Misplaced Pages

Revision as of 23:26, 3 November 2013

This sandbox is in the Misplaced Pages talk namespace. Either move this page into your userspace, or remove the {{User sandbox}} template.

Photoredox catalysis is a branch of catalysis where a chemical reaction is accelerated by harnessing the energy of visible light. This branch is named as a combination of “photo-” referring to light and “redox”, a condensed expression for the chemical processes of reduction and oxidation. In particular, photoredox catalysis employs small quantities of a light sensitive chemical that, when excited by light, can mediate the transfer of electrons between chemical compounds that would otherwise not react. One advantage of using these catalysts rather than stronger reagents are that photoredox catalysts and the mild reagents used alongside them are generally bench-stable compounds that can be stored for long periods of time, whereas more reactive redox agents are naturally more prone to decomposition through exposure to the atmosphere over long periods of time. For the same reason, photoredox catalysts tend to be more chemoselective than more reactive redox agents and can perform certain chemical transformations more reliably on sensitive and complex substrates. Finally, in certain cases such as reductive dehalogenation (discussed below) the use of photoredox catalysts allows highly toxic chemicals to be replaced with much less dangerous reagents. Photoredox catalysts are generally drawn from three classes of materials: transition-metal complexes, organic dyes, and semiconductors. While each class of materials has advantages, transition-metal complexes are used most often because they are homogeneous catalysts unlike semiconductors and because they tend to have stronger redox capabilities than most organic dyes.

Photochemistry of Ru(bipy)₃ and Similar Photocatalysts

The activity of a photoredox catalyst can be described in three steps. First, a molecule of the catalyst in its ground state, where the electrons are distributed among the lowest-energy combination of states, interacts with light and moves into a long-lived “excited state”, where the electrons are not distributed among the lowest energy combination of available states. Second, the photoexcited catalyst interacts by an outer-sphere single electron transfer (redox) process to “quench” the excited state and to activate one of the other components of the chemical reaction. Finally, a second single electron transfer occurs to regenerate the ground-state catalyst.

The first step of this process, photoexcitation, is initiated by absorption of a photon, promoting an electron from the highest occupied molecular orbital (HOMO) of the photocatalyst to the lowest energy spin-allowed state. Thus, since the ground state of the photocatalyst is a singlet state (a state with no total electron spin), the photon absorption excites the catalyst to another singlet state with the next lowest energy. Formally this excitation consists of a metal-to-ligand charge transfer, where the electron moves from an orbital centered on the metal (e.g. a d orbital) to an orbital localized on the ligands (e.g. the π* orbital of an aromatic ligand). This singlet excited state can relax by two distinct processes: the catalyst may radiate a photon and return to the singlet ground state, a process termed fluorescence, or it can move to the lowest energy triplet excited state (a state where two unpaired electrons have the same spin) by a non-radiative process termed intersystem crossing. Relaxation from the triplet excited state to the ground state requires both radiation of a photon and inversion of the spin of the excited electron via a process termed phosphorescence. The spin-forbidden nature of this pathway means that it is a slow process and therefore that the triplet excited state has a substantial average lifetime. For the common photocatalyst tris-(2,2’-bipyridyl)ruthenium chloride (also known as Ru(bipy)3), the lifetime of the triplet excited state has been measured to be approximately 1100 ns, long enough that other relaxation pathways, specifically electron-transfer pathways, can occur more rapidly than decay of the catalyst to its ground state.

