A kinetic study of concomitant addition and deprotonation reactions of ferrocenyl-stabilised carbocations in aqueous acetonitrile and of the reverse reactions

Abstract

Rate constants for addition of water and for deprotonation in aqueous acetonitrile have been measured for a series of ferrocenyl-stabilised carbocation salts. Both alcohol and alkene are formed from FcC(Ph)CH₂R (Fc = ferrocenyl; R = Ph and Bu^t) and Fc₂CCH₂Ph, but FcC(Ph)CH₂CPh₃ reacts exclusively by deprotonation and Fc₂CCH₂R (R = H and Me) mainly (> 90%) by addition. Bulky β-substituents (Bu^t and CPh₃) retard both reactions by steric and conformational effects which disfavour addition and proton loss. The usual rate-accelerating effect of a β-phenyl group upon deprotonation is strongly attenuated by steric hindrance to styryl conjugation. Rates of water addition and deprotonation are slowed by perchlorate ions (from NaClO₄ or HClO₄). Chloride ions (from NaCl or HCl) retard water addition but substantially accelerate deprotonation; the magnitude of the latter effect is reduced in the presence of perchlorate ions showing that ion-pairing must be extensive and that both anions compete for carboactions, with rates of elimination from the (R⁺)(Cl^–) ion-pairs enhanced. At a given acidity, diferrocenylalkanols Fc₂C(OH)CH₂R undergo acid heterolysis in aqueous acetonitrile faster than the monoferrocenyl substrates FcC(Ph)(OH)CH₂R, but there is no obvious correlation between these rates and those for water addition to the resulting carbocations. For a given acidity and solvent, protonation of the alkenes FcC(R¹)= CHR² is faster for the diferrocenyl (R¹= Fc) than for the monoferrocenyl compounds (R¹= Ph), and is slowed when R² is phenyl or t-butyl. Rates of both alcohol heterolysis and alkene protonation are increased by chloride and perchlorate ions and by an increase in the water content of the solvent from 1 : 1 to 3 : 1 w/w H₂O–MeCN.