δ-Selective Functionalization of Alkanols Enabled by Visible-Light-Induced Ligand-to-Metal Charge Transfer

Benzylic C–H heteroarylation of N-(benzyloxy)phthalimides with cyanopyridines enabled by photoredox 1,2-hydrogen atom transfer

Visible light initiated α-C(sp³)–H hetroarylation of N-(benzyloxy)phthalimides with cyanopyridines via 1,2-hydrogen atom transfer is depicted.

Bisphosphonium salt: an effective photocatalyst for the intramolecular hydroalkoxylation of olefins

A photocatalytic, intramolecular hydroalkoxylation of alkenes has been achieved by utilizing simple and readily available biphosphonium salts as an effective organic photocatalyst. The unique photocatalytic properties of biphosphonium salt enables facile oxidations of substituted alkenes to facilitate the streamline and regioselective synthesis of a series of saturated cyclic ethers from simple alkenes under mild conditions.

Visible-Light-Induced Alkoxyl Radicals Enable α-C(sp3)-H Bond Allylation

The alkoxyl radical is an essential reactive intermediate in mechanistic studies and organic synthesis with hydrogen atom transfer (HAT) reactivity. However, compared with intramolecular 1,5-HAT or intermolecular HAT of alkoxyl radicals, the intramolecular 1,2-HAT reactivity has been limited to theoretical studies and rarely synthetically utilized. Here we report the first selective 1,2-HAT of alkoxyl radicals for α-C(sp³)-H bond allylation of α-carbonyl, α-cyano, α-trifluoromethyl, and benzylic N-alkoxylphthalimides. The mechanistic probing experiments, electron paramagnetic resonance (EPR) studies, and density functional theory (DFT) calculations confirmed the 1,2-HAT reactivity of alkoxyl radicals, and the use of protic solvents lowered the activation energy by up to 10.4 kcal/mol to facilitate the α-C(sp³)-H allylation reaction.

Photocatalytic carbanion generation from C-H bonds - reductant free Barbier/Grignard-type reactions

We report a redox-neutral method for the generation of carbanions from benzylic C-H bonds in a photocatalytic Grignard-type reaction. The combination of photo- and hydrogen atom transfer (HAT) catalysis enables the abstraction of a benzylic hydrogen atom, generating a radical intermediate. This radical is reduced in situ by the organic photocatalyst to a carbanion, which is able to react with electrophiles such as aldehydes or ketones, yielding homobenzylic secondary and tertiary alcohols.

Light-Driven Intramolecular C-N Cross-Coupling via a Long-Lived Photoactive Photoisomer Complex

Reported herein is a visible-light-driven intramolecular C-N cross-coupling reaction under mild reaction conditions (metal- and photocatalyst-free, at room temperature) via a long-lived photoactive photoisomer complex. This strategy was used to rapidly prepare the N-substituted polycyclic quinazolinone derivatives with a broad substrate scope (>50 examples) and further exploited to synthesize the natural products tryptanthrin, rutaecarpine, and their analogues. The success of gram-scale synthesis and solar-driven transformation, as well as promising tumor-suppressing biological activity, proves the potential of this strategy for practical applications. Mechanistic investigations, including control experiments, DFT calculations, UV-vis spectroscopy, EPR, and X-ray single-crystal structure of the key intermediate, provides insight into the mechanism.

Remote C(sp3 )-H Arylation and Vinylation of N-Alkoxypyridinium Salts to δ-Aryl and δ-Vinyl Alcohols

The reaction of readily available and bench-stable N-alkoxypyridinium salts with arylboronic and vinylboronic acids afforded δ-aryl and δ-vinyl alcohols, respectively, in the presence of fac-Ir(ppy)₃ and Cu(OTf)₂ dual catalysts. The reaction takes place through a domino process involving the reductive generation of alkoxyl radicals, 1,5-hydrogen atom transfer (1,5-HAT) and the copper-catalyzed cross-coupling reaction of the resulting translocated carbon radicals with boronic acids. Complementary to the Minisci reaction, this method allows for the arylation of nucleophilic alkyl radicals with both electron-rich and electron-poor arenes under mild reaction conditions.

