A kinetic study of the hydrolysis of ferrocenyliminium lons

Abstract

Rate constants have been detemined for reaction of PF₆^– salts of ferrocenliminium ions FcC(R¹)[graphic omitted]R₂(1) FcC(R¹)[graphic omitted]HR (3)(Fc = ferrocenyl; R¹= H, Me, or aryl; R = H or alkyl) with hydroxide ion in water and in H₂O–MeCN (1:1 W/W). The reactions, which are faster in the mixed solvent, give acylferrocenes quantitatively and proceed by rate-limiting nucleophilic addition to the iminium ions. Increasing bulk of groups attached to the C[graphic omitted] system retards reaction, with a much larger steric effect exerted by C-substituents than by [graphic omitted]-substituents. Electronic effects of substituents on rates are relatively small. Reaction is very slow in the absence of added hydroxide ion and is not catalysed by general bases, and the reactivity of water relative to that of hydroxide ion towards the iminium ions is much lower than predicted by the N₊ nucleophilicity scale. Added inorganic salts retard reaction. The reativities of the ion (1) and (3) towards water and hydroxide ion are lower than those of aryl analogues, and the ions (3) are slightly weaker acids than protonated imines derived from substitued benzaldehydes and benzophenones.