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DOI: 10.1055/a-2161-9607
Redox-Tag-Guided Radical Cation Diels–Alder Reactions: Use of Enol Ethers as Dienophiles
This work was supported in part by JSPS KAKENHI Grants Nos. 22K05450 (to Y.O.), 23KJ0870 (to H.M.), and No. 21J12556 (to K.N.), and by the TEPCO Memorial Foundation (to Y.O.).
Abstract
Although radical cation Diels–Alder reactions enable the formation of cyclohexene ring systems between electronically mismatched (both electron-rich) dienes and dienophiles, which is otherwise difficult or impossible to achieve under thermal conditions, the substrate scope has been limited. Herein, we disclose that a radical cation Diels–Alder reaction using an enol ether as an electron-rich (and therefore oxidizable) dienophile is possible through a rationally designed redox tag strategy. Electrochemical and TiO2 photochemical approaches are effective in driving the reaction, where both intermolecular and intramolecular electron transfers are the key.
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Key words
redox tag - radical cation - Diels–Alder reaction - enol ether - electrochemistry - photochemistryFor more than half a century, the Diels–Alder reaction has been the first option for constructing six-membered ring systems in the field of synthetic organic chemistry. Cyclohexene rings and heterocycles can be assembled by using dienes and dienophiles as starting materials. The reactivity between the diene and dienophile is controlled by electronic matching, in which an electron-rich (nucleophilic) diene effectively reacts with an electron-deficient (electrophilic) dienophile. Conversely, electron-deficient (electrophilic) dienes and electron-rich (nucleophilic) dienophiles can also be a productive combination in constructing six-membered ring systems; this is referred to as an inverse-electron-demand Diels–Alder reaction. The radical cation Diels–Alder reaction is a third class that can access six-membered ring systems by the cycloaddition between pairs of electron-rich (nucleophilic) dienes and dienophiles.[1] The key to realizing such an electronically mismatched Diels–Alder reaction is the use of single-electron oxidation, where a dienophile (or diene) radical cation is generated and is effectively trapped by a neutral diene (or dienophile).
In 2011, the pioneering work by Bauld[2] was revisited by Yoon[3] in the realm of photoredox chemistry[4] by using a ruthenium complex as a sensitizer. Since then, the reaction of trans-anethole as a dienophile has been the benchmark for the radical cation Diels–Alder cycloaddition, which has been studied with various single-electron-oxidation methodologies, including the use of transition-metal complexes or organic dyes as sensitizers (Scheme [1]).[5] Although the radical cation Diels–Alder reaction permits the formation of cyclohexene ring systems between electronically mismatched (both electron-rich) dienes and dienophiles, which is otherwise difficult or impossible under thermal conditions, the substrate scope has been limited. To extend the synthetic utility, it is necessary to have a wide variety of dienophile (diene) radical cation precursors available for the reaction.
We have been developing electrochemical[6] and TiO2 photochemical[7] radical cation cycloadditions in lithium perchlorate (LiClO4)/nitromethane (CH3NO2) solution,[8] where carbon–carbon bond formations are facilitated. Electron-rich alkenes, such as styrene derivatives (including trans-anethole) or aryl vinyl ethers, serve as the radical cation precursors for the cycloadditions. To control the reactivity of such radical cations, the tethering of an electron-rich aryl group, a structural motif we call a redox tag, is the key.[9] The redox tag can thus function as both an electron donor and an electron acceptor during carbon–carbon bond formations. For example, an enol ether tethered to a methoxyphenyl group can serve as the radical cation precursor for a [2+2] cycloaddition (Scheme [2a]). We thus questioned whether it could also serve as the radical cation precursor for the Diels–Alder reaction (Scheme [2b]). In both scenarios, single-electron oxidation is expected to initiate the reaction, and reduction completes the net-redox-neutral cycloaddition. Described herein is the radical cation Diels–Alder reaction rationally designed by a redox tag strategy.
a All reactions were carried out on a 0.20 mmol scale of the enol ether (2) with 2,3-dimethylbuta-1,3-diene (6, 2 equiv) in 1 M LiClO4/CH3NO2 under Ar.
b Yields were determined by GC/MS analysis.
c 2,3-Dimethyl-1,3-butadiene (6; 5.0 equiv) was used.
