| Revision as of 16:15, 2 November 2013 editAMayfield (talk | contribs)156 edits →Applications of Photoredox Catalysis in Organic Chemistry← Previous edit |
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'''Introduction''' |
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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”, itself a condensed expression for reduction and oxidation, that refers to electron transfer processes. In particular, photoredox catalysis employs small quantities of a light sensitive chemical that, when excited by light, can readily donate or remove an electron from another chemical in order to improve its rate of reaction. |
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While photoredox catalysts do not remove the need for a stoichiometric reducing or oxidizing agent, the mediating effect of the catalyst allows the use of milder stoichiometric reagents. Similarly, photoredox catalysts allow the use of visible light as a source of energy for some reactions rather than higher energy ultraviolet light. |
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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. |
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==Photochemistry of Ru(bipy)<sub>3</sub><sup>2+</sup> and Similar Photocatalysts== |
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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. |
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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. |
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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. |
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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. |
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'''Photophysical Properties of Common Photoredox Catalysts''' |
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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.<ref>{{cite journal|last=Tucker|first=Joseph W.|coauthors=Stephenson, Corey R. J.|title=Shining Light on Photoredox Catalysis: Theory and Synthetic Applications|journal=The Journal of Organic Chemistry|year=2012|volume=77|issue=4|pages=1617-1622|doi=10.1021/jo202538x}}</ref> |
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{| class="wikitable sortable" |
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! Photocatalyst !! E<sub>1/2</sub>(C<sup>+</sup>/C) (V vs SCE) !! E<sub>1/2</sub>(C/C<sup>-</sup>) (V vs SCE) !! E<sub>1/2</sub>(C<sup>+</sup>/C*) (V vs SCE) !! E<sub>1/2</sub>(C*/C<sup>-</sup>) (V vs SCE) !! Excited-State Lifetime (ns) !! Peak Excitation Wavelength (nm) !! Peak Emission Wavelength (nm) !! Reference |
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| tris-(2,2'-bipyrimidine)ruthenium<sup>2+</sup> || 1.69 || -0.91 || -0.21 || 0.99 || 131 || 454 || 639 ||<ref>{{cite journal|last=Rillema|first=D. Paul|coauthors=Allen, G.; Meyer, T. J.; Conrad, D.|title=Redox properties of ruthenium(II) tris chelate complexes containing the ligands 2,2'-bipyrazine, 2,2'-bipyridine, and 2,2'-bipyrimidine|journal=Inorganic Chemistry|year=1983|volume=22|issue=11|pages=1617-1622}}</ref> |
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| tris-(2,2'-bipyrazine)ruthenium<sup>2+</sup> || 1.86 || -0.80 || -0.26 || 1.45 || 740 || 443 || 591 || Example |
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| tris-(2,2'-bipyridine)ruthenium<sup>2+</sup> || 1.29 || -1.33 || -0.81 || 0.77 || 1100 || 452 || 615 || Example |
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| tris-(1,10-phenanthroline)ruthenium<sup>2+</sup> || 1.26 || -1.36 || -0.87 || 0.82 || 500 || 422 || 610 || Example |
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| bis-(2-(2',4'-difluorophenyl)-5-trifluoromethylpyridine)(di-tert-butylbipyridine)iridium<sup>+</sup> || 1.69 || -1.37 || -0.89 || 1.21 || 2300 || 380 || 470 || Example |
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| bis-(2-phenylpyridine)(di-tert-butylbipyridine)iridium<sup>+</sup> || 1.21 || -1.51 || -0.96 || 0.66 || 557 || || 581 || Example |
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| (2,9-bis-(p-anisyl)-1,10-phenanthroline)copper<sup>2+</sup> || 0.62 || || -1.43 || || 270 || || 670 || Example |
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| tris-(2,2'-phenylpyridine)iridium<sup>+</sup> || 0.77 || -2.19 || -1.73 || 0.31 || 1900 || 375 || 494 || Example |
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==Applications of Photoredox Catalysis in Organic Chemistry== |
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'''Reductive Transformations:''' |
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Reductive Dehalogenation |
