Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Dec 11;26(50):10752–10756. doi: 10.1021/acs.orglett.4c03723

Photocatalyst for Visible-Light-Driven Sm(II)-Mediated Reductions

Monika Tomar 1, Caroline Bosch 1, Jules Everaert 1, Rohan Bhimpuria 1, Anders Thapper 1, Andreas Orthaber 1, K Eszter Borbas 1,*
PMCID: PMC11667727  PMID: 39661866

Abstract

graphic file with name ol4c03723_0006.jpg

Commercially available coumarin 343 in combination with reducible Sm(III) ions catalyzed divalent lanthanide-mediated C=O, C–halogen, P–Cl, and N=N reductions at ambient temperature in aqueous solvent mixtures. The catalyst absorbs visible light efficiently. The active divalent species is formed by photoinduced electron transfer from coumarin 343 to the stable trivalent precursor, and the coumarin could be regenerated by strictly 1 equiv of ascorbic acid.

The role of lanthanides (Ln) in catalysis has long been restricted to Lewis acid catalysis.1 It is only recently that cycling between stable Ln(III) and a less stable Ln(IV) or Ln(II) state to promote a redox reaction by catalytic amounts of a lanthanide has become possible. Ce(III) photocatalysts can mediate dehalogenations, C–H activations, borylations, and C–C bond formations.2 Ln(II) compounds are versatile one-electron reductants that can affect functional group interconversions and C–C and C–X bond formations with high selectivity even in complex substrates.3 SmI2, the most common Ln(II) reagent, is typically used in large excess, which makes such reactions environmentally burdensome. Approaches toward Ln(II) catalysis include chemical4 or electrochemical5 Ln(II) generation or the reliance on radical propagation.6 These attempts suffer from a limited substrate scope, diminished selectivity, or the need for toxic additives.

Ln(III) can be reduced by excited-state chromophores.7 The chromophore can be regenerated by a reductant, closing the catalytic cycle.8,9 The catalysts in Figure 1 could reduce benzyl and aryl halides and nitro, C=O, and P=O groups, and the reductions could initiate C–C and C–N bond formations in yields comparable to the analogous stoichiometric reactions. While promising, LnL1, LnL2, and LnL3 had limitations: weak or no visible absorption, the need for Zn or N,N-diisopropylethylamine (DIPEA) sacrificial reductants, and multistep ligand syntheses in the case of LnL1 and LnL2.

Figure 1.

Figure 1

Open in a new tab

Lanthanide photocatalysts, with Ln = Eu or Sm.

Here, we report that a simple combination of commercially available coumarin 343 (C343; Figure 1) and a reducible Ln(III) salt is an excellent photocatalyst for several Ln(II)-mediated reductions. Reactions proceed under ambient conditions in aqueous solvent mixtures without Ir or Ru (co)catalysts. The absorption maximum of C343 is at λabs = 446 nm, matching the output of the blue light-emitting diode (LED) common in photoreactors.10 Unlike L1 and L2, C343 does not have to be modified with a synthetically appended metal binding site. As a sacrificial reductant, 1 equiv of cheap ascorbic acid could be used. This catalyst afforded significantly improved yields of previously low-yielding reactions and catalyzed reactions that did not take place using LnL1–LnL3, greatly improving the operational simplicity of photocatalytic Ln(II)-mediated reductions.

Benzaldehyde (1a) was chosen as a model substrate. Irradiation with blue LED of a solution containing Sm(OTf)3 and C343 (1:1, 0.1 equiv) under conditions optimized for LnL2 [N,N-dimethylformamide (DMF)/H2O (4:1), [1a] = 30 mM, DIPEA (5 equiv), and LiCl (5 equiv)]9 afforded 15% conversion to compound 1c in 24 h, which increased to 42% after 48 h. Longer reaction times did not improve upon this result (entry 1 in Table 1). Solvents and proton sources were then screened. Almost no product formed when increasing the water content to 50% (entry 2) or in tetrahydrofuran (THF)/H2O (4:1, entry 3). Conversion improved to 77% in MeCN/H2O (4:1, entry 4). The reaction gave a mixture of dl and meso isomers. Chiral additives [proline, 2,2′-dihydroxy-1,1′-binaphthyl, and 3-(heptafluoropropylhydroxymethylene)-(+)-camphorate] did not alter the product distribution (Table S2 of the Supporting Information). The optimized conditions (condition A) are thus 10% catalyst loading [Sm(OTf)3/C343, 1:1], 5 equiv each of DIPEA and LiCl, in MeCN/H2O (4:1, 35 mM), affording compound 1c selectively in 77% yield; compound 1b was not detected.