The present work began with the synthesis of a series of enol ethers tethered to a (methoxy)phenyl group 1–5 (Scheme [3]). In general, a Weinreb amide (prepared from the corresponding carboxylic acid) was reduced by lithium aluminum hydride to give an amide that was then converted into an enol ether by a Wittig reaction with (methoxymethyl)(triphenyl)phosphonium chloride. Enol ether 2 was used as a model substrate in combination with 2,3-dimethylbuta-1,3-diene (6) to test our working hypothesis (Table [1]). To our delight, when the electrochemical reaction was carried out in LiClO4/CH3NO2 solution using carbon felt electrodes under potentiostatic conditions in an undivided cell, the desired cycloadduct 7 was obtained in 71% yield as a cis/trans mixture (Table [1], entry 1).
It should be noted that only a substoichiometric amount of electricity was required for full conversion, indicating the involvement of a radical cation chain pathway (Scheme [4]). A cyclic-voltammetry study suggested that the oxidation potential of the cycloadduct (7) was slightly higher than that of the enol ether 2, enabling the radical cation chain pathway to operate (Supporting Information; Figure S1). Based on the electrochemical analysis, an electron transfer/chemical reaction/backward electron transfer (EC-backward-E) process might also be possible for the reaction to be completed with a substoichiometric amount of electricity, wherein both a single-electron oxidation and reduction can occur at the anode (Figure [1]).[10] The oxidative current derived from the enol ether 2 decreased and the peak potential shifted negatively when measured in the presence of 2,3-dimethylbuta-1,3-diene (6). Furthermore, a significantly large current was observed at around 1.4 V vs Ag/AgCl and higher potentials; this can be assigned to the oxidative current derived from the cycloadduct 7, continuously formed through the radical cation chain pathway.
In control studies, the applied potentials were found to be critical for the reaction (Table [1], entries 2 and 3), whereas galvanostatic conditions were somewhat less effective (entries 4 and 5). The yield of the cycloadduct 7 decreased when the reaction was carried out with a higher concentration of 2,3-dimethylbuta-1,3-diene (6) (entry 6), presumably due to its polymerization. In addition, TiO2 photochemical conditions were found to be productive for the reaction, and full conversion was achieved within an hour (entry 7). Because relatively mild potentiostatic conditions, and therefore low-current situations, were preferred in the electrochemical reaction, it took more than several hours to achieve completion. In this sense, TiO2 photochemistry would be more beneficial than electrochemistry to drive the reaction.
In most electrochemical radical cation cycloadditions reported by us so far, an electron-rich aryl group, the redox tag, is crucial for the reactions as an intramolecular electron donor, whereas a nonsubstituted phenyl group cannot be an alternative structural motif. However, the TiO2 photochemical reaction might potentially overcome this limitation, since excited electrons generated at a conduction band upon irradiation can function as donors.[11] When the enol ether 5 was used instead under the electrochemical conditions, the desired cycloadduct 8 was not obtained (Scheme [5]). This result suggests that an intramolecular electron transfer from the tethered methoxyphenyl group is the key to forming the cyclohexene ring system. On the other hand, TiO2 photochemical conditions were found to be productive in affording cycloadduct 8 in 55% yield as a cis/trans mixture. Although mechanistic details remain unclear, TiO2 photochemistry can expand the substrate scope of the reaction.
