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Cyclopropylcarbinyl cations static

The tertiary 2-methyl-8,9-dehydro-2-adamantyl cation 25, R = Me is stable up to 10 °C, and shows a much deshielded absorption for the cationic center (<5I3C 274.4), which is relatively shielded compared with the 2-methyl-2-adamantyl cation (<513C 323) showing some charge delocalization into the cyclopropane ring. The C8 and C9 carbons are also deshielded (<5I3C 100.7), comparable to other static cyclopropylcarbinyl cations. [Pg.827]

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. [Pg.837]

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). [Pg.838]

The parent 3-homonortricyclyl cation [147] underwent three-fold degenerate rearrangements in superacids, as shown by its temperature dependent nmr spectra, but only at higher temperatures (—85°C to 20 C) than for the corresponding dehydrohomoadamantyl [144 R = H] and dehydroadamantyl cations [126 X = H], The lower rearrangement rate of [147] was explained by a less favourable formation of the puckered cyclobutyl cation intermediates [159] in this geometrically more constrained system. The assignment of a symmetrical cyclopropylcarbinyl cationic structure to [147] was confirmed by comparison of its and C-nmr spectra with the static counterparts [157]. [Pg.281]

Considerable support for Roberts suggestion has come from later studies, notably those involving NMR studies in superacid media. Thus, while the spectrum of the a,a-dimethylcyclopropylcarbinyl cation is best rationalized in terms of a classical structure that for the parent system is consistent with neither static nor rapidly equilibrating classical structures The parent system seems best to be described as a mixture of the equilibrating bicyclobutonium ions and, less than 4 kcal mol higher in energy another structure that might be the classical cyclopropylcarbinyl cation ... [Pg.1069]

Cyclobutyl-Cyclopropylmethyl-Allylmethyl Systems. The long-accepted picture that interconversions amongst the cations in this system take place via a rapid equilibrium of classical structures has recently been criticized by Olah. He has argued, on the basis of n.m.r. evidence, in favour of non-classical a-delocalized structures for primary cyclopropylcarbinyl cations, although recognizing that localized, and even static, structures are involved in secondary and tertiary cases, and especially in the 8,9-dehydro-2-adamantyl series. Hehre and Hiberty have now questioned the (T-delocalized picture of the parent ion presented by Olah, and provide ab... [Pg.158]


See other pages where Cyclopropylcarbinyl cations static is mentioned: [Pg.836]    [Pg.838]    [Pg.113]    [Pg.836]    [Pg.274]    [Pg.276]    [Pg.279]    [Pg.281]    [Pg.186]    [Pg.158]    [Pg.816]    [Pg.824]    [Pg.816]    [Pg.824]    [Pg.141]    [Pg.254]   
See also in sourсe #XX -- [ Pg.824 , Pg.825 , Pg.826 , Pg.827 , Pg.828 , Pg.829 , Pg.830 , Pg.831 , Pg.832 , Pg.833 ]

See also in sourсe #XX -- [ Pg.824 , Pg.825 , Pg.826 , Pg.827 , Pg.828 , Pg.829 , Pg.830 , Pg.831 , Pg.832 , Pg.833 ]




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Cyclopropylcarbinyl

Cyclopropylcarbinyl cation

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