Letter pubs.acs.org/OrgLett
Visible Light as a Sole Requirement for Intramolecular C(sp3)−H Imination Jingjing Li, Pengxiang Zhang, Min Jiang, Haijun Yang, Yufen Zhao, and Hua Fu* Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: A novel, simple, and practical visible-light-mediated intramolecular αC(sp3)−H imination of tertiary aliphatic amines containing β-O-aryl oximes leading to Nheterocycles has been developed. The reaction was performed well at rt with tolerance of some functional groups. Importantly, the selective C−H functionalization did not require added catalyst, oxidant, additive, acid, and base; visible light was the sole requirement. Scheme 1. Proposed Mechanism for Intramolecular αC(sp3)−H Imination of Tertiary Aliphatic Amines Containing β-O-Aryl Oximes (1)
D
irect and selective formation of carbon−carbon and carbon−heteroatom bonds via carbon−hydrogen (C− H) activation has attracted much attention in organic synthesis, and this strategy can decrease the number of steps to the target products and reduce cost and waste.1 However, achieving regioselective C−H activation is a great challenge because C−H bonds are ubiquitous in organic molecules. The formation of C− N bonds is a very important transformation because many bioactive and medicinal molecules are nitrogen-containing compounds.2 In the past 10 years, there has been great progress in the formation of C−N bonds via the C−H activation strategy.3 However, transition-metal catalysts, oxidants, additives, acids, bases, and/or heating conditions are often necessary, which are unfavorable for the construction of diverse functional molecules. Recently, photopromoted methods with visible light as a renewable energy source have become a powerful strategy in organic synthetic chemistry.4 In previous visible-light photoredox research, the chemistry of C-centered radicals has been widely investigated. However, the generation and synthetic utility of N-centered radicals is an exciting and emerging field of research with many novel transformations remaining unexplored.5 The N-radical species are classically generated under the assistance of stoichiometric radical initiators, thermolysis, or UV photolysis.6 Yet, the harsh conditions greatly hinder broad applications of N-radical species in organic synthesis. Recently, several novel and interesting C(sp2)−H functionalizations via the formation of N-radical intermediates have been developed.7 In particular, Leonori et al. have represented a divergent strategy for the hydroimination and iminohydroxylation/cyclization of unactivated olefins containing O-aryl oximes.8 Obviously, the more difficult C(sp3)−H functionalization is a great challenge via N-radical intermediates. Inspired by Leonori’s research,8 we designed a novel, intramolecular α-C(sp3)−H imination of tertiary aliphatic amines containing β-O-aryl oximes. A detailed mechanism is shown in Scheme 1. Reversible binding of both O-2,4-dinitrophenyl and tertiary amine in 1 makes 1 become a photosensitizer,8 and irradiation of 1 with visible light gives the excited state (A). A single electron transfer (SET) from N of the tertiary amine9 to the aromatic ring can form B, and cleavage of the N−O bond in B produces C with an iminyl radical © 2017 American Chemical Society
and amine radical cation, freeing anion D. Intramolecular migration of a hydrogen radical from the α-C(sp3)−H of the amine radical cation in C provides cation E, and intramolecular nucleophilic attack in E in the presence of D provides the desired target product (2), leaving 2,4-dinitrophenol (F). As part of our continuing investigation on the visible light photoredox reactions,10 we here report the novel, simple, and practical visible-light-mediated intramolecular α-C(sp3)−H imination of tertiary aliphatic amines containing β-O-aryl oximes at rt. Initially, visible light photoredox intramolecular α-C(sp3)−H imination of 1-phenyl-2-(piperidin-1-yl)ethanone O-(2,4dinitrophenyl)oxime (1a) was applied as the model reaction to optimize conditions, including solvents, atmosphere, and time. As shown in Table 1, five solvents were tested under irradiation of visible light with a 23 W CFL bulb and an argon atmosphere at room temperature for 5 h (entries 1−5), and dimethyl sulfoxide (DMSO) provided the highest yield (entry 5). Surprisingly, the resulting product under the present conditions was the oxidation product (3a) of 2a, whose structure was confirmed by 1H, 13C NMR and mass spectrometry. Meanwhile, 2,4-dinitrophenol and 2-amino-4-nitrophenol were isolated in 33% and 22% yields (Scheme 2), respectively, which showed that 2,4-dinitrophenol Received: February 21, 2017 Published: March 31, 2017 1994
DOI: 10.1021/acs.orglett.7b00533 Org. Lett. 2017, 19, 1994−1997
Letter
Organic Letters Table 1. Optimization of Conditions for Visible Light Photoredox Intramolecular α-C(sp3)−H Imination of 1Phenyl-2-(piperidin-1-yl)ethanone O-(2,4Dinitrophenyl)oxime (1a)a
entry
solvent
atmos.