With these observations in hand, the enol ethers 1, 3, and 4 with varying alkylene linker lengths were subjected to the electrochemical conditions to further investigate mechanistic aspects of the reaction, with a particular focus on the efficacy of the intramolecular electron transfer (Scheme [6]). An elongation effect of the linker is reasonably understood. The yield of the cycloadduct 10 decreased when the linker was elongated from ethylene (2) to propylene (3), and only a trace of product 11 was obtained with butylene (4). The electron donor (the methoxyphenyl group) and acceptor (the cyclohexene radical cation) should be in close proximity to shorten the distance and/or time for the electron to travel. On the other hand, the shortening effect of the linker among the substrates tested is somewhat difficult to interpret. Although the donor and acceptor were in the closest proximity to promote efficient intramolecular electron transfer, the yield of the cycloadduct 9 was significantly reduced. Although we are not able to rationalize this observation at the moment, the allylic and benzylic carbons might have a negative impact on the reaction.
In conclusion, we have demonstrated that the radical cation Diels–Alder reaction using an enol ether as a dienophile can be rationally designed by a redox tag strategy, extending the synthetic utility of this class. Both electrochemical and TiO2 photochemical conditions were effective in driving the reaction. The substrate scope can be expanded when TiO2 photochemical conditions are employed, whereas electrochemical approaches have enabled us to understand various mechanistic aspects, including the radical cation chain pathway and the EC-backward-E process. An intramolecular electron transfer is found to be the key to the success of the reaction, which can be regulated by the redox tag. To have a variety of radical cation precursors available for the reaction, further experimental and theoretical studies are underway in our laboratory.
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Conflict of Interest
The authors declare no conflict of interest.
Supporting Information
- Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2161-9607.
- Supporting Information
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References and Notes
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- 1b Ohmura S, Isogai R, Ishihara K. Asian J. Org. Chem. 2021; 10: 2534
- 1c Horibe T, Ohmura S, Katagiri K, Ishihara K. Asian J. Org. Chem. 2020; 9: 395
- 1d Horibe T, Ishihara K. Chem. Lett. 2020; 49: 107
- 1e Horibe T, Ohmura S, Ishihara K. J. Am. Chem. Soc. 2019; 141: 1877
- 1f Tanaka K, Kishimoto M, Tanaka Y, Kamiyama Y, Asada Y, Sukekawa M, Ohtsuka N, Suzuki T, Momiyama N, Honda K, Hoshino Y. J. Org. Chem. 2022; 87: 3319
- 1g Morse PD, Nguyen TM, Cruz CL, Nicewicz DA. Tetrahedron 2018; 74: 3266
