Cooperative β-cyclodextrin/TBD acylation/deacylation in the ring-opening of ε-caprolactone - a 1H NMR and MALDI MS kinetic study.

Abstract

Cyclodextrins (CDs) are widely employed in supramolecular chemistry for their ability to form inclusion complexes, yet the connection between cyclic ester complexation and its influence on ring-opening processes remains insufficiently understood. Here, we investigate the ring-opening oligomerization of ε-caprolactone (ε-CL) in the presence of β-cyclodextrin (β-CD) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). The monomer conversion proceeds via monomer acylation to form cyclodextrin-oligocaprolactone intermediates, followed by their deacylation to homooligoesters. Complementary 1H NMR and MALDI MS kinetic analyses were used to simultaneously monitor the distribution of monomer fractions. The kinetic study took into consideration the influence of β-CD, TBD, and the relative amounts in the feed. Special attention is given to the influence of the cyclodextrin structure (α-, β-, γ-CD, and TMS-β-CD - a 6-O-tert-butyldimethylsilyl-substituted β-CD). The obtained results reveal that the two hydroxyl rims of β-CD perform distinct functions. The secondary C2,3 rim acts as the reactive catalytic face, undergoing selective acylation and deacylation, whereas the primary C6 rim serves as a stabilization site reached through intramolecular C2,3/C6 acyl migration, regenerating the reactive C2,3 hydroxyls. Blocking this migration suppresses active-site regeneration and polymer formation. These findings establish the molecular basis of β-CD pseudo-enzymatic behavior and provide mechanistic principles for designing carbohydrate-based catalysts.