Comparison of the Rates of Solvolysis of Representative Norbornyl and Cyclopentyl Derivatives. A Critical Examination of Rates as a Basis for the Postulated Nonclassical Structure of Norbornyl Cations

Brown, Herbert C.; Chloupek, Frank J.; Rei, Min-Hon

doi:10.1021/ja01060a059

Cited by 14 publications

(9 citation statements)

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“…In 1949, Winstein et al [2] . proposed a C s symmetrically bridged non‐classical carbocation structure with a three‐center two‐electron bond (Scheme 1 a) for the 2NB + ; while in early 1960s, Brown [3] advocated understanding 2NB + as a pair of rapidly equilibrating classical carbocation structures (Scheme 1 b). Afterwards, vehement debates continued for around 20 years, and many great chemists were involved.…”

Section: Methodsmentioning

confidence: 99%

“…Zuschriften definitely contributed to the non-classical 2NB + ; however, when d 1 or d 2 is smaller than 1.64 , a traditional sp 3 hybridized covalent bond (2c-2e bond or s bond) is highly localized between C 1 and C 6 atoms, characterizing its classical structure. Thus, the C 1 -C 6 (and/or C 2 -C 6 ) distances can be used to distinguish non-classical and classical 2NB + .…”

Section: Angewandte Chemiementioning

confidence: 99%

“…[1] One of the most famous carbocations is 2-norbornyl cation (C 7 H 11 + , 2NB + ), since it was the first proposed non-classical carbocation, and its exact structure, being non-classical or classical form, had been controversial and hotly debated for decades. In 1949, Winstein et al [2] proposed a C s symmetrically bridged nonclassical carbocation structure with a three-center twoelectron bond (Scheme 1 a) for the 2NB + ; while in early 1960s, Brown [3] advocated understanding 2NB + as a pair of rapidly equilibrating classical carbocation structures (Scheme 1 b). Afterwards, vehement debates continued for around 20 years, and many great chemists were involved.…”

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In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

Tang

Chen

et al. 2021

Angewandte Chemie

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Carbonium ions are an important class of reaction intermediates, but their dynamic evolution is difficult to be monitored by in situ techniques under experimental conditions because of their extremely short lifetime. Probably the most famous case is 2‐norbornyl cation (2NB+): its existing form (classical or non‐classical) had been debated for decades, until the concrete proof of non‐classical geometry was achieved by X‐ray crystallographic characterization at ultra‐low temperature (40 K) and super acidic environment. However, we lack the understanding about 2NB+ at ambient conditions. Herein, by taking advantage of the confinement effect and delocalized acidic environment of zeolites, we successfully stabilized 2NB+ and unequivocally confirmed its “non‐classical” structure inside the ZSM‐5 zeolite by ab initio molecular dynamics simulations and 13C solid‐state nuclear magnetic resonance experiments. It is the first time to in situ observe the non‐classical 2NB+ without the super acidic environment at ambient temperature, which provides a new strategy to expand the carbocation chemistry.

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Section: Methodsmentioning

confidence: 99%

Section: Angewandte Chemiementioning

confidence: 99%

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confidence: 99%

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In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

Tang

Chen

et al. 2021

Angewandte Chemie

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“…Communications definitely contributed to the non-classical 2NB + ; however, when d 1 or d 2 is smaller than 1.64 , a traditional sp 3 hybridized covalent bond (2c-2e bond or s bond) is highly localized between C 1 and C 6 atoms, characterizing its classical structure. Thus, the C 1 -C 6 (and/or C 2 -C 6 ) distances can be used to distinguish non-classical and classical 2NB + .…”

Section: Angewandte Chemiementioning

confidence: 99%

In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

Tang

Chen

et al. 2021

Angew Chem Int Ed

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“…The oxidation of bicyclo[2.2.2]oct-2-ene with performic acid is known to afford bicyclo[2.2.2]-octanetrans-2,3-diol, whereas the oxidation of bicyclo[2.2.1]-hept-2-ene under the same conditions is accompanied by Wagner-Meerwein rearrangement and 1,3-hydride shift [1][2][3].…”

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Spatial arrangement of norbornanedicarboxylic acids N-arylimides

2010

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The effect of substituents on the spatial arrangement of norbornane compounds was investigated. Norbornanedicarboxylic acids N-arylimides with OCH 3 , Cl, NO 2 groups in the ortho-position of the aromatic ring form as conformational isomers with hindered rotation. The specifi c spatial isomerism is confi rmed by the quantum-chemical calculations. The oxidation of bicyclo[2.2.2]oct-2-ene with performic acid is known to afford bicyclo[2.2.2]-octanetrans-2,3-diol, whereas the oxidation of bicyclo[2.2.1]-hept-2-ene under the same conditions is accompanied by Wagner-Meerwein rearrangement and 1,3-hydride shift [1-3].The introduction into the norbornene ring of electronacceptor substituents act as a stabilizing factor fi xing the boat-like conformation of the cyclohexene fragment and thus prevents the hydride shifts and rearrangements [4,5]. In the oxidation of norbornenes containing reactive groups in the endo-position forms as a rule a new lactone ring, whereas the presence of these groups in the exo-position of the norbornene results in normal oxidation products [6,7]. This rule is conserved in the oxidation of norbornene dicarboxylic acids, their anhydrides I, II, and the nitrogen-containing derivatives with the endo-and exo-confi guration [8][9][10]. The epoxidation of exo-compounds proceeds stereoselectively with the formation of an oxirane ring of exo-confi guration, a lactone is formed from the corresponding endo-derivatives [11-13] (Scheme 1).Our investigation of the oxidation of norbornenedicarboxylic acids N-arylimides showed that independent of the endo-or exo-confi guration of the imide ring and also of the electronic nature of the substituents in the ortho-, meta, or para-positions in the N-arylimide fragment only stereoisomeric epoxy or dihydroxy derivatives formed, and no lactonization and Wagner-Meerwein rearrangement was observed [14,15]. The strong electron-acceptor imide moiety decreases the electron density of the π-bond of norbornene and diminishes the ionization of the C-O bond which is the fi rst stage in the Wagner-Meerwein rearrangement, and the low strain of the fi ve-membered imide ring and its chemical inertness prevents the lactonization (Scheme 2).The role of electronic and sterical effects and the positions (ortho-, meta-, para-) of substituents in the formation of the conformational structure of norbornane N-arylimides remains unclear.Our quantum-chemical calculations, spectral analysis, and the consideration of model structures of N-arylimides III-LXVI of bicyclo[2.2.1]hept-5-ene-endo-and -exo-2,3-dicarboxylic acid showed that when in the orthoposition of the aromatic ring were located OCH 3 , Cl, NO 2 groups the large bulk of the substituents hampered the free rotation, and as a result a stable conformational isomer of N-arylimide formed where the benzene ring at the nitrogen atom was perpendicular to the plane of the fi ve-membered heterocycle, and the N-C bond suffered elongation compared to its average length [16]. This fact is confirmed by the presence of two

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Comparison of the Rates of Solvolysis of Representative Norbornyl and Cyclopentyl Derivatives. A Critical Examination of Rates as a Basis for the Postulated Nonclassical Structure of Norbornyl Cations

Cited by 14 publications

References 1 publication

In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

Spatial arrangement of norbornanedicarboxylic acids N-arylimides

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