Table 1. Optimization and Selected Control Experiments.

graphic file with name ol4c03723_0005.jpg

entry Lna solvent conversion (%)b dl/meso (%)c
1 Sm DMF/H2O (4:1) 42 32/68
2 Sm DMF/H2O (1:1) <1  
3 Sm THF/H2O (4:1) <1  
4 Sm MeCN/H2O (4:1) 77 49/51
5d Sm MeCN/H2O (4:1) <1  
6e   MeCN/H2O (4:1) 4 43/57
7 Gd MeCN/H2O (4:1) 7 31/69
8 Eu MeCN/H2O (4:1) 91 38/62
9 Yb MeCN/H2O (4:1) 98 36/64
Open in a new tab
a

As Ln(OTf)3.

b

Determined by gas chromatography–mass spectrometry (GC–MS).

c

On the basis of the ratio of integrated signal areas of the separated product peaks.

d

In the dark.

e

With only C343.

The roles of the different components were probed. Changing the proton source to hexafluoroisopropanol or tert-BuOH decreased the yield, which is consistent with water-coordinating Sm(II) serving as a proton-coupled electron transfer agent.11 Diglyme and tetraglyme additives were detrimental to the yield; these bind Sm ions and block substrate access (Table S1 of the Supporting Information).12 The reaction did not work without light; therefore, Sm(II) was not generated by DIPEA directly (entry 5 in Table 1). Only 4–7% conversion was observed in the absence of Sm(OTf)3 (entry 6 in Table 1) or with Gd(OTf)3 instead of Sm(OTf)3 (entry 7 in Table 1). Both EuC343 and YbC343 afforded the pinacol product in excellent yields (entries 8 and 9). Differences between reactions carried out with reducible Ln(III) (Ln = Eu, Yb, or Sm) and non-reducible Gd(III) enable the identification of those processes that are dependent upon Ln redox activity. The results confirm that photochemical Ln(II) formation is essential for reactivity.

The substrate scope was explored under the optimized conditions but with a shortened reaction time, as most substrates were consumed within 24 h (Figure 1). SmC343 was used as the catalyst because Sm(II) is a stronger reductant that Yb(II) and Eu(II);13 compounds 2a and 7a were unreactive with YbC343. Aldehydes bearing electron-neutral and electron-poor (3a and 4a) substituents were selectively reduced in good to excellent yields to the corresponding pinacol products. Substrates carrying electron-donating p-methyl and p-F substituents were unreactive, likely due to their low electrophilicity. Free alcohols (20a and 21a), a tertiary amine (6a), and a thioether (5a) were tolerated. The electron-donating ability of the tertiary amine may be reduced by the Lewis acidic additive. Reducible C=O functionalities, such as methyl ester (10a) and methyl ketone (15a), were retained. One aldehyde could be reduced selectively in 1,4-benzodialdehyde (16a). Multiple spots were present in the reaction mixture with 4-acetyl benzaldehyde, suggesting the reactivity of both the aldehyde and ketone groups. The pinacol product was isolated in a 40% yield. Low solubility of the starting material or the product was a problem in some cases, especially on a larger scale. For 4-formylbenzonitrile, changing the solvent to a DMF-based solvent resulted in a faster reaction (8 h versus 16 h) and good isolated yield (63%). Most products were obtained after purification with column chromatography, but compound 3c precipitated out of the reaction mixture and was isolated by simple filtration.

Some reactions yielded the alcohol rather than the pinacol product. Both the nitro and aldehyde groups were reactive in 4-nitrobenzaldehyde; 4-nitrobenzylalcohol was the major product (36%). 4-(Methylsulfonyl)benzaldehyde was reduced to the alcohol in 22% yield in 24 h; the yield did not increase upon continuing the reaction for 48 h. The low yield of compounds 2b and 2c was due to the sluggish reaction of compound 2a, as 70% of the starting naphthaldehyde remained after 48 h. Cinnamaldehyde formed the pinacol product in a 43% yield. Some of the low yields were due to purification problems, as a C343-derived byproduct was difficult to remove.