- 1h Tan JS. J, Hirvonen V, Paton RS. Org. Lett. 2018; 20: 2821
- 1i Yu Y, Fu Y, Zhong F. Green Chem. 2018; 20: 1743
- 1j Lim HN, Parker KA. J. Org. Chem. 2014; 79: 919
- 1k Moore JC, Davies ES, Walsh DA, Sharma P, Moses JE. Chem. Commun. 2014; 50: 12523
- 2a Bellville DJ, Wirth DW, Bauld NL. J. Am. Chem. Soc. 1981; 103: 718
- 2b Reynolds DW, Bauld NL. Tetrahedron 1986; 42: 6189
- 2c Bauld NL, Bellville DJ, Harirchian B, Lorenz KT, Pabon RA. Jr, Reynolds DW, Wirth DD, Chiou HS, Marsh BK. Acc. Chem. Res. 1987; 20: 371
- 2d Bauld NL. Tetrahedron 1989; 45: 5307
- 3a Lin S, Ischay MA, Fry CG, Yoon TP. J. Am. Chem. Soc. 2011; 133: 19350
- 3b Lin S, Padilla CE, Ischay MA, Yoon TP. Tetrahedron Lett. 2012; 53: 3073
- 3c Lin S, Lies SD, Gravatt CS, Yoon TP. Org. Lett. 2017; 19: 368
- 4a Silvi M, Melchiorre P. Nature 2018; 554: 41
- 4b Twilton J, Le C, Zhang P, Shaw MH, Evans RW, MacMillan DW. C. Nat. Rev. Chem. 2017; 1: 0052
- 4c Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
- 4d Skubi KL, Blum TR, Yoon TP. Chem. Rev. 2016; 116: 10035
- 4e Kärkäs MD, Porco JA. Jr, Stephenson CR. J. Chem. Rev. 2016; 116: 9683
- 4f Prier CK, Rankic DA, MacMillan DW. C. Chem. Rev. 2013; 113: 5322
- 5a Stevenson SM, Shores MP, Ferreira EM. Angew. Chem. Int. Ed. 2015; 54: 6506
- 5b Higgins RF, Fatur SM, Shepard SG, Stevenson SM, Boston DJ, Ferreira EM, Damrauer NH, Rappé AK, Shores MP. J. Am. Chem. Soc. 2016; 138: 5451
- 5c Alpers D, Gallhof M, Stark CB. W, Brasholz M. Chem. Commun. 2016; 52: 1025
- 5d Zhao Y, Antonietti M. Angew. Chem. Int. Ed. 2017; 56: 9336
- 5e Stevenson SM, Higgins RF, Shores MP, Ferreira EM. Chem. Sci. 2017; 8: 654
- 5f Yang Y, Liu Q, Zhang L, Yu H, Dang Z. Organometallics 2017; 36: 687
- 5g Shin JH, Seong EY, Mun HJ, Jang YJ, Kang EJ. Org. Lett. 2018; 20: 5872
- 5h Tanaka K, Kishimoto M, Sukekawa M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 3361
- 5i Farney EP, Chapman SJ, Swords WB, Torelli MD, Hamers RJ, Yoon TP. J. Am. Chem. Soc. 2019; 141: 6385
- 5j Huber N, Li R, Ferguson CT. J, Gehrig DW, Ramanan C, Blom PW. M, Landfester K, Zhang KA. I. Catal. Sci. Technol. 2020; 10: 2092
- 5k Tang M, Cameron L, Poland EM, Yu L.-J, Moggach SA, Fuller RO, Huang H, Sun J, Thickett SC, Massi M, Coote ML, Ho CC, Bissember AC. Inorg. Chem. 2022; 61: 1888
- 6a Okada Y. Electrochemistry 2020; 88: 497
- 6b Okada Y, Chiba K. Chem. Rev. 2018; 118: 4592
- 7a Adachi S, Horiguchi G, Kamiya H, Okada Y. Eur. J. Org. Chem. 2022; e202201207
- 7b Hashimoto Y, Horiguchi G, Kamiya H, Okada Y. Chem. Eur. J. 2022; 28: e202202018
- 7c Nakayama K, Kamiya H, Okada Y. Beilstein J. Org. Chem. 2022; 18: 1100
- 7d Maeta N, Kamiya H, Okada Y. J. Org. Chem. 2020; 85: 6551
- 7e Maeta N, Kamiya H, Okada Y. Org. Lett. 2019; 21: 8519
- 7f Nakayama K, Maeta N, Horiguchi G, Kamiya H, Okada Y. Org. Lett. 2019; 21: 2246
