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

Abstract

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...

Teaching an old carbocation new tricks: Intermolecular C–H insertion reactions of vinyl cations

“…1B) (21, 22). In this reaction, and in analogous reports from Olah and Mayr (23) and Caple and others (24), this unusual C–C bond–forming process was rationalized via a carbocation rebound mechanism. Here the vinyl cation 5 is quenched via 1,5-hydride shift, and the neutralized alkene (6) attacks the newly formed alkyl carbocation.…”

“…Subjection of butenyl triflate 21 to the reaction conditions led to high-yielding reductive alkylation of cyclohexane, providing 2-cyclohexylbutane (22) in 85% yield (Table 2, entry 1). We found that the treatment of a cyclohexane solution of but-1-en-2-yl triflate (23) with 2 mol % of [Ph 3 C] + [HCB 11 Cl 11 ] − and 1.5 equivalents of triethylsilane led to formation of a 1:1 mixture of 1-cyclohexylbutane (24) and 2-cyclohexylbutane (22) (Table 2, entry 2). Moreover, use of a 1:1 mixture of C 6 D 12 :C 6 H 12 in this system led to exclusive formation of D 12 and D 0 isomers, without any detectable amount of crossover products (that is, C 10 H 8+ n D 12- n or C 10 H 20– n D n , n ≠ 0) that would be expected in a stepwise process (fig.…”

Kinetic Studies that Evaluate the Solvolytic Mechanisms of Allyl and Vinyl Chloroformate Esters

“…Additionally due to the possibility of increased resonance stabilization in the cation, allyl, benzyl, and benzoyl substrates typically tend to favor stepwise unimolecular S N 1 (dissociative) reactions [29,33,51,52,62–65]. An allyl cation with two resonance contributors is shown to have approximately the same stability as a secondary alkyl cation [66]. On the other hand reactivity at an sp 2 carbon of vinyl substrates is dependent on whether the substrate is activated or unactivated, for alkyl-substituted vinyl triflates are shown to solvolyze through ion-pair mechanisms where nucleophilic solvation is important in substrates where the β hydrogen is trans to the leaving group [66–73].…”

Brønsted Acid‐Catalyzed Enantioselective Cycloisomerization of Arylalkynes

“…To assess the vinyl cation character of intermediate C/C',w e decided to investigate its ability to promote CÀHi nsertion. Indeed, as experimentally observed [5] and then rationalized by DFT studies, [37] vinyl cationsp ossess ac arbenic reactivity [38] resulting in their insertion into inert CÀHb onds. We thus prepared substrate 1v bearing an isopropyl chain ands ubmitted it to cycloisomerization in the presence of 5mol %o fB rønsted acid catalyst 7a as model catalyst (Scheme 3).…”