Oxazoline-/Copper-Catalyzed Alkoxyl Radical Generation: Solvent-Switched to Access 3a,3a'-Bisfuroindoline and 3-Alkoxyl Furoindoline

We report the first example of oxazoline-/copper-catalyzed alcohol oxidation to generate the alkoxyl radical under additive-free conditions. The resulting alkoxyl radical addition to alkene enables useful C-O bond-forming and selective C(sp³)-C(sp³) radical-radical dimerization/radical-trapping reactions, providing direct access to the 3a,3a'-bisfuro[2,3-b]indoline scaffold for the first time and a wide range of 3-alkoxyl furoindolines with high efficiency.

γ-Alkylation of Alcohols Enabled by Visible-Light Induced 1,6-Hydrogen Atom Transfer

Site-selective remote alkylation of alcohol is attractive but challenging in organic synthesis. Herein, we report a novel visible-light mediated γ-alkylation of alcohol derivatives via the formation of C_sp3-C_sp3 bond through C_sp3-H bond functionalization under mild conditions. The use of sulfamate esters enables the directed, otherwise rare 1,6-HAT to generate γ-selective C-centered radical, which is complementary to δ-selective 1,5-HAT of alcohols. This redox-neutral protocol provides a general and operationally simple method to access γ-alkylated alcohols.

Sulfamyl Radicals Direct Photoredox-Mediated Giese Reactions at Unactivated C(3)-H Bonds

Alcohol-anchored sulfamate esters guide the alkylation of tertiary and secondary aliphatic C(3)-H bonds. The transformation proceeds directly from N-H bonds with a catalytic oxidant, a contrast to prior methods which have required preoxidation of the reactive nitrogen center, or employed stoichiometric amounts of strong oxidants to obtain the sulfamyl radical. These sulfamyl radicals template otherwise rare 1,6-hydrogen-atom transfer (HAT) processes via seven-membered ring transition states to enable C(3)-H functionalization during Giese reactions.

Visible-Light-Promoted Ring-Opening Alkynylation, Alkenylation, and Allylation of Cyclic Hemiacetals through β-Scission of Alkoxy Radicals

The alkoxy radicals that are derived from cyclic hemiacetals have been generated through the visible-light-promoted reaction of the corresponding N-alkoxyphthalimides with Hantzsch ester as the reductant. The alkoxy radicals subsequently undergo β-scission of the C-C bond to generate carbon-centered radicals, which are trapped by alkynyl-, alkenyl-, or allylsulfones.

Homogeneous visible light mediated transition metal catalysis other than Ruthenium and Iridium

The field of photoredox chemistry is dominated by ruthenium- or iridium based metal complexes or organic dyes that are employed as catalysts. Other metal based coordination compounds provide a cost efficient alternative, however, the much shorter excited lifetimes generally observed for such complexes make their application more challenging. Nevertheless, a growing number of successful examples with metal complexes based on chromium, iron, nickel, zirconium, cerium, rhenium, platinum, uranium, and especially on copper exist, which is being reviewed in this chapter.

Transition-Metal- and Light-Free Directed Amination of Remote Unactivated C(sp3)-H Bonds of Alcohols

Due to the great value of amino alcohols, new methods for their synthesis are in high demand. Abundant aliphatic alcohols represent the ideal feedstock for the method development toward this important motif. To date, transition-metal-catalyzed approaches for the directed remote amination of alcohols have been well established. Yet, they have certain disadvantages such as the use of expensive catalysts and limited scope. Very recently, transition-metal-free visible-light-induced radical approaches have emerged as new powerful tools for directed remote amination of alcohols. Relying on 1,5-HAT reactivity, these methods are limited to β - or δ-amination only. Herein, we report a novel transition-metal- and visible-light-free room-temperature radical approach for remote β -, γ-, and δ-C(sp3)-N bond formation in aliphatic alcohols using mild basic conditions and readily available diazonium salt reagents.

