A Very Strong Methylation Agent: [Me2 Cl][Al(OTeF5 )4 ]

[Xe(OTeF5)(pyF)]+: a strong oxidizing xenonium(II) teflate cation with N-donor bases

Herein we report on the formation of the adduct salts [Xe(OTeF5)(pyF)][Al(OTeF5)4] (pyF = C5F5N, C5H3F2N) by abstraction of an -OTeF5 group from Xe(OTeF5)2 with the Lewis superacid Al(OTeF5)3 and subsequent adduct formation of the generated [XeOTeF5]+ cation with fluorinated pyridines. These salts represent the first xenonium cations with the weakly coordinating [Al(OTeF5)4]- anion. The strong oxidizing property of these compounds is further assessed.

A computational tool to accurately and quickly predict 19F NMR chemical shifts of molecules with fluorine-carbon and fluorine-boron bonds

We report the evaluation of density-functional-theory (DFT) based procedures for predicting ¹⁹F NMR chemical shifts at modest computational cost for a range of molecules with fluorine bonds, to be used as a tool for assisting the characterisation of reaction intermediates and products and as an aid to identifying mechanistic pathways. The results for a balanced learning set of molecules were then checked using two further testing sets, resulting in the recommendation of the ωB97XD/aug-cc-pvdz DFT method and basis set as having the best combination of accuracy and computational time, with a RMS error of 3.57 ppm. Cationic molecules calculated without counter-anion showed normal errors, whilst anionic molecules showed somewhat larger errors. The method was applied to the prediction of the conformationally averaged ¹⁹F chemical shifts of 2,2,3,3,4,4,5,5-octafluoropentan-1-ol, in which gauche stereoelectronic effects involving fluorine dominate and to determining the position of coordination equilibria of fluorinated boranes as an aid to verifying the relative energies of intermediate species involved in catalytic amidation reactions involving boron catalysts.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

One-step methylation of aromatic phosphorus heterocycles: synthesis and crystallographic characterization of a 1-methyl-phosphininium salt

For the first time, the direct synthesis of 1-methyl-phosphininium salts has been achieved by reacting aromatic λ³,σ²-phosphinines with the readily available dimethyl chloronium salt [(CH₃)₂Cl]⁺[Al(OTeF₅)₄]^-. The remarkably high electrophilicity of the alkylation reagent in combination with the weakly coordinating pentafluoro-orthotelluratoaluminate anion offers excellent conditions for this one-step approach. Our simple and quantitative access to 1-methyl-phosphininium salts will pave the way to explore the chemistry of such reactive species in more detail.

Structural proof of a [C-F-C]+ fluoronium cation

Organic fluoronium ions can be described as positively charged molecules in which the most electronegative and least polarizable element fluorine engages in two partially covalent bonding interactions to two carbon centers. While recent solvolysis experiments and NMR spectroscopic studies on a metastable [C–F–C]⁺ fluoronium ion strongly support the divalent fluoronium structure over the alternative rapidly equilibrating classical carbocation, the model system has, to date, eluded crystallographic analysis to confirm this phenomenon in the solid state. Herein, we report the single crystal structure of a symmetrical [C–F–C]⁺ fluoronium cation. Besides its synthesis and crystallographic characterization as the [Sb₂F₁₁]⁻ salt, vibrational spectra are discussed and a detailed analysis concerning the nature of the bonding situation in this fluoronium ion and its heavier halonium homologues is performed, which provides detailed insights on this molecular structure.

Unlike other halogen atoms, the ability for fluorine to exist in a [C–X–C]⁺ connectivity pattern has only been shown in spectroscopic studies. Here the authors present a single crystal structure of a fluoronium cation, characterized by X-ray diffraction.

Friedel-Crafts Type Methylation with Dimethylhalonium Salts

The dimethylchloronium salt [Me2 Cl][Al(OTeF5 )4 ] is used to methylate electron-deficient aromatic systems in Friedel-Crafts type reactions as shown by the synthesis of N-methylated cations, such as [MeNC5 F5 ]+ , [MeNC5 F4 I]+ , and [MeN3 C3 F3 ]+ . To gain a better understanding of such fundamental Friedel-Crafts reactions, the role of the dimethylchloronium cation has been evaluated by quantum-chemical calculations.

Bioinspired design of a robust d 3-methylating agent

Methods to incorporate deuterium atoms into organic molecules are valuable for the pharmaceutical industry. The introduction of deuterium atoms by a synthetic method enables the direct tracing of the drug molecule without substantially altering its structure or function. The methyl group is one of the most commonly occurring carbon fragments in biologically active molecules. Here, a biomimetic design reagent, 5-(methyl-d ₃)-5H-dibenzo[b,d]thiophen-5-ium trifluoromethane sulfonate (DMTT), as an analog of S-adenosylmethionine (SAM), has been developed for the selective d ₃-methylation of complex molecules bearing several possible reactive sites with excellent selectivity and high-level deuterium incorporation. A series of d ₃-methylated organic molecules and deuterated pharmaceuticals were synthesized under the mild system with excellent functional group compatibility.

N-Methylation and Trideuteromethylation of Amines via Magnesium-Catalyzed Reduction of Cyclic and Linear Carbamates

A new reduction of carbamates to N-methyl amines is presented. The magnesium-catalyzed reduction reaction allows the conversion of cyclic and linear carbamates, including N-Boc protected amines, into the corresponding N-methyl amines and amino alcohols which are of significant interest due to their presence in many biologically active molecules. Furthermore, the reduction can be extended to the formation of N-trideuteromethyl labeled amines.