The low yields of some of the reactions and the purification problems prompted a search for a more inexpensive and water-soluble sacrificial reductant. Satisfyingly, 1 equiv of ascorbic acid could replace 5 equiv of DIPEA/LiCl (condition B in Figure 2). The absorption of SmC343 in the presence of ascorbic acid remains at λabs = 446 nm, while DIPEA shifts it to λabs = 412 nm (Figure S4 of the Supporting Information). Similar or higher yields of the products were obtained in the same reaction time after a simpler workup procedure. Some products precipitated after aqueous–organic workup, eliminating the need for column chromatographic purification. The formation of C343 degradation products was suppressed under condition B. C343 could be recovered and reused with similar efficiency (12c, 40% using 2 mol %, section 2 of the Supporting Information). Compounds 3a–6a, which were reduced efficiently under condition A, afforded the products in comparable yields under condition B. The pinacol products formed selectively even when a mixture of the alcohol and the pinacol product was formed under condition A (2a, 12a, and 13a). Several substrates that reacted only sluggishly under condition A gave the product under condition B in good to excellent yields (10a–12a, 18a, and 21a). 4-Methylbenzaldehyde (7a), 3,4,5-trimethoxybenzaldehyde (8b), 4-fluorobenzaldehyde (14a), and 4-methoxyacetophenone (18a) did not react under condition A but did under condition B. The catalyst loading could be decreased to 5 mol % and even 2 mol % (5a, 11a, 12a, and 15a).

Figure 2.

Figure 2

Open in a new tab

SmC343-catalyzed C=O reduction under condition A or B. 2c*, with both DIPEA:LiCl and ascorbic acid (2.5:2.5:1); 11c*, with Gd(OTf)3 (5 mol %), condition B.

SmI2 is a versatile reagent, and a useful catalytic alternative should promote a variety of transformations. Under condition B, C–halogen and N=N bonds could be reduced with 2–5% catalyst loading, while a P–Cl bond was reduced under condition A (Figure 3). The reductions could be followed by C–C and P–P bond formations. Benzyl bromide (22a) underwent dimerization (41%, 22b) and intramolecular cyclization (10%, 22c) under condition B. Decreasing the catalyst loading to 2 mol % improved the yield of compounds 22b and 22c to 56 and 11%, respectively. The reaction between 2,3-dimethoxybenzyl chloride and 6,7-dimethoxyisoquinoline yielded papaverine (23b) in 77% isolated yield, improving upon the previous photocatalyzed procedure (56%).9 Diazo compound 24a was reduced to hydrazine (24b). trans-1,2-Diol (25b) was obtained in a highly stereoselective reaction. Catalytic halophosphine reduction afforded diphosphine (26b) and Ph2PH (26c) in 35 and 23% yields, respectively. Thus, SmC343 catalyzes several typical reductions carried out by stoichiometric SmI2.

Figure 3.

Figure 3

Open in a new tab

C–X, X=X, P–Cl, and C=O reductions using SmC343.

The proposed mechanism is shown in Scheme 1. Ln(III) is highly oxophilic and has high coordination numbers.14 The ultraviolet–visible (UV–vis) spectrum of C343 changes upon the addition of 1 equiv of Sm(OTf)3 (Figures S2 and S3 of the Supporting Information). Eu(III) has information-dense luminescence15 and similar chemical properties to Sm(III); therefore, luminescence spectroscopic experiments were carried out with Eu(OTf)3. The luminescence lifetime of Eu(III) was 0.31 ms without C343 and 0.24 ms with C343 in MeCN (Table S4 of the Supporting Information). The shortening of the lifetime is consistent with C343 binding bringing a quenching O–H oscillator16 into the inner sphere of Eu(III), and suggests that Ln(III) may coordinate to C343. Carbonyl reductions did not proceed without LiCl. Li+ likely acts as a Lewis acid (LA; Scheme 1), increasing the electrophilicity of the C=O group.

Scheme 1. Proposed Mechanism of the Ln-Catalyzed C=O Reduction.

Scheme 1

Open in a new tab

Electron transfer from the S1 state of C343 is exergonic.17C343 fluorescence is quenched by Ln(III) (Ln = Gd, Sm, Eu, or Yb). Quenching is largest for Eu, Sm, and Yb, which can deplete excited-state C343 (C343*) by electron and energy transfer in addition to the heavy atom effect, which is also possible for Gd(III). The C343 fluorescence lifetime decreased in the presence of Eu(III), Yb(III), and Sm(III) (Figures S5–S12, S15, and S17–S24 and Table S3 of the Supporting Information). These observations are consistent with C343* being quenched by electron transfer to Ln(III) (path a in Scheme 1).

