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 But) and Fc2CCH2Ph, but FcC(Ph)CH2CPh3 reacts exclusively by deprotonation and Fc2CCH2R (R = H and Me) mainly (> 90%) by addition. Bulky β-substituents (But 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.