time (h)
yield (%)b
1 2 3 4 5 6 7 8 9 10 11
MeCN CH2Cl2 THF DMF DMSO DMSO DMSO DMSO DMSO DMSO DMSO
Ar Ar Ar Ar Ar Ar Ar air O2 Ar Ar
5 5 5 5 5 3 12 5 5 5 5
53 14 22 39 75 48 73 NR NR NRc 73d
Table 2. Substrate Scope of Visible-Light Photoredox Intramolecular α-C(sp3)−H Imination of Tertiary Aliphatic Amines Containing β-O-Aryl Oximes (1)a
a
Reaction conditions: irradiation of visible light with 23 W CFL and argon atmosphere, 1-phenyl-2-(piperidin-1-yl)ethanone O-(2,4dinitrophenyl)oxime (1a) (0.2 mmol), solvent (2.0 mL), temperature (rt, ∼25 °C), time (3−12 h) in a sealed Schlenk tube. bIsolated yield. c The reaction was carried out in the dark. dAddition of acetic acid (0.2 mmol). NR = no reaction. CFL = compact fluorescent light.
Scheme 2. Visible-Light-Mediated Reaction of 1a Leading to 3a and Byproducts
here acted as an oxidant during oxidation of 2a to 3a. Reaction times shorter than 5 h were deleterious to yield (entry 6), and extension to 12 h did not improve the yield (entry 7). The reaction did not proceed under an air (entry 8) or oxygen atmosphere (entry 9). No reaction occurred without irradiation of visible light (entry 10). Addition of a stoichiometric acetic acid did not affect this reaction (entry 11). With these optimized photoredox conditions in hand, we investigated the substrate scope for visible-light-mediated intramolecular α-C(sp3)−H imination of tertiary aliphatic amines containing β-O-aryl oximes. As shown in Table 2, we first attempted different substituted aromatic units (Ar parts) in 1 (see 1a−s), and they provided the imidazoles in moderate to good yields (see 3a−s), but the substrates containing nitro (see 1o) and ortho-site fluoro (see 1g) gave lower yields. When tertiary aliphatic amine parts in 1 were changed (see 1t−aa), we found that yields displayed more significant differences. In general, the substrates containing six-membered cycles provided higher yields than the seven-membered ring (see 1u) and acyclic substrates (see 1x−aa). 2-(3,4-Dihydro-1H-isoquinolin-2-yl)-1phenyl-ethanone O-(2,4-dinitro-phenyl)-oxime (1t) with
a
Reaction conditions: irradiation of visible light with 23 W CFL and argon atmosphere, oxime (1) (0.2 mmol), DMSO (2.0 mL), temperature (rt, ∼25 °C), time (5 or 7 h) in a sealed Schlenk tube. b Isolated yield. cAddition of triethylamine (NEt3) (0.4 mmol).
1,2,3,4-tetrahydroisoquinoline produced 3t in 85% yield. Intramolecular imination of 1w provided two products 3w (major) and 3w′ (minor). For acyclic substrates in the tertiary aliphatic 1995
DOI: 10.1021/acs.orglett.7b00533 Org. Lett. 2017, 19, 1994−1997
Letter
Organic Letters amine parts, those containing N-benzyl (see 1y and 1aa) exhibited higher reactivity than the others because of the higher activity of benzyl C−H bonds. In addition, addition of 2 equiv of triethylamine promoted the reactivity of several substrates (see 1u, 1x−z). The visible-light-mediated intramolecular α-C(sp3)− H imination of tertiary aliphatic amines containing β-O-aryl oximes displayed tolerance of various functional groups, including ethers (see 3c and 3d), C−F bonds (see 3e−3g), C−Cl bonds (see 3h and 3i), C−Br bond (see 3j), CF3 (see 3l and 3m), cyano (see 3n), nitro (see 3o), ester (see 3p), and O- or S-heterocycles (see 3r and 3s). It is well-known to all that the imidazole derivatives exhibit diverse functions such as acting as the enzyme inhibitors,11 drugs,12 dyes,13 and polymers.14 Therefore, the present method via visible-light-mediated C−H activation provides a novel, simple, and practical strategy for construction of N-heterocycles. To explore the mechanism of visible-light-mediated intramolecular α-C(sp3)−H imination of tertiary aliphatic amines containing β-O-aryl oximes, we investigated the types of the radicals produced under different conditions by electron spin resonance (ESR) using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trapping agent. A mixture of triethylamine (100 mM), Ir(ppy)3 (2 mM), and DMPO (150 mM) in DMSO (30 μL) was bubbled with Ar for 10 min, and then a small amount of the mixture was transferred to a capillary under Ar. After the capillary was irradiated with visible light for 5 min, ESR determination was performed. A signal of a nitrogen-centered DMPO radical adduct appeared (Figure 1a), which indicated that the nitrogen-centered