Hydrogen Atom Transfer Catalyst Research Articles

Electrochemical Silylation of Electron‐Deficient Heterocycles Using N‐Hydroxyphthalimide as HAT Catalyst

AbstractA practical electrochemical Minisci‐type reaction for the synthesis of silylated heterocycles is described. Using N‐hydroxyphthalimide (NHPI) as the hydrogen atom transfer (HAT) catalyst and trialkylsilanes as the silyl radical sources, a wide range of electron‐deficient heterocycles in‐cluding quinoline, quinoxaline, phthalazine, 3,6‐dichloropyridazine, phenanthridine, 6‐chloroimidazo[1,2‐b]pyridazine and coumarin, etc. were easily converted to the corresponding silylated products in 37–83% yields.magnified image

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 Disulfide-Catalyzed Intramolecular Anti-Markovnikov Hydroamination through in Situ N-S Species.

The photoinduced anti-Markovnikov hydroamination of olefins typically required photocatalysts with a high oxidative ability to initiate the single-electron process. Herein, we alternatively utilize bis(2,4,6-triisopropylphenyl) disulfide, an inexpensive reagent with relatively low oxidative ability, as a photo and hydrogen atom transfer catalyst to achieve intramolecular hydroamination. The mechanistic studies as well as the DFT calculations are consistent with a novel process involving N-centered radical generation through the homolysis of the in situ formed N-S species and subsequent cyclization. An array of diverse nitrogen-containing cycles could be obtained.

Photochemical synthesis of pyrano[2,3-d]pyrimidine scaffolds using photoexcited organic dye, Na2 eosin Y as direct hydrogen atom transfer (HAT) photocatalyst via visible light-mediated under air atmosphere

The Knoevenagel-Michael cyclocondensation of barbituric acid/1,3-dimethylbarbituric acid, malononitrile, and arylaldehyde derivatives was used to construct a multicomponent green tandem method for the metal-free synthesis of pyrano[2,3-d]pyrimidine scaffolds. At room temperature in aqueous ethanol, photo-excited state functions generated from Na2 eosin Y were employed as direct hydrogen atom transfer (HAT) catalysts by visible light mediated in the air atmosphere. This research looks towards expanding the use of a non-metallic organic dye that is both affordable and readily available. Because of its good yields, energy-effectiveness, high atom economy, time-saving qualities of the reaction, and operational simplicity, Na2 eosin Y is photochemically produced with the least amount of a catalyst. As a result, various ecological and sustainable chemical properties are met. Surprisingly, such cyclization may be carried out on a gram scale, indicating the reaction's potential industrial application.

Anti-Markovnikov hydrochlorination and hydronitrooxylation of α-olefins via visible-light photocatalysis

Conventional hydrofunctionalization of α-olefins with mineral acids proceeds with Markovnikov selectivity to afford branched isomers. The direct formation of linear constitutional isomers is challenging, yet anti-Markovnikov addition would be valuable for the synthesis of commodity chemicals, such as primary alcohols, which are currently only accessible via stoichiometric redox reactions, with a full equivalent of waste of both oxidant and reductant. Strategies that utilize radical intermediates have been demonstrated, but only for activated alkenes, and the direct use of aqueous mineral acids remains elusive. Here we present anti-Markovnikov addition reactions of aqueous hydrochloric and nitric acid to unactivated alkenes. The transformation is enabled by the in situ generation of photoredox-active ion pairs, derived from acridine and the mineral acid, as a combined charge- and phase-transfer catalyst. The introduction of a hydrogen atom transfer catalyst enabled us to bypass the challenging chain propagation by hydrochloric and nitric acids that originates from the high bond dissociation energy.

Visible Light-Induced Cross-Dehydrocoupling of 3-Cyanocoumarins with Unactivated Aliphatic Aldehydes Enables Access to 4-Acylated Coumarins.

We report visible light-induced cross-dehydrocoupling of 3-cyanocoumarins with unactivated aliphatic aldehydes for the direct synthesis of 4-acylated coumarins. Inexpensive and readily available (n-Bu)4NBr was used as a precursor of the hydrogen atom transfer (HAT) catalyst and [Ir(ppy)2(dtbbpy)PF6] as a photocatalyst. A variety of aldehydes including linear, branched, cyclic, and α,β-unsaturated aliphatic aldehydes were suitable for this reaction yielding the desired acylated coumarin derivatives in 41-98% yields. This method provides a mild, atom-economical, and environmentally friendly synthetic route for the direct acylation of 3-cyanocoumarins. This is the first example so far of C-4 aliphatic acylation of coumarins.

