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Merging Photoredox PCET with Ni-Catalyzed Cross-Coupling: Cascade Amidoarylation of Unactivated Olefins
摘要： A rapid, highly diastereoselective amidoarylation of unactivated olefins was achieved to render medicinally privileged pyrrolidinone structures. Taking advantage of a photoredox proton-coupled electron transfer process, amidyl radicals were obtained from non-prefunctionalized N–H bonds under mild conditions, which were subsequently trapped by pendant olefins, delivering alkyl radicals for nickel-catalyzed cross-coupling. Mechanistic studies revealed the key balance between thermodynamically-driven radical generation and kinetically-driven cyclization, which led to expanding the scope toward urea and carbamate substrates.
关键词： urea,nickel catalysis,carbamate,unactivated olefins,proton-coupled electron transfer,amidoarylation,photoredox catalysis,pyrrolidinone
更新于2025-09-23 15:23:52
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Visible Light Driven Reductive (Cyclo)Dimerization of Chalcones Over Heterogeneous Carbon Nitride Photocatalyst
摘要： Single electron reduction of chalcones to the respective radical anions is a useful technique to activate these molecules toward subsequent transformations. Herein, a metal free photocatalytic version of chalcones reduction in the presence of triethanolamine as a convenient electron donor and using heterogeneous carbon nitride visible light photocatalyst is presented. The reaction proceeds via a long lived radical species of the heterogeneous organic semiconductor. The scope of the reaction was studied and regioselectivity of the chalcone radicals coupling was investigated. (1) Ten chalcones gave selectively poly substituted cyclopentanoles with 31-73% isolated yield; (2) Two chalcones bearing electron-donor groups, 4-MeOC6H4 and 2-thienyl, gave selectively the β-ketodienes in 42% and 53% isolated yield, respectively; (3) Pentafluorophenyl substituted chalcone gave exclusively the product of the radicals coupling followed by hydrogen transfer from triethanolamine – hexane-1,6-dione in 65% isolated yield. Reductive cross cyclodimerization of a mixture of two different chalcones proceeded regioselectively with the formation of one product out of four possible. The mechanism was investigated by cyclic voltammetry and linear sweep voltammetry and suggests that the reaction proceed through proton coupled electron transfer.
关键词： cyclopetanole,organic photoredox catalysis,chalcone,carbon nitride,proton coupled electron transfer
更新于2025-09-23 15:22:29
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Photoinduced Proton-Coupled Electron Transfer in Supramolecular Sn <sup>IV</sup> Di( <scp>l</scp> -tyrosinato) Porphyrin Conjugates
摘要： Proton-coupled electron transfer (PCET) plays a key role in many biological processes, and a thorough comprehension of its subtle mechanistic complexity requires the synthesis and characterization of suitable artificial systems capable of mimicking this fundamental, elementary step. Herein, we report on a detailed photophysical investigation of conjugate 1, based on a tin(IV) tetraphenylporphyrin (SnTPP) chromophore bound to two L-tyrosinato amino acids, in CH2Cl2 in combination with organic bases of different strength and the preparation of a novel conjugate 3, based on a tin(IV) octaethylporphyrin (SnOEP) in place of the tetraphenyl analogue, and its photophysical characterization in CH2Cl2 in the presence of pyrrolidine. In the case of compound 1 with all bases examined, quenching of both the singlet and triplet excited states is observed and attributed to the occurrence of concerted proton?electron transfer (CPET). Rates and quenching yields decrease with the strength of the base used, consistent with the decrease of the driving force for the CPET process. Conjugate 3 with pyrrolidine is quenched only at the triplet level by CPET, albeit with slower rates than its parent compound 1, ascribable to the smaller driving force as a result of SnOEP being more difficult to reduce than SnTPP. For both systems, the quenching mechanism is confirmed by suitable blank experiments, specific kinetic treatments, and the observation of kinetic isotope effects (KIEs). Differently from what has been previously proposed, a detailed reinvestigation of the triplet quenching of 1 with pyrrolidine shows that no long-lived radical pair state is formed, as diradical recombination is always faster than formation. This is true for both 1 and 3 and for all bases examined. The kinetics of the CPET pathways can be well described according to Marcus theory and point toward the involvement of substantial reorganization energy as typically observed for PCET processes of concerted nature.
关键词： tin(IV) porphyrin,photophysical characterization,L-tyrosinato amino acids,CPET,PCET,Proton-coupled electron transfer,kinetic isotope effects,concerted proton?electron transfer,Marcus theory
更新于2025-09-23 15:19:57
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Excited-state proton-coupled electron transfer within ion pairs
摘要： The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.
关键词： proton-coupled electron transfer,salicylate,ion pairs,ruthenium complexes,Marcus theory,excited-state
更新于2025-09-19 17:13:59
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Proton release process during the S2-to-S3 transition of photosynthetic water oxidation as revealed by the pH dependence of kinetics monitored by time-resolved infrared spectroscopy
摘要： Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II via light driven cycle of intermediates called S states (S0–S4). Clarifying how electron and proton transfer reactions are coupled with each other in the S2→S3 transition, which occurs just before the O-O bond formation, is crucial to understand the water oxidation mechanism. Here, we investigated the pH dependence of the kinetics of the S2→S3 transition using time-resolved infrared (TRIR) spectroscopy to identify the proton release phase in this transition. TRIR measurements of YD-less PSII core complexes from the D2-Y160F mutant of Thermosynechococcus elongatus showed that the last phase in this transition (τ = ~350 μs at pH 6) was strongly pH dependent, and its time constant at pH 5 was larger than that at pH 8 by a factor of more than three. In contrast, the earlier phase with a ~100 μs time constant was virtually independent of pH. These results strongly support the view that proton release is a rate-limiting step of the proton-coupled electron transfer in the last phase of the S2→S3 transition. This proton release enables electron transfer by removing an excessive positive charge from the catalytic center and hence lowering its redox potential.
