Kinetic Studies on the Nucleophilic Addition Reactions of Vinylic ￥a-Diketones

摘要

The kinetics of the addition of X-substituted benzylamines (BA) to Y-substituted Benzylideneacetylacetones (BAA) have been investigated in acetonitrile at 25.0∩. The reaction is studied under pseudo-first-order conditions by keeping a large excess of BA over BAA. The addition of BA to BAA occurs in a single step in which the addition of BA to Cメ of BAA and proton transfer from BA to Cモ of BAA take place concurrently with a four-membered cyclic transition state structure. The magnitude of the Hammett ( ヱx) and Bronsted ( モx) coefficients are rather small suggesting an early tansition state (TS). The sign and magnitude of the cross-interaction constant, ヱ_XY (= -0.49), is comparatible to those found in the normal bond formation processes in the S_N2 and addition reactions. The normal kinetic isotope effect (k_H/k_D > 1.0) and relatively low DH⒃ and large negative DS⒃ values are also consistent with the mechanism proposed.