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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39226072
10.1021/jacs.4c10053
Communication
Photodriven Sm(III)-to-Sm(II) Reduction for Catalytic Applications
https://orcid.org/0000-0003-0066-4424
Johansen Christian M. ‡
https://orcid.org/0000-0003-0150-5396
Boyd Emily A. ‡
Tarnopol Drew E.
https://orcid.org/0000-0002-6610-4414
Peters Jonas C. *
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States
* Email: jpeters@caltech.edu
03 09 2024
18 09 2024
146 37 2545625461
23 07 2024
26 08 2024
23 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The selectivity of SmI2 as a one electron-reductant motivates the development of methods for reductive Sm-catalysis. Photochemical methods for SmI2 regeneration are desired for catalytic transformations. In particular, returning SmIII-alkoxides to SmII is a crucial step for Sm-turnover in many potential applications. To this end, photochemical conditions for reduction of both SmI3 and a model SmIII-alkoxide to SmI2(THF)n are described here. The Hantzsch ester can serve either as a direct photoreductant or as the reductive quencher for an Ir-based photoredox catalyst. In contrast to previous SmIII reduction methodologies, no Lewis acidic additives or byproducts are involved, facilitating selective ligand coordination to Sm. Accordingly, SmII species can be generated photochemically from SmI3 in the presence of protic, chiral, and/or Lewis basic additives. Both the photoreductant and photoredox methods for SmI2 generation translate to intermolecular ketone-acrylate coupling as a proof-of-concept demonstration of a photodriven, Sm-catalyzed reductive cross-coupling reaction.

National Institutes of Health 10.13039/100000002 GM153322 document-id-old-9ja4c10053
document-id-new-14ja4c10053
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pmcSamarium diiodide (SmI2) is an exceptionally versatile single-electron reductant. The large and labile coordination sphere of SmII can recruit one or multiple substrates and additives to achieve selectivity in both organic synthesis and small-molecule reductions (Figure 1A).1−4 However, SmI2 is employed stoichiometrically in all but a few select cases5−8 because its reactions typically terminate in the formation of highly stable SmIII–alkoxide species. Catalytic regeneration of the SmII state requires abstraction of OR– by a stoichiometric oxophile (EX) to generate a SmIII species that can be reduced by a relatively mild reductant (Figure 1A). The difficulty associated with this transformation has been cited as a motivation for the development of a variety of alternative photo- and electrochemically driven methods for ketyl radical generation.9−13

Figure 1 Summary of key challenges for Sm-turnover; prior studies exploiting LnIII/II photochemistry and photoreductions with HEH2 and Lewis acidic metals; and this work describing photodriven generation of SmI2.

Early strategies for reductive Sm catalysis relied on harsh combinations of halosilane oxophiles (R3SiX) and low valent metals (Mg0 for X = Cl; Zn0 for X = I) or an applied electrochemical potential as the reductant.14−22 In a collaborative effort with the Reisman laboratory, we recently disclosed comparatively mild silane-free thermal and electrochemical conditions for catalytic turnover of SmI2 in reductive coupling of ketones and acrylates through combination of cationic Brønsted acids with either Zn0 or an applied potential of −1.55 V vs Fc+/0 (Fc+/0 = ferrocenium/ferrocene; all potentials referenced to Fc+/0).23

Given the growing interest in (metalla)photoredox catalysis,24 photodriven strategies for LnIII/II catalysis remain surprisingly underexplored.25,26 In a strategy recently showcased by the groups of Borbas27 and Nemoto,28 photosensitizers are incorporated into the secondary coordination spheres of LnIII complexes (Ln = Sm, Eu; Figure 1B). Intramolecular oxidative quenching of the excited sensitizer by the LnIII center produces a potent LnII reductant which can carry out a variety of transformations.

While this and other strategies show promise,25−28 the chelating ligand platforms used thus far in photodriven LnIII/II catalysis (cryptands, bidentate phosphine oxides) restrict the coordination sphere and/or shift E°(LnIII/II) to strongly negative potentials, belying direct translation to the rich stoichiometric chemistry of SmI2(L)n as an inner sphere reductant (L = solvent molecule, typically THF).

