CF Activation of Fluorobenzene by Silylium Carboranes: Evidence for Incipient Phenyl Cation Reactivity

Abstract

Silicon and fluorine are an unequal pair, and yet they form the strongest couple of all elements. The unmatched SiÀF bond dissociation energy [1] of 662 kJ mol À1 offers the possibility of abstracting fluoride from almost any fluorine-containing species, given a suitable silyl Lewis acid. In particular, fluoride abstraction from organofluorine compounds by a silylium ion R 3 Si + is a thermodynamically favored process. [2] Silylium ion-based Lewis acids have been shown to effect CÀF activation in aliphatic fluorocarbons, affording isolable carbocationic intermediates [3] or hydrocarbons R 3 C-H in the presence of a reducing agent. [4] The extension of such reactivity to the heterolytic activation of fluoroarenes, however, remains a fundamental challenge. [

Al(ORF)3 (RF = C(CF3)3) activated silica: a well-defined weakly coordinating surface anion

“…Silylium ions are strong Lewis acids that catalyze or mediate numerous chemical reactions. [65][66][67] Silylium ions activate C-F bonds to form R 3 Si-F and carbocation intermediates, [68][69][70][71][72] which are rapidly quenched in the presence of excess silane to form C-H bonds. 3 activates C-F bonds in 1-adamantyluoride in the presence of Et 3 SiH at 0 C to give adamantane (TON ¼ 18).…”

Towards Nonalternant Nanographenes through Self‐Promoted Intramolecular Indenoannulation Cascade by C−F Bond Activation

“…In 2011 Siegel et al. have demonstrated alternative indenoannulation by C−F bond activation under transition‐metal free conditions, preparing fluoranthrene 1 . Even though this transformation yields 1 in nearly 95 % yield, it requires even more expensive silylium carborane catalyst limiting its broad application.…”

Terpene Tail-to-Head Polycyclization Mediated by Small Molecule Catalysts: A Weakly-Coordinating Anion Approach

“…At the outset of our efforts, we hypothesized that the use of WCAs would allow for the generation of persistent carbocations with sufficient lifetimes to partake in polycyclization events before fast, counteranion-mediated-E1 elimination or trapping by solvent. [21][22][23] Previous reports from our lab have demonstrated the competency of R3Si + /WCA-derived catalysts in generating long-lived carbocationic species that engage in intermolecular C-H insertion reactions. 24 Hence, we hypothesized that allylic fluoride 11, (Figure 1e) could be readily ionized under analogous R3Si + /WCA conditions to generate an allylic carbocation (4, Figure 1b) poised to undergo rapid 1,6-cyclization to generate the bisabolyl cation (6).…”