- 7g Okada Y, Maeta N, Nakayama K, Kamiya H. J. Org. Chem. 2018; 83: 4948
- 8a Shida N, Imada Y, Okada Y, Chiba K. Eur. J. Org. Chem. 2020; 570
- 8b Imada Y, Yamaguchi Y, Shida N, Okada Y, Chiba K. Chem. Commun. 2017; 53: 3960
- 9a Okada Y. J. Org. Chem. 2019; 84: 1882
- 9b Okada Y, Yamaguchi Y, Ozaki A, Chiba K. Chem. Sci. 2016; 7: 6387
- 9c Okada Y, Nishimoto A, Akaba R, Chiba K. J. Org. Chem. 2011; 76: 3470
- 10a Feldberg SW, Jeftic L. J. Phys. Chem. 1972; 76: 2439
- 10b Nelsen SF, Akaba R. J. Am. Chem. Soc. 1981; 103: 2096
- 10c Nelsen SF, Kapp DL, Akaba R, Evans DH. J. Am. Chem. Soc. 1986; 108: 6863
- 10d Kabe Y, Takata T, Ueno K, Ando W. J. Am. Chem. Soc. 1984; 106: 8174
- 10e Tsuchiya M, Akaba R, Aihara S, Sakuragi H, Tokumaru K. Chem. Lett. 1986; 10: 1727
- 10f Okada Y, Akaba R, Chiba K. Tetrahedron Lett. 2009; 50: 5413
- 10g Nakayama K, Kamiya H, Okada Y. J. Electrochem. Soc. 2020; 167: 155518
- 10h Francke R, Little RD. ChemElectroChem 2019; 6: 4373
- 10i Roesel AF, Ugandi M, Huyen NT. T, Májek M, Broese T, Roemelt M, Francke R. J. Org. Chem. 2020; 85: 8029
- 11 Okada Y. Chem. Rec. 2021; 21: 2223
- 12 Electrochemical Radical Cation Diels–Alder Reactions; General Procedure The appropriate enol ether (0.20 mmol) and 2,3-dimethylbuta-1,3-diene (45.0 μL, 0.40 mmol) were added with stirring to a solution of 1.0 M solution of LiClO4 in CH3NO2 (20 mL) at r.t. The mixture was then electrolyzed with stirring at 1.2 V vs Ag/AgCl using carbon felt electrodes (10 × 10 mm) in an undivided cell under Ar. The solution was then diluted with H2O and extracted with EtOAc. The combined organic layers were dried (Na2SO4), filtered, and concentrated in vacuo. The reported yields were determined by GC/MS analysis. The residue was purified by column chromatography [silica gel, 0.40 mmol scale (2 batches), hexane–EtOAc (24:1)]. All the reactions gave the corresponding cycloadducts as cis/trans mixtures. 1-Methoxy-4-[2-(6-methoxy-3,4-dimethylcyclohex-3-en-1-yl)ethyl]benzene (7) Colorless oil; yield: 27.0 mg (0.0985 mmol, 49%). 1H NMR (600 MHz, CDCl3): δ = (major) 7.11 (d, J = 7.2 Hz, 2 H), 6.83–6.81 (m, 2 H), 3.78 (s, 3 H), 3.42 (dt, J = 4.6, 2.3 Hz, 1 H), 3.31 (s, 3 H), 2.68–2.48 (m, 2 H), 2.31–2.20 (m, 1 H), 2.13–2.04 (m, 1 H), 1.96–1.91 (m, 2 H), 1.77–1.74 (m, 2 H), 1.61–1.36 (m, 7 H); (minor): 7.11 (d, J = 7.2 Hz, 2 H), 6.83–6.81 (m, 2 H), 3.78 (s, 3 H), 3.35 (s, 3 H), 3.15 (dt, J = 7.9, 5.5 Hz, 1 H), 2.68–2.48 (m, 2 H), 2.31–2.20 (m, 1 H), 2.13–2.04 (m, 1 H), 1.96–1.91 (m, 2 H), 1.77–1.74 (m, 2 H), 1.61–1.36 (m, 7 H). 13C{1H} NMR (150 MHz, CDCl3): δ = (major) 157.8, 135.1, 129.4, 124.9, 122.4, 113.8, 78.4, 56.5, 55.4, 36.6, 36.1, 35.1, 34.5, 32.8, 19.1, 18.9; (minor) 157.8, 135.2, 129.4, 124.7, 122.7, 113.9, 80.3, 57.0, 55.4, 38.0, 36.0, 34.2, 32.5, 32.1, 19.2, 19.1. HRMS (DART): m/z [M + H]+ calcd for C18H27O2: 275.2006; found: 275.2005.
