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DOI: 10.1039/D4GC02737J (Paper) Green Chem., 2024, 26, 9749-9756

Visible light-induced cobalt-catalyzed 1,3-diphosphination of alkenes†

Wenlong Shan a, Zemin Wang a, Chenxia Gao a, Xiaowei Li a, Wenli Zhuang a, Ruihua Liu a, Cong Shi a, Hongyun Qin a, Xiangqian Li a and Dayong Shi *ab
aState Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, Shandong, P. R. China. E-mail: shidayong@sdu.edu.cn
bLaboratory for Marine Drugs and Bioproducts, Qingdao Marine Science and Technology Center, Qingdao, 266237, Shandong, P. R. China

Received 6th June 2024 , Accepted 6th August 2024

First published on 9th August 2024

Abstract

1,3-Difunctionalization of alkenes, as a novel route for the modern development of alkene transformations, has drawn significant research attention in recent years. Herein, we disclose a novel cobalt-catalyzed radical 1,3-diphosphination of alkenes, which enables straightforward access to 1,3-diphosphine skeleton compounds under mild conditions without additional oxidants and photosensitizers. This transformation features excellent functional group tolerance, operational simplicity, and high atom economy, and is amenable for late-stage functionalization of complex molecule skeletons. Preliminary bioactivity studies reveal that these valuable 1,3-diphosphine products show potential antitumor activity.

Introduction

Organophosphorus compounds play important roles as bioactive molecules,1 functionalized materials,2 and ligands (Scheme 1a).3 As indispensable ancillary ligands in organic catalysis, diphosphines can greatly affect the activity and selectivity of transition-metal salts in catalytic reactions.4 Among them, the structural motif of 1,3-diphosphine has attracted significant attention as a bidentate chelating ligand for the fabrication of interesting coordinated six-membered metallacycles with transition metals, which have accelerated the development of the homogeneous transition metal catalysis field.5 Despite their importance, the number of effective methods for their preparation is quite limited. A well-known process for the preparation of 1,3-diphosphine compounds is based on SN2 nucleophilic substitution between 1,3-dihalopropane and alkali metal diarylphosphides.6 In recent years, the strategies to access such molecular architectures involved tandem double hydrophosphination under conditions of a strong base or high temperature,7 ring opening diphosphination using methylenecyclopropanes,8etc.9 Unfortunately, these methods are greatly limited by the need for prefunctionalized starting materials, air-sensitive reagents, and harsh conditions, which could cause side reactions, low atom-economy, and environmental problems (Scheme 1b). Therefore, developing a green synthesis method for 1,3-diphosphine compounds with high atom- and step-economy from simple and readily available starting materials is a highly desirable yet challenging task.
image file: d4gc02737j-s1.tif
Scheme 1 Cobalt-catalyzed 1,3-diphosphination of alkenes.

As a sustainable and versatile strategy for constructing complex chemical structures from simple feedstock, the direct difunctionalization of alkenes has received a great deal of attention and interest.10 Compared to the abundant studies on the 1,2-difunctionalization of alkenes, the nascent area of 1,3-functionalization has emerged as a novel route for the modern development of alkene transformations, which has been significantly applied in the synthesis of various complex molecules and pharmaceutical targets.11 Cobaloxime, as a model system of vitamin B12,12 has been established as a powerful catalyst for alkenylation13 and aromatization14via hydrogen evolution. Given that the organocobalt(III) intermediate undergoes β-H elimination, we propose a strategy for the cobalt-catalyzed radical 1,3-difunctionalization of alkenes. Radical addition to the alkene forms a carbon-centered radical intermediate, which could be captured using a cobalt catalyst, followed by β-H elimination13d and radical readdition to realize the 1,3-difunctionalization of alkenes (Scheme 1c).

