Photoinduced Hydrogen Atom Transfer Research Articles

Synthesis of β-Silyl Amines via Merging Photoinduced Energy and Hydrogen Atom Transfer in Flow.

The development of efficient methods for synthesizing β-silyl amines has long been a significant goal in organic synthesis. Previous methods mainly relied on the use of prefunctionalized substrates or special reagents. Herein, we present a visible-light-promoted synthesis approach for β-silyl amines, utilizing a combination of photoinduced energy and hydrogen atom transfer processes. Using flow chemistry technology, a variety of valuable skeletons, including β-silyl amines and α-amino esters, can be produced from readily available feedstocks such as hydrosilanes and simple alkanes. Moreover, the strategy's full-process fluidized production capability highlights its potential for industrial-scale manufacturing. Mechanistic studies revealed that oxime esters can act as radical precursors as well as hydrogen atom transfer reagents.

Zwitterionic Acridinium Amidate: A Nitrogen‐Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer

AbstractThe development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C−H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited‐state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C−H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen‐bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Zwitterionic Acridinium Amidate: A Nitrogen-Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer.

The development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C-H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited-state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C-H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen-bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Covalent Organic Frameworks Based Photoenzymatic Nano-reactor for Asymmetric Dynamic Kinetic Resolution of Secondary Amines.

Bio-catalysis represents a highly efficient and stereoselective method for the synthesis of valuable chiral compounds, however, the poor stability and limited reaction types of free enzymes restrict their wide application in industrial production. In this work, to overcome these problems, a multifunctional photoenzymatic nanoreactor CALB@COF-Ir was developed through the encapsulation of Candida antarctica lipase B (CALB) in a photosensitive covalent organic framework COF-Ir. This bio-nanocluster serves as efficient catalysts in asymmetric dynamic kinetic resolution (DKR) of secondary amines to give a series of chiral amines in high yields (up to 99 %) and enantioselectivities (up to 99 % ee). The well-designed COF-Ir not only acts as safety cover to prevent CALB from deactivation but promotes racemization of secondary amines via photo-induced hydrogen atom transfer (HAT) process. Photoelectric characterization and TDDFT calculation revealed that (ppy)2Ir units in COF-Ir play crucial role in this photocatalytic system which enhance its photo-redox properties through facilitating the separation between photoelectrons (e-) and holes (h+). Furthermore, the heterogeneous photoenzymatic nanoreactor could be recycled for five rounds with slight decline of catalytic reactivity.

Silylation of 2H-indazoles by photoinduced hydrogen atom transfer catalysis.

A metal-free, visible-light-mediated C-H silylation of 2H-indazoles with triphenylsilane has been developed employing 4CzIPN as a photocatalyst and triisopropylsilanethiol as a hydrogen atom transfer (HAT) reagent under aerobic reaction conditions. This method shows tolerance toward many functional groups and affords a variety of silylated indazoles at up to 89% yield. The experimental results suggest that the reaction progresses through a radical pathway.

Photocatalyzed regioselective hydrosilylation for the divergent synthesis of geminal and vicinal borosilanes

Geminal and vicinal borosilanes are useful building blocks in synthetic chemistry and material science. Hydrosilylation/hydroborylation of unsaturated systems offer expedient access to these motifs. In contrast to the well-established transition-metal-catalyzed methods, radical approaches are rarely explored. Herein we report the synthesis of geminal borosilanes from α-selective hydrosilylation of alkenyl boronates via photoinduced hydrogen atom transfer (HAT) catalysis. Mechanistic studies implicate that the α-selectivity originates from a kinetically favored radical addition and an energetically favored HAT process. We further demonstrate selective synthesis of vicinal borosilanes through hydrosilylation of allyl boronates via 1,2-boron radical migration. These strategies exhibit broad scopes across primary, secondary, and tertiary silanes and various boron compounds. The synthetic utility is evidenced by access to multi-borosilanes in a diverse fashion and scaling up by continuous-flow synthesis.

Photoinduced Radical Sulfinylation of C(sp3)-H Bonds with Sulfinyl Sulfones.

A direct C(sp3)-H sulfinylation reaction of alkanes with sulfinyl sulfones via decatungstate photocatalysis is reported. The sulfinyl sulfones generated in situ from sulfinates in the presence of an acylating reagent were able to trap the alkyl radicals that were produced via the photoinduced direct hydrogen atom transfer of alkanes, leading to a range of sulfoxides. This radical sulfinylation process provides an efficient and concise method for the synthesis of sulfoxides from abundant alkanes under mild conditions. Using the same strategy, aldehydes can also be transferred to the corresponding sulfoxides via decarbonylative sulfinylation.

