Resonance stabilization cations

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

The protonation of 1-methyl-1,2,4-triazole [88] leads to a cation stabilized by amidinium type resonance [89] only if the proton adds on to N-4. Chemical shifts caused by protonation confirm this. 

The site of protonation of amino-substituted pyridazones [145] is not firmly established (Cookson and Cheeseman, 1972). Protonation on the ring nitrogen leading to a cation stabilized by amidinium-type resonance appears unlikely, since it would be expected to produce a bathochromic shift in the ultraviolet absorption. The reverse is observed and therefore oxygen protonation is more likely. 

On the other hand, for RCXJ and R2CF+ the cation stability increases along with the increase of resonance (crR) effect of a halogen F > Cl > Br > I [59]. The significant stabilizing effect of fluorine substituent was explained as a result of back-donation of an unshared electron pair of F on the vacant orbital of carbon. Stability of substituted fluoromethyl cations in gas phase increases going from CF3 to 12 [15] ... 

The SHM predicts the propenyl cation, radical and anion to have the same resonance energy (stabilization energy). Actually, we expect the resonance energy to decrease as we add ji electrons why should this be the case ... 

The difficulties encountered in using the analysis of substituent effects in solvolyses as a mechanistic probe mostly arise from the mechanistic involvement of the solvent (Shorter, 1978, 1982 Tsuno and Fujio, 1996). Consequently, the behaviour of benzylic carbocations in the gas phase should be the best model for the behaviour of the solvolysis intermediate in solution (Tsuno and Fujio, 1996). The intrinsic substituent effects on the benzylic cation stabilities in the gas phase have also been analysed by equation (2), and they will be compared here with the substituent effects on the benzylic solvolysis reaction. In our opinion, this provides convincing evidence for the concept of varying resonance demand in solvolysis. Finally, we shall analyse the mechanisms of a series of benzylic solvolysis reactions by using the concept of a continuous spectrum of varying resonance demand. 

Fig. 32 The resonance demand r vs. stability relationship for gas-phase cation stabilities. Reproduced with permission from Matsumoto etal. (1995). Copyright 1995...

The benzylic carbocation is stabilized by resonance delocalization of the cationic charge into the ring65 and as a result the benzylic carbon becomes much less positive. As the cation stability increases the isokinetic point shifts to a more positive value. 

Substituents capable of resonance stabilization of a cyclopropyl cation can lead to the formation of substitution products with a conserved three-membered ring vide supra). /S-Cation stabilization by a trimethylsilyl substituent can also inhibit silver(I)-assisted ring opening of several 7,7-dihalo-l-(trimethylsilyl)bicyclo[4.1.0]heptanes, e.g. 16, but not the analogous bi-cyclo[3.1.0]hexanes and -[b.l.OJnonanes. ... 

The utility of the ct+ and cr values in comparison to the a values could be understood from the plots of log10 (kR/kH) for the bromination of monosubstituted benzenes versus a and ct+. While the plot logio (kR/kH) versus cr is a scatter of points, the plot log10 ( r/ h) versus ct+ is a straight line [76]. Bromination of anisole, at both ortho and para positions, demonstrates how a substituent, electron-donating by resonance, can stabilize the positive charge in the intermediate cations 181 and 182 and, therefore, the respective transition states. 

From another point of view, we can say that protonation of silylamines destroys resonance structure II, that is, resonance energy stabilizes silylamines in a manner that is not possible with protonated silylamines. In essence, resonance lowers the energy content of silylamines more than it lowers the energy content of the corresponding cation. 

Both fragmentations give cations stabilized by resonance from an oxygen lone pair. This is, in fact, the correct answer. 

The correlation with suggests that the equilibrium population of the benzylic carbocation (23) is enhanced by a substituent X that can stabilize the carbocation through a resonance interaction. The correlation with o-y suggests that substituent Y has a much weaker effect. This result is not compatible with an alternative mechanism involving rate-limiting formation of a bridged cationic intermediate such as 24, since in that case the X and Y substituents would be expected to have a similar effect on cation stability. ... 

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