Based on this strategy, we report a novel cobalt-catalyzed radical 1,3-diphosphination of alkenes, which allows dual C–P bond formation through double addition of a phosphinoyl radical to alkenes in one pot without additional oxidants and photosensitizers (Scheme 1d). Our protocol opens up a green and highly efficient route for the construction of 1,3-diphosphine skeleton compounds with only H2 as the byproduct. The synthetic utility of the protocol has been further corroborated through functionalization of complex molecule skeletons and modifications of the products. Importantly, preliminary bioactivity studies reveal that the synthesized compounds possess potential anticancer activity. Moreover, this work via a radical addition–dehydrogenation–readdition process provides a new strategy for the 1,3-difunctionalization of alkenes.

Results and discussion

We initiated our studies by employing Co-I (10 mol%) as a catalyst and pyridine as a base in 1,2-dichloroethane. N-Phenylmethacrylamide 1 and commercially available diphenylphosphine oxide 2 were used as model substrates for the reaction development (Table 1). After systematic optimization of the reaction conditions, product 3 was obtained in 89% isolated yield (entry 1). Several structurally similar Co catalysts, such as Co-II and Co-III, were tested but 3 was obtained in unsatisfactory yields (entries 2 and 3). Variations of other bases and solvents led to diminished yields of product 3 (entries 4–11). The absence of pyridine resulted in a dramatic decrease in efficiency (entry 12). Furthermore, control experiments revealed that the Co catalyst and light were both essential for this transformation (entries 13 and 14).
Table 1 Optimization of the reaction conditionsa

image file: d4gc02737j-u1.tif
Entry Deviation from the standard conditionsa Yieldb of 3 (%)
a Reaction conditions: alkene 1 (0.1 mmol), diphenylphosphine oxide 2 (0.25 mmol), Co-I (0.01 mmol), pyridine (0.2 mmol), DCE (1 mL), 6 W LED (420–430 nm), 25 °C for 24 h under an Ar atmosphere. b Yields were determined by 1H NMR analysis (with CH2Br2 as the internal standard). Isolated yield in parentheses.
1 None 90 (89)
2 Co-II as the catalyst 34
3 Co-III as the catalyst n.d
4 HCOONa instead of pyridine 41
5 CsCO3 instead of pyridine 46
6 DBU instead of pyridine 32
7 DABCO instead of pyridine 23
8 DCM instead of DCE 78
9 MeCN instead of DCE 27
10 DMSO instead of DCE n.d.
11 MeOH instead of DCE n.d.
12 No pyridine 17
13 No Co-I n.d.
14 No light n.d.
image file: d4gc02737j-u2.tif

Having established optimal reaction conditions, we then explored the reactivities of various alkenes, and the results are summarized in Table 2. A series of methacrylamides were observed to be suitable substrates, including those with alkyl and phenyl groups located at the nitrogen core, and gave good yields (3–7). Substrates with either electron-donating or electron-withdrawing substituents on the benzene rings could lead to the formation of products in moderate to excellent yields (8–13). We were pleased to find that methacrylamides and dialkylamides were well tolerated in this transformation (14–15). The five-, six-, and seven-membered heterocycles could be effectively transformed to the desired products (16–18). Moreover, other heterocyclic ring systems, which have an active C–H bond bound to either the O or the S atom, were also found to be compatible with this reaction (19–21). In particular, indole could smoothly afford the corresponding product, without the product of C-3 substitution (22).15

Table 2 Substrate scope of alkenesa
a Reaction conditions: alkene (0.1 mmol), diphenylphosphine oxide (0.25 mmol), Co(dmgH)2(DMAP)Cl (0.01 mmol), pyridine (0.2 mmol), DCE (1 mL), 6 W LED (420–430 nm), 25 °C for 24 h under an Ar atmosphere. Isolated yield. b The reaction time was 48 h. c 1.0 mmol-scale reaction was performed.
image file: d4gc02737j-u3.tif

