Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Photocatalyst-Free Visible-Light Photoredox Dearomatization of Phenol Derivatives Containing Ketoximes: An Easy Access to Spiropyrrolines Yongzhen Han, Yunhe Jin, Min Jiang, Haijun Yang, and Hua Fu* Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China
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S Supporting Information *
ABSTRACT: A novel and simple visible-light photoredox intramolecular dearomatization of phenol derivatives containing ketoximes leading to spiropyrrolines has been developed. The protocol uses readily available 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base and electron-donor, visible light as the light source, and the reaction was performed well at room temperature without need of a photocatalyst. Therefore, the present method should provide a useful strategy for synthesis of spiropyrrolines.
S
simple aromatic molecules.8 Phenols and their derivatives are readily available and inexpensive chemicals,9 and their dearomatization affords multifunctionalized cyclic enones that are often found as a structural core in natural products.10 In previous methods, common protocols for dearomatization of phenols include metal- and hypervalent-iodine-mediated oxidations,11 transition-metal-catalyzed alkylation and arylation,12 and phase-transfer chiral-anion-catalyzed fluorination.13 However, related C−N bond-forming dearomatization of phenols is still in its infancy, so it is highly desirable to develop novel dearomative processes. Readily available oximes are usually stable to air and moisture, easy to store, and simple to handle, and they are important intermediates in various chemical transformations through N−O bond cleavage14 because of its relatively low bond energy.15 Recently, visiblelight photoredox organic reactions have become a powerful strategy under mild conditions.16 However, the generation and utilities of nitrogen-centered radical species are limited via visible-light photocatalysis thus far.17 Very recently, several visible-light photoredox methods involving nitrogen-centered radicals have been developed.18 Meanwhile, we have also represented a useful visible-light-mediated intramolecular αC(sp3)-H imination of tertiary aliphatic amines containing β-Oaryl oximes.19a As part of our continuing study on the visiblelight photoredox reactions,19 here, we report a novel, simple, and practical photocatalyst-free visible-light photoredox dearomatization of phenol derivatives containing ketoximes leading to spiropyrrolines at room temperature. At first, visible-light photoredox intramolecular dearomatization of 1-(4′-hydroxy-[1,1′-biphenyl]-2-yl)ethanone O-(2,4dinitrophenyl) oxime (1a) leading to 3′-methylspiro[cyclohexa[2,5]diene-1,1′-isoindol]-4-one (2a) was used as the model to
pirocycles with inherent three-dimensional (3D) conformational constraint nature and rich structural diversity and complexity widely occur in natural products, biologically and pharmacologically active molecules, and they are assigned as the privileged scaffolds in the drug discovery.1 In this regard, spiropyrrolines are often found as core motifs in many bioactive natural products and pharmacologically relevant synthetic compounds.2−8 Several representative examples (A−I) are shown in Figure 1, and they show diverse biological acitities.3−7
Figure 1. Representative examples for biologically and pharmacologically active spiropyrroline molecules.
Therefore, it is highly desirable to develop conceptually novel, economical and efficient synthetic methods for the synthesis of spiropyrrolines with readily available building blocks. Aromatic compounds are widely found in nature and synthetic materials. Dearomatization reaction has emerged as a powerful chemical transformation in the synthesis of complex compounds (including heterocyclic skeletons) from relatively © XXXX American Chemical Society
Received: January 29, 2019
A
DOI: 10.1021/acs.orglett.9b00372 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
entry 11). Subsequently, inexpensive DBU was used in the following optimization of conditions. Another seven solvents were screened, and they were inferior to MeCN (see Table S1 in the Supporting Information). When amount of DBU was reduced to one equivalent (relative to amount of 1a), the reaction did not complete until 24 h (Table 1, entry 12). In addition, we found that small amount of precipitate appeared when DBU was added to the solution of 1a in MeCN, which showed that 1a and DBU formed salt (see I in Scheme 3, presented later in this work). Comparing the conditions in entries 7 and 12 in Table 1, we found that the reaction rate in entry 7 was greatly promoted in the presence of 1.1 equiv of DBU. The result showed that an excess amount of DBU was more favorable for this reaction. We attempted the reaction in the presence of 0.5 equiv of DBU (Table 1, entry 13), and found that a longer reaction time was needed (96 h) with only 72% yield of 2a, which indicated that the formed salt between 1a and DBU was reversible. The same yield as that in entry 7 was afforded when 1.5 equiv of DBU was used (Table 1, entry 14). No reaction occurred in the dark (Table 1, entry 15). The reaction worked well when 6 W blue LEDs was used as light source instead of 23 W CFL (see Table S1), and the result exhibited