Ab Initio Study of the Adamantonium Cations:  the Protonated Adamantane

Esteves, Pierre M.; Alberto, Gabriel G. P.; Ramı́rez-Solı́s, A.; Mota, Cláudio J. A.

doi:10.1021/jp003431c

Cited by 18 publications

(20 citation statements)

References 14 publications

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“…The 1-adamantyl cation is a stable species 19 and can be synthesized from the 1-adamantanol and fluorosulfonic acid-antimony pentafluoride. 20 On the zeolite surface, this species can be also generated from its halide.…”

Section: R⊕ + R'-h → R-h + R'⊕mentioning

confidence: 99%

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

Oliveira¹,

Rosenbach²,

Santos³

et al. 2010

J. Braz. Chem. Soc.

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Foi realizado um estudo teórico da transferência de hidreto entre o cátion 1-adamantila e isopentano, usando a teoria do funcional da densidade em nível PBE1PBE/6-31G (d,p). Os resultados indicaram que há formação de um íon carbônio como intermediário, que é mais estável que os complexos de van der Waals entre os íons carbênios e os alcanos. Os fatores entrópicos são bastante importantes e sobrepujam o termo entálpico à temperatura ambiente.A density functional teory study of the hydride transfer between 1-adamantyl cation and isopentane has been performed at the PBE1PBE/6-31G(d,p) theoretical level. The results indicate that a carbonium ion is involved as intermediate and is more stable than the van der Waals complexes between the carbenium ions and the alkanes. Entropic factors are important and overcome the enthalpic term at room temperature. Keywords: hydride transfer, carbocations, adamantane, isopentane IntroductionIntermolecular or bimolecular hydride transfer (HT) is a crucial step in acid-catalyzed hydrocarbon transformations. This reaction is involved in the propagation of the catalytic cycle of cracking 1,2 and alkylation 3 of hydrocarbons, as well as in the formation of aromatic hydrocarbons 4 in gasoline and coke, 5,6 which are carbonaceous materials deposit over the catalyst surface that may lead to loss of activity. Conceptually, HT involves the bimolecular transfer of a hydride from an alkane to a carbenium ion (Scheme 1). R⊕ + R'-H → R-H + R'⊕ Scheme 1. Schematic representation of hydride transfer reactions.In the gas phase, HT is much slower than proton transfer 7 and shows a negative temperature dependence. [8][9][10] The reaction is believed to occur via formation of a "tight" complex, 11,12 whose structure may resemble a carbonium ion, 13 formed by the interaction of the empty p-orbital of a carbenium ion with the C-H bond of the alkane. The reaction is used 14,15 to assess the carbenium ion stability in the gas phase.In condensed phase, especially on zeolites and other solid acid catalysts, there are few studies regarding hydride transfer. 16,17 Product distribution obtained from bimolecular alkane reactions is potentially interesting for the evaluation of the HT capability of solid acids. However, the validity of the results depends on the system chosen as model reaction. Indeed, variation of the hydride acceptor seems to affect the rates of hydride transfer to a considerably greater extent, than an equal change of the thermodynamic driving force caused by variation of the hydride donor. 18 In addition, some systems involve simultaneous reactions competing with HT, and may significantly affect the interpretation of the results.The 1-adamantyl cation is a stable species 19 and can be synthesized from the 1-adamantanol and fluorosulfonic acid-antimony pentafluoride. 20 On the zeolite surface, this species can be also generated from its halide. 21 In addition de Oliveira et al. 2181 Vol. 21, No. 12, 2010 to the stabilization by sC-C bond hyperconjugation, the 1-adamantyl cation has a cage ...

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Section: R⊕ + R'-h → R-h + R'⊕mentioning

confidence: 99%

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

Oliveira¹,

Rosenbach²,

Santos³

et al. 2010

J. Braz. Chem. Soc.

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“…However, our computations [26] at this level indicate that there is a low-energy path from 5 via transition structure (TS) 6 to the lower-lying cation 7, which can subsequently rearrange to the more stable bicyclo[3.3.0]octadienyl 8 via TS 9. The discrepancies between experiment and theory are simply unacceptable and require careful scrutiny of both the high-energy part of the C 8 H 9 + potencial enery surface (PES) and the theoretical treatment.First of all, the gas-phase protonation of hydrocarbons with tertiary CÀH bonds results in weakly bound complexes (R + ···H 2 ) as shown both experimentally [2] and computationally [5] for isobutane as well as computationally for adamantane; [11] the situation for cubane is expected to be similar. Small, strained hydrocarbons like cyclopropane [8] and cyclobutane [4] form structures protonated at a CÀC bond.…”

