Photoreactivity of some .alpha.-arylvinyl bromides in acetic acid. Selectivity toward bromide versus acetate ions as a mechanistic probe

Ginkel, Frits I. M. Van; Cornelisse, J.; Lodder, Gerrit

doi:10.1021/ja00011a032

Cited by 24 publications

(6 citation statements)

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“…ipso -Substitution (at C-4 of the anisyl group), which has been observed in reactions of photochemically generated vinyl cations 2 with cyanide or alkoxide, , but not with alcohols, , did not occur under the conditions described in Scheme . We therefore conclude that all nucleophiles were added to the cationic sp-center of 2 and 3 in our kinetic experiments, in line with results from earlier studies carried out under similar conditions. ,,,,,,− …”

Section: Product Characterizationsupporting

confidence: 89%

Why Are Vinyl Cations Sluggish Electrophiles?

et al. 2017

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The kinetics of the reactions of the vinyl cations 2 [PhC═C-(4-MeO-CH)] and 3 [MeC═C-(4-MeO-CH)] (generated by laser flash photolysis) with diverse nucleophiles (e.g., pyrroles, halide ions, and solvents containing variable amounts of water or alcohol) have been determined photometrically. It was found that the reactivity order of the nucleophiles toward these vinyl cations is the same as that toward diarylcarbenium ions (benzhydrylium ions). However, the reaction rates of vinyl cations are affected only half as much by variation of the nucleophiles as those of the benzhydrylium ions. For that reason, the relative reactivities of vinyl cations and benzhydrylium ions depend strongly on the nature of the nucleophiles. It is shown that vinyl cations 2 and 3 react, respectively, 227 and 14 times more slowly with trifluoroethanol than the parent benzhydrylium ion (Ph)CH, even though in solvolysis reactions (80% aqueous ethanol at 25 °C) the vinyl bromides leading to 2 and 3 ionize much more slowly (half-lives 1.15 yrs and 33 days) than (Ph)CH-Br (half-life 23 s). The origin of this counterintuitive phenomenon was investigated by high-level MO calculations. We report that vinyl cations are not exceptionally high energy intermediates, and that high intrinsic barriers for the sp ⇌ sp rehybridizations account for the general phenomenon that vinyl cations are formed slowly by solvolytic cleavage of vinyl derivatives, and are also consumed slowly by reactions with nucleophiles.

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Section: Product Characterizationsupporting

confidence: 89%

Why Are Vinyl Cations Sluggish Electrophiles?

et al. 2017

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“…18 For actinometry, the photolysis of 3-nitroanisole in an aqueous 0.1 M NaOH solution was used.lg 254-nm excitation was achieved with a Hanau TNN 15/32 lowpressure mercury lamp, 313 nm, with a Hanau TQ 81 highpressure mercury lamp combined with a filter solution. 6 The concentrations of the vinyl halides 1X in methanol (MeOH, spectroscopic grade) were always 0.5 mM. The solutions were argon or oxygen saturated by bubbling with argon or oxygen, respectively, for 30 min prior to and during the irradiation.…”

Section: Methodsmentioning

confidence: 99%

Photochemical Formation and Electrophilic Reactivities of Vinyl Cations. Influence of Substituents, Anionic Leaving Groups, Solvents, and Excitation Wavelength on Photoheterolysis and Photohomolysis of 1-(p-R-Phenyl)-2-(2,2'-biphenyldiyl)vinyl Halides