- 13 TiO2 Photochemical Radical Cation Diels–Alder Reactions; General Procedure TiO2 nanoparticles (100 mg) were added to a solution of the appropriate enol ether (0.20 mmol) and 2,3-dimethylbuta-1,3-diene (45.0 μL, 0.40 mmol) in a 1.0 M solution of LiClO4 in CH3NO2 (4.0 mL). The mixture was stirred at r.t. 5 cm away from of a 15 W UV lamp (λ = 365 nm) under air. The solution was then diluted with H2O and extracted with EtOAc. The combined organic layers were dried (Na2SO4), filtered, and concentrated in vacuo. The reported yields were determined by GC/MS analysis. The residue was purified by column chromatography [silica gel, 0.40 mmol scale (2 batches), hexane–EtOAc (24:1)]. All the reactions gave the corresponding cycloadducts as cis/trans mixtures.
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Corresponding Author
Publication History
Received: 14 July 2023
Accepted after revision: 29 August 2023
Accepted Manuscript online:
29 August 2023
Article published online:
23 October 2023
© 2023. Thieme. All rights reserved
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References and Notes
- 1a Ohmura S, Katagiri K, Kato H, Horibe T, Miyakawa S, Hasegawa J, Ishihara K. J. Am. Chem. Soc. 2023; 145: 15054
- 1b Ohmura S, Isogai R, Ishihara K. Asian J. Org. Chem. 2021; 10: 2534
- 1c Horibe T, Ohmura S, Katagiri K, Ishihara K. Asian J. Org. Chem. 2020; 9: 395
- 1d Horibe T, Ishihara K. Chem. Lett. 2020; 49: 107
- 1e Horibe T, Ohmura S, Ishihara K. J. Am. Chem. Soc. 2019; 141: 1877
- 1f Tanaka K, Kishimoto M, Tanaka Y, Kamiyama Y, Asada Y, Sukekawa M, Ohtsuka N, Suzuki T, Momiyama N, Honda K, Hoshino Y. J. Org. Chem. 2022; 87: 3319
- 1g Morse PD, Nguyen TM, Cruz CL, Nicewicz DA. Tetrahedron 2018; 74: 3266
- 1h Tan JS. J, Hirvonen V, Paton RS. Org. Lett. 2018; 20: 2821
- 1i Yu Y, Fu Y, Zhong F. Green Chem. 2018; 20: 1743
- 1j Lim HN, Parker KA. J. Org. Chem. 2014; 79: 919
- 1k Moore JC, Davies ES, Walsh DA, Sharma P, Moses JE. Chem. Commun. 2014; 50: 12523
- 2a Bellville DJ, Wirth DW, Bauld NL. J. Am. Chem. Soc. 1981; 103: 718
- 2b Reynolds DW, Bauld NL. Tetrahedron 1986; 42: 6189
- 2c Bauld NL, Bellville DJ, Harirchian B, Lorenz KT, Pabon RA. Jr, Reynolds DW, Wirth DD, Chiou HS, Marsh BK. Acc. Chem. Res. 1987; 20: 371
- 2d Bauld NL. Tetrahedron 1989; 45: 5307
- 3a Lin S, Ischay MA, Fry CG, Yoon TP. J. Am. Chem. Soc. 2011; 133: 19350
- 3b Lin S, Padilla CE, Ischay MA, Yoon TP. Tetrahedron Lett. 2012; 53: 3073
- 3c Lin S, Lies SD, Gravatt CS, Yoon TP. Org. Lett. 2017; 19: 368
- 4a Silvi M, Melchiorre P. Nature 2018; 554: 41
- 4b Twilton J, Le C, Zhang P, Shaw MH, Evans RW, MacMillan DW. C. Nat. Rev. Chem. 2017; 1: 0052
- 4c Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
- 4d Skubi KL, Blum TR, Yoon TP. Chem. Rev. 2016; 116: 10035
