Influence of Donor/Withdrawing Groups in an 3,5-Aryl-Substituted Pyrazole Organocatalyst for the Chemical Fixation of CO₂

A series of 13 pyrazole derivatives, each featuring varied aryl groups in the 3,5-positions, were synthesized and characterized, including the determination of crystalline structures for compounds <b>11</b> and <b>13</b>. These pyrazoles, in the presence of tetrabutylammonium bromide (TBAB), selectively produced propylene cyclic carbonate (PC), with conversions reaching up to 90% and turnover frequency (TOF) = 75 h^-1 under optimal conditions (0.4 mol % of <b>13</b> and TBAB, 120 °C, 3 h, 30 bar). A 3³ Box-Behnken experimental design, with triplicate in the central point, was employed to evaluate the effects of temperature, catalyst, and cocatalyst loading. The nature of the aryl substituent significantly influenced the conversion rates, with electron-withdrawing groups (e.g., NO₂) yielding higher conversion than electron-donating groups (e.g., Me, MeO). Notably, pyrazoles featuring strong electron-donating <i>p</i>-OH-C₆H₄ (<b>11</b>-<b>13</b>) achieved the highest conversions, suggesting that the hydroxyl group also acts as a catalytic site. Density functional theory (DFT) calculations provided insight into the reaction mechanism and energy profiles, highlighting the roles of both the hydroxyl (OH) and amino (NH) groups in the catalytic cycle of compound <b>13</b>.