Impact of the α‐Ferrocenyl Group on the Solvolytic Reactivity – Electrofugality – of Ferrocenylphenylmethyl Cations

Abstract

The electron‐donating effect of the ferrocenyl group in α‐position to the reaction center on SN1 solvolytic reactivity has been quantified by determining the electrofugalities (Ef) of a series of ferrocenyl‐X‐phenylmethyl cations according to the LFER equation: log k = sf(Ef + Nf). Due to highly stabilized transition state, the Ef values are about eight units higher than those of the corresponding benzhydryl cations. Impact of the phenyl group in ferrocenylphenylmethyl derivatives on stabilization of the positive charge is somewhat leveled by the ferrocenyl group, so the rate effect of the substituents on the phenyl ring is suppressed, causing narrow range of Ef parameters and small absolute values of the ρ+ constant. Calculations at B2PLYP‐D3/may‐cc‐pVTZ level of theory showed that the positive charge in ferrocenylphenylmethyl cation is largely shifted from the phenyl to ferrocenyl moiety, and that the substituents on the phenyl ring have small impact on its stability.

Electrofugality of Some Ferrocenylphenylmethyl Cations

“…To extract the nucleofugality parameters (N f and sf) for carboxylates a-d, the logarithms of the first-order solvolysis rate constants in the given solvents were plotted against published Ef, values of ferrocenylphenylmethyl cations 5. [6] The correlation lines are presented in Figure S1a and S1b in the Supporting Information. The nucleofugespecific parameters (Nf and sf) are presented in Table 1.…”

“…Kinetic results obtained earlier with the series of ferrocenylphenylmethyl derivatives 5 as well as quantum chemical calculations showed that the effect of the substituents on the phenyl ring is suppressed due to very strong electron donating ability of the α-ferrocenyl group, i.e., leveling occurs. [6] The question arose if this applied to all X substituents (1)(2)(3)(4)(5), or electron-donating substituents on the phenyl in substrates 1-3 had somewhat more pronounced effect than in 5. This assumption was tested with Hammett-Brown correlation.…”

“…[5] In our previous work we determined the electrofugality of some ferrocenylphenylmethyl cations, mostly those with electron-accepting groups on the phenyl ring (5 in Scheme 1). [6] In this work we chose to extend the spectrum of Ef parameters for ferrocenylphenylmethyl substrates by investigating some more reactive ferrocenylphenylmethyl electrofuges (1)(2)(3)(4)Scheme 1). The aim was to (a) collect additional Ef parameters that can be used for estimation of the absolute first-order heterolysis rates in a given solvent for variety of substrates that are combination of ferrocenylphenylmethyl moiety and any nucleofuge of known sf and Nf parameters, by applying Equation (1), (b) to determine if the stabilizing effect of the ferrocenyl group is similar to that in less reactive substrates, and (c) to make feasible comparison of reactivities of the series of ferrocenylphenylmethyl cations with numerous electrofuges with known Ef values.…”

“…Up to now the least reactive nucleofuge on the scale is acetate anion (Nf from -3.55 to -4.8 in various solvents). [6,7] It turned out that acetates of ferrocenylphenylmethyl cations 1-4 solvolyze too fast, so electrofuges 1-4 should be combined with less reactive nucleofuges. Having in mind that according a qualitative rule of thumb, the abilities of leaving groups are arranged in the same order as the acidities of their conjugate Brönsted acids, we assumed that carboxylates with longer alkyl chain, as are butyrate, isobutyrate, valerate, and isovalerate (a-d, Scheme 1), would be appropriate.…”

“…Therefore, instead using reactive benzhydryl substrates (Ef > 0) to get the nucleofugality parameters from log k vs. Ef plots, we used ferrocenylphenylmethyl derivates whose Ef values have already been detemined. [6]…”