Single Electron Transfer Reduction Research Articles

Photocatalytic oxidative coupling of terminal alkynes with p-benzoquinone to diaryl ketones by the Cu(I)-PS-POM assembling system through synergistic effect

Photocatalytic C–C bonds formation is an effective approach to synthesize the significant drug intermediates diaryl ketones. Herein, a new polyoxometalate-based metal–organic framework (POMOF), CuW6–TPT, was synthesized by assembling an oxidation catalyst [W6O19]2− into photoactive MOF constructed by photosensitizer (PS) 4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) and binuclear Cu(I) units. CuW6–TPT exhibits excellent light absorption ability and unique conductivity due to the coordination bond, strong π∙∙∙π interactions and H-bonding among Cu(I) ions, TPT moieties and [W6O19]2− in the defined spatial. The abundant exposed Cu(I) catalytic centres are conductive to activate phenylacetylene in situ formation Cu(I)-phenylacetylide, which then undergoes a reductive single electron transfer (SET) process to obtain the key intermediate Cu(II)-phenylactetylide for the Paterno-Buchi (2+2) cycloaddition with benzoquinone to form the labile Cu(II)-oxetane ring. It is a direct route to synthesize diaryl ketones with a hydroxyl group from simple starting materials such as terminal alkynes with p-benzoquinone.

Visible-Light Induced C(sp2 )-H Amidation with an Aryl-Alkyl σ-Bond Relocation via Redox-Neutral Radical-Polar Crossover.

We report an approach for the intramolecular C(sp2 )-H amidation of N-acyloxyamides under photoredox conditions to produce δ-benzolactams with an aryl-alkyl σ-bond relocation. Computational studies on the designed reductive single electron transfer strategy led us to identify N-[3,5-bis(trifluoromethyl)benzoyl] group as the most effective amidyl radical precursor. Upon the formation of an azaspirocyclic radical intermediate by the selective ipso-addition with outcompeting an ortho-attack, radical-polar crossover was then rationalized to lead to the rearomative ring-expansion with preferential C-C bond migration.

Visible‐Light Induced C(sp2)−H Amidation with an Aryl–Alkyl σ‐Bond Relocation via Redox‐Neutral Radical–Polar Crossover

AbstractWe report an approach for the intramolecular C(sp2)−H amidation of N‐acyloxyamides under photoredox conditions to produce δ‐benzolactams with an aryl‐alkyl σ‐bond relocation. Computational studies on the designed reductive single electron transfer strategy led us to identify N‐[3,5‐bis(trifluoromethyl)benzoyl] group as the most effective amidyl radical precursor. Upon the formation of an azaspirocyclic radical intermediate by the selective ipso‐addition with outcompeting an ortho‐attack, radical–polar crossover was then rationalized to lead to the rearomative ring‐expansion with preferential C−C bond migration.

Eosin Y‐Catalyzed Visible‐Light‐Mediated Synthesis of 2‐Mercaptobenzothiazoles from 2‐Azidoarenediazonium Tetrafluoroborates and Carbon Disulfide

AbstractA visible‐light photocatalyzed process for the synthesis of 2‐mercaptobenzothiazoles from 2‐azidoarenediazonium tetrafluoroborates and carbon disulfide has been discovered. The reaction proceeded via single electron transfer reduction of azidoarenediazonium ion to corresponding azidoarene radical followed by carbon disulfide addition and intramolecular cyclization (with azide moiety) reactions. This method is very practical owing to the use of inexpensive eosin Y, mild reaction conditions and easy purification step thus delivering the target compounds in high yields.

Cascades, Catalysis and Chiral Ligand Control with SmI₂; The Rebirth of a Reagent.

This review focuses on recent developments from our laboratory in the field of radical reactions mediated by the archetypal reductive single electron transfer (SET) reagent, SmI₂. Namely, we have expanded the scope of reducible carbonyl moieties to esters and amides and have exploited the resultant ketyl radicals in radical cascade reactions that generate unprecedented scaffolds. Moreover, we have taken the first steps to address the long-standing challenges of catalysis and chiral ligand control associated with the reagent.

