Letter Cite This: Org. Lett. 2018, 20, 2382−2385
pubs.acs.org/OrgLett
Autoxidation Photoredox Catalysis for the Synthesis of 2‑Phosphinoylindoles Chun-Hai Wang,† Yong-Hong Li,† and Shang-Dong Yang*,†,‡ †
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
‡
S Supporting Information *
ABSTRACT: A new approach to the synthesis of 2phosphinoylindoles through photoredox catalysis without external oxidants has been developed. Promoted by a ruthenium photoredox catalyst, a broad scope of 2phosphinoylindoles can be synthesized through phosphinoylation/cyclization of diphenylphosphine oxide at room temperature under irradiation without external oxidants. The estrone skeleton isocyan is also an amenable substrate for this cyclization, yielding a molecule that has potential medicinal applications.
I
Scheme 1. Photoredox Catalysis for the Synthesis of 2Phosphinoylindoles
n recent years, P-radicals have been widely applied to build a range of organophosphorus compounds because they are easily generated in reactions and their high activity allows them to be trapped by many unsaturated linkages such as olefins, alkynes and isonitriles.1 Early methods for producing P-radicals required stoichiometric amounts of a radical initiator such as AIBN, SnBu4, and others,2 which meant that the reaction was neither economic nor functional-group compatible. Recently, the use of transition-metal-catalyzed P-radical reactions3 effectively solved these problems, but the use of an extra equivalent of oxidizer and invariably high reaction temperature prompted us to search for more economic and facile methods of producing Pradicals. The emergence of photoredox catalysis provides an alternative strategy considering its mild reaction temperature and environmentally compatible conditions.4 At present, increasing numbers of photoredox-based P-radical reactions have been developed to synthesize a range of organophosphorus compounds.5 Indole skeletons are common in natural products and drug molecules.6 Among the members of the indole family, phosphinoylindoles display the best biological activity, and they have some attractive prospects as pharmaceuticals and materials (Scheme 1a).6d,i,11 As a result, the development of sustainable catalytic strategies for the synthesis of phosphinoylindoles is considered highly desirable. In past decades, although many strategies have been used to synthesize different structural phosphinoylindoles,7 to our knowledge, the application of photocatalytic approaches to the synthesis of phosphinoylindoles has not been reported to date. Herein, we demonstrate a Ru(bpy)3Cl2·6H2O-prompted autoxidation photoredox catalysis that can be used to form 2-phosphinoylindoles (Scheme 1b). Compared with the existing phosphorus radical reactions, this photoredox catalysis process does not need an additional © 2018 American Chemical Society
oxidant, and the formed benzyl radical F is not only the key reaction intermediate but also acts as oxidant to oxidize ruthenium(I) to regenerate the ground-state photoredox catalyst ruthenium(II) and complete the catalytic cycle.8 Furthermore, by using this method, we can easily synthesize the pharmaceutical compound zafirlukast derivative of 3-benzyl-2-phosphinoylindoles in a single step. In the initial study, we first selected 1-isocyano-2-styrylbenzene (1a) and diphenylphosphine oxide (2) as substrates and Received: March 4, 2018 Published: April 6, 2018 2382
DOI: 10.1021/acs.orglett.8b00722 Org. Lett. 2018, 20, 2382−2385
Letter
Organic Letters DBU as base to explore various photocatalysts, including eosin Y, rose bengal, anthraquinone, rhodamine B, fac-Ir(ppy)3, and Ru(bpy)3Cl2·6H2O, in the proposed phosphinoylation/cyclization reaction in CH3CN at 25 °C irradiated with 5 W blue LEDs. We were delighted to obtain the desired product 3a in 30% yield by using Ru(bpy)3Cl2·6H2O as the photoredox catalyst (Table 1,
Scheme 2. Substrate Scope for the Oxidant-Free Photoredox Catalysis Protocola
Table 1. Investigation of the Reaction Conditionsa
entry
base
solvent
yielda,b (%)
1 2 3 4 5 6 7 8 9 10c 11d
DBU DABCO DBU DBU DBU DBU DBU DBU DBU DBU DBU
CH3CN CH3CN DCM DMF DMSO CH3CN/H2O (0.10 mL) CH3CN/H2O (0.12 mL) CH3CN/H2O (0.14 mL) CH3CN/H2O (0.15 mL) CH3CN/H2O (0.14 mL) CH3CN/H2O (0.14 mL)
30 26 25 NR NR 69 79 91 89 79 54
a
All reactions were carried out with 1a (0.1 mmol), 2 (0.15 mmol), Ru(bpy)3Cl2·6H2O (0.005 mmol), base (0.2 mmol), and solvent (1.0 mL), irradiated with 5 W blue LEDs for 4 h under Ar at 25 °C. b Isolated yields. NR = no reaction. cRu(bpy)3Cl2·6H2O (0.04 equiv). d 1a (0.12 mmol), 2 (0.1 mmol).