Lewis acidic metal ions are commonly used to template substrates in photodriven reductive coupling reactions.10,29,30 Recently, in contrast to the use of photocatalysts, several Lewis acid-mediated photoreductions utilize the blue-light absorbing Hantzsch ester (HEH2) as a photoreductant (E(HEH2+•/*HEH2) = −2.5 V).31−34 Photoexcited HEH2 (*HEH2) carries out CrIII reduction in a catalytic-in-Cr photodriven Nozaki–Hiyama–Kishi reaction (Figure 1C).35 Alternatively, HEH2 acts as a photoreductant in a Gd(OTf)3-mediated Giese addition of an N-hydroxyphthalimide (NHPI) ester-derived alkyl radical into α,β-unsaturated ketones or a lactone (Figure 1C).36 In the latter study, an interaction between Gd and HEH2 is observed, but GdIII reduction to GdII is not accessible even by *HEH2.23

Based on these precedents we noted that *HEH2 should be capable of reducing SmIII-species such as SmI3 (E°(SmI3/(SmI2 + I–)) = −1.58 V; Figure S35). Because Sm and Gd are similar in size and oxophilicity, we envisioned that photoexcitation of HEH2 bound to SmIII could result in intramolecular oxidative quenching to produce SmII (Figure 1D). Crucially, however, a more dynamic Sm-chromophore interaction might allow access to coordinatively unsaturated SmI2(L)n species which could carry out inner-sphere reduction in a photodriven Sm-catalyzed cross-coupling reaction. Importantly, both HEH2 and its 2H+/2e– oxidized congener, HE, are weak bases and are therefore compatible with the acidic conditions necessary for recovery of inactive SmIII–OR species by protonolysis.

Gratifyingly, HEH2 proved competent as a photoreductant for SmIII-to-SmII conversion. Monitoring the UV–visible absorption spectrum of a solution of SmI3 (2 mM), HEH2 (60 mM) and 2,6-lutidine base (Lut, 60 mM) following irradiation at 440 nm for 5 min in THF reveals the characteristic profile of blue SmI2(THF)n with λmax at 555 and 618 nm (Figure 2A, left panel). Extended irradiation (120 min) results in increasing SmI2 generation, with maximum yield ∼25%. Interestingly, in the absence of base this reaction does not proceed (Figure S17), likely due to rapid back-electron transfer (BET) between HEH2•+ and SmI2. However, HEH2•+ can be deprotonated in the presence of base, circumventing BET.

Figure 2 (A) UV–vis spectra following photoreduction of SmI3 (left) and SmI2(OiPr)(L)n to form SmI2. CVs of Sm(OiPr)3 (2 mM) in the presence of iodide and proton sources in THF.

We next evaluated conditions for photogeneration of SmI2(THF)n from Sm(OiPr)3 as a model SmIII-alkoxide. Irradiation of Sm(OiPr)3 (2 mM), tetra-n-heptylammonium iodide (nHep4NI, 6 mM), and HEH2 (60 mM) at 440 nm in THF shows no evidence of SmI2 formation (Figure S19). However, upon the addition of only 1.5 equiv of the acid bis-trifluoromethylsulfonylimide (HTFSI) to Sm(OiPr)3, SmI2(THF)n is generated upon irradiation with nHep4NI and HEH2 (Figure 2A, right panel). Parallel CV studies demonstrate that no SmI3 is generated from Sm(OiPr)3 at this acid loading (Figure 2B, compare light and dark blue traces), and current attributable to SmIII reduction (presumably of an intermediate mixture of solvated “SmI(OiPr)2” and “SmI2OiPr”) does not onset until −2.3 V. In contrast to SmI3, no external base is needed, suggesting that the Sm-bound alkoxide might additionally serve the role of deprotonating HEH2•+ to avoid BET. UV–vis studies reveal that addition of the colorless SmIII–OiPr species (gray trace in Figure 2A) gives rise to a significantly red-shifted shoulder in the HEH2 absorption profile (compare light and dark red traces in Figure 2A), consistent with preassociation.

The modest yields and rates of these reactions motivated the study of SmIII reduction with a photoredox catalyst to overcome the low quantum yield and excited state lifetime (220 ps in MeCN)37 of HEH2.

