Vinyl cations as SNV1 intermediates

Radioactive tritium ( H, half-life, 12.26 years) spontaneously decays to He with emission of a /3 particle to give vibrationally excited [H2C=CH- He] , which converts to the free vinyl cation by heterolysis of the very weak C- He bond. In benzene as solvent, the Friedel-Crafts product, styrene, is detected after 10-14 months. Photoexcitation of appropriate precursors also generates primary vinyl cations as discussed in the last part of Section 3 and in Section 5. 

In the gas phase, vinyl cations are easily produced by mass spectrometric techniques, which also allow measurement of their heats of formation.Gas-phase spectroscopic studies show that the smallest vinylic cation, C2HJ, has a structure of protonated acetylene (28b) and not that of the classical vinyl cation (28a). 

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.  

The kinetic stability of vinyl(phenyl)iodonium salts is largely due to the instability of the corresponding vinyl cations formed on departure of the iodobenzene nucleofuge. Thus, o -phenyl- or a-alkyl-substituted (secondary) vinyl iodonium ions so far could not be prepared as stable salts.  

In addition to these expected solvolysis products, (4- -butyl-l-cyclohexenyl)io-dobenzenes (33) were obtained in yields of 13-15% in methanol and aqueous solutions and of 30-40% in TFE. The ortho derivative o-33 is the predominant isomer of these products, and evidently is formed via the internal return of the contact ion-molecule pair of cyclohexenyl cation (31) and iodobenzene. The rates 

---