entry 1),9 whereas the other catalysts did not promote this transformation (Table S1).5a,d Subsequent base screening showed that only DBU and DABCO promoted this reaction, with DBU being much more effective (Table 1, entries 1 and 2). We also further examined a range of solvents and established that CH3CN was a more competent solvent (Table 1, entries 3−5). Based on the above results, we considered whether an appropriate amount of water may be beneficial to this reaction because rapid protonation may accelerate the reaction rate. When 0.1 mL of H2O was added into the reaction system, we were delighted to obtain the desired product in moderate yield (69%; entry 6). Further screening of the amount of water revealed that CH3CN/H2O (1.0/0.14 mL) was the optimal choice, giving the desired product 3a in excellent yield (91%; Table 1, entries 7−9). Decreasing the loading of Ru(bpy)3Cl2· 6H2O or changing the proportion of the two substrates was not helpful (Table 1, entries 10 and 11). Finally, control experiments run in the absence of either photoredox catalyst or a visible light source did not generate the phosphonylation/cyclization product 3a, which indicated that the photoredox process is a crucial process in the reaction (Table S1). Having identified optimal conditions, a wide range of isonitriles were submitted to the photoredox catalysis protocol. First, we investigated the influence of substituted arene isonitriles. As shown in Scheme 2, when the strong electronwithdrawing groups OCF3 and Cl were introduced to the paraposition of isonitriles (3c, 3d), the yields dropped to 49% and 42% (the structure of 3c was confirmed by X-ray crystallographic analysis; see the Supporting Information (SI)). To our delight, when methyl or methoxy groups were introduced into the benzene ring of the isonitriles, moderate or excellent yields of
a
All reactions were carried out with 1 (0.1 mmol), 2 (0.15 mmol), catalyst (0.005 mmol), DBU (0.2 mmol), and CH3CN/H2O (1.0/0.14 mL), irradiated by 5 W blue LEDs for 4 h under Ar at 25 °C; yield of isolated product is given. b2 (3.0 equiv), Ru(bpy)3Cl2·6H2O (0.10 equiv), DBU (4.0 equiv), CH3CN/H2O (2.0/0.28 mL). cCH3CN (1.0 mL).
