Theoretical and experimental studies of 1,5,7-triazabicyclo[4.4.0]dec-5-ene-catalyzed ring opening/ring closure reaction mechanism for 5-, 6-and 7-membered cyclic esters and carbonates

The ring-opening polymerizations (ROP) of five-, six-, and seven-membered ring cyclic substrates, as well as the transesterification of acyclic esters, with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst precursor in the presence of methanol as an initiator were investigated using density functional theory calculations at the B3LYP/6-311G(d) level of theory. The calculated energy barriers for the ROP are in the range from 12.5 to 21.5 kcal/mol. By analyzing the energy profiles, the overall polymerization process can be classified into exergonic for trimethylene carbonate, tetramethylene carbonate, caprolactone and 1,4-dioxane-2,5-dione, almost athermic for valerolactone, energetically unfavorable for 1,4-dioxanone, and thermodynamically forbidden for ethylene carbonate and gamma-butyrolactone. The study of the kinetics of ROP of cyclic substrates and transesterification of acyclic esters using the TBD-benzyl alcohol catalyst demonstrated that the experimentally determined rate constants for the reactions of cyclic substrates are qualitatively correlated with the calculated activation barriers. It was also experimentally found that TBD efficiently catalyzes the irreversible cyclization to give ethylene carbonate and gamma-butyrolactone, the catalyst productivity being 2-3 orders of magnitude higher than that of conventional basic catalysts. The obtained results confirm that the donor-acceptor mechanism is common for different ester-type substrates.