that the present reaction was performed in the region of visible light rather than in a region of UV light. An air atmosphere was disadvantageous for this reaction (Table 1, entry 16). When the amount of 1a was increased to 0.2 mmol from 0.1 mmol, 2a was obtained in 89% isolated yield (Table 1, entry 17). With this optimized photoredox conditions in hand, we investigated the substrate scope on the photocatalyst-free dearomatization of phenol derivatives containing ketoximes. As shown in Table 2, we attempted variation of three units in 1. First, when R2 units were aromatic structures, substrates 1 with different substituted aliphatic alkyls R1 provided the corresponding target products (see 2a−2k) in good to excellent yields, in which substrates containing neutral and electron-donating groups in R2 gave higher yields than those containing electron-withdrawing groups in R2. For the substrate with aliphatic R1 and R2, a lower yield was afforded (see 2l). Subsequently, we investigated varation of aryl R1 when R2 were aliphatic groups (see 2m−2y), and the results showed that the yields were closely relative to electronic effect. Substrates with neutral, electron-donating, and weak electron-withdrawing groups on aryl R1 provided good yields (see 2m−2t), but lower yields were observed (see 2u and 2v) for aryl R1 with strong electron-withdrawing groups. The substrates containing O, S, and N-heterocycles also were suitable, and the corresponding products (2w−2y) were provided in reasonable yields. Introduction of two substituents R3 in phenol ring did not affect this reaction, and the corresponding products 2z and 2aa were prepared. The photocatalyst-free visible-light photoredox dearomatization of phenol derivatives containing ketoximes exhibited tolerance of various functional groups, including ester groups (see 2d, 2i, and 2v), ether groups (see 2e, 2o, and 2aa), and cyan groups (see 2h and 2u), C−F bonds (see 2f and 2r), C−Cl bonds (see 2g and 2s), C−Br bonds (see 2t), and O-, S-, and N-heterocycles (see 2j, 2k, 2w−2y). As shown in Scheme 1A, the photocatalytic dearomatization of 1a (1.0 mmol, 393 mg) was performed under the standard conditions, and 2a was prepared in 88% yield (184 mg) almost without loss of efficiency, comparing with the small-scale reaction in Table 2. The result showed that the present method was of good practicability. Furthermore, reaction of 2a with
optimize conditions including amines, solvents, and atmosphere. As shown in Table 1, 10 amines (1.1 equiv) as bases and Table 1. Optimization of Conditions for Visible-Light Photoredox Dearomatization of 1-(4′-Hydroxy-[1,1′biphenyl]-2-yl)ethanone O-(2,4-dinitrophenyl)oxime (1a) Leading to 3′-Methylspiro[cyclohexa[2,5]diene-1,1′isoindol]-4-one (2a)a
entry
amine
solvent
atmosphere
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12c 13d 14e 15f 16g 17h
Et3N DIPEA CyNMe2 BnNMe2 PhNMe2 DABCO DBU DBN DTHP MDPA − DBU DBU DBU DBU DBU DBU
MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN
Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar air Ar
38 32 25 10 trace 23 93 92 93 31 NR 92 72 93 NR NR 91 (89i)
a
Reaction conditions: irradiation of visible light with 23 W CFL and argon atmosphere, 1-(4′-hydroxy-[1,1′-biphenyl]-2-yl) ethanone O(2,4-dinitrophenyl) oxime (1a) (0.1 mmol), amine (0.11 mmol), solvent (1.0 mL), temperature (rt, ∼25 °C), time (6 h) in a sealed Schlenk tube. bConversion yield from 1a to 2a by 1H NMR determination using 1,3,5-methoxybenzene as the internal standard. c 0.1 mmol of DBU for 24 h. d0.05 mmol of DBU for 96 h. e0.15 mmol of DBU. fThe reaction was performed in darkness. gIn air. h1a (0.2 mmol), DBU (0.22 mmol), MeCN (2.0 mL). iIsolated yield. Legend: CFL, compact fluorescent light; DIPEA, N,N-diisopropylethylamine; CyNMe2, N,N-dimethylcyclohexylamine; BnNMe2, N,Ndimethylbenzylamine; DABCO, 1,4-diazabicyclo[2.2.2]octane; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DBN, 1,5-diazabicyclo[4.3.0]non-5-ene; DTHP, 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine; MDPA, 1-methyldecahydropyrimido[1,2-a]azepine; NR, no reaction.
electron donors were tested in MeCN under irradiation of visible light with 23 W compact fluorescent light (CFL) bulb and argon atmosphere at room temperature for 6 h (Table 1, entries 1−10), and three amines, 1,8-diazabicyclo[5.4.0]undec7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine (DTHP), provided higher conversion yields from 1a to 2a (Table 1, entries 7−9). No reaction was observed in the absence of amine (Table 1, B
DOI: 10.1021/acs.orglett.9b00372 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Table 2. Substrate Scope on Visible-Light Photoredox Dearomatization of Phenol Derivatives Containing Ketoximes (1)a
Scheme 1. (A) Scale-up Experiment on Visible-Light Photoredox Intramolecular Cyclization of 1a; (B) Reaction of 2a with MeMgI
Scheme 2. (A) Reaction of 1l in the Presence of Cs2CO3 (1 equiv) and DBU (0.1 equiv); (B) Treatment of 1l in the Presence of Cs2CO3 (1 equiv) without Addition of DBU; and (C) Reaction of 1l in the Presence of 2 equiv of TEMPO
a
Reaction conditions: irradiation of visible light with 23 W CFL and argon atmosphere, phenol derivative (1) (0.2 mmol), DBU (0.22 mmol for 2a−2j, 2z, and 2aa; 0.3 mmol for 2k−2y), MeCN (2.0 mL), temperature (rt, ∼25 °C) in a sealed Schlenk tube. bIsolated yield.
