The sensitization of stilbene isomerization

The regioselectivity in the oxidative electron-transfer cycloreversion (CR) of 2,3-diaryloxetanes depends on the substitution of the aryl groups and on the nature of the electrontransfer photosensitizer. The reaction occurs with fragmentation of the C2−C3 and C4−O bonds either in the presence of electron-releasing substituents in the 3-aryl groups, or when chloranil is used as photosensitizer. Thus, CR of the methoxysubstituted derivative trans,trans-3-(4-methoxyphenyl)-4-methyl-2-phenyloxetane (1b) results in the production of trans-anethole and benzaldehyde. In this case, the trans-ane-

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“…The triplet energies for 2a and 2b are 53 and 52 kcal/mol, respectively, [27] and the coulombic factor (E coul. ) can be neglected in this case.…”

Section: Effect Of Substitution On the Cr Pathwaymentioning

confidence: 99%

Electron‐Transfer Cycloreversion of 2,3‐Diaryloxetanes: Influence of the Substitution and the Photosensitizer on the Regioselectivity

Izquierdo

Miranda

2004

Eur J Org Chem

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“…This energy was reported to be 2.30 eV according to ref. 28. For better fitting we permitted ourselves to vary it, but only a little bit and finally chose E T ¼ 1.8 eV.…”

Section: B Triplet Channel Of Backward Electron Transfermentioning

confidence: 99%

The crucial role of triplets in photoinduced charge transfer and separationElectronic Supplementary Data (ESI) available: Reversible triplet transfer (Appendix A). See http://www.rsc.org/suppdata/cp/b2/b201784a/

Burshteǐn

Ivanov

2002

Phys. Chem. Chem. Phys.

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“…Common organic photosensitizers employed in transition-metal-free photocatalytic transformations and their ground- and excited-state redox properties. − Potentials highlighted in red correspond to the reduction potentials of the photosensitizers, while potentials highlighted in blue refer to their corresponding oxidation potentials.…”

Section: Introductionmentioning

confidence: 99%

Understanding the Kinetics and Spectroscopy of Photoredox Catalysis and Transition-Metal-Free Alternatives

2016

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Over the past decade, the field of photoredox catalysis has gained increasing attention in synthetic organic chemistry because of its wide applicability in sustainable free-radical-mediated processes. Numerous examples have shown that under carefully optimized conditions, efficient and highly selective processes can be developed through excitation of a photosensitizer using inexpensive, readily available light sources. However, despite all of these recent advancements, some generalizations and/or misconceptions have become part of the photoredox culture, and often many of these discoveries lack in-depth investigations into the excited-state kinetics and underlying mechanisms. In this Account, we begin with a tutorial for understanding both the redox properties of excited states and how to measure the kinetics of excited-state processes. We discuss the generalization of direct excitation of closed-shell species to generate more potent reductive or oxidative excited states, using the helium atom as a quantitative example. We also outline how to apply redox potentials to calculate whether the proposed electron transfer events are thermodynamically feasible. In the second half of our tutorial, we discuss how to measure the kinetics of excited-state processes using techniques such as steady-state and time-resolved fluorescence and transient spectroscopy and how to apply the data using Stern-Volmer and kinetic analysis. Then we shift gears to discuss our recent contributions to the field of photoredox catalysis. Our lab focuses on developing transition-metal-free alternatives to ruthenium and iridium bipyridyl complexes for these transformations, with the goal of developing systems in which the reaction kinetics is more favorable. We have found that methylene blue, a member of the thiazine dye family, can be employed in photoredox processes such as oxidative hydroxylations of arylboronic acids to phenols. Interestingly, we were able to demonstrate that methylene blue is more efficient for this reaction than Ru(bpy)3Cl2, which upon further examination using transient spectroscopic techniques we were able to relate to the reductive quenching ability of the aliphatic amine. Recently we were also successful in applying methylene blue for radical trifluoromethylation reactions, which is discussed in detail. Finally, we have also demonstrated that common organic electron donors, such as α-sexithiophene, can be used in photoredox processes, which we demonstrate using the dehalogenation of vic-dibromides as a model system. This is a particularly interesting system because well-defined, long-lived intermediates allowed us to fully characterize the catalytic cycle. Once again, through an in-depth kinetic analysis we were able to gain valuable insights into our reaction mechanism, which demonstrates how powerful a tool proper kinetic analysis can be in the design and optimization of photoredox processes.

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The sensitization of stilbene isomerization

Cited by 18 publications

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Electron‐Transfer Cycloreversion of 2,3‐Diaryloxetanes: Influence of the Substitution and the Photosensitizer on the Regioselectivity

Electron‐Transfer Cycloreversion of 2,3‐Diaryloxetanes: Influence of the Substitution and the Photosensitizer on the Regioselectivity

The crucial role of triplets in photoinduced charge transfer and separationElectronic Supplementary Data (ESI) available: Reversible triplet transfer (Appendix A). See http://www.rsc.org/suppdata/cp/b2/b201784a/

Understanding the Kinetics and Spectroscopy of Photoredox Catalysis and Transition-Metal-Free Alternatives

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