Stabilization energy and ion size in carbocations in the gas phase.

Lossing, F. P.; Holmes, John L.

doi:10.1021/ja00335a008

Cited by 132 publications

(75 citation statements)

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“…If this is true, S N 1 and S N 2 rate profiles should merely reflect the hydride affinities of cycloalkylcarbenium ions (i.e., the C ´´´HÀ heterolytic dissociation energies of cycloalkanes). Such thermodynamic quantities, though readily available for bicyclic structures [9] [10], have never been established systematically for cycloalkanes [11] [12]. This obligated us to calculate these hydride affinities using ab initio methods (see Exper.…”

Section: Results ± the Benzenethiolate (Phsmentioning

confidence: 99%

The Effect of Ring Size on Reactivity: The Diagnostic Value of ‘Rate Profiles’

Masson

Leroux²

2005

Helvetica Chimica Acta

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Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthdayThe rates of cycloalkyl phenyl sulfide formation of a series of homologous bromocycloalkanes upon treatment with sodium benzenethiolate have been determined to ascertain the effect of ring size on reactivity. The rate profile, i.e., reaction rate vs. ring size, for these nucleophilic substitutions (S N 2) was determined. A linear free-energy relationship could be derived from computed hydride affinities of cycloalkanes and rates of typical S N 1 reactions, whereas rates of S N 2 reactions exhibited a strong discrepancy from the seven-up to the twelve-membered rings. This discrepancy was rationalized by a careful examination of the geometry of the intermediates and transition states involved in these reactions.1. Introduction. ± In the early fifties, Brown et al. [1 ± 3] recognized characteristic effects of ring size on the reactivity of a series of model substrates. They suggested to use plots of rates vs. ring size, so called rate profiles, as tools to diagnose mechanistic details, in particular the geometry of key intermediates or transition states. According to this concept, first-and second-order substitution reactions (S N 1 and S N 2, resp.) should exhibit like ring dependences. As both processes involve a planarized C-atom as a high-energy species, the principal reactivity-controlling factor should be the extra strain (internal strain or I-strain) caused by the change of a tetragonal to a trigonal C-center.This simplified view raised objections by Sicher [4]. When comparing the rate profiles of two archetypical S N 1 reactions (Fig. 1a, b) [3] [5] with intramolecular ( Fig. 1,c) [6] and intermolecular (Fig. 1,d) [5] S N 2 displacements, he spotted significant discrepancies in the C 8 ± C 15 range. This comparison was flawed by two experimental shortcomings, however. On one hand, the kinetic work monitored only the consumption of the substrate, but did not rigorously quantify the products formed. Thus, important side reactions may have remained undetected. On the other hand, the juxtaposition was not continued into the area of the smallest rings, cyclobutyl and cyclopropyl derivatives. Such an extension should be especially meaningful if one considers the coincidence of five-, six-, and seven-membered-ring reactivities (Fig. 1). We, thus, set out to bridge these gaps and to propose a detailed and comprehensive justification of the measured rate constants.

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Section: Results ± the Benzenethiolate (Phsmentioning

confidence: 99%

The Effect of Ring Size on Reactivity: The Diagnostic Value of ‘Rate Profiles’

Masson

Leroux²

2005

Helvetica Chimica Acta

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“…17 The tertbutyl cation is stable and is characterized by X-ray diffraction. 18 The high Lewis acidity of the cationic [CCH] subunit and high stability of the resulting tert-butyl cation enhanced heterolytic cleavage of the carbonnitrogen bond.…”

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confidence: 99%

Activation of Isocyanides and Hydrosilanes by a Cationic [CCH] Subunit in a Tetrairon–Tetracarbon Cluster

