Cations cyclopropylcarbinyl

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

A closely related stable cation which also exhibits a strong conformational preference is cyclopropylcarbinyl cation. 

Display electrostatic potential maps for both bisected and perpendicular conformers of cyclopropylcarbinyl cation. For which is the charge more delocalized Is the more delocalized cation also the lower-energy cation ... 

The cyclopropylcarbinyl cation is characterized, like the previous molecules, by the existence of a high labile 2p orbital which can interact with orbitals of appropriate symmetry in the ring. We can single out the important interactions in the two extreme configura-... 

Cyclopropylcarbinyl cation, 48 bisected, 35, 206 bond lengths, 37 perpendicular, 35, 208 rotational barrier, 35... 

The various carbenium ions /erf-alkyl, bridgehead-, norbornyl-, allyl-, benzyl- or cyclopropylcarbinyl-cations, which are assumed to be intermediates in these decarboxylations are compiled in ref. [293]. 

A cyclopropylcarbinyl cation can be trapped to form either a cyclopropane product (Path a, Eq. 3) or a homoallyl product (Path b, Eq. 3). The latter has proven useful to create acyclic units containing olefins of defined geometry as in the synthesis... 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

The crystal structures of several different cyclopropylcarbinyl cations, each with a hydroxy function on the carbinyl carbon, have been reported59"61. Of these structures some five represent relatively simple systems and in each of these cases a bisected or close to... 

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