Chapter 3
Unusually Stable Vinyl Cations 1
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Thomas Müller , Dominik Margraf , Yvonne Syha , Hamid Reza Nasiri , Christian Kaiser , Rita Maier , Bettine Boltres , Mark Juhasz , and Christopher A. Reed 2
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Institut für Reine und Angewandte Chemie der Carl von Ossietzky, Universität Oldenburg, D-26211 Oldenburg, Germany Institut für Anorganische und Analytische Chemie der Goethe, Universität Frankfurt, D-60439 Frankfurt/Main, Germany Department of Chemistry, University of California, Riverside, C A 92521-0403
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Recent results on the chemistry of persistent vinyl cations are summarized, β, β-Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and N M R spectroscopy and by X-ray crystallography. The experimental results for the α-alkyl- and α-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged α-carbon atom and a trigonal planar coordinated β-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, β-silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C≡C triple bonds are discussed.
2007 American Chemical Society
In Recent Developments in Carbocation and Onium Ion Chemistry; Laali, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Introduction Vinyl cations (/), the dicoordinated unsaturated analogues of tri valent carbenium ions, were first detected by Grob and coworkers in the early 1960s in solvolysis reactions of α-arylvinyl halides. (2, 3) In the 1970s, numerous investi gations established vinyl cations as transient intermediates in solvolysis reactions of activated alkenyl halides (4, 5), and of alkenes bearing superleaving groups like the triflate, nonaflate or phenyliodonio groups.(5-5) Similarly, the addition of electrophiles to alkynes and aliènes was shown to proceed via vinyl cations.(9) A landmark in the chemistry of stable vinyl cations was their direct N M R detection under superacidic reaction conditions at temperatures below -100 °C, which was achieved in the early 1990s by Siehl and co-workers. In combination with quantum mechanical computations, their work provided important information concerning structures and the electronic properties of persistent vinyl cations.(70-7
