Tert-Butyl cation nucleophilic substitution

The distinctive feature of any ionization mechanism for nucleophilic substitution is the generation of a tricoordinate carbocation in the rate-determining step. It is essential, then, that such a species not be prohibitively high in energy. The production of carbonium ions in the gas phase is a particularly unfavorable process. The heat of formation of (CH3)3C (tert-butyl cation) is +169 kcal/mol, compared with -32 kcal/mol for (CH3)3CH. The reaction... 

Rate constants for the reaction of substituted pyridines with the 4-methoxystyrene radical cations have also been measured (Table 6). The bulky 2,6-di-tert-butyl pyridine reacts with the two methyl-substituted radical cations with rate constants of approximately 10 M s , but is substantially less reactive towards 4-methoxystyrene. This reaction has been attributed to deprotonation since electron transfer would be endergonic by -0.7 V and since the effects of methyl substitution at the P-carbon are opposite to those observed for other nucleophilic additions. 2,6-Dimethylpyridine also reacts with the two methyl-substituted radical cations with rate constants of 10 M s, but is approximately I order of magnitude more reactive towards the 4-methoxystyrene radical cation. The latter reaction must be nucleophilic addition since this radical cation cannot undergo deprotonalion. Product studies have confirmed that the reaction of 2,6-dimethylpyridine with the p-methyl-4-methoxystyrene radical cation is deprotonation. The major product of irradiation of a mixture of 1,4-dicyanobenzene, 4-methoxystyrene, and 2,6-dimethylpyridine is the rearranged tautomer, 3-(4-methoxyphenyljpropene, formed by a deprotonation, reduction, protonation sequence as shown in Eq. 19. By contrast to these... 

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