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Readily offer substrates for ring-closing metathesis
Access to tunable enones and cycloenones
The bigger picture
Alkenes are much preferred building blocks in organic synthesis. They are widely used in developing new reaction patterns and constructing sophisticated molecules. This inspires researchers to develop more alkene synthesis methodology from diverse substrates. Our methodology paved the way to preparation of terminal alkenes from cycloalkanols. Enones and cycloenones were thus achieved with variable chain lengths or ring sizes accordingly. As olefins are important building blocks, this transformation can serve as a powerful tool to enrich the arsenal of organic synthesis.
We have developed a dual-catalytic strategy for the synthesis of linear alpha olefins with variable chain lengths from readily available cycloalkanols. This strategy was realized by combining photocatalyzed ring opening of cycloalkanols and Cu-mediated β-H elimination. The terminal desaturation was achieved at aryl and alkyl ketones’ γ, δ, ε, or more remote sites under mild conditions with remarkable functional group tolerance. The resulting diene products via this approach have the potential to be transformed into challenging tunable medium-sized cycloenones.
Therefore, the generation of olefins is still synthetically attractive. During the past decades, deconstructive transformations into olefins through C–C bond cleavage emerged as a great success using Fe,
as catalysts (Figure 1A ). Such methodology extended the raw materials for preparation of olefins to acids, esters, ketones, amides, etc. Their key driving force was mostly evolution of small molecules during reactions such as CO2, HCN, etc. Inspired by the impressive advancements, a photodriven destructive transformation to olefins was developed to extend the olefination substrate scope beyond the above-mentioned materials to cycloalkanols.
Cycloalkanols have been reported as privileged precursors to deliver the distally functionalized ketones in a radical ring-opening process. The alkoxy radical species could be generated via a preactivated mode
Encouraged by this, activated cycloalkanols by hypervalent iodines were then designed to undergo a photoredox-catalyzed ring-opening process, subsequently enabling β-scission and Cu-mediated β-hydride elimination, affording distally unsaturated ketones (Figure 1D). The generality of this approach was demonstrated with wide functional group tolerance. In particular, products with two terminal alkenyl groups underwent ring-closing metathesis and delivered challenging medium-sized cycloenones.
Results and discussion
At the outset of our studies, we selected 1-phenylcyclopentanol 1a as the model substrate to react with 3.0 equiv of phenyliodine(III) diacetate (PIDA; Figure 2) in the presence of 2 mol % of 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN; Figure 2) and 20 mol % CuCl2. The ring-opening olefination readily proceeded under blue LED light irradiation, affording the desired enone 2a in 25% yield (Table 1, entry 1). Our initial screening investigated various copper catalysts and found that Cu(acac)2 was the best choice (entries 1–5; see Table S1 for details). The effect of the Cu(acac)2 amount was also studied. We were pleased to find that increasing the loading of Cu(acac)2 to 25 mol % decreased the reaction time to 4 h without any loss of yield (entry 6). We then tested a range of photocatalysts (transition metal and organic photoredox catalysts) and found that 9-mesityl-10-methyl acridinium (PC2; Figure 2) provided the highest yield of ring-opening olefination (entry 7). A solvent screening revealed that the optimal solvent was CH2Cl2 (for details, see Table S3). To further improve the yield, a careful screening of hypervalent iodine (HI) (PIDA and Figures 2A–2D) reagents was performed (entries 8–12), indicating that D (Figure 2) was the most efficient one over other HIs such as hydroxyl-benziodoxole (BI-OH), 2-iodoxybenzoic acid (IBX), and Dess-Martin periodinane (DMP), perhaps due to its better solubility in CH2Cl2. Then, we optimized the amount of D and found that 3.0 equiv of D showed the best result, giving 2a in 72% yield. Furthermore, control experiments were established in the absence of copper salts, HI D, or visible-light irradiation, respectively. No desired product was detected in all these cases (entries 13–15), demonstrating that all these components were necessary in this system. Without thephotocatalyst , only traces of the product were obtained under light condition (entry 16).
Reaction conditions: 1a (0.2 mmol, 1.0 equiv), HI (0.6 mmol, 2.0 equiv), PC (0.004 mmol, 0.02 equiv), copper salt (0.05 mmol, 0.25 equiv) in 2.0 mL CH2Cl2 under argon with 15 W blue LED irradiation at room temperature for 4 h.
a Reaction conditions: 1a (0.2 mmol, 1.0 equiv), HI (0.6 mmol, 2.0 equiv), PC (0.004 mmol, 0.02 equiv), copper salt (0.05 mmol, 0.25 equiv) in 2.0 mL CH2Cl2 under argon with 15 W blue LED irradiation at room temperature for 4 h.
With these optimized conditions in hand, we turned to assess the substrates tolerance of our reaction. Outstandingly, our methodology could be better tolerated with many functional groups including variously substituted aromatic rings (1a–1s, 1w–1y, and 1ab–1ag), alkynes (1z), cycloalkanes (1aa and 1aj), and alkenes (1v and 1ak).
