Generation of vinyl cations by solvolysis reactions. A lesser known genre of carbenium ions

Subramanian, L. R.; Hanack, M.

doi:10.1021/ed052p80

J. Chem. Educ.

1975

DOI: 10.1021/ed052p80

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Generation of vinyl cations by solvolysis reactions. A lesser known genre of carbenium ions

L. R. Subramanian¹,

M. Hanack²

Abstract: Generation of Vinyl Cations by Solvolysis ReactionsA lesser known genre of carbenium ions Among the reactive intermediates in organic chemistry, carbenium ions are the most important and extensively studied (1). Although they were discovered shortly after 1900 (2), carbenium ions have been well recognized only

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Cited by 25 publications

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References 22 publications

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“…During the last ten years, however, extensive studies on the solvolysis of vinyl derivatives, the plurality of which have been a-phenyl substituted, have provided indirect evidence for the intermediacy of vinyl cations. 5 Recently, the reaction of -aryl vinyl fluorides with SbFs has been reported to yield vinyl cations as stable species in low nucleophilic media by both Hanack63 and Masamune.6b Discrepancies of results and the question of whether real vinyl cations were indeed observed, however, remain to be resolved.…”

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confidence: 99%

Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

Oláh¹,

Mayr²

1976

J. Am. Chem. Soc.

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A series of 12 alkynes has been protonated with FSO3H-SbFs (magic acid) in SO2 or S02CIF solution under stable ion conditions. Whereas the formation of oligomeric products was observed at -78 OC, allyl cations were formed in high yields at higher temperatures. In many cases this way of preparing allyl cations is superior to other methods. While alkynes, which are branched in the a position to the triple bond, underwent rearrangements to allyl cations after protonation in SO2 at -20 O C , the corresponding reactions of the unbranched systems demanded higher temperatures. Only the sterically crowded ditert-butylacetylene rearranged to stereoisomeric allyl cations upon protonation even at -78 O C . The nonequilibrium nature of the protonation step and the intermediacy of vinyl cations was demonstrated by deuterium labeling.Whereas trivalent alkyl cations have been recognized as reactive intermediates since Ingold's definition of SN 1 reactions2 and have been extensively studied as long lived species within the last 15 years,3 evidence for the existence of vinyl cations4 has proved to be more elusive. Indeed, the low reactivity of vinyl halides under S N~ conditions has suggested that vinyl cations are not formed as reactive intermediates. During the last ten years, however, extensive studies on the solvolysis of vinyl derivatives, the plurality of which have been a-phenyl substituted, have provided indirect evidence for the intermediacy of vinyl cation^.^ Recently, the reaction of a-aryl vinyl fluorides with SbFS has been reported to yield vinyl cations as stable species in low nucleophilic media by both Hanack6a and Masamune.6b Discrepancies of results and the question of whether real vinyl cations were indeed observed, however, remain to be resolved.Evidence for alkyl substituted vinyl cations has been obtained from the stereoselective nature of H X additions to allenes and alkynes7 Furthermore, skeletal rearrangements, accompanying the solvolysis of some vinyl derivatives and electrophilic attack on alkynes, have been interpreted on the basis of the intermediacy of vinyl cations4 In contrast to the large number of 1,2-alkyl migrations observed in alkyl cations, there were only isolated reports on 1,2-alkyl shifts occurring in vinyl cations (Le., 1 -+ 2, Scheme I).8 Moreover, while 1,2-hydrogen shifts are extremely fast in simple carbenium ions, the formation of a small amount of 3-(2,2,2-trifluoroethoxy)-3-methyl-l-butene in the solvolysis of 3-methyl-lbuten-2-yl triflate in trifluoroethanol is the only experimental evidence that has been provided for a 1,2-hydrogen migration to a vinylic carbenium center (1 -+ 2, R = H).9 Though the reaction paths via vinyl cations (A and B, Scheme I) seem to be probable, synchronous ionization and alkyl (hydrogen) The formation of allyl cations from vinyl cations has not been detected under stable ion conditions.I0 In our previous work it has been observed that the protonation of several alkynes with FS03H at -78 O C yields cyclobutenyl cations' I and vinyl fluoro~ul...

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Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

Oláh¹,

Mayr²

1976

J. Am. Chem. Soc.

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“…[Traduit par le journal] There is considerable interest in vinyl cations and a nuniber of reviews on the subject are available (1)(2)(3)(4). Recently in this laboratory, ' T-labeling coupled with cmr analysis was used to nieasure the extents of degenerate rearrangenients arising froni 1,2-aryl shifts in triarylvinyl cations (5,6).…”

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confidence: 99%

The reaction of cis- and trans-1,2-dianisyl-2-phenylvinyl-2-¹³C bromides with acetic acid and silver acetate

Lee¹,

Oka²

1976

Can. J. Chem.

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The reaction of cis- or trans-1,2-dianisyl-2-phenylvinyl-2-13C bromide with HOAc–AgOAc gave a 1:1 mixture of cis- and trans-1,2-dianisyl-2-phenylvinyl-2-13C acetates with no isotopic scrambling according to pmr and cmr analyses. The lack of degenerate 1,2-phenyl shift in this system is in contrast with the finding of about 7% scrambling in the analogous reaction with triphenylvinyl-2-13C bromide and mechanistic implications of this difference are discussed.

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Mechanistische und präparative Aspekte der Chemie der Vinylkationen

Hanack

1978

Angewandte Chemie

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In den letzten Jahren ist umfangreiches experimentelles Material erschlossen worden, das zwingend auf Vinylkationen als Zwischenstufen deutet. Vinylkationen können durch elektrophile Additionen an Alkine und Allene sowie unter Beteiligung dieser Gruppierungen bei Solvolysereaktionen erzeugt werden. Die heterolytische Bindungsspaltung von Vinylverbindungen führt ebenfalls zu Vinylkationen. Beispiele für präparative Anwendungen sind Synthesen von substituierten Indenen, Cyclobutanonen und Cyclopropylketonen.

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Generation of vinyl cations by solvolysis reactions. A lesser known genre of carbenium ions

Cited by 25 publications

References 22 publications

Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

The reaction of cis- and trans-1,2-dianisyl-2-phenylvinyl-2-¹³C bromides with acetic acid and silver acetate

Mechanistische und präparative Aspekte der Chemie der Vinylkationen

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Generation of vinyl cations by solvolysis reactions. A lesser known genre of carbenium ions

Cited by 25 publications

References 22 publications

Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations

The reaction of cis- and trans-1,2-dianisyl-2-phenylvinyl-2-13C bromides with acetic acid and silver acetate

Mechanistische und präparative Aspekte der Chemie der Vinylkationen

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The reaction of cis- and trans-1,2-dianisyl-2-phenylvinyl-2-¹³C bromides with acetic acid and silver acetate