Sulfonamide as Photoinduced Hydrogen Atom Transfer Catalyst for Organophotoredox Hydrosilylation and Hydrogermylation of Activated Alkenes

AbstractReadily available, sterically, and electronically tunable sulfonamides have been developed as effective photoinduced hydrogen atom transfer (HAT) catalysts for selective Si−H functionalizations of a broad range of silanes. N‐centered radicals, catalytically generated from sulfonamides by photoredox catalyzed single‐electron oxidation, are the key intermediates that enable an effective HAT process for silyl radical generation to achieve hydrosilylation of activated alkenes. Additionally, this catalytic system can also be applied for the activation of Ge−H bond for hydrogermylation of activated alkenes through hydrogen atom transfer by sulfonamide derived nitrogen centered radical. The ability to generate silyl and germyl radical using this photochemical HAT process offers new avenue towards the sustainable synthesis of organosilicon and organogermanium compounds.magnified image

Photoinduced Acylations Via Azolium-Promoted Intermolecular Hydrogen Atom Transfer.

Photoinduced hydrogen atom transfer (HAT) has been developed as a powerful tool to generate synthetically valuable radical species. The direct photoexcitation of ketones has been known to promote HAT or to generate acyl radicals through Norrish-type pathways, but these modalities remain severely limited by radical side reactions. We report herein a catalyst- and transition metal-free method for the acylation of C-H bonds that leverages the unique properties of stable, isolable acyl azolium species. Specifically, acyl azolium salts are shown to undergo an intermolecular and regioselective HAT upon LED irradiation with a range of substrates bearing active C-H bonds followed by C-C bond formation to afford ketones. Experimental and computational studies support photoexcitation of the acyl azolium followed by facile intersystem crossing to access triplet diradical species that promote selective HAT and radical-radical cross-coupling.

Zwitterionic Diphenylphosphinyl Amidate as a Powerful Photoinduced Hydrogen-Atom-Transfer Catalyst for C-H Alkylation of Simple Alkanes.

The chemical and physical properties of amides change substantially when the electron-withdrawing groups attached to the nitrogen are varied. Herein, we report the superior performance of N-diphenylphosphinyl 1,2,3-triazolium amidate as a photoinduced hydrogen-atom transfer catalyst compared to its N-benzoyl analog. A binary catalyst system of the phosphinyl amidate and an Ir-based photocatalyst enables the alkylation of unbiased C-H bonds.

Catalytic acceptorless dehydrogenative coupling mediated by photoinduced hydrogen-atom transfer

Catalytic acceptorless dehydrogenative coupling mediated by photoinduced hydrogen-atom transfer

Metal-free photoinduced hydrogen atom transfer assisted C(sp3)–H thioarylation

A mild metal-free approach for C(sp3)–H thioarylation of organic building blocks.

Facile synthesis of 1,2-aminoalcohols via α-C–H aminoalkylation of alcohols by photoinduced hydrogen-atom transfer catalysis

A photocatalytic method for the conversion of aliphatic alcohols and N-sulfinyl α-iminoesters into 1,2-aminoalcohols is reported. A cationic hydrogen-atom transfer catalyst enables site-selective C–H aminoalkylation of primary and secondary alcohols.

3+2] Cycloaddition of alkyl aldehydes and alkynes enabled by photoinduced hydrogen atom transfer

[3+2] Cycloaddition is a step- and atom-economic method for the synthesis of five-membered rings. Despite the great success of 1,3-dipolar cycloadditions, the radical [3+2] annulation of alkynes remains a formidable challenge. Herein, a photoinduced decatungstate-catalyzed [3+2] cycloaddition of various internal alkynes using abundant aliphatic aldehydes as a three-carbon synthon is developed, producing elaborate cyclopentanones in 100% atom economy with excellent site-, regio-, and diastereoselectivity under mild conditions. The catalytic cycle consists of hydrogen atom abstraction from aldehydes, radical addition, 1,5-hydrogen atom transfer, anti-Baldwin 5-endo-trig cyclization, and back hydrogen abstraction. The power of this method is showcased by the late-stage elaboration of medicinally relevant molecules and total or formal synthesis of (±)-β-cuparenone, (±)-laurokamurene B, and (±)-cuparene.

Transition Metal‐Free Radical α‐Oxy C−H Cyclobutylation via Photoinduced Hydrogen Atom Transfer

AbstractA transition‐metal‐free photoinduced radical‐mediated α‐oxy C−H cyclobutylation reaction of bicyclo[1.1.0]butane (BCB) compounds was described. In this protocol, α‐oxy C(sp3)−H motifs including ethers and alcohols were activated via photocatalytic conditions, releasing the ring strain of BCBs and generating 1,3‐disubstituted cyclobutanes in 40–80% yields. Control experiments and Stern‐Volmer quenching experiments indicated that the reaction proceeded through the generation of α‐oxy carbon‐centered radical intermediates by photoinduced hydrogen atom transfer (HAT) process.magnified image

A practical FeCl3/HCl photocatalyst for versatile aliphatic C–H functionalization