We selected [Ir(dtbbpy)(ppy)2]+ ([IrIII]+)38 as a photosensitizer, which could undergo reductive quenching by a sacrificial electron donor to generate IrII. IrII is thermodynamically capable of reducing SmI3 to SmI2 (E°(IrIII/II) = −1.94 V, Figure 2B and Figure S36).

Irradiating SmI3 or SmI2OiPr (2 mM) with [IrIII]PF6 (0.2 mM), HEH2 (60 mM) as sacrificial reductant, and Lut (60 mM) rapidly generates SmI2 (80% or 30% conversion in 2 min, Figure 3A). Again, the weak base Lut enhances the process (Figures S20–S21).

Figure 3 (A) Photoreductions of SmIII species with [Ir]PF6 photocatalyst. (B) Rationale for net photoinduced proton- and electron-transfer from HEH2 to [SmIII–OR] species.

The accelerated reduction of SmI2OiPr is curious, as electron transfer from IrII to this SmIII species is uphill by 400 mV (Figure 2B). A rationale for these observations is provided in Figure 3B: reductive quenching of *[IrIII]+ by HEH2 generates not only the strong reductant IrII, but also the strong acid HEH2•+ (pKa – 1 in MeCN),39,40 the combination of which can carry out net proton-coupled electron transfer to SmIII–OiPr.41 Proton transfer from HEH2•+ to a SmIII–OiPr species, likely via proton relay mediated by Lut, liberates iPrOH and [SmI2]+.42 The latter can then be reduced to SmI2 by IrII.

Development of Sm-catalysis leveraging diverse ligand coordination to modulate reactivity is an attractive goal. Exploration of SmII generation in the presence of potential coligands was carried out pursuant to these interests.

Satisfyingly, SmII is readily photogenerated from SmI3 by [IrIII]+ and quencher (HEH2 or Et3N) in the presence of several protic additives (ethylene glycol, N,N-dimethylaminoethanol, Figures S23–S24),3,43−45 including a chiral aminediol (Figure 4A, Figure S25) that has been utilized in several enantioselective SmI2 transformations.46−48

Figure 4 (A) Ligand coordinated Sm-species generated by a photoredox approach. See SI for relevant electrochemical data. Choice of a sufficiently reducing photocatalyst remains crucial to observe SmII. (B) UV–vis spectra following photogeneration of SmBr2 and Sm(HMPA)42+.

The reduction potential and reactivity of SmII is highly sensitive to coordination of Lewis-basic additives (HMPA, Br–; Figure 4A).49 While [IrII] is insufficiently reducing to access such species, the more reducing photocatalyst 3DPA2FBN,50 when paired with the more reducing quencher 9,10-dihydroacridine and Et3N as base, mediates generation of both SmBr2 and Sm(HMPA)42+ (Figure 4B). 3DPA2FBN also facilitates SmIII reduction and binding to the chiral BINAPO ligand (Figures 4A and S34).51,52

Having established two different photochemical approaches to SmII generation, we targeted an intermolecular ketone-acrylate coupling as a model reaction to benchmark photodriven Sm-catalysis (Table 1). This reaction is representative of the qualities that set SmI2 apart as a stoichiometric reductant. Inner-sphere electron transfer to one or both of the carbonyl substrates is obligatory based on comparison of outer-sphere reduction potentials.23 Importantly, a Sm-alkoxide is generated as the byproduct of lactonization, enabling evaluation of the ability of a set of conditions to overcome this critical barrier to generalizable Sm catalysis.

Table 1 Photodriven Sm-Catalyzed Coupling of Ketones and Phenyl Acrylate to Form Lactone Productsa

a Yields were determined by 1H NMR analysis. For additional reaction data, see Table S2.

b tert-Butyl acrylate used as coupling partner; lactonization observed only upon acidic workup.

Irradiation of ketone 1 (0.04 mmol), phenyl acrylate (2 equiv), and SmI2(THF)2 (10 mol %) in the presence of HEH2 (4.0 equiv) in 2-MeTHF (0.02 M) at 440 nm for 90 min yields lactone 2 in 76% yield (Table 1, entry 1, method A). Addition of a photoredox catalyst ([Ir]PF6, 1 mol %) with pyridine (2 equiv) results in an increase in yield to 89% (entry 1, method B). Light and Sm were required for catalytic formation of 2 by either method (entries 3 and 4). Sm(OTf)3 is a competent precatalyst with 50 mol % MgI2 included as an iodide source (entry 4). Substitution of Gd(OTf)3 for Sm(OTf)3 results in trace product formation, supporting a key role for SmII in catalysis (entry 5).

Both methods are competent in the presence/absence of pyridine (entries 1, 6, and 7), but yields are greatly diminished in the presence of a stronger base (Et3N, entry 8). This suggests that the dynamics of Sm-alkoxide protonation play an important role in turnover.23 Interestingly, the use of a dihydropyridine without carbonyl groups, 5,6-dihydrophenanthridine, only shows product formation with [Ir]+ (entry 9). In the absence of Ir, the specific interaction between Sm and HEH2 appears to be required. The Ir-catalyzed reaction is also faster, achieving 60% conversion in 15 min, compared to 29% by method A (entry 10).

Methods A and B were tested against alternative coupling partners to assess their relative efficacies. When using less activated substrate pairs (aliphatic ketones and alkyl acrylates, entries 1, 11 and 12), method B is favored, perhaps because these slower cross-couplings require rapid SmIII-to-SmII conversion. Method A is preferred when using aryl ketones (entries 13–15), as method B gives considerable pinacol-coupled side-products (Table S3). With method A, selective inner-sphere photogeneration of SmII by SmIII–HEH2 may favor SmII-mediated cross-coupling, while with method B background Ir-mediated substrate reduction to homocoupled products can dominate.

A proposed mechanism for this photodriven lactonization reaction (by method A) is presented in Figure 5. The mechanism can be divided into two parts, a photoreduction side in which SmIII is reduced to SmII, and a SmI2 cross-coupling side where the organic substrates are coupled. Starting from SmI2(OPh), coordination to HEH2 (as demonstrated in Figure 2A) followed by excitation to *HEH2 allows for the proton and electron transfer required to generate SmI2, with PhOH and HEH• as additional products. Subsequently, SmI2 couples the acrylate and ketone to form a radical intermediate.53,54 HEH• is capable of reducing this intermediate as a potent H atom donor, although alternative schemes for reduction of the radical intermediate can be envisioned (Figure S45). Following reduction and lactonization, 2 is formed along with SmI2(OPh).

Figure 5 Proposed mechanism of Sm cross-coupling under Ir-free conditions (method A).

With [Ir]+, a similar mechanism is proposed, differing in the regeneration of SmII, which can be regenerated from SmIII-alkoxide as depicted in Figure 3B (see Figure S46 for full scheme).

In summary, we have demonstrated photodriven generation of SmI2(THF)2 from SmIII precursors using both a photoreductant and a photoredox catalyst. These conditions translate to proof-of-concept photodriven reductive Sm-catalyzed ketone-acrylate coupling. Distinct from reported methods, photodriven Sm-catalysis occurs in the absence of competing Lewis-acidic metal additives and byproducts (e.g., Mg2+ and Zn2+ salts),14−23 which may be of utility in development of Sm-catalysis with ligands.3,18,43,46−51 These findings are anticipated to facilitate applications of Sm-catalysis beyond the types of thermally driven transformations studied thus far.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c10053.Experimental methods, data from individual catalysis experiments, and additional spectra as referenced in the text. (PDF)

Supplementary Material

ja4c10053_si_001.pdf

Author Contributions

‡ C.M.J. and E.A.B. contributed equally.

The authors declare no competing financial interest.

Acknowledgments

We thank the National Institutes of Health (R35GM153322). E.A.B. and D.E.T. thank the National Science Foundation for a Graduate Research Fellowship under Grant No. DGE-1745301 and 2139433, respectively. C.M.J. is grateful for support from the Aker Scholarship foundation. We also acknowledge the Resnick Sustainability Institute at Caltech for support of enabling facilities. We thank the Reisman laboratory for supplying ligand samples.
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CV experiments suggest that speciation of 1:1 SmI3:BINAPO is a complex mixture with SmIII/II redox waves negative of −2 V (Figure S43).

Substrate coupling could be initiated either by ketone or acrylate reduction, leading to either an α-ester radical or an alkoxy radical intermediate, respectively, following addition to the corresponding coupling partner. In the case of difficult-to-reduce ketone substrates such as 1, neither pathway can be reliably ruled out. See the following and ref 54 for detailed examination of this mechanistic question:

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