A predictive model for unimolecular reactions: Reactions of unsaturated carbonium ions

The major slow unimolecular reactions undergone by C₄H₇⁺, C₅H₉⁺ and C₆H⁺₁₁ are discussed in terms of a potential surface approach and the organic chemist's concept of mechanism. It is shown that the observed decompositions which do not involve σ-bond formation in the dissociation step are precisely those expected from the model. Further use of the model correctly predicts the slow reactions of C₇H⁺₁₃ which have not previously been reported. The approach also permits useful limits to be set on the transition state energies for reactions involving σ-bond formation in the dissociation step (H₂,CH₄ loss). It is concluded that stepwise addition of ethylene to the allyl cation is preferred to a concerted 4-electron process which is symmetry forbidden.     

Reaction of tetraalkoxysilanes with stable carbonium ions

Conclusions Tetramethoxy-, tetraethoxy-, and tetra-n-propoxysilanes split out an alkoxy group under the influence of such stable carbonium ions as the triphenylmethyl and tropylium carbonium ions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 678–680, March, 1972.     

Theoretical study of the structure and stability of cage-substituted octahedral boranes,alanes, and gallanes

The equilibrium geometric parameters and energetic and spectroscopic characteristics of low-lying isomers for a series of successively cage-substituted octahedral closo-borane, alane, and gallane dianions Al_{6 − i} B _i H₆²⁻ and Ga_{6 − i} Al _i H₆²⁻ (i = 0−6) and mono- and disubstituted anions and neutral molecules of boranes, XB₅H₆⁻ and XYB₄H₆ and alanes, XAl₅H₆⁻ and XYAl₄H₆, with the same or different cage heteroatoms of Group IV elements (X and Y = C, Si, Ge, Ti) were calculated at the B3LYP level of the density functional theory using the 6-31G* and 6-311+G** basis sets. Differences in structure and stability between borane and alane clusters of like composition are revealed. The mutual influence of the X and Y heteroatoms in the trans and cis isomers and its manifestations in the behavior of molecular characteristics are considered for the disubstituted octahedral clusters XYB₄H₆ and XYAl₄H₆.     

The electronic structure of carbonium ions

A large number of carbonium ions are calculated with a modified CNDO procedure. The study of the electronic structure of these electron deficient compounds using energy partitioning and localized orbital techniques leads to an understanding of the observed trends of the stabilities of carbonium ions. The stabilization of a positive charge by methyl substituents is broken down into hyperconjugative, hybridization and inductive effects. Nonclassical structures characterized by three center bonds are also investigated. The chemical reactivity of carbonium ions is explained by the ability of the empty orbital to combine steadily with bond orbitals.

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Zusammenfassung Ein modifiziertes CNDO-Verfahren wurde auf eine große Zahl von Carboniumionen angewandt. Die Untersuchung der elektronischen Struktur dieser Elektronenmangel-Verbindungen mit einem Verfahren zur Energieaufteilung und mit lokalisierten Orbitalen führen zum Verständnis der beobachteten Trends in der Stabilität von Carboniumionen. Die Stabilisierung einer positiven Ladung durch Methylsubstituenten wird aufgeteilt in Beiträge der Hyperkonjugation, der Hybridisierung und des induktiven Effekts. Außerdem wurden nichtklassische Strukturen, charakterisiert durch eine Dreizentrenbindung, untersucht. Die chemische Reaktivität von Carboniumionen wird durch die Fähigkeit des leeren Orbitals erklärt, mit einem Bindungsorbital zu kombinieren.

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Résumé Un procédé CNDO modifié est utilisé pour le calcul d'un grand nombre d'ions carbonium. L'étude de la structure électronique de ces composés à déficience d'électrons en utilisant des techniques de partition de l'énergie et d'orbitales localisées conduit à la compréhension des tendances observées dans la stabilité des ions carbonium. La stabilisation d'une charge positive par les substituants méthyles est décomposée en effets d'hyperconjugaison, d'hybridation et d'induction. Les structures non classiques caractérisées par des liaisons à trois centres sont aussi étudiées. La réactivité chimique des ions carbonium est expliquée par la capacité de l'orbitale vide à se lier fortement avec des orbitales de liaison.

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The calculations were performed on a CDC 3300 computer using a FORTRAN program based on Pople and Segal's CNDO/2 program. Our program includes sections for minimizing the total energy with respect to the geometry, approximating reaction coordinates, transforming the canonical orbitals to localized orbitals, and calculating the energy matrices containing the terms of the energy partitioning.     

Quantitative treatment of the effect of counterions on ctab-catalyzed reactions of stabilized carbonium ions with cyanide ion

The reactions of stabilized carbonium ions of setoglaucin, methyl violet, and ethyl violet with cyanide ions are largely catalyzed by the cationic micelles of cetyltrimethylammonium bromide (CTAB) in aqueous media. Added counterions (anions in this case) have strong inhibitory effects on the CTAB-catalyzed reactions in the following order: N > NO > Br^? > Cl^? > F^? > no salt. The inhibitory effects of the counterions have been attributed to the exchange between added anions and reagent (CN^?) in the micellar media. The data have been analyzed by the model schemes, and mathematical formulations were developed. Various parameters associated with the exchange process, such as equilibrium exchange constant, number of surfactant molecules per substrate molecule, number of added anions, and a factor related to the binding of additives to the catalytic micellar aggregates, have been evaluated.     