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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.<ref>{{cite journal|last=Narayanam|first=Jagan M. R.|coauthors=Joseph W. Tucker, and Corey R. J. Stephenson|title=Electron-Transfer Photoredox Catalysis: Development of a Tin-Free Reductive Dehalogenation Procedure|journal=JACS|date=06/05/2009|volume=131|issue=25|pages=8756-8757|doi=10.1021/ja9033582|accessdate=10/28/2013}}</ref> This method employs Ru(bipy)<sub>3</sub><sup>2+</sup> 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 amine 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. |
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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)<sub>3</sub><sup>2+</sup>).<ref>{{cite journal|last=Nguyen|first=John D.|coauthors=D'Amato, Erica M.; Narayanam, Jagan M. R.; Stephenson, Corey R. J.|title=Engaging unactivated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions|journal=Nature Chemistry|year=2012|volume=4|issue=10|pages=854-859|doi=10.1038/nchem.1452|accessdate=10/28/2013}}</ref> This updated reaction is mechanistically distinct from the previous transformation of activated bromides and chlorides. In this version of the reaction, fac-Ir(ppy)<sub>3</sub> is used as a photocatalyst. The increased reduction potential of this catalyst compared to Ru(bipy)<sub>3</sub><sup>2+</sup> 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. |
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Figure: |
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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.<ref>{{cite journal|last=Tucker|first=Joseph W.|coauthors=Nguyen, John D.; Narayanam, Jagan M. R.; Krabbe, Scott W.; Stephenson, Corey R. J.|title=Tin-free radical cyclization reactions initiated by visible light photoredox catalysis|journal=Chemical Communications|date=28 May 2010|volume=46|issue=27|pages=4985-4987|doi=10.1039/c0cc00981d}}</ref> 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. |
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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.<ref>{{cite journal|last=Furst|first=Laura|coauthors=Narayanam, Jagan M. R.; Stephenson, Corey R. J.|title=Total Synthesis of (+)-Gliocladin C Enabled by Visible-Light Photoredox Catalysis|journal=Angewandte Chemie International Edition|date=4 October 2011|volume=50|issue=41|pages=9655–9659|doi=10.1002/anie.201103145}}</ref> |
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'''Oxidative Transformations:''' |
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These reactions take advantage of the elevated oxidation potential of the photocatalyst excited state and employ a stoichiometric oxidant to regenerate the catalyst. |
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Generation of Iminium Ions from Amines |
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Iminium ions are potent electrophiles useful for generating C-N bonds and particularly C-N stereocenters. However, the condensation of sterically bulky secondary amines with aldehydes can be quite slow and often requires harsh dehydration conditions. This reaction is even more difficult for condensation with ketones. This methodology allows formation of the iminium from the oxidation of a C-N bond rather than from the condensation of a carbonyl with a primary or secondary amine. This general procedure takes place by single-electron oxidation of a tertiary amine to an aminium radical. This nitrogen-centered radical can be oxidized further to the iminium cation either by hydrogen-atom abstraction from an adjacent carbon atom, or in two steps by deprotonation to create a two-center-three-electron pi system, followed by a second single-electron oxidation. |
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The iminium cations catalytically generated in this manner can be quenched by a wide variety of nucleophiles. In particular, methods have been developed for adding the following nucleophiles under photoredox conditions: nitromethane (aza-Henry reaction), cyanide (Strecker reaction), silyl enol ethers (Mannich reaction), allyl silanes, and indole (Friedel-Crafts reaction).<ref>{{cite journal|last=Condie|first=Allison G.|coauthors=Gonzalez-Gomez, Jose C.; Stephenson, Corey R. J.|title=Visible-Light Photoredox Catalysis: Aza-Henry Reactions via C-H Functionalization|journal=JACS|year=2010|volume=132|issue=5|pages=1464-1465}}</ref> <ref>{{cite journal|last=Rueping|first=Magnus|coauthors=Zhu, Shaoqun; Koenigs, Rene M.|title=Visible-light photoredox catalyzed oxidative Strecker reaction|journal=Chemical Communications|year=2011|volume=47|pages=12709-12711}}</ref> <ref>{{cite journal|last=Zhao|first=Guolei|coauthors=Yang, Chao; Guo, Lin; Sun, Hongnan; Chen, Chao; Xia, Wujiong|title=Visible light-induced oxidative coupling reaction: easy access to Mannich-type products|journal=Chemical Communications|year=2012|volume=48|pages=2337-2339}}</ref> <ref>{{cite journal|last=Freeman|first=David B.|coauthors=Furst, Laura; Condie, Allison G.; Stephenson, Corey R. J.|title=Functionally Diverse Nucleophilic Trapping of Iminium Intermediates Generated Utilizing Visible Light|journal=Organic Letters|year=2012|volume=14|pages=94-97}}</ref> |