As shown in Scheme 1, the scope of various cyclopentanols was firstly tested. Gratifyingly, substrates bearing substituents such as –F, –Cl, –Br, –I, –Me, –CN, –CF3, and –CO2Me on the phenyl rings gave the corresponding products in moderate to good yields (1a–1n, 1w–1y, 1aa–1ag, 1ah, and 1ai). The aryl bromide and iodide (1f and 1g) remained intact under the condition, allowing the further transformation of product into other derivatives through a cross-coupling reaction. The change in the position (meta-, ortho-, para-) of substituents did not affect the reaction (1b and 1k; 1d and 1l), giving the corresponding products in comparable yields. With respect to heteroaromatic substrates, thianaphthenyl (1p), thiophenyl (1q), furyl (1r), and methylpyrazolyl (1s) cycloalkanols could also work well with good yields, while pyridyl ones unluckily were not suitable substrates (see supplemental experimental procedures section 3.5). To our delight, the alkyl (1t and 1al) and alkenyl (1u and 1ak) groups were nicely tolerated and delivered the desired products in moderate yields. Alkyl substrate (1v) with polar sulfonyl group afforded the desired product in the 15% yield. Moreover, the use of six-membered cyclohexanols 1w–1aa also furnished the corresponding products. We also found that the reaction was efficient for four-membered (1ab–1ag) and medium- to large-sized rings (7-, 8-, and even 12-membered) (1ah–1al), furnishing the products in moderate to good yields.
As a particular feature, our methodology allows facile access to terminal dienes, which could be employed in olefin metathesis for the synthesis of more sophisticated olefins.
Site-selective medium-sized cycloenones 3a–3e were readily synthesized in high yields through ring-closing metathesis using 5 mol % of Hoveyda-Grubbs II catalyst (Scheme 2). The position of the double bond in cycloenones (3a versus 3b versus 3e) and the ring size of cycloenones (3c versus 3d) were all amenable with the use of cycloalkanols with different ring sizes.
To extend the application scope and demonstrate the utility of this dual-catalytic approach, the products were transformed into potentially valuable compounds (Scheme 3; for details, see supplemental information). First, 2b could easily react with TMSN3, forming a new C–N3 bond in good yield. The addition of 2b with CF2H-COOH in THF gave the corresponding CF2H-containing ketone 3g in 62% yield. Sulfonated derivative 3h was obtained in the presence of PhSO2Na under blue-light irradiation. Subsequently, epoxidation by m-CPBA gave the product 3i in 55% yield. Finally, the reduction of 2b with NaBH4 in ethanol led to the corresponding alcohol 3j in 95% yield.
Next, we conducted mechanistic experiments to gain insight into the mechanism of this transformation. Radical trapping experiments by TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) were conducted (Scheme 4, Reaction a; for details, see supplemental information). Only 8% of product 2a generated. When we removed the copper salt and added the functional reagent CBr4 or TsN3 (Scheme 4, Rxn b and c), terminally functional products 3k and 3l were collected in 52% and 28% yields, respectively. This indicated the generation of radical species. The copper salts could capture such radical species and led to terminal alkenes. Further control reactions demonstrated that no product could be obtained in the absence of PC or HI (see Tables S6 and S7). On this basis,
we propose a possible reaction mechanism (Scheme 4). First, the ground state of MesAcrMe+ gives a highly oxidizing singlet excited state MesAcrMe+∗ under visible-light irradiation, which undergoes a single electron transfer with the Cu(I) giving the reduced photocatalyst MesAcrMe ⋅. MesAcrMe ⋅ is then oxidized by intermediate I, which was formed by the ligand exchange of cycloalkanol 1a with HI reagent D. In this process, intermediate I generates the alkoxy radical II via single-electron transfer.
Subsequently, the β-scission of the alkoxy radical II leads to the alkyl radical III. Radical III is trapped by a Cu(II) salts, giving a Cu(III) intermediate. Further β-H elimination via TSI leads to corresponding olefin 2a and Cu(I) species.
It is worthy to note that the role of acac anion in TSI could be replaced by other carboxylate ions from the HI.
In summary, we have successfully designed and validated a dual photoredox/copper-catalyzed ring-opening olefination process under mild conditions. The reaction serves as a viable strategy for the synthesis of terminal olefins with highly functional group tolerance. Moreover, this methodology provides a high-efficiency process for the ring-opening of less-strained or unstrained cycloalkanols. The reaction mechanism is supported by radical trapping experiments. As alkenyl ketone is an essential functional group, this reaction has broad applications in organic synthesis.
Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Junliang Wu ( email@example.com ).
Any materials used are freely available from the lead contact upon request.
Data and code availability
Data are freely available upon request from the lead contact. No code was generated. Full experimental procedures are provided in the supplemental information.
This work was supported by grants from the Top Youth Talent Fund of Zhengzhou University and The State Key Laboratory of Bio-organic and Natural Products Chemistry, CAS (no. SKLBNPC18440 ), National Natural Science Foundation of China (no. 22101266 ), and International Postdoctoral Exchange Fellowship Program (Talent-Introduction Program, no. YJ20210304 ).
B.W. conducted most of experiments and collected the data. H.L. synthesized some substrates and performed control reactions. L.W. coordinated the project and drafted the manuscript. Y.-G.L. reviewed and revised the manuscript with assistance from all authors. J.W. conceptualized and directed the project with Y.-G.L.