Kinetics of the Oxidation of Isoquinolinium Cations by Ferricyanide Ion

Article abstract of DOI:10.1021/jo00361a024

The rates of the ferricyanide oxidation of a number of isoquinolinium cations have been investigated in the range pH 11-14 in 20percent CH3CN-80percent H2O at 25 deg C (ionic strength 1.0).The oxidation of the 2-methylisoquinolinium cation is kinetically first order in each of ferricyanide ion and isoquinolinium cation but is subject to pronounced inhibition by the ferrocyanide ion reaction product.Second-order rate constants evaluated from initial reaction rates display a pH dependence which is consistent with rate-determining ferricyanide attack on the pseudobase alkoxide ion derived from the isoquinolinium cation.Ferrocyanide ion inhibition is shown to be consistent with this initial electron transfer being a reversible process.Similar observations are made for a number of substituted isoquinolinium cations.Substituent effects are shown to be consistent with electron abstraction from the endocyclic nitrogen atom rather than the exocyclic oxyanion of these pseudobase alkoxide ions.The initially formed radical then undergoes a base-catalyzed deprotonation from C-1, and this is confirmed by deuterium kinetic isotope effects when a deuterium label is present at C-1.The combination of these pH effects leads to ferrocyanide ion inhibition becoming less pronounced with increasing pH.These observations have prompted us to reinvestigate the ferricyanide ion oxidation of some of the 5-nitroisoquinolinium cations that we have previously studied.We find that ferrocyanide ion inhibition is observable when the initial rates of these reactions are examined in the stopped-flow spectrophotometer.Detailed kinetic analysis of these oxidations is shown to be consistent with the reaction scheme detailed above for the 2-methylisoquinolinium cation.In the light of this work, we must now withdraw our earlier claim for rate-determining hydride abstraction in these reactions.Substituent effects on the oxidation of these isoquinolinium cations allow the development of a relationship which can be used to predict the pH-rate profile for the ferricyanide ion oxidation of any isoquinolinium cation for which pK_R+ for pseudobase formation is available, provided that steric hindrance from the N substituent is not significant.