Cyclohexenyl cations, rearrangement

Sorensen s group has investigated ways in which alkyl groups may migrate in cyclohexenyl cations. Rearrangement of (249) to (250) has been noted in the low-temperature n.m.r. spectra by observation of the C-5 methyl peak at... 

Attempts have been made to generate a number of 2-bicyclo[ .l. OJalkyl cations under stable ion conditions55. Ionization of bicyclo[3.1.0]hexan-2-ol 57 in SbF5/S02ClF even at -140 °C gave the rearranged cyclohexenyl cation 59, presumably through the bicy-clo[3.1.0]hex-2-yl cation 60 (equation 40). However, ionization of bicyclo[4.1.0]heptan-2-... 

The presence of cyclohexenyl cation intermediates was firmly established by the observation of carbocation rearrangement during the solvolysis of 25, in which an initially generated bent vinyl cation 26 with sp2 hybridization rearranges to a more stable linear vinyl cation 27 with sp hybridization [Eq. (16)]. The ratio of the rearranged to the unrearranged ketones depends on the nature of solvents used and changed from 14 86 (27a 26a) in 60% aqueous ethanol to 46 54 in the less nucleophilic 2,2,2-trifluoroethanol. 

The mechanism for solvolysis of A3-iodane 22a involves generation of the intimate cyclohexenyl cation-iodobenzene pair 28. Friedel-Crafts vinylation of iodobenzene within the intimate ion-molecular pair 28 will produce a mixture of rearranged products 23 with selective formation of the ortho isomer [Eq. (17)]. The fact that solvolysis of 22a in methanol in the presence of an excess amount (50 equiv) of p-methyliodobenzene affords the exchanged vinyliodane 22b (4 %) in addition to the formation of 23 (o m p = 86 5 9) and the recovered vinyliodane 22a (49 %) suggests the reversible formation of the free cation 29 during solvolysis. 

Harmata, M., Bohnert, G., Kurti, L., Barnes, C. L. Intramolecular 4+3 cycloadditions. A cyclohexenyl cation, its halogenated congener and a quasi-Favorskii rearrangement. Tetrahedron Lett. 2002,43, 2347-2349. 

Evidence for the bisected structure of cyclopropylcarbinyl cations has been provided by solvolysis of deuterium-labelled derivatives. Attempts to determine the stereochemistry of the degenerate rearrangement of cyclohexenyl cations via cyclopropylcarbinyl intermediates have not been successful. ... 

Formation of the 3-cyclohexenyl cation from the alcohol in superacid media is followed by extensive rearrangement, giving the methylcyclopentenyl ion, which is tertiary and allylic. ... 

General reactivities of vinyl iodonium salts are summarized, and reactions of cyclohexenyl, 1-alkenyl, styryl, and 2,2-disubstituted vinyl iodonium salts are discussed in relation to possible formation of vinyl cation intermediates. Primary vinyl cation cannot be generated thermally but rearrangement via neighboring group participation often occurs. Photosolvolysis to give primary vinyl cation is also discussed. 

Oxidation of cyclohexene by peroxydisulfate in the presence of copper(II) salts results in the formation of cyclopentanecarboxaldehyde as the main product in an aqueous acetonitrile solution (equation 261), and 2-cyclohexenyl acetate in an acetic acid solution (equation 262).588,589 Reaction (261) has been interpreted as the formation of a radical cation (186) by oxidation of cyclohexene with S2Og, followed by hydrolysis of (186) to the /3-hydroxy alkyl radical (187), which is oxidized by copper(II) salts to the rearranged aldehydic product (188 equation 263).589... 

The oxidation of cyclohexene under the same conditions afforded a variety of products including cyclohexenyl and cyclohexyl trifluoroacetates, 1,2-cyclo-hexanediol ditrifluoroacetate, and a number of unidentified components. The complex mixture of products is probably formed via rearrangement of various cationic intermediates subsequent to the initial electron transfer process. As a preparative method, the oxidation of alkenes under these conditions also suffers from the competitive addition of trifluoroacetic acid to the olefin ... 

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