Stereoselective synthesis of Gly-Gly-type (E)-methylalkene and (Z)-chloroalkene dipeptide isosteres and their application to 14-mer RGG peptidomimetics.

Stereoselective and efficient synthesis of Gly-Gly-type (E)-methylalkene and (Z)-chloroalkene dipeptide isosteres is realized by organocuprate-mediated single electron transfer reduction. The synthetic isosteres can be used in Fmoc-based solid phase peptide synthesis, resulting in the preparation of the 14-mer RGG peptidomimetics containing an (E)-methylalkene or a (Z)-chloroalkene unit.

Photoinduced Sulfonylation Reactions through the Insertion of Sulfur Dioxide

Recent advances in the photoinduced sulfonylation reactions through the insertion of sulfur dioxide are summarized. Among the approaches developed in the sulfonylation reactions, the insertion of sulfur dioxide via radical process can be accomplished with the assistance of photoredox catalysis in the presence of visible light or under ultraviolet irradiation, depending the substrates used in the reactions. Under ultraviolet irradiation, the conversion can be completed under metal‐ and additive‐free conditions. It is noteworthy that alkyl halides possessing eliminable β‐hydrogen atom are effective as alkyl radical precursors with the insertion of sulfur dioxide. Two parts, according to the photoinduced reaction pathways, are introduced: (a) photo‐initiated formation of radical with the subsequent capture of sulfur dioxide; (b) photo‐assisted single electron transfer (SET) after insertion of sulfur dioxide. During the transformation, an oxidative or reductive single electron transfer would occur under visible light photoredox catalysis.

Samarium(II) folding cascades involving hydrogen atom transfer for the synthesis of complex polycycles

The expedient assembly of complex, natural product-like small molecules can deliver new chemical entities with the potential to interact with biological systems and inspire the development of new drugs and probes for biology. Diversity-oriented synthesis is a particularly attractive strategy for the delivery of complex molecules in which the 3-dimensional architecture varies across the collection. Here we describe a folding cascade approach to complex polycyclic systems bearing multiple stereocentres mediated by reductive single electron transfer (SET) from SmI2. Simple, linear substrates undergo three different folding pathways triggered by reductive SET. Two of the radical cascade pathways involve the activation and functionalization of otherwise inert secondary alkyl and benzylic groups by 1,5-hydrogen atom transfer (HAT). Combination of SmI2, a privileged reagent for cascade reactions, and 1,5-HAT can lead to complexity-generating radical sequences that unlock access to diverse structures not readily accessible by other means.

Dual Roles for Potassium Hydride in Haloarene Reduction: CSNAr and Single Electron Transfer Reduction via Organic Electron Donors Formed in Benzene.

Potassium hydride behaves uniquely and differently than sodium hydride toward aryl halides. Its reactions with a range of haloarenes, including designed 2,6-dialkylhaloarenes, were studied in THF and in benzene. In THF, evidence supports concerted nucleophilic aromatic substitution, CSNAr, and the mechanism originally proposed by Pierre et al. is now validated through DFT studies. In benzene, besides this pathway, strong evidence for single electron transfer chemistry is reported. Experimental observations and DFT studies lead us to propose organic super electron donor generation to initiate BHAS (base-promoted homolytic aromatic substitution) cycles. Organic donor formation originates from deprotonation of benzene by KH; attack on benzene by the resulting phenylpotassium generates phenylcyclohexadienylpotassium that can undergo (i) deprotonation to form an organic super electron donor or (ii) hydride loss to afford biphenyl. Until now, BHAS reactions have been triggered by reaction of a base, MO tBu (M = K, Na), with many different types of organic additive, all containing heteroatoms (N or O or S) that enhance their acidity and place them within range of MO tBu as a base. This paper shows that with the stronger base, KH, even a hydrocarbon (benzene) can be converted into an electron-donating initiator.

Photocatalytic Reaction of Potassium Alkyltrifluoroborates and Sulfur Dioxide with Alkenes.

A three-component reaction of potassium alkyltrifluoroborates, the sulfur dioxide surrogate of DABCO·(SO2)2, and alkenes under photocatalysis in the presence of visible light is developed. This reaction works efficiently at room temperature with the insertion of sulfur dioxide under mild conditions, affording diverse sulfones in good to excellent yields. The alkyl radical and alkylsulfonyl radical are generated as key intermediates, and a reductive single-electron transfer is involved in the reaction process.

