Figure 2. Intrinsic reaction coordinate (IRC) curve of TS1 and energy level diagram of elementary reaction 1.
Simultaneously, H(12) of isatin (2a ), which is connected to N(4), leaves in the presence of triethylamine, leading to an increase in the charge on N(4). The C(3)–O(11) bond length increases from 1.213 to 1.236 Å, while the N(4)–C(5) bond is shortened from 1.404 to 1.379 Å. The latter bond length is between that of a C–N single bond and C–N double bond; thus, N(4)–C(5) has a partial double bond character. The N(4)–C(3) bond is shortened from 1.380 to 1.358 Å, while the C(3)–N(4)–C(5) bond angle changes from 112.08° to 106.46°. The negative charge on N(4) increases when intermediate 2a’ , which has a net negative charge, is formed. Because of the strong electron-withdrawing effect of the carbonyl groups, the H atom on the amide is more acidic. It leaves more easily and bonds to the N atom of the weak organic base, triethylamine. This process occurs spontaneously without a transition state. The geometry of each stationary point is shown in Figure 3.