Entropy controlled reaction of piperidine with isatin derivatives in 80% aqueous methanol

The reactions of 5- and 7-substituted isatin derivatives (H, Me, Cl, Br) with piperidine in 80% aqueous methanol gave the corresponding 1-(2’-amino-5’- or 3’- substituted phenyl)-2-(piperidin-1-yl) ethane-1,2-dione derivatives. The reaction is proceeded through nucleophilic attack on C-2 followed by ring opening process. The reaction showed a second order and the isokinetic temperature is 169 K indicating that the reaction is entropy controlled. The constant ∆G# value, the linear plot of ∆H# versus ∆S#, plot of log kN values at high temperature versus those at low temperatures indicated that a unified mechanism of the reactions for all substituents. The σo-Taft's constants are linearly correlated with log kN values and the ρ values are (2.01–1.86) pointing out that inductive effect of substituent is predominant and the rate-limiting is the breakdown of the tetrahedral intermediate T− and the ring opening is catalyzed by water.