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confidence: 93%

“…), 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?…”

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confidence: 99%

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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“…As a result the other two aligned bonds (C5OC10 and C7OC8) are elongated to 1.653 Å. Disso- ciation of 2f into 1,3-adamantanediyl dication 1b and H 2 , however, was calculated to be favored by 8.4 kcal͞mol. In a related study, Esteves et al (14) calculated the structures of protioadamantane cation by an ab initio method. The most stable structure was found to be the van der Waals complex between H 2 and 1-adamantyl cation.…”

Section: Resultsmentioning

confidence: 99%

Density functional theory study of adamantanediyl dications C₁₀H₁₄²⁺andprotio-adamantyl dications C₁₀H₁₆²⁺

Rasul

Oláh

Prakash

2004

Proc. Natl. Acad. Sci. U.S.A.

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Structures of the isomeric adamantanediyl dications C10H14 2؉ and protio-1-and protio-2-adamantyl dications C10H16 2؉ were investigated by using the density functional theory (DFT) method at the B3LYP͞6 -31G** level. Four structures, 1 b-e, were found to be minima on the potential energy surface of C10H14 2؉ . The 1,3-adamantanediyl dication 1b with two bridgehead tertiary carbocationic centers was found to be the most stable structure. On the potential energy surface of C10H16 2؉ (protonated adamantly cation), five structures, 2 b-f, were found to be minima. Each of the structure contains a two-electron, three-center bond. The COC protonated 1-adamantyl dication, 2f, was characterized as the most stable structure. 13 C NMR chemical shifts of the structures were also calculated by using gaugeincluding atomic orbital-density functional theory and gauge-including atomic orbital-self-consistent field methods.A damantane, C 10 H 16 i, has been used extensively as a model compound to investigate electrophilic reactions of saturated hydrocarbons. The unique structural features of adamantane are ideal for a systematic study of its cations, including both their structure and chemical reactivity. The tight interlocking of cyclohexane rings into the rigid, relatively strain free-chair conformation in adamantane makes it stable against deprotonation, prohibiting the easy formation of double bonds and back-side (nucleophilic or electrophilic) attack (1).Of particular interest is the bridgehead 1-adamantyl cation, which has been prepared and characterized under long-lived stable ion conditions (2). The bridgehead 1-adamantyl cation is stabilized by COC hyperconjugation. X-ray crystal structural studies of the 3,5,7-trimethyl-1-adamantyl cation demonstrates the COC hyperconjugative effect elegantly (3). A series of 2,6-disubstituted 2,6-adamantanediyl dications ii (Scheme 1) (R ϭ C 6 H 5 , c-C 3 H 5 ) were also prepared in superacid media by Prakash et al. (4). The dications were stable only with stabilizing groups such as phenyl and cyclopropyl. Attempts to generate the unsubstituted secondary dication ii (R ϭ H) were unsuccessful (4). The previously attempted preparation of the 1,3-adamantanediyl dication in superacid solutions were also unsuccessful (3).We now report density functional theory (DFT) studies (5) of the possible isomeric adamantanediyl dications C 10 H 14 2ϩ and protio-adamantyl dications C 10 H 16 2ϩ , hitherto not yet observed as persistent long-lived species. Superelectrophilic (6) COH or COC protonation of the adamantyl cation is also possible and can lead to extremely reactive protio-adamantyl dications. Previously, we have been able to show by hydrogen͞deuterium exchange experiments and theoretical calculations that the tertbutyl (7) as well as isopropyl (8) cations can undergo COH protonation in superacids to form the activated highly electrondeficient gitonic carbenium-carbonium dications protio-tertbutyl and protio-iso-propyl dications, respectively (Scheme 2). ‡

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Ab Initio Study of the Adamantonium Cations: the Protonated Adamantane

Cited by 18 publications

References 14 publications

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

The Protonation of Cubane Revisited

Density functional theory study of adamantanediyl dications C₁₀H₁₄²⁺andprotio-adamantyl dications C₁₀H₁₆²⁺

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Ab Initio Study of the Adamantonium Cations: the Protonated Adamantane

Cited by 18 publications

References 14 publications

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

The Protonation of Cubane Revisited

Density functional theory study of adamantanediyl dications C10H142+andprotio-adamantyl dications C10H162+

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Density functional theory study of adamantanediyl dications C₁₀H₁₄²⁺andprotio-adamantyl dications C₁₀H₁₆²⁺