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Oxidative Removal of the PMB protecting group |
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Research by Stephenson, et al. has shown that the selective deprotection of para-methoxybenzyl protecting groups can be cleaved in the presence of less electron-rich benzyl groups through the use of the iridium complex Ir<sub>2</sub>(dtbbpy)PF<sub>6</sub>.<ref>{{cite journal|last=Tucker|first=Joseph W.|coauthors=Narayanam, Jagan M. R.; Shah, Pinkey S.; Stephenson, Corey R. J.|title=Oxidative photoredox catalysis: mild and selective deprotection of PMB ethers mediated by visible light|journal=Chemical Communications|year=2011|volume=47|issue=17|pages=5040-5042}}</ref> This complex will oxidize to Ir(IV) in the presence of bromotrichloromethane, here used as a stoichiometric oxidant. The strongly electron-poor nature of the ligands means that this iridium complex can be readily oxidized: in particular, by an electron-rich arene such as a para-methoxy benzyl ether. |
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'''Redox-Neutral Transformations:''' |
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This class of reactions takes advantage of both single electron transfers necessary to regenerate the photocatalyst from its excited state. Thus, one reaction partner is reduced by the photocatalyst and the other is oxidized. Alternatively, the photoredox catalyst allows the reactive substrate to access an unusual redox state, participate in the reaction, and then return to its more stable redox form. Since the overall reaction in this class is redox-neutral, a stoichiometric reductant or oxidant is not necessary. |
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Photoredox Organocatalysis |
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Tandem activation with a nucleophilic secondary amine catalyst. Condensation of a carbonyl compound, typically an aldehyde, with the amine catalyst gives an enamine. The activated photocatalyst can oxidize this enamine participate in a SOMO coupling, or transfer of the radical to the adjacent position for beta-activation of the carbonyl compound. Furthermore, the use of a chiral amine cocatalyst allows the subsequent addition to proceed asymmetrically. |
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Alkylation and Benzylation of Aldehydes |
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This challenging asymmetric is usually performed on carbonyl compounds in a higher oxidation state (usually as amides) and bound to a chiral auxiliary. This method allows lower loadings of chiral material than with an auxiliary and allows further functionalization of the product, e.g. by addition to the aldehyde, without first removing the auxiliary and changing the oxidation state of the carbonyl group. |
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Cycloadditions |
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Work by the Yoon group has explored the use of photoredox reactions to access unusual cycloaddition reactions. The Woodward-Hoffmann rules and other equivalent methods of evaluating reaction geometries predict that only certain cycloadditions are allowed under thermal conditions because of the symmetry of the populated molecular orbitals. In order to activate molecules to participate in thermally-disallowed cycloadditions, previous research has focused on ultraviolet excitation of the substrate molecules in order to change the molecular orbital population and therefore the selectivity of the reactions. Yoon has accomplished the required alteration of the molecular orbital populations by electron transfer from a photocatalyst, giving access to 2+2 cyclizations, cyclizations, and non-traditional Diels-Alder reactions that are orthogonal in scope to traditional Diels-Alder cycloadditions: particularly radical-anion hetero-Diels-Alder reactions and radical-cation Diels-Alder reactions. |
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==References== |
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{{Reflist}} |
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