Nucleophilic addition to ferrocenyl-stabilised carbocations: the nature of the transition state

Abstract

Rate constantsfor reactions ofp-MeOCsH4dHFc (la) (Fc = ferrocenyl), Fc,~H (1 b), Fc,tPh (lc), and Fc6HCH=CHFc (1 d) with nucleophiles in water and water-acetonitrile can be approximately correlated with the N+ scale.There is a good correlation with rate constants for nucleophilic addition to (p-MeOC6H4),C+ in water, except for bulky amines. Reactivities of bulky amines towards a secondary carbocation are always greater than predicted by comparison with reactivities towards a tertiary carbocation, suggesting steric hindrance to the latter reaction. These steric effects are much smaller if comparisons of reactivity are between two secondary carbocations, e.g. (1 a and b). The secondary a-hydrogen kinetic isotope effects (kH/kD) for reactions of ( l a and b) are ca. 1.0 with anionic nucleophiles, ca. 0.96 with primary amines, and ca. 0.9 with water, and depend on the nature of the nucleophile and not its reactivity.

Cation–anion combination reactions. 26. A review

“…A difference in mechanism of the water and hydroxide ion reactions is also indicated by the results of a study of the a-deuterium isotope effect on the reactions of di-ferrocenylmethyl cation (38). The k, shows k H / k D = 0.91, koH shows k H / k D = 0.99, and KR shows KH/KD = 0.85.…”

“…8.26; p-dimethylaminophenyltropylium cation + thiophenoxide ion (23) McClelland's very recent results for reactions of triphenylmethyl cation are particularly interesting (36). Although this study was actually carried out in 1 : 2 acetonitrile: water solvent, it does not seem likely that the results would be appreciably different in pure water (see, for example, the study by Bunton et al (38) in a very comparable solvent). The rate constant for reaction of azide ion with the trityl cation, k,, = 4 x lo9 M-' s ' .…”

“…Bunton et al (38,67) have extended the N+ relationship to reactions of ferrocenyl-substituted methyl cations, and to a few nucleophiles, such as borohydride ion, which are not simple Lewis bases. His studies of the a-isotope effects on the reactions have been discussed in earlier sections of this paper.…”

A comprehensive N+ scale of nucleophilicity in an equation including a Swain‐Scott response/selectivity parameter

“…In contrast, N in Eqns (1, 2) refers to a floating scale, defined less clearly by multi‐parameter correlations . Steric effects are less important for these N + scales, based on secondary cations (e.g. 2 and 3 ), compared with tertiary cations originally chosen …”

Limitations of the s(E+N) and Related Equations: Solvent Dependence of Electrophilicity

“…[32] For cation-nucleophile recombinations in protic solvents, anion desolvation is a dominant process for both anions [33] and amines; [34] for cations R 1 R 2 CH + , k H /k D % 1.00, [35] thus suggesting that there is little change in hybridization at the carbenium center in the transition state. [8] In contrast, reactions of alkenes in dichloromethane show k H /k D % 0.80, thus suggesting that "rehybridization of the carbenium ion center is far advanced in the transition state."…”