Subsequently, various acrylates were tested. The reaction of tert-butyl 2-methyl acrylate could result in a good yield (23). Phenyl and benzyl 2-methyl acrylates were tolerated as well (24–25). Furthermore, it was found that the substrates with a bulky group could give the desired products in good yields (26–31). To broaden the generality of the reaction, we turned our attention to other types of alkenes to explore the potential of this transformation. Methacrylonitrile could react under optimized reaction conditions to provide the corresponding product (32). Heterocyclic aromatic alkenes were also smoothly transformed into diphosphine products (33–36). To further demonstrate the potential utility of the protocol, we evaluated the method in the late-stage modification of complex and biologically active compounds, all of which were suitable candidates and afforded the desired products in good yields (37–44). In addition, a 1 mmol scale reaction was conducted and a glucose derivative 40 was obtained with 80% yield. These results demonstrated that the present method would be valuable for late-stage functionalization in synthetic sequences.

We further investigated the scope of phosphine oxides (Table 3). An array of phosphine oxides with electron-donating substituents were found to be suitable reaction partners, giving the corresponding products (45–50). Notably, diheteroarylphosphine oxide was proved to be tolerant of this system (51). It is worth mentioning that unsymmetrical phosphine oxide was also a good partner for this reaction and afforded the desired products (52). Unfortunately, when diethyl phosphite or dicyclohexylphosphine oxide was used, it could not furnish the corresponding products, probably due to lower reactivity.

Table 3 Substrate scope of phosphine oxidesa
a Reaction conditions: alkene (0.1 mmol), diphenylphosphine oxide (0.25 mmol), Co(dmgH)2(DMAP)Cl (0.01 mmol), pyridine (0.2 mmol), DCE (1 mL), 6 W LED (420–430 nm), 25 °C for 48 h under an Ar atmosphere. Isolated yield. b Determined by 31P NMR spectroscopy of the mixtures.
image file: d4gc02737j-u4.tif

After establishing the scope of 1,3-diphosphination of alkenes, we subsequently showed the synthetic utility of this protocol (Scheme 2a). With MeMgBr, the acylamide moiety of compound 15 could be readily converted to ketone 15a in good yield. Considering the importance of 1,3-diphosphine skeleton compounds in organic synthesis, compound 15 could be easily transformed to a lower valent diphosphine 15bvia the reductive reaction using the HSiCl3/Et3N system. A further reduction of the acylamide moiety with LiAlH4 could deliver the potential tripod ligands 15c.


image file: d4gc02737j-s2.tif
Scheme 2 Synthetic applications and mechanistic investigations.

To gain more insight into the reaction mechanism, a series of mechanistic experiments was conducted. The reactions in the presence of radical scavengers, such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 1,1-diphenylethylene, were significantly inhibited, indicating that this transformation might involve radical processes. Noticeably, radical adducts were detected using HRMS or 1H NMR, which indicated a phosphinoyl radical-involved mechanistic scenario (Scheme 2b). Subsequently, we speculated that 3a might be an intermediate of the reaction. When 3a as a substrate was subjected to this transformation, the desired product 3 was generated in good yields (Scheme 2c). To get a further understanding of the mechanism, we studied the reaction profile of this procedure. As shown in Scheme 2e, product 3 was generated with the formation of 3a under the standard reaction conditions. These results together indicated that 3a may be the key intermediate and this dual C–P bond formation is a cascade process. The light on-and-off experiment showed that continuous irradiation was essential for product formation (Scheme 2f). The deuterium-labeling experiment with D2O under standard conditions demonstrated that the metallic intermediate probably underwent the process of protonation (Scheme 2d).

