Caibocation secondary

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the caibocation stability order and is readily accommodated by the mechanism we have outlined. 

Additional evidence for caibocation intermediates in certain nucleophilic substitutions comes from observing reanangements of the kind normally associated with such species. For exfflnple, hydrolysis of the secondary alkyl bromide 2-bromo-3-methylbutane yields the rearianged tertiary alcohol 2-methyl-2-butanol as the only substitution product. 

Competition studies reported by Kuwajima, ° which also complement the results of Nakai, illustrate the limitations of the -effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone product (8c) arising from the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary B-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary caibocations appear to be more stable than secondary P-silyl carbocations, as judged by the formation of compound 9c). 

Caibocation rearrangements can also occur by the shift of an alkyl group with its electron pair. For example, reaction of 3,3-dimethyl-l-butene with HCl leads to an equal mixture of unrearranged 2-chloro-3,3-dimethylbutane and rearranged 2-chloro-2,3-dimethylbutane. In this instance, a secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group. 

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