Iron-Catalysed Remote C(sp3 )-H Azidation of O-Acyl Oximes and N-Acyloxy Imidates Enabled by 1,5-Hydrogen Atom Transfer of Iminyl and Imidate Radicals: Synthesis of γ-Azido Ketones and β-Azido Alcohols

In the presence of a catalytic amount of iron(III) acetylacetonate [Fe(acac)₃ ], the reaction of structurally diverse ketoxime esters with trimethylsilyl azide (TMSN₃ ) afforded γ-azido ketones in good to excellent yields. This unprecedented distal γ-C(sp³ )-H bond azidation reaction went through a sequence of reductive generation of an iminyl radical, 1,5-hydrogen atom transfer (1,5-HAT) and iron-mediated redox azido transfer to the translocated carbon radical. TMSN₃ served not only as a nitrogen source to functionalise the unactivated C(sp³ )-H bond, but also as a reductant to generate the catalytically active Fe^II species in situ. Based on the same principle, a novel β-C(sp³ )-H functionalisation of alcohols via N-acyloxy imidates was subsequently realised, leading, after hydrolysis of the resulting ester, to β-azido alcohols, which are important building blocks in organic and medicinal chemistry.

Remote Allylation of Unactivated C(sp3)-H Bonds Triggered by Photogenerated Amidyl Radicals

The allylation reaction is a highly versatile transformation in chemical synthesis. While many elegant direct C(sp²)-H allylation reactions have been developed, the direct allylation of unactivated C(sp³)-H bonds is underdeveloped. By applying photoredox catalysis and a [1,5]-HAT process, herein we report a direct allylation of unactivated C(sp³)‒H bonds. This photocatalyzed transformation is tolerant of several functional groups in the amide and allylic chloride substrates. Various allyl-substituted amide products were obtained with good yields and high δ-selectivity.

Modular Synthesis of Alkylarylazo Compounds via Iron(III)-Catalyzed Olefin Hydroamination

A novel Fe-catalyzed olefin hydroamination with aryldiazo sulfones for accessing alkylarylazo compounds has been successfully developed. Aryldiazo sulfones are used as radical acceptors, and N-N double bonds will be regenerated when an arene sulfonyl group leaves. The reaction features mild reaction conditions and a broad substrate scope, allowing access to many azo compounds that would be difficult or perhaps impossible to access using other methods.

Decarboxylative hydrazination of unactivated carboxylic acids by cerium photocatalysis

We report the cerium photocatalyzed radical decarboxylative hydrazination of carboxylic acids with di-tert-butylazodicarboxylate (DBAD). The operationally simple protocol provides rapid access to synthetically useful hydrazine derivatives and overcomes current scope limitations in the photoredox-catalyzed decarboxylation of carboxylic acids.

Tuning the Photophysical and Excited State Properties of Phosphorescent Iridium(III) Complexes by Polycyclic Unit Substitution

Two novel N-embedded polycyclic units functionalized phosphorescent iridium(III) complexes (Ir-1 and Ir-2) with substituents in different positions have been prepared. Complex Ir-1 bearing the substituent at the 3-position shows a distinct blue shift single-peak emission (524 nm) with a higher luminescence efficiency (ΦPL=42 %) and shorter emission lifetime (τ=282 ns) by comparison with 4-position substitution based complex Ir-2 (ΦPL=23 %, τ=562 ns), which exhibits a dual-peak emission (564 nm and 602 nm), and phosphorescence color can be tuned from green to yellow. In addition, DFT calculations demonstrate that unusual ligand-to-metal charge transfer (3LMCT) excited state property can be found in Ir-2, which is in contrast to metal-to-ligand charge transfer (3MLCT) excited state character in Ir-1. This result can be attribute to strong electron-donating character and 4-position substitution effect of the unit.

Generation of Dithianyl and Dioxolanyl Radicals Using Photoredox Catalysis: Application in the Total Synthesis of the Danshenspiroketallactones via Radical Relay Chemistry

Visible-light-induced generation of dithianyl and dioxolanyl radicals via selective hydrogen atom transfer (HAT) has been achieved. This radical relay tactic enables remote C(sp³)-H functionalization to permit rapid access to polyol and spiroketal segments, and in turn has been exploited as a key synthetic construct in the total synthesis of the danshenspiroketallactones. The conformational stability of the danshenspiroketallactones has also been defined via experiments and DFT calculations.

