Reduction of 1‐benzyl‐3‐cyanoquinolinium ion by phosphonate: Solvent effects in propan‐2‐ol–water mixed solvents

초록

Rate constants (k<inf>2</inf>) were determined for the reduction of 1‐benzyl‐3‐cyanoquinolinium ion by phosphonate dianion in mixed solvents consisting of propan‐2‐ol and water. The reduction product was mostly 1‐benzyl‐3‐cyano‐1,4‐dihydroquinoline with a trace of the 1,2‐isomer. The solvent properties were varied by increasing the fraction of water in the mixed solvent, which increases the polarity of the solvent. Increasing the proportion of alcohol makes the solvent a better electron pair donor. The higher fraction of propan‐2‐ol in the mixed solvent gives rise to a substantial increase in k<inf>2</inf>. A quantitative comparison of the solvent effect on this reaction with the solvent effects on related reactions suggests a solvent Brønsted α value of 0·45. This suggests that the PO<inf>3</inf>− unit of the transition state interacts only weakly with the solvent although the final oxidized product is phosphate. Copyright © 1992 John Wiley & Sons, Ltd.