Nucleophilicity towards a vinylic carbon atom: rate constants for the addition of amines to the 1-methyl-4-vinylpyridinium cation in aqueous solution

Abstract

Second-order rate constants (KNu) have been measured for the addition of 44 primary amines (including five α-effect amines), 28 secondary amines, 19 tertiary amines, ammonia and hydroxide ion to the vinyl group of the 1-methyl-4-vinylpyridinium cation (1) in aqueous solution at 25° C (ionic strength 0.1 mol dm–3). Nucleophilic attack is shown to be rate-determining for primary and secondary amines, with secondary amines being generally more reactive than primary amines of the same basicity. After classification of these species in terms of structure, they describe a number of Bronsted-type correlations having βnuc in the range 0.35–0.54 for six structural classes of primary amine, βnuc= 0.48 for α-effect amines, and βnuc in the range 0.23–0.34 for four structural classes of secondary amine. Substitution upon the α-carbon atom reduces amine nucleophilicity of both primary and secondary amines. The presence of an unsaturated carbon atom (either sp2- or sp-hybridized) as the β-carbon atom leads to an enhanced reactivity relative to the corresponding β-sp3 species in all cases. Tertiary amines are in general less reactive than other amines of the same basicity. Bronsted-type plots for tertiary amines present the appearance of random scatter which is not readily decipherable in terms of structure. β-Hydroxy and β-amino tertiary amines are unusually reactive relative to their basicity. All of these phenomena suggest that protonation of the carbanionic intermediate by a molecule of water is the rate-determining step for the addition of tertiary amines to 1.Rate constants for the attack of primary and secondary amines on 1 are shown to correlate with literature data for a variety of other reactions involving rate-determining nucleophilic attack of amines upon electrophilic carbon. These kNu for primary and secondary amines reacting with 1 are also shown to correlate with Ritchie's N+ parameters for nucleophilic attack at electrophilic sp2-carbon. N+ parameters for amine nucleophiles have not been widely available previously; the parameters that have been available for selected amines are known to be sensitive to the nature of the defining electrophile. The minimal steric hindrance at the electrophilic centre in nucleophilic attack upon 1 suggests that this species is an appropriate electrophile for the definition of N+ parameters for amine nucleophiles; these parameters are evaluated for 70 primary and secondary amines and ammonia and are suggested to provide an appropriate data base for future investigations of the reactivity and selectivity of amine attack upon sp2-carbon electrophiles in aqueous solution.