radical of trialkylamine occurred during the process. Next, to investigate formation of the iminyl radical, a mixture of 1-phenyl-ethanone O-(2,4-dinitro-phenyl)-oxime (4) (100 mM), Ir(ppy)3 (2 mM), and DMPO (150 mM) in DMSO (30 μL) was monitored by ESR. Fortunately, a clear quartet peak signal indicative of a nitrogen-centered DMPO radical adduct was observed (Figure 1b), which demonstrated the formation of iminyl radical C-4. Subsequently, a mixture of 2-(3,4-dihydro1H-isoquinolin-2-yl)-1-phenyl-ethanone O-(2,4-dinitro-phenyl)-oxime (1t) (100 mM) and DMPO (200 mM) in DMSO (30 μL) was bubbled with Ar for 10 min, then a small amount of the mixture was transferred to a capillary under Ar, and ESR determination was carried out. No signal was observed in the dark (Figure 1c). After the capillary was irradiated with visible light for 2 min, the ESR superhyperfine spectrum exhibited signals of two nitrogen-centered DMPO radical adducts (Figure 1d), which indicated the formation of imine and tertary amine radicals C-1t. The hyperfine and spectral splitting constants of two nitrogen-centered DMPO radical adducts were obtained according to Figure 1d (AN = 1.060 mT, AH = 1.274 mT, AN′ = 0.132 mT, g = 2.006 for nitrogen-centered DMPO radical adduct from imine; AN = 1.060 mT, AH = 1.252 mT, AN′ = 0.117 mT, g = 2.006 for nitrogen-centered DMPO radical adducts from tertary amine). The results above indicate that the process for visiblelight-mediated, intramolecular α-C(sp3)−H imination in Scheme 1 is reasonable. Subsequently, oxidation of 2 by 2,4dinitrophenol gives 3, leaving 2-amino-4-nitrophenol. We extended the substrate scope to substituted 1-phenyl-2(pyrrolidin-1-yl)ethanone O-(2,4-dinitrophenyl)oximes (1ab− 1af). Unexpectedly, products 2ab−2af were obtained in good yields, but their oxidation products were not found (Scheme 3). The results showed that the products (2) containing pyrrolidinyl were more stable than those containing six-membered cycles and acyclic tertiary amines under the standard conditions.
Figure 1. Investigations of mechanism for the visible light photoredox intramolecular α-C(sp3)−H imination by ESR. ESR spectra of the mixtures under different conditions: (a) ESR spectrum for a mixture of triethylamine (100 mM), Ir(ppy)3 (2 mM), and DMPO (150 mM) in DMSO after irradiation of visible light with 23 W CFL for 5 min. (b) ESR spectrum for a mixture of 1-phenyl-ethanone O-(2,4-dinitrophenyl)-oxime (4) (100 mM), Ir(ppy)3 (2 mM), and DMPO (150 mM) in DMSO after irradiation of visible light with 23 W CFL for 1 min. (c) ESR spectrum for a mixture of 1t (100 mM) and DMPO (150 mM) in DMSO without irradiation of light. (d) ESR spectrum and superhyperfine spectrum for a mixture of 1t (100 mM) and DMPO (200 mM) in DMSO after irradiation of visible light with 23 W CFL for 2 min.
Scheme 3. Visible-Light-Mediated Intramolecular α-C(sp3)− H Imination of Substituted 1-Phenyl-2-(pyrrolidin-1yl)ethanone O-(2,4-dinitrophenyl)oximes (1ab−1af) under the Standard Conditions
In summary, we have developed a novel, simple, and practical visible light-mediated intramolecular α-C(sp3)−H imination of 1996
DOI: 10.1021/acs.orglett.7b00533 Org. Lett. 2017, 19, 1994−1997
Letter
Organic Letters tertiary aliphatic amines containing β-O-aryl oximes. The reaction only required irradiation with visible light, and no added catalyst, oxidant, additive, acid, and base were required. This is a very easy visible light photoredox process, and the simple C−H activation strategy makes this method very practical. In addition, the obtained products are imidazole derivatives that are key building blocks and useful molecules in natural products, medicinal chemistry, and materials science. We believe that the present method with good tolerance of functional groups will find wide applications in synthesis of N-heterocyclic compounds.
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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.7b00533. Experimental details, 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 in this department for her great help in analysis of high resolution mass spectrometry and the National Natural Science Foundation of China (Grant No. 21372139) for financial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b00533 Org. Lett. 2017, 19, 1994−1997