The long-lived triplet excited state accessible by photoexcitation is chemically interesting because it is both a more potent reducing agent and a more potent oxidizing agent than the ground state of the catalyst molecule. In other words, the catalyst can more readily give up one its electrons or accept an electron from an external source. The catalyst excited state is a stronger reductant because its highest energy electron has been excited to an even higher energy state through the photoexcitation process. Similarly, the excited catalyst is a stronger oxidant because one of the catalyst’s lowest energy orbitals, which is fully occupied in the ground state, is only singly occupied after photoexcitation and is therefore available for an external electron to occupy. Since organometallic photocatalysts consist of a coordinatively saturated metal complex, i.e. a structure that cannot form any additional bonds, electron transfer cannot take place by an “inner-sphere” mechanism through a direct bond of the metal complex to another reagent. Instead, electron transfer must take place via an “outer sphere” process, where the electron tunnels between the catalyst and another molecule. The theory of outer sphere electron transfer, developed by Rudolph Marcus, predicts that such a tunneling process will occur most quickly in systems where the transfer is not only thermodynamically favorable (i.e. between strong reductants and oxidants) but also in rigid systems that will undergo only a small amount of reorganization during the electron transfer. Photocatalysts such as Ru(bipy)3, are held in a rigid arrangement by flat, bidentate ligands arranged in an octahedral geometry around the metal center. Therefore, the complex will not undergo much reorganization during an electron transfer and the process is therefore likely to be fast. Since electron transfer of these complexes is fast, it is very likely to take place within the duration of the catalyst’s active state, i.e. during the lifetime of the triplet excited state.

The final step in the photocatalytic cycle is the regeneration of the photocatalyst in its ground state. At this stage, the catalyst exists as the ground state of either its oxidized or reduced forms, depending on whether it acted as a reductant or oxidant in the electron transfer step of the cycle. In order to regenerate the original ground state, the catalyst must participate in a second outer-sphere electron transfer. In many cases, this electron transfer takes place with a stoichiometric two-electron reductant or oxidant, although in some recent cases this step has also been implemented to activate a second reagent.

Photophysical Properties of Common Photoredox Catalysts

In the study of photoredox catalysis, it is essential to choose a catalyst for which the reduction and oxidation potentials are matched to the other components of the reaction. While ground state redox potentials are easily measured by cyclic voltammetry or other electrochemical methods, measuring the redox potential of an electronically excited state cannot be accomplished directly by these methods. However, it is possible to calculate the excited state redox potentials from ground state redox potentials and spectroscopic data.

The relative reducing and oxidizing natures of these photocatalysts can be understood intuitively by considering the electronegativity of the ligands and the metal center of the catalyst complex. More electronegative metals and ligands will tend to stabilize electrons better than their less electronegative counterparts. Therefore, complexes with more electronegative complexes will be more easily reduced (i.e. be more powerfully oxidizing) than more electropositive complexes. For example, the ligands 2,2'-bipyridine and 2,2'-phenylpyridine are isoelectronic structures, containing the same number and arrangement of electrons. However, phenylpyridine replaces one of the nitrogen atoms in bipyridine with a carbon atom. Carbon is less electronegative than nitrogen is, so it holds electrons less tightly than nitrogen. Since the remainder of the ligand molecule is identical, this effect is transferred to the structure as a whole: phenylpyridine holds is electrons less tightly than bipyridine, i.e. it is more strongly electron-donating and less electronegative as a ligand. Complexes with phenylpyridine ligands will therefore be more strongly reducing and less strongly oxidizing than complexes with bipyridine ligands because the phenylpyridine complexes are more electron-rich and hold their electrons with slightly less strength. In the same way, a fluorinated derivative of phenylpyridine will be more electronegative than the corresponding simple phenylpyridine and complexes with fluorine-containing ligands will be more strongly oxidizing and less strongly reducing than the unsubstituted phenylpyridine complex. The electronic influence of the metal center on the complex is somewhat more complex than the effect of the ligands. According to the Pauling scale of electronegativity, both ruthenium and iridium have an electronegativity of 2.2. If this was the sole factor relevant to the redox potentials, then complexes of ruthenium and iridium with the same ligands should be equally powerful photoredox catalysts. However, in determining the excited-state redox potentials by the Rehm-Weller equation, spectroscopic terms related to the energy of the excited state are incorporated in the zero-zero excitation energy of the catalyst. This value is related to the maximum emission of the complex, and therefore to the size of the Stokes shift - the difference in energy between the maximum absorption and emission of a molecule. Ruthenium complexes typically have large Stokes shifts, and therefore have low energy emission wavelengths and small zero-zero excitation energies, when compared to iridium complexes. In effect, this means that although ground-state ruthenium complexes can be potent reductants, that the excited-state complex will be a much less potent reductant or oxidant than a comparable iridium complex. This makes iridium photocatalysts more favorable for the development of general organic transformations because the stronger redox potentials of the excited catalyst allows the use of weaker stoichiometric reductants and oxidants or the use of less reactive substrates.