Eosin Y as a direct HAT photocatalyst for the synthesis of tetrahydrodipyrazolopyridines under white LED irradiation

A facile and efficient synthetic protocol has been developed for the synthesis of biologically privileged tetrahydrodipyrazolopyridines via visible light mediated one pot pseudo-six component reaction of ethyl acetoacetate, hydrazine hydrate, multifarious aldehydes and ammonium acetate (Knoevenagel condensation-Michael addition) by using metal-free, inexpensive eosin Y organic dye as a direct hydrogen atom transfer (HAT) catalyst at room temperature in open air condition. Mild condition, good yields, environment-friendly, high atom economy and good functional group tolerance of an extensive range of both electron-withdrawing and electron-donating groups are extra features of this approach. Additionally, this methodology is also appropriate for multi-gram scale, which enlighten the potentiality of using this reaction for industrial purposes.

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.

Visible-light induced eosin Y catalysed C(sp2)-H alkylation of carbonyl substrates via direct HAT

Visible light-driven organic transformations has been proposed, using eosin Y, a widely used and affordable substitute for metal catalysts, which can also act as an effective direct hydrogen atom transfer (HAT) catalyst for C–H activation. A study that utilized the alkylation of C–H bonds with electron-deficient alkenes as a paradigm provided a very wide substrate scope that made it simple to access a number of significant synthons. This green and sustainable eosin Y-based photocatalytic hydrogen atom transfer method promises to diversify the functionalization of a variety of native C–H bonds.

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.

TEMPO as a Hydrogen Atom Transfer Catalyst for Aerobic Dehydrogenation of Activated Alkanes to Alkenes.

2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) has been extensively utilized as a radical scavenger or an oxidation catalyst. In contrast, TEMPO as a hydrogen atom transfer (HAT) catalyst has rarely been studied. Here, we report that TEMPO, as the HAT catalyst, homolytically cleaves benzylic or allylic C-H bonds to give the corresponding alkyl radicals. Benefiting from the dual roles played by TEMPO as the HAT catalyst and the radical scavenger, the highly challenging aerobic dehydrogenation of activated alkanes to alkenes is successfully developed.

Pyrimidopteridine-catalyzed Photo-mediated Hydroacetoxylation.

Herein we report a photo-mediated formal addition of carboxylic acids to activated alkenes catalyzed by a pyrimidopteridine photoredox catalyst. The decarboxylation of aliphatic carboxylic acids upon single-electron oxidation is countered in the presence of electron-rich alkenes and a hydroacetoxylation is observed. Mechanistic proposals have been made based on CV measurements, competitive Stern-Volmer quenching and EPR experiments. Evidence that tetra-N-substituted pyrimidopteridines function as dual photoredox and hydrogen atom transfer catalyst was supported by spectroscopic means.

Site-Selective α-Alkylation of 1,3-Butanediol Using a Thiophosphoric Acid Hydrogen Atom Transfer Catalyst.

Herein, we developed secondary-alcohol-selective C-H alkylation of 1,3-butane diol by combining an acridinium photoredox catalyst and a thiophosphoric acid hydrogen atom transfer (HAT) catalyst. The use of non-coordinating solvent such as dichloromethane (DCM) improved secondary α-alkoxy C-H selectivity by lowering bond dissociation energy (BDE) through intramolecular hydrogen bonding.

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp 3 )–H Bond Activation

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp ³ )–H Bond Activation

New Role for Photoexcited Na2 Eosin Y via the Direct Hydrogen Atom Transfer Process in Photochemical Visible-Light-Induced Synthesis of 2-Amino-4H-Chromene Scaffolds Under Air Atmosphere.

The Knoevenagel–Michael cyclocondensation of malononitrile, aryl aldehydes, and resorcinol was used as a multicomponent green tandem strategy for the metal-free synthesis of 2-amino-4H-chromene scaffolds. Through a visible-light-induced process, the photo-excited state functions derived from Na2 eosin Y were used as direct hydrogen atom transfer catalysts in aqueous ethanol at ambient temperature. The purpose of this study was to examine the further use of an organic dye that does not contain metal and is inexpensive and commercially available. Na2 eosin Y is synthesized by photochemical means using the least amount of catalyst, which results in excellent yields, energy efficiency, and environmental friendliness, high atom economy, time-saving features, and ease of operation. As a result, some properties of green and sustainable chemistry are met. This kind of cyclization can be performed on a gram scale, indicating the potential utility of this reaction in industry.

