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.