Magic acid

Abstract: Owing to the ready availability of the starting materials, the functionalization of saturated hydrocarbons for preparing chemical intermediates and products represents an attractive goal. The possibility of converting alkane in a controlled way into alcohols, ketones etc., i.e. to oxyfunctionalize them, is particularly challenging. After a short account of previous methods used for the oxidation of alkanes, particular emphasis will be given to recently developed reactions of alkanes with oxygenating agents in superacidic media.

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 a t 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 a t -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) Scheme I X b migration (C and D, Scheme I) have not been rigorously excluded in any of these 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 f luoro~ulfates .~ Treating several alkynes with FSO3HSbF5 a t -78 \"C, we obtained only complex mixtures of unidentified products, probably oligomers. We now wish to report that protonation of alkynes with FS03H-SbF5 a t more elevated temperatures results in the predominant formation of the related allyl cations. Moreover, unequivocal evidence has been obtained for these transformations proceeding through the intermediacy of vinyl cations.

Abstract: Cationic σ-complexes (or benzenium ions) of aromatic hydrocarbons with various electrophiles are commonly accepted as transient (Wheland) intermediates in electrophilic aromatic substitutions.1 Olah and co-workers were the first to obtain nearly 30 years ago2 the direct spectroscopic (NMR) evidence for the existence of the NO2 and Cl+ complexes in magic acid solutions at -70 °C by using hexamethylbenzene as the prototypical aromatic donor. However, the isolation of these highly labile cations as crystalline salts for definitive structural characterization by X-ray crystallography has remained an experimental challenge. As such, we now report that the low-temperature chlorination of hexamethylbenzene (1) with antimony pentachloride in dichloromethane (under an argon atmosphere) leads to a bright yellow solution, which upon the addition of cold (-78 °C) hexane affords a highly unstable precipitate according to the stoichiometry in eq 1.3