A theoretical study on the stereoconvergency of the intramolecular radical cation [2+2] cycloadditions of bis(styrenes)

Guo, Chenchen; Cui, Lijie; Chen, Bo‐Zhen; Yuan, Jianhua; Zhang, Tian

doi:10.1039/c2ra21236f

“…Following this study, Metzger and co-workers in 2008 detected a distonic (where charge and radical sites are separated) radical cationic intermediate via extractive electrospray ionisation mass spectrometry, lending direct support to the stepwise mechanism. 59 Further computational mechanistic investigations looking into intramolecular cyclobutanation of (bis)styrene 60 and intermolecular cyclobutanation of unsubstituted styrene 61 via radical cation catalysis further confirmed the stepwise nature of such mechanism.…”

Section: Radical Cations Constitute An Important Class Of Reactive Inmentioning

confidence: 83%

Stereoretention in Styrene Heterodimerisation Promoted by One-Electron Oxidants

Zhang

¹

,

Paton

²

2020

Preprint

0

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Radical cations generated from the oxidation of C=C p-bonds are synthetically useful reactive intermediates for C–C and C–X bond formation. Radical cation formation, induced by sub-stoichiometric amounts of external oxidant, are important intermediates in the Woodward-Hoffmann thermally disallowed [2+2] cycloaddition of electron-rich alkenes. Using density functional theory (DFT), we report the detailed mechanisms underlying the intermolecular heterodimerisation of anethole and β-methylstyrene to give unsymmetrical, tetra-substituted cyclobutanes. Reactions between trans-alkenes favour the all-trans adduct, resulting from a kinetic preference for anti-addition reinforced by reversibility at ambient temperatures since this is also the thermodynamic product; on the other hand, reactions between a trans-alkene and a cis-alkene favour syn-addition, while exocyclic rotation in the acyclic radical cation intermediate is also possible since C-C forming barriers are higher. Computations are consistent with the experimental observation that hexafluoroisopropanol (HFIP) is a better solvent than acetonitrile, in part due to its ability to stabilize the reduced form of the hypervalent iodine initiator by hydrogen bonding, but also through the stabilisation of radical cationic intermediates along the reaction coordinate.

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“…[43][44][45][46][47] All of the above-mentioned study concerns the first step of the reaction, the SEO step, and seems to us in need for further understandings despite the subsequent steps that lead to the all trans cyclobutane ring are not considered, at least to the best of our knowledge, by other workers under these conditions. 26,[48][49][50] An important question that should be raised is the number of electrons to be transferred to the iodine reagent to initiate the reaction. At this point, the reaction mechanism and reactivity of HVIR-mediate dimerization exclusively appears incomplete and warrants further attentions ( Figure 2).…”

Section: Introductionmentioning

confidence: 99%

Hypervalent Iodine-Mediated Styrene Hetero- and Homodimerization Initiation Proceeds with Two-Electron Reductive Cleavage

Hussein¹,

Ma²,

Al-Yasari³

2020

Preprint

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<a>A mechanistic insight into </a>the hetero- and homodimerizations (HETD and HOMD) of styrenes promoted by hypervalent iodine reagents (HVIRs; DMP and PIDA) and facilitated by HFIP to yield all trans cyclobutanes is reported using density functional theory (DFT) calculations. The initialization involving direct bimolecular one-electron transfer is found to be highly unfavored, especially for the PIDA system. At this point, we suggest that the reaction is initiated with an overall two-electron reductive cleavage of two I─O bond cleavages, affording I(III) (iodinane) and I(I) (iodobenzene) product with DMP and PIDA as oxidant, respectively. The resulting acetate groups are stabilized by the solvent HFIP through strong hydrogen bonding interaction, which promotes the electron transfer process. The nature of the electron transfer is studied in detail and found that the overall two-electron transfer occurs within tri-molecular complex organized by π-stacking interactions and as a stepwise and concerted mechanism for I(III) and I(V) oxidants, respectively. The reaction rate is determined by the initialization step: for I(III), the initiation is thermodynamically endergonic, whereas the endergonicity for I(V) is modest. Upon initialization, the reaction proceeds through a stepwise [2+2] pathway, involving a radical-cationic π-π stacked intermediate, either hetero- or homodimerized. DFT results supported by quasiclassical molecular dynamics simulations show that HOMD is dynamically competing pathway to HETD although the latter is relatively faster, in accordance with experimental observations.

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“…[43][44][45][46][47] All of the above-mentioned study concerns the first step of the reaction, the SEO step, and seems to us in need for further understandings despite the subsequent steps that lead to the all trans cyclobutane ring are not considered, at least to the best of our knowledge, by other workers under these conditions. 26,[48][49][50] An important question that should be raised is the number of electrons to be transferred to the iodine reagent to initiate the reaction. At this point, the reaction mechanism and reactivity of HVIR-mediate dimerization exclusively appears incomplete and warrants further attentions ( Figure 2).…”

Section: Introductionmentioning

confidence: 99%

Hypervalent Iodine-Mediated Styrene Hetero- and Homodimerization Initiation Proceeds with Two-Electron Reductive Cleavage

Hussein

¹

,

Ma²,

Al-Yasari³

2020

Preprint

0

View full text Add to dashboard Cite

<a>A mechanistic insight into </a>the hetero- and homodimerizations (HETD and HOMD) of styrenes promoted by hypervalent iodine reagents (HVIRs; DMP and PIDA) and facilitated by HFIP to yield all trans cyclobutanes is reported using density functional theory (DFT) calculations. The initialization involving direct bimolecular one-electron transfer is found to be highly unfavored, especially for the PIDA system. At this point, we suggest that the reaction is initiated with an overall two-electron reductive cleavage of two I─O bond cleavages, affording I(III) (iodinane) and I(I) (iodobenzene) product with DMP and PIDA as oxidant, respectively. The resulting acetate groups are stabilized by the solvent HFIP through strong hydrogen bonding interaction, which promotes the electron transfer process. The nature of the electron transfer is studied in detail and found that the overall two-electron transfer occurs within tri-molecular complex organized by π-stacking interactions and as a stepwise and concerted mechanism for I(III) and I(V) oxidants, respectively. The reaction rate is determined by the initialization step: for I(III), the initiation is thermodynamically endergonic, whereas the endergonicity for I(V) is modest. Upon initialization, the reaction proceeds through a stepwise [2+2] pathway, involving a radical-cationic π-π stacked intermediate, either hetero- or homodimerized. DFT results supported by quasiclassical molecular dynamics simulations show that HOMD is dynamically competing pathway to HETD although the latter is relatively faster, in accordance with experimental observations.

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A theoretical study on the stereoconvergency of the intramolecular radical cation [2+2] cycloadditions of bis(styrenes)

Cited by 13 publications

References 44 publications

Stereoretention in Styrene Heterodimerisation Promoted by One-Electron Oxidants

Stereoretention in Styrene Heterodimerisation Promoted by One-Electron Oxidants

Hypervalent Iodine-Mediated Styrene Hetero- and Homodimerization Initiation Proceeds with Two-Electron Reductive Cleavage

Hypervalent Iodine-Mediated Styrene Hetero- and Homodimerization Initiation Proceeds with Two-Electron Reductive Cleavage

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