A carbon-13 NMR spectroscopic study of ferrocenylmethyl carbonium ions

The ¹³C NMR spectra of three ferrocenylmethyl carbonium ions and of the alcohols from which they were derived have been obtained. The chemical shifts and coupling constants of the carbonium ions are explicable in terms of a structure that has the positive charge extensively delocalized throughout the entire molecule, with a fulvenelike structure for the substituted cyclopentadienyl ring.     

Synthesis and reactions of N-heterocyclic carbene boranes

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit "borane chemistry", but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.     

Electronic nature of carbonium ions and their silicon analogues

Nonclassical ions or carbonium ions have multi-center bonding from delocalized sigma or pi electrons. The 2-norbornyl cation, its derivative 6,6-difluoro-2-norbornyl cation, tris-homocyclopropenyl cation, 7-norbornenyl cation, and 4-cyclopentenyl cation and their corresponding silicon analogues were studied in this work. All carbocations have topologically different 3c-2e systems. The magnitude of all delocalization indexes between each atomic pair of the 3c-2e bond can be used to predict homoaromaticity. The silicon analogues have a topologically different 3c-2e bond from their corresponding carbocation.     

Strong dissimilarities between the gas-phase acidities of saturated and alpha,beta-unsaturated boranes and the corresponding alanes and gallanes

The effect that unsaturation has on the intrinsic acidity of boranes, alanes, and gallanes, was analyzed by B3 LYP and CCSD(T)/6-311+G(3df,2p) calculations on methyl-, ethyl-, vinyl-, and ethynylboranes, -alanes and -gallanes, and on the corresponding hydrides XH3. Quite unexpectedly, methylborane, which behaves as a carbon acid, is predicted to have an intrinsic acidity almost 200 kJ mol(-1) stronger than BH3, reflecting the large reinforcement of the C--B bond, which upon deprotonation becomes a double bond through the donation of the lone pair created on the carbon atom into the empty p orbital of the boron. Also unexpectedly, and for the same reason, the saturated and alpha,beta-unsaturated boranes are much stronger acids than the corresponding hydrocarbons, in spite of being carbon acids as well. The Al derivatives also behave as carbon acids, but in this case the most favorable deprotonation process occurs at C beta, leading to the formation of rather stable three-membered rings, again through the donation of the C beta lone pair into the empty p orbital of Al. For Ga-containing compounds the deprotonation of the GaH2 group is the most favorable process. Therefore only Ga derivatives behave similarly to the analogues of Groups 14, 15, and 16 of the periodic table, and the saturated derivatives exhibit a weaker acidity than the unsaturated ones. Within Group 13, boranes are stronger acids than alanes and gallanes. For ethyl and vinyl derivatives, alanes are stronger acids than gallanes. We have shown, for the first time, that acidity enhancement for primary heterocompounds is not only dictated by the position of the heteroatom in the periodic table and the nature of the substituent, but also by the bonding rearrangements triggered by the deprotonation of the neutral acid.     

On the coexistence of the “reactivity-selectivity” and the “constant selectivity” relationships in reactions of carbonium ions with nucleophiles

It is noted that the “reactivity-selectivity” and the “constant selectivity” relationships for the R⁺ + Nu reactions can coexist if diffusion-controlled rates for one nucleophile govern the behaviour at the reactivity-selectivity region.     

Ethynyl carbonium ions. 3. Diene synthesis with 4-phenylethynylpyrylium salts

Norbornadiene derivatives of pyrylium salts were obtained for the first time by [4+2] cycloaddition of cyclopentadienes and 2,6-diaryl-4-phenylethynylpyrylium salts with and without methyl groups in the 3 and 5 positions. Their photochemical transformations were studied.See [1] for Communication 2.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 307–311, March, 1990.The authors thank M. I. Knyazhanskii for discussing the photochemical results.     

A PMR investigation of organic cations—II: Mixed p-tolyl-thienyl carbonium ions

The PMR spectra of isomeric p-tolyl-thienyl carbonium ions and the corresponding carbinols were studied in order to compare the relative abilities of 2-thienyl, 3-thienyl and p-tolyl groups to stabilize a carbonium ion. The low field shifts in the ions relative to the neutral molecules are discussed in terms of positive charge delocalisation, as seen by a comparison with calculated π charge densities. Additionally, vicinal coupling constants fit a correlation with π bond order, but this applies only to the thienyl groups in the ionic species.     

A voltammetric study of several oxygen,sulfur and selenium heterocyclic carbonium ions in acetonitrile

Cyclic voltammetric data are reported on seven new oxygen, sulfur and selenium heterocyclic carbonium ions. Each compound undergoes electrochemical reduction to form a dimer. The reaction mechanisms are discussed. The compounds with selenoethoxy substituents formed heterofulvalenes at room temperature.     

A 13C nuclear magnetic resonance study of cyclobutadieneirontricarbonyl-substituted carbonium ions

The ¹³C NMR spectra of (hydroxymethyl)-, (α-hydroxyethyl)- and (α-hydroxybenzyl)cyclobutadieneiron tricarbonyl (in CDCl₃) and of their derived carbonium ions (in concentrated H₂SO₄) are reported. The data suggest extensive donation of electron density from the metal, via the cyclobutadiene unit, to the exocyclic, electron-deficient carbon atom, thus giving highly stabilized carbonium ions.