Reduction and Reductive Deuteration of Tertiary Amides Mediated by Sodium Dispersions with Distinct Proton Donor-Dependent Chemoselectivity.

A practical and scalable single electron transfer reduction mediated by sodium dispersions has been developed for the reduction and reductive deuteration of tertiary amides. The chemoselectivity of this method highly depends on the nature of the proton donor. The challenging reduction via C-N bond cleavage has been achieved using Na/EtOH, affording alcohol products, while the use of Na/NaOH/H2O leads to the formation of amines via selective C-O scission. Sodium dispersions with high specific surface areas are crucial to obtain high yields and good chemoselectivity. This new method tolerates a range of tertiary amides. Moreover, the corresponding reductive deuterations mediated by Na/EtOD- d1 and Na/NaOH/D2O afford useful α,α-dideuterio alcohols and α,α-dideuterio amines with an excellent deuterium content.

Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation.

Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B12-dependent biodegradation and zerovalent metal-mediated dehalogenation. Compound-specific isotope effect (13C/12C, 37Cl/35Cl) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO2 radical anions, S2--doped graphene oxide in water) caused strong carbon (εC = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (εCl = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl3 (εC = -7.7‰, εCl = -2.6‰), and in CEs when the exergonicity of C-Cl bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π* orbital followed by C-Cl cleavage) in ethenes. The nonexistent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced Cl isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.

A Single Electron Transfer (SET) Approach to C–H Amidation of Hydrazones via Visible-Light Photoredox Catalysis

The reductive single electron transfer (SET) umpolung amination of aldehyde-derived hydrazones has been developed through visible-light-promoted photoredox catalysis. The ideal transformation of hydrazones into the corresponding hydrazonamide through selective carbon-hydrogen (C-H) bond functionalization represents one of the most step- and atom-economical methods. This SET umpolung strategy features mild conditions and a remarkably broad substrate scope, offering an entirely new substrate class to direct C-H amination.

Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals.

Visible‐light‐mediated photoredox‐catalyzed aldimine–aniline and aldehyde–aniline couplings have been realized. The reductive single electron transfer (SET) umpolung of various carbonyl derivatives enabled the generation of intermediary ketyl and α‐amino radical anions, which were utilized for the synthesis of unsymmetrically substituted 1,2‐diamines and amino alcohols.

Exocyclic Bond Cleavage in Oxaphosphirane Complexes?

A first computational insight into the intrinsic strength of exocyclic bonds to phosphorus in oxaphosphirane κP-pentacarbonylmetal(0) complexes 1a-f (M=Cr, Mo) is provided using a set of P-R derivatives (R=Me, tBu, CPh(3)). Whereas homolytic cleavage of the exocyclic P-R bond was found to be always unfavored (for neutral complexes), heterolytic cleavage leading to a carbocation R(+) moiety and the oxaphosphiranide complex 2(-) constitutes the lowest-energy process, especially if R is bulky and can stabilize the positive charge, that is, triphenylmethyl (trityl), efficiently. The energies required for P-M bond cleavage are about 30 kcal mol(-1), and decrease with the increasing bulk of the R substituent (from Me to trityl) and ongoing from Cr to Mo. The reactivities of complexes 1a-f towards oxidative and reductive single electron transfer (SET) reactions were analyzed using the facile variation of bond-strength-related descriptors (VBSD) methodology, thus enabling the design of synthetically useful strategies addressing decomplexation and P-functionalization. Reductive SET reactions with sodium naphthalenides enable selective P-M bond cleavage (i.e., decomplexation) for the case of P-Me and P-tBu substitution, whereas reductive P-R bond cleavage is favored in the case of the P-trityl complexes 1c,f, and results in the formation of the (anionic) oxaphosphiranide complex 2(-), which may be regarded as a potential key intermediate for further P-functionalization.

ChemInform Abstract: m‐Terphenyl Ethers, a New Hydroxy Protecting Group Cleavable under Reductive Single Electron Transfer Reaction Conditions.