- 4e Kärkäs MD, Porco JA. Jr, Stephenson CR. J. Chem. Rev. 2016; 116: 9683
- 4f Prier CK, Rankic DA, MacMillan DW. C. Chem. Rev. 2013; 113: 5322
- 5a Stevenson SM, Shores MP, Ferreira EM. Angew. Chem. Int. Ed. 2015; 54: 6506
- 5b Higgins RF, Fatur SM, Shepard SG, Stevenson SM, Boston DJ, Ferreira EM, Damrauer NH, Rappé AK, Shores MP. J. Am. Chem. Soc. 2016; 138: 5451
- 5c Alpers D, Gallhof M, Stark CB. W, Brasholz M. Chem. Commun. 2016; 52: 1025
- 5d Zhao Y, Antonietti M. Angew. Chem. Int. Ed. 2017; 56: 9336
- 5e Stevenson SM, Higgins RF, Shores MP, Ferreira EM. Chem. Sci. 2017; 8: 654
- 5f Yang Y, Liu Q, Zhang L, Yu H, Dang Z. Organometallics 2017; 36: 687
- 5g Shin JH, Seong EY, Mun HJ, Jang YJ, Kang EJ. Org. Lett. 2018; 20: 5872
- 5h Tanaka K, Kishimoto M, Sukekawa M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 3361
- 5i Farney EP, Chapman SJ, Swords WB, Torelli MD, Hamers RJ, Yoon TP. J. Am. Chem. Soc. 2019; 141: 6385
- 5j Huber N, Li R, Ferguson CT. J, Gehrig DW, Ramanan C, Blom PW. M, Landfester K, Zhang KA. I. Catal. Sci. Technol. 2020; 10: 2092
- 5k Tang M, Cameron L, Poland EM, Yu L.-J, Moggach SA, Fuller RO, Huang H, Sun J, Thickett SC, Massi M, Coote ML, Ho CC, Bissember AC. Inorg. Chem. 2022; 61: 1888
- 6a Okada Y. Electrochemistry 2020; 88: 497
- 6b Okada Y, Chiba K. Chem. Rev. 2018; 118: 4592
- 7a Adachi S, Horiguchi G, Kamiya H, Okada Y. Eur. J. Org. Chem. 2022; e202201207
- 7b Hashimoto Y, Horiguchi G, Kamiya H, Okada Y. Chem. Eur. J. 2022; 28: e202202018
- 7c Nakayama K, Kamiya H, Okada Y. Beilstein J. Org. Chem. 2022; 18: 1100
- 7d Maeta N, Kamiya H, Okada Y. J. Org. Chem. 2020; 85: 6551
- 7e Maeta N, Kamiya H, Okada Y. Org. Lett. 2019; 21: 8519
- 7f Nakayama K, Maeta N, Horiguchi G, Kamiya H, Okada Y. Org. Lett. 2019; 21: 2246
- 7g Okada Y, Maeta N, Nakayama K, Kamiya H. J. Org. Chem. 2018; 83: 4948
- 8a Shida N, Imada Y, Okada Y, Chiba K. Eur. J. Org. Chem. 2020; 570
- 8b Imada Y, Yamaguchi Y, Shida N, Okada Y, Chiba K. Chem. Commun. 2017; 53: 3960
- 9a Okada Y. J. Org. Chem. 2019; 84: 1882
- 9b Okada Y, Yamaguchi Y, Ozaki A, Chiba K. Chem. Sci. 2016; 7: 6387
- 9c Okada Y, Nishimoto A, Akaba R, Chiba K. J. Org. Chem. 2011; 76: 3470
- 10a Feldberg SW, Jeftic L. J. Phys. Chem. 1972; 76: 2439
- 10b Nelsen SF, Akaba R. J. Am. Chem. Soc. 1981; 103: 2096
- 10c Nelsen SF, Kapp DL, Akaba R, Evans DH. J. Am. Chem. Soc. 1986; 108: 6863
- 10d Kabe Y, Takata T, Ueno K, Ando W. J. Am. Chem. Soc. 1984; 106: 8174
- 10e Tsuchiya M, Akaba R, Aihara S, Sakuragi H, Tokumaru K. Chem. Lett. 1986; 10: 1727
- 10f Okada Y, Akaba R, Chiba K. Tetrahedron Lett. 2009; 50: 5413
- 10g Nakayama K, Kamiya H, Okada Y. J. Electrochem. Soc. 2020; 167: 155518