Alkylation of Allyl/Alkenyl Sulfones by Deoxygenation of Alkoxyl Radicals

A challenging deoxygenation of alkoxyl radicals from readily accessible alcohol derivatives was developed, affording facile synthesis of functionalized alkenes with good functional group tolerance under mild reaction conditions. Because alkoxyl radicals can easily undergo β-fragmentations or hydrogen abstractions, this new strategy for deoxygenation of alkoxyl radicals is highly valuable. Moreover, mechanistic studies revealed that the electron-neutral phosphine acts as the deoxygenation reagent.

Visible light-induced direct α C-H functionalization of alcohols

Considering the synthetic value of introducing active alcoholic hydroxyl group, developing C-H functionalization of alcohols is of significance. Herein, we present a photochemical method that under visible light irradiation, selectfluor can effectively promote the oxidative cross-coupling between alcohols and heteroarenes without the external photocatalysis, achieving the selective α sp³ C-H arylation of alcohol, even in the presence of ether. The N-F activation of selectfluor under blue LEDs irradiation is evidenced by electron paramagnetic resonance (EPR) study, which is the key process for the oxidative activation of α sp³ C-H alcohols. The observed reactivity may have significant implications for chemical transformations.

Photoredox-mediated remote C(sp3)-H heteroarylation of free alcohols

We report an efficient and economical method for remote δ C(sp3)-H heteroarylation of free aliphatic alcohols using a hypervalent iodine PFBI-OH oxidant under photoredox catalysis. The reaction sequence involves in situ alcoholysis of PFBI-OH with alcohol, generation of an alkoxy radical intermediate by SET reduction, 1,5-HAT, and Minisci-type C-C bond formation. This method uses a slight excess of alcohols, can facilitate reaction at δ methyl and methylene positions, and has been successfully applied to modification of complex drug molecules.

Catalytic β C-H amination via an imidate radical relay

An iodine-catalyzed strategy for β C–H amination of alcohols is enabled by a chemo-, regio-, and stereo-selective H-atom transfer mechanism.

The first catalytic strategy to harness imidate radicals for C–H functionalization has been developed. This iodine-catalyzed approach enables β C–H amination of alcohols by an imidate-mediated radical relay. In contrast to our first-generation, (super)stoichiometric protocol, this catalytic method enables faster and more efficient reactivity. Furthermore, lower oxidant concentration affords broader functional group tolerance, including alkenes, alkynes, alcohols, carbonyls, and heteroarenes. Mechanistic experiments interrogating the electronic nature of the key 1,5 H-atom transfer event are included, as well as probes for chemo-, regio-, and stereo-selectivity.

Visible-light mediated C-C bond cleavage of 1,2-diols to carbonyls by cerium-photocatalysis

We describe a photocatalytic method for the cleavage of vicinal diols to aldehydes and ketones. The reaction is catalyzed by blue light and a cerium-catalyst and the scope includes aryl as well as alkyl substituted diols. The simple protocol which works under air and at room temperature enables the valorization of abundant diols.

Transition-metal-free Intramolecular C–H amination of sulfamate esters and N-alkylsulfamides

The transition-metal-free intramolecular C–H amination of sulfamate esters and N-alkylsulfamides using iodine oxidants, tert-butyl hypoiodite (t-BuOI) and N-iodosuccinimide (NIS) is reported.

Minisci C–H alkylation of N-heteroarenes with aliphatic alcohols via β-scission of alkoxy radical intermediates

A generally applicable method for Minisci C–H alkylation of N-heteroarenes with aliphatic alcohols via β-scission of alkoxy radical intermediates under photoredox-catalyzed conditions.

Application of dialkyl azodicarboxylate frameworks featuring multi-functional properties

The application of dialkyl azodicarboxylates as versatile reagents in Mitsunobu, oxidation, electrophilic, amination and carbonylation reactions is reviewed.