AbstractIt is demonstrated, that m‐terphenyl ethers such as (III) and (VIII) and, to a lesser extent analogous 2,6‐dimethoxyphenyl ethers (V), can be cleaved under SET conditions using alkali metals in THF.

M-Terphenyl Ethers, a New Hydroxy Protecting Group Cleavable under Reductive Single Electron Transfer Reaction Conditions

m-Terphenyl ethers and, to a lesser extent, 2,6-dimethoxyphenyl ethers, were tested as protected hydroxy derivatives under a variety of reaction conditions. These ethers underwent regioselective cleavage of the aromatic C(1)―O bond under reductive SET reaction conditions using alkali metals in tetrahydrofuran at room temperature. This deprotective procedure was efficiently realized in the presence of several other functional groups, including an acetal, a phenyl alkyl ether, an unprotected alcohol, and carbon―carbon double and triple bonds. Furthermore, m-terphenyl ethers proved stable under different reaction conditions, including acidic hydrolysis and formation and/or employment of different organometallic reagents.

Enantioselective Synthesis of 4-Acetylaminocyclopent-2-en-1-ols from Tricyclo[5.2.1.02,6]decenyl Enaminones. Precursors for 5‘-Norcarbocyclic Nucleosides and Related Antiviral Compounds

An efficient synthesis of both (1S,4R) and (1R,4S)-4-N-acetylamino-1-benzoylcyclopent-2-enes 33 has been accomplished starting from enantiopure 5-(1'-phenylethylamino)-endo-tricyclo[5.2.1.0(2,6)]deca-4,8-dien-3-ones 14 and 15. N-Acetylation of both 15 and 14 followed by single electron-transfer reduction using lithium in liquid ammonia afforded diastereomeric mixtures of beta-amino ketones 26 and 27 and of ent-26 and ent-27 in high yields and with high diastereoselectivity. In this reduction process, the enaminone double bond is reduced with the concomitant removal of the alpha-methylbenzyl group as the chiral auxiliary. Thermolysis of the respective diastereomic mixtures of 26 and 27 in the gas phase (FVT) or in solution afforded 4-N-acetylaminocyclopent-2-ene-1-ones 30 in high optical and chemical yields. Acidic hydrolysis of (+)-30 gave (R)-(+)-4-aminocyclopentenone 31 as its hydrochloride. Stereoselective reduction of 30 using sodium borohydride and cerium chloride heptahydrate furnished amido alcohol 32, which was isolated and characterized as its benzoyl derivative 33.

Free Radical Rearrangements in Uracil Derivatives

As part of an effort to develop general probes for radical reactions involving DNA bases, several uracil derivatives were synthesized. The rates of the cyclopropyl carbinyl rearrangement in these systems were evaluated by means of competition experiments. The results indicate that when a cyclopropyl group is substituted in the 5-position of uracil, the rearrangement occurs very slowly-with a rate constant of <2.5×10 4 s -1 . On the other hand, the analog of the 5-hexenyl radical cyclization onto the 5,6-double bond of uracil derivatives occurs with rates which were similar to the parent process: (4.0-8.9)×10 4 s -1 . The experimental results along with semiempirical calculations show that radicals 23 and 25 are unusually stable species. These results explain why no rearrangements are observed when a cyclopropyl-substituted thymine dimer is cleaved by reductive single electron transfer

Benzoin enediol dianion and hydroxide ion in DMSO: a single electron transfer reduction system driven by the irreversible benzilic acid rearrangement

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTBenzoin enediol dianion and hydroxide ion in DMSO: a single electron transfer reduction system driven by the irreversible benzilic acid rearrangementJ. Michael Robinson, Emmette T. Flynn, Terry L. McMahan, Sharai L. Simpson, John C. Trisler, and Kenneth Bruce ConnCite this: J. Org. Chem. 1991, 56, 23, 6709–6712Publication Date (Print):November 1, 1991Publication History Published online1 May 2002Published inissue 1 November 1991https://doi.org/10.1021/jo00023a043RIGHTS & PERMISSIONSArticle Views482Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (528 KB) Get e-Alerts Get e-Alerts