# Aromatic Amines in Organic Synthesis Part III; \(p\)-Aminocinnamic Acids and Their Methyl Esters
## 2 Results and Discussion
The analysis of cyclic voltammograms of the tested compounds indicates that their electrochemical oxidation is irreversible. The position of the oxidation peaks depends on the amino substituents. For example, the values of the oxidation potential increase with the elongation of the aliphatic substituents at the amino group (2a is methyl groups, 2b is ethyl groups, and 2c is butyl groups). The lowest value is observed for compound 2j, which is a morpholine derivative. This means that 2j has the greatest tendency to lose electrons. The oxidation potentials of all the tested acids range from 963 to 1285 mV, with the highest value of 1285 mV for compound 2f with a methyl substituent in the _ortho_ position to the \(N\),_N_-dimethylamino group responsible for its coplanar conformation and the decoupling effect between the dimethylamino group and the electron-accepting part of the molecule [50]. Moreover, the oxidation potentials of cinnamic acids containing cyclic terminal groups, i.e., pyrrolidine, piperidine, and morpholine, depend on the change in aromatic ring-nitrogen resonance interaction as the saturated heterocyclic is varied. In general, the extent of overlap of the nitrogen lone-pair electrons with the \(\pi\)-electrons of the aromatic ring in N-phenyl-substituted saturated heterocycles may depend upon the molecular conformation, the nature of the heterocyclic, the angle of rotation of the aromatic ring about the N-C(phenyl) bond, and any _p_-substituent present [51]. The transformation of cinnamic acids into the corresponding methyl esters leads to an increase in oxidation potentials, e.g., from 993 and 1084 mV for 2h and 2i to 1306 and 1210 mV for 3h and 3i, respectively. This effect is probably related to the decrease in the dipole moment when the COOH group is replaced with COOMe (\(\Delta\)Hg = 0.58 D for 2h\(\rightarrow\)3h and \(\Delta\)Hg = 1.07 D for 2i\(\rightarrow\)3i). Furthermore, alkane bridges between C\({}_{ortho}\) and N\({}_{amino}\), e.g., in julolidine derivatives, cause the nitrogen atom to reveal weaker electron-donor properties than that in the 1-pyrrolidinino group (E\({}_{OX}\) = 1171 mV vs. 993 mV). This is consistent with the \(\sigma_{R}\)\({}^{0}\) substituent constants of the 1-N[2,6-[(CH\({}_{2}\))\({}_{3}\)]\({}_{2}\)] (\(-\)0.59) and N(CH\({}_{2}\))\({}_{4}\) (\(-\)0.63) amino groups [52]. The modification of cinnamic acids with stiffened electron-donating substituents leads to a planar conformation of  
Figure 1:1 Cyclic voltammograms of compounds 2a, 2b, and 2c in anhydrous acetonitrile. Solution contained tetra-_n_-butylammonium perchlorate as the supporting electrolyte and was deoxygenated prior to analysis by purging with helium for 15 min. Scan rate was 300 mVs\({}^{-1}\). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Abbr.** & **E\({}_{ox}\) (mV)** & **Abbr.** & **E\({}_{ox}\) (mV)** & **Abbr.** & **E\({}_{ox}\) (mV)** & **Abbr.** & **E\({}_{ox}\) (mV)** \\ \hline
2a & 1132 & 2f & 1285 & 2k & 1072 & 3a & 1183 \\
2b & 1213 & 2g & 1264 & 2l & 1150 & 3h & 1306 \\
2c & 1261 & 2h & 993 & 2m & 1171 & 3i & 1210 \\
2d & 1192 & 2i & 1084 & 2n & 1147 & 3l & 1240 \\
2e & 1177 & 2j & 963 & 2o & 1066 & 3n & 1171 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The measured oxidation potential of the tested compounds. the molecules, which increases the probability of charge separation and leads to a decrease in oxidation potentials. The importance of the electronic effect of amino groups is also confirmed by the linear correlation of E\({}_{\text{OX}}\) values with the corresponding \(\sigma_{\text{R}}\)\({}^{0}\) substituent constants [52; 53; 54] (see Figure S13 in Supplementary File). Cinnamic acids, which bear a stronger electron-releasing substituent, have a lower oxidation potential.