Kinetic and equilibrium study of the ring opening of 2-aryl-1-methyl-1-pyrrolinium ions in aqueous solution

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Abstract
Ultraviolet spectral and kinetic measurements are reported for the transformations in aqueous solution undergone by the cyclic iminium ion, the 2-aryl-1-methyl-1-pyrrolinium ion I+. In solutions with pH < 9.5 this ion equilibrates with a ring-opened hydrated form, the protonated 4-methylaminobutyrophenone SH+. The concentration of the latter at equilibrium is small, but it can be observed using nuclear magnetic resonance spectroscopy. A kinetic analysis provides a measure of the I+ .dblarw. SH+ equilibrium constant. Values range from 0.06 for 4-methoxyphenyl to 0.17 for 3-chlorophenyl. In acid solutions (pH < 5) the equilibration is slow (half-life .apprxeq. 20 min for phenyl) and pH independent. Above pH 5 this process becomes first order in hydroxide ion. In strongly basic solutions a neutral species is formed in equilibrium with these two cations, with pK(app) for the transformation cations .dblarw. neutral + H+ ranging from 11.8 to 11.3. Spectral arguments imply that the base species is the ring-opened 4-methylaminobutyrophenone S, with less than ten percent of the pseudobase of I+ present.