Reactions of Substituted Vinyl Cations in Acetonitrile Solution as Studied by Flash Photolysis

Kobayashi, Shinjiro; Zhu, Qin Qin; Schnabel, W.

doi:10.1515/znb-1988-0707

Cited by 16 publications

(3 citation statements)

References 9 publications

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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%

“…If instead high intrinsic barriers for the sp 2 ⇌ sp rehybridization were responsible for the slow solvolyses of vinyl derivatives, the discussion of Figure implies that the reverse reaction should also be slow, and vinyl cations should not be extraordinarily reactive intermediates but rather sluggish electrophiles. Although rate constants for the reactions of vinyl cations generated by laser-flash photolysis with a variety of nucleophiles have previously been reported, , a systematic comparison of the electrophilic reactivities of vinyl cations and tricoordinated carbenium ions has not yet been performed. We now report on a systematic analysis of the reactivities of vinyl cations and show that exceptionally high intrinsic barriers account for their extraordinarily slow formation in S N 1 reactions as well as for their slow reactions with nucleophiles.…”

Section: Introductionmentioning

confidence: 99%

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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%

Section: Introductionmentioning

confidence: 99%

Why Are Vinyl Cations Sluggish Electrophiles?

et al. 2017

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“…12 Vinyl cations, however, studied by LFP in solvents such as 2,2,2-trifluoroethanol (TFE) and acetonitrile, do not show a substantial difference from sp 2 -hybridized analogs. [13][14][15][16][17][18] For example, the lifetime of the 4-methoxyphenylvinyl cation in TFE is very similar to that of the 4-methoxyphenethyl cation. 18 In this paper we report a flash photolysis study of the N-isopropylbenzonitrilium ion 3.…”

Section: Introductionmentioning

confidence: 99%