关键词： Mn4CaO5 cluster,proton-coupled electron transfer,S2→S3 transition,photosynthetic water oxidation,time-resolved infrared spectroscopy
更新于2025-09-16 10:30:52
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Excited state structural dynamics of 4-cyano-4′-hydroxystilbene: deciphering the signatures of proton-coupled electron transfer using ultrafast Raman loss spectroscopy
摘要： The photo-initiated proton-coupled electron transfer (PCET) process plays a crucial role in the context of light harvesting in various biological and chemical systems. Molecular model systems are typically employed to understand the mechanisms underlying the functioning of complex biological systems. Some molecular dyads based on the PCET property have been particularly designed to achieve efficient sunlight-to-fuel production. Organic photoacids are potential sources for such applications since they exhibit an enhancement in their acidity upon photoexcitation, facilitating the mimicking of some of the biological processes. p-Hydroxybenzylideneimidazolinone (p-HBI), an organic photoacid, is a key chromophore in green fluorescent protein, which exhibits green emission due to excited state proton transfer. Herein, we investigate the structural changes and dynamics of 4-cyano-40-hydroxystilbene (CHSB), an analogue of p-HBI, in the presence of an external base, t-butylamine (TBA), using the techniques of ultrafast transient absorption, emission and ultrafast Raman loss spectroscopy.
关键词： excited state dynamics,Raman loss spectroscopy,proton-coupled electron transfer,photoacids,ultrafast spectroscopy
更新于2025-09-12 10:27:22
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Design and synthesis of benzimidazole phenol-porphyrin dyads for the study of bioinspired photoinduced proton-coupled electron transfer
摘要： Benzimidazole phenol-porphyrin dyads have been synthesized to study proton-coupled electron transfer (PCET) reactions induced by photoexcitation. High-potential porphyrins have been chosen to model P680, the photoactive chlorophyll cluster of photosynthetic photosystem II (PSII). They have either two or three pentafluorophenyl groups at the meso positions to impart the high redox potential. The benzimidazole phenol (BIP) moiety models the Tyrz-His190 pair of PSII, which is a redox mediator that shuttles electrons from the water oxidation catalyst to P680?+. The dyads consisting of a porphyrin and an unsubstituted BIP are designed to study one-electron one-proton transfer (E1PT) processes upon excitation of the porphyrin. When the BIP moiety is substituted with proton-accepting groups such as imines, one-electron two-proton transfer (E2PT) processes are expected to take place upon oxidation of the phenol by the excited state of the porphyrin. The bis-pentafluorophenyl porphyrins linked to BIPs provide platforms for introducing a variety of electron-accepting moieties and/or anchoring groups to attach semiconductor nanoparticles to the macrocycle. The triads thus formed will serve to study the PCET process involving the BIPs when the oxidation of the phenol is achieved by the photochemically produced radical cation of the porphyrin.
关键词： pentafluorophenyl porphyrin,benzimidazole derivatives,proton-coupled electron transfer (PCET),photosystem II
更新于2025-09-11 14:15:04
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Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer
摘要： The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and the trans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cis and cis,cis-isomers with blue light (405 nm) affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum yield. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex (38% yield) and furnish ammonia in 74% yield.
关键词： light,dinitrogen reduction,ammonium,proton-coupled electron transfer,rhenium
更新于2025-09-11 14:15:04
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Proton-Coupled Electron Transfer Kinetics for the Photoinduced Generation of a Cobalt(III)-Hydride Complex
摘要： Studying the formation of transition metal hydride complexes via proton-coupled electron transfer is important for developing next-generation molecular catalysts for hydrogen evolution. We report herein the study of stepwise photoinduced reduction and protonation of [CoIICp-(dppe)]+ (Cp = cyclopentadienyl, dppe = 1,2-bis-(diphenylphosphino)ethane) to form the corresponding hydride complex [HCoIIICp(dppe)]+. Reaction intermediates were optically tracked using transient absorption spectroscopy, and a combination of experimental fitting and kinetic simulations was used to determine apparent rate constants for electron transfer and proton transfer with a range of acid sources. A linear free energy relationship is observed between measured apparent proton transfer rate constants and acid strength, but marked differences from previously electrochemically determined protonation rate constants are found. These deviations, which stem from ground-state reactivity present in photochemical experiments, highlight the challenges in comparing mechanistic studies using different techniques.
关键词： kinetic simulations,transient absorption spectroscopy,transition metal hydride complexes,proton-coupled electron transfer,hydrogen evolution
更新于2025-09-11 14:15:04
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Spectro-electrochemical Studies on [Ru(TAP) <sub/>2</sub> (dppz)] <sup>2+</sup> —Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides
摘要： [Ru(TAP)2(dppz)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP)2(dppz)]2+ in acetonitrile. The UV?vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP)2(dppz)]2+ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λmax at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP)2(dppz)]2+, the reduced complex [Ru(TAP)2(dppz)]+, and four isomers of [Ru(TAP)(TAPH)(dppz)]2+ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP)2(dppz)]+ but not for [Ru(TAP)(TAPH)(dppz)]2+. Both the UV?vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP)2(dppz)]+ agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP)2(dppz)]2+ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)]2+, as is proposed for the reaction with 5′-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λmax at 1750 and 2360 nm).
关键词： proton-coupled electron transfer,DNA oxidation,photoelectron transfer,Ruthenium complexes,spectro-electrochemistry
更新于2025-09-04 15:30:14