Taniwaki

Ohta

Okazaki

2015

Bulletin of the Chemical Society of Japan

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The reactivity of [(η5-C5H4Me)4Fe4(HCCH)(HCC-NC4H4N)](PF6)2 (1) toward isocyanides and hydrosilanes was examined. The reaction of 1 with tert-butyl isocyanide led to C(sp3)-N(sp) bond cleavage to give [(η5-C5H4Me)4Fe4(HCCH)(HCC-CN)](PF6) (2), 1-tert-butylpyrazinium salt, and isobutene. In the reactions with cyclohexyl and benzyl isocyanides, isocyanide adducts [(η5-C5H4Me)4Fe4(HCCH)(HCC-CNR)](PF6)2 (R = Cy (5), CH2Ph (6)) were obtained and further transformed to 2 (70 °C, 10 h for 5; RT, 120 h for 6). The reactions of 1 with aryl isocyanides such as 2,6-dimethylphenyl and 2,4,6-tri-tert-butylphenyl isocyanides yielded thermally stable [(η5-C5H4Me)4Fe4(HCCH)(HCC-CNAr)](PF6)2 (Ar = 2,6-Me2C6H3 (8), 2,4,6-tBu3C6H2 (9)). The Si–H bonds of R3SiH (R = Et, Ph), Ph2SiH2, and PhSiH3 were activated to give [(η5-C5H4Me)4Fe4(HCCH)2](PF6) together with the corresponding fluorosilanes. Coordination of the substrates with a cationic [CCH] subunit promoted the cleavage of the carbon–nitrogen and silicon–hydrogen bonds to give the products.

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“…This observation is only consistent with a comparable efficiency of the two competing processes analogous to [4a,b]. In these cases, in fact, an 0-phenylated propene oxide intermediate (14) is involved, which isomerizes exclusively to the protonated 3-methyldihydrobenzofuran structure (IS), reaction [6a], the alternative isomerization pathway leading to 2-methyldihydrobenzofuran being essentially prevented. Following the previous conclusion concerning isomerization of 7, this observation is consistent with an isomerization transition state wherein a partial positive charge is developed at the side-chain carbon center before significant interaction with the phenyl ring n orbital is established.…”

Section: Discussionmentioning

confidence: 73%

Arylation of cyclic ethers by gaseous phenylium ions. Formation and behavior of phenoxenium ions in the gas phase

Fornarini¹,

Speranza²

1988

Can. J. Chem.

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SIMONETTA FORNARINI and MAUR~ZIO SPERANZA. Can. J. Chem. 66, 2506Chem. 66, (1988. Free, unsolvated phenylium ions formed by the spontaneous decay of 1,4-ditritiobenzene have been allowed to react with gaseous cyclic ethers (oxirane, propene oxide, and oxetane) and acetaldehyde in the pressure range 30-250 Torr and in the presence of a thermal radical scavenger (02, 4 Torr). The effects of a gaseous base (NMe3, 20 Ton) and of an energy moderator (He, 630-720 Torr) were also investigated. Phenylium ion confirms its considerable site selectivity, demonstrated by the distinct preference for the n-type center of the substrate, although appreciable insertion into the carbocyclic structure of propene oxide and oxetane is observed as well. The stability features of the ionic intermediates from addition of phenylium ion to selected substrates have been evaluated as well as their fragmentation and isomerization mechanisms. The behavior of gaseous phenylium ion toward cyclic ethers, in particular its ability to formally abstract an oxygen atom from the ether to give the phenoxenium ion, a reaction first observed in the present study, is discussed and compared with previous mechanistic investigations carried out in the gas phase and in solution. (He,. L'ion phCnylium confirme sa haute sClectivitC qui est dCmontrCe par sa prCfCrence marquCe pour le centre de type n du substrat m&me si on observe aussi une quantitC appreciable d'insertion dans la structure carbocyclique de l'oxyde de propene et de 1'oxCtanne. On a CvaluC les caracteristiques de stabilitC ainsi que les mCcanismes de fragmentation et dlisomCrisation des intermediaires ioniques obtenus par l'addition de l'ion phCnylium sur des substrats choisis. On discute du comportement de l'ion phCnylium gazeux vis-a-vis des Cthers cycliques, en particulier de sa facilitC d'arracher formellement un atome d'oxygene de 1'Cther pour donner l'ion phCnoxCnium, une reaction qui a CtC observee pour la premiere fois dans cette Ctude; on compare ce comportement avec celui observe dans des Ctudes mkcanistiques rapportCes antkrieurement et effectuees soit en phase gazeuse soit en solution.[Traduit par la revue]

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Stabilization energy and ion size in carbocations in the gas phase.

Cited by 132 publications

References 0 publications

The Effect of Ring Size on Reactivity: The Diagnostic Value of ‘Rate Profiles’

The Effect of Ring Size on Reactivity: The Diagnostic Value of ‘Rate Profiles’

Activation of Isocyanides and Hydrosilanes by a Cationic [CCH] Subunit in a Tetrairon–Tetracarbon Cluster

Arylation of cyclic ethers by gaseous phenylium ions. Formation and behavior of phenoxenium ions in the gas phase

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