Optimizing the Least Nucleophilic Anion. A New, Strong Methyl⁺ Reagent

Abstract

The icosahedral carborane anions H-CB11X6H5 - (X = Cl, Br, I) are among the most inert, least coordinating, and least basic anions known. These properties are enhanced by 2,3,4,5,6-pentamethylation with methyl triflate. The resulting anions, H-CB11X6Me5 -, are more inert than their unmethylated precursors, have improved NMR handles, and their salts have higher solubility in low dielectric media. They sustain superacidity in H(H-CB11X6Me5). Protonated benzene has been isolated and characterized by X-ray crystallography, moving Wheland intermediates from the status of spectroscopically observable transients to weighable reagents. The new anions sustain extreme Lewis acidity in silylium ion-like R3Si(H-CB11X6Me5) species. Treatment of Et3Si(H-CB11Br6Me5) with methyl triflate leads to a new methyl+ reagent CH3(H-CB11Br6Me5) that is more potent than methyl triflate. It methylates benzene without heating or acid catalysis to give the toluenium ion. The H-CB11X6Me5 - anions come as close as any to the concept of a univeral weakly coordinating anion and, with cheaper starting materials now available, promise to become specialty chemicals of wide usage.

Molecular Structure of the Solvated Proton in Isolated Salts. Short, Strong, Low Barrier (SSLB) H-bonds

“…Carborane anions, [62][63][64] protonated arene salts, 45,58 and H9O4 + salts 27 were prepared as previously described. Solvents were dried by standard methods.…”

Intermediates in the Methanol-to-hydrocarbons (MTH) Reaction: A Gas Phase Study of the Unimolecular Reactivity of Multiply Methylated Benzenium Cations

“…The benzenium ion corresponds to a side-on adduct of the phenyl cation and H 2 having C 2v symmetry [46], and H 2 loss proceeds with monotonically increasing potential energy towards the dissociation limit. Isolable arenium ions are known, and the benzenium ion itself, C 6 H 7 + , has been inferred from NMR spectroscopy of superacidic solutions [47] and on the basis of X-ray crystallography [48].…”

The Strongest Isolable Acid

“…The nNÀH ranking brings out a number of features: a) For each of the pairs of hexahalo carboranes [CHB 11 H 5 X 6 ] À (X = Cl, Br, I,) and their pentamethylated counterparts [CHB 11 Me 5 X 6 ] À , [18] the difference in nNÀH values is 5 or 6 cm À1 . This consistency gives confidence in the scale, suggesting that the CCl 4 solvation environment remains constant over the series and that mass differences and steric effects have negligible influence.…”