The reduction of Ln(III) by C343* yields Ln(II) and C343• +. This process is thermodynamically downhill for Ln = Eu, Sm, and Yb (ΔGeT = −1.47, −0.63, and −0.72 eV, respectively; Table S5 of the Supporting Information). Irradiation of solutions containing equimolar amounts of C343 and Ln(OTf)3 (Ln = Eu or Sm) in the presence of a radical trap, N-tert-butyl-α-phenylnitrone (PBN) gave a N-based radical from a N adduct visible in the electron paramagnetic resonance (EPR) spectrum recorded at room temperature, which is consistent with the formation of the C343• + radical cation (Figure S28 of the Supporting Information). In the sample with Eu(OTf)3 and C343 an additional wider EPR signal is present that we tentatively assign to a Eu(II) species (Figure S28 of the Supporting Information).

An alternative pathway (path b in Scheme 1) leading to Ln(II) involves the formation of C343• – through the reduction of C343* by DIPEA [Eox = 0.86 V versus saturated calomel electrode (SCE)18] or ascorbate (Eox = 0.35 V versus SHE19). C343• –, in turn, could reduce Ln(III) and return to C343. All of these steps were calculated to be exergonic, with ΔGeT values of −0.25 and −0.99 eV for the electron transfers from DIPEA and ascorbate to C343, respectively, and −0.33 eV for Sm(III) reduction by C343• –. Neither the C343 fluorescence intensity nor lifetime changed in the presence of Gd(III) and increasing amounts of ascorbate (Figures S13, S14, and S16 and Table S3 of the Supporting Information). Thus, C343* is not quenched by ascorbate, and this pathway is not operational. These results support electron transfer taking place from C343* to Ln(III) under the reaction conditions to form a reactive Ln(II) species.

From photochemically generated Sm(II), inner-sphere electron transfer to the substrate is likely, as is the case for the analogous stoichiometric reactions.20 Irradiation of trifluoromethyl benzaldehyde in the presence of 3 equiv of the radical quencher (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) formed compound 27 (m/z 330.1 [M + H]+), which is consistent with the intermediacy of the corresponding substrate-derived radical in the reaction.

In conclusion, a simple photocatalyst was identified for the promotion of a range of Sm(II)-mediated reductions and subsequent P–P and C–C bond formations. The catalyst consists of commercially available Sm(OTf)3 and C343 that can be added directly to the reaction mixture. DIPEA and ascorbic acid were suitable terminal reductants, with the latter affording better yields for several substrates. This simple, cheap, and efficient catalyst renders photocatalytic Ln(II)-mediated reductions accessible and significantly more sustainable than the corresponding stoichiometric reactions.

Acknowledgments

This work was supported by the Swedish Research Council (Project Grant 2021-04625 to K. Eszter Borbas), the Knut och Alice Wallenbergs Stiftelse (Dnr: KAW 2019.0071), the French Engineering Programme (to Caroline Bosch), and the EU Erasmus Programme (to Jules Everaert). Caroline Bosch and Jules Everaert were exchange M.Sc. students from ENSICAEN (Caen, France) and the University of Ghent (Belgium), respectively.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.4c03723.

  • Reaction optimization, chemical characterization of products, photophysical and electrochemical characterization of the catalyst, 1H, 13C, and 19F nuclear magnetic resonance (NMR) spectra for new compounds, and X-ray crystallography of compound 21c (PDF)

The authors declare no competing financial interest.

Supplementary Material

ol4c03723_si_001.pdf (2.7MB, pdf)

