Solvolytic behavior of some ferrocenylphenylmethyl‐4‐methoxypyridinium cations in various solvents

Abstract

The nucleofugalities of 4‐methoxypyridine (4‐OMePy) in pure and aqueous solvents were derived from the rate constants for SN1 solvolysis with respect to the corresponding ferrocenylphenylmethyl‐4‐methoxypyridinium salts by applying the linear free energy relationship (LFER) equation, log k (25°C) = sf (Nf + Ef). In more polar solvents, pyridinium ions become more stabilized by solvation in the reactant ground state than in the corresponding transition state and the reactivity (nucleofugality, Nf) of 4‐OMePy decreases increasing solvent polarity. Due to the strong electron‐donor effect of the ferrocenyl group, it seems that 4‐methoxypyridine is a somewhat better leaving group (stronger nucleofuge) if the substrate is benzhydryl‐4‐methoxypyridinium salts rather than ferrocenylphenylmethyl‐4‐methoxypyridinium salts. Given that the electron‐withdrawing substituents on the 4‐position of the pyridine ring increase the rate of the heterolysis step of solvolysis for benzhydryl and ferrocenylphenylmethyl pyridinium salts, the Nf value of substituted pyridine decreases in order Nf (4‐ClPy) > Nf (Py) > Nf (4‐MePy) > Nf (4‐OMePy) across in all examined solvents.