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Why Are Vinyl Cations Sluggish Electrophiles?

Author:
Byrne, Peter A., Kobayashi, Shinjiro, Würthwein, Ernst-Ulrich, Ammer, Johannes, Mayr, Herbert
Source:
Journal of the American Chemical Society 2017 v.139 no.4 pp. 1499-1511
ISSN:
1520-5126
Subject:
bromides, cations, energy, ethanol, half life, ionization, photolysis, pyrroles, solvents
Abstract:
The kinetics of the reactions of the vinyl cations 2 [Ph₂CC⁺–(4-MeO–C₆H₄)] and 3 [Me₂CC⁺–(4-MeO–C₆H₄)] (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)₂CH⁺, 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)₂CH-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 sp² ⇌ 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.