Stepwise radical cation Diels–Alder reaction via multiple pathways

Shimizu, Ryo; Okada, Yohei; Chiba, Kazuhiro

doi:10.3762/bjoc.14.59

Cited by 16 publications

(8 citation statements)

References 34 publications

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“…Radical cation ( 7 ) can then be cyclized via single electron reduction to form both six‐ and four‐membered rings. Although mechanistic aspects remain unclear, single electron oxidation may trigger the rearrangement of the vinylcyclobutane ( 6 ) into the cycloadduct ( 5 ), which accords well with our previous observations [12] …”

Section: Resultssupporting

confidence: 91%

Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

et al. 2022

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Radical cation cycloadditions have long been known as unique reactions and yet are intensively studied chemical transformations in the field of modern synthetic organic chemistry. The key to developing new reactions of this class is the generation and control of highly reactive radical cation intermediates. Herein, we report TiO2 photochemical [2+2] and [4+2] cycloadditions using aryl vinyl ethers as radical cation precursors. The constructions of various four‐ and six‐membered rings are made possible by these radical cation cycloadditions. Although detailed mechanistic aspects remain unclear, 2,3‐dimethyl‐1,3‐butadiene (3) is found to be more radicalophilic than 2‐ethyl‐1‐butene (2) and effectively traps the radical cation of the aryl vinyl ethers to construct six‐membered rings.

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Section: Resultssupporting

confidence: 91%

Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

et al. 2022

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“…Recently, for aryl vinyl ethers the initiating electrochemical oxidation was proposed to occur at these dienophiles, while the reaction again proceeds through an intermediate distonic radical cation. 67 As can unequivocally be seen from the theoretical calculations in the present work, without making an a priori assumption about the role of the radical cation species, 2 •+ generated in the initial electrochemical electron transfer must be identified as the dienophile in the reaction with the diene 2. This preference has been termed "role selectivity" before 68 and contrasts the one deduced by Mlcoch and Steckhan from experiments previously 62 and formulated by Saettel et al in their computational work.…”

Section: ■ Results and Discussionmentioning

confidence: 71%

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

et al. 2023

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6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product’s electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation–cycloaddition–reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

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“…Meanwhile, many studies have shown that formaldehyde was a key intermediate to form the early (or the first) ethene or propene in the induction period of MTO. , In addition, it was found that formaldehyde generated from methanol dehydrogenation could also react with propene and butene, viz., through the Prins reaction, forming dienes, such as butadiene and pentadiene. − Inspired by these observations, we propose that MCHs with a six-membered ring skeleton can be constructed through the (4 + 2) cycloaddition between an electron-rich conjugated diene like 1,3-butadiene and an electron-poor dienophile like propene; this is a typical Diels–Alder (D–A) reaction. , …”

Section: Resultsmentioning

confidence: 98%

“…37−39 Inspired by these observations, we propose that MCHs with a six-membered ring skeleton can be constructed through the (4 + 2) cycloaddition between an electron-rich conjugated diene like 1,3-butadiene and an electron-poor dienophile like propene; this is a typical Diels−Alder (D−A) reaction. 40,41 To prove the presence of the D−A reaction, as shown in Figure 3, the coreaction of 1,3-butadiene and propene was conducted at 325−375 °C for 3 min over various H-ZSM-5 zeolites with different Si/Al ratios, compared with a blank test over the quartz sand. For the blank test over quartz sand (Figure 3A), only two signals are detected in the gas effluents by GC−MS at a retention time of 9.4 and 14.8 min, corresponding to 4-methylcyclohex-1-ene and 4-vinylcyclohex-1-ene, respectively, with the six-membered ring skeleton.…”

Section: Formation Of Mchs In the Initial Period Of Mtomentioning

confidence: 99%

Formation and Evolution of Methylcyclohexene in the Initial Period of Methanol to Olefins over H-ZSM-5

Fan

Wang

et al. 2022

ACS Catal.

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Identification of the reaction intermediates and probing into their evolution in the conversion of methanol to light olefins (MTO) are rather challenging but meaningful in unraveling the hydrocarbon pool (HCP) mechanism as well as in exploring more efficient catalysts in MTO. Hence, we would like to report a finding made in this regard that methylcyclohexene (MCH) is a crucial reaction intermediate in the initial period of MTO over H-ZSM-5 through various measures, including transient pulse experiment, 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR), gas chromatography–mass spectrometry (GC–MS), and density-functional theory (DFT) calculation. The early alkenes (mostly ethene and propene) produced in the induction stage can rapidly construct the long-chain alkenes through oligomerization and create dienes through the Prins reaction with formaldehyde from methanol dehydrogenation. After that, MCH is facilely formed via the Diels–Alder (D–A) reaction between dienes and monoenes. Compared with the oligomerization and cyclization of alkenes, the D–A reaction shows higher activity with a lower energy barrier in building larger molecule products. MCH is highly reactive and can be quickly transformed either into methylcyclopentene (MCP) via ring contraction or into methylbenzene (MB) via hydride transfer and deprotonations; the formation of MCP may take the priority over that of MB at a low temperature, as the former reaction needs a lower energy barrier. Accordingly, MCH acts as a bridge connecting MCP and MB and plays a vital role in the establishment of initial HCP in MTO. These findings should be helpful for an in-depth understanding of the MTO reaction mechanism and then benefit further research into MTO.

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Stepwise radical cation Diels–Alder reaction via multiple pathways

Cited by 16 publications

References 34 publications

Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

Formation and Evolution of Methylcyclohexene in the Initial Period of Methanol to Olefins over H-ZSM-5

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