Visible‐Light Photoredox‐Catalyzed Hydrodecarboxylation and Deuterodecarboxylation of Fatty Acids

Comprehensive SummaryWe develop an efficient visible‐light photoreodox‐catalyzed hydrodecarboxylation and deuterodecarboxylation of fatty acids for alkanes and deuterium alkanes. The key to the efficient transformation should attribute to the co‐catalysis of the suitable methoxy substituted acridine photocatalyst (Mes‐1,3,6,8‐tetramethoxy‐Acr‐3”,5”‐dimethoxy‐Ph) and the hydrogen atom transfer catalyst 4′,4′‐dimethyl diphenyldisulfane, possibly via facilitating of quenching the active alkyl radicals to overcome the competitive homocoupling products, olefins via the disproportionation, cracking products via the C—C bond β‐scission.

Recent Advancements in Development of Radical Silylation Reactions

Abstract: Transition metal-free radical hydrosilylation of alkynes and factionalized alkenes to vinylsilanes, organosilanes from alkenes, radical silylative cyclizations for sila heterocycles, aromatic dehydrogenative radical silylation, sila-amino acids, silyl hydroperoxides by using transition metal-free reaction conditions such as peroxides, photocatalyst, hydrogen atom transfer catalyst, radical initiators have become one of the most important and useful methodologies for the construction of C-Si bonds for their synthetic applications. In order to demonstrate the growth in this area, this review highlights the last twenty years of success in the fields of silylation of aromatics, silylative cyclizations of sila heterocycles, synthesis of vinylsilanes by hydrosilylation of alkynes, and functionalized alkenes, organosilanes from alkenes and dehydrogenative approaches for vinylsilanes involving radical mechanisms. We believe that summarizing these methods would be very useful for the chemists who are interested in the synthesis of organosilicon compounds for drug discovery or the development of silicon-based materials for industrial applications.

Hydrogen-Atom Transfer Catalysis for C-H Alkylation of Benzylic Fluorides.

A photoinduced catalytic C(sp3)-H alkylation of benzylic fluorides is developed. The use of zwitterionic 1,2,3-triazolium amidate as a hydrogen-atom transfer catalyst is uniquely effective for promoting this transformation. The combination of C-H alkylation with subsequent displacement of the C-F bond enables 1,1-difunctionalization of benzylic fluorides, providing rapid access to an array of functionalized molecular entities.

Arenethiolate as a Dual Function Catalyst for Photocatalytic Defluoroalkylation and Hydrodefluorination of Trifluoromethyls

Thiol is known to act as a hydrogen atom transfer catalyst working in synergy with a photocatalyst in photoredox catalysis, but we report herein that an arene thiolate with an appropriate substituent can be photoactivated under visible light to function as both a strongly reducing electron-donating redox catalyst and a HAT catalyst to enable catalytic C–F activation of trifluoromethyl substrates for selective hydrodefluorination and coupling with various alkenes in the presence of formate salts. These reactions demonstrate the promising utility of arenethiolates as dual function photocatalysts. The synthetic utility of this method is demonstrated by the broad scope of amenable trifluoromethyl substrates, including trifluoromethylated (hetero)arenes, trifluoroacetates, and trifluoroacetamides, which exhibited high levels of chemoselectivity. The reaction efficacy allows site-selective late-stage functionalization of multitrifluoromethylated bioactive compounds and pharmaceuticals

Visible-Light-Induced Radical Condensation Cyclization to Synthesize 3,4-Dihydropyrimidin-2-(1H)-ones/thiones Using Photoexcited Na2 Eosin Y as a Direct Hydrogen Atom Transfer (HAT) Catalyst.

The data suggests that Na2 eosin Y-derived photoinduced states act as a HAT catalyst for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones/thiones in ethanol at room temperature. This research establishes a novel function for using a nonmetallic natural dye, Na2 eosin Y, available commercially and at a cheap cost in the photochemical synthesis using the least amount of catalyst, obtaining good results, speeding up the process, and achieving a high atom economy. The TON and TOF of 3,4-dihydropyrimidin-2-(1H)-ones/thiones are computed. Furthermore, this cycle runs on the gram scale as well, indicating the possibility of industrial purposes.