The Stability of Bridgehead Carbocations

Abboud, José‐Luis M.; Herreros, M.; Notario, Rafael; Lomas, John S.; Mareda, Jiri; Müller, Paul; Rossier, Jean-Claude

doi:10.1021/jo990724x

Cited by 46 publications

(43 citation statements)

References 49 publications

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“…The metric parameters around the [UO 2 Cl 4 ] 2-anion are similar to other examples in the literature with cesium [17] or tetraphenylphosphonium [18] counterions. The arrangement of the dication is unremarkable.…”

Section: Methodssupporting

confidence: 71%

“…The mechanism of this reaction is unknown at present, but it was noted that UO 2 [12]. Halide abstraction by a number of uranium molecules were noted, e.g.…”

Section: Methodsmentioning

confidence: 99%

“…The reaction of tertiary amines with alkyl halides to form quaternary ammonium salts, or the Menschutkin reaction [1], has been subjected to extensive experimental [2] and theoretical [3] studies as alkyl transfer processes are important in biochemical processes. Moreover, the activation of halogen-substituted alkanes has been widely studied because of the interest in the destruction and conversion of chlorine-containing hydrochlorofluorocarbon (HCFC) analogues [4].…”

Section: Introductionmentioning

confidence: 99%

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The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂

Baker

Hashem

Motevalli

et al. 2010

Zeitschrift anorg allge chemie

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Section: Methodssupporting

confidence: 71%

“…The mechanism of this reaction is unknown at present, but it was noted that UO 2 [12]. Halide abstraction by a number of uranium molecules were noted, e.g.…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂

Baker

Hashem

Motevalli

et al. 2010

Zeitschrift anorg allge chemie

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“…18 Values of pK R and ΔΔG o f are also summarized in Table 3. 34 or bridgehead cycloalkyl carbocations, 35 and, with somewhat greater scatter, a wider structural range of cations. 37 From the figure it is clear that methyl substituent effects are also correlated between gas and solution, and that points for benzyl and methoxymethyl cations are well represented by straight lines.…”

Section: Resultsmentioning

confidence: 99%

The protonation of carbenes: structural effects on α-proton acidity of carbocations

Keeffe¹,

O’Ferrall²

2008

Arkivoc

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Ionisation constants (pK a s) are reported for proton loss from the α-carbon of carbocations forming singlet methyl, dimethyl, phenyl, phenylmethyl, diphenyl, methoxy and methoxymethyl carbenes. Equilibrium constants have been evaluated for isomerisation of the carbenes to alkenes or their reaction with water (hydration) to form alcohols based on G3 calculations and estimates of free energies of transfer from gas to solution. Combining equilibrium constants pK H2O for hydration (R 2 C: + H 2 O = R 2 CHOH) with pK R for hydrolysis of the carbocations (R 2 CH + + H 2 O = R 2 CHOH + H + ) provides the desired pK a s as pK R -pK H2O . For the methyl cation an approximate value of pK R = -43 is extrapolated from a correlation of hydride ion affinities of ethyl, isopropyl and t-butyl cations between the gas phase and solution. Based on pK H2O = -70 for H 2 C: the pK a for the methyl cation is 27. The range of equilibrium constants for different carbenes (50 log units in pK H2O ) points to wide variations in stability, in contrast to kinetic measurements which reflect a uniform lack of selectivity. Remarkably, a comparison of substituent effects on pK H2O and pK R shows that methyl and phenyl substituents stabilise the carbocation and carbene similarly but that methoxy substituents are more stabilizing for the carbene. The latter finding is consistent with calculation of a larger barrier to rotation of a methoxy group in dimethoxy carbene than in the dimethoxymethyl cation and reports that O-substituted carbenes react less rapidly with acids than diarylcarbenes. The behaviour is understandable if inductive effects of electronegative atoms complement donation of an electron pair by resonance in the case of carbenes and oppose it for carbocations. Although errors of 2-3 units are likely for individual pKs, these are mitigated by the large span of values considered and cancellations of systematic errors when structurally related values are compared.

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“…Attachment of the anionic moiety to 1 must then proceed by Menshutkin alkylation [8] forming a chemically quite stable quaternary ammonium salt as the product. Though these nucleophilic substitutions generally give good yields, the four fold reiteration of this process to furnish the tetra-substituted product from 1 might require a particularly reactive alkylation reagent for success.…”

Section: Resultsmentioning

confidence: 99%

A novel electroneutral anion host based on organoboron betaines

Cattani¹,

Schmidtchen²

1999

J. prakt. Chem.

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291The design of artificial molecular hosts for anions [1] heavily depends on the solvation conditions applied in the molecular recognition event. While in strongly solvating solvents (e.g. in water or alcohols) the accumulation of positive charge in the host may furnish a viable route for enhancing guest binding affinity and selectivity this path is generally blocked when solvents of low polarity are to be used. This is due to the low solubility of highly charged species in these solvents as well as to the tendency to form ion pairs with whatever anion is present thereby limiting the attainable selectivity. The shining goal of maximum discriminatory power requires a well balanced design of attractive and repulsive interactions in the host-guest complex. In principle a satisfactory host design will be more readily accomplished the larger the individual energetic contributions are (Coulombic-, van der Waals-, solvophobic-, hydrogen bonding-interaction, etc.) in an absolute sense. For this reason strong and far reaching Coulombic attractions resting on full positive charges in the host may be preferable over weaker dipole interactions like hydrogen bonding. Making use of this notion, however, mandates a host design that enables a unique guest binding mode, ensures high charge density, and avoids its annihilation by inter-or intramolecular unspecific ion pairing. On top all charged sites must be readily solvated to allow sufficient solubility in less polar solvents.Good candidates to meet these requirements are zwitterionic compounds built on the tetrahedral macrocyclic core structure 1. Cationic derivatives of this cage compound were shown to bind a variety of simple anions in protic solutions [2, 3] Abstract. Host-guest binding of anions in organic solvents may find attractive applications in ion sensing and transport, if the host can extract the negatively charged guest species from its solvation sphere. The most powerful means to dismantle the anionic guest from solvent molecules that is required for selective binding is to provide positive charges in the host compound. They have to be counterbalanced by negatively charged subsites in order to conserve electroneutrality, electroneutral borane (BH 3 ) adduct of 1 exploited the attraction of guest anions to a fixed array of dipoles to allow selective anion binding in apolar solvents [5]. An extension of the original idea was the attachment of anionic sites to the macrotricycle [6] to give the zwitterionic compound 2 having a zero overall charge. The but must avoid collapse into ion pairing with the cationic sites. According to this concept the cage compound 3 was constructed by attachment of anionic tetraphenylborate moieties to the parent macrotricyclic amine 1. The betainic product 3 was shown to be soluble in water-immiscible dipolar organic solvents like propylene carbonate and formed inclusion complexes with chloride and bromide in the gas phase as well as in solution.

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The Stability of Bridgehead Carbocations

Cited by 46 publications

References 49 publications

The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂

The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂

The protonation of carbenes: structural effects on α-proton acidity of carbocations

A novel electroneutral anion host based on organoboron betaines

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The Stability of Bridgehead Carbocations

Cited by 46 publications

References 49 publications

The Coupling of Pyridine and Dichloromethane Mediated by UO2Cl2

The Coupling of Pyridine and Dichloromethane Mediated by UO2Cl2

The protonation of carbenes: structural effects on α-proton acidity of carbocations

A novel electroneutral anion host based on organoboron betaines

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Product

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The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂

The Coupling of Pyridine and Dichloromethane Mediated by UO₂Cl₂