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

Abstract

Rate constants for reactions of p-MeOC₆H₄CHFc (1a)(Fc = ferrocenyl), Fc₂CH (1b), Fc₂CPh (1c), and FcCHCHCHFc (1d) 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)₃C⁺ 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.(1a and b). The secondary α-hydrogen kinetic isotope effects (k^H/k^D) for reactions of (1a 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.