available enantiomeric lactate ester was developed. With these catalysts, an asymmetric anti-Markovnikov alkene hydroamination-cyclization reaction was established, affording a variety of pharmaceutically interesting 3-substituted piperidines with moderate to high enantioselectivity. Results of the designed control experiments and theoretical computation rationalized the origin of stereocontrol and disclosed the spatial effect of the moiety of chiral thiols on the enantioselectivity. We believed the facile synthesis, flexible tunability, and effective enantioselectivity-controlling capability of these catalysts would shed light on the development of versatile chiral HAT catalysts and related asymmetric reactions.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

Asymmetric synthesis of oxazolines bearing α-stereocenters through radical addition–enantioselective protonation enabled by cooperative catalysis

An efficient radical conjugate addition/enantioselective protonation process was developed for the asymmetric synthesis of chiral oxazolines bearing an α-stereocenter through cooperative photoredox catalysis and asymmetric organocatalysis.

Photoenzymatic Generation of Unstabilized Alkyl Radicals: An Asymmetric Reductive Cyclization

Flavin-dependent "ene"-reductases can generate stabilized alkyl radicals when irradiated with visible light; however, they are not known to form unstabilized radicals. Here, we report an enantioselective radical cyclization using alkyl iodides as precursors to unstabilized nucleophilic radicals. Evidence suggests this species is accessed by photoexcitation of a charge-transfer complex that forms between flavin and substrate within the protein active site. Stereoselective delivery of a hydrogen atom from the flavin semiquinone to the prochiral radical formed after cyclization provides high levels of enantioselectivity across a variety of substrates. Overall, this transformation demonstrates that photoenzymatic catalysis can address long-standing selectivity challenges in the radical literature.

Conjugate addition-enantioselective protonation reactions

The addition of nucleophiles to electron-deficient alkenes represents one of the more general and commonly used strategies for the convergent assembly of more complex structures from simple precursors. In this review the addition of diverse protic and organometallic nucleophiles to electron-deficient alkenes followed by enantioselective protonation is summarized. Reactions are first categorized by the type of electron-deficient alkene and then are further classified according to whether catalysis is achieved with chiral Lewis acids, organocatalysts, or transition metals.

Asymmetric allylic alkylation of isatin-derived Morita-Baylis-Hillman carbonates with nitroalkanes

A stereoselective allylic alkylation of isatin-derived Morita-Baylis-Hillman (MBH) carbonates with nitroalkanes has been developed. In the presence of 10 mol % β-isocupreidine (β-ICD), 3,3'-disubstituted oxindoles were prepared with moderate diastereoselectivities and excellent enantioselectivities.

Hydrogen atom abstraction selectivity in the reactions of alkylamines with the benzyloxyl and cumyloxyl radicals. The importance of structure and of substrate radical hydrogen bonding

A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(•), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO(•), where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO(•) were in all cases significantly higher than those measured for the corresponding reactions of CumO(•), and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive α-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO(•))/k(H)(CumO(•)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(•) were described in all cases as direct hydrogen atom abstractions. With BnO(•) the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical α-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate α-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.

Pyrones to pyrans: enantioselective radical additions to acyloxy pyrones

This paper describes a highly site-, diastereo-, and enantioselective intermolecular radical addition/hydrogen atom transfer to hydroxypyrone pyromeconic and kojic acids. The methodology can be extended to the formation of chiral quaternary centers. The products obtained are densely functionalized pyran moieties. The products contain structural features amenable for the introduction of additional substituents.