References

  1. Dissanayake P.; Allen M. J. Dynamic Measurements of Aqueous Lanthanide Triflate-Catalyzed Reactions Using Luminescence Decay. J. Am. Chem. Soc. 2009, 131 (18), 6342–6343. 10.1021/ja900630d. [DOI] [PMC free article] [PubMed] [Google Scholar]; a Sasai H.; Bougauchi M.; Arai T.; Shibasaki M. Enantioselective synthesis of α-hydroxy phosphonates using the LaLi3tris(binaphthoxide) catalyst (LLB), prepared by an improved method. Tetrahedron Lett. 1997, 38 (15), 2717–2720. 10.1016/S0040-4039(97)00437-1. [DOI] [Google Scholar]; b Hamada T.; Manabe K.; Ishikawa S.; Nagayama S.; Shiro M.; Kobayashi S. Catalytic Asymmetric Aldol Reactions in Aqueous Media Using Chiral Bis-pyridino-18-crown-6–Rare Earth Metal Triflate Complexes. J. Am. Chem. Soc. 2003, 125 (10), 2989–2996. 10.1021/ja028698z. [DOI] [PubMed] [Google Scholar]; c Robinson J. R.; Fan X.; Yadav J.; Carroll P. J.; Wooten A. J.; Pericas M. A.; Schelter E. J.; Walsh P. J. Air- and Water-Tolerant Rare Earth Guanidinium BINOLate Complexes as Practical Precatalysts in Multifunctional Asymmetric Catalysis. J. Am. Chem. Soc. 2014, 136 (22), 8034–8041. 10.1021/ja502568g. [DOI] [PubMed] [Google Scholar]
  2. Qiao Y.; Schelter E. J. Lanthanide Photocatalysis. Acc. Chem. Res. 2018, 51 (11), 2926–2936. 10.1021/acs.accounts.8b00336. [DOI] [PubMed] [Google Scholar]
  3. Kagan H. B. Twenty-five years of organic chemistry with diiodosamarium: An overview. Tetrahedron 2003, 59 (52), 10351–10372. 10.1016/j.tet.2003.09.101. [DOI] [Google Scholar]; a Nicolaou K. C.; Ellery S. P.; Chen J. S. Samarium diiodide mediated reactions in total synthesis. Angew. Chem., Int. Ed. 2009, 48 (39), 7140–7165. 10.1002/anie.200902151. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Szostak M.; Procter D. J. Beyond Samarium Diiodide: Vistas in Reductive Chemistry Mediated by Lanthanides(II). Angew. Chem., Int. Ed. 2012, 51 (37), 9238–9256. 10.1002/anie.201201065. [DOI] [PubMed] [Google Scholar]
  4. Corey E. J.; Zheng G. Z. Catalytic reactions of samarium(II) iodide. Tetrahedron Lett. 1997, 38 (12), 2045–2048. 10.1016/S0040-4039(97)00263-3. [DOI] [Google Scholar]; a Jenks T. C.; Bailey M. D.; Hovey J. L.; Fernando S.; Basnayake G.; Cross M. E.; Li W.; Allen M. J. First use of a divalent lanthanide for visible-light-promoted photoredox catalysis. Chem. Sci. 2018, 9 (5), 1273–1278. 10.1039/C7SC02479G. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Sun L.; Sahloul K.; Mellah M. Use of Electrochemistry to Provide Efficient SmI2 Catalytic System for Coupling Reactions. ACS Catal. 2013, 3 (11), 2568–2573. 10.1021/cs400587s. [DOI] [Google Scholar]; a Röckl J. L.; Lundberg H. Samarium and Ytterbium in Organic Electrosynthesis. Synthesis 2023, 55 (09), 1375–1384. 10.1055/a-1997-0939. [DOI] [Google Scholar]; b Boyd E. A.; Shin C.; Charboneau D. J.; Peters J. C.; Reisman S. E. Reductive samarium (electro)catalysis enabled by SmIII-alkoxide protonolysis. Science 2024, 385 (6711), 847–853. 10.1126/science.adp5777. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Huang H.-M.; McDouall J. J. W.; Procter D. J. SmI2-catalysed cyclization cascades by radical relay. Nat. Catal. 2019, 2 (3), 211–218. 10.1038/s41929-018-0219-x. [DOI] [Google Scholar]; a Mansell J. I.; Yu S.; Li M.; Pye E.; Yin C.; Beltran F.; Rossi-Ashton J. A.; Romano C.; Kaltsoyannis N.; Procter D. J. Alkyl Cyclopropyl Ketones in Catalytic Formal [3 + 2] Cycloadditions: The Role of SmI2 Catalyst Stabilization. J. Am. Chem. Soc. 2024, 146 (18), 12799–12807. 10.1021/jacs.4c03073. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Kovacs D.; Borbas K. E. The role of photoinduced electron transfer in the quenching of sensitized Europium emission. Coord. Chem. Rev. 2018, 364, 1–9. 10.1016/j.ccr.2018.03.004. [DOI] [Google Scholar]
