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Photoredox Catalysis of Polypyridyl
Transition Metal Complexes
Using Visible Light as a Reagent in Organic Synthesis
Methods for Photoredox Catalysis
Properties of Ru(bpy)32+

d6 (RuII); D3 symmetry (“pinwheel” chirality)
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s-donating; p-donating, accepting ligands (bpy)
Photochemistry (N.B. potentials v. SCE)

Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Top. Curr. Chem. 2007, 280, 117-214; Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.;
Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85-277; Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159-244
Nature of MLCT
Electronic Absorption Spectrum (Ground State)
Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani,
V. Top. Curr. Chem. 2007, 280, 117-214; Juris, A.; Balzani, V.;
Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord.
Chem. Rev. 1988, 84, 85-277
Electronic Emission Spectrum
Emission Lifetime v. T
@77K (solid), rt (dashed)
Consistent with MLCT, not LC
Methods for Photoredox Catalysis
SOMO Catalysis: Photochemical Advances
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a-Alkylation, Trifluoroalkylation, Benzylation of Aldehydes
Ru(bpy)32+
Nicewicz, D.A.; MacMillan, D.W.C. Science 2008, 322, 77-80; Nagib, D.A.; Scott, M.E.; MacMillan, D.W.C. J. Am. Chem. Soc. 2009, 131, 10875-10877; Shih, H.;
Vander Wal, M.N.; Grange, R.L.; MacMillan, D.W.C. J. Am. Chem. Soc. 2010, 132, 13600-13603
Photo-Trifluoromethylation of Silyl Enol Ethers
Pham, P.V.; Nagib, D.A.; MacMillan, D.W.C. Angew. Chem. Int. Ed. 2011, 50, 6119-6122
Reductive Intermolecular C-H Alkylation
Furst, L.; Matsuura, B.S.; Narayanam, J.M.R.; Tucker, J.W.; Stephenson, C.R.J. Org. Lett. 2010, 12, 3104-3107
Proposed Mechanism
Tucker, J.W.; Narayanam, J.M.R.; Krabbe, S.W.; Stephenson, C.R.J. Org. Lett. 2010, 13, 368-371; Furst, L.; Matsuura, B.S.; Narayanam, J.M.R.; Tucker, J.W.;
Stephenson, C.R.J. Org. Lett. 2010, 12, 3104-3107
Arylation of Dehalogenation Radical: Total
Synthesis of Gliocladin C


10 Steps, 30% Yield from Boc-D-Tryptophan Methyl Ester
Key Photoarylation step:
Furst, L.; Narayanam, J.M.R.; Tucker, J.W.; Stephenson, C.R.J. Angew. Chem. Int. Ed. 2011, 50, 9655-9659
Methods for Photoredox Catalysis
Reductive Dehalogenation
Narayanam, J.M.R.; Tucker, J.W.; Stephenson, C.R.J. J. Am. Chem. Soc. 2009, 131, 8756-8757
Alcohols to Halides: Phosphorus-Free Appel
Reactions
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Original Appel Reaction
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Arrow-Pushing Mechanism
Alcohols to Halides: Phosphorus-Free Appel
Reactions
Dai, C.; Narayanam, J.M.R.; Stephenson, C.R.J. Nature Chem. 2011, 3, 140-145
Proposed Mechanism: Appel Reaction

Photochemical Cycle
Dai, C.; Narayanam, J.M.R.; Stephenson, C.R.J. Nature Chem. 2011, 3, 140-145
Proposed Mechanism: Appel Reaction

Fate of Imidate

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Product obtained as racemate
Side product obtained during interrupted cycle, sterically encumbered alcohols
Dai, C.; Narayanam, J.M.R.; Stephenson, C.R.J. Nature Chem. 2011, 3, 140-145
Methods for Photoredox Catalysis
Oxidative [2+2] Cycloadditions
Ischay, M.A. ; Lu, Z.; Yoon, T.P. J. Am. Chem. Soc. 2010, 132, 8572-8574
Proposed Mechanisms: Oxidative [2+2]
Ischay, M.A. et al. J. Am. Chem. Soc. 2010, 132, 8572-8574; Teply, F. Collect. Czech. Chem. Commun. 2011, 76, 859-917
Reductive [2+2] Intramolecular Cycloadditions
Ischay, M.A.; Anzovino, M.E.; Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2008, 130, 12886-12887
Proposed Mechanisms: Reductive [2+2]
Ischay, M.A.; Anzovino, M.E.; Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2008, 130, 12886-12887; Teply, .F. Collect. Czech. Chem. Commun. 2011, 76, 859-917
Crossed Intermolecular [2+2] Cycloadditions
Key Challenges to Previous Precedent:
 Unproductive Events: Cis-trans isomerization

Self-reaction: Efficiently Generates Homodimers
Morrison, H.; Rodriguez, O. J. Photochem. 1974/75, 3, 471-474; Ischay, M.A.; Anzovino, M.E.; Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2008, 130, 12886-12887
Crossed Intermolecular [2+2] Cycloadditions
Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2009, 131, 14604-14605
Control and Scalability

