Cobalt bis(acetylacetonate)-tert-butyl hydroperoxide-triethylsilane: a general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

Reversal of the Regioselectivity of Iron-Promoted Hydrogenation and Hydrohalogenation of gem-Difluoroalkenes

The reaction regioselectivity of gem-difluoroalkenes is dependent on the intrinsic polarity. Thus, the reversal of the regioselectivity of the addition reaction of gem-difluoroalkenes remains a formidable challenge. Herein, we described an unprecedented reversal of regioselectivity of hydrogen atom transfer (HAT) to gem-difluoroalkenes triggered by Fe-H species for the formation of difluoroalkyl radicals. Hydrogenation of the in situ generated radicals gave difluoromethylated products. Mechanism experiments and theoretical studies revealed that the kinetic effect of the irreversible HAT process resulted in the reversal of the regioselectivity of this scenario, leading to the formation of a less stable α-difluoroalkyl radical regioisomer. On basis of this new reaction of gem-difluoroalkene, the iron-promoted hydrohalogenation of gem-difluoroalkenes for the efficient synthesis of aliphatic chlorodifluoromethyl-, bromodifluoromethyl- and iododifluoromethyl-containing compounds was developed. Particularly, this novel hydrohalogenation of gem-difluoroalkenes provided an effect and large-scale access to various iododifluoromethylated compounds of high value for synthetic application.

Advancements in hydrochlorination of alkenes

The hydrochlorination of alkenes has been extensively studied in research and is commonly featured in organic chemistry textbooks as an exemplification of the Markovnikov rule. However, the application of this reaction is typically limited to specific alkenes, such as highly substituted ones, styrenes, or strained systems. Conversely, monosubstituted or 1,2-disubstituted alkenes do not readily react with HCl gas or solutions of HCl gas at practical rates. The challenges associated with hydrochlorination reactions for these "non-activated" alkenes have spurred considerable research efforts over the past 30 years, which constitute the primary focus of this review. The discussion begins with classical polar hydrochlorinations, followed by metal-promoted radical hydrochlorinations, and concludes with a brief overview of recent anti-Markovnikov hydrochlorinations.

Thiyl Radical Trapped by Cobalt Catalysis: An Approach to Markovnikov Thiol-Ene Reaction

In the presence of a thiyl radical species, the catalytic Markovnikov thiol-ene reaction is challenging because it prefers to proceed via a radical pathway, thereby leading to anti-Markovnikov selectivity. In this work, a rare example of thiyl radical engaged in Markovnikov thiol-ene reaction enabled by cobalt catalysis is reported. This protocol features the avoidance of unique oxidants, exclusive regioselectivity, and broad substrate scope. Scalable synthesis and late-stage modification of complex molecules demonstrate the practicability of the protocol.

Short Scalable Route to Apiaceae Sesquiterpene Scaffolds: Total Synthesis of 4-epi-Epiguaidiol A

The oxy-Cope/ene reaction cascade to form a locked elemane conformer allows the short scalable synthesis of versatile Apiaceae scaffolds. The divergent fate of the obtained macrocyclic germacrane is surveyed under cationic and dioxygen-induced Prins-type reaction conditions to allow the diastereoselective synthesis of oxidized Apiaceae guaiane congeners and the total synthesis of 4-epi-epiguaidiol A. Additionally, the unprecedented reduction of a hydrogen-bond-biased guaiane substrate permits the chemoselective synthesis of desoxo-jungiaguaiane.

An Azo Coupling-Based Chemoproteomic Approach to Systematically Profile the Tyrosine Reactivity in the Human Proteome

The tyrosine residue of proteins participates in a wide range of activities including enzymatic catalysis, protein-protein interaction, and protein-ligand binding. However, the functional annotation of the tyrosine residues on a large scale is still very challenging. Here, we report a novel method integrating azo coupling, bioorthogonal chemistry, and multiplexed proteomics to globally investigate the tyrosine reactivity in the human proteome. Based on the azo-coupling reaction between aryl diazonium salt and the tyrosine residue, two different probes were evaluated, and the probe with the best performance was employed to further study the tyrosine residues in the human proteome. Then, tagged tyrosine-containing peptides were selectively enriched using bioorthogonal chemistry, and after the cleavage, a small tag on the peptides perfectly fits for site-specific analysis by MS. Coupling with multiplexed proteomics, we quantified over 5000 tyrosine sites in MCF7 cells, and these quantified sites displayed a wide range of reactivity. The tyrosine residues with high reactivity were found on functionally and structurally diverse proteins, including those with the catalytic activity and binding property. This method can be extensively applied to advance our understanding of protein functions and facilitate the development of covalent drugs to regulate protein activity.

