Cyclohexenyl cation

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

These results show that cyclohexenyl iodonium salt readily gives cyclohexenyl cation under poorly nucleophilic/basic conditions. When a stronger nucleophile like bromide is added to the solution of 3, 1-bromocyclohexene 13 is formed as a main product (eq 5). It is noteworthy here that the rate of the reaction is strongly retarded by the added bromide salt as a nucleophilic reagent (Figure 1), although bromide does react with the substrate to give substitution product 13.6... 

The infra-red spectra of the trimethyl, dimethyl- and dimethylethyl-carbonium salts in excess antimony pentaduoride are shown in Figs. 4a, b, and c. The IRTRAN cells used are not transparent below 770 cm , thus obscuring the 650 cm SblY absorption which would, however, be overlapped by the solvent SbFs absorption. The broad, intense absorption band which appears in all the spectra near 1550 cm is present in the solvent spectrum. It was found to be dependent on the purity of the SbFs, but the nature of the impurity was not established. It should also be mentioned that Deno found an intense absorption at 1533 cm in cyclohexenyl cations thus, secondary carbonium ions formed from the reaction with olefins (which arise from deprotonation) could add to this broad absorption. 

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-... 

Farnum, Mehta, and co-workers have shown that longifolene 218 and isolongifolene 219 in HSO3F media give a mixture of cyclohexenyl cations at different temperatures. Quenching these cations provides some unusual sesquiterpene like Cis-hexahydronaphthalenes 220-225 (Scheme 5.82). 

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. 

However, when a linear geometry cannot be achieved, as in the case of the cyclohexenyl cation 167, a bridged geometry of type 168 (in a nonclassical formulation) may become more stable, at least when R = Alkyl. This type of bridged ion is likely to be generated in the triple... 

A symmetrical ailyl cation can give one product only by the S jl reaction. We have already discussed the formation of the cyclohexenyl cation (Chapter 7) and that is a good example. The two delocalized structures are identical and the 7t bond is shared equally among the three atoms. 

Bishomo-square pyramidal cation, 2-bicyclo[3.2.1]octa-3,6-dienyl cation and related cations 302 Cyclobutenyl cations 308 Cyclopentenyl cations 310 Cyclohexenyl cations 311 Benzenium ions and related cations 313 9,l0-Dihydro-10-phenanthrenium ions and related cations 323 Acenaphthenium ions 332 Vinyl cations 334... 

Reaction mechanisms in the cyclohexenyl cation system have been further investigated by Rajeswari and Sorensen (1972). Two different processes (178)... 

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. 

In contrast,when deuterium was introduced into single minimum, nonequilibrating species such as the cyclohexenyl cation 4 [Eq. (5.4)], an allylic cation with allylic resonance, no large splittings were observed. Unlike the... 

Because of the preferred linearity of vinyl cations, the stability of cyclic vinyl cations depends on the ring strain or the ring size. Rates of solvolysis of Inflates show that the 1-cyclohexenyl cation is 6 X 10 -fold less readily generated than the 1-methyl-1-propenyl cation. The 1-cyclopentenyl cation is not formed at all. On the other hand, 1-cyclobutenyl triflate is 3700 times more reactive than 1-cyclohexenyl triflate.The stability of the cyclobutenyl cation is due to resonance involving cyclopropyl-stabilization of the positive charge. ... 

---