Recent advances in alkoxy radical-promoted C–C and C–H bond functionalization starting from free alcohols

This feature article summarizes our recent achievements in alkoxy radical-promoted C–C and C–H bond functionalization starting from free alcohols.

A strategy to improve the performance of cerium(iii) photocatalysts

A structural modification strategy to improve the photocatalytic performance of molecular cerium(iii) luminophores was demonstrated.

Lanthanide Photocatalysis

The use of earth-abundant, cheap, potent, and readily available lanthanide photocatalysts provides an opportunity to complement or even replace rare and precious metal photosensitizers. Moreover, lanthanide photosensitizers have been demonstrated for the generation of a variety of reactive species, including aryl radicals, alkyl radicals, and others, by single-electron-transfer (SET) and hydrogen atom transfer (HAT) pathways under mild reaction conditions. Some lanthanide photocatalysts have unprecedented reducing power from their photoexcited states, achieving the activation of challenging organic substrates that have not otherwise been activated by reported organic or transition-metal photosensitizers. In this Account, we describe our recent advances in the rational design and strategic application of lanthanide photo(redox)catalysis. Our research goals include understanding the photophysics of lanthanide luminophores and incorporating them into new photocatalysis. Among the lanthanides, we have focused on cerium because of the doublet to doublet 4f → 5d excitation and emission, which affords good conservation of energy without losses through spin-state changes, as well as a large natural abundance of that element. We have performed structural, spectroscopic, computational, and reactivity studies to demonstrate that luminescent Ce(III) guanidinate-amide complexes can mediate photocatalytic C(sp³)-C(sp³) bond forming reactions. Taking advantage of the strong reducing power of the cerium excited states and the cerium-halogen bond forming enthalpies, we determined that the reactive, excited-state cerium metalloradical abstracts chloride anion from benzyl chloride to generate the benzyl radical. To control and predict the photocatalytic reactivities, we have also performed photophysical and photochemical studies on a series of mixed-ligand Ce(III) guanidinate-amide and guanidinate-aryloxide complexes to establish structure-property relationship for Ce(III) photocatalysts. We discovered that the emission color is directly related to ligand type and rigidity of the coordination sphere and that the photoluminescent quantum yield is correlated to variation in steric encumbrance around the cerium centers. The low excited-state reduction potentials ( E_1/2^* ≈ -2.1 to -2.9 V versus Cp₂Fe^0/+) and relatively fast quenching rates ( k_q ≈ 10⁷ M^-1 s^-1) toward aryl halides enabled the Ce(III) guanidinate-amide complexes to participate in photocatalytic C(sp²)-C(sp²) bond forming reactions through either inner-sphere or outer-sphere SET processes. We have also reported a simple, potent, and air-stable ultraviolet A photoreductant, the hexachlorocerate(III) anion ([Ce^IIICl₆]^3-). This complex is a potent photoreductant ( E_1/2^* ≈ -3.45 V versus Cp₂Fe^0/+) and exhibits a fast quenching kinetics ( k_q ≈ 10⁹-10¹⁰ M^-1 s^-1) toward organohalogens. The [Ce^IVCl₆]^2- redox partner can also act as a potent photo-oxidant though a (presumably) long-lived chloride-to-cerium(IV) charge transfer excited state (ε = ∼6000 M^-1 cm^-1), that was used to turnover photocatalytic dechlorination and Miyuara borylation reactions. With [Ce^IIICl₆]^3-, we achieved efficient photoinduced dehalogenation and borylation of unactivated aryl chlorides with broad substrate scope, through formally two-photon cycles where both Ce(III) and Ce(IV) act as photocatalysts. Lanthanide photoredox catalysis is now being applied in several contexts for reactions including photocatalytic dehydrogenation of amines, alkoxy-radical-mediated C-C bond cleavage and amination of alkanols, and C-H activation of alkanes. Overall, simple and potent lanthanide photocatalysis is expected to find practical applications in organic synthesis, pharmaceutical development, and small molecule activation.