The practical and diversified functionalization of ubiquitous C(sp 3 )–H bonds is an economically attractive yet chemically challenging approach to build molecular complexity from abundant chemical raw materials. Herein, we have identified a very robust and practical FeCl 3 /HCl photocatalytic system that enables greatly efficient C–H alkylation, oxidation, chlorination, fluorination, amination, alkynylation, and sulfonylation with total turnover number (TON) of up to 9,900. The aliphatic C–H activation is mediated by chlorine radical, in situ generated from a photoactive tetrahedral [Fe III Cl 4 ] − via ligand-to-metal charge transfer excitation, and the use of HCl as co-catalyst significantly improves the total TON by more than one order of magnitude. • Efficient and cheap photocatalyst generated from FeCl 3 and hydrochloric acid • Practical and diversified functionalization of aliphatic C–H bonds • Detailed mechanistic investigation for photoactive species The aliphatic C–H functionalization has been recognized as an ideal springboard to build molecular complexity from simple molecules with minimal manipulations. In recent years, photoinduced hydrogen atom transfer (HAT) catalysis has emerged as a powerful tool for aliphatic C–H activation by taming the light energy. There has been increasing interest in this prevailing research area, as witnessed by the blowout growth of efficient HAT photocatalysts. Here, we go a step further to report a practical and cheap FeCl 3 /HCl photocatalyst, allowing the diversified functionalization of numerous feedstock chemicals. Notably, the use of HCl as promotor can improve the total turnover number by more than one order of magnitude. Mechanistic investigations shed light on a photoactive tetrahedral [Fe III Cl 4 ] − species that is responsible for generating a chlorine radical via ligand-to-metal charge transfer excitation. The diversified functionalization of aliphatic C–H bonds has been readily established using a practical and economical FeCl 3 /HCl photocatalytic system, in which the use of HCl as promotor can improve the total turnover number by more than one order of magnitude. Mechanistic investigations suggest that this aliphatic C–H functionalization is mediated by a chlorine radical, generated from a photoactive tetrahedral [Fe III Cl 4 ] − species through ligand-to-metal charge transfer (LMCT) excitation.

A radical crossover into C–H functionalization

In this issue of Chem Catalysis , Musacchio and coworkers achieve a formal hydride abstraction of strong C sp 3 –H bonds through sequential photoinduced hydrogen atom transfer and radical-polar crossover events. The potential of this strategy is initially showcased via nucleophilic fluorination, but preliminary studies suggest wide nucleophile generality. In this issue of Chem Catalysis , Musacchio and coworkers achieve a formal hydride abstraction of strong C sp 3 –H bonds through sequential photoinduced hydrogen atom transfer and radical-polar crossover events. The potential of this strategy is initially showcased via nucleophilic fluorination, but preliminary studies suggest wide nucleophile generality.

Cationic DABCO-Based Catalyst for Site-Selective C–H Alkylation via Photoinduced Hydrogen-Atom Transfer

Cationic DABCO-Based Catalyst for Site-Selective C–H Alkylation via Photoinduced Hydrogen-Atom Transfer

Visible light-driven carbamoyloxylation of the α-C(sp3)-H bond of arylacetones via radical-initiated hydrogen atom transfer.

Photocatalytic synthesis has emerged as an efficient route to transform CO2 into functionalized organic carbamates by photocatalysis. Herein, a catalyst-free carbamoyloxylation of arylacetones with CO2 and amines under visible light was developed for the synthesis of O-β-oxoalkyl carbamates in yields up to 93%. This protocol proceeded smoothly with the assistance of inexpensive carbon tetrabromide at room temperature under atmospheric CO2 pressure, leading to simultaneous construction of C-O and C-N bonds. Mechanism studies suggested the photoinduced hydrogen atom transfer (HAT) pathway followed by radical addition or single electron transfer (SET).

Acenaphthoquinoxaline Derivatives as Dental Photoinitiators of Acrylates Polymerization.

A series of dyes based on the acenaphthoquinoxaline skeleton was synthesized. Their structure was modified by introducing electron-withdrawing and electron-donating groups, increasing the number of conjugated double bonds and the number and position of nitrogen atoms, as well as the arrangement of aromatic rings (linear or angular). The dyes were investigated as a component in the photoinitiating systems of radical polymerization for a potential application in dentistry. They acted as the primary absorber of visible light and the acceptor of an electron, which was generated from a second component being an electron donor. Thus, the radicals were generated by the photoinduced intermolecular electron transfer (PET) process. Electron donors used differed in the type of heteroatom, i.e., O, S and N and the number and position of methoxy substituents. To test the ability to initiate the polymerization reaction by photoinduced hydrogen atom transfer, we used 2-mercaptobenzoxazole as a co-initiator. The effectiveness of the photoinitiating systems clearly depends on both the modified acenaphthoquinocaline structure and the type of co-initiator. The lower amount of heat released during the chain reaction and the polymerization rate comparable to this achieved for the photoinitiator traditionally used in dentistry (camphorquinone) indicates that the studied dyes may be valuable in this field.