Carbocations reactivity, quantitative studies

The propensity of S-S dications to undergo dealkylation was found to decrease in the order of methyl > ethyl > benzyl. This order of reactivity parallels the increase in the stability of the corresponding carbocations.94 Dealkylation of dication 77 affords thiosulfonium salt 78 in quantitative yield.95 Kinetic studies suggest SN1 mechanism of dealkylation. In addition, reaction of sulfoxide 79 with a substituent chiral at the a-carbon results in racemic amide 80 after hydrolysis. 

The correlation results for the bromination of diarylethylenes [31(X,Y)] summarized in Table 14 also involve the same serious problem. The p value increases significantly as the fixed substituent Y becomes more EW. This behaviour is indeed what is expected for the quantitative reactivity-selectivity relationship. However, in Table 14, the range of variable substituents X involved in the correlation of the respective Y sets is evidently different from set to set. The correlation for the Y = p-MeO set giving p = -2.3 should be referred to as the correlation for the T-conformation where X is more EW than Y, correlations for Y = p-Me, H and p-Br sets giving p = -3.6 may be referred to the E-conformation, and those for Y = m-Hal, especially P-NO2, refer without doubts to the P-conformation. The variation of p value cited in Table 14 demonstrates nothing other than the dependence of the selectivity p upon the propeller conformation of the diaryl carbocations. While there is no doubt regarding the importance of RSR in the mechanistic studies, these results lead to the conclusion that the RSR, or most of the non-additivity behaviour of a,a-diarylcarbocation systems which have been cited as best examples of quantitative RSR, may indeed be only an artifact. 

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