Cerium-Catalyzed Formal Cycloaddition of Cycloalkanols with

Communication Cite This: J. Am. Chem. Soc. 2018, 140, 13580−13585

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Cerium-Catalyzed Formal Cycloaddition of Cycloalkanols with Alkenes through Dual Photoexcitation Anhua Hu,† Yilin Chen,† Jing-Jing Guo, Na Yu, Qing An, and Zhiwei Zuo* School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

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S Supporting Information *

ABSTRACT: We describe a synergistic utilization of cerium photocatalysis and photoinduced electron transfer catalysis that enables an atom- and step-economical ring expansion of readily available cycloalkanols. This operationally simple protocol provides rapid access to privileged and synthetically challenging bridged lactones. The mild catalytic manifold has been adapted to continuous flow for scale-up applications and employed for the concise synthesis of polycyclic core of nepalactones.

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elective C−C bond cleavage and functionalizations have long been considered as one of the most challenging transformations due to the inherent inertness of the C−C bond,1 while concurrently presenting advantageous opportunities to significantly streamline synthetic routes.2,3 Recent advances in photoredox catalysis have enabled the modern utilization of β-scissions for a myriad of straightforward C−C bond cleavage transformations,4,5 as well as various synthetically intriguing formal [3+2] and [4+2] cycloadditions.6 Here, we describe an efficient dual catalyst system that can employ readily available cycloalkanols in this paradigm, enabling formal cycloadditions for the rapid assembly of synthetically challenging bridged-lactone scaffolds that commonly found in several classes of natural products, such as nepalactones,7 dendromonilisides,8 tutin,9 and hushinone,10 among others (Figure 1). Over the past decade, polypyridyl complexes of iridium and ruthenium have emerged as highly versatile and popular photocatalysts, due to their efficient metal-to-ligand chargetransfer (MLCT) photoexcitation process, as well as wide and useful redox windows for a variety of photoinduced electron transfer (PET) reactions.11 In the pursuit of more sustainable photocatalysts, a mechanistically distinct photoexcitation manifold caught our attention, namely ligand-to-metal charge-transfer (LMCT), which can lead to the homolysis of a metal−ligand bond to generate a ligand-based radical from metal-coordinated substrates.12 In contrast to PET events, LMCT mode provides a direct and more selective pathway to achieve targeted oxidation on heteroatom functionalities, and has long been exploited to access high-energy alkoxy radicals from simple alkanols (Ep > 2.0 V versus SCE in acetonitrile) via the utilization of stoichiometric metal oxidants.13 More importantly, a variety of Earth-abundant metals such as Pb,13d Ni,12g,h Co,12d,e and Ce,12c have been investigated in LMCT excitations, providing vast opportunities for the development of more affordable and sustainable photocatalysts.5e,14 Never© 2018 American Chemical Society

Figure 1. Synergistic combination of LMCT and PET catalysts enables rapid assembly of bridged lactones.

theless, the development of LMCT photocatalytic platforms has been rendered rather challenging by difficulties with catalyst regeneration due to the high oxidation potentials of those metal species (for example, lead(IV) acetate, E1/2 (PbIV/ PbII) = 0.83 V versus SCE in acetic acid),15 thus undermining the synthetic potential of LMCT excitation mode. Recently, we found that simple cerium complexes could be employed in the LMCT catalysis platform for the alkoxy radical-mediated aminations of feedstock alcohols, wherein a SET event between cerium(III) and an oxidizing nitrogencentered radical was crucial for regenerating the photoactive Ce(IV) species.14c Despite the economical and operational advantage of using readily available and inexpensive cerium trichloride as the catalyst,16 the narrow redox window of cerium (E1/2 (CeIV/CeIII) = 0.40 V versus SCE in acetonitrile) poses significant challenges for the further development of diverse catalytic transformations, currently limited to amination reactions. An electron shuttle, capable of transporting electrons from less reducing Ce(III) species to a wide range of electron acceptors with low redox potential, could be utilized to facilitate the turnover of the cerium catalytic cycle and the reductive quenching of the radical intermediates, to further Received: August 15, 2018 Published: October 5, 2018 13580

DOI: 10.1021/jacs.8b08781 J. Am. Chem. Soc. 2018, 140, 13580−13585

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Journal of the American Chemical Society Table 1. Reaction Optimization

Figure 2. Mechanistic studies and proposed catalytic cycle. (A) Control experiments on PET activation using narrow-band LED (395 nm) and laser (450 nm), reactions were run with 0.5 mol % DPA for 2 h. See Supporting Information for detailed description. (B) Proposed mechanism.

a

Yields were determined by GC analysis of the crude reaction mixtures. bReaction was run with CeCl3·7H2O. cReaction was run in the absence of (n-Bu)4PCl. dReactions were run in the absence of CeCl3.