  8. Meyer A. U.; Slanina T.; Heckel A.; König B. Lanthanide Ions Coupled with Photoinduced Electron Transfer Generate Strong Reduction Potentials from Visible Light. Chem. - Eur. J. 2017, 23 (33), 7900–7904. 10.1002/chem.201701665. [DOI] [PubMed] [Google Scholar]; a Johansen C. M.; Boyd E. A.; Tarnopol D. E.; Peters J. C. Photodriven Sm(III)-to-Sm(II) Reduction for Catalytic Applications. J. Am. Chem. Soc. 2024, 146, 25456. 10.1021/jacs.4c10053. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Kuribara T.; Kaneki A.; Matsuda Y.; Nemoto T. Visible-Light-Antenna Ligand-Enabled Samarium-Catalyzed Reductive Transformations. J. Am. Chem. Soc. 2024, 146 (30), 20904–20912. 10.1021/jacs.4c05414. [DOI] [PubMed] [Google Scholar]
  9. Tomar M.; Bhimpuria R.; Kocsi D.; Thapper A.; Borbas K. E. Photocatalytic Generation of Divalent Lanthanide Reducing Agents. J. Am. Chem. Soc. 2023, 145 (41), 22555–22562. 10.1021/jacs.3c07508. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Mutra M. R.; Li J.; Wang J.-J. Light-mediated sulfonyl-iodination of ynamides and internal alkynes. Chem. Commun. 2023, 59 (43), 6584–6587. 10.1039/D3CC00842H. [DOI] [PubMed] [Google Scholar]
  11. Chciuk T. V.; Anderson W. R. Jr.; Flowers II R. A. High-Affinity Proton Donors Promote Proton-Coupled Electron Transfer by Samarium Diiodide. Angew. Chem., Int. Ed. 2016, 55 (20), 6033–6036. 10.1002/anie.201601474. [DOI] [PubMed] [Google Scholar]; a Chciuk T. V.; Flowers R. A. Proton-Coupled Electron Transfer in the Reduction of Arenes by SmI2–Water Complexes. J. Am. Chem. Soc. 2015, 137 (35), 11526–11531. 10.1021/jacs.5b07518. [DOI] [PubMed] [Google Scholar]
  12. Sadasivam D. V.; Teprovich J. A. Jr.; Procter D. J.; Flowers R. A. II. Dynamic Ligand Exchange in Reactions of Samarium Diiodide. Org. Lett. 2010, 12 (18), 4140–4143. 10.1021/ol101722c. [DOI] [PubMed] [Google Scholar]
  13. Cotton S.The Lanthanides—Principles and Energetics. Lanthanide and Actinide Chemistry; John Wiley & Sons, Inc.: Hoboken, NJ, 2006; Chapter 2, pp 9–22, 10.1002/0470010088.ch2. [DOI] [Google Scholar]
  14. Parker D.; Dickins R. S.; Puschmann H.; Crossland C.; Howard J. A. K. Being Excited by Lanthanide Coordination Complexes: Aqua Species, Chirality, Excited-State Chemistry, and Exchange Dynamics. Chem. Rev. 2002, 102 (6), 1977–2010. 10.1021/cr010452+. [DOI] [PubMed] [Google Scholar]
  15. Binnemans K. Interpretation of europium(III) spectra. Coord. Chem. Rev. 2015, 295, 1–45. 10.1016/j.ccr.2015.02.015. [DOI] [Google Scholar]
  16. Horrocks W. D. Jr.; Sudnick D. R. Lanthanide ion probes of structure in biology. Laser-induced luminescence decay constants provide a direct measure of the number of metal-coordinated water molecules. J. Am. Chem. Soc. 1979, 101 (2), 334–340. 10.1021/ja00496a010. [DOI] [Google Scholar]
  17. Tomar M.; Thapper A.; Orthaber A.; Borbas K. E. Divalent intermediates in lanthanide-based photocatalysts: Spectroscopic characterization and reactivity. Inorg. Chem. 2024, 10.1021/acs.inorgchem.4c03926. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Hämmerling L.; Zysman-Colman E. Building a photocatalyst library of MR-TADF compounds with tunable excited-state redox potentials. Chem. Catal. 2024, 4 (8), 101061. 10.1016/j.checat.2024.101061. [DOI] [Google Scholar]
  19. Matsui T.; Kitagawa Y.; Okumura M.; Shigeta Y. Accurate Standard Hydrogen Electrode Potential and Applications to the Redox Potentials of Vitamin C and NAD/NADH. J. Phys. Chem. A 2015, 119 (2), 369–376. 10.1021/jp508308y. [DOI] [PubMed] [Google Scholar]
  20. Agasti S.; Beattie N. A.; McDouall J. J. W.; Procter D. J. SmI2-Catalyzed Intermolecular Coupling of Cyclopropyl Ketones and Alkynes: A Link between Ketone Conformation and Reactivity. J. Am. Chem. Soc. 2021, 143 (9), 3655–3661. 10.1021/jacs.1c01356. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ol4c03723_si_001.pdf (2.7MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Articles from Organic Letters are provided here courtesy of American Chemical Society

RESOURCES