Attempt for Direct Access to Photoexcited Enone
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Cis-trans isomerization disfavors direct photochemical coupling
Scalability
Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2009, 131, 14604-14605
Du, J.; Yoon, T.P. J. Am. Chem. Soc. 2009, 131, 14604-14605
Methods for Photoredox Catalysis
An Interesting Observation
Hurtley, A.E.; Cismesia, M.A.; Ischay, M.A.; Yoon, T.P. Tetrahedron, 2011, 67, 4442-4448
Reductive Formal [4+2] Heterocycloadditions
Hurtley, A.E.; Cismesia, M.A.; Ischay, M.A.; Yoon, T.P. Tetrahedron, 2011, 67, 4442-4448
Mechanism: Formal [4+2]
Hurtley, A.E.; Cismesia, M.A.; Ischay, M.A.; Yoon, T.P. Tetrahedron, 2011, 67, 4442-4448
Learning from Observation
Hurtley, A.E.; Cismesia, M.A.; Ischay, M.A.; Yoon, T.P. Tetrahedron, 2011, 67, 4442-4448
Reductive Cyclizations of Enones
Du, J.; Espelt, L.R.; Guzei, I.A.; Yoon, T.P. Chem. Sci. 2011, 2, 2115
Mechanism: Reductive Enone Cyclization
Du, J.; Espelt, L.R.; Guzei, I.A.; Yoon, T.P. Chem. Sci. 2011, 2, 2115
Where Can We Go From Here?
Reductive Formal [3+2] Cycloadditions
Lu, Z.; Shen, M.; Yoon, T.P. J. Am. Chem. Soc. 2011, 133, 1162-1164
Proposed Mechanism: Formal [3+2]
Lu, Z.; Shen, M.; Yoon, T.P. J. Am. Chem. Soc. 2011, 133, 1162-1164
Intermolecular Formal 3+2 Cycloadditions?

In development, but similar challenges to intermolecular [2+2]
Lu, Z.; Shen, M.; Yoon, T.P. J. Am. Chem. Soc. 2011, 133, 1162-1164
Methods for Photoredox Catalysis
Possible Future Directions

Enantioselective Perfluoroalkylation of Chiral Ketenamines
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Photoredox Substitution of Unactivated 1o & 2o Alcohols
Acknowledgements

Prof. Teshik Yoon & Prof. Corey Stephenson



Photographs
Prof. M. Christina White & CHEM 535
Prof. Martin D. Burke & the Burke Group
Methods for Photoredox Catalysis
ASAPs

Radical Cation [4+2] Cycloadditions of Unactivated Dienes
Ru(bpz)32+
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Oxidative C-H Arylation with Diazobenzene Using Pd(OAc)2/Ru(bpy)3Cl2
Lin, S.; Ischay, M.A.; Fry, C.G.; Yoon, T.P. J. Am. Chem. Soc. 2011 ASAP; Kalyani, D.; McMurtrey, K. B.; Neufeldt, S. R.; Sanford, M. S. J. Am. Chem. Soc. 2011
ASAP
Reductive Intramolecular C-H Alkylation
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Sn-Free Reductive Radical Cyclizations of p-Rich Heteroaromatics
Radical Cascade Cyclizations
Tucker, J.W.; Narayanam, J.M.R.; Krabbe, S.W.; Stephenson, C.R.J. Org. Lett. 2010, 13, 368-371
Mechanistic Evidence: Reductive Dehalogenation
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Internal Radical Clock
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Deuterium Incorporation
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Proposed Mechanism
Narayanam, J.M.R.; Tucker, J.W.; Stephenson, C.R.J. J. Am. Chem. Soc. 2009, 131, 8756-8757
Mechanistic Evidence: Appel Reaction

Evidence for
involvement of DMF

Evidence against
carbene mechanism
Dai, C.; Narayanam, J.M.R.; Stephenson, C.R.J. Nature Chem. 2011, 3, 140-145
Mechanistic Evidence: Appel Reaction

Evidence against
solvolysis of
oxyiminium ion

Figure taken from
reference
Dai, C.; Narayanam, J.M.R.; Stephenson, C.R.J. Nature Chem. 2011, 3, 140-145
Mechanistic Evidence: Oxidative Cycloaddition
Rxn charged with pure EZ
Fast erosion of EZ/EE;
Fast Olefin Isomerization
Relative to Cyclization
Erosion of EZ corresponds
to Formation of Minor
Diastereomer;
Stereospecific Cyclization;
Cycloaddition
Ischay, M.A. et al. J. Am. Chem. Soc. 2010, 132, 8572-8574
Mechanistic Evidence: Formal [3+2] Cycloaddition


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No reaction if photocatalyst, Lewis acid, amine (reductive quencher), or
light source is omitted
MgSO4 not necessary, contributes to reproducibility, slightly increased yield
(ca. 5%)
Diastereomeric cyclopropanes stereoconvergent in cyclization
Lu, Z.; Shen, M.; Yoon, T.P. J. Am. Chem. Soc. 2011, 133, 1162-1164
Early Examples

Photocatalytic Pschorr
Reaction (Cano-Yelo,
1984)

Debromination of
dibromostilbene (Willner,
1983)
Cano-Yelo, H.; Deronzier, A. J. Chem. Soc. Perkin Trans. 2; 1984, 1093-1098; Maidan, R.; Goren, Z.; Becker, J.Y.; Willner, I. J. Am. Chem. Soc. 1984, 106, 6217;
Goren, Z.; Willner, I. J. Am. Chem. Soc. 1983, 105, 7764
Early Examples

Radical Addition Of
Selenosulfonates to Olefins
(Barton, 1994)
Barton, D.H.R.; Csiba, M.A.; Jaszberenyi, J.C. Tet. Lett. 1994, 35, 2869