Proton‐Coupled Electron Transfer in Photoredox Catalytic Reactions

Multiple-site concerted proton electron transfer (MS-CPET) reactions were studied in a three-component system. 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer to a single e−/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, which is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second order rate constants, measured using stopped-flow kinetic techniques, spanned four orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Brønsted plot of ln(kMS-CPET) vs. ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outlier to the Brønsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related hydrogen atom transfer reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the PCET process.

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“…1,9 Recently a number of catalytic processes utilizing MS-CPET have been developed for organic synthetic applications. 10-14 …”

Section: Introductionmentioning

confidence: 99%

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions

2017

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“…7 An alternative approach to generate the hydroxy radical would be the reduction of carbonyl compounds by a photocatalytic proton coupled electron transfer process. 8 However, the direct cross-coupling of α-hydroxyalkyl radicals with aryl halides remains unexplored to this point in time. Therefore, we set out to use α-hydroxyalkyltrifluoroborates as radical precursors, generating α-hydroxyalkyl radicals under suitable photoredox/Ni dual catalytic conditions (Figure 2B).…”

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confidence: 99%

“…As illustrated in Table 2, different electron-poor aryl bromides provided moderate to high yields of the desired products ( 6a-k ). Of note, a number of electrophilic functional groups that are intolerant of Grignard reactions 14 or even reducing conditions 15 could be employed to afford nitrile ( 6a ), aldehyde ( 6b ) 8a , ketone ( 6c ), ester ( 6f ), and lactone ( 6g )-containing secondary benzylic alcohols. Furthermore, the reaction is also scalable; on scaling the reaction 11-fold to 5.50 mmol, the coupling of 4-bromobenzonitrile ( 5a ) and 3a afforded an uncompromised yield under the same reaction time.…”

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confidence: 99%

Direct Synthesis of Secondary Benzylic Alcohols Enabled by Photoredox/Ni Dual-Catalyzed Cross-Coupling

Alam

Molander

2017

J. Org. Chem.

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An operationally simple, mild, redox-neutral method for the cross-coupling of α-hydroxyalkyltrifluoroborates is reported. Utilizing an Ir photocatalyst, α-hydroxyalkyl radicals are generated from the single-electron oxidation of the trifluoroborates, and these radicals are subsequently engaged in a nickel-catalyzed C-C bond-forming reaction with aryl halides. The process is highly selective, functional group tolerant, and step economical, which allows the direct synthesis of secondary benzylic alcohol motifs.

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“…Upon visible-light irradiation, the excited photocatalyst [Ir III ]* is formed, and is reductively quenched by single electron transfer from Cy2NMe, resulting in the generation of the highly reducing [Ir II ] and the radical cation A. The formation of 2a might be attributed to the protoncoupled electron transfer [62][63][64][65][66] from [Ir II ] to imine 1a, where the radical cation A donates a proton to 1a to form the -amino radical intermediates B and C, which underwent cross-coupling to give the desired unsymmetrical vicinal diamine 2a. On the other hand, in CH3OH, 1a preferentially abstracts a proton from CH3OH, than from A, which in turn prevents the generation of the -amino radical C from Cy2NMe, resulting in the homocoupling of B to selectively form the symmetrical diamine product 3a.…”

Section: Resultsmentioning

confidence: 99%

Distinctive Reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under Photoredox Catalysis

Tambe¹,

Min²,

Iqbal³

et al. 2020

Preprint

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A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the unprecedented reductive coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of phenyl substituent in the aniline moiety of the substrate was critical for the reactivity. The reaction proceeded via radical-radical cross-coupling of a-amino radicals generated by proton-coupled single-electron transfer in the presence of an Ir-photocatalyst. On the other hand, symmetrical 1,2-diamines were selectively produced from the same starting materials by judicious choice of the reaction conditions, showcasing the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines. The developed method can be employed for the synthesis of various bulky vicinal diamines, which are potential ligands in stereoselective synthesis.

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Proton‐Coupled Electron Transfer in Photoredox Catalytic Reactions

Cited by 104 publications

References 174 publications

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions

Direct Synthesis of Secondary Benzylic Alcohols Enabled by Photoredox/Ni Dual-Catalyzed Cross-Coupling

Distinctive Reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under Photoredox Catalysis

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