Aminolyses of Y-substituted phenyl 2-furoates and cinnamates: Effect of nonleaving group substituent on reactivity and mechanism

Abstract

Second-order rate constants (kN) have been determined spectrophotometrically for reactions of Y-substituted phenyl 2-furoates (1a-h) with piperidine and morpholine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The Brønsted-type plot exhibits a downward curvature for the reactions with strongly basic piperidine but is linear for the reactions with weakly basic morpholine. The slope of the curved Brønsted-type plot changes from -1.25 to -0.28 as the pKa of the conjugate acid of the leaving aryloxides decreases. The pKa at the center of the Brønsted curvature, defined as pKao, was determined to be 6.4. The aminolysis of 1a-h has been concluded to proceed through a stepwise mechanism on the basis of the curved Brønsted-type plot. The reactions of Ysubstituted phenyl cinnamates (2a-g) with piperidine resulted in a curved Brønsted-type plot with a pKa o values of 6.4. However, the curved Brønsted-type plot has been suggested to be not due to a change in the RDS but due to a normal Hammond effect of a concerted mechanism, since the Brønsted-type plot for the corresponding reactions with morpholine results in also a curved Brønsted-type plot with a pKao values of 6.1. The furoates with a basic leaving group (i.e., 1b-g) are less reactive than the corresponding cinnamates (i.e., 2b-g). The k 2/k-1 ratios for the reactions of 1b-h are much smaller than unity, which has been suggested to be responsible for their low reactivity.