Effects of the alkyl substituent in the π-donor heteroatom on the kinetic and thermodynamic acidities of Fischer thiocarbene complexes

Abstract

Rate constants for the proton transfer reaction from Fischer thiocarbene complexes (CO)₅MC(SR₁)CH₃ (M = Cr or W; R₁ = n-butyl, isopropyl, tert-butyl, cyclohexyl) to OH^– and various primary and secondary amines were determined in 50% acetonitrile–50% water at 25 °C. These measurements allowed the determination of the thermodynamic and kinetic acidities for these substrates. The results obtained show that there is a slight effect of the substituent on the thermodynamic acidity, which is governed by its hydrophobicity; whereas the effect on the kinetic acidity is more noticeable and is due to the steric effect of the substituent.