[Me3Si][R‐CB11F11]—Synthesis and Properties

Küppers, Torsten; Bernhardt, Eduard; Eujen, R.; Willner, Helge; Lehmann, Christian W.

doi:10.1002/anie.200701136

Cited by 107 publications

(76 citation statements)

References 23 publications

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“…Thus, the undeca chloro reagent R 3 Si(CHB 11 Cl 11 ) is a stronger silylating reagent than the corresponding hexa chloro reagent, which is stronger than the corresponding hexabromo reagent. The fluorinated carborane13 and borane14 anions developed in the Strauss lab should give even stronger electrophilic reagents15 but these anions are only available to those willing to work with F 2 . The easiest carboranes to prepare are the hexa- and undeca-halo anions of the more conveniently used halogens.…”

Section: Introductionmentioning

confidence: 99%

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

2009

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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.

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Section: Introductionmentioning

confidence: 99%

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

2009

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“…14 To investigate both the basicity and the redox inertness of these remarkable anions, all five carborane anions were reacted with several reference electron transfer reagents with known electron affinities. No reactions were observed with nitrogen dioxide (NO 2 , EA ) 2.273 ( 0.005 eV), 2-chloroacrylonitrile (CH 2 dC(Cl)CN, vertical detachment energy (VDE) ) 4.50 eV), molybdenum hexafluoride (MoF 6 , EA ) 3.82 ( 0.19 eV), and rhenium hexafluoride (ReF 6 , EA ) >5.14 ( 0.48 eV). 11 These results indicate that the electron binding energies of all five carborane anions are >5.14 ( 0.48 eV.…”

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confidence: 99%

Investigating the Weak to Evaluate the Strong: An Experimental Determination of the Electron Binding Energy of Carborane Anions and the Gas phase Acidity of Carborane Acids

et al. 2009

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“…[14] The resulting sodium salt with a ratio thf/Na ≈ 1 reacts instantaneously with triarylchloromethanes in CD 2 Cl 2 , leading to yellow to orange thermally stable solutions (Scheme 4). 13 C{ 1 H} NMR spectra display carbocation signals. The "frustrated" trityl Lewis acid/altebate Lewis base pair, however, is easily and rapidly decomposed by water.…”

Section: Resultsmentioning

confidence: 99%

A Lipophilic, Fluorine‐Free, Thermostable, Inexpensive, S₄‐Symmetric, Highly Soluble, Weakly Coordinating, Protolabile Aluminate

et al. 2010

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This communication presents the facile and cost-effective synthesis as well as properties of salts of a lipophilic aluminate anion. Its unprecedented solubility in pentane makes possible tetrabutylammonium salt extraction with pentane. Nevertheless, its symmetry leads to high melting points of its thermostable salts. Several single crystals for X-ray diffrac- IntroductionThe term "lipophilic anion" appears to be a paradox. The electric charge separation of salts in cations and anions is best stabilized by a polar, hydrophilic environment. However, lipophilic anions are characterized by a low negative charge, large molecular dimensions, and a lipophilic molecular surface. Related weakly coordinating anions (WCA) usually comprise fluorine substituents, [1] resulting in more fluorophilic behavior. The most important structural features of WCAs are, on the one hand, multiple C-F fragments in order to electronically conceal and, on the other hand, bulky substituents to sterically conceal nucleophilic sites within the anion. In contrast, the main principles of designing lipophilic anions are the minimization of electrostatic contact with the positive counterion and maximization of van der Waals interactions with nonpolar solvents. The permethylated carboranes of the Michl group can be considered as lipophilic anions.[2] Our strategy for a lipophilic aluminate anion avoids expensive and persistent C-F bonds and uses large and inexpensive tert-butylated 2,2Ј-biphenols for an extended hydrocarbon molecular surface. Results and Discussion3,3Ј,5,5Ј-Tetra-tert-butyl-2,2Ј-biphenol can be synthesized under ambient conditions by a modified literature protocol comprising oxidative C-C coupling of commercially available 2,4-di-tert-butylphenol with MnO 2 and recrystallization from acetic acid (Scheme 1). [3] [a]

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[Me₃Si][R‐CB₁₁F₁₁]—Synthesis and Properties

Cited by 107 publications

References 23 publications

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

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

Investigating the Weak to Evaluate the Strong: An Experimental Determination of the Electron Binding Energy of Carborane Anions and the Gas phase Acidity of Carborane Acids

A Lipophilic, Fluorine‐Free, Thermostable, Inexpensive, S₄‐Symmetric, Highly Soluble, Weakly Coordinating, Protolabile Aluminate

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[Me3Si][R‐CB11F11]—Synthesis and Properties

Cited by 107 publications

References 23 publications

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

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

Investigating the Weak to Evaluate the Strong: An Experimental Determination of the Electron Binding Energy of Carborane Anions and the Gas phase Acidity of Carborane Acids

A Lipophilic, Fluorine‐Free, Thermostable, Inexpensive, S4‐Symmetric, Highly Soluble, Weakly Coordinating, Protolabile Aluminate

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[Me₃Si][R‐CB₁₁F₁₁]—Synthesis and Properties

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

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

A Lipophilic, Fluorine‐Free, Thermostable, Inexpensive, S₄‐Symmetric, Highly Soluble, Weakly Coordinating, Protolabile Aluminate