Applications of Photoredox Catalysis in Organic Chemistry

Reductive Dehalogenation:

Reductive dehalogenation is a convenient method for removing halogen atoms that are introduced into a molecule by a method such as halolactonization. However, the standard method for removing halogen atoms requires the use of stoichiometric organotin reagents, such as tributyltin hydride. While this reaction is very powerful and orthogonal to other functional groups commonly present in organic molecules, organotin reagents are highly toxic and the development of alternative reagents is desirable. In 2009, the Stephenson lab developed the first photoredox approach to the problem of reductive dehalogenation. This method employs Ru(bipy)₃ as the photocatalyst and a stoichiometric amine reductant to reduce "activated" carbon-halogen bonds, such as those with an adjacent carbonyl group or arene. These bonds are considered to be activated because the radical they produce upon fragmentation is stabilized by conjugation with the carbonyl group or arene, respectively. The stoichiometric reductant present in this reaction transfers an electron to reduce the excited-state catalyst to the Ru(I) oxidation state. The reduced catalyst can then shuttle the transferred electron to the halogenated substrate, reducing the weak C-X bond and inducing fragmentation. In subsequent work published in 2012, Stephenson accomplished the reduction of unactivated carbon-iodine bonds using the strongly reducing photocatalyst tris-(2,2’-phenylpyridine)iridium (Ir(ppy)₃). This updated reaction is mechanistically distinct from the previous transformation of activated bromides and chlorides. In this version of the reaction, fac-Ir(ppy)₃ is used as a photocatalyst. The increased reduction potential of this catalyst compared to Ru(bipy)₃ allows direct reduction of the carbon-iodine bond without first interacting with a stoichiometric reductant. Thus, the iridium complex transfers an electron to the substrate, causing fragmentation of the substrate and oxidizing the catalyst to the Ir(IV) oxidation state. The oxidized photocatalyst is then easily returned to its original oxidation state through interaction with one of the reaction additives.

Just as tin-mediated radical dehalogenation reactions can be used to initiate cascade cyclizations to rapidly generate molecular complexity, Stephenson's tin-free reductive dehalogenation allows access to the same types of complex products. In this work, Stephenson et al. presented a radical cascade cyclization that closed two five-membered rings and formed two new stereocenters, proceeding in good yield. Furthermore, the Stephenson group has made use of their reductive dehalogenation protocol in a key step of their total synthesis of the natural product (+)-Gliocladin C.

Generation of Iminium Ions from Amines Iminium ions are potent electrophiles useful for generating C-N bonds in complex molecules. However, the condensation of amines with carbonyl compounds to form iminium ions is often an unfavorable process, sometimes requiring harsh dehydration conditions. For this reason, alternative methods for the generation of iminium ions, particularly by oxidation from the corresponding amine, is a valuable synthetic tool. The discovery by the Stephenson lab that amines could act as the stoichiometric reductant in their photoredox reductive dehalogenation suggested that photoredox catalysis would be an effective means of oxidatively generating iminium ions from the corresponding amine. In fact, the Stephenson lab achieved the generation of iminium ions from activated amines by the use of Ir(dtbbpy)(ppy)₂PF₆ as a photoredox catalyst. The substrates investigated for this reaction were aryltetrahydroisoquinolines, substrates which would stabilize generation of nitrogen-centered radicals through conjugation with one of the arenes and would promote rapid and regioselective H-atom abstraction to generate the conjugated benzylic imine. Stephenson et al. propose that this transformation occurs by oxidation of the amine to the aminium radical cation by the excited photocatalyst, which is strongly oxidizing due to the electrophilicity of iridium and due to the electron-poor nature of the fluorinated ligands, followed by H-atom transfer to a superstoichimetric oxidant, such as nitromethane (also functioning in the reaction as a nucleophile and as the solvent) or molecular oxygen. Finally, the reactive iminium ion formed by the H-atom transfer is quenched by reaction with any nucleophile present in the reaction. Among the nucleophiles for which photoredox addition to iminium ions has been investigated are nitromethane (aza-Henry reaction), cyanide (Strecker reaction), silyl enol ethers (Mannich reaction), allyl silanes, and indole (Friedel-Crafts reaction).