Verbeek¹,

Stapper²,

Krijnen³

et al. 1994

J. Phys. Chem.

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Using product analysis and time-resolved laser flash photolysis techniques, the photochemistry at A, , , X 250 and 310 nm (exc = excitation) of l-(p-R-phenyl)-2-(2,2'-biphenyldiyl)vinyl halides (R = H, Me, MeO) in methanol and acetonitrile at room temperature was studied. The title compounds undergo photoheterolysis and photohomolysis to give vinyl cations =C+-(carbenium ions) and vinyl radicals =c'--. The cation: radical ratio increases with electron-donating strength of the substituent R, H < Me < MeO, and with anionic leaving group power of the halide, F-< C1-< Br-< I-, indicating that the cleavage of the C-X bond to yield cation and halide anion proceeds, in the rate-determining step, by heterolysis and not by homolysis followed by electron transfer in the radical pair. The cation:radical ratio is solvent dependent: e.g., for the vinyl bromide with R = M e 0 and with A, , , = 308 nm, in CH2Clz as solvent, only radical is observed, in comparison with only cation in the much more polar solvent acetonitrile. In acetonitrile-methanol mixtures, the absolute yields of both cation and radical go through a maximum as the methanol content is increased, however, the cation:radical ratio decreases continuously with increasing [MeOH]. For a particular R, the quantum yield for C-X bond cleavage is higher at ~2 5 0 than at ~3 1 0 nm. Also, the cation:radical ratio is wavelength-dependent: at A, , , x 310 nm there is relatively more heterolysis than at A, , , z 250 nm. Rate constants for reaction of the cations with nucleophiles were determined in acetonitrile. Those for reaction of the cation with R = M e 0 (lifetime in acetonitrile 7 ps) with anionic nucleophiles such as the halides are at the diffusion limit in this solvent, x 2 x 1O'O M-' s-l, whereas those for reaction with water, alcohols, and cyclic ethers are of the order 105-106 M-' s-l. The less stabilized cation (R = Me) reacts with alcohols faster by the factor ~1 0 0 , and it decays in acetonitrile e l 0 0 times more rapidly. Also, l-(p-R-phenyl)-2-dimethylvinyl bromides (R = H, Me, MeO) were photolyzed in acetonitrile. In the case of R = MeO, the cation p-CH30-c&-c+=CMe2 was seen (lifetime 770 ns), and its reactivity with alcohols (k values of 106-107 M-' s-') and halides (k values of 1O' O M-' s-l) was determined. The vinyl radicals =c'-react with 0 2 to yield vinylperoxyl radicals =C-Oi which have absorption maxima at ~3 9 0 nm.

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“…[1] It can provide an efficient route to aryl alkyne derivatives following a Sonogashira reaction [2] or to triarylethylenes following hydroarylation. [3] Triarylethylenes show interesting spectroscopic and electronic properties [4] and have applications in medicinal chemistry as a core for some selective estrogen receptor modulators (SERM) like Tamoxifen  and Clomifene  , respectively used for breast cancer chemotherapy [5] and in treatment of infertility caused by polycystic ovary syndrome therapy [6] (Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

Selectivity studies in the reaction between iodobenzene and phenylacetylene: Sonogashira coupling vs hydroarylation

Barros

Souza

Lima

et al. 2008

Applied Organom Chemis

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The selectivity of the coupling reaction between iodobenzene and phenylacetylene was evaluated. Several palladium catalysts, ligands and reaction conditions were tested, showing that supported catalysts, room temperature or ionic liquids (NHC precursors) favor Sonogashira coupling, while the non-supported ones, higher temperature and PPh 3 as ligand, favor hydroarylation. Neither excess of iodobenzene nor phenylboronic acids are required; and it is possible to avoid the use of PPh 3 , although this lowers selectivity.

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Photoreactivity of some .alpha.-arylvinyl bromides in acetic acid. Selectivity toward bromide versus acetate ions as a mechanistic probe

Cited by 24 publications

References 1 publication

Why Are Vinyl Cations Sluggish Electrophiles?

Why Are Vinyl Cations Sluggish Electrophiles?

Photochemical Formation and Electrophilic Reactivities of Vinyl Cations. Influence of Substituents, Anionic Leaving Groups, Solvents, and Excitation Wavelength on Photoheterolysis and Photohomolysis of 1-(p-R-Phenyl)-2-(2,2'-biphenyldiyl)vinyl Halides

Selectivity studies in the reaction between iodobenzene and phenylacetylene: Sonogashira coupling vs hydroarylation

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