- 10h Francke R, Little RD. ChemElectroChem 2019; 6: 4373
- 10i Roesel AF, Ugandi M, Huyen NT. T, Májek M, Broese T, Roemelt M, Francke R. J. Org. Chem. 2020; 85: 8029
- 11 Okada Y. Chem. Rec. 2021; 21: 2223
- 12 Electrochemical Radical Cation Diels–Alder Reactions; General Procedure The appropriate enol ether (0.20 mmol) and 2,3-dimethylbuta-1,3-diene (45.0 μL, 0.40 mmol) were added with stirring to a solution of 1.0 M solution of LiClO4 in CH3NO2 (20 mL) at r.t. The mixture was then electrolyzed with stirring at 1.2 V vs Ag/AgCl using carbon felt electrodes (10 × 10 mm) in an undivided cell under Ar. The solution was then diluted with H2O and extracted with EtOAc. The combined organic layers were dried (Na2SO4), filtered, and concentrated in vacuo. The reported yields were determined by GC/MS analysis. The residue was purified by column chromatography [silica gel, 0.40 mmol scale (2 batches), hexane–EtOAc (24:1)]. All the reactions gave the corresponding cycloadducts as cis/trans mixtures. 1-Methoxy-4-[2-(6-methoxy-3,4-dimethylcyclohex-3-en-1-yl)ethyl]benzene (7) Colorless oil; yield: 27.0 mg (0.0985 mmol, 49%). 1H NMR (600 MHz, CDCl3): δ = (major) 7.11 (d, J = 7.2 Hz, 2 H), 6.83–6.81 (m, 2 H), 3.78 (s, 3 H), 3.42 (dt, J = 4.6, 2.3 Hz, 1 H), 3.31 (s, 3 H), 2.68–2.48 (m, 2 H), 2.31–2.20 (m, 1 H), 2.13–2.04 (m, 1 H), 1.96–1.91 (m, 2 H), 1.77–1.74 (m, 2 H), 1.61–1.36 (m, 7 H); (minor): 7.11 (d, J = 7.2 Hz, 2 H), 6.83–6.81 (m, 2 H), 3.78 (s, 3 H), 3.35 (s, 3 H), 3.15 (dt, J = 7.9, 5.5 Hz, 1 H), 2.68–2.48 (m, 2 H), 2.31–2.20 (m, 1 H), 2.13–2.04 (m, 1 H), 1.96–1.91 (m, 2 H), 1.77–1.74 (m, 2 H), 1.61–1.36 (m, 7 H). 13C{1H} NMR (150 MHz, CDCl3): δ = (major) 157.8, 135.1, 129.4, 124.9, 122.4, 113.8, 78.4, 56.5, 55.4, 36.6, 36.1, 35.1, 34.5, 32.8, 19.1, 18.9; (minor) 157.8, 135.2, 129.4, 124.7, 122.7, 113.9, 80.3, 57.0, 55.4, 38.0, 36.0, 34.2, 32.5, 32.1, 19.2, 19.1. HRMS (DART): m/z [M + H]+ calcd for C18H27O2: 275.2006; found: 275.2005.
- 13 TiO2 Photochemical Radical Cation Diels–Alder Reactions; General Procedure TiO2 nanoparticles (100 mg) were added to a solution of the appropriate enol ether (0.20 mmol) and 2,3-dimethylbuta-1,3-diene (45.0 μL, 0.40 mmol) in a 1.0 M solution of LiClO4 in CH3NO2 (4.0 mL). The mixture was stirred at r.t. 5 cm away from of a 15 W UV lamp (λ = 365 nm) under air. The solution was then diluted with H2O and extracted with EtOAc. The combined organic layers were dried (Na2SO4), filtered, and concentrated in vacuo. The reported yields were determined by GC/MS analysis. The residue was purified by column chromatography [silica gel, 0.40 mmol scale (2 batches), hexane–EtOAc (24:1)]. All the reactions gave the corresponding cycloadducts as cis/trans mixtures.
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