Oxidative Removal of the PMB protecting group The development of orthogonal protecting group chemistry is a crucial problem in organic synthesis because it is the use of these protecting groups that allows each instance of a common functional group, such as the hydroxyl group, to be distinguished during the synthesis of a complex molecule. One very common protecting group for the hydroxyl functional group is the para-methoxy benzyl (PMB) ether. This protecting group is chemically very similar to the less electron-rich benzyl ether. The usual method for selective cleavage of a PMB ether in the presence of a benzyl ether is through the use of strong stoichiometric oxidants such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or ceric ammonium nitrate (CAN). PMB ethers are far more susceptible to oxidation than benzyl ethers because they are more electron-rich. Research by Stephenson, et al. has shown that the selective deprotection of PMB ethers can be achieved through the use of the iridium complex Ir₂(dtbbpy)PF₆ and a mild stoichiometric oxidant such as CBrCl₃. The photoexcited iridium catalyst is reducing enough to fragment the polyhalomethane compound to form trichloromethyl radical, bromide anion and the Ir(IV) oxidation state of the catalyst. The electron-poor nature of the fluorinated ligands means that this iridium complex can be readily reduced: in particular, by an electron-rich arene such as a para-methoxy benzyl ether. After the arene is oxidized, it will readily participate in H-atom transfer with trichloromethyl radical to form chloroform and an oxocarbenium ion, which is readily hydrolyzed to reveal the free hydroxide. This reaction was demonstrated to be orthogonal to many common protecting groups, especially with the addition of a base to counteract the buildup of HBr during the reaction.

Cycloadditions

Cycloadditions and other pericyclic reactions are powerful transforms in organic synthesis because of their potential to rapidly generate complex molecular architectures and particularly because of their capacity to set multiple adjacent stereocenters in a highly controlled manner. However, only certain cycloadditions are allowed under thermal conditions according to the Woodward-Hoffmann rules of orbital symmetry, or other equivalent models such as frontier molecular orbital theory (FMO) or the Dewar-Zimmermann model. Cycloadditions which are not thermally allowed, such as the cycloaddition, can be enabled by photochemical activation of the reaction. Under uncatalyzed conditions, this activation requires the use of high energy ultraviolet light capable of altering the orbital populations of the reactive compounds. Alternatively, metal catalysts such as cobalt and copper have been reported to catalyze thermally-forbidden cycloadditions via single electron transfer. Recent work by the Yoon lab has demonstrated that the required change in orbital populations can be achieved by electron transfer with a photocatalyst sensitive to lower energy visible light. Yoon's work has demonstrated the efficient intra- and intermolecular cycloadditions of activated olefins: particularly enones and styrenes. Enones, or electron-poor olefins, were discovered to react via a radical-anion pathway, utilizing diisopropylethylamine as a transient source of electrons. For this electron-transfer, Ru(bipy)₃ was discovered to be an efficient photocatalyst. Conversely, electron-rich styrenes were found to react via a radical-cation mechanism, utilizing methyl viologen or molecular oxygen as a transient electron sink. While Ru(bipy)₃ proved to be a competent catalyst for intramolecular cyclizations using methyl viologen, it could not be used with molecular oxygen as an electron sink or for intermolecular cyclizations. For intermolecular cyclizations, Yoon et al. discovered that the more strongly oxidizing photocatalyst Ru(bpm)₃ and molecular oxygen provided a catalytic system better suited to access the radical cation necessary for the cycloaddition to occur. Ru(bpz)₃, a still more strongly oxidizing photocatalyst, proved to be problematic because it was not only strong enough to oxidize the reagents and catalyze the desired cycloaddition, but also was strong enough to oxidize the cycloadduct and catalyze the retro- reaction. This comparison of photocatalysts highlights the importance of tuning the redox properties of a photocatalyst to the reaction system as well as demonstrating the value of polypyridyl compounds as ligands due to the ease with which they can be modified to adjust the redox properties of their complexes.

