Nonclassical cation

Thiazine has been formulated as 1 rather than 2 because it does not form a sulfonamide under Hinsberg conditions. Symmetrical azines can form only one classical monocation, e.g., pyrazine forms 3. The nonclassical cation 4 has been postulated for pyridazine, but there is no compelling evidence in its favor. 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

This approach has been applied to the norbornyl cation, where the NMR even at very low temperatures showed equivalent NMR signals for Cl and C2. The monodeuterated 26 was prepared, which showed a difference of only 2 ppm in the signals for Cl and C2. This small splitting is inconsistent with rapidly equilibrating ions but is expected for the symmetrical nonclassical cation. 

Heilbroner and co-workers found the UV spectra of A-protonated 1,2-diazocine 8 and M-protonated (Z)-azobenzene were very similar. This and other spectroscopic and chemical evidence indicated that upon protonation (Z)- and ( )-azobenzene retain their configuration and are classical rather than bridged or nonclassical cations (60HCA1890). Haselbach and co-workers studied protonated 8 (and other azo compounds) using electron spectroscopy for chemical analysis (ESCA) and concluded that the cation has classical structure 69 as opposed to a nonclassical structure (70) (72HCA705). 

Bicyclo[2.2.1]heptane (norbomane) and bicyclo[2.2.2]octane, when treated with nitronium tetrafluoroborate in nitrile-free nitroethane, unexpectedly gave no nitro products. Instead, only bicyclo[2.2.1]heptane-2-one and bicyclo[2.2.2]octan-l-ol were isolated, respectively.500 Observation of bicyclo[2.2.1]heptane-2-yl nitrite as an intermediate and additional information led to the suggestion of the mechanism depicted in Scheme 5.48. In the transformation of norbomane the first intermediates are the 2-norbornyl cation 126 formed by hydride abstraction and nonclassical cation 127 formed through insertion of N02+ into the secondary C—H bond. In the case of bicyclo [2.2.2]octane, the oxidation of bridgehead tertiary C—H bond takes place and no further transformation can occur under the reaction conditions. Again these electrophilic oxygenation reactions testify to the ambident character of the nitronium ion. 

For example, acetolysis of exo-2-norbornyl brosylate 254 produces exclusively exo-2-norbornyl acetate 255. The exo-brosylate 254 is more reactive than the endo-brosylate 256 by a factor of 350 and the acetolysis of optically active exo-brosyl ate gave completely racemic exo-acetate 255. Thus, the carbonium ion produced from exo-254 is more rapidly (thus more easily) formed than that from endo-256. These results were originally rationalized in term of a bridged (nonclassical) cation 257 (Winstein approach) (97) or as the rapidly equilibrating classical carbonium ions 258 and 259 (Brown approach (98, 99)). 

Fig. 7.9 The classical/nonclassical 2-norbornyl cation problem. Grey a pair of rapidly equilibrating classical cations with a nonclassical, bridged transition structure black the nonclassical cation as the minimum cation. The fully MP2/6-31G(d) optimised 2-norbornyl cations are depicted the nonclassical ion is 13.6 kcal mofi1 more stable at this and comparable levels of theory.

See S. Weininger. What s in a name From designation to denunciation - the nonclassical cation controversy," Bulletin for the History of Chemistry 25 (2000) 123-131 and references therein C. Walling, "An innocent bystander looks at the 2-norbornyl cation, Accounts of Chemical Research 16 (1983) 448-454. 

Thus, it seems, there are such things as nonclassical cations. What is still to be settled is just how much they are involved in the chemistry of ordinary solvo lytic reactions. 

Winstein proposed that the much faster rate for the solvolysis of the exo-nor-bornyl brosylate (11) was the result of participation by the 1,6-bonding pair in the transition state, leading to the formation of a a-bridged (nonclassical) cation intermediate (15) (7). The nonclassical cation 15 possesses a plane of symmetry. Consequently, the acetate product, 12, must be optically inactive. 

The Goering-Schewene diagram (Fig. 2) establishes that the two transition states differ in energy by 5.8 kcal mol-1. If this is the result of nonclassical stabilization of the exo transition state, the fully developed nonclassical cation should be more stable than the classical ion by a quantity significantly larger than 5.8 kcal mol"1. 

Participation by remote double bonds has often been adressed as the tt route to a-bridged ( nonclassical ) cations. Detailed examination of the intermediates is deferred to Section 7.6 where the generation of such species from other sources is discussed. 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

Weininger, S.J. 2000, What s in a Name From Designation to Denunciation - The Nonclassical Cation Controversy , Bulletin for the History of Chemistry, 25,123-31. 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

Although this, as Gassman rightly points out, is no argument that in the latter case a classical 2-norbomyl cation is formed as well, the high exoiendo rate ratio or the exclusive formation of exo products is not yet an absolute criterion for the formation of a nonclassical cation in 2-norbomyl tosylate solvolysis. 

On the basis of all the data obtained Olah has ascribed to the 2-norbomyl cation the structure of a comer-protonated nonclassical cation. The bond nature can be best described with a two-electron three-centre molecular orbital (2e3C) ... 

One of the first representatives of homoallylic ions of the nonhoraoaromatic type — ion 396 — was generated by Kamschei et al. The spectral of this ion and a comparison with model compounds, as well as the temperature independent NMR spectra character support the nonclassical cation to predominate over a superrapid equilibrium of two classical ions. 

Carbonium ions Compounds containing high coordinate carbon bearing a positive charge with multicenter bonding. Also called nonclassical cations. 

Camphene is found in higher concentrations in the essential oils of common coniferous trees, for example, Abies alba or Tetraclinis articulata, in the rhizome of Zingiber officinale as well as in S. officinalis and R. officinalis (Bornscheuer et al 2014). There is only one publication on various biotransformation products in the urine of rabbits after its oral administration. As shown in Figure 9.2, camphene is metabolized into two diastereomeric glycols (camphene-2,10-glycols). Their formation obviously involves two isomeric epoxide intermediates, which are hydrated by epoxide hydrolase. The monohydroxylated camphene and tricyclene derivatives were apparently formed through the nonclassical cation intermediate (Ishida et al., 1979). So far, there are no studies available about the biotransformation of camphene in humans. 

Compounds 3 and 4 are structurally related to 1 and it has been observed experimentally that exo-triflates 4 react three to four-times more rapidly flian endo-triflates 3. These two arguments could be invoked to suggest the involvement of a nonclassical carbocation in the solvolysis process of both compounds, as we have discussed previously for 1. This time, nonclassical cation 10 should be formed respectively by direct ionization of cntfo-triflates 3 and by assisted departure of the OTf group in the case of exo-isomers 4. The attack of flie nucleophile in 10 should finally lead to the solvolysis compounds 11 and 12 (Scheme 9.5). 

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