LiCB<sub>11</sub>Me<sub>12</sub>:  A Catalyst for Pericyclic Rearrangements

Moss, Stefan; King, Benjamin T.; Meijere, Armin de; Kozhushkov, Sergei I.; Eaton, and Philip E.; Michl, Josef

doi:10.1021/ol0161864

Cited by 81 publications

(86 citation statements)

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“…[22] Electrophilic cubane conversions are limited to reactions of soft electrophiles like metal ions, [18] and a number of fascinating rearrangements (e.g., to syn-tricyclooctadiene (2) with Pd 2+ and to cuneane (3) with Ag + and Li + ) are preparatively useful. [18,23] The attack of a proton on cubane is highly exergonic and all of our and previous [9,15,16,24] ]oct-5-yl cation (5) [25] upon optimization without symmetry constrains. This structure is energetically much too low to account for the "experimental" gas-phase basicity of 1 (ca.…”

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confidence: 94%

The Protonation of Cubane Revisited

et al. 2004

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Protonation of saturated hydrocarbons is the simplest and practically most important electrophilic reaction.[1] Small alkanes like methane, ethane, and isobutane [2] form welldefined protonated species, and their experimental proton affinities (PAs) are excellently reproduced computationally. [3][4][5] However, the current data on the protonations of strained hydrocarbons are rather controversial, and facile rearrangements are thought to account for the difficulties encountered in measuring the PAs.[6] Cyclopropane (strain energy(E str ) = 27.2 kcal mol À1 ) is an exception because the number of energetic downhill paths is very limited and the barrier for the ring opening of corner-protonated C 3 H 7 + to the 2-propyl cation is high. The experimental PA of cyclopropane (179.4 kcal mol À1 [7] ) is well reproduced computationally (179.3 kcal mol À1 at CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVDZ for corner-protonated C 3 H 7 + ).[8] In contrast to cyclopropane which displays secondary CÀH bonds only, a surprisingly small range of PAs is characteristic for a number of hydrocarbons with tertiary C À H bonds. These hydrocarbons differ considerably in their strain energies, for example, dodecahedrane (PA = 201.7 kcal mol À1 , [9] E str = 65.4 kcal mol), adamantane (PA = 175.7 kcal mol À1 , [11] E str = 6.3 kcal mol À1 [12] ), and cubane [13] (PA = ca 200 kcal mol À1 , [7,9] E str = 161 kcal mol À1 [14] ). For these systems the question remains: How much does the strain energy affect the proton affinity of a saturated hydrocarbon and to what extent should isomerizations be considered? The experimental PA of cubane (1) measured by ion-cyclotron resonance spectroscopy [9] is surprisingly low and was recently questioned by Koppel et al. [15] on the basis that 1 rearranges rapidly to cuneane so that the PA is attributed to the energy change of the entire protonation/rearrangement/deprotonation process. However, cuneane is also highly strained, so where one does stop? Are there any other rearrangement paths for protonated cubane? Herein we address both computationally and experimentally the question to what extent the cubane cage is able to survive electrophilic attack [16] and what the most favorable paths for the reaction of 1 with a proton are.Despite enormous strain, cubane [15,17,18] is kinetically quite stable because breaking just one CÀC bond causes only minor structural changes and hence only little relief of strain. However, cleavage of a second CÀC bond is highly exothermic and followed by rapid rearrangements. For instance, thermolysis [16,19] and single-electron oxidation [20] of 1 leads to cyclooctatetraene. In contrast, mild radical reagents [21] lead to substitution products with conservation of the cubane cage. [22] Electrophilic cubane conversions are limited to reactions of soft electrophiles like metal ions, [18] and a number of fascinating rearrangements (e.g., to syn-tricyclooctadiene (2) with Pd 2+ and to cuneane (3) with Ag + and Li + ) are preparatively useful. [18,23] The attack of a proton on cubane i...

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confidence: 94%

The Protonation of Cubane Revisited

et al. 2004

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“…Lithium salts are also used in conjunction with aprotic dipolar solvents for cellulose dissolution and as reaction media [6][7][8][9][10]. Further examples of condensed-phase applications can be found in the domain of catalysis by weakly coordinated lithium cation [11][12][13][14][15] and hydrogen storage [16][17][18][19][20]. Analytical mass spectrometry makes use of the formation of gas-phase adducts with metal cations (a process often called cationization), and Li + has been extensively examined in this regard.…”

Section: Introductionmentioning

confidence: 99%

Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

Mayeux

Burk

Gal

et al. 2014

J. Am. Soc. Mass Spectrom.

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According to high level calculations, the upper part of the previously published FT-ICR lithium cation basicity (LiCB at 373 K) scale appeared to be biased by a systematic downward shift. The purpose of this work was to determine the source of this systematic difference. New experimental LiCB values at 373 K have been measured for 31 ligands by proton-transfer equilibrium techniques, ranging from tetrahydrofuran (137.2 kJ mol(-1)) to 1,2-dimethoxyethane (202.7 kJ mol(-1)). The relative basicities (ΔLiCB) were included in a single self-consistent ladder anchored to the absolute LiCB value of pyridine (146.7 kJ mol(-1)). This new LiCB scale exhibits a good agreement with theoretical values obtained at G2(MP2) level. By means of kinetic modeling, it was also shown that equilibrium measurements can be performed in spite of the formation of Li(+) bound dimers. The key feature for achieving accurate equilibrium measurements is the ion trapping time. The potential causes of discrepancies between the new data and previous experimental measurements were analyzed. It was concluded that the disagreement essentially finds its origin in the estimation of temperature and the calibration of Cook's kinetic method.

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“…This salt is stable to >150°C. Conferring this measure of stability on an arenium ion, where traditional anions such as SbF 6 -fail to stabilize them much above dry ice temperatures, illustrates the powerful effect of choosing a carborane as counterion. 14 Oxidative stability is also noted.…”

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

2002

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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.

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LiCB₁₁Me₁₂: A Catalyst for Pericyclic Rearrangements

Cited by 81 publications

References 13 publications

The Protonation of Cubane Revisited

The Protonation of Cubane Revisited

Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

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

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LiCB11Me12: A Catalyst for Pericyclic Rearrangements

Cited by 81 publications

References 13 publications

The Protonation of Cubane Revisited

The Protonation of Cubane Revisited

Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

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

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Product

Resources

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LiCB₁₁Me₁₂: A Catalyst for Pericyclic Rearrangements

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