Cooperative Hydrogen Atom Transfer: From Theory to Applications

Hydrogen atom transfer (HAT) is one of the fundamental transformations of organic chemistry, allowing the interconversion of open- and closed-shell species through the concerted movement of a proton and an electron. Although the value of this transformation is well appreciated in isolation, with it being used for homolytic C–H activation via abstractive HAT and radical reduction via donative HAT, cooperative HAT (cHAT) reactions, in which two hydrogen atoms are removed or donated to vicinal reaction centers in succession through radical intermediates, are comparatively unknown outside of the mechanism of desaturase enzymes. This tandem reaction scheme has important ramifications in the thermochemistry of each HAT, with the bond dissociation energy (BDE) of the C–H bond adjacent to the radical center being significantly lowered relative to that of the parent alkane, allowing each HAT to be performed by different species. Herein, we discuss the thermodynamic basis of this bond strength differential in cHAT and demonstrate its use as a design principle in organic chemistry for both dehydrogenative (application 1) and hydrogenative (application 2) reactions. We hope that this overview will highlight the exciting reactivity that is possible with cHAT and inspire further developments with this mechanistic approach.

1 Introduction and Theory

2 Application: Dehydrogenative Transformations

3 Application: Alkene Hydrogenation

4 Future Applications of cHAT

Silane- and peroxide-free hydrogen atom transfer hydrogenation using ascorbic acid and cobalt-photoredox dual catalysis

Hydrogen atom transfer (HAT) hydrogenation has recently emerged as an indispensable method for the chemoselective reduction of unactivated alkenes. However, the hitherto reported systems basically require stoichiometric amounts of silanes and peroxides, which prevents wider applications, especially with respect to sustainability and safety concerns. Herein, we report a silane- and peroxide-free HAT hydrogenation using a combined cobalt/photoredox catalysis and ascorbic acid (vitamin C) as a sole stoichiometric reactant. A cobalt salophen complex is identified as the optimal cocatalyst for this environmentally benign HAT hydrogenation in aqueous media, which exhibits high functional-group tolerance. In addition to its applicability in the late-stage hydrogenation of amino-acid derivatives and drug molecules, this method offers unique advantage in direct transformation of unprotected sugar derivatives and allows the HAT hydrogenation of unprotected C-glycoside in higher yield compared to previously reported HAT hydrogenation protocols. The proposed mechanism is supported by experimental and theoretical studies.

Hydrogenation of Alkenes via Cooperative Hydrogen Atom Transfer

Radical hydrogenation via hydrogen atom transfer (HAT) to alkenes is an increasingly important transformation for the formation of thermodynamic alkane isomers. Current single-catalyst methods require stoichiometric oxidant in addition to hydride (H^-) source to function. Here we report a new approach to radical hydrogenation: cooperative hydrogen atom transfer (cHAT), where each hydrogen atom donated to the alkene arrives from a different catalyst. Further, these hydrogen atom (H•) equivalents are generated from complementary hydrogen atom precursors, with each alkane requiring one hydride (H^-) and one proton (H⁺) equivalent and no added oxidants. Preliminary mechanistic study supports this reaction manifold and shows the intersection of metal-catalyzed HAT and thiol radical trapping HAT catalytic cycles to be essential for effective catalysis. Together, this unique catalyst system allows us to reduce a variety of unactivated alkene substrates to their respective alkanes in high yields and diastereoselectivities and introduces a new approach to radical hydrogenation.

Sterically encumbered β-diketonates and base metal catalysis

Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn. All four form ML2-type complexes with typical coordination behavior for late-metal β-diketonates, however the effects on established electrochemistry and reactivity vary somewhat per metal. For example, the striking chemical and electrochemical inertness of CoII(esac)2 to oxidation and disproportionation is atypical. Conversely, the behavior of CuII(esac)2 is rather typical relative to other CuII(β-diketonate)2 complexes. These data suggest a relative disfavoring of certain reaction pathways, and represent an important step in modulating the catalysis of the base metals via sterically hindered β-diketonates.

Cobalt-catalyzed cyclization with the introduction of cyano, acyl and aminoalkyl groups

An efficient synthesis of carbo- and heterocycles using C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds under cobalt catalysis is described. The substituents on olefins are key for controlling the regio- and chemoselectivity in the initial hydrogen atom transfer step and quaternary carbons are efficiently constructed under mild conditions. Cyclopropane cleavage and tandem cyclization give highly functionalized bicyclic skeletons in a single operation.

Cobalt-Catalyzed Intermolecular Hydrofunctionalization of Alkenes: Evidence for a Bimetallic Pathway

A functional group tolerant cobalt-catalyzed method for the intermolecular hydrofunctionalization of alkenes with oxygen- and nitrogen-based nucleophiles is reported. This protocol features a strategic use of hypervalent iodine(III) reagents that enables a mechanistic shift from conventional cobalt-hydride catalysis. Key evidence was found supporting a unique bimetallic-mediated rate-limiting step involving two distinct cobalt(III) species, from which a new carbon-heteroatom bond is formed.