In addition to research on the thermally-forbidden cycloaddition, research in the Yoon group has also studied photoredox catalysis of the cyclization, also known as the Diels-Alder reaction. Yoon et al. discovered that bis-enones, similar to the substrates used for the photoredox cyclization, but with a longer linker joining the two enone functional groups, underwent intramolecular radical-anion hetero-Diels-Alder reactions more rapidly than cycloaddition. Similarly, Yoon et al. discovered that electron-rich styrenes participated in intra- or intermolecular Diels-Alder cyclizations via a radical cation mechanism. Conversely to the Yoon's lab discovery that Ru(bipy)₃ was competent for intramolecular, but not intermolecular cyclizations, Yoon et al. discovered that Ru(bipy)₃ was a competent catalyst for intermolecular, but not intramolecular, Diels-Alder cyclizations.

References

1. Tucker, Joseph W. (2012). "Shining Light on Photoredox Catalysis: Theory and Synthetic Applications". The Journal of Organic Chemistry. 77 (4): 1617–1622. doi:10.1021/jo202538x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
2. Rillema, D. Paul (1983). "Redox properties of ruthenium(II) tris chelate complexes containing the ligands 2,2'-bipyrazine, 2,2'-bipyridine, and 2,2'-bipyrimidine". Inorganic Chemistry. 22 (11): 1617–1622. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
3. Crutchley, R. J. (1980). "Ruthenium(II) tris(bipyrazyl) dication - a new photocatalyst". Journal of the American Chemical Society. 102 (23): 7128–7129. doi:10.1021/ja00543a053. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
4. Juris, Alberto (4 November 1981). "Characterization of the Excited State Properties of Some New Photosensitizers of the Ruthenium (Polypyridine) Family". Helvetica Chimica Acta. 64 (7): 2175–2182. doi:10.1002/hlca.19810640723. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
5. Young, Roger C. (1976). "Electron transfer quenching of excited states of metal complexes". Journal of the American Chemical Society. 98 (1): 286–287. doi:10.1021/ja00417a073. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
6. Lowry, Michael S. (2005). "Single-Layer Electroluminescent Devices and Photoinduced Hydrogen Production from an Ionic Iridium(III) Complex". Chemistry of Materials. 17 (23): 5712–5719. doi:10.1021/cm051312. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
7. Lowry, Michael S. (2005). "Single-Layer Electroluminescent Devices and Photoinduced Hydrogen Production from an Ionic Iridium(III) Complex". Chemistry of Materials. 17 (23): 5712–5719. doi:10.1021/cm051312. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
8. Flamigni, Lucia (2007). "Photochemistry and Photophysics of Coordination Compounds: Iridium". Topics in Current Chemistry: 143–203. doi:10.1007/128_2007_131. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
9. Tucker, Joseph W. (2012). "Shining Light on Photoredox Catalysis: Theory and Synthetic Applications". The Journal of Organic Chemistry. 77 (4): 1617–1622. doi:10.1021/jo202538x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
10. Tucker, Joseph W. (2012). "Shining Light on Photoredox Catalysis: Theory and Synthetic Applications". The Journal of Organic Chemistry. 77 (4): 1617–1622. doi:10.1021/jo202538x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
11. Narayanam, Jagan M. R. (06/05/2009). "Electron-Transfer Photoredox Catalysis: Development of a Tin-Free Reductive Dehalogenation Procedure". JACS. 131 (25): 8756–8757. doi:10.1021/ja9033582. {{cite journal}}: |access-date= requires |url= (help); Check date values in: |accessdate= and |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
12. Nguyen, John D. (2012). "Engaging unactivated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions". Nature Chemistry. 4 (10): 854–859. doi:10.1038/nchem.1452. {{cite journal}}: |access-date= requires |url= (help); Check date values in: |accessdate= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