67−91% were obtained (3b, 3e−h). These results demonstrated that the electronic nature of the group on the arene isonitriles can affect the reaction to different degrees. We then investigated the use of substituted styrene substrates. We were delighted that the reactions proceeded in good yields of 73% and 79% (3i and 3j) when thiophene and pyridine were used instead of benzene. Furthermore, a wide range of substituted styrene derivatives were tolerated in the photoredox catalysis process. Both electron-withdrawing groups (3l, 3m, 3o, and 3q) and electron-donating groups (3k, 3n, 3p, and 3r) could be present on the styrene, and good to excellent yields of 71% to 96% were obtained. Notably, the phosphinoylation/cyclization of 2,3,4,5,6-pentafluorostyrene was carried out with excellent yield (90%, 3q). In spite of the steric bulk of the disubstituted styrene 1t, the reaction also proceeded with good yield (71%, 3t). Importantly, estrone skeleton isocyan was also an amenable substrate; the transformation proceeded with moderate yield (57%, 3s) to give a molecule that may have potential medicinal applications. Finally, we have also investigated some different 2383
DOI: 10.1021/acs.orglett.8b00722 Org. Lett. 2018, 20, 2382−2385
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Organic Letters
reaction indicated that a radical process might be involved (eq 1). Thereafter, we obtained the deuterium-labeled product 3a-d with the deuterium at the benzyl position by using deuteroxide instead of water (eq 2). Furthermore, when o-vinylphenyl isocyanide 1z, which lacks the benzene ring, was used as the substrate, the photoredox catalysis process did not proceed (eq 3). These results indicate that benzyl anion G is an important intermediate involved in the reaction and thereby demonstrates that the reduction occurred as expected, with the resulting benzyl radical F reduced by SET from available Ru(I) species C to generate benzyl anion G and regenerate the ground-state photoredox catalyst A, completing the proposed catalytic cycle (Figure 1).8,9c
P(O)−H compounds. We found that the substituted diphenylphosphine oxides could be successfully used as an efficient precursors of P-radicals in this transformation with good to excellent yields (94%, 3u and 73%, 3v). If one aryl group was replaced by the butyl group, the yield of phosphinoylation/ cyclization was dropped to 8% (3w), and when diethyl phosphonate was used as the P-source, only a trace amount of product was detected by NMR analysis (3x). Zafirlukast10 is a well-known oral leukotriene receptor antagonist (LTRA) for the maintenance treatment of asthma. The key factor for its synthesis is the construction of the indole ring. Thus, the late-stage functionalization of drug molecule 5y was designed to highlight the synthetic value of our photoredox catalysis strategy (Scheme 3). Not only good efficiency but also Scheme 3. Synthesis of a New Zafirlukast Derivative
Figure 1. Proposed mechanistic pathway.
Based on the experimental results of the mechanistic investigation, details of the mechanism of the proposed phosphonylation/cyclization reaction are outlined in Figure 1. Irradiation of photoredox catalyst Ru(bpy)3Cl2·6H2O A with visible light leads to the formation of a long-lived (τ = 1.1 μs) excited-state *RuII B, which is a suitable oxidant (E1/2red = 0.77 V vs SCE in CH3CN).4b On this basis, we hypothesized that, first, oxidation of the conjugate base of C should be thermodynamically feasible, generating P radical D, following the loss of a proton from diphenylphosphine oxide 2 under the basic conditions.5a,d Then P radical D is rapidly trapped by 1isocyano-2-styrylbenzene (1a) to generate alkene radical E,1g,i which immediately forms benzyl radical F through 5-exo-trig cyclization.11 Finally, we expected that reduction of the resulting benzyl radical F by SET from available RuI species C (E1/2red = −1.37 V vs SCE in CH3CN) should generate benzyl anion G and regenerate the ground-state photoredox catalyst A, completing the proposed catalytic cycle.4b,8,9 At the same time, benzyl anion G should yield the product 3a after protonation and isomerization. In summary, we have demonstrated a new, mild approach to the synthesis of 2-phosphinoylindoles through photoredox catalysis without the use of external oxidant. A range of 1isocyano-2-styrylbenzenes can be applied efficiently in this transformation, making it appealing for late-stage synthesis strategies.
functional-group tolerance was demonstrated for the successful realization of 3y with a moderate yield of 64%. According to the published procedure, 5y can be obtained through the late-stage transformation of 3y. Thus, a new zafirlukast derivative may be obtained that could allow the development of new drug therapies for the maintenance treatment of asthma. These pioneering results are expected to offer new opportunities in drug discoveries. Several experiments were implemented to test our mechanistic hypothesis (Scheme 4). In the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 2.0 equiv) or 2,6-di-tert-butyl-4methylphenol (BHT, 2.0 equiv), the observed inhibition of the Scheme 4. Mechanistic Studies
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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/acs.orglett.8b00722. Experimental details and characterization data for all new compounds (PDF) 2384
DOI: 10.1021/acs.orglett.8b00722 Org. Lett. 2018, 20, 2382−2385
Letter
Organic Letters Accession Codes
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CCDC 1817885 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shang-Dong Yang: 0000-0002-4486-800X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the NSFC (Nos. 21472076 and 21532001) and International Joint Research Centre for Green Catalysis and Synthesis, Gansu Provincial Sci. & Tech. Department (No. 2016B01017) for financial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b00722 Org. Lett. 2018, 20, 2382−2385
