Organic Molecules, Photoredox, and. Catalysis

Transcription

1 Organic Molecules, Photoredox, and Catalysis 1

2 What is Photoredox Catalysis 2

3 Transition Metal vs Organic Photoredox Transition Metal Catalysts Organic Catalyst Reprinted (2017) with permission from (Wangelin, A. J. V., Majek, M. Acc. Chem. Res. 2016, ). Copyright (2016) American Chemical Society. Similar range of reduction potentials Ru(bpy) $/g Ir(ppy) $/g eosiny - 1 $/g 3

4 Why Photoredox? 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 4

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

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

7 Outline-Photophysical Process Photophysical Properties Catalyst Design Methods Absorption Decay ISC PET Cage escape Back electron transfer S 1 vs T 1 7

8 Absorption and Types of Decay Absorption of a photon causing an electron to excite from the ground state (S 0 ) to the singlet excited state (S 1 ) Radiative decay- Loss of radiation in the form of a photon. S 1 to S 0 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, 2008 Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of molecular photochemistry: an introduction; Viva Books: New Delhi,

9 Inter System Crossing (ISC) and Electron Transfer ISC is a radiationless spin relaxation from S 1 to the triplet T 1 and is a forbidden process Transition from T 1 to S 0 is a forbidden process. Relaxation from S 1 to ground state S 0 is an allowed transition A reductant can donate an electron to the excited substrate This transfer causes the formation of a radical ion pair 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,

10 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 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,

11 Outline-Photocatalyst Design Photophysical Properties Catalyst Design ph Quantum Yield Free Rotor Effect Heavy Atom Effect Donor-Acceptor Catalysts Methods 11

12 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 Romero, N. A.,Nicewicz, D. A. Chem. Rev. 2016, 116,

13 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. 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,

14 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 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,

15 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 T 1 state This increase is due to Spin Orbit Coupling and is called the heavy atom effect 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,

16 Donor-Acceptor Electron Transfer p-omettpp + λ max = 455nm E red =1.9V 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 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,

17 2004: A New Organic Photoredox Catalyst Acr + -Mes 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 S 1 and T 1 state Commercially available 1g/$170 Fukuzumi, et al. J. Am. Chem. Soc., 2004, 126, 1600.; Fukuzumi, et al. J. Am. Chem. Soc., 2004, 126, ; 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, 17 et al. Chem. Commun., 2010, 46, 601.; Fukuzumi, et al. Chem. Commun., 2011, 47, 8515.; Fukuzumi, et al. Chem. Sci., 2011, 2, 715.

18 Acr + -Mes Photochemical Applications Photophysical Properties Catalyst Design Methods 18

19 Anti-Markovnikov Alkene Hydrofunctionalization 19

20 Anti-Markovnikov Hydroetherification Z=(CH 2 ) n Nicewicz, D. A., et al. J. Am. Chem. Soc. 2012, 134,

21 Mechanism icewicz, D. A., et al. J. Am. Chem. Soc. 2012, 134,

22 Intramolecular Anti-Markovnikov Hydroamination Nicewicz, D. A., et al. J. Am. Chem. Soc. 2013, 135,

23 Intermolecular Anti-Markovnikov Hydroaminaation Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2014, 53,

24 Anti-Markovnikov Hydrofluoronation Nicewicz, D. A., et al. Nat. Chem., 2014, 6,

25 Anti-Markovnikov Hydrochloronation Nicewicz, D. A., et al. Nat. Chem., 2014, 6,

26 Anti-Markovnikov Hydrofunctionalization Nicewicz, D. A., et al. J. Am. Chem. Soc., 2013, 135, Nicewicz, D. A., et al. Chem. Sci. 2013, 4,

27 Polar Radical Crossover Additions 27

28 Polar Radical Crossover Additions Group Question Provide a reasonable mechanism, and a stereochemical rationale for the formation of the syn and anti products. Finally, indicate which will be the major product. Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2013, 52,

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

30 Group Question Answer Nicewicz, D. A., et al. Angew. Chem. Int. Ed., 2013, 52,

31 Decarboxylative Functionalization 31

32 Hydrodecarboxylation Nicewicz, D. A., et al. J. Am. Chem. Soc., 2015, 137, pp

33 Hydrodecarboxylation Nicewicz, D. A., et al. J. Am. Chem. Soc., 2015, 137, pp

34 Fluorination Ye, J., et al.chem. Commun. 2015,

35 Formal [3+2] Cycloadditions 35

36 Formal [3+2] Cycloaddition for Pyrrole Synthesis Xiao, W.-J., et al. Angew. Chem. Int. Ed., 2014, 53,

37 Proposed Mechanism Xiao, W.-J., et al. Angew. Chem. Int. Ed., 2014, 53,

38 Oxazole Synthesis via Azirine Aldehyde Cycloaddition Xiao, W. J., et al. Org. Lett., 2015, 17,

39 Aryl C-H Functionalization 39

40 Arene C-H Amination Nicewicz, D. A., et al. Science, 2015, 349,

41 Mechanism Nicewicz, D. A., et al. Science, 2015, 349,

42 Arene Cyanation Nicewicz, D. A., et al. J. Am. Chem. Soc., 2017, asaps 42

43 Conclusion Photophysical Properties 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 43

44 References Reviews Chanon. M, Julliard. M., Chem. Rev. 1983, 83, Albini. A, et al., Chem. Soc. Rev., 2013, 42, Wangelin, A. J. V., Majek, M. Acc. Chem. Res. 2016, 49, Romero, N. A.,Nicewicz, D. A. Chem. Rev. 2016, 116, 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,

45 Questions? 45