13. Tucker, Joseph W. (28 May 2010). "Tin-free radical cyclization reactions initiated by visible light photoredox catalysis". Chemical Communications. 46 (27): 4985–4987. doi:10.1039/c0cc00981d. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
14. Furst, Laura (4 October 2011). "Total Synthesis of (+)-Gliocladin C Enabled by Visible-Light Photoredox Catalysis". Angewandte Chemie International Edition. 50 (41): 9655–9659. doi:10.1002/anie.201103145. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
15. Condie, Allison G. (10 February 2010). "Visible-Light Photoredox Catalysis: Aza-Henry Reactions via C−H Functionalization". Journal of the American Chemical Society. 132 (5): 1464–1465. doi:10.1021/ja909145y. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
16. Condie, Allison G. (10 February 2010). "Visible-Light Photoredox Catalysis: Aza-Henry Reactions via C−H Functionalization". Journal of the American Chemical Society. 132 (5): 1464–1465. doi:10.1021/ja909145y. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
17. Rueping, Magnus (2011). "Visible-light photoredox catalyzed oxidative Strecker reaction". Chemical Communications. 47: 12709–12711. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
18. Zhao, Guolei (2012). "Visible light-induced oxidative coupling reaction: easy access to Mannich-type products". Chemical Communications. 48: 2337–2339. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
19. Freeman, David B. (6 January 2012). "Functionally Diverse Nucleophilic Trapping of Iminium Intermediates Generated Utilizing Visible Light". Organic Letters. 14 (1): 94–97. doi:10.1021/ol202883v. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
20. Tucker, Joseph W. (2011). "Oxidative photoredox catalysis: mild and selective deprotection of PMB ethers mediated by visible light". Chemical Communications. 47 (17): 5040–5042. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
21. Ischay, Michael A. (2008). "Efficient Visible Light Photocatalysis of Enone Cycloadditions". Journal of the American Chemical Society. 130 (39): 12886–12887. doi:10.1021/ja805387f. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
22. Du, Juana (21 October 2009). "Crossed Intermolecular Cycloadditions of Acyclic Enones via Visible Light Photocatalysis". Journal of the American Chemical Society. 131 (41): 14604–14605. doi:10.1021/ja903732v. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
23. Ischay, Michael A. (30 June 2010). " Cycloadditions by Oxidative Visible Light Photocatalysis". Journal of the American Chemical Society. 132 (25): 8572–8574. doi:10.1021/ja103934y. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
24. Tyson, Elizabeth L. (17 February 2012). "Photocatalytic Cycloadditions of Enones with Cleavable Redox Auxiliaries". Organic Letters. 14 (4): 1110–1113. doi:10.1021/ol3000298. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
25. Ischay, Michael A. (2012). "Crossed intermolecular cycloaddition of styrenes by visible light photocatalysis". Chemical Science. 3 (9): 2807. doi:10.1039/c2sc20658g. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
26. Hurtley, Anna E. (2011). "Visible light photocatalysis of radical anion hetero-Diels–Alder cycloadditions". Tetrahedron. 67 (24): 4442–4448. doi:10.1016/j.tet.2011.02.066. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
27. Lin, Shishi (7 December 2011). "Radical Cation Diels–Alder Cycloadditions by Visible Light Photocatalysis". Journal of the American Chemical Society. 133 (48): 19350–19353. doi:10.1021/ja2093579. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
28. Lin, Shishi (2012). "Visible light photocatalysis of intramolecular radical cation Diels–Alder cycloadditions". Tetrahedron Letters. 53 (24): 3073–3076. doi:10.1016/j.tetlet.2012.04.021. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)