H⁺, CH₃⁺, and R₃Si⁺ Carborane Reagents: When Triflates Fail

Abstract

For decades, triflic acid, methyl triflate, and trialkylsilyl triflate reagents have served synthetic chemistry well as clean, strong electrophilic sources of H + , CH 3 + , and R 3 Si + , respectively. However, a number of weakly basic substrates are unreactive toward these reagents. In addition, triflate anion can express undesired nucleophilicity toward electrophilically activated substrates.In this Account, we describe methods that replace triflatebased electrophilic reagents with carborane reagents. Using carborane anions of type CHB 11 R 5 X 6 -(R ) H, Me, X; X ) Br, Cl), members of a class of notably inert, weakly nucleophilic anions, significantly increases the electrophilicity of these reagents and shuts down subsequent nucleophilic chemistry of the anion. Thus, H(carborane) acids cleanly protonate benzene, phosphabenzene, C 60 , etc., while triflic acid does not. Similarly, CH 3 (carborane) reagents can methylate substrates that are inert to boiling neat methyl triflate, including benzene, phosphabenzenes, phosphazenes, and the pentamethylhydrazinium ion, which forms the dipositive ethane analogue, Me 6 N 2 2+. Methyl carboranes are also surprisingly effective in abstracting hydride from simple alkanes to give isolable carbocation salts, e.g., t-butyl cation.Trialkylsilyl carborane reagents, R 3 Si(carborane), abstract halides from substrates to produce cations of unprecedented reactivity. For example, fluoride is extracted from freons to form carbocations; chloride is extracted from IrCl(CO)(PPh 3 ) 2 to form a coordinatively unsaturated iridium cation that undergoes oxidative addition with chlorobenzene at room temperature; and silylation of cyclo-N 3 P 3 Cl 6 produces a catalyst for the polymerization of phosphazenes that functions at room temperature. Although currently too expensive for widespread use, carborane reagents are nevertheless of considerable interest as specialty reagents for making reactive cations and catalysts.

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

“…With improvements in the preparation of CHB 11 Cl 11 À , [24] silylium carboranes can be expected to become more common reagents in synthetic chemistry than they are at present. [25] Figure 2. Calculated transition state for 2 + + PhF!5 (B98/DZ-(2df,pd)).…”

Selective Removal of Alkali Metal Cations from Multiply-Charged Ions via Gas-Phase Ion/Ion Reactions Using Weakly Coordinating Anions

“…Lithium chloride, sodium chloride, potassium chloride, cesium chloride, the sodium salt of benzene sulfonic acid (BMSA), ammonium hexafluorophosphate (PF 6 − ), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF), and lithium tetrakis(pentafluorophenyl)borate ethyl etherate (TPPB) were purchased from Sigma Aldrich (St. Louis, MO, USA). The cesium salt of carborane, Cs(CHB 11 Cl 11 ), was synthesized [44] by Professor C. A. Reed's group of the Department of Chemistry, University of California, Riverside. All peptide stock solutions for positive nanoelectrospray were prepared in a 49.5/49.5/1 (vol/vol/vol) solution of water/methanol/metal chloride salt at an initial concentration of~1 mg/mL and diluted 100-fold prior to use.…”

Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation

“…32 Treatment of 5-bromo-2-oxaadamantane 7 with 1 equiv. of Et 3 Si(CHB 11 H 5 Br 6 ) in odichlorobenzene-d 4 gave a new product whose 1 H NMR spectrum is consistent not with halide abstraction, but with silylation at the ether O atom, forming the oxonium ion 17 (Scheme 6).…”