Based on the above experimental results and previous reports,13a,e,f,16 a plausible reaction mechanism was proposed and is illustrated in Scheme 2g. Under visible light irradiation, photoexcited Co(III) oxidizes diphenylphosphine oxide 2 into a phosphinoyl radical with the formation of Co(II) species. Subsequent addition of the phosphinoyl radical to the electron-deficient olefin 1 coupling partner forms a carbon-centered radical intermediate A. In the meantime, Co(II) can accept the radical to afford B. With irradiation, the Co–C bond cleavage and subsequent β-hydride elimination will furnish intermediate 3a and hydrocobalt C, which can react with another proton to release H2.13e,f,16c,d A similar process for the formation of the second C–P bond is proposed. The addition of a phosphinoyl intermediate gives rise to the carbon center radical D, which combines with Co(II) to form intermediate E. Protonation of intermediate E finally provides the H-phosphorylation product 3, accompanied by the regeneration of the initial Co(III) complex, which closes the catalytic cycle without an external oxidant or reductant.

Inspired by the significant biological activities of phosphorus-containing molecules, the antiproliferative activity of compounds was measured in vitro using the CCK-8 method on four human cancer cell lines (HCT116, A549, HeLa and SK-OV-3) and two normal human cell lines (H8 and NCM460). The commercially available antitumor drug 5-FU was used as the positive control (for more details, see the ESI†). The results indicated that compounds 28 and 30 exhibited excellent antitumor potential with IC50 values of 6.11 μM and 4.86 μM, respectively. Moreover, these two compounds possess lower cytotoxicity in normal cells (Fig. 1a). These results indicate that compounds 28 and 30 possess a certain degree of selectivity and safety. Next, the expression of apoptosis-related proteins after treatment with different concentrations of compounds 28 and 30 was detected by western blot analysis. The results showed that the expression level of the apoptosis marker protein C-PARP was increased and the expression level of the anti-apoptotic protein Bcl-2 was decreased with the increase in the concentration of the compounds, which indicated that compounds 28 and 30 induced apoptosis in HCT116 cells (Fig. 1b). These results demonstrate the potential application of 1,3-diphosphine compounds in antitumor drug development.


image file: d4gc02737j-f1.tif
Fig. 1 In vitro antiproliferative activity of compounds. (a) Antiproliferative activities of compound 28 and compound 30, with 5-FU as the positive control in cell proliferation inhibition tests. (b) Western blot analysis of apoptosis-related protein levels in HCT116 cells treated with compounds 28 and 30 for 48 h at the doses indicated.

Conclusions

In summary, we developed a novel strategy for radical 1,3-diphosphination of alkenes via cascade cobalt catalysis under mild reaction conditions. The reaction is operationally simple and high yielding and offers a broad scope across the pool of 1,3-diphosphine skeleton compounds in a step- and atom-economical manner. Notably, this reaction model has been successfully used in the introduction of double phosphoryl groups into many complex molecule skeletons. Mechanistic studies indicate that the reaction likely proceeds through radical addition, β-H elimination, and radical readdition processes. It is also worth mentioning that 1,3-diphosphine skeleton compounds exhibit excellent anticancer activity against HCT116 cells, demonstrating their potential utility in drug discovery. Further investigation of this strategy and the application of these compounds is underway in our laboratory.

Author contributions

Conceptualization: D. S., Z. W., R. L., and W. S.; methodology and investigation (reaction optimization, substrate scope and compound characterization): W. S.; biological activity of the compounds: C. G.; writing – original draft: W. S. and Z. W.; writing – reviewing and editing: W. S., Z. W., R. L., X. w. L., W. Z., C. S., H. Q. and X. q. L.; and supervision and funding acquisition: D. S.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2022YFC2804105), the Joint Fund of Shandong Natural Science Foundation (ZR2021LSW013), the Key R&D Program of Shandong Province, China (2023CXGC010413), the Natural Science Foundation of Shandong Province (ZR2023MH245 and ZR2022QB090), the Qingdao Emerging Industry Cultivation Project in 2023 (23-1-4-xxgg-19-nsh), and the Shandong Provincial Science and Technology SME Innovation Capacity Improvement Project (2022TSGC2204). We thank Haiyan Sui and Xiaoju Li from Shandong University Core Facilities for Life and Environmental Sciences for their help with the NMR studies.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc02737j
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