Grignard reagent MeMgI provided 3 in 87% yield with 2.5:1 diastereomeric ratio (dr) (Scheme 1B). To explore roles of DBU in this reaction, 1 equiv of Cs2CO3 and 0.1 equiv of DBU were used instead of 1.1 equiv of DBU in Table 1, and a similar yield was provided (Scheme 2A). However, small amount of product was observed in the absence of DBU (Scheme 2B). The results showed that DBU acted as two roles including base and electron-donor (see I in Scheme 3). When 2 equiv of 2,2,6,6-tetramethylpiperidinooxy (TEMPO) as the quenching agent was added to the reaction system of 1l, only trace amount of product 2l was observed (Scheme 2C), which indicated that the present reaction underwent a radical process. To further explore the mechanisms involved in the photocatalyst-free visible-light photoredox dearomatization of phenol derivatives containing ketoximes, several control experiments were performed as follows. (a) Solutions of 1a in MeCN were detected by ultraviolet−visible light (UV-vis) spectrometer when different amounts of DBU were added to the system, and
we found that the absorption intensity in the range of 400−450 nm obviously increased together with moderate absorption spectra appearing between 450 and 600 nm (Figure 2A). (b) We recorded absorption spectra of 1a′ in MeCN in the presence of different amounts of DBU. Similar to Figure 2A, strong absorption spectra between 400 and 450 nm were observed, but without absorption between 450 and 600 nm (Figure 2B). (c) Solution of 1a″ in MeCN in the presence of 1.5 equiv of DBU was detected, and no absorption peak was observed between 400 and 700 nm (Figure 2C). The results above showed that the absorption between 400 and 450 nm was attributed to the salt formed between phenol in 1a′ and DBU (see I in Scheme 3). (d) After O-methylation of phenol in 1a, C
DOI: 10.1021/acs.orglett.9b00372 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
N−O bond in the ketoxime unit of III affords radical anion (VI), leaving salt (V). Subsequently, treatment of the nitrogen radical with phenol anion in VI forms radical anion VII, and a SET from VII to IV affords 2 with releasing DBU. The results above showed that DBU plays two roles: base and electron donor. In summary, we have developed a novel, simple, and practical photocatalyst-free visible-light photoredox intramolecular dearomatization of phenol derivatives containing ketoximes leading to spiropyrrolines. The mechanism investigations showed that the EDA complex between 2,4-dinitrophenyl and DBU acted as the visible-light photosensitizer. The method showed some advantages including readily available starting materials, simple and mild conditions, wide substrate scope, and good tolerance of functional groups. We believe that the present method will find wide applications in the synthesis of spiropyrrolines.
Scheme 3. Proposed Mechanism on the Visible-Light Photoredox Dearomatization of Phenol Derivatives Containing Ketoximes
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00372. Experimental details and NMR data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hua Fu: 0000-0001-7250-0053 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Dr. Haifang Li (Department of Chemistry, Tsinghua University) for her great help in highresolution mass spectrometric analysis, and the National Natural Science Foundation of China (Grant No. 21772108) is acknowledged for financial support.
Figure 2. Visible absorption spectra of (A) 1a, (B) 1a′, (C) 1a″, and (D) 1a′′′, in the presence of different amounts of DBU in MeCN.
leading to 1a′′′, solutions of 1a′′′ in MeCN with different amounts of DBU were tested by using a UV-vis spectrometer, medium absorption spectra between 400 and 450 nm were observed with obvious absorption appearing between 450 and 600 nm (Figure 2D), which was attributed to an electron donor−acceptor (EDA) complex between 2,4-dinitrophenyl in 1a′′′ and DBU (the similar EDA complex between 2,4dinitrophenyl and Et3N was reported by Leonori and coworkers18c). According to the results above, we speculated that the absorption intensity between 400 and 450 nm in Figure 2A was from absorption addition of both the salt between phenol in 1a′ and DBU, and the EDA complex formed between DBU and 2,4-dinitrophenyl in 1a, while the absorption intensity between 450 and 600 nm in Figure 2A was attributed to the EDA complex between 2,4-dinitrophenyl in 1a and DBU. On the basis of these investigations above, a possible mechanism was proposed in Scheme 3 on the visible-light photoredox dearomatization of phenol derivatives containing ketoximes (1). First, reversible treatment of DBU with 1 formed I, in which DBU grabs a proton of phenol in 1 to form salt together with an EDA complex between DBU and 2,4dinitrophenyl of 1 appearing. Irradiation of I as a photosensitizer with visible light provides the excited-state II, and a single electron transfer (SET) from DBU to 2,4-dinitrophenyl in II gives radical anion (III) and radical cation (IV).18c Homolysis of
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