tetrabutylphosphonium chloride, 8% of the desired bridged lactone was observed together with 85% indanol remaining (entry 1). Inspired by the work done by the König group, we started to evaluate a variety of cocatalysts (see Supporting Information for detailed optimization results). Interestingly, anthracene additives revealed a significant acceleration effect, among which, 9,10-diphenylanthracene (DPA) exhibited the optimal activity (entries 2−6). Modifications of the electronic property of the arene moiety of DPA via the installation of electron donating or electron withdrawing groups did not provide any further improvement on catalytic efficiency (entries 7 and 8). In addition, a commonly employed iridium photocatalyst was also found to be effective in facilitating the radical-mediated cycloaddition (entry 9). From a practical point of view, the reaction could be performed using cerium chloride heptahydrate (entry 10). We also found that exogenous chloride was essential to this catalytic system (entry 11). Furthermore, control experiments revealed that no product was formed in the absence of cerium catalyst or light (entries 12−14), indicating the synergistic cooperation between cerium salts and DPA. Although the photophysical properties of anthracene compounds have been extensively studied for electrochemiluminescence and other fluorescence applications,18 the application of those inexpensive aromatic compounds for modern photocatalytic transformations remains largely underinvestigated.19 To elucidate the acceleration effect of DPA, additional mechanistic studies were carried out. According to the absorption spectra of DPA (338−415 nm) and the LMCT band (312−500 nm) of the premade Ce(IV) alkoxide species,

expand the range of application of inexpensive cerium catalysts. Importantly, it could be used catalytically without interference with LMCT activities. Recently, the König group demonstrated a sensitization-initiated electron transfer strategy where Ru(bpy)3Cl2 and polycyclic aromatic hydrocarbons were utilized to convert light energy into redox energy for the single electron reduction of aryl halides,17 which falls outside the redox window of Ru(bpy)3Cl2 catalyst. Inspired by this dual catalyst system, we questioned whether the synergistic merger of a LMCT catalyst and a PET catalyst could expand the inherently narrow redox window of high-valent metals, comparable to iridium and ruthenium, to enable a wide range of radical cross-coupling transformations unattainable with the use of one single catalyst. Herein, we report the utilization of a synergistic combination of cerium and anthracene catalysts to enable a formal insertion of alkene units into C−C single bonds of cycloalkanols. Recognizing the foundational research on cycloadditions of cyclopropane and cyclobutane derivatives,1e as well as the challenging activation of less-strained cyclopentane substrates, we first sought to develop a formal [5+2] cycloaddition employing abundant cyclopentanols. We posited that the αhydroxy C−C bond of 2-indanol could be selectively cleaved and inserted with electron-deficient alkene 2 in stepwise fashion to form a cycloheptanol, which could subsequently undergo lactonization upon acid treatment to generate a functionally dense [4.2.1]-bridged lactone. As shown in Table 1, in the presence of inexpensive cerium trichloride and 13581


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Journal of the American Chemical Society Table 2. Reaction Scope*

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General reaction conditions: alcohol substrate (0.2 mmol), alkene substrate (0.3 mmol), CeCl3 (0.008 mmol), DPA (0.004 mmol), tetrabutylphosphonium chloride (0.016 mmol), benzonitrile (0.5 mL), 90W blue LEDs. All yields are isolated yields. Diastereoselectivity was determined by 1H NMR analysis or HPLC analysis, X-ray crystal structures are correlated to the major isomers. See Supporting Information for experimental details. aAlkene 2 was employed. bCyclobutanol was employed.

both DPA and Ce(IV) intermediate in the reaction can be effectively photoexcited by the broad-band blue LED light (380−530 nm) we used, thus rendering PET activity and LMCT oxidation possible. In addition to the control results presented in Table 1 (entries 1, 6 and 9), we further employed a narrow-band LED centered at 395 nm (380−418 nm

wavelength) and a 450 nm laser (446−453 nm wavelength) in paralleled reactions. Based on the absorption spectra of the two photocatalysts, a narrow-band LED centered at 395 nm should be efficient at photoexciting both catalysts, while one centered at 450 nm should only be capable of exciting the Ce complex (Figure 2A). Indeed, the disparate results obtained with the 13582


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[4.2.1] ring systems were effectively synthesized utilizing substituted indanols and alkene 2 in an atom and stepeconomical fashion. Simple cyclopentanols could be cleaved and alkylated with high efficiency, however, the corresponding bridged-lactone was rendered problematic by the entropically challenging intramolecular aldol reaction, thus only limited success was achieved under basic conditions (see Supporting Information for cycylopentanol scope). Aside from cyclopentanols, this mild condition was successfully applied to an array of readily available cyclobutanols for rapid constructions of valuable [3.2.1]-bridged scaffolds. Critically, several complex bridged structures have been unambiguously determined by Xray crystallography. It is worth noting the cycloaddition proceeds with high regioselectivity for unsymmetrical cycloalcohols, as the β-scissions of alkoxy radicals are highly selective for the formation of the most stable radical intermediate. Furthermore, a wide range of electron-deficient alkenes were formally inserted into the cyclobutanol ring to generate bridged-lactones with high efficiency. To demonstrate the synthetic potential of this simple and inexpensive protocol, we first carried out a 100 times scaled-up reaction using continuous flow technology using the standard catalytic condition (Figure 3). A parallel of 10 glass microreactors (4.5 mL total internal volume) were employed to ensure maximum utilization of photons to achieve high efficiency. At a flow rate of 3 mL/min in a closed-loop mode, the desired cycloaddition product was produced at a remarkable productivity of 5 mmol/h. Recognizing the prevalence of bridged-lactone scaffolds in complex natural products, we further developed a concise route for the rapid access to the challenging polycyclic skeleton of nepalactones. From commercially available methylcyclopentene, a ketene [2+2] cycloaddition followed by simple reductions would provide a fused cyclobutanol 29. Under the standard condition, the ring system of 29 was readily reconstructed to generate 30, the polycyclic bridged-lactone core structure of nepalactone A and B. In summary, we have developed a general photocatalytic protocol to access bridged lactones via the formal cycloaddition of cycloalkanols with alkenes. This atom- and stepeconomical transformation takes advantage of dual photoexcitation of inexpensive and robust cerium salts and DPA. The mild reaction conditions have been successfully adapted to continuous flow reactors for scale-up applications. Furthermore, the synthetic value of this operationally simple transformation has been demonstrated in the rapid synthesis of the polycyclic core skeleton of the nepalactones. We postulate that the merger of LMCT and PET catalysis could be extended to more Earth-abundant metals for the development of sustainable photocatalytic transformations.

Figure 3. Scale-up applications in continuous flow reactors and rapid access to the core structure of nepalactones.

395 nm LED vs 450 nm laser exhibited rate acceleration with the inclusion of DPA with irradiation at 395 nm, but no such effect when irradiated at 450 nm. These results are indicative of the excited DPA playing a catalytic role, whereas without excited DPA in the reaction mechanism, the Ce catalytic cycle evidently stalls after all dissolved oxygen in the reaction is consumed. Furthermore, Stern−Volmer quenching studies revealed that the emission of photoexcited DPA (excited state lifetime, τ = 9 ns) can be quenched by electron-deficient alkene 2, but not by Ce(III) complex, and thus, we speculated that the radical cation of DPA (E1/2 = 1.13 V versus SCE in acetonitrile) is serving as the crucial oxidant to regenerate Ce(IV) from Ce(III) (E1/2 (CeIV/CeIII) = 0.40 V versus SCE in acetonitrile). In light of these findings, a detailed mechanism for this dual photoexcitation promoted cycloaddition is proposed in Figure 2B. Upon irradiation, LMCT would promote bond homolysis to generate the key secondary alkoxy radical, triggering a fast βscission process to cleave the α-hydroxy C−C bond of cycloalkanol and yield a nucleophilic alkyl radical. The subsequent radical cross-coupling with electron-deficient alkene 2 would forge a new C−C bond and an α-acyl radical. Single electron reduction of the α-acyl radical (E1/2 = −0.60 V versus SCE in acetonitrile),20 which falls out of the narrow redox window of Ce(III), can be easily accomplished by the strongly reducing photoexcited DPA (E1/2 = −1.77 V versus SCE in acetonitrile, calculated following the Rehm−Weller formalism) to generate a stable enolate and the radical cation of DPA. A facile intramolecular aldol reaction would afford an isolable cycloheptanol, which can be subsequently converted to the desired bridged-lactone product upon acid treatment. The single electron transfer between the radical cation of DPA and Ce(III) would concurrently close the two catalytic cycles. With the inexpensive catalyst combination in hand, next we explored this simple protocol for facile bridged-lactone assembly (Table 2). Pleasingly, a variety of functional groups including ethers, amides, and halogen atoms are well-tolerated. Notably, functional groups prone to transition-metal insertions, such as cyclopropane ring, cyanoarene and aryl bromide, stay intact in this cerium catalyzed system. A variety of rigid

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b08781. Experimental procedures and compound characterization data (PDF) Crystallographic data for 3(CIF) Crystallographic data for 12 (CIF) Crystallographic data for 13 (CIF) Crystallographic data for 15 (CIF) Crystallographic data for 18 (CIF) 13583


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Journal of the American Chemical Society

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Crystallographic data for 20 (CIF) Crystallographic data for 24 (CIF) Crystallographic data for 27 (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Zhiwei Zuo: 0000-0002-3361-3220 Author Contributions †

A.H. and Y.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21772121) and the “Thousand Plan” Youth program. J.-J.G. thanks the Shanghai Sailing Program (18YF1416900) for financial support.

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Cerium-Catalyzed Formal Cycloaddition of Cycloalkanols with

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