On the Gelation of Profens as a Property Causing Their Oscillatory Transenantiomerization

We have previously revealed, for the first time, the tendency of some profens (S-(+)-ibuprofen, S-(+)-naproxen, and S,R-(±)-2-phenylpropionic acid) to undergo oscillatory transenantiomerization when dissolved in some low-molecular-weight solvents. Such oscillatory reactions, which are relatively rare among chemical reactions, can be stimulated by several different physical and chemical conditions. It was the objective of this study to reveal the reason for the oscillatory transenantiomerization of profens. The analytical techniques used in our study embraced measurement of the viscosity of the profen solutions and acquiring their HPLC chromatograms and H NMR spectra. It has been firmly established that the physical property leading to oscillatory transenantiomerization of the profens is their ability to act as low-molecular-weight gelators, which results in substantial differences between the viscosity of their solutions and that of the pure solvents. One well-known cause of oscillatory reactions is diffusion-driven instability which appears in the form of oscillations. This occurs when, because of high solution viscosity, the rate of diffusion of an intermediate reaction product becomes less its rate of formation. For the profens investigated it seems most likely that their ability to substantially increase the viscosity of the solution results in the rate of diffusion of an intermediate keto–enol product being lower than its rate of formation from the respective profen enantiomer.