organic molecules, redox, and catalysis · inter system crossing (isc) and electron transfer...

Organic Molecules, Photoredox, and Catalysis

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What is Photoredox Catalysis

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Transition Metal vs Organic Photoredox

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• Similar range of reduction potentials

• Ru(bpy)33+ 104 $/g

• Ir(ppy)3+ 1300 $/g

• eosinY- 1 $/gReprinted (2017) with permission from (Wangelin, A. J. V., Majek, M. Acc. Chem. Res. 2016, 2316-2327). Copyright (2016) American Chemical Society.

Transition Metal Catalysts

Organic Catalyst

• Facilitates unique/exotic bond construction

• Occurs under mild conditions

• Orthogonal to polar or two electron processes

• Tunable catalysts

• Excited state regulates reactivity

• Redox neutral processes allow catalytic use of external oxidants and reductants

Why Photoredox?

4

Fukuzumi, et al. Org. Lett., 2005, 42655

Fukuzumi, et al. Org. Lett., 2005, 42656

Outline-Photophysical Process

• Absorption

• Decay

• ISC

• PET

• Cage escape

• Back electron transfer

• S1 vs T1

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PhotophysicalProperties

Catalyst Design

Methods

Absorption and Types of Decay

• Absorption of a photon causing an electron to excite from the ground state (S0) to the singlet excited state (S1)

• Radiative decay- Loss of radiation in the form of a photon. S1 to S0 is called fluorescence

• Non-radiative decay (Interconversion): loss of energy as heat/ vibrations/ and rotations.

Anslyn, E. V.; Dougherty, D. A. Modern physical organic chemistry; Univ. Science Books: Sausalito, CA, 2008Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of molecular photochemistry: an introduction; Viva Books: New Delhi, 2015.

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Inter System Crossing (ISC) and Electron Transfer


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• ISC is a radiationless spin relaxation from S1 to the triplet T1 and is a forbidden process

• Transition from T1 to S0 is a forbidden process.

• Relaxation from S1 to ground state S0 is an allowed transition

• A reductant can donate an electron to the excited substrate

• This transfer causes the formation of a radical ion pair

Cage Escape and Back Electron Transfer (BET)

• Cage Escape-The process of solvation and separation of a radical ion pair into free ions

• Only possible if the excited state has a long lifetime

• BET-The decay of an excited electron back to the ground state of the reductant

• BET causes no net reaction between the photocatalyst and the reductant


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Outline-Photocatalyst Design

• pH

• Quantum Yield

• Free Rotor Effect

• Heavy Atom Effect

• Donor-Acceptor Catalysts

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Catalyst Design

Methods

pH Effects on Absorption

• Isomer A and B are not catalytically active species and do not absorb in the visible light region

• pH is often neglected when catalytic species are reported in the literature and can drastically effect the absorption of a catalyst

12

Romero, N. A.,Nicewicz, D. A. Chem. Rev. 2016, 116, 10075−10166

Quantum Yield

• 𝜙 =#𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑡ℎ𝑎𝑡 𝑢𝑛𝑑𝑒𝑟𝑔𝑜 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑝𝑟𝑜𝑐𝑒𝑠𝑠

# 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑

• Quantum Yield is an indicator of a processes efficiency

• Quantum yield is usually measured in terms of relative rates to different processes and can be misleading

• Example: both of these have the same quantum yield but vastly different rates of fluorescence. The rate of ISC for pyrene-3-carboxaldahyde is also much faster then benzene.


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Free Rotor Effect

• Addition of substituents with many degrees of rotational/ vibrational freedom increases the efficiency of internal conversion

• The substitution of a simple alkyl group can drastically effect the quantum yield of a catalyst


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Why the bromines?

• Significant numbers of efficient photocatalysts contain multiple heavy atoms such as bromine or iodine.

• Heavy atoms lead to an increased population of the T1state

• This increase is due to Spin Orbit Coupling and is called the heavy atom effect

15Anslyn, E. V.; Dougherty, D. A. Modern physical organic chemistry; Univ. Science Books: Sausalito, CA, 2008Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of molecular photochemistry: an introduction; Viva Books: New Delhi, 2015.

Donor-Acceptor Electron Transfer

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• Efficient photocatalysts have charge separation or areas of high and low electron density separated by a lack of conjugation

• These donor-acceptor pairs facilitate electron transfer due to Marcus Theory


p-OMeTTPP+

λmax

= 455nm

Ered=1.9V

2004: A New Organic Photoredox Catalyst

• Strong absorption band at λ=430nm

• Strong donor-acceptors regions

• Catalyst’s excited state possesses high oxidation potential

• Reduced catalyst participates in further single electron transfer

• Mix between S1 and T1 state

• Commercially available 1g/$170

Acr+-Mes

Fukuzumi, et al. J. Am. Chem. Soc., 2004, 126, 1600.; Fukuzumi, et al. J. Am. Chem. Soc., 2004, 126, 15999.; Fukuzumi, et al.

Org. Lett., 2005, 7, 4265.; Fukuzumi, et al. Org. Lett., 2006, 8, 6079.; Griesbeck, A. G., et al. Org. Lett., 2007, 9, 611.; Fukuzumi, et

al. Chem. Commun., 2010, 46, 601.; Fukuzumi, et al. Chem. Commun., 2011, 47, 8515.; Fukuzumi, et al. Chem. Sci., 2011, 2, 715.

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Acr+-Mes Photochemical Applications

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Catalyst Design

Methods

Anti-Markovnikov Alkene Hydrofunctionalization

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Anti-Markovnikov Hydroetherification

Nicewicz, D. A., et al. J. Am. Chem. Soc. 2012, 134, 18577. 20

Z=(CH2)n

Mechanism

Nicewicz, D. A., et al. J. Am. Chem. Soc. 2012, 134, 18577. 21

Intramolecular Anti-Markovnikov Hydroamination

22Nicewicz, D. A., et al. J. Am. Chem. Soc. 2013, 135, 9588−9591

Intermolecular Anti-Markovnikov Hydroaminaation

Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2014, 53, 6198 23

Anti-Markovnikov Hydrofluoronation

Nicewicz, D. A., et al. Nat. Chem., 2014, 6, 720. 24

Anti-Markovnikov Hydrochloronation

Nicewicz, D. A., et al. Nat. Chem., 2014, 6, 720. 25

Anti-Markovnikov Hydrofunctionalization

Nicewicz, D. A., et al. J. Am. Chem. Soc., 2013, 135, 10334.Nicewicz, D. A., et al. Chem. Sci. 2013, 4, 3160–3165. 26

Polar Radical Crossover Additions

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Polar Radical Crossover Additions

Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2013, 52, 3967. 28

Group Question

Provide a reasonable mechanism, and a stereochemicalrationale for the formation of the syn and anti products. Finally, indicate which will be the major product.

Mechanism

Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2013, 52, 3967.29

Group Question Answer

Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2013, 52, 3967.30

Decarboxylative Functionalization

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Hydrodecarboxylation

Nicewicz, D. A., et al. J. Am. Chem. Soc., 2015, 137, pp 11340–1134832

Hydrodecarboxylation

Nicewicz, D. A., et al. J. Am. Chem. Soc., 2015, 137, pp 11340–1134833

Fluorination

Ye, J., et al.Chem. Commun. 2015, 11864. 34

Formal [3+2] Cycloadditions

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Formal [3+2] Cycloaddition for Pyrrole Synthesis

Xiao, W.-J., et al. Angew. Chem. Int. Ed., 2014, 53, 565336

Proposed Mechanism

Xiao, W.-J., et al. Angew. Chem. Int. Ed., 2014, 53, 565337

Oxazole Synthesis via Azirine–Aldehyde Cycloaddition

Xiao, W. J., et al. Org. Lett., 2015, 17, 4070.38

Aryl C-H Functionalization

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Arene C-H Amination

Nicewicz, D. A., et al. Science, 2015, 349, 6254.40

Mechanism

Nicewicz, D. A., et al. Science, 2015, 349, 6254.41

Arene Cyanation

Nicewicz, D. A., et al. J. Am. Chem. Soc., 2017, asaps42

Conclusion

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Catalyst Design

Methods

• Organic photoredox catalysis is an expanding field

• Many privileged catalytic scaffolds other than the acridinium family exist

• Applications towards oxidation, reduction, C-H functionalization, and decarboxylative functionalization have been discovered

• New dual catalytic systems should be developed to enable enantioselective catalysis

• Increased oxidation and reduction potentials are necessary for the discovery of new types of synthetic disconnections

References

Books

• Anslyn, E. V.; Dougherty, D. A. Modern physical organic chemistry; Univ. Science Books: Sausalito, CA, 2008

• Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of molecular photochemistry: an introduction; Viva Books: New Delhi, 2015.

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Reviews

• Chanon. M, Julliard. M., Chem. Rev. 1983, 83, 426-506.

• Albini. A, et al., Chem. Soc. Rev., 2013, 42, 97-113

• Wangelin, A. J. V., Majek, M. Acc. Chem. Res. 2016, 49, 2316-2327

• Romero, N. A.,Nicewicz, D. A. Chem. Rev. 2016, 116, 10075−10166

Questions?

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